Note: Descriptions are shown in the official language in which they were submitted.
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SYSTEMS AND METHODS OF ENCODING USING A REDUCED CODEBOOK
WITH ADAPTIVE RESETTING
FIELD
This application relates to systems and methods of encoding using a
reduced codebook with adaptive resetting, for example for use in wireless
communication systems.
BACKGROUND
Draft IEEE 802.16m System Description Document, IEEE 802.16m-
08/003r1, dated April 15th 2008, it is stated that:
This [802. 16m] standard amends the IEEE 802.16 WirelessMAN-
OFDMA specification to provide an advanced air interface for operation in
licensed
bands. It meets the cellular layer requirements of IMT-Advanced next
generation
mobile networks. This amendment provides continuing support for legacy
WirelessMAN-QFDMA equipment.
And the standard will address the following purpose:
i. The purpose of this standard is to provide performance
improvements necessary to support future advanced services and applications,
such as those described by the ITU in Report ITU-R M.2072.
Providing feedback in a multiple-input multiple-output (MIMO)
communication environment has proven to significantly benefit performance. In
early systems that employed feedback, weighting factors were created at the
MIMO receiver based on channel conditions. A codeword was fed back from the
MIMO receiver to the transmitter, which would apply the weighting factors to
the
respective signals to be transmitted from the different MIMO transmitter
antennas.
The weighting factors effectively pre-distorted the signals to be transmitted
to
reverse the effects of the communication channel. Accordingly, the signals
received at the MIMO receiver's antennas approximated those that would have
been received without weighting through a clear channel. Unfortunately,
channel
conditions frequently if not continuously change, and the weighting factors to
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represent channel conditions are data-intensive. In light of the limited
bandwidth
available for feedback, providing weighting factors for changing channel
conditions
became unfeasible.
In an effort to reduce the bandwidth required to provide feedback for
a MIMO channel, designers developed codebooks, which are maintained at the
MIMO transmitter and MIMO receiver. The codebooks include codewords, which
are pointers to predefined weighting factors (which could include amplitude
and
phase information). These predefined weighting factors are configured to cover
the range of possible channel conditions through a fixed number of discrete
weighting factors. Each of the codewords in a codebook is associated with a
codeword index. In operation, the MIMO receiver will systematically determine
the
channel conditions and select the most appropriate codeword in light of the
channel conditions. Instead of sending the codeword, which includes the
weighting factors, the MIMO receiver will send the codeword index to the MIMO
transmitter over an appropriate feedback channel. The MIMO transmitter will
receive the codeword index, obtain the weighting factors of the corresponding
codeword, and apply those weighting factors to the signals to be transmitted
from
the different transmit antennas.
The number of codewords in the codebook impacts the bandwidth
required for feedback and the performance enhancement associated with using
feedback. Unfortunately, as the number of codewords increases, the bandwidth
required to feed back the codeword index to the MIMO transmitter from the MIMO
receiver increases. Incorporating the use of codebooks and providing the
codeword index instead of the entire weighting factors as feedback has
provided
performance enhancement over systems providing the entire weighting factors as
feedback.
FIGs. 7-13 of the present application correspond to Figures 1-7 of
IEEE 802.16m-08/003r1.
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SUMMARY
Aspects and features of the present application will become
apparent to those ordinarily skilled in the art upon review of the following
description of specific embodiments of a disclosure in conjunction with the
accompanying drawing figures and appendices.
According to one aspect of the present invention, there is provided a
method comprising: in respect of a plurality of codewords of a full codebook
and a
plurality of codeword subsets, each codeword subset containing a respective
plurality of the codewords of the full codebook, repeatedly: determining a
best
codeword from the full codebook; adaptively selecting a codeword subset of the
plurality of codeword subsets to be a current codeword subset by selecting,
based
on a threshold criterion, between a) the most recently selected current
codeword
subset and b) another codeword subset that contains the best codeword from the
full codebook; if adaptively selecting results in a change in the current
codeword
subset, transmitting an indication of the current codeword subset;
transmitting an
index of a codeword within the current codeword subset of the plurality of
codeword subsets.
According to another aspect of the present invention, there is
provided an apparatus comprising: receive circuitry associated with a
plurality of
antennas; transmit circuitry associated with the plurality antennas; and a
memory
containing codeword subsets; a subset selector based on threshold criterion
that
causes the apparatus to perform a method comprising: in respect of a plurality
of
codewords of a full codebook and a plurality of codeword subsets, each
codeword
subset containing a respective plurality of the codewords of the full
codebook,
repeatedly: determining a best codeword from the full codebook; adaptively
selecting a codeword subset of the plurality of codeword subsets to be a
current
codeword subset by selecting, based on a threshold criterion, between a) the
most
recently selected current codeword subset and b) another codeword subset that
contains the best codeword from the full codebook; if adaptively selecting
results
in a change in the current codeword subset, transmitting an indication of the
current codeword subset; transmitting an index of a codeword within the
current
codeword subset of the plurality of codeword subsets.
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BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the present application will now be described, by
way of example only, with reference to the accompanying drawing figures
wherein:
FIG. 1 is a block diagram of a cellular communication system;
FIG. 2 is a block diagram of an example base station that might be
used to implement some embodiments of the present application;
FIG. 3 is a block diagram of an example wireless terminal that might
be used to implement some embodiments of the present application;
FIG. 4 is a block diagram of an example relay station that might be
used to implement some embodiments of the present application;
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;
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 present application;
FIG. 7 is Figure 1 of IEEE 802. 16m-081003r1, an Example of overall
network architecture;
FIG. 8 is Figure 2 of IEEE 802. 16m-08/003r1, a Relay Station in
overall network architecture;
FIG. 9 is Figure 3 of IEEE 802.1 6m-08/003r1, a System Reference
Model;
FIG. 10 is Figure 4 of IEEE 802.16m-08/003r1, The IEEE 802.16m
Protocol Structure;
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FIG. 11 is Figure 5 of IEEE 802. 16m-08/003r1, The IEEE 802. 16m
MS/ES Data Plane Processing flow;
FIG. 12 is Figure 6 of IEEE 802. 16m-08/003r1, The IEEE 802. 16m
MS/BS Control Plane Processing Flow;
5 FIG. 13 is Figure 7 of IEEE 802. 16m-08/003r1, Generic protocol
architecture to support multicarrier system.
FIG. 14 is a block diagram of a MIMO system;
FIG. 15 is a table showing a set of codewords and corresponding
codeword indices;
FIG. 16 is a table showing codewords and associated codeword
subsets;
FIG. 17 is a table showing a codeword subset;
FIG. 18 is a flowchart of a method of transmitting codeword indices;
FIGS. 19 and 20 are flowcharts of two methods of transmitting
codeword indices;
FIG. 21 is a flowchart of a method of receiving codeword indices;
and
FIG. 22 is a block diagram of a receiver.
DETAILED DESCRIPTION
Referring to the drawings, 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 or zones (not shown). In
general, each base station 14 facilitates communications using OFDM with
mobile
and/or wireless terminals 16, which are within the cell 12 associated with the
corresponding base station 14. The movement of the mobile terminals 16 in
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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. In
some
configurations, relay stations 15 may assist in communications between base
stations 14 and wireless terminals 16. Wireless terminals 16 can be handed off
from any cell 12. sector 13, zone (not shown), base station 14 or relay 15 to
an
other cell 12, sector 13, zone (not shown), base station 14 or relay 15. In
some
configurations, base stations 14 communicate with each and with another
network
(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.
With reference to FIG. 2, an example of a base station 14 is
illustrated. The base station 14 generally includes a control system 20, a
baseband processor 22, transmit circuitry 24, receive circuitry 26, multiple
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.
Down conversion and digitization circuitry (not shown) will then down convert
the
filtered, received signal to an intermediate or baseband frequency signal,
which is
then digitized into one or more digital streams.
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.
On the transmit side, the baseband processor 22 receives digitized
data, which may 20 represent voice, data, or control information, from the
network
interface 30 under the control of control system 20, and encodes the data for
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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.
With reference to FIG. 3, an example of a mobile terminal 16 is
illustrated. Similarly to the base station 14, the mobile terminal 16 will
include a
control system 32, a baseband processor 34, transmit circuitry 36, receive
circuitry
38, multiple antennas 40, and user interface circuitry 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. Down conversion and digitization circuitry (not shown) will then
down
convert the filtered, received signal to an intermediate or baseband frequency
signal, which is then digitized into one or more digital streams.
The 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).
For transmission, the 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
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transmission between the mobile terminal and the base station, either directly
or
via the relay station.
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.
OFDM modulation utilizes the performance of an Inverse Fast
Fourier Transform (IFFT) on the information to be transmitted. For
demodulation,
the performance of a Fast Fourier Transform (FFT) on the received signal
recovers the transmitted information. In practice, the IFFT and FFT are
provided
by digital signal processing carrying out an Inverse Discrete Fourier
Transform
(IFFT) 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.
In operation, 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 >=I), and each mobile
terminal 16 is equipped with "in" receive antennas 40 (m>=1). Notably, the
respective antennas can be used for reception and transmission using
appropriate
duplexers or switches and are so labeled only for clarity.
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.
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With reference to FIG. 4, an example of a relay station 15 is
illustrated. Similarly to the base station 14, and the mobile terminal 16, the
relay
station 15 will include a control system 132, a baseband processor 134,
transmit
circuitry 136, receive circuitry 138, multiple antennas 130, and relay
circuitry 142.
The relay circuitry 142 enables the relay 14 to assist in communications
between
a base station 15 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.
The baseband processor 134 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 134 is generally implemented in one or more
digital signal processors (DSPs) and application specific integrated circuits
(ASICs).
For transmission, the baseband processor 134 receives digitized
data, which may represent voice, video, data, or control information, from the
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.
With reference to FIG. 5, a logical OFDM transmission architecture
will be described. Initially, the base station controller 10 will send data to
be
transmitted to various mobile terminals 16 to the base station 14, either
directly or
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with the assistance of a relay station 15. The base station 14 may use the
channel
quality indicators (CQls) associated with the mobile terminals to schedule the
data
for transmission as well as select appropriate coding and modulation for
transmitting the scheduled data. The CQIs may be directly from the mobile
5 terminals 16 or determined at the base station 14 based on information
provided
by the mobile terminals 16. In either case, the CQI foreach mobile terminal 16
is a
function of the degree to which the channel amplitude (or response) varies
across
the OFDM frequency band.
Scheduled data 44, which is a stream of bits, is scrambled in a
10 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. Again, the channel coding for a particular mobile terminal 16 is
based
on the CQI. 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.
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.
The degree of modulation is preferably chosen based on the CQI 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.
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
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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 respect 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.
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 STC 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 IFFT 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 (DUC) and digital-to-
analog (D/A) 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.
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.
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Analog-to-digital converter (ADC) 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.
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 FFT 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 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.
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-
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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.
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 the STC decoder 100
sufficient to
remove the effects of the transmission channel when processing the respective
frequency domain symbols.
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 dc-mapping logic 104. The bits are then dc-
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 dc-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
descrambling
using the known base station de-scrambling code to recover the originally
transmitted data 116.
In parallel to recovering the data 116, a CQI or at least information
sufficient to create a CQI at the base station 14, is determined and
transmitted to
the base station 14. As noted above, the CQI may be a function of the carrier-
to-
interference ratio (CR). as well as the degree to which the channel response
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varies across the various sub-carriers in the OFDM frequency band. 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. In some embodiments, a relay station may operate in a
time
division manner using only one radio, or alternatively include multiple
radios.
FIGs. 1 to 6 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 of the application 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.
FIGs. 7-13 of the present application correspond to Figures 1-7 of
IEEE 802.16m-08/003r1.
The description of these figures in IEEE 802. 16m-08/003r1 is
incorporated herein by reference.
The embodiments set forth below represent the necessary
information to enable those skilled in the art to practice the invention and
illustrate
the best mode of practicing the invention. Upon reading the following
description
in light of the accompanying drawing figures, those skilled in the art will
understand the concepts of the invention and will recognize applications of
these
concepts not particularly addressed herein. It should be understood that these
concepts and applications fall within the scope of the disclosure and the
accompanying claims.
Embodiments of the present invention may in some cases, be used
to improve performance of multiple-input multiple-output (MIMO) communication
systems. As shown in FIG. 14, a MIMO transmitter 202 has multiple transmit
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antennas 204 from which MIMO signals are transmitted over a MIMO
communication channel to a MIMO receiver 206. The MIMO signals may be
space-time coded (STC) signals, which are defined to be any spatially diverse
signals or spatially multiplexed signals. As such, the signals transmitted
from
5 different transmit antennas 204 may be the same or different signals. The
MIMO
signals are received at the MIMO receiver by multiple receive antennas 208.
In closed loop MIMO (CL-MIMO), the channel matrix is determined
at the receiver such as a mobile station, and this is quantized to a codeword
within
a predefined codebook, and a codeword index is fed back to the transmitter
such
10 as a base station. Such a codebook C, such as that illustrated in FIG. 15,
is
provided in the MIMO transmitter 202 as well as the MIMO receiver 206. The
codebook C will include M codewords c;, where i is a unique codeword index for
each codeword c;. An index is simply any identifier of the codeword. Rather
than transmitting a codeword of interest per se, the index of the codeword of
15 interest can be transmitted, and assuming both ends of the communication
are
aware of the mapping between codewords and indexes, the codeword of interest
can be recovered.
An approach is provided that can be used to reduce the amount of
signaling required to convey codeword indices from the receiver to the
transmitter.
The approach is based on the expectation that more often than not, each time
the
channel matrix is determined and a new codeword from the defined codeword
selected, the channel matrix, and the associated codeword will be relatively
close
to the previous channel matrix and associated codeword. However, it is to be
clearly understood that the approaches described herein can be used to convey
other types of information.
In the detailed examples that follow, it is assumed that the indices
are simply integers, and that binary representations of the indices are
transmitted,
but other forms for the indices can be employed. In this example, there are 64
codewords c; (M=64), and the codeword indices are simply integers i that range
from zero to M-1.
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In some embodiments, each codeword defines weighting factors to
apply to the MIMO signals, and may correspond to channel matrices or vectors
to
apply to the MIMO signals prior to transmission from the respective antennas
204
of the MIMO transmitter 202. In some embodiments, each codeword defined pre-
coding to be applied in the MIMO transmitter 202.
Referring now to Figure 16, a method for execution, for example, by
a MIMO receiver, will be described. The method is performed based on a
plurality
of codewords of a full codebook and a plurality of codeword subsets, each
codeword subset containing a respective plurality of the codewords of the full
codebook. The steps 500,504,506 are performed repeatedly for each of a
plurality
of periods, which may, for example each relate to a respective MIMO channel
feedback instance.
A best codeword from the full codebook is determined at step 500.
Next, at step 502, a codeword subset is adaptively selected of the plurality
of
codeword subsets to be a current codeword subset by selecting, based on a
threshold criterion, between a) the most recently selected current codeword
subset and b) another codeword subset that contains the best codeword from the
full codebook. At step 504, if adaptively selecting results in a change in the
current codeword subset, an indication of the current codeword subset is
transmitted. At step 506, an index of a codeword within the current codeword
subset of the plurality of codeword subsets is transmitted.
In some embodiments, the adaptively selecting involves determining
if the best codeword from the full codebook also belongs to the most recently
selected codeword subset, and if so, selecting the most recently selected
current
codeword subset to be the current codeword subset.
In some embodiments, adaptively selecting further involves,
(optionally only if the best codeword from the full codebook does not also
belong
to the most recently selected codeword subset):
selecting a best codeword from the most recently selected codeword
subset;
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if the best codeword from the most recently selected codeword
subset and the best codeword from the full codebook together satisfy the
threshold criterion, selecting the current codeword subset to be the most
recently
selected current codeword subset and otherwise selecting the current codeword
subset to be the codeword subset that contains the best codeword from the full
codebook.
In some embodiments, the best codeword from the most recently
selected current codeword subset and the best codeword from the full codebook
together satisfy the threshold criterion when a correlation between the best
codeword from the most recently selected current codeword subset and the best
codeword from the full codebook is higher than a defined threshold.
