The present application is a divisional application of the chinese patent application with the application number of 200580027388.3, application date of 2005, 8/11/entitled "method and apparatus for implementing space frequency block coding in an ofdm wireless communication system".
Detailed Description
Hereinafter, the term "station" (STA) includes, but is not limited to, user equipment, wireless transmit/receive units, fixