Note: Descriptions are shown in the official language in which they were submitted.
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NETWORK-RELAY SIGNALING FOR DOWNLINK TRANSPARENT RELAY
FIELD OF THE INVENTION
[0001] The present invention relates to wireless communications and more
particularly to methods and systems for providing DL retransmissions to mobile
stations in wireless communication networks employing transparent relay.
BACKGROUND
[0002] Wireless communication systems are widely deployed to provide various
types of communication content such as voice, data, and other content. These
systems may be multiple-access systems capable of simultaneously supporting
communication for multiple wireless terminals by sharing the available
transmission
resources (e.g., frequency channel and/or time interval). Since the
transmission
resources are shared, efficient allocation of the transmission resources is
important
as it impacts the utilization of the transmission resources and the quality of
service
perceived by individual terminal users. One such wireless communications
system is
the Orthogonal Frequency-Division Multiple Access (OFDMA) system in which
multiple wireless terminals perform multiple-access using Orthogonal Frequency-
Division Multiplexing (OFDM).
[0003] OFDM is a multi-carrier modulation technique that partitions the
overall
system bandwidth into multiple orthogonal frequency subchannels, each of which
is
associated with a respective subcarrier that may be modulated with data.
Because
the subchannels are made orthogonal, some spectral overlap between the
subchannels is permitted, leading to a high spectral efficiency. In OFDM
systems,
the user data stream is split into parallel streams of reduced rate, and each
obtained
substream then modulates a separate subcarrier.
[0004] In OFDMA, access to the shared wireless medium is scheduled using
frames that extend over two dimensions: time, in units of symbols, and
frequency, in
units of logical sub-channels. Data bursts are conveyed in two-dimensional
(i.e. time
and frequency) data regions within the frame which are scheduled by the BS via
specific control messages. Each frame is divided into downlink (DL) and uplink
(UL)
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subframes. The former is used by the BS to transmit data to the MSs, whereas
the
MSs transmit to the BS in the latter.
[0005] Examples of OFDM communication systems include, but are not limited
to,
wireless protocols such as the wireless local area network ("WLAN") protocol
defined
according to the Institute of Electrical and Electronics Engineering ("IEEE")
standards radio 802.11a, b, g, and n (hereinafter "Wi-Fi"), the Wireless
MAN/Fixed
broadband wireless access ("BWA") standard defined according to IEEE 802.16
(hereinafter "WiMAX"), the mobile broadband 3GPP Long Term Evolution ("LTE")
protocol having air interface High Speed OFDM Packet Access ("HSOPA") or
Evolved UMTS Terrestrial Radio Access ("E-UTRA"), the 3GPP2 Ultra Mobile
Broadband ("UMB") protocol, digital radio systems Digital Audio Broadcasting
("DAB") protocol, Hybrid Digital ("HD") Radio, the terrestrial digital TV
system Digital
Video Broadcasting-Terrestrial ("DVB-T"), the cellular communication systems
Flash-
OFDM, etc. Wired protocols using OFDM techniques include Asymmetric Digital
Subscriber Line ("ADSL") and Very High Bitrate Digital Subscriber Line
("VDSL")
broadband access, Power line communication ("PLC") including Broadband over
Power Lines ("BPL"), and Multimedia over Coax Alliance ("MoCA") home
networking.
[0006] 3GPP LTE defines the following physical channels:
= Downlink (DL)
O Physical Broadcast Channel (PBCH): This channel carries system
information for mobile stations (referred to as user equipment, or UE)
requiring access to the network.
O Physical Downlink Control Channel (PDCCH): The main purpose of
this physical channel is to carry scheduling information.
O Physical Hybrid ARQ Indicator Channel (PHICH): This channel is
used to report the Hybrid ARQ status.
O Physical Downlink Shared Channel (PDSCH): This channel is used
for unicast and paging functions.
O Physical Multicast Channel (PMCH): This physical channel carries
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system information for multicast purposes.
o Physical Control Format Indicator Channel (PCF1CH): This
channel provides information to enable the UEs to decode the
PDSCH.
=
Uplink (UL)
O Physical Uplink Control Channel (PUCCH): This channel is used to
transport user signalling data from one or more UE that can transmit
on the control channel. The PUCCH transports, for example,
acknowledgment responses and retransmission requests, service
scheduling requests, and channel quality information measured by the
UE to the system.
O Physical Uplink Shared Channel (PUSCH): This channel is used to
transport user data from one or more mobiles that can transmit on the
shared channel.
O Physical Random Access Channel (PRACH): This uplink physical
channel allows a UE to randomly transmit access requests when the
UE attempts to access the wireless communication system.
[0007] Wireless communication systems may employ a relay scheme to relay
user data and possibly control information between a base station (BS) and a
mobile
station (MS) through one or more relay stations (RS). A relay scheme may be
used
to enhance coverage, range, throughput and/or capacity of a base station. The
relay
stations can repeat transmissions to/from the BS so that MSs within
communication
range of a relay can communicate with the BS through the relay. The relays do
not
need a backhaul link because they can communicate wirelessly with both BSs and
MSs. This type of network may be referred to as a multihop network because
there
may be more than one wireless connection between the MS and a hardwired
connection. Depending upon the particular network configuration, a particular
MS
may gain network access via one or more neighbour relays and/or one or more
neighbour BSs. In addition, relays themselves might have one or more available
path
options to connect to a particular BS. The radio link between a BS or RS and
an MS
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is called an access link, while the link between a BS and an RS or between a
pair of
RSs is called a relay link.
[0008] Conventional relays operate in one of two different modes:
transparent
and non-transparent. A transparent RS does not transmit control information,
such
that a MS connected to a transparent RS receives control information directly
from
the BS, and the RS relays only data traffic. A non-transparent RS transmits
control
information and relays data traffic as well.
[0009] Hybrid automatic repeat-request (HARQ) operations can be used for
error
control in wireless communication systems. With HARQ, the receiver detects an
error in a message and automatically requests a retransmission of the message
from
the transmitter. In response to receiving the HARQ request (a "NACK"), the
transmitter retransmits the message until it is received correctly, unless the
error
persists. In one variation, HARQ combines forward error correction (FEC) with
an
error-correction code.
[0010] LTE uses asynchronous HARQ transmission on the DL. In asynchronous
HARQ, the receiver does not know ahead of time when the retransmission is
being
sent, and therefore control information must be sent along with the data. This
is
accomplished by sending resource allocation messages on the PDCCH
simultaneous to the corresponding PDSCH transmission. The advantage of this
scheme is that the scheduling algorithm has considerable freedom in deciding
which
MSs are sent data during any subframe.
[0011] In LTE systems where transparent relays are used, a RS could help
improve system performance by sending DL HARQ retransmissions to the MS at the
same time as the BS. However, an issue arises as to how the BS and the RS can
coordinate concurrent DL HARQ retransmission. Prior to retransmission, the RS
has
to know which physical resources (time and frequency) are used for
retransmission
of the packet by the BS so that the RS can use the same resources to transmit
the
same packet concurrently. However, since DL HARQ retransmissions are
asynchronous, the BS sends PDCCH and PDSCH in one subframe for
retransmission when a NACK is received. As the control signalling region and
data
transmission region are multiplexed contiguously in time division multiplexing
(TDM)
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fashion, there is no guard time between the two regions. The PDCCH is
transmitted
in the first n (where n = 1, 2 or 3) OFDM symbols in each subframe, and the
PDSCH
is transmitted through the remaining (N-n) OFDM symbols (where N is the number
of
OFDM symbols in each subframe). It is difficult for the RS to switch from
reception
mode to transmission mode between contiguous symbols. It is also difficult for
the
RS to both decode retransmission control information in the PDCCH and prepare
retransmission in the PDSCH in the same subframe. Additionally, in some
situations
the number of PDCCH carried by PCFICH could vary from subframe to subframe,
requiring the RS to decode PCFICH, determine the start of PDCCH and prepare
retransmission in the PDSCH in the same subframe.
[0012] While use of synchronous HARQ (i.e. retransmissions are scheduled on
predetermined subframes) might alleviate some of the aforementioned
difficulties,
such an approach could introduce undesirable restrictions on the scheduler.
[0013] A need exists for an improved scheme for downlink retransmissions in
transparent relay systems.
SUMMARY OF THE INVENTION
[0014] In accordance with an aspect of the present invention, there is
provided a
method of providing downlink retransmissions to a mobile station in a wireless
communication network, the wireless communication network comprising a base
station communicatively linked to a transparent relay station. According to
the
method, the base station receives a request for a retransmission from the
mobile
station; schedules resources for the retransmission; signals scheduling
information
for the retransmission to the transparent relay station via a control link;
and the
transparent relay station receives the scheduling information for the
retransmission
on the control link; and sends the retransmission to the mobile station in a
retransmit
subframe on a retransmit frequency band.
[0015] In a further aspect of the present invention, there is provided a
base
station in a wireless communication network, the base station comprising a
controller
operable to: receive a request for a retransmission from a mobile station;
schedule
resources for the retransmission; signal scheduling information for the
retransmission to a transparent relay station via a control link; and wherein
the
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signalling of the scheduling information enables the transparent relay station
to send
the retransmission to the mobile station in a retransmit subframe on a
retransmit
frequency band.
[0016] In a further aspect of the present invention, there is provided a
transparent
relay station in a wireless communication network, the transparent relay
station
comprising a controller operable to: receive, on a control link from a base
station,
scheduling information for a retransmission to a mobile station; and send the
retransmission to the mobile station in the retransmit subframe on the
retransmit
frequency band.
[0017] Other aspects and features of the present invention will become
apparent
to those ordinarily skilled in the art upon review of the following
description of specific
embodiments of the invention in conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] In the figures which illustrate embodiments of the invention by
example
only,
[0019] FIG. 1 is a block diagram of a cellular communication system;
[0020] FIG. 2 is a block diagram of an example base station that might be
used to
implement some embodiments of the present application;
10021] FIG. 3 is a block diagram of an example mobile terminal that might
be
used to implement some embodiments of the present application;
[0022] FIG. 4 is a block diagram of an example relay station that might be
used to
implement some embodiments of the present application;
[0023] FIG. 5 is a block diagram of a logical breakdown of an example OFDM
transmitter architecture that might be used to implement some embodiments of
the
present application;
[00241 FIG. 6 is a block diagram of a logical breakdown of an example OFDM
receiver architecture that might be used to implement some embodiments of the
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present application;
[0025] FIG. 7A is an example SC-FDMA transmitter;
[0026] FIG. 7B is an example SC-FDMA receiver;
[0027] FIG. 8 illustrates an example DL HARQ retransmission scheme in
accordance with embodiments of the present application;
[0028] FIG. 9 shows a flow diagram illustrating the steps for a DL HARQ
retransmission according to the scheme of FIG. 8;
[0029] FIG. 10A illustrates another example IX HARQ retransmission scheme
in
accordance with embodiments of the present application; and
[0030] FIG. 10B illustrates yet another example DL HARQ retransmission
scheme in accordance with embodiments of the present application.
DETAILED DESCRIPTION
[0031] Referring now to the drawing figures in which like reference
designators
refer to like elements, FIG. 1 shows a base station controller (BSC) 10 which
controls wireless communications within multiple cells 12, which cells are
served by
corresponding base stations (BS) 14. In some configurations, each cell is
further
divided into multiple sectors 13 (not shown). In general, each base station 14
facilitates communications using OFDM with mobile terminals 16, which are
within
the cell 12 associated with the corresponding base station 14. The movement of
the
mobile terminals 16 in relation to the base stations 14 results in significant
fluctuation
in channel conditions. As illustrated, the base stations 14 and mobile
terminals 16
may include multiple antennas to provide spatial diversity for communications.
As
described in more detail below, relay stations 15 may assist in communications
between base stations 14 and mobile terminals 16. Mobile terminals 16 can be
handed off 18 from any cell 12, sector 13 (not shown), base station 14 or
relay 15 to
an other cell 12, sector 13 (not shown), base station 14 or relay 15. In some
configurations, base stations 14 communicate with each and with another
network
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(such as a core network or the Internet, both not shown) over a backhaul
network 11.
In some configurations, a base station controller 10 is not needed.
[0032] FIG. 2 depicts an example of a base station 14. Base station 14
generally
includes a control system 20, a baseband processor 22, transmit circuitry 24,
receive
circuitry 26, antennas 28, and a network interface 30. The receive circuitry
26
receives radio frequency signals bearing information from one or more remote
transmitters provided by mobile terminals 16 (illustrated in FIG. 3) and relay
stations
15 (illustrated in FIG. 4). A low noise amplifier and a filter (not shown) may
cooperate
to amplify and remove broadband interference from the signal for processing.
