Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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OFDM/OFDMA FRAME STRUCTURE FOR
COMMUNICATION SYSTEMS
Cross-Reference to Related Applications
[0001] This application claims priority to U.S. Provisional Patent Application
No.
60/980,760 filed on October 17, 2007, U.S. Provisional Patent Application No.
61/020,690
filed January 11, 2008 and U.S. Provisional Patent Application No. 61/032,032
filed on
February 27, 2008, the contents of which are incorporated by reference herein
in their
entirety.
Field of the Invention
[0002] The present invention relates generally to digital communications and
more
particularly to Orthogonal Frequency Division Multiplexing (OFDM) and
Orthogonal
Frequency Division Multiple Access (OFDMA) systems.
Background of the Invention
[0003] There is an increasing need for mobile high speed communication systems
to
provide a variety of services such as the Internet, television, photo sharing,
and
downloading music files. In order to provide such services, a mobile high
speed
communication system must be able to overcome a variety of difficult operating
conditions caused by the environment. Among these operating conditions are
multipath
signals, inter-symbol interference (ISI), and inter-channel interference
(ICI). In mobile
high speed communication systems, multipath is interference resulting from
radio signals
reaching the receiving antenna by two or more paths. Causes of multipath
include
atmospheric ducting, ionospheric reflection and refraction, and reflection
from terrestrial
objects, such as mountains and buildings. In telecommunications, ISI is a form
of
distortion of a signal in which one symbol interferes with subsequent symbols.
ICI is a
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form of distortion of a signal caused by transmission of signals on adjacent
channels that
may interfere with one another.
[0004] Figure 1 illustrates a mobile radio channel operating environment 100.
The mobile
radio channel operating environment 100 may include a base station (BS) 102, a
mobile
station (MS) 104, various obstacles 106/108/110, and a cluster of notional
hexagonal cells
126/130/132/134/136/138/140 overlaying a geographical area 101. Each cell
126/130/132/134/136/138/140 may include a base station operating at its
allocated
bandwidth to provide adequate radio coverage to its intended users. For
example, the base
station 102 may operate at an allocated channel transmission bandwidth to
provide
adequate coverage to the mobile station 104. The base station 102 and the
mobile station
104 may communicate via a downlink radio frame 118, and an uplink radio frame
124
respectively. Each radio frame 118/124 may be further divided into sub-frames
120/126
which may include data symbols 122/128. In this mobile radio channel operating
environment 100, a signal transmitted from a base station 102 may suffer from
the
operating conditions mentioned above. For example, multipath signal components
112
may occur as a consequence of reflections, scattering, and diffraction of the
transmitted
signal by natural and/or man-made objects 106/108/110. At the receiver antenna
114, a
multitude of signals may arrive from many different directions with different
delays,
attenuations, and phases. Generally, the time difference between the arrival
moment of
the first received multipath component 116 (typically the line of sight
component), and the
last received multipath component (possibly any of the multipath signal
components 112)
is called delay spread. The combination of signals with various delays,
attenuations, and
phases may create distortions such as ISI and ICI in the received signal. The
distortion
may complicate reception and conversion of the received signal into useful
information.
For example, delay spread may cause ISI in the useful information (data
symbols)
contained in the radio frame 124.
[0005] Orthogonal Frequency Division Multiplexing (OFDM) is one technique that
is
being developed for high speed communications that can mitigate delay spread
and many
other difficult operating conditions. OFDM divides an allocated radio
communication
channel into a number of orthogonal subchannels of equal bandwidth. Each
subchannel is
modulated by a unique group of subcarrier signals, whose frequencies are
equally and
minimally spaced for optimal bandwidth efficiency. The group of subcarrier
signals are
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chosen to be orthogonal, meaning the inner product of any two of the
subcarriers equals
zero. In this manner, the entire bandwidth allocated to the system is divided
into
orthogonal subcarriers.
[0006] Orthogonal Frequency Division Multiple Access (OFDMA) is a multi-user
version
of OFDM. For a communication device such as the base station 102, multiple
access is
accomplished by assigning subsets of orthogonal sub-carriers to individual
subscriber
devices. A subscriber device may be a mobile station 104 with which the base
station 102
is communicating.
[0007] An inverse fast Fourier transform (IFFT) is often used to form the
subcarriers, and
the number of orthogonal subcarriers determines the fast Fourier transform
(FFT) size
(NFFT) to be used. An information symbol (e.g., data symbol) in the frequency
domain of
the IFFT is transformed into a time domain modulation of the orthogonal
subcarriers. The
modulation of the orthogonal subcarriers forms an information symbol in the
time domain
with a duration Tu . Duration Tõ is generally referred to as the OFDM useful
symbol
duration. For the subcarriers to remain orthogonal, the spacing between the
orthogonal
subcarriers Of is chosen to be T , and vice versa the OFDM symbol duration Tu
is o ff
T.
The number of available orthogonal subcarriers Nc (an integer less than or
equal to
NFFT) is the channel transmission bandwidth (BW) divided by the subcarrier
spacing
Of , or BW * Tu .
[0008] Figure 2 illustrates principles of an OFDM/OFDMA multicarrier
transmission with
four subcarriers. The principle of multi-carrier transmission is to convert a
serial high-rate
data stream 202 into multiple parallel low-rate sub-streams 204 by a serial-to-
parallel
converter. Each parallel sub-stream is modulated on to one of Nc orthogonal
sub-carriers
206, where Nc is an integer that, for example, can be greater than or equal to
128. The
Nc sub-streams are modulated onto the Nc sub-carriers 206 with a spacing of Of
in
order to achieve orthogonality between the signals on the Nc sub-carriers 206.
The
resulting Nc parallel modulated data symbols 210 are referred to as an OFDM
symbol.
Since the symbol rate on each sub-carrier 206 is much less than the symbol
rate of the
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initial serial data 202, the OFDM symbols are less sensitive to timing. Thus,
the effects of
symbol overlap (i.e., ISI) caused by delay spread decrease for the channel.
[0009] Figure 3 illustrates ISI between OFDM/OFDMA symbols. As shown in Figure
3,
OFDM/OFDMA symbols Si - S3 may be transmitted on the sub-frame 120 of the
downlink radio sub-frame 118 from the base station (BS) 102 to the mobile
station (MS)
104 (Figure 1). Multipath components 112 (Figure 1) may cause a delay spread
302 of the
symbols Sl - S3. The delay spread may cause the OFDM/OFDMA symbols Sl - S3 to
overlap each other, such that ISI 304 occurs between OFDM/OFDMA symbols S1-S2
and
S2-S3. If the ISI is large enough, the signal reception may be disrupted.
[0010] In order to make an OFDM/OFDMA system more robust to multipath signals,
an
extension is made to the information symbol called a cyclic prefix. The cyclic
prefix 402
is generally inserted between adjacent OFDM/OFDMA symbols as shown in Figure
4.
The cyclic prefix 402 is typically pre-pended to each OFDM/OFDMA symbol and is
used
to compensate for the delay spread introduced by the radio channel as
explained below.
The cyclic prefix 402 can also compensate for other sources of delay spread
such as that
from pulse shaping filters often used in transmitters. By significantly
reducing or avoiding
the effects of ISI and ICI, the cyclic prefix 402 also helps to maintain
orthogonally
between the OFDM/OFDMA signals on the sub-carriers 206 (Figure 2). The cyclic
prefix
402 has a duration TG, which may be added to the useful symbol duration Tõ .
Thus, a total
OFDM/OFDMA symbol duration TSYM may be T,, + TG . Although, in this example, a
total OFDM/OFDMA symbol duration of TSYM = Tõ + TG may be employed for
transmitting an OFDM/OFDMA symbol, only the useful symbol duration Tu (Figure
2)
may be available for user's data symbol transmission.
[0011] As mentioned above, the cyclic prefix 402 is a cyclic extension of each
OFDM/OFDMA symbol, which is obtained by extending the duration of an
OFDM/OFDMA symbol. Figure 5 shows an exemplary cyclic prefix. In Figure 5, a
sinusoidal curve 504 corresponds to an original sinusoid where one cycle of
the sinusoid is
of duration 3.2 s (i.e., 64 samples with 20 MHz sampling rate). For this
example, the
subcarrier frequency is 312.5 KHz. A cyclic prefix 502 of 16 samples (0.8 s)
is pre-
appended to the original subcarrier 504 which still has the original sinusoid
of frequency
312.5 KHz. The sinusoid is now of duration 4.0 s, which allows the receiver
to choose
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one period (3.2 s) of the subcarrier 504 from the bigger window (4.0 s). In
this manner,
the cyclic prefix 502 acts as a buffer region. The receiver at the mobile
station 104
(Figure 1) may exclude samples from the cyclic prefix 502/402 that are
corrupted by the
previous symbol when choosing samples for OFDM/OFDMA symbols (e.g., Si - S3
(Figure 3)). The cyclic prefix 502/402 duration should be optimized to
increase
bandwidth efficiency (i.e., bit/Hz).
[0012] In telecommunications, a frame is a fixed or variable length packet of
data, which
has been encoded by a communications protocol for digital transmission. A
frame
structure is the way a communication channel is divided into frames (e.g.,
118/124 in
Figure 1) or sub-frames (e.g., 120/126 in Figure 1) for transmission. The
frame structure
of an OFDM or OFDMA system contributes to determining the performance of a
communication system. In a communications system, the size and timing of a
cyclic
prefix in a frame is specified by a frame structure.
[0013] In existing OFDM/OFDMA systems, such as Wireless Interoperability for
Microwave Access (WiMAX), the cyclic prefix is configurable, but it is fixed
when a
system is deployed. This limits configuration of the system for efficient
bandwidth
utilization since the cyclic prefix cannot be reconfigured. Additionally, in
existing frame
structures, there are no mechanisms to allow a base station to change or
reconfigure the
cyclic prefix duration for different communication usage scenarios. For
example, when
communication in a channel suffers from severe multipath effects (i.e., large
delay
spread), a longer cyclic prefix can be used to eliminate the ISI and ICI. In
less severe
channel conditions, with fewer multipath issues, a short cyclic prefix can be
used in order
to reduce overhead and transmission power. Therefore, there is a need for
systems and
methods that provide a frame structure for high performance OFDM and OFDMA
systems
that more efficiently use the cyclic prefix.
Summary of the Invention
[0014] An OFDM/OFDMA fra me structure technology for communication systems is
disclosed. The OFDM/OFDMA frame structure technology comprises a variable
length
sub-frame structure with efficiently sized cyclic prefixes, and efficient
transition gap
durations operable to effectively utilize OFDM/OFDMA bandwidth. Furthermore,
the
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frame structure provides compatibility with multiple wireless communication
systems. An
uplink frame structure and a downlink frame structure are provided.
[0015] A first embodiment comprises an OFDM/OFDMA communication system. The
OFDM/OFDMA communication system comprises a plurality of radio frequency (RF)
channels, wherein the RF channels comprise dissimilar bandwidths. The system
also
comprises a transmitter for providing a plurality of OFDM subcarriers. The
OFDM
subcarriers comprise a fixed subcarrier spacing chosen such that the OFDM
subcarriers
are scalable in number to utilize any of the RF channels. In one embodiment,
all RF
channel bandwidths in the communication system can be divided evenly by the
subcarrier
spacing.