In some embodiments, each codeword subset has a corresponding
unique identifier, and transmitting an indication of the current codeword
subset
comprises transmitting the unique identifier associated with the current
codeword
subset. The embodiments described below that do not employ header codewords
are examples of this.
In some embodiments, for each codeword subset, there is an
associated header codeword belonging to the full codebook or a coarse subset
of
the full codeword. Transmitting an indication of the current codeword subset
comprises transmitting an index of the associated header codeword of the
current
codeword subset within the full codebook or the coarse subset. The embodiments
described below that employ header codewords are examples of this.
In some embodiments, a respective codeword subset Si is created
for each codeword c; of the full codebook C, as illustrated in FIG. 17. The
codewords in the codeword subset Si will differ from one codeword subset Si to
another. In another embodiment, rather than creating a codeword subset Si for
each and every codeword c; of the full codebook C, a respective codeword
subset
Si is created for each codeword c; of a coarse subset of the full codebook. In
either case, a codeword for which there is an associated codeword subset is
referred to herein as a header codeword. The coarse subset contains some but
not all of the codewords of the full codebook C. In a specific example, the
coarse
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subset might contain N of the M codewords of the full codebook. N might, for
example be 1/8 x M.
The codeword subset Si for codeword c; contains L codewords c,,
which are selected from all the codewords in the codebook C. In some
embodiments, the codewords ci in a codeword subset Si are L codewords from the
full codebook that are best correlated with the codeword c; as defined by a
correlation criterion. The codewords in each codeword subset are indexed using
respective subset indices. Every codeword has an index into the full codebook
C
(or coarse subset) that uniquely identifies the codeword among all other
codebooks of the full codebook C (or coarse subset), and a subset index for a
codeword subset to which the codeword belongs that uniquely identifies the
codeword among other codewords of the codeword subset. Each index of a
codeword of the full codebook C (or coarse subset) in effect points to the
respective codeword subset.
In a first example, consider a full codebook with sixteen codewords,
a coarse codebook containing four codewords from the full codebook, with each
codeword of the course codebook pointing to a respective codeword subset
containing four codewords of the full codebook. The full codebook contains 16
codewords CAO, CA1, CA2, CA3, CB1, CB2, CB3, CCO, CC1, CC2, CC3, CDO,
CD1, CD2, CD3. The coarse subset includes four codewords from the full
codebook, namely CA1, CB1, CC1 and CD1, and the associated codeword
subsets might be defined as:
codeword subset for CA1: CAO to CA3
codeword subset for CB1: CBO to CB3
codeword subset for CC1: CCO toCC3
codeword subset for CD1: CDO to CD3.
When an index of a codeword in the coarse subset is to be transmitted, two
bits
can be used to select between CA1,CB1,CC1,CD1. When an index of a
codeword in a codebook subset is to be transmitted, two bits can be used to
select
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a codeword from the current codeword subset, namely the codeword subset
associated with the coarse codeword for which an index was most recently
transmitted. For example, if the most recent coarse codeword was CAI, then the
two bits can be used to select between CAO,CA1,CA2,CA3.
A second example will now be described with reference to Figure 18.
A codeword subset S7, which corresponds to the codeword c7, is defined to
include L=8 codewords cj. The L codewords have respective subset codeword
indices 0,...,7. The codewords c, in the codeword subset S7 are the eight
codewords that are best correlated with the codeword c7 as defined by a
correlation criterion. In this example, the codeword subset S7 associated with
codeword C7 is defined to include codewords C3, C13, C16, C25, C37, C41, C49,
and C60=
In some embodiments, such as is the case in the first example, each
codeword of the full codebook (or course subset) for which an associated
codeword subset is defined is included in the associated codeword subset. In
the
first example, CAI has an associated codeword subset containing codewords
CAO,CA1,CA2,CA3, and CAI is included in the associated codeword subset.
In some embodiments, such as in the case of the second example,
each codeword of the full codebook (or couse subset) for which an associated
codeword subset is defined is not necessarily included in the associated
codeword
subset. Specifically, c7 has an associated codeword subset c3, c13, c16, c25,
c37,
c41, c49, and c60. and c7 is not included in the associated codeword subset.
As indicated above, in some embodiments, the codeword subsets Si
are created by identifying the L codewords cc that are best correlated with
the
codeword c; as defined by some correlation criterion. Those skilled in the art
will
recognize various techniques for determining codewords c;, cj, that have the
highest correlations. For example, identifying the codewords c;, q having the
highest correlations may be determined using the normalized interproduct
criterion
defined as:
ci cc
c j) i)c IIC;II*IIc;II
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In some embodiments, where a respective codebook subset is
defined for every codeword of the full codebook, the subset index, which is
used
to identify each of the codewords cj of the codeword subset Si, will require
log2(L)
bits, whereas the full codebook index requires log2(M) bits. There is a
savings in
5 terms of the amount of bits required to feedback the codeword index that is
a
function of how much smaller L is than M.
In one embodiment, where a respective codebook subset is defined
for every codeword of a coarse subset having N codewords out of the M
codewords of the full codebook, the subset index, which is used to identify
each of
10 the codewords cj of the codeword subset Si, will require 1og2(L) bits,
whereas the
coarse subset index requires log2(N) bits.
In the example described above, the number of codewords in a
codeword subset (L) is eight times less than the number of codewords in the
full
codebook C (M). The codebook C, including the codeword subsets, is provided in
15 the MIMO transmitter 202 as well as in the MIMO receiver 206. The codebook
C
will remain substantially fixed, and if there is a change to a codebook C in
the
MIMO transmitter 202, there must be corresponding changes in the MIMO
receiver 206 to ensure that the codebooks C remain the same.