Downconversion and digitization circuitry (not shown) will then downconvert
the
filtered, received signal to an intermediate or baseband frequency signal,
which is
then digitized into one or more digital streams.
[0033] The baseband processor 22 processes the digitized received signal to
extract the information or data bits conveyed in the received signal. This
processing
typically comprises demodulation, decoding, and error correction operations.
As
such, the baseband processor 22 is generally implemented in one or more
digital
signal processors (DSPs) or application-specific integrated circuits (ASICs).
The
received information is then sent across a wireless network via the network
interface
30 or transmitted to another mobile terminal 16 serviced by the base station
14,
either directly or with the assistance of a relay 15.
[0034] On the transmit side, baseband processor 22 receives digitized data,
which may represent voice, data, or control information, from the network
interface
30 under the control of control system 20, and encodes the data for
transmission.
The encoded data is output to the transmit circuitry 24, where it is modulated
by one
or more carrier signals having a desired transmit frequency or frequencies. A
power
amplifier (not shown) will amplify the modulated carrier signals to a level
appropriate
for transmission, and deliver the modulated carrier signals to the antennas 28
through a matching network (not shown). Modulation and processing details are
described in greater detail below.
[0035] FIG. 3 illustrates an example of a mobile terminal 16. Similarly to
the base
station 14, the mobile terminal 16 will include a control system 32, a
baseband
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processor 34, transmit circuitry 36, receive circuitry 38, antennas 40, and
user
interface circuity 42. The receive circuitry 38 receives radio frequency
signals
bearing information from one or more base stations 14 and relays 15. A low
noise
amplifier and a filter (not shown) may cooperate to amplify and remove
broadband
interference from the signal for processing. Downconversion and digitization
circuitry
(not shown) will then downconvert the filtered, received signal to an
intermediate or
baseband frequency signal, which is then digitized into one or more digital
streams.
[0036] Baseband processor 34 processes the digitized received signal to
extract
the information or data bits conveyed in the received signal. This processing
typically
comprises demodulation, decoding, and error correction operations. The
baseband
processor 34 is generally implemented in one or more digital signal processors
(DSPs) and application specific integrated circuits (ASICs).
[0037] For transmission, baseband processor 34 receives digitized data,
which
may represent voice, video, data, or control information, from the control
system 32,
which it encodes for transmission. The encoded data is output to the transmit
circuitry 36, where it is used by a modulator to modulate one or more carrier
signals
that is at a desired transmit frequency or frequencies. A power amplifier (not
shown)
will amplify the modulated carrier signals to a level appropriate for
transmission, and
deliver the modulated carrier signal to the antennas 40 through a matching
network
(not shown). Various modulation and processing techniques available to those
skilled in the art are used for signal transmission between the mobile
terminal and
the base station, either directly or via the relay station.
[0038] In OFDM modulation, the transmission band is divided into multiple,
orthogonal carrier waves. Each carrier wave is modulated according to the
digital
data to be transmitted. Because OFDM divides the transmission band into
multiple
carriers, the bandwidth per carrier decreases and the modulation time per
carrier
increases. Since the multiple carriers are transmitted in parallel, the
transmission
rate for the digital data, or symbols, on any given carrier is lower than when
a single
carrier is used.
[0039] OFDM modulation utilizes the performance of an Inverse Fast Fourier
Transform (IFFT) on the information to be transmitted. For demodulation, the
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performance of a Fast Fourier Transform (FFT) on the received signal recovers
the
transmitted information. In practice, the 1FFT and FFT are provided by digital
signal
processing carrying out an Inverse Discrete Fourier Transform (IDFT) and
Discrete
Fourier Transform (DFT), respectively. Accordingly, the characterizing feature
of
OFDM modulation is that orthogonal carrier waves are generated for multiple
bands
within a transmission channel. The modulated signals are digital signals
having a
relatively low transmission rate and capable of staying within their
respective bands.
The individual carrier waves are not modulated directly by the digital
signals. Instead,
all carrier waves are modulated at once by IFFT processing.