[0016] In addition, the system can further comprises a processor coupled to
the transmitter
and operable to provide a flexible radio frame structure comprising a
plurality of variable
length cyclic prefixes operable for the RF channels.
[0017] A second embodiment comprises a communication system. The communication
system comprises at least one base station supporting variable cyclic prefix
durations. The
variable cyclic prefix durations are chosen based on a cell coverage area of
the at least one
base station. The system also comprises a processor for providing a flexible
radio frame
structure utilizing the variable size cyclic prefix durations. The flexible
radio frame
structure is used by the at least one base station for transmitting data to a
mobile station.
The processor may also be operable to calculate a plurality of timing gaps
associated with
at least one of the sub-frames, wherein the timing gaps are calculated based
in part on the
variable cyclic prefix durations.
[0018] A third embodiment comprises an OFDM/OFDMA radio frame structure for
communication in an RF channel in a wireless network. The radio frame
structure
comprises a plurality of OFDM symbols each comprising a variable cyclic prefix
duration
and at least one OFDM data symbol. The frame structure also comprises a
plurality of
variable size sub-frames formed from a subset of the OFDM symbols, and a
plurality of
radio frames for transmitting a subset of the variable sub-frames through the
RF channel.
The frame structure further comprises a plurality of timing gaps associated
with the radio
frames for providing a protection for timing variations at signal reception.
The timing
gaps are calculated based, at least in part, on the variable cyclic prefix
duration.
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[0019] A fourth embodiment comprises a communication system. The communication
system comprises a plurality of RF channels, wherein a subset of the RF
channels have
dissimilar channel bandwidths. The system also comprises an inverse fast
Fourier
transform (IFFT) module operable for transforming a plurality of frequency
domain data
symbols into a plurality of time domain data symbols respectively. The system
further
comprises a cyclic prefix selector operable for selecting a cyclic prefix from
a plurality of
variable length cyclic prefixes to obtain a selected cyclic prefix. The system
also
comprises an add cyclic prefix module operable for adding the selected cyclic
prefix to
each of the time domain data symbols to obtain a plurality of OFDM frames.
[0020] The system may also comprise a processor operable for providing a
plurality of
variable size sub-frames formed from a subset of the OFDM frames. The
processor is also
operable for providing a plurality of radio frames for transmitting a subset
of the variable
size sub-frames through at least one of the RF channels. The processor is
further operable
for calculating a plurality of timing gaps associated with at least one of the
variable size
sub-frames for providing a protection for timing variations at signal
reception. The timing
gap is calculated based, at least in part, on a cyclic prefix duration of the
selected cyclic
prefix.
[0021] A fifth embodiment comprises a method for communication in a
communication
system. The method comprises receiving a time domain data symbol for
transmission on a
radio channel, and selecting a cyclic prefix from a plurality of variable
length cyclic
prefixes to obtain a selected cyclic prefix. The method also comprises adding
the selected
cyclic prefix into each of the time domain data symbols to obtain a plurality
of OFDM
frames.
[0022] A sixth embodiment comprises a computer-readable medium for a
communication
system. The computer-readable medium comprises program code for receiving a
time
domain data symbol for transmission on a radio channel. The program code also
selects a
cyclic prefix from a plurality of variable length cyclic prefixes for the
radio channel to
obtain a selected cyclic prefix. The program code also adds the selected
cyclic prefix into
each of the time domain data symbols to obtain a plurality of OFDM frames.
[0023] The computer-readable medium may further comprise program code for
adding the
OFDM frames to a flexible sub-frame prior to transmitting the OFDM frames on
the radio
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channel. The program code may also provide a plurality of variable size sub-
frames
formed from a subset of the OFDM frames, and provide a plurality of radio
frames for
transmitting a subset of the variable size sub-frames through the radio
channel. The
program code may also calculate a plurality of timing gaps associated with at
least one of
the variable size sub-frames for providing a protection for timing variations
at signal
reception. The timing gap is calculated based, at least in part, on a cyclic
prefix duration
of the selected cyclic prefix.
[0024] Further features and advantages of the present disclosure, as well as
the structure
and operation of various embodiments of the present disclosure, are described
in detail
below with reference to the accompanying drawings.
Brief Description of the Drawings
[0025] The present disclosure, in accordance with one or more various
embodiments, is
described in detail with reference to the following Figures. The drawings are
provided for
purposes of illustration only and merely depict exemplary embodiments of the
disclosure.
These drawings are provided to facilitate the reader's understanding of the
disclosure and
should not be considered limiting of the breadth, scope, or applicability of
the disclosure.
It should be noted that for clarity and ease of illustration these drawings
are not necessarily
made to scale.
[0026] Figure 1 is an illustration of an OFDM/OFDMA mobile radio channel
operating
environment.
[0027] Figure 2 is an illustration of principles of an OFDM/OFDMA multicarrier
transmission with four subcarriers.
[0028] Figure 3 is an illustration of exemplary OFDM/OFDMA symbols distorted
due to
ISI.
[0029] Figure 4 is an illustration of exemplary OFDM/OFDMA symbols with cyclic
prefix insertions in the time domain.
[0030] Figure 5 is an illustration of an exemplary cyclic prefix extension to
an
OFDM/OFDMA symbol in the frequency domain.
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[0031] Figure 6 is an illustration of an exemplary OFDM/OFDMA exemplary
communication system according to an embodiment of the invention.
[0032] Figure 7 is an illustration of an exemplary OFDM/OFDMA digital
transceiver
according to an embodiment of the invention.
[0033] Figure 8 is an illustration of an exemplary OFDM/OFDMA signal
definition in the
frequency domain.
[0034] Figure 9 is an illustration of an exemplary OFDM /OFDMA symbol
structure in
the time domain.
[0035] Figure 10 is an illustration of an exemplary OFDM/OFDMA sub frame
structure
according to an embodiment of the invention.
[0036] Figure 11 is an illustration of an exemplary OFDM/OFDMA uplink and
downlink
radio frame structure according to an embodiment of the invention.
[0037] Figure 12 is an illustration of an exemplary OFDM/OFDMA uplink and
downlink
sub-frame structure according to an embodiment of the invention.
[0038] Figure 13 is an illustration of an exemplary OFDM/OFDMA optional radio
frame
structure according to an embodiment of the invention.
[0039] Figure 14 is an illustration of an exemplary table of basic OFDM/OFDMA
parameters for a nx 1.25 MHz bandwidth series according to an embodiment of
the
invention.
[0040] Figure 15 is an illustration of an exemplary table of basic OFDM/OFDMA
parameters for a nx3.5 MHz bandwidth series according to an embodiment of the
invention.
[0041] Figure 16 is an illustration of an exemplary table of basic OFDM/OFDMA
parameters for a nx1.25 MHz bandwidth series for 0.5, 0.675, 1, 1.5, 2, and
2.5 ms sub-
frames according to an embodiment of the invention.
[0042] Figure 17 is an illustration of an exemplary table of basic OFDM/OFDMA
parameters for a nx3.5 MHz bandwidth series according to an embodiment of the
invention.
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[0043] Figure 18 is an illustration of an exemplary table of basic OFDM/OFDMA
parameters for a 0.5 ms sub-frame for a nx 1.25 MHz bandwidth series, with a
subcarrier
frequency spacing Of =12.5 KHz, according to an embodiment of the invention.
[0044] Figure 19 is an illustration of an exemplary table of basic OFDM/OFDMA
parameters for a 0.675 ms sub-frame for a nx1.25 MHz bandwidth series, with a
subcarrier frequency spacing Of =12.5KHz, according to an embodiment of the
invention.
[0045] Figure 20 is an illustration of an exemplary table of basic OFDM/OFDMA
parameters for a 1.0 ms sub-frame for a nx1.25 MHz bandwidth series, with a
subcarrier
frequency spacing Of =12.5 KHz, according to an embodiment of the invention.
[0046] Figure 21 is an illustration of an exemplary table of basic OFDM/OFDMA
parameters for a 1.5 ms sub-frame for a nx1.25 MHz bandwidth series, with a
subcarrier
frequency spacing Of = 12.5 KHz, according to an embodiment of the invention.
[0047] Figure 22 is an illustration of an exemplary table of basic OFDM/OFDMA
parameters for a 2 ms sub-frame for a n x 1.25 MHz bandwidth series, with a
subcarrier
frequency spacing Of 12.5 KHz, according to an embodiment of the invention.
[0048] Figure 23 is an illustration of an exemplary table of basic OFDM/OFDMA
parameters for a 2.5 ms sub-frame for a nx 1.25 MHz bandwidth series, with a
subcarrier
frequency spacing Of 12.5 KHz, according to an embodiment of the invention.
[0049] Figure 24 is an illustration of an exemplary table of basic OFDM/OFDMA
parameters for a 5 MHz bandwidth series, with a subcarrier frequency spacing
Of & 10.94 KHz according to an embodiment of the invention.
[0050] Figure 25 is an illustration of exemplary table of basic OFDM/OFDMA
parameters
for a 5 MHz bandwidth series, with a subcarrier frequency spacing Of '& 12.5
KHz,
according to an embodiment of the invention.
[0051] Figure 26 is an illustration of an exemplary table of basic OFDM/OFDMA
parameters for a 5 MHz bandwidth series, with a subcarrier frequency spacing
Of 25 KHz, according to an embodiment of the invention.
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[0052] Figure 27 is an illustration of a flowchart showing an OFDM/OFDMA
process for
creating a frame structure with a variable cyclic prefix, according to
embodiments of the
invention.
Detailed Description of Exemplary Embodiments
[0053] The following description is presented to enable a person of ordinary
skill in the art
to make and use the invention. Descriptions of specific devices, techniques,
and
applications are provided only as examples. Various modifications to the
examples
described herein will be readily apparent to those of ordinary skill in the
art, and the
general principles defined herein may be applied to other examples and
applications
without departing from the spirit and scope of the invention. Thus, the
present invention is
not intended to be limited to the examples described herein and shown, but is
to be
accorded the scope consistent with the claims.
[0054] The present disclosure is directed toward systems and methods for
OFDM/OFDMA frame structure technology for communication systems. Embodiments
of the invention are described herein in the context of one practical
application, namely,
communication between a base station and a plurality of mobile devices. In
this context,
the example system is applicable to provide data communications between a base
station
and a plurality of mobile devices. Embodiments of the disclosure, however, are
not
limited to such base station and mobile device communication applications, and
the
methods described herein may also be utilized in other applications such as
mobile-to-
mobile communications, or wireless local loop communications. As would be
apparent to
one of ordinary skill in the art after reading this description, these are
merely examples
and the invention is not limited to operating in accordance with these
examples.
[0055] As explained in additional detail below, the OFDM/OFDMA frame structure
comprises a variable length sub-frame structure with an efficiently sized
cyclic prefix
operable to effectively utilize OFDM/OFDMA bandwidth. The frame structure
provides
compatibility with multiple wireless communication systems.
[0056] Figure 6 shows an exemplary wireless communication system 600 for
transmitting
and receiving OFDM/OFDMA transmissions in accordance with the present
invention.
The system 600 may include components and elements configured to support known
or
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conventional operating features that need not be described in detail herein.
In the
exemplary embodiment, system 600 can be used to transmit and receive
OFDM/OFDMA
data symbols in a wireless communication environment such as the wireless
communication environment 100 (Figure 1). System 600 generally comprises a
base
station transceiver module 602, a base station antenna 606, a mobile station
transceiver
module 608, a mobile station antenna 612, a base station processor module 616,
a base
station memory module 618, a mobile station memory module 620, a mobile
station
processor module 622, and a network communication module 626.