In some embodiments, the MIMO receiver 206 will initially identify
20 the channel conditions of the MIMO channel and search the full codebook C
for a
first codeword based on the channel conditions of the MIMO channel that is the
best codeword of the full codebook C. The MIMO receiver 206 will then transmit
the codeword index of the first codeword in the full codebook back to the MIMO
transmitter 202 as feedback. The MIMO transmitter 202 will use the codeword
index to identify the first codeword to employ when weighting and/or pre-
coding
the MIMO signals to be transmitted to the MIMO receiver 206. For subsequent
feedback instances, the MIMO receiver will either transmit an index
corresponding
to a selected codeword of the codeword subset corresponding to the codeword of
the full codebook (or coarse codebook) for which an index was most recently
transmitted, or an index corresponding to a newly selected codeword determined
from a search of the entire codebook C (or coarse codebook). Initially, the
codeword subset corresponding to the codeword of the full codebook (or coarse
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codebook) for which an index was most recently will be the codeword subset of
the first codeword. However, from time to time, as determined by the
adaptation
performed by the MIMO receiver, an index of a newly selected codeword
determined from a search of the entire codebook C (or coarse codebook) is
transmitted. The codeword subset of the most recently selected codeword from
the full codebook C (or coarse codebook) is used when transmitting an index of
a
subset codeword.
The following approach is taken to determine whether to send an
index corresponding to a selected codeword of the codeword subset or to send
an
index of a codeword from the full codebook C (or coarse subset). Note that
this is
a particular example of step 502 of Figure 16 in the sense that if an index of
a
codeword in the current codeword subset is transmitted, then the most recently
selected current codeword subset is (again) selected as the current codeword
subset. If instead an index of a codeword in the full codebook (or coarse
subset),
then the current codeword subset is selected to be the codeword subset of
another codeword subset that contains the best codeword from the full
codebook.
Update Channel Conditions
To begin, The MIMO receiver 206 will again identify channel
conditions for the MIMO channel.
Identify codeword from full codebook (or coarse subset)
The full codebook C is searched (or the coarse codebook), and from
the M codewords of the codebook C (or the N codewords of the coarse subset),
the MIMO receiver 206 will select the best codeword, hereinafter the "second
codeword". The index of the second codeword in the full codebook (or coarse
subset) is available for transmission back to the MIMO transmitter 202 as
feedback. However, as described below, it may or may not actually be
transmitted.
Identify codeword from codeword subset
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The MIMO receiver 206 conducts a search for the best codeword
that belongs to the codeword subset associated with the codeword of the full
codebook (or coarse subset) for which an index was most recently transmitted.
From the L codewords of the codeword subset, the MIMO receiver
206 will select the best codeword, hereinafter the "third codeword". The index
of
the third codeword in the codeword subset is available for transmission back
to
the MIMO transmitter 202 as feedback. However, as described below, this index
may or may not actually be transmitted.
In the event the index of the codeword in the codeword subset is
transmitted, the MIMO transmitter 202 will use the index to identify the
second
codeword from the codeword subset associated with the most recently
transmitted
index of a codeword in the full codebook (or coarse subset). Once the third
codeword is obtained, the corresponding weighting factors/pre-coding are
provided to the MIMO signals being transmitted to the MIMO receiver 206 from
the
MIMO transmitter 202.
Apply Threshold Criterion
Next, a threshold criterion is applied to the second codeword and the
third codeword to select either the second codeword or the third codeword.
This
may, for example involve, applying a threshold criterion to the second
codeword
and the third codeword. For example, if they are sufficiently similar, as
defined by
an appropriate threshold criterion such as a correlation criterion, then the
third
codeword is selected and the index of the third codeword in the current
codeword
subset (namely the codeword subset associated with the codeword of the
full/coarse codebook for which an index was most recently transmitted) is
transmitted, and otherwise the second codeword is selected and the index of
the
second codeword in the full/coarse codebook is transmitted, as well as an
indication that the index being transmitted is from the full codebook (or
coarse
subset), and that therefor the codeword from the full codebook (or coarse
subset)
for which an index was most recently transmitted should be updated to be the
second codeword, and'accordingly the current codeword subset should be
switched to be the subset that corresponds with second codeword.
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In a specific example, a correlation metric is used that is the same
as that used to select the codeword subsets in the first place. In the
specific
example given above, the correlation metric was the normalized interproduct
criterion defined as:
c; cj
(c`' c' ) N Ci 11 * 11 ci
If the normalized interproduct of the second codeword and the third codeword
exceeds a threshold, then the threshold criterion is satisfied, and the index
of the
third codeword in the codeword subset is transmitted, and otherwise the index
of
the second codeword in the full codebook (or coarse subset) is transmitted.
Alternatively, if they are sufficiently dis-similar, as defined by an
appropriate criterion such as a correlation criterion, then the second
codeword is
selected and the index of the second codeword in the full/coarse codebook is
transmitted, and otherwise the third codeword is selected and the index of the
third codeword in the current codeword subset (namely the codeword subset
associated with the codeword of the full/coarse codebook for which an index
was
most recently transmitted) is transmitted. If the index of the second codeword
is
transmitted, an indication that the index being transmitted is from the full
codebook
(or coarse codebook) is also sent, this also indicating that the codeword from
the
full/coarse codebook for which an index was most recently transmitted should
be
updated to be the second codeword, and accordingly the current codeword subset
should be reset to be the subset that corresponds with second codeword.