[0040] In one embodiment, OFDM is preferably used for at least downlink
transmission from the base stations 14 to the mobile terminals 16. Each base
station
14 is equipped with "n" transmit antennas 28 (n >= 1), and each mobile
terminal 16 is
equipped with "m" receive antennas 40 (m>= 1). Notably, the respective
antennas
can be used for reception and transmission using appropriate duplexers or
switches
and are so labelled only for clarity.
[0041] When relay stations 15 are used, OFDM is preferably used for
downlink
transmission from the base stations 14 to the relays 15 and from relay
stations 15 to
the mobile terminals 16.
[0042] FIG. 4 illustrates an example relay station 15. Similarly to the
base station
14, and the mobile terminal 16, the relay station 15 includes a control system
132, a
baseband processor 134, transmit circuitry 136, receive circuitry 138,
antennas 130,
and relay circuitry 142. The relay circuitry 142 enables the relay 14 to
assist in
communications between a base station 16 and mobile terminals 16. The receive
circuitry 138 receives radio frequency signals bearing information from one or
more
base stations 14 and mobile terminals 16. A low noise amplifier and a filter
(not
shown) may cooperate to amplify and remove broadband interference from the
signal for processing. Downconversion and digitization circuitry (not shown)
will then
downconvert the filtered, received signal to an intermediate or baseband
frequency
signal, which is then digitized into one or more digital streams.
[0043] Baseband processor 134 processes the digitized received signal to
extract
the information or data bits conveyed in the received signal. This processing
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comprises demodulation, decoding, and error correction operations. Baseband
processor 134 is generally implemented in one or more digital signal
processors
(DSPs) and application specific integrated circuits (ASICs).
[0044] For transmission, baseband processor 134 receives digitized data,
which
may represent voice, video, data, or control information, from control system
132,
which it encodes for transmission. The encoded data is output to the transmit
circuitry 136, where it is used by a modulator to modulate one or more carrier
signals
that is at a desired transmit frequency or frequencies. A power amplifier (not
shown)
will amplify the modulated carrier signals to a level appropriate for
transmission, and
deliver the modulated carrier signal to the antennas 130 through a matching
network
(not shown). Various modulation and processing techniques available to those
skilled in the art are used for signal transmission between the mobile
terminal and
the base station, either directly or indirectly via a relay station, as
described above.
[0045] With reference to FIG. 5, a logical OFDM transmission architecture
will be
described. Initially, base station controller 10 will send data to be
transmitted to
various mobile terminals 16 to base station 14, either directly or with the
assistance
of a relay station 15. As described in more detail below, base station 14 uses
the
channel quality indicators (CQI) values associated with the mobile terminals
to
schedule the data for transmission as well as select an appropriate modulation
and
coding scheme (MCS) level for transmitting the scheduled data. The CQI values
may
be received directly from the mobile terminals 16 or determined at the base
station
14 based on information provided by the mobile terminals 16. In either case,
the CQI
value associated with each mobile terminal 16 may for example be a function of
the
signal-to-interference ratio (SIR), as well as of the degree to which the
channel
amplitude (or response) varies across the OFDM frequency band.
[0046] Scheduled data 44, which is a stream of bits, is scrambled in a
manner
reducing the peak-to-average power ratio associated with the data using data
scrambling logic 46. A cyclic redundancy check (CRC) for the scrambled data is
determined and appended to the scrambled data using CRC adding logic 48. Next,
channel coding is performed using channel encoder logic 50 to effectively add
redundancy to the data to facilitate recovery and error correction at the
mobile
terminal 16. As described in more detail below, the channel coding for a
particular
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mobile terminal 16 is based on the current CQI value associated with that
mobile
terminal. In some implementations, the channel encoder logic 50 uses known
Turbo
encoding techniques. The encoded data is then processed by rate matching logic
52
to compensate for the data expansion associated with encoding.
[0047] Bit interleaver logic 54 systematically reorders the bits in the
encoded data
to minimize the loss of consecutive data bits. The resultant data bits are
systematically mapped into corresponding symbols depending on the chosen
baseband modulation by mapping logic 56. Preferably, Quadrature Amplitude
Modulation (QAM) or Quadrature Phase Shift Key (QPSK) modulation is used. As
described in more detail below, the degree of modulation is chosen based on
the
CQI value for the particular mobile terminal. The symbols may be
systematically
reordered to further bolster the immunity of the transmitted signal to
periodic data
loss caused by frequency selective fading using symbol interleaver logic 58.