[0057] System 600 may comprise any number of modules other the modules shown
in
Figure 6. Furthermore, these and other elements of system 600 may be
interconnected
together using a data communication bus (e.g., 628, 630), or any suitable
interconnection
arrangement. Such interconnection facilitates communication between the
various
elements of wireless system 600. Those skilled in the art will understand that
the various
illustrative blocks, modules, circuits, and processing logic described in
connection with
the embodiments disclosed herein may be implemented in hardware, computer-
readable
software, firmware, or any practical combination thereof. To clearly
illustrate this
interchangeability and compatibility of hardware, firmware, and software,
various
illustrative components, blocks, modules, circuits, and steps are described
generally in
terms of their functionality. Whether such functionality is implemented as
hardware,
firmware, or software depends upon the particular application and design
constraints
imposed on the overall system. Those familiar with the concepts described
herein may
implement such functionality in a suitable manner for each particular
application, but such
implementation decisions should not be interpreted as causing a departure from
the scope
of the present invention.
[0058] In the exemplary OFDM/OFDMA system 600, the base station transceiver
602 and
the mobile station transceiver 608 each comprise a transmitter module and a
receiver
module (not shown in Figure 6). Operation of the transmitter and receiver
modules is
explained in more detail in the context of the discussion of Figure 7. For
this example, the
transmitter and receiver modules are coupled to a shared antenna to form a
time division
duplex (TDD) system. The base station transceiver 602 is coupled to the base
station
antenna 606 and the mobile station transceiver 608 is coupled to the base
station antenna
612. Although in a simple time division duplex (TDD) system only one antenna
is
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required, more sophisticated systems may be provided with multiple and/or more
complex
antenna configurations. Additionally, although not shown in this figure, those
skilled in
the art will recognize that a transmitter may transmit to more than one
receiver, and that
multiple transmitters may transmit to the same receiver. In a TDD system,
transmit and
receive timing gaps exist as guard bands to protect against transitions from
transmit to
receive and vice versa. For example, a transmission timing gap (TTG) is
designed to
separate the downlink transmission period TTG(DL) from uplink transmission
period
TTG(UL). The downlink TTG(DL) provides a protection for timing variations at
signal
reception in downlink transmission. The TTG(DL) portion of timing gap is also
used to
prevent the downlink radio signal colliding with uplink signals due to
propagation delay.
The TTG(UL) portion of timing gap is used to offset uplink radio signal
propagation delay
so that all uplink signals synchronized at the base station (BS) receiver(s).
The TTG(DL)
may allow sufficient time for a TDD system to transition from a downlink to an
uplink.
Similarly, a TTG for the uplink TTG(UL) may allow sufficient time for a TDD
system to
transition from an uplink to a downlink. According to an embodiment of the
invention,
the TTG(DL) and TTG(UL) can be calculated based on the cyclic prefix duration
as
explained in more detail in the context of discussing of Figure 14.
[0059] In the particular example of the OFDM/OFDMA system depicted in Figure
6, an
"uplink" transceiver 608 includes an OFDM/OFDMA transmitter that shares an
antenna
with an uplink receiver. A duplex switch may alternatively couple the uplink
transmitter
or receiver to the uplink antenna in time duplex fashion. Similarly, a
"downlink"
transceiver includes an OFDM/OFDMA receiver which shares a downlink antenna
with a
downlink transmitter. A downlink duplex switch may alternatively couple the
downlink
transmitter or receiver to the downlink antenna in time duplex fashion. The
operation of
the two transceivers 602/608 is coordinated in time such that the uplink
OFDM/OFDMA
receiver is coupled to the uplink antenna 612 for reception of transmissions
over the
wireless transmission link 614 at the same time that the downlink OFDM/OFDMA
transmitter is coupled to the downlink antenna 606. Preferably there is close
time
synchronization with only a minimal guard time between changes in duplex
direction.
[0060] Although many OFDM/OFDMA systems will use OFDM/OFDMA technology in
both directions, those skilled in the art will recognize that the present
embodiments of the
invention are applicable to systems using OFDM/OFDMA technology in only one
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direction, with an alternative transmission technology (or even radio silence)
in the
opposite direction. Furthermore, it should be understood by a person of
ordinary skill in
the art that the OFDM/OFDMA transceiver modules 602/608 may utilize other
communication techniques such as, without limitation, a frequency division
duplex (FDD)
communication technique.
[0061] The mobile station transceiver 608 and the base station transceiver 602
are
configured to communicate via a wireless data communication link 614. The
mobile
station transceiver 608 and the base station transceiver 602 cooperate with a
suitably
configured RF antenna arrangement 606/612 that can support a particular
wireless
communication protocol and modulation scheme. In the exemplary embodiment, the
mobile station transceiver 608 and the base station transceiver 602 are
configured to
support industry standards such as the Third Generation Partnership Project
Long Term
Evolution (3GPP LTE), Third Generation Partnership Project 2 Ultra Mobile
Broadband
(3Gpp2 UMB), Time Division-Synchronous Code Division Multiple Access (TD-
SCDMA), and Wireless Interoperability for Microwave Access (WiMAX), and the
like.
The mobile station transceiver 608 and the base station transceiver 602 may be
configured
to support alternate, or additional, wireless data communication protocols,
including future
variations of IEEE 802.16, such as 802.16e, 802.16m, and so on. In an
exemplary
embodiment, a mobile station transceiver 608 may be used in a user device such
as a
mobile phone. Alternately, the mobile station transceiver 608 may be used in a
personal
digital assistant (PDA) such as a Blackberry device, Palm Treo, MP3 player, or
other
similar portable device. In some embodiments the mobile station transceiver
608 may be a
personal wireless computer such as a wireless notebook computer, a wireless
palmtop
computer, or other mobile computer devices. In further embodiments, the
invention can
be implemented in a mobile station as well as a base station. The transmitter
at the mobile
station can add the variable length cyclic prefixes and understand the changes
of the
timing gaps accordingly. However, in the current intended practices, the
dynamic
configuration of the variable length cyclic prefixes of mobile stations is set
by the base
station. The mobile stations can negotiate with the base station for the
preferred cyclic
prefix. The base station can assign it to a particular uplink sub-frame to
transmit with the
preferred cyclic prefix.
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[0062] Processor modules 616/622 may be implemented, or realized, with a
general
purpose processor, a content addressable memory, a digital signal processor,
an
application specific integrated circuit, a field programmable gate array, any
suitable
programmable logic device, discrete gate or transistor logic, discrete
hardware
components, or any combination thereof, designed to perform the functions
described
herein. In this manner, a processor may be realized as a microprocessor, a
controller, a
microcontroller, a state machine, or the like. A processor may also be
implemented as a
combination of computing devices, e.g., a combination of a digital signal
processor and a
microprocessor, a plurality of microprocessors, one or more microprocessors in
conjunction with a digital signal processor core, or any other such
configuration.
Processor modules 616/622 comprise processing logic that is configured to
carry out the
functions, techniques, and processing tasks associated with the operation of
OFDM/OFDMA system 600. In particular, the processing logic is configured to
support
the OFDM/OFDMA frame structure parameters described herein. For, example the
processor modules 616/612 may be suitably configured to compute cyclic prefix
durations
and timing transitions (TDD (UL) and TDD (DL)), as explained below, to provide
a
flexible size frame structure. For example, a frame may be constructed from
one or
multiple sub-frames, each sub-frame is consisted of one or multiple symbols
and timing
gaps. A timing gap is the period of idle transmission time, such as TTG(DL),
TTG(UL), or
RTG. Based on this new definition, the gap time periods, TTG and RTG, have
been
included in the sub-frames. This way to define a frame and sub-frame (also
known as
"slot" in LTE) has greatly simply the design of a frame, and make it much more
flexible
for different sub-frame designs. The newly defined sub-frame has been self-
contained
within its time period and boundary. Sub-frames with different cyclic prefixes
can co-exist
in the same system and the same deployment. As mentioned above, in some
embodiments
the processing logic may be resident in the base station and/or may be part of
a network
architecture that communicates with the base station transceiver 602.
[0063] Furthermore, the steps of a method or algorithm described in connection
with the
embodiments disclosed herein may be embodied directly in hardware, in
firmware, in a
software module executed by processor modules 616/622, or in any practical
combination
thereof. A software module may reside in memory modules 618/620, which may be
realized as RAM memory, flash memory, ROM memory, EPROM memory, EEPROM
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memory, registers, a hard disk, a removable disk, a CD-ROM, or any other form
of storage
medium known in the art. In this regard, memory modules 618/620 may be coupled
to the
processor modules 618/622 respectively such that the processors modules
616/620 can
read information from, and write information to, memory modules 618/620. As an
example, processor module 616, and memory modules 618, processor module 622,
and
memory module 620 may reside in their respective ASICs. The memory modules
618/620
may also be integrated into the processor modules 616/620. In an embodiment,
the
memory module 618/620 may include a cache memory for storing temporary
variables or
other intermediate information during execution of instructions to be executed
by
processor modules 616/622. Memory modules 618/620 may also include non-
volatile
memory for storing instructions to be executed by the processor modules
616/620.
[0064] Memory modules 618/620 may include a frame structure database (not
shown) in
accordance with an exemplary embodiment of the invention. Frame structure
parameter
databases may be configured to store, maintain, and provide data as needed to
support the
functionality of system 600 in the manner described below. Moreover, a frame
structure
database may be a local database coupled to the processors 616/622, or may be
a remote
database, for example, a central network database, and the like. A frame
structure
database may be configured to maintain, without limitation, frame structure
parameters as
explained below. In this manner, a frame structure database may include a
lookup table
for purposes of storing frame structure parameters.
[0065] The network communication module 626 generally represents the hardware,
software, firmware, processing logic, and/or other components of system 600
that enable
bi-directional communication between base station transceiver 602, and network
components to which the base station transceiver 602 is connected. For
example, network
communication module 626 may be configured to support internet or WiMAX
traffic. In a
typical deployment, without limitation, network communication module 626
provides an
802.3 Ethernet interface such that base station transceiver 602 can
communicate with a
conventional Ethernet based computer network. In this manner, the network
communication module 626 may include a physical interface for connection to
the
computer network (e.g., Mobile Switching Center (MSC)).
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[0066] Figure 7 is a block diagram of an exemplary OFDM/OFDMA transceiver
system
700 (e.g., transceivers 602 or 608 in Figure 6) that can be configured in
accordance with
an exemplary embodiment of the invention. Figure 7 represents a method for
adding an
efficiently sized cyclic prefix to an OFDM/OFDMA frame structure operable to
effectively utilize OFDM/OFDMA channel transmission bandwidth. It is
understood that,
the system 700 may include additional components and elements configured to
support
known or conventional operating features. For the sake of brevity,
conventional
techniques and components related to digital signal processing such as channel
encoding/decoding, correlation techniques, spreading/dispreading, pulse
shaping, radio
frequency (RF) technology, and other functional aspects and the individual
operating
components of the system 700 are not described in detail herein. The
OFDM/OFDMA
system 700 digitally transmits and receives data wirelessly to and from
infrastructure
devices using IFFT/FFT techniques. A discrete Fourier transform (DFT) and an
inverse
discrete Fourier transform (IDFT) may be used as an alternative to an FFT and
IFFT
respectively.