In a specific example, a correlation metric is used that is the same
as that used to select the codeword subsets in the first place. In the
specific
example given above, the correlation metric was the normalized interproduct
criterion defined as: C, Ci
CiIC=/ -
IlCill*IIc; II
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If the normalized interproduct of the second codeword and the third codeword
is
below a threshold, then the threshold criterion is satisfied, and the index of
the
second codeword in the full codebook (or coarse subset) is transmitted, and
otherwise the index of the third codeword in the current codeword subset is
transmitted.
In another embodiment that results in the same index being
transmitted, but which results in a slightly reduced searching some of the
time, the
second codeword (in the full or coarse codebook) is computed, and if that
codeword belongs to the current codeword subset, then by definition, the same
codeword would be selected if a search were to be conducted of the current
codeword subset, and as such, it is not necessary to search the current
codeword
subset. Rather, the index of the second codeword within the current codeword
subset is looked up and transmitted as feedback. Typically, the index of the
second codeword in the current codeword subset will be different from the
index of
the second codeword in the full or coarse codebook. If the second codeword
does
not belong to the current codeword subset S;, then the method continues as in
the
previously described embodiment with the determination of a codeword in the
codeword subset, and application of the threshold criterion to select between
the
second codeword and the third codeword.
With reference to FIG. 19, a flow diagram illustrating a method
provided by an embodiment of the invention. This may, for example, be
implemented in a MIMO receiver, for example MIMO receiver 206, to feedback
information to a MIMO receiver. More generally, these methods may be employed
whenever the identity of a codeword is to be communicated through the use of
indexes. The flowchart and description assumes that each codeword of the full
codebook has an associated codeword subset. However, this method can also be
applied with a coarse codeword codebook. Operation begins where the selection
of a first codeword from a full codebook search (step 252). This may, for
example,
be based on channel estimates, but more generally, it can be selected based on
any appropriate selection criteria. The codeword index of the first codeword
is
then transmitted (step 254). The method continues with the selection of a
second
codeword from a full codebook search (step 256). Next, a search of the current
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codeword subset is made for a third codeword (step 262). The same selection
criteria are applied in searching for the third codeword as was applied when
searching for the second codeword. Next, adaptively determine whether to
transmit an index of the second codeword in the full codebook, or to transmit
an
5 index of the third codeword in the current codeword subset (step 264).
Following
this, depending on the determination, either an index of the third codeword in
the
current codeword subset is transmitted (step 266) or an index of the second
codeword in the full codebook is transmitted (step 268). In a specific
example,
step 264 involves comparing the second codeword and the third codeword, and if
10 they are sufficiently similar then the index of the third codeword in the
current
codeword subset is transmitted (step 266) and otherwise the index of the
second
codeword in the full codebook is transmitted (step 268). Further, the current
codeword subset will not change until a new full codebook index is fed
transmitted. When a full codebook index is transmitted (and becomes the most
15 recently transmitted index of a codeword in the full codebook), the current
codebook subset is updated accordingly (step 270). Following any one of method
steps 466,270, the method continues at step 256.
With reference to FIG. 20, a flow diagram illustrating a method
provided by an embodiment of the invention. This may, for example, be
20 implemented in a MIMO receiver, for example MIMO receiver 206, to feedback
information to a MIMO receiver. More generally, these methods may be
employed whenever the identity of a codeword is to be communicated through the
use of indexes. The flowchart and description assumes that each codeword of
the
full codebook has an associated codeword subset. However, this method can
25 also be applied with a coarse codeword codebook. Operation begins where the
selection of a first codeword from a full codebook search (step 302). This
may, for
example, be based on channel estimates, but more generally, it can be selected
based on any appropriate selection criteria. The codeword index of the first
codeword is then transmitted (step 304). The method continues with the
selection of a second codeword from a full codebook search (step 306). Again,
this may, for example, be based on channel estimates, but more generally, it
can
be selected based on any appropriate selection criteria. If the second
codeword
belongs to the current codeword subset (namely the codeword subset associated
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with the codeword of the full codebook for which an index was most recently
transmitted) (yes path step 308), then the index of the second codeword in the
current codeword subset is determined and transmitted (step 310). If the
second
codeword does not belong to the current codeword subset (no path step 308), a
search of the current codeword subset is made for a third codeword (step 312).
The same selection criteria are applied in searching for the third codeword as
was
applied when searching for the second codeword. Next, adaptively determine
whether to transmit an index of the second codeword in the full codebook, or
to
transmit an index of the third codeword in the current codeword subset (step
314).
Following this, depending on the determination, either an index of the third
codeword in the current codeword subset is transmitted (step 316) or an index
of
the second codeword in the full codebook is transmitted (step 318). In a
specific
example, step 314 involves comparing the second codeword and the third
codeword (step 314), and if they are sufficiently similar then the index of
the third
codeword in the current codeword subset is transmitted (step 316) and
otherwise
the index of the second codeword in the full codebook is transmitted (step
318).
Further, the current codeword subset will not change until a new full codebook
index is fed transmitted. When a full codebook index is transmitted (and
becomes the most recently transmitted index of a codeword in the full
codebook),
the current codebook subset is updated accordingly (step 320). Following any
one of method steps 310,316,320, the method continues at step 308.