[0048] At this point, groups of bits have been mapped into symbols
representing
locations in an amplitude and phase constellation. When spatial diversity is
desired,
blocks of symbols are then processed by space-time block code (STC) encoder
logic
60, which modifies the symbols in a fashion making the transmitted signals
more
resistant to interference and more readily decoded at a mobile terminal 16.
The STC
encoder logic 60 will process the incoming symbols and provide "n" outputs
corresponding to the number of transmit antennas 28 for the base station 14.
The
control system 20 and/or baseband processor 22 as described above with
reference
to FIG. 5 will provide a mapping control signal to control STC encoding. At
this point,
assume the symbols for the "n" outputs are representative of the data to be
transmitted and capable of being recovered by the mobile terminal 16.
[0049] For the present example, assume the base station 14 has two antennas
28 (n=2) and the STC encoder logic 60 provides two output streams of symbols.
Accordingly, each of the symbol streams output by the SIC encoder logic 60 is
sent
to a corresponding IFFT processor 62, illustrated separately for ease of
understanding. Those skilled in the art will recognize that one or more
processors
may be used to provide such digital signal processing, alone or in combination
with
other processing described herein. The IFFT processors 62 will preferably
operate
on the respective symbols to provide an inverse Fourier Transform. The output
of the
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TUFT processors 62 provides symbols in the time domain. The time domain
symbols
are grouped into frames, which are associated with a prefix by prefix
insertion logic
64. Each of the resultant signals is up-converted in the digital domain to an
intermediate frequency and converted to an analog signal via the corresponding
digital up-conversion (DUG) and digital-to-analog (DIA) conversion circuitry
66. The
resultant (analog) signals are then simultaneously modulated at the desired RF
frequency, amplified, and transmitted via the RF circuitry 68 and antennas 28.
Notably, pilot signals known by the intended mobile terminal 16 are scattered
among
the sub-carriers. The mobile terminal 16, which is discussed in detail below,
will use
the pilot signals for channel estimation.
[0050] Reference is now made to FIG. 6 to illustrate reception of the
transmitted
signals by a mobile terminal 16, either directly from base station 14 or with
the
assistance of relay 15. Upon arrival of the transmitted signals at each of the
antennas 40 of the mobile terminal 16, the respective signals are demodulated
and
amplified by corresponding RF circuitry 70. For the sake of conciseness and
clarity,
only one of the two receive paths is described and illustrated in detail.
Analog-to-
digital (AID) converter and down-conversion circuitry 72 digitizes and
downconverts
the analog signal for digital processing. The resultant digitized signal may
be used by
automatic gain control circuitry (AGC) 74 to control the gain of the
amplifiers in the
RF circuitry 70 based on the received signal level.
[00511 Initially, the digitized signal is provided to synchronization logic
76, which
includes coarse synchronization logic 78, which buffers several OFDM symbols
and
calculates an auto-correlation between the two successive OFDM symbols. A
resultant time index corresponding to the maximum of the correlation result
determines a fine synchronization search window, which is used by fine
synchronization logic 80 to determine a precise framing starting position
based on
the headers. The output of the fine synchronization logic 80 facilitates frame
acquisition by frame alignment logic 84. Proper framing alignment is important
so
that subsequent PET processing provides an accurate conversion from the time
domain to the frequency domain. The fine synchronization algorithm is based on
the
correlation between the received pilot signals carried by the headers and a
local
copy of the known pilot data. Once frame alignment acquisition occurs, the
prefix of
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the OFDM symbol is removed with prefix removal logic 86 and resultant samples
are
sent to frequency offset correction logic 88, which compensates for the system
frequency offset caused by the unmatched local oscillators in the transmitter
and the
receiver. Preferably, the synchronization logic 76 includes frequency offset
and clock
estimation logic 82, which is based on the headers to help estimate such
effects on
the transmitted signal and provide those estimations to the correction logic
88 to
properly process OFDM symbols.