[0067] The OFDM/OFDMA digital transceiver system 700 includes a transmitter
701 and
a receiver 703. The transmitter 701 further includes a serial-to-parallel
converter 702, an
OFDM/OFDMA module 704, and a digital-to-analog converter (D/A) module 712. The
OFDM/OFDMA module 704 includes an IDFT/IFFT module 706, a parallel-to-serial
converter 708, and an add cyclic prefix module 710 coupled to a cyclic prefix
selector 709.
The receiver 703 includes an analog-to-digital converter (A/D) module 716, an
inverse
OFDM/OFDMA receiver module 718, and a parallel-to-serial converter 726. The
inverse
OFDM/OFDMA receiver module 718 includes a remove cyclic prefix module 720, a
serial-to-parallel converter 722, and a DFT/FFT module 724. In this example,
the
transmitter 701 and the receiver 703 can send and receive data and other
communication
signals via a multipath propagation channel 714 or other channels (e.g., 614
in Figure 6).
[0068] In the transmitter 701, a serial stream of N. source data symbols Dn
(corresponding to serial data symbols 202 in Figure 2) is converted into Nc
parallel data
symbols (corresponding to parallel data symbols 210 in Figure 2) by the serial-
to-parallel
converter 702. The source data symbols D,, may, for example, be obtained from
an
original data source (e.g., a text message) after source and channel coding,
interleaving,
and symbol mapping. After serial to parallel conversion 702, the source data
symbol
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duration Td of the Nc serial data symbols results in the OFDM/OFDMA symbol
duration
T = Nc * Td'
[0069] The parallel data symbols are then modulated on to Nc different sub-
carriers (206
in Figure 2) via the IDFT/IFFT module 706. As explained above, an OFDM system
modulates the Nc parallel data sub-streams on to Nc sub-carriers (206 in
Figure 2). The
Nc sub-carriers have a frequency spacing of Of = in order to achieve
orthogonality
T.
between the signals on the Nc sub-carriers. The Nc parallel modulated signals
form an
OFDM symbol (210 in Figure 2). Depending on the transmission media and the
network
bandwidth, system 700 can employ, for example, 64 to 4096 subcarriers as
explained in
more detail below.
[0070] The add cyclic prefix module 710 is then used to add a cyclic prefix to
the output
of the parallel-to-serial converter 708. In order to completely avoid or
significantly reduce
the effects of ISI, a cyclic prefix of duration TG may be inserted between
adjacent
OFDM/OFDMA symbols (402 in Figure 4). The cyclic prefix duration parameter TG
may
be set to various values in order to efficiently size the cyclic prefix to
effectively utilize
OFDM/OFDMA bandwidth. As explained above, a cyclic prefix is a cyclic
extension of
each OFDM symbol which is obtained by extending the duration of an OFDM symbol
to
Tsym = Tu + TG in accordance with one embodiment of the invention.
[0071] For example, according to an embodiment of the invention, the cyclic
prefix values
are selected based on RF channel conditions. In this manner, the cyclic prefix
is
configurable even if a system is deployed, thereby allowing efficient use of
bandwidth.
Accordingly, a communication system can select various effective cyclic prefix
lengths for
the base stations in a network, and may support different cyclic prefix
lengths for different
base stations in the network. Furthermore, a communication system may support
different
cyclic prefix lengths in different downlink and/or uplink sub-frames for the
same base
station. A variable cyclic prefix length allows a base station to change or
configure the
cyclic prefix duration for different communication usage scenarios, thereby
increasing the
bandwidth efficiency (bit/Hz) of the system. For example, when communication
is in a
channel with a severe multipath (i.e., larger delay spread), longer cyclic
prefixes can be
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used to eliminate the ISI and ICI. In less severe channel conditions with
fewer multipaths,
a short cyclic prefix can be used in order to increase data rate (bits/sec),
and to reduce
overhead and transmission power.
[0072] The add cyclic prefix module 710 may receive the cyclic prefix values
from the
cyclic prefix selector 709. The cyclic prefix selector 709 may communicate
with the
processor 622 and or the memory module 620 to obtain values for the cyclic
prefix. For
example, the value may correspond to the cyclic prefix duration needed for
base station
coverage. The cyclic prefix selector 709 will then provide the appropriate
cyclic prefix
value to the add cyclic prefix module 710. For example, the cyclic prefix
duration can be
associated with multipath delay spread. When a Femto base station is deployed,
for
example, due to low transmit power of the base station power amplifier (PA) it
only
covers a small area or a hot-spot. The delay spread becomes very small and
therefore this
Femto BS and the associated mobile stations should select a small cyclic
prefix (out of the
different length options) for the downlink and uplink sub-frames transmission.
On the
other hand, if a Macro base station is deployed, due to the large transmit
power of the base
station power amplifier (PA), the base station can group the serving mobile
stations into
different groups by their delay spreads (determined by base station or
requested by
individual mobile station). The base station can allocate these different
groups of mobile
stations into different sub-frames with appropriate settings of cyclic
prefixes. The cyclic
prefix is just a copy of the end portion of the useful symbol (T"), it is
calculated and
copied on the fly. The cyclic prefix values may be chosen to efficiently size
the cyclic
prefix to effectively utilize OFDM/OFDMA bandwidth, while providing a frame
structure
compatible with multiple wireless communication systems. The specific cyclic
prefix
values that are used are discussed below in the context of discussion of
Figures 14-26.
[0073] The output of the add cyclic prefix module 710 is then passed through
the D/A 712
to create an analog signal for transmission. The output of D/A 712 comprises
the signal
waveform X (t) with duration Tsym. The signal waveform X (t) is up converted
(not
shown) and the RF signal is transmitted to the channel 714.
[0074] The output of the channel 714, after RF down conversion (not shown), is
the
received signal waveform Y(t) which may include ISI from the channel and RF
processing. The received signal Y(t) is passed through analog-to-digital
convert 716,
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whose output sequence Y, is the received signal Y(t) sampled at rate T . A
sampling
d
rate of T or greater is necessary to insure proper Nyquist sampling for the
data rate of
d
the data symbols Dõ with duration Td .
[0075] Since ISI may be present within the cyclic prefix time period, cyclic
prefix samples
are removed via remove cyclic prefix module 720 before DFT/FFT demodulation.
The
ISI-free part of Y(t) may be converted to parallel data symbols via serial-to-
parallel
converter 722, and demodulated by DFT/FFT module 724. The output of the
DFT/FFT
module 724 is a sequence R, which is the received replica of the original data
symbols
Dõ along with any transmission errors. The receiver may incorporate other
techniques
which are not illustrated here, such as channel estimation, maximum receive
ratio
combining, etc.
[0076] Figure 8 is an illustration of an exemplary OFDM/OFDMA signal frequency
domain definition 800. As will be explained below, the choice of sub-carrier
frequency in
the OFDM/OFDMA signal frequency domain may be used, according to an embodiment
of the invention, to insure frequency compatibility with wireless
communication
standards. The OFDM/OFDMA signal frequency domain definition 800 may comprise
a
nominal channel transmission bandwidth (BW) 802, a subset of signal
subcarriers (NsJG )
804, a signal bandwidth (BWS,G) 808, a DC sub-carrier (DC) 810, guard
subcarriers 812,
and a sampling frequency (Fs) 814. In some systems, the DC sub-carrier may not
be
defined.
[0077] For a given nominal channel transmission bandwidth (BW) 802, a subset
of signal
subcarriers 804 out of a plurality of subcarriers 806 may be used to match the
bandwidth
of the subcarriers 804 to the channel transmission bandwidth BW 802. The
subset of
signal subcarriers 804 is referred as signal bandwidth (BWS,G) 808. The
plurality of
subcarriers 806 may include the DC sub-carrier (DC) 810, which contains no
data.
Subcarriers outside the signal bandwidth (BWSJG) 808 that are not used may
serve as
guard subcarriers 812. The purpose of the guard subcarriers 812 is to enable
the signal to
have a smooth roll off in the time domain.
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[0078] Figure 9 is an illustration of an exemplary time domain symbol
structure of an
OFDM/OFDMA signal. Figure 9 illustrates the positioning of a cyclic prefix in
the
exemplary time domain OFDM symbol. The time domain symbol structure 900
comprises
a useful symbol time (T, or TIFFT) 902, a cyclic prefix 904, a windowing
period (TWIN )
908, and a total symbol time (TSYM) 910. In some systems, the windowing period
may
not be defined, which can be treated as windowing period having a value of
zero: TWIN =
0.
[0079] A time duration of a set of OFDM data symbols to be transmitted by an
OFDM/OFDMA system is referred to as the useful symbol time (Tu or TIFFT) 902.
A
copy of the end section of the symbol period 906 is used to produce the cyclic
prefix (CP)
904. By using a cyclic extension, the samples used to perform the FFT at the
receiver can
be taken anywhere over the length of the extended symbol. This provides
multipath
immunity as well as a tolerance for symbol time synchronization errors. A
small
windowing period (TWIN) 908 can be added before the cyclic prefix 904 and at
the end of
symbol time 902 to reduce signal in-band and out-of-band emission. In this
example, the
total symbol time (TSYM) 910 may include the useful symbol time (Tõ or TIFFT)
902, the
cyclic prefix duration T. 904, and a windowing period (TWIN) 908. An inverse
Fourier
transform (IFFT) of a set of OFDM data symbols in the time duration TSYM
creates an
OFDM/OFDMA waveform.
[0080] As explained above, many interference sources such as ISI, ICI, and
multipath, can
have an effect on OFDM/OFDMA system performance. Furthermore, the choice of
the
frame structure and the parameters that define the frames may also determine
the
performance of an OFDM/OFDMA system. A tradeoff must generally be made between
resistance to interference and data transmission capacity. The tradeoff is
determined by
the choice of the parameters of the radio frames. For example, a long cyclic
prefix may
improve the multipath performance, but reduce the overall throughput of the
system and
overall frequency efficiency.
[0081] Figure 10 is an illustration of an exemplary OFDM/OFDMA sub-frame
structure
according to an embodiment of the invention. For this example, the OFDM/OFDMA
sub-
frame structure comprises a short sub-frame 1002, a regular sub-frame 1004, a
long sub-
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frame 1006, and an optional low chip rate (LCR) sub-frame 1008. A 10 ms radio
frame
may be divided into twenty or more short sub-frames 1002, ten regular sub-
frames 1004,
or five long sub-frames 1006. For a 10 ms radio frame divided in this way, a
short sub-
frame 1002 has a duration of 0.5 ms, a regular sub-frame 1004 has a duration
of 1 ms, and
a long sub-frame 1006 has a duration of 2 ms. Other numbers of sub-frames that
don't
necessarily divide the 10 ms radio frame evenly may also be used. In this
case, a gap
remains in the radio frame. For example, a long sub-frame may have six long
sub-frames
each with a duration of 1.5 ms. Then, the total time of the sub-frames is 9
ms, which
leaves a gap of I ms in the radio frame. The optional low chip rate sub-frame
1008 may
also be used. A low chip rate sub-frame 1008 may have a duration of 0.675 ms,
and a 10
ms radio frame may be divided into 14 or more low chip rate sub-frames 1008
with a 0.55
ms gap. These sub-frame duration options may allow a communication system such
as the
system 600 to reduce interference with other systems that are based on various
industry
standards as mentioned in the context of Figure 6 above.