With reference to FIG. 21, a flow diagram of another method
provided by an embodiment of the invention will now be described. The
flowchart
and description assumes that each codeword of the full codebook has an
associated codeword subset.. However, this method can also be applied with a
coarse subset. In a specific example, this method may be implemented in a
MIMO transmitter, such as MIMO transmitter 202. More generally, these methods
may be employed whenever the identity of a codeword is to be communicated
through the use of indexes. Operation begins with the reception of a codeword
index for a codeword in a full codebook C (step 400). A codeword from the full
codebook C based on the codeword index (step 402) is obtained. In
embodiments where the codewords are representative of weighting or pre-coding
to be applied to MIMO signals to be transmitted by a MIMO transmitter, the
MIMO
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transmitter will then apply the codeword to the MIMO signals to be transmitted
to
the MIMO receiver (step 403). Next, a further codeword index is received.
(step
404). If the index is an index of a codeword in the current codeword subset
(namely the codeword subset associated with the codeword of the full codeset
for
which an index was most recently transmitted) (yes path step 406), then the
current codeword subset is consulted, and using the index, the appropriate
codeword is obtained (step 408). If the index is not an index of a codeword in
the
current codeword subset subset index (no path, step 406), then it is an index
into
the full codebook, and the index is used to obtain the appropriate codeword
from
the full codebook (step 410). In addition, the current codebook subset is
reset to
be that associated with the codeword thus obtained (step 412). After step 412
or
418, in embodiments where the codewords are representative of weighting or pre-
coding to be applied to MIMO signals to be transmitted by a MIMO transmitter,
the
MIMO transmitter will then apply the codeword to the MIMO signals to be
transmitted to the MIMO receiver 206 (step 416). The method continues back at
step 404 with the receipt of another codeword index.
Those skilled in the art will recognize that codewords may have
various configurations, but in any case will effectively define weighting
factors in
the form of matrices, vectors, and the like to apply to MIMO signals to be
transmitted from the various antennas 204 of the MIMO transmitter 202.
Further, for OFDM systems, the codeword subsets may be arranged
by time and frequency. As such, the subset indices may relate to and be
selected
from codebook codewords having the best correlations in the time or frequency
direction in the OFDM spectrum.
Subset Switching Mechanism - Full codebook
In some of the above described embodiments, some of the time the
MIMO receiver transmits a codeword (or index) from the full codebook, while
other
times, the MIMO transmits an index of a codeword from the codeword subset
associated with the codeword of the full codebook for which an index was most
recently transmitted. After the index of a codeword from the full codebook is
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transmitted, the current codeword subset is switched to be the codeword subset
associated with the codeword of the full codebook.
In some embodiments, a first index length, in bits, is used for
indexing a codeword in a codeword subset, and a second index length, in bits,
is
used for indexing a codeword in the full codebook. Since there are fewer
codewords in the codeword subset, the first length can be made less than the
second length. For example, a p-bit index may be transmitted for a codeword in
the codeword subset, and an m-bit index may be transmitted for a codeword in
the
full codebook.
The receiver (more generally the device that is encoding the
codewords) will transmit information from which a determination can be made of
whether a given index is an index of a codeword in the full codebook as
opposed
to an index of a codeword in the current codeword subset. In some embodiments,
this is sent each time an index of a codeword in the full codebook is
transmitted,
but other implementations are possible, although they may be less efficient.
For
example, a single bit could be transmitted with every index that indicates
what
type of index it is, namely an index into the full codebook or an index into a
codebook subset.
If a p-bit index is used for the codeword subset, then the there are
2p unique p-bit indexes. In some embodiments, a codeword subset is defined to
contain up to 2p-1 codewords (using up 2p-1 indices), and the remaining p-bit
index is used to indicate that the next index is an index of a codeword from
the full
codebook. This may, for example, be an all zeros index, or any other
predefined
index. An index of a codeword from the full codebook is then transmitted, and
the
current codebook subset is updated to be the codebook subset associated with
that codeword.
In a specific example, to indicate a switch in the codeword subset,
the receiver transmits feedback that contains the remaining p-bit index, plus
an m-
bit index representing normal encoding of a codeword belongs to the entire
codebook, where it is assumed that the whole codebook contains up to 2m
codewords.
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This mechanism can be applied to dynamic or adaptive reset. With
dynamic reset, the codeword subset is switched each time the best codeword
from the overall codeword does not belong to the current codeword subset.
Subset Switching Mechanism - Coarse codebook
In some of the above described embodiments, some of the time the
MIMO receiver transmits a codeword from a coarse subset of the full codebook,
while other times, the MIMO transmits an index of a codeword from the codeword
subset associated with the codeword of the coarse subset of the full codebook
for
which an index was most recently transmitted. After the index of a codeword
from.
the coarse codebook is transmitted, the current codeword subset is switched to
be
the codeword subset associated with the codeword of the coarse codebook.
In some embodiments, a first index length, in bits, is used for
indexing a codeword in a codeword subset, and a second index length, in bits,
is
used for indexing a codeword in the full codebook. There may or may not be
fewer codewords in the codeword subset than in the coarse subset. For example,
a p-bit index may be transmitted for a codeword in the codeword subset, and an
k-
bit index may be transmitted for a codeword in the coarse subset.
The receiver (more generally the device that is encoding the
codewords) will transmit information from which a determination can be made of
whether a given index is an index of a codeword in the coarse codebook as
opposed to an index of a codeword in the current codeword subset. In some
embodiments, this is sent each time an index of a codeword in the full
codebook is
transmitted, but other implementations are possible, although they may be less
efficient. For example, a single bit could be transmitted with every index
that
indicates what type of index it is, namely an index into the coarse codebook
or an
index into a codebook subset.