[0052] At this point, the OFDM symbols in the time domain are ready for
conversion to the frequency domain using FFT processing logic 90. The results
are
frequency domain symbols, which are sent to processing logic 92. The
processing
logic 92 extracts the scattered pilot signal using scattered pilot extraction
logic 94,
determines a channel estimate based on the- extracted pilot signal using
channel
estimation logic 96, and provides channel responses for all sub-carriers using
channel reconstruction logic 98. In order to determine a channel response for
each
of the sub-carriers, the pilot signal is essentially multiple pilot symbols
that are
scattered among the data symbols throughout the OFDM sub-carriers in a known
pattern in both time and frequency. Continuing with FIG. 6, the processing
logic
compares the received pilot symbols with the pilot symbols that are expected
in
certain sub-carriers at certain times to determine a channel response for the
sub-
carriers in which pilot symbols were transmitted. The results are interpolated
to
estimate a channel response for most, if not all, of the remaining sub-
carriers for
which pilot symbols were not provided. The actual and interpolated channel
responses are used to estimate an overall channel response, which includes the
channel responses for most, if not all, of the sub-carriers in the OFDM
channel.
[0053] The frequency domain symbols and channel reconstruction information,
which are derived from the channel responses for each receive path are
provided to
an STC decoder 100, which provides STC decoding on both received paths to
recover the transmitted symbols. The channel reconstruction information
provides
equalization information to STC decoder 100 sufficient to remove the effects
of the
transmission channel when processing the respective frequency domain symbols.
The -relay station could act as another base station or as a terminal in the
context of
this invention.
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(0054] The recovered symbols are placed back in order using symbol de-
interleaver
logic 102, which corresponds to the symbol interleaver logic 58 of the
transmitter. The de-
interleaved symbols are then demodulated or de-mapped to a corresponding
bitstream using
de-mapping logic 104. The bits are then de-interleaved using bit de-
interleaver logic 106,
which corresponds to the bit interleaver logic 54 of the transmitter
architecture. The de-
interleaved bits are then processed by rate de-matching logic 108 and
presented to channel
decoder logic 110 to recover the initially scrambled data and the CRC
checksum.
Accordingly, CRC logic 112 removes the CRC checksum, checks the scrambled data
in
traditional fashion, and provides it to the de-scrambling logic 114 for de-
scrambling using the
known base station de-scrambling code tdecover the originally transmitted data
116.
[0055] In parallel to recovering the data 116, a CQI value 120, or at least
information
sufficient to determine a CQI value at the base station 14, is determined and
transmitted to the base station 14. As noted above, the CQI value may be a
function
of the signal-to-interference ratio (SIR) 122, as well as the degree to which
the channel
response varies across the various sub-carriers in the OFDM frequency band as
determined
by channel variation analysis 118. For this embodiment, the channel gain for
each sub-
carrier in the OFDM frequency band being used to transmit information is
compared relative
to one another to determine the degree to which the channel gain varies across
the OFDM
frequency band. Although numerous techniques are available to measure the
degree of
variation, one technique is to calculate the standard deviation of the channel
gain for each
sub-carrier throughout the OFDM frequency band being used to transmit data.
[0056] In some embodiments, Single Carrier Frequency Division Multiple
Access
(SC-FDMA) is used for uplink transmissions from mobile station 16. SC-FDMA is
a
modulation and multiple access scheme introduced for the uplink of 3GPP LTE
broadband wireless fourth generation (4G) air interface standards, and the
like.
Referring to FIGS. 7A and 7B, an example SC-FDMA transmitter and receiver for
single-in single-out (SISO) configuration is illustrated provided in
accordance with
one embodiment of the present application. In SISO, mobile stations transmit
on one
antenna and base stations and/or relay stations receive on one antenna. FIGS.
7A
and 7B illustrate the basic signal processing steps needed at the transmitter
and
receiver for the LTE SC-FDMA uplink. There are several similarities in the
overall
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transceiver processing of SC-FDMA and OFDMA. Those common aspects between
OFDMA and SC-FDMA are depicted generally as "OFDMA transmit circuitry" and
"OFDMA receive circuitry", as they will be obvious to a person having ordinary
skill in
the art in view of the present specification. SC-FDMA is distinctly different
from
OFDMA because of the DFT pre-coding of the modulated symbols, and the
corresponding 1DFT of the demodulated symbols. Because of this pre-coding, the
SC-FDMA subcarriers are not independently modulated as in the case of the
OFDMA subcarriers. As a result, the peak-to-average power ratio (PAPR) of the
SC-
FDMA signal is lower than the PAPR of the OFDMA signal. Lower PAPR greatly
benefits the mobile terminal in terms of transmit power efficiency.