[0082] The frame structure provides compatibility with multiple wireless
communication
systems. For example, the low chip rate sub-frame 1008 duration of 0.675 ms
may allow
compatibility with the Time Division-Synchronous Code Division Multiple Access
(TD-
SCDMA) OFDM/OFDMA radio frame structure. The long sub-frame 1006 duration of 2
ms may allow compatibility with the Third Generation Partnership Project Long
Term
Evolution (3GPP LTE) OFDM/OFDMA radio frame structure, and the like.
[0083] Figure 11 is an illustration of an exemplary OFDM/OFDMA radio frame
structure
1100 according to an embodiment of the invention. The OFDM/OFDMA radio frame
structure 1100 may include five exemplary frame structures 1102, 1104, 1106,
1108, and
1112. The sub-frames may be allocated for uplink or downlink transmission. The
first
frame structure 1102 illustrates a series of alternating uplink regular sub-
frames (shown by
arrows pointing up) and downlink regular sub-frames (shown by arrows pointing
down).
The second exemplary frame structure 1104 illustrates a series of alternating
uplink short
sub-frames and downlink short sub-frames. This can give the same uplink data
rate and
downlink data rate, but in contrast to the first exemplary frame structure
1102, the second
exemplary frame structure 1104 would have a lower overall data rate (bits/sec)
because of
an increase in overhead, and a lower latency because of the delay between sub-
frames.
Lower latency is useful for some applications like vocoders, where time delay
is critical.
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The third exemplary frame structure 1106 illustrates a series of alternating
uplink regular
sub-frames and downlink long sub-frames. This can give a greater downlink data
rate than
uplink data rate. The fourth exemplary frame structure 1108 illustrates a
series of
alternating uplink short sub-frames and downlink long sub-frames. This can
give an even
greater downlink data rate than uplink data rate. The fifth exemplary frame
structure 1110
illustrates a series of alternating downlink long sub-frames with a uplink
short sub-frame
and a downlink short sub-frame. This would be useful, for applications like an
internet
download, where small control commands alternate with large webpage downloads.
In
many real world situations, and particularly for multiple access systems like
OFDMA, the
frame synchronization on uplinks and downlinks to the various communicating
devices
may have timing variations.
[0084] Figure 12 is an illustration of an exemplary OFDM/OFDMA uplink and
downlink
frame structure 1200 according to an embodiment of the invention. The
OFDM/OFDMA
uplink and downlink frame structure 1200 includes a downlink sub-frame 1202, a
single
uplink sub-frame 1204, a last uplink sub-frame 1206, and an uplink sub-frame
1208. Each
sub-frame 1202/1204/1206/1208 includes a plurality of symbols 1210.
[0085] The downlink sub-frame 1202 has a transmission timing gap (TTG) for the
downlink (TTG(DL)) 1212. Transmit and receive timing gaps exist as guard
periods to
protect against transitions from transmit to receive and vice versa in a TDD
system. The
TTG(DL) 1212 provides a protection for timing variations at signal reception,
and allows
sufficient time for a TDD system to transition from a downlink to an uplink. A
TTG(DL)
is a portion of the transmit/receive transition gap contributed from the
downlink sub-
frame.
[0086] For the single uplink sub-frame 1204, there is a transmission-timing
gap at each
end of the transmission for single uplink sub-frame 1204, since a single
uplink sub-frame
1204 is both the start and end of its series in a frame. The single uplink sub-
frame 1204
has a transmission timing gap for the uplink (TTG(UL)) 1214 and a receive-
transmit
transition gap (RTG) 1216 according to an embodiment of this invention. The
TTG(UL)
1214 and RTG 1216 provide a protection for timing variations at signal
reception, and the
TTG(UL) 1214 allows sufficient time for a TDD system to transition from a
downlink to
an uplink. A TTG(UL) is the portion of the transmit/receive transition gap
contributed
from the uplink sub-frame. The RTG 1216 allows a TDD system (Figure 6) time to
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transition from an uplink back to a downlink. Since the necessary timing gap
period for
RTG is often very short, it is optional in the system design. In some systems,
RTG can be
set to 0. In theory the base station may take up very small portion of the
cyclic prefix time
for switching from transmitting to receiving mode, but it is typically up to
the base station
to adjust when the uplink frame starts. In one embodiment, the uplink frame is
sent in
advance in time to offset propagation delay, therefore there is more than
sufficient time for
the mobile station to switch from transmitting mode to receiving mode without
sacrificing
the cyclic prefix for transaction. When RTG is set to 0, then the system
design is further
simplified, then "single uplink sub-frame" 1204 and "last uplink sub-frame"
1206 become
the same design as "uplink sub-frame" with only TTG(UL) 1208.
[0087] If there is a series of uplink sub-frames in a frame, then uplink sub-
frame 1208
starts the series and the last uplink sub-frame 1206 ends the series. The
uplink sub-frame
1208 begins the series with a TTG(UL) 1214, which provides a time gap to allow
a TDD
radio system base station to transition from transmit mode to receive mode,
and a TDD
radio system mobile station to transition from receive mode to transmit mode.
After the
transition, subsequent sub-frames transmitted on the uplink may be sub-frames
without
time gaps. For the last sub-frame in the uplink series, a last uplink sub-
frame 1206 is
transmitted as explained above, which ends the series with the RTG 1216. The
RTG 1216
provides a time gap to allow a TDD radio system base station to transition
from receive
mode to transmit mode, and a TDD radio system mobile station to transition
from transmit
mode to receive mode. According to an embodiment of the invention values for
the
TTG(DL) and TTG (UL) can be calculated based on the cyclic prefix duration as
explained below in the context of discussion of Figure 14.
[0088] Figure 13 is an illustration of an exemplary OFDM/OFDMA optional radio
frame
1300 according to an embodiment of the invention. The optional radio frame
1300 is 5 ms
in length 1302. It starts with a 0.675 ms optional sub-frame 1304. Then a 75
s downlink
pilot (DwPTS) 1306 is transmitted. A 75 s gap period (GP) 1308 is allowed
between
transmissions, and then a 125 s transmitted uplink pilot (UpPTS) 1310 is
transmitted.
Then 0.675 ms optional sub-frame 1312 is transmitted up to the end of the
frame 1300. In
one embodiment of the invention, the DwPTS, GP, UpPTS are used to provide
downlink
and uplink transmission periods that are synchronized/lined-up with the TD-
SCDMA for
adjacent RF channel deployment.
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[0089] Figures 14-26 illustrate exemplary tables of basic OFDM/OFDMA
parameters for
several channel transmission bandwidth series according to various embodiments
of the
invention. The OFDM/OFDMA parameters detail the variable length sub-frame
parameters of the OFDM/OFDMA frame structure. As explained above, the frame
structure may provide compatibility with multiple wireless communication
systems using
an efficiently sized cyclic prefix to efficiently utilize OFDM/OFDMA
bandwidth. Note
that numerology specified in these tables is for exemplary purposes only and
other values
for the OFDM/OFDMA parameters may be used.
[0090] Figure 14 illustrates an exemplary table of basic OFDM/OFDMA parameters
for a
nx 1.25 MHz bandwidth series according to an embodiment of the invention. A nx
1.25
bandwidth series includes channel transmission bandwidths of 1.25, 2.5, 5, 10,
20, and 40
MHz based on multiples of 1.25 MHz. Figure 14 shows sub-frame duration,
subcarrier
spacing, sampling frequency, FFT size NFFT, number of occupied subcarriers,
number of
OFDM/OFDMA symbols per sub-frame, cyclic prefix durations of each of the sub-
frames,
and the cyclic prefix duration of the TTG(DL), TTG(UL), and RTG of each the
sub-
frames.
[0091] The FFT size NFFT may be the smallest power of two that is greater than
the
required number of signal subcarriers (804 in Figure 8) needed for the
sampling frequency
Fs (814 in Figure 8) for the OFDM/OFDMA system. For example, for a
transmission
BW of 1.25 MHz, and a carrier spacing Af = 12.5kHz, the required number of
signal
subcarriers (804 in Figure 8) can be 100. Then, the FFT size NFFT is equal to
128 which
is the smallest power of two (i.e., 27) that is less than 100.
[0092] In this example, the FFT size NFFT is scalable from 128 to 4096. When
the
available channel transmission bandwidth BW increases, the NFFT also increases
such
that Af is constant. This keeps the OFDM/OFDMA symbol duration Tu fixed, which
is
independent of channel system bandwidth BW. TSYM (TsYM = Tr, + TG) is
configurable
based on different deployment scenarios, and therefore makes scaling have a
minimal
impact on higher layers. For example, a 7MHz system may have the same
performance as
a 10MHz system, except for that the maximum data throughput is proportional to
the
channel bandwidth (BW). A 5MHz system can migrate to a 10MHz system by adding
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another 5MHz channel BW right next to it without a guard band and without
causing
adjacent channel interference by simply making all subcarriers orthogonal to
each other.
The migration can be done with the same base station and mobile station, as
long the
bandwidth filter has been designed for a 10MHz channel. All frequency bands
and rasters
(200KHz and 250KHz) in the world can be divided by 12.5KHz evenly, with no
extra
bandwidth and banding constraints. A scalable design also keeps the costs low.
[0093] In this example embodiment, an OFDM/OFDMA system with a fixed
subcarrier
spacing value Of = 12.5kHz may be used. The Af = 12.5kHz is chosen because it
can not
only divide the common channel raster of 200KHz evenly, but also divide the
alternative
common channel raster of 250KHz evenly. Thus, a frequency spacing of Af =
12.5kHz
can divide all RF channel evenly without unnecessary residue bandwidth.
Additionally,
the adjacent bands that are adopting the same technology will have minimum
inter-
channel interference (ICI), simply all adjacent sub-carriers are orthogonal to
each other.
Similarly Of =lOkHz, 20kHz, 25kHz can serve the same purpose. The higher the
Of is
selected the higher the Doppler shift, often caused by mobility, the system
can tolerate.
As mentioned above, in the frequency domain an OFDM or OFDMA signal is made up
of
orthogonal subcarriers, and the number of used subcarriers may be less than or
equal to the
FFT size (NFFT). For example, the FFT size (NFFT) may be in a range comprising
128,
256, 512, 1024, 2048, or 4096 subcarriers.
[0094] The sampling frequency (e.g., FS = 1.6, 3.2, 6.4, 12.8, 25.6, and 51.2
MHz) can
be calculated based on the NFFT and Of using the following equation:
FS = Of X NFFT
[0095] With this particular subcarrier spacing, RF channels with different
channel
transmission bandwidths are scalable. They can be defined with accordant used
subcarriers within a fast Fourier transform size NFFT . A subcarrier spacing
of
Of = 12.5kHz has a property of good trade-off of cyclic prefix overhead with
mobility
support and achieving reasonable frequency efficiency.
[0096] For a given nominal channel transmission bandwidth BW (802 in Figure 8)
only a
subset of subcarriers NSIG out of NFFT is occupied for signal bandwidth BWSJG
. For
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example, according to an embodiment of the invention the number of occupied
subcarriers
for a channel transmission bandwidth BW of 1.25, 2.5, 5, 10, 20, and 40 MHz
can be 20,
100, 200, 400, 800, 1600 and 3200 respectively.