If a p-bit index is used for the codeword subset, then the there are
2P unique p-bit indexes. In some embodiments, a codeword subset is defined to
contain up to 2P-1 codewords (using up 2P-1 indices), and the remaining p-bit
index is used to indicate that the next index is an index of a codeword from
the
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coarse codebook. This may, for example, be an all zeros index, or any other
predefined index. An index of a codeword from the full codebook is then
transmitted, and the current codebook subset is updated to be the codebook
subset associated with that codeword.
5 In a specific example, to indicate a switch in the codeword subset,
the receiver transmits feedback that contains the remaining p-bit index, plus
a k-bit
index representing normal encoding of a codeword belongs to the coarse
codebook, where it is assumed that the coarse codebook contains up to 2k
codewords.
10 This mechanism can be applied to dynamic or adaptive reset. With
dynamic reset, the codeword subset is switched each time the best codeword
from the overall codeword does not belong to the current codeword subset.
In the above described embodiments, every index fed back from the
receiver is associated with a codeword from full codebook. Either the index is
an
15 index into the full codebook/coarse codebook, or the index is into the
current
codeword subset. Whenever the transmitted index is an index of a codeword in
the full codebook/coarse codebook, the current codeword subset is switched to
be
the codeword subset associated with the codeword in the full codebook/coarse
codebook. For the coarse subset embodiments, each codeword of the coarse
20 subset can be viewed as a header codeword for the associated codeword
subset.
For the case where a codeword subset is defined for every codeword of the full
codebook, each codeword of the full codebook can be viewed as a header
codeword for the associated codeword subset. Both types of embodiments can
be collectively referred to as embodiments that employ header codewords.
25 In another class of embodiments, header codewords are not
employed. Rather, a set of codeword subsets are defined such that the
codewords of a given subset are relatively close to each other, for example
using
the correlation criterion defined previously. The receiver picks the best
codeword
from the full codebook, and determines which codebook subset this belongs to.
30 The receiver then feeds back to the transmitter an indication of which
codebook
subset is being used. The indication on its own is not an indication of any
specific
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31
codeword in the codebook subset, but rather simply indicates the codebook
subset as a whole. This becomes the "current codebook subset". The receiver
then feeds back to the transmitter an index of the best codeword in the
current
codebook subset. Then, the next time feedback is to be performed, the best
codeword from the full codebook is selected and the best codeword from the
current codeword subset is selected. If the best codeword from the current
codeword subset is close enough to the best codeword from the full codebook,
as
determined by a threshold criterion, then an index of the best codeword of the
current codeword subset is transmitted. Otherwise, the receiver feeds back an
indication of a different codebook subset which is to become the new current
codebook subset, namely the codebook subset to which the best codeword
selected from the full codebook belongs, followed by an index of the best
codeword into the new current codebook subset.
In some such embodiments, when transmitting indexes into
codebook subsets, a specific index is reserved to indicate that the next
feedback
concerns a switch in the current codebook subset. Alternatively, this can be
conveyed in some other manner.
For example, assuming a codebook having 2^(N1+N2) codewords,
this can be divided into 2AN1 codeword subsets via a correlation criteria,
each
having 2^N2 members. N1 bits can be used to signal a particular current
codeword subset, and N2 bits can be used to index a particular codeword within
the current codeword subset. With this approach, the N1 +N2 bits together
identify a specific codeword from the full codeword. The N1 bits can be viewed
as
a coarse index, and the N2 bits viewed as a differential index.
The threshold criterion for switching between codeword subsets for
these embodiments can be the same as described previously for embodiments
that make use of header codewords. For example, a switch may be made when
the best codeword from the full codebook differs from the best codeword from
the
current codeword subset by more than a threshold amount, or conversely a
switch
may not be made when the best codeword from the full codebook is similar
enough to the best codeword from the current codeword subset as defined for
example by a correlation criterion exceeding a threshold.
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It will be appreciated that that any module, component, or device
exemplified herein that executes instructions may include or otherwise have
access to computer readable media such as storage media; computer storage
media, or data storage devices (removable and/or non-removable) such as, for
example, magnetic disks, optical disks, or tape. Computer storage media may
include volatile and non-volatile, removable and non-removable media
implemented in any method or technology for storage of information, such as
computer readable instructions, data structures, program modules, or other
data.
Examples of computer storage media include RAM, ROM, EEPROM, flash
memory or other memory technology, CD-ROM, digital versatile disks (DVD) or
other optical storage, magnetic cassettes, magnetic tape, magnetic disk
storage or
other magnetic storage devices, or any other medium which can be used to store
the desired information and which can be accessed by an application, module,
or
both. Any such computer storage media may be part of the device or accessible
or
connectable thereto. Any application or module herein described may be
implemented using computer readable/executable instructions that may be stored
or otherwise held by such computer readable media.
Figure 22 is a block diagram of a receiver generally indicated at 600.
The receiver 600 has at least one antenna 602 and at least one wireless access
radio 604. The receiver has a memory 606 containing codebook subsets 608 as
per any of the embodiments described herein. The receiver has a subset
selector
610 that performs subset selection based on a threshold criterion, for example
using one of the methods described herein.
Numerous modifications and variations of the present disclosure are
possible in light of the above teachings. It is therefore to be understood
that within
the scope of the appended claims, the disclosure may be practiced otherwise
than
as specifically described herein.