[0057] FIGS. 1 to 7 provide one specific example of a communication system
that
could be used to implement embodiments of the application. It is to be
understood
that embodiments can be implemented with communications systems having
architectures that are different than the specific example, but that operate
in a
manner consistent with the implementation of the embodiments as described
herein.
[0058] In accordance with embodiments of the present application, relay
station
15 is capable of assisting DL retransmissions (e.g. DL HARQ retransmissions)
while
operating in transparent mode. More specifically, base station 14 is
configured to
signal retransmission information to relay station 15 over a control link
(herein
referred to as a "network-to-relay link"), which may be either in-band or out-
of-band,
prior to sending a retransmission so that relay station 15 may send the
retransmission concurrently with base station 14 (e.g. within the same OFDMA
subframe).
[0059] FIG. 8 shows a flow diagram illustrating the steps for a DL
retransmission
assisted by a transparent relay according to embodiments of the present
application.
As shown, at step 802 a base station (BS) receives a request for a
retransmission
(e.g. a HARQ NACK) from a mobile station (MS). At step 804, the BS identifies
the
MS as being at or near the cell edge and potentially requiring the assistance
of a
transparent relay station (RS) for the retransmission. At step 806, the BS
schedules
resources for the retransmission, and at step 808 the BS signals the
scheduling
information for the retransmission to the RS via the network-to-relay link. As
explained in more detail below, in some embodiments resources for the
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retransmission may be scheduled one subframe ahead of the retransmission. It
is
noted that with the assistance of the RS for retransmission, the requirement
on the
scheduler to capture instantaneous channel variations is eased. At step 810,
BS
sends the scheduled retransmission to the MS. At the RS, at step 812 the RS
obtains the retransmission information, and at step 814 the RS sends the
scheduled
retransmission to the MS concurrently with, and on the same frequency band as,
the
BS.
[00601 FIG. 9 illustrates a DL retransmission scheme where the network-to-
relay
link is in-band; that is, the network-to-relay link occupies the same
frequency band
F1 as the network-to-mobile access link. As shown, in subframe (n) NR receives
retransmission information from the base station (eNB) on frequency band F1,
and in
subframe (n+1) the relay station (NR) sends the retransmission data to UE
concurrently with base station 14, with both retransmissions occurring on the
same
frequency band F1. The in-band network-to-relay link could use some reserved
resources in PDSCH or PDCCH. A new control channel format may defined, for
example, a PDCCH for a group of cell edge mobile stations may be defined.
[0061] FIGS. 10A and 10B illustrate DL HARQ retransmission schemes where
the network-to-relay link is out-of-band; that is, the network-to-relay link
and the
network-to-mobile access link occupy different frequency bands F2 and F1,
respectively. In some embodiments, frequency band F2 assigned for the network-
to-
relay link may be a dedicated frequency band. For example, in some
embodiments,
F2 may be 'new' spectrum such as the 2.5GHz band. As shown, NR receives
signals
from eNB and transmits signals to UE on different frequency bands. Two options
are
presented. In a first option illustrated in FIG. 10A, eNB transmits the HARQ
related
PDCCH in subframe (n), and NR transmits the retransmission data to the UE in
subframe (n+1). In a second option illustrated in FIG. 10B, eNB transmits the
HARQ
related PDCCH in subframe (n), and NR transmits the retransmission data to the
UE
in subframe (n). In embodiments adopting the second option, a different
control
channel format may be defined for NR oriented PDCCH to provide sufficient
guard
time to allow NR to decode its PDCCH before the corresponding PDSCH is to be
sent.
(0062] Advantageously, the schemes herein described enable relay stations
15
17
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operating in transparent mode to send DL retransmissions to the mobile
stations 16
concurrently with base station 14, thus increasing the robustness of the
transparent
relay system and enhancing its performance.
[00631 Other modifications will be apparent to those skilled in the art
and,
therefore, the invention is defined in the claims.
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