[0097] As explained above, in addition to the useful symbol duration Tu which
is
available for user data transmission, an additional period of time TG can be
used for
transmission of a cyclic prefix. The cyclic prefix duration is prepended to
each useful
symbol duration Tõ and is used to compensate for the dispersion introduced by
the
channel response and by the pulse shaping filter used at the transmitter.
Thus, although a
total OFDM/OFDMA symbol duration of TSYM = Tu + TG + TWIN is employed for
transmitting an OFDM symbol, the useful symbol duration: T. = 1 is available
for user
data transmission. T,, is therefore called the useful OFDM/OFDMA symbol
duration.
The windowing time period TWIN is optional, it can be set to 0 in some
communication
systems, such as the IEEE 802.16e version of WiMAX.
[0098] As mentioned above, in the existing systems the cyclic prefix is
configurable, but it
is fixed when a system is deployed, thereby constraining system configuration
for efficient
bandwidth utilization. In these existing systems, cyclic prefix length may not
be variable
and one type of cyclic prefix may exist. In this manner, existing systems may
not allow a
base station to change or configure the cyclic prefix duration to adjust to
varying channel
conditions.
[0099] According to one embodiment of the invention, for example, when a
communication channel has a severe multipath delay spread (i.e., larger delay
spread), a
longer cyclic prefix duration can be used to eliminate the ISI. In a less
severe channel
conditions with less multipath delay spread, a short cyclic prefix can be used
in order to
reduce radio overhead and improve overall throughput and spectral efficiency.
In this
manner, various cyclic prefix types may be used for small, regular, and large
cell site
deployment as explained in more detail below. The various cyclic prefix types
are
referred to as Short, Normal, and Long respectively.
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[01001 A cyclic prefix duration TG may be calculated based on the following
relationship:
TG = CPF ples , where Fs is the sampling frequency as shown above and CP is
the
samples
s
number of samples per cyclic prefix. Where, CPsamples = TG X Fs can be
obtained from
knowing TG and Fs.
The value of TG is selected in the particular sub-frame configuration, such as
Short,
Normal, or Long.
[01011 An initial TG (plus TWIN) value may be selected for an initial channel.
As shown
in Figure 14, at a 10 MHz channel one uplink sub-frame can be configured for
Normal
Cyclic Prefix (Normal CP), TG =10 s, in which CPsamples may comprise 128
samples.
Another uplink sub-frame could be configured for Short Cyclic Prefix (Short
CP), TG
=3.125 s, in which CPsamples may comprise only 40 samples. For example, an
initial
Normal TG =10 s is selected based on a typical cell site coverage, and the
downlink
control channel, multicast and broadcast sub-frames and the Normal TG is used
so that all
mobile stations are able to listen to the base station. However, once the base
station has
identified that one or multiple mobile stations are close enough to the base
station with
small delay spreads, the base station can allocate these mobile stations to
transmit in the
uplink sub-frames with Short Cyclic Prefix (Short CP) TG =3.125 s. The base
station can
also transmit the unicast or multicast information so these mobile stations
can also use a
downlink sub-frame with Short Cyclic Prefix (Short CP) TG =3.125 s. For
example, for a
channel transmission bandwidth of 2.5 MHz and a Short cyclic prefix, a
CPsamples value of
(3.125x3.2) may be obtained. In this manner, for a given bandwidth series,
CPsamples
may be scaled by the sampling frequencies so as to keep the cyclic prefix
duration TG
constant. For example, for a Short cyclic prefix, CPsamples may be
5/10/20/40/80/160 for
channel transmission bandwidth of 1.25/2.5/5/10/20/40 MHz respectively, while
TG
remains at 3.125 s. A system with different bandwidths will have the same
performance
and user experience. In IEEE 802.16e version of WIMAX, a subscriber moves from
a
7MHz system to a 10MHz system, TG is reduced accordingly with the bandwidth
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increase. The same subscriber may experience more dropped calls in the 10MHz
system.
It has imposed great constraints on the cell site planning, and it is hard to
maintain the
same user experience across different bandwidth systems.
[0102] Alternatively, variable cyclic prefix durations may be chosen for a
given channel
transmission bandwidth based on the same sub-carrier spacing Af =12.5kHz . For
example, cyclic prefix durations of TG &3.125/10/16.875 s may be chosen for
the Short,
Normal, and Long cyclic prefixes respectively. These cyclic prefix durations
can be used
for small, regular, and large cell site deployment as explained above.
Selecting different
cyclic prefixes for OFDM/OFDMA symbols in a sub-frame for a base station
allows for
supporting different types of base station cells and cell coverage areas.
Thereby, network
deployment may be simplified by eliminating the need for the entire network to
select the
same cyclic prefixes regardless of the different requirements on each base
station for its
cell coverage area.
[0103] For a TDD system, downlink and uplink radio propagation are reciprocal,
the base
stations can detect and determine whether a smaller size of cyclic prefix is
sufficient for a
particular mobile station. On the other hand, a mobile station also can
measure the
downlink signals from a base station to determine what size of cyclic prefix
is sufficient
for the uplink transmission. The mobile station can report to the base station
the preferred
size of the cyclic prefix.
[0104] Different sub-frame (e.g., Short, Reg., Long) durations TS,,b-frame can
be designed
based on different cyclic prefix durations such as Short, Normal, and Long
cyclic prefix
durations. For example, in Figure 14 sub-frame durations may be TSub_frame =
0.5, 1, and
1.5ms for the Short, Regular, and Long durations respectively. Also in Figure
14, the
cyclic prefix duration TG + T1 for a 1.25 MHz channel transmission bandwidth
may be
approximately 3.125/10/16.875 s for Short, Normal, and Long cyclic prefix
durations
respectively. Accordingly, useful bandwidth can be allocated for data
transmission instead
of cyclic prefix transmission, thereby increasing the bandwidth efficiency
(bits/Hz). In
this manner, the overhead from the cyclic prefix duration can be minimized.
[0105] For a given Tsub-frame and cyclic prefix type, the number of OFDM/OFDMA
symbols per sub-frame (NSYM ) can be a function of the sampling frequency FS
and FFT
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size NFFT . Fs and NFFT may be chosen so that NSYM can remain the same across
the
bandwidth series. For example, for a 1.25 MHz transmission bandwidth NSYM may
be
calculated based on the following relationship:
1
T Sub -frame x ]~ l S - 2 TTG min samples 1.5 x 1000x 1.6 - 5
N SYM < _ =18.01
N FFT + CP sample 128+5
[0106] When the carrier spacing 4f is fixed, the length of the useful symbol
duration
becomes constant, T,, = 1 . For a given cyclic prefix duration, the length of
symbol
Af
duration is also determined (assuming TWIN = 0), TSYM = Tu +Tw,N +TG = Off +
TG . For a
given sub-frame duration Tsub_Frame , the Tsub_Frame comprises transmission
time and idle
time. The transmission time is occupied by radio signal of multiple of
symbols,
NSYM x TsYM . The leftover idle time is used for transmit transition gap (TTG)
time
TTGSub_Frame and receive transition gap (RTG) time RTGsub_Frame, the latter is
typically
applicable to only uplink sub-frame. The value of RTG is often small. The
number of
OFDM/OFDMA symbols per sub-frame (NsYM ) can be calculated as following:
[0107] N sym = T Sub - frame - TTG Sub - frame - RTG Sub -frame
1
+T,G
[0108] In the table of FIG 25, a sub-frame length of 1.5ms Tsub_Frame =1500 s
is used as an
example to demonstrate how the number of symbols in the sub-frame is
calculated. Since
Of is fixed at 12.5kHz, Tu = Qf, =80 s, we can pick a Normal CP TG = 10 t for
this sub-
frame. We can also assume RTGSub_Frame=0, and TTGSub_Frame>10 to accommodate
additional propagation delays to derive the following relationship:
T Sub- frame - l T G Sub- frame -RTG Sub-frame 1500-10-0
_
[0109] NSYM = 1 +TG < 80+10 - 16.56
of
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[0110] In one embodiment, the number of symbols in the sub-frame is 16, as
shown in the
table of Figure 25.
[0101] From this example, regardless of the size of the transmission bandwidth
to 5MHz
to 20MHz, the number of symbols in a particular sub-frame is the same when a
particular
CP length is chosen. For a particular deployment channel condition, a
particular CP is
chosen; the RF performance related to mobility for different transmission
bandwidth
remains roughly the same with the same RF overhead. The subscriber will enjoy
the
similar user experience in the system.
[0102] Additionally, as shown in the table of Figure 25, the shorter the
cyclic prefix the
higher number of symbols can be fitted into a particular sub-frame, thereby
providing
higher throughput for the sub-frame. For a Pico or Femto cell deployment, the
delay
spreads and round trip delays are often small, so the base station and the
associated mobile
stations can be configured to transmit with short cyclic prefixes (Short CP)
to improve
frequency efficiency. For a Macro cell deployment, the coverage is often the
important
limitation. Due to high transmit RF power, Macro cell naturally have large
cell site, which
has increased the delay spreads and round trip delays for most radio signals.
We can
configure the base station and mobile stations and the associated mobile
stations with long
cyclic prefixes (Long CP) to combat rich multipath and large delay spread so
as to reduce
inter-symbol interference (ISI). The Femto Cells, Pico Cells, and Macro Cells
can be
deployed simultaneously and each can have optimized CP selections and
frequency
efficiency.
[0103] As discussed in relation to Figure 12, the TTG(DL), TTG(UL), and RTG
may vary
based upon to the size of the sub-frame and the corresponding cyclic prefix.
The
processor modules 616/622 may be suitably configured to compute TTG (DL),
TTG(UL)
and the RTG values as follows:
[0104] The TTG (DL) may be calculated by using the following relationship:
TTG(DL) = (DL sub-frame duration) - (num of symbols in the DL sub-frame)
(OFDM/OFDMA symbol duration (TsYM)), where
OFDM/OFDMA symbol duration = (cyclic prefix duration (TG )) + (IFFT time
(T,,)) + (Windowing time (TWIN) )
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[0105] Similarly, the TTG (UL) may be calculated by using the following
relationship:
TTG(UL) = (UL sub-frame duration) - (num of symbols in the UL sub-
frame) * (OFDM/OFDMA symbol duration (TsYM)), where TSYM is calculated as
shown above.
[0106] RTG is usually small and may be obtained by switching the time period
from
transmit to receive mode. If RTG is required in the system design, RTG should
also be
defined in a time unit which can be divided evenly by all sampling times.
Using the table
in FIG 14 as an example, the minimum time unit is Ts =3.125/5 =0.625 s. RTG
for
Normal CP sub-frame is 1.25 s in this particular example. Alternatively, RTG
can also be
set to zero for the sub-frame.
[0107] As shown in Figure 14, the TTG(DL), TTG(UL), and RTG, for a Long sub-
frame
and a Short cyclic prefix (CP), may be 3.75 s, 2.5 s, and 1.25 is
respectively.
Similarly, the TTG(DL), TTG(UL), and RTG, for the Long sub-frame and a Normal
cyclic
prefix, may be 60 s, 58.75 s, and 1.25 .ts respectively, and so on.
[0108] OFDM/OFDMA parameters in Figures 15-26 may share same OFDM/OFDMA
parameters definition and functionality as Figure 14, therefore these
definitions and the
functionalities are not redundantly explained herein.
[0109] Figure 15 illustrates an exemplary table of basic OFDMA parameters for
a 3.5
bandwidth series (channel transmission bandwidths 3.25, 7, 14, 28, 56, and 112
MHz)
according to an embodiment of the invention. The FFT size NFFT is scalable
from 512 to
16384. Similar to the 1.25 bandwidth series explained above, an OFDM/OFDMA
system
with a fixed subcarrier spacing value Of =12.5kHz may be used for the 3.5
bandwidth
series. As shown in Figure 15, the sampling frequencies can be 6.4, 12.8,
25.6, 51.2,
102.4, and 204.8 MHz. For this example, the number of occupied subcarriers for
channel
transmission bandwidth BW of 3.25, 7, 14, 28, 56, and 112 MHz are 281, 561,
201, 1121,
2241, 4481 and 8961 respectively.
[0110] Variable cyclic prefix durations plus a windowing time (e.g., TG + TWIN
2.97/10/16.72 s) can be chosen based on the same Of 12.5kHz condition. The
variable
cyclic prefix durations are referred to as Short, Normal, and Long cyclic
prefix
respectively, and can be used for small, regular, and large cell site (Figure
1) deployment.
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Different cyclic prefixes may be selected for OFDM/OFDMA symbols in a sub-
frame
(e.g., short, large, and long). Different sub-frames (e.g., Short, Regular,
Long) durations
Tsub-frame can be designed based on different cyclic prefix durations such as
Short, Normal,
and Long cyclic prefix durations respectively. For example, for Tsõb-frame =
0.5, 1, and
1.5 ms (Figure 8), MHz, the TG + TWIN duration (overhead) for a 3.5 MHz
channel
transmission bandwidth may be about 2.96/10/16.875 s respectively.
[0111] As mentioned above, according to an embodiment of the invention, the
TTG(DL),
TTG(UL), and RTG may vary based upon to the size of the sub-frame and the
corresponding cyclic prefixes. For example, as shown in Figure 15, the cyclic
prefix
duration of the TTG(DL), TTG(UL), and RTG, for a Long sub-frame and a Short
cyclic
prefix, may be about6.56, 6.25, and 0.31 s respectively. Similarly, the
cyclic prefix
duration of the TTG(DL), TTG(UL), and RTG, for the Long sub-frame and a Normal
cyclic prefix, may be about 60 is, 59.68 s, and 0.31 ps respectively, and so
on.
[0112] Figure 16 illustrates an exemplary table of basic OFDM/OFDMA parameters
for a
1.25 MHz bandwidth series showing additional optimized overhead (TG + TWIN )
values
that may be used according to embodiments of the invention. For example,
overhead
values for a 1.25 MHz channel transmission bandwidth BW may be about
2.5/9.3716.87
for a Short cyclic prefix, Normal cyclic prefix, and Long cyclic prefix
respectively
(compared to 3.125/10/16.875 ps in Figure 14), and so on.
[0113] Figure 17 illustrates an exemplary table of basic OFDMA parameters for
a 3.5
bandwidth series showing additional overhead values that may be used according
to
embodiments of the invention. For example, TG + TWIN durations that are
similar to the
1.25 MHz bandwidth series explained above (Figure 16), but are used for a 3.5
MHz
bandwidth series.
[0114] Figures 18-23 are extensions of Figure 16 showing values for the TTG
(DL),
TTG(UL) and RTG for 0.5, 0.675, 1, 1.25 , 2, and 2.5 sub-frames according to
various
embodiments of the invention.
[0115] Figure 18 illustrates an exemplary table of basic OFDM/OFDMA parameters
for a
1.25 MHz bandwidth series with a 0.5 ms sub-frame according to an embodiment
of the
invention. As shown in Figure 18, the duration for the TTG(DL), TTG(UL), and
RTG for
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a 0.5 ms sub-frame and a Short cyclic prefix (2.5 s), may be 5, 2.5, and 2.5
s
respectively. Similarly, the duration for the TTG(DL), TTG(UL), and RTG for a
0.5 ms
sub-frame and a Long cyclic prefix (15 s), may be about 112.5, 110, and 2.5
s
respectively, and so on.
[0116] Figure 19 illustrates an exemplary table of basic OFDM/OFDMA parameters
for a
1.25 MHz bandwidth series with a 0.675 ms sub-frame according to an embodiment
of
the invention. For example, as shown in Figure 19, the duration for the
TTG(DL),
TTG(UL), and RTG for a 0.675 ms sub-frame and a Short cyclic prefix (2.5 s),
may be
15, 12.5, and 2.5 s respectively. Similarly, the duration for the TTG(DL),
TTG(UL), and
RTG for a 0.675 ms sub-frame and a Long cyclic prefix (15 s), may be 93.75,
91.25, and
2.5 is respectively, and so on.
[0117] Figure 20 illustrates an exemplary table of basic OFDM/OFDMA parameters
for a
1.25 MHz bandwidth series with a 1 ms sub-frame according to an embodiment of
the
invention. For example, as shown in Figure 20, the duration for the TTG(DL),
TTG(UL),
and RTG for a 1 ms sub-frame with a Short cyclic prefix (2.5 s), may be 10,
7.5, and 2.5
s respectively. Similarly, the duration for the TTG(DL), TTG(UL), and RTG for
a 0.675
ms sub-frame and a Long cyclic prefix (15 s), may be 31.25, 28,75, and 2.5 s
respectively, and so on.
[0118] Figure 21 illustrates an exemplary table of basic OFDM/OFDMA parameters
for a
1.25 MHz bandwidth series with a 1.5 ms sub frame according to an embodiment
of the
invention. For example, as shown in Figure 21, the TTG(DL), TTG(UL), and RTG
for a
1.5 ms sub-frame with a Short cyclic prefix (2.5 s), may be 15, 12.5, and 2.5
is
respectively. Similarly, the duration for the TTG(DL), TTG(UL), and RTG for a
1.5 ms
sub-frame and a Long cyclic prefix (15 s), may be 46.875, 44.375, and 2.5 is
respectively, and so on.
[0119] Figure 22 illustrates an exemplary table of basic OFDM/OFDMA parameters
for a
1.25 MHz bandwidth series with a 2 ms sub frame according to an embodiment of
the
invention. For example, as shown in Figure 22, the duration of the TTG(DL),
TTG(UL),
and RTG for a 2 ms sub-frame with a Short cyclic prefix (2.5 s), may be 20,
17.5, and
2.5 s respectively. Similarly, the duration for the TTG(DL), TTG(UL), and RTG
for a 2
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ms sub-frame and a Long cyclic prefix (15 s), may be about 62.5, 60, and 2.5
s
respectively, and so on.
[0120] Figure 23 illustrates an exemplary table of basic OFDM/OFDMA parameters
for a
MHz bandwidth series with a 2.5 ms sub-frame for transmission in a channel
with a
channel transmission bandwidth BW of 20 MHz according to an embodiment of the
invention. For example, as shown in Figure 23, the duration of the TTG(DL),
TTG(UL),
and RTG for a 2.5 ms sub-frame with a Short cyclic prefix (2.5 s), may be 25,
22.5, and
2.5 s respectively. Similarly, the duration for the TTG(DL), TTG(UL), and RTG
for a
2.5 ms sub-frame and a Long cyclic prefix (15 s), may be about 78.125,75.625,
and 2.5
s respectively, and so on.
[0121] Alternatively, systems 600 and 700 may operate with different fixed
subcarrier
spacing and hence provide different scalability properties. In this manner
embodiments of
the invention can offer compatibility with various communication systems. For
example,
Figures 24-26 illustrate exemplary tables of basic OFDM/OFDMA parameters for a
5
MHz bandwidth series (channel transmission bandwidths 5, 7, 8.75, 10, 14, and
20 MHz)
for a sub-carrier spacing Of = 10.9375 KHz, 12.5 KHz , and 25 KHz. A sub-
carrier
spacing of Of = 10.9375 KHz corresponds to that used in IEEE 802.16e (WiMAX).
[0122] Figure 24 illustrates an exemplary table of basic OFDMA parameters for
a MHz
bandwidth series for a sub-carrier spacing Of = 10.9375 which can not divide
all the RF
bandwidths evenly, therefore it is not a good choice according to embodiments
of the
invention. However, IEEE 802.16e version of Mobile WiMAX has chosen Af =
10.9375
kHz for 5MHz and 10MHz channel bandwidths deployment. For some backward
compatibility consideration, Af = 10.9375 or Of =21.875kHz can be used for
other
channel bandwidths deployment for IEEE 802.16m version of future WiMAX.
According
to this embodiment of the invention, the FFT size NFFT is scalable from 512 to
2048. The
sampling frequency (e.g., Fs = 5.6, 11.2, 11.2, 11.2, 22.4, and 22.4 MHz) is
calculated for
the 5, 7, 8.75, 10, 14, and 20 MHz channel transmission bandwidths
respectively as
explained above. The number of occupied subcarriers for channel transmission
bandwidths of 5, 7, 8.75, 10, 14, and 20 MHz may be 421, 589, 735, 841, 1177,
1681
respectively in this example. A Short cyclic prefix , a Normal cyclic prefix,
a Long cyclic
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prefix , and another Long cyclic prefix (CP2) durations of 2.857, 11.428,
17.142, and
22.857 s can be chosen for the 5 MHz bandwidth series.
[0123] Different sub-frames durations Tsub_frame can be designed based on
different cyclic
prefix durations such as Short, Long, and Normal cyclic prefix durations. For
example,
sub-frame durations may be Tsub-frame = 0.5, 0.675, 1, 1.5, 2, and 2.5, and
the cyclic prefix
duration for these sub-frames may be selected from the above cyclic prefix
values. For
example, for a 0.5 ms sub-frame duration at 5 MHz channel transmission
bandwidth
BW, a Short cyclic prefix of 2.857 s can be selected thereby allowing 5
OFDM/OFDMA
symbol per frame to be transmitted. The duration for the TTG(DL), TTG(UL), and
RTG
(Figure 12) may vary based upon to the size of the sub-frames and the
corresponding
cyclic prefixes. For example, as shown in Figure 25, for the 5 MHz bandwidth
series, the
duration for the TTG(DL) or TTG(UL), for a 0.5 ms sub-frame and a Short cyclic
prefix
(2.857 .is), may be 28.571 s. Similarly, the duration for the TTG(DL) or
TTG(UL), for a
0.5 ms sub-frame and a Long cyclic prefix (17.142 s), may be 65.71 s, and so
on.
[0124] Figure 25 illustrates an exemplary table of basic OFDM/OFDMA parameters
for a
MHz bandwidth series according to an embodiment of the invention. In this
example,
the FFT size N,;,;.,. is scalable from 512 to 2048. A fixed subcarrier spacing
value
Af =12.5kHz (similar to the 1.25 bandwidth series) may be used for the 5 MHz
bandwidth series.
[0125] The sampling frequency (e.g., Fs = 6.4, 12.8, 12.8, 12.8, 25.6, and
25.6 MHz) is
calculated for the 5, 7, 8.75, 10, 14, and 20 MHz channel transmission
bandwidth BW
respectively as explained above. For this example, the number of occupied
subcarriers for
channel transmission bandwidth BW of 5, 7, 8.75, 10, 14, and 20 MHz is 401,
561, 701,
801, 1121, 4481 and 1601 respectively. A Short cyclic prefix, a Normal cyclic
prefix, a
Long cyclic prefix, and another Long cyclic prefix (CP2) durations of 2.5, 10,
15, and 20
s can be chosen for these channel transmission bandwidths.
[0126] Different sub-frames durations Tsub_f,.ame can be designed based on
different cyclic
prefix durations such as Short, Long, and Normal cyclic prefix durations. For
example,
Tsub-frame = 0.5, 0.675, 1, 1.5, 2, and 2.5, and the cyclic prefix duration
for these sub-
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frames may be selected from 2.5, 10, 15, and 20 s cyclic prefix duration TG
values. For
example, for a 0.5 ms sub-frame duration at a channel transmission bandwidth
of 5 MHz,
a cyclic prefix duration of 2.5 s can be selected. As mentioned above, the
TTG(DL),
TTG(UL), and RTG (Figure 12) may vary based upon to the size of the sub-frame
and the
corresponding cyclic prefixes. For example, as shown in Figure 18, for the
transmission
bandwidth BW of 5 MHz, the duration for the TTG(DL) or TTG(UL), for a 0.5 ms
sub-
frame and a Short cyclic prefix (2.5 s), may be 5 s. Similarly, the duration
for the
TTG(DL) or TTG(UL), for a 0.5 ms sub-frame and a Long cyclic prefix (15 s),
may be
120 s, and so on.
[0127] Figure 26 illustrates an exemplary table of basic OFDM/OFDMA parameters
for a
MHz bandwidth series with a subcarrier spacing 4f 25 KHz according to an
embodiment of the invention. The FFT size NFFT is scalable from 256 to 1024
(e.g., 256,
512, 512, 512, 1024, and 1024). The sampling frequency (e.g., Fs = 6.4, 12.8,
12.8, 12.8,
25.6, and 25.6 MHz) is calculated for the 5, 7, 8.75, 10, 14, and 20 MHz
channel
transmission bandwidths BW respectively as explained above. The number of
occupied
subcarriers for these channel transmission bandwidths can be 201, 281, 351,
401, 561, and
801 respectively. A Short cyclic prefix , a Normal cyclic prefix , a Long
cyclic prefix, and
another Long cyclic prefix (CP2) durations of 2.857 s, 11.428 s, 17.142 .is,
and 22.857
s can be chosen for these channel transmission bandwidths.
[0128] Different sub-frames durations Tsub-frame can be designed based on
different cyclic
prefix durations such as Short, Long, and Normal cyclic prefix durations. For
example,
sub-frame durations may be Tsub_frame = 0.5, 0.675, 1, 1.5, 2, and 2.5, and
the cyclic prefix
duration for these sub-frames may be selected from the above cyclic prefix
values. For
example, for a 0.5 ms sub-frame duration at 5 MHz bandwidth, a Short cyclic
prefix of
2.5 s can be selected thereby allowing 11 OFDM/OFDMA symbols per frame to be
transmitted in this frame. The duration for the TTG(DL), TTG(UL), and RTG
(Figure 12)
may vary based upon to the size of the sub-frames and the corresponding cyclic
prefixes.
For example, as shown in Figure 26, for a channel transmission bandwidth of 5
MHz, the
duration for the TTG(DL), or TTG(UL), for a 0.5 ms sub-frame and a Short
cyclic prefix
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(2.5 s), may be 32.5 s. Similarly, the duration for the TTG(DL), or TTG(UL),
for a 0.5
ms sub-frame and a Long cyclic prefix (15 s), may be 60 s, and so on.
[0129] Figure 27 illustrates a flowchart showing an OFDM/OFDMA process 2700
for
creating a frame structure with a variable cyclic prefix according to
embodiments of the
invention. The various tasks performed in connection with these processes may
be
performed by software, hardware, firmware, a computer-readable medium having
computer executable instructions for performing the process method, or any
combination
thereof. It should be appreciated that process 2700 may include any number of
additional
or alternative tasks. The tasks shown in Figures 27 need not be performed in
the
illustrated order, and these processes may be incorporated into a more
comprehensive
procedure or process having additional functionality not described in detail
herein. For
illustrative purposes, the following description of process 2700 may refer to
elements
mentioned above in connection with Figures 6-26. In various embodiments,
portions of
process 2700 may be performed by different elements of systems 600-700 e.g.,
transceivers and processors. OFDM/OFDMA process 2700 may share same
OFDM/OFDMA definitions and functionalities as explained above in the context
of
Figures 6-26, therefore these definitions and the functionalities are not
redundantly
explained herein.
[0130] Process 2700 may begin with the OFDM/OFDMA transmitter 701 receiving
time
domain OFDM data symbols for transmission on an RF channel (task 2702). Netx,
the
cyclic prefix selector 709 selects a cyclic prefix from a plurality of
variable length cyclic
prefixes (task 2704). The cyclic prefix may be selected from a plurality of
cyclic prefixes
available for the RF channel. For example, as shown in Figure 14, the RF
channel may
comprise a plurality of variable length cyclic prefixes that range from 5-864
samples for
various channel transmission bandwidths in the RF channel.
[0131] As shown in Figure 14, for a 1.25 MHz channel transmission bandwidth
BW, a
set of variable length cyclic prefix comprises a Short, a Normal and a Long
cyclic prefix
length comprising 5, 16, and 27 samples respectively. These cyclic prefixes
may be scaled
to obtain the set of cyclic prefixes for each of the other channel
transmission BW (RF
channels). For example, a short cyclic prefix (e.g., 5 samples) can be scaled
to obtain a
10, 20, 40 and 80 samples for the channel transmission bandwidths BW of 2.5,
5, 10, 20,
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and 40 MHz respectively, and so on. For these channel transmission bandwidths,
the
cyclic prefix duration of 3.125 s can then be calculated as explained above
in the context
of discussion of Figure 14.
[0132] Process 2700 then adds the selected cyclic prefix into each of the time
domain
OFDM/OFDMA data symbols to obtain a plurality of OFDM frames (task 2706) using
the
add cyclic prefix module 710. The selected cyclic prefix may be in the form of
digital
samples of the corresponding cyclic prefix duration. Process 2700 may then
transmits the
OFDM frames on the radio channel such as the radio channel 714 (task 2708). In
this
manner, process 2700 adds the OFDM frames to a variable size sub-frame prior
to
transmitting the OFDM frames on the channel.
[0133] According to embodiments of the invention, these variable length cyclic
prefixes
can be used for small, regular, and large cell site deployment as explained
above to
improve bandwidth efficiency (bit/Hz) of the system. Furthermore, selecting
different
cyclic prefixes for OFDM/OFDMA symbols in a sub-frame for a base station
allows for
supporting different types of base station cells and cell coverage areas.
Thereby, network
deployment may be simplified and made more flexible by eliminating the need
for the
entire network to select the same cyclic prefixes regardless of the different
requirements
on each base station for its cell coverage area.
[0134] While various embodiments of the invention have been described above,
it should
be understood that they have been presented by way of example only, and not by
way of
limitation. Likewise, the various diagrams may depict an example architectural
or other
configuration for the disclosure, which is done to aid in understanding the
features and
functionality that can be included in the disclosure. The disclosure is not
restricted to the
illustrated example architectures or configurations, but can be implemented
using a variety
of alternative architectures and configurations. Additionally, although the
disclosure is
described above in terms of various exemplary embodiments and implementations,
it
should be understood that the various features and functionality described in
one or more
of the individual embodiments are not limited in their applicability to the
particular
embodiment with which they are described. They instead can, be applied, alone
or in
some combination, to one or more of the other embodiments of the disclosure,
whether or
not such embodiments are described, and whether or not such features are
presented as
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being a part of a described embodiment. Thus the breadth and scope of the
present
disclosure should not be limited by any of the above-described exemplary
embodiments.
[0135] In this document, the term "module" as used herein, refers to software,
firmware,
hardware, and any combination of these elements for performing the associated
functions
described herein. Additionally, for purpose of discussion, the various modules
are
described as discrete modules; however, as would be apparent to one of
ordinary skill in
the art, two or more modules may be combined to form a single module that
performs the
associated functions according embodiments of the invention.
[0136] In this document, the terms "computer program product", "computer-
readable
medium", and the like, may be used generally to refer to media such as, memory
storage
devices, or storage unit. These, and other forms of computer-readable media,
may be
involved in storing one or more instructions for use by processor to cause the
processor to
perform specified operations. Such instructions, generally referred to as
"computer
program code" (which may be grouped in the form of computer programs or other
groupings), when executed, enable the computing system.
[0137] It will be appreciated that, for clarity purposes, the above
description has described
embodiments of the invention with reference to different functional units and
processors.
However, it will be apparent that any suitable distribution of functionality
between
different functional units, processors or domains may be used without
detracting from the
invention. For example, functionality illustrated to be performed by separate
processors or
controllers may be performed by the same processor or controller. Hence,
references to
specific functional units are only to be seen as references to suitable means
for providing
the described functionality, rather than indicative of a strict logical or
physical structure or
organization.
[0138] Terms and phrases used in this document, and variations thereof, unless
otherwise
expressly stated, should be construed as open ended as opposed to limiting. As
examples
of the foregoing: the term "including" should be read as meaning "including,
without
limitation" or the like; the term "example" is used to provide exemplary
instances of the
item in discussion, not an exhaustive or limiting list thereof; and adjectives
such as
"conventional," "traditional," "normal " "standard " "known", and terms of
similar
meaning, should not be construed as limiting the item described to a given
time period, or
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to an item available as of a given time. But instead these terms should be
read to
encompass conventional, traditional, normal, or standard technologies that may
be
available, known now, or at any time in the future. Likewise, a group of items
linked with
the conjunction "and" should not be read as requiring that each and every one
of those
items be present in the grouping, but rather should be read as "and/or" unless
expressly
stated otherwise. Similarly, a group of items linked with the conjunction "or"
should not
be read as requiring mutual exclusivity among that group, but rather should
also be read as
"and/or" unless expressly stated otherwise. Furthermore, although items,
elements or
components of the disclosure may be described or claimed in the singular, the
plural is
contemplated to be within the scope thereof unless limitation to the singular
is explicitly
stated. The presence of broadening words and phrases such as "one or more,"
"at least,"
"but not limited to", or other like phrases in some instances shall not be
read to mean that
the narrower case is intended or required in instances where such broadening
phrases may
be absent.
[01391 Additionally, memory or other storage, as well as communication
components,
may be employed in embodiments of the invention. It will be appreciated that,
for clarity
purposes, the above description has described embodiments of the invention
with
reference to different functional units and processors. However, it will be
apparent that
any suitable distribution of functionality between different functional units,
processing
logic elements or domains may be used without detracting from the invention.
For
example, functionality illustrated to be performed by separate processing
logic elements,
or controllers, may be performed by the same processing logic element, or
controller.
Hence, references to specific functional units are only to be seen as
references to suitable
means for providing the described functionality, rather than indicative of a
strict logical or
physical structure or organization.
[01401 Furthermore, although individually listed, a plurality of means,
elements or method
steps may be implemented by, for example, a single unit or processing logic
element.
Additionally, although individual features may be included in different
claims, these may
possibly be advantageously combined. The inclusion in different claims does
not imply
that a combination of features is not feasible and/or advantageous. Also, the
inclusion of a
feature in one category of claims does not imply a limitation to this
category, but rather the
feature may be equally applicable to other claim categories, as appropriate.
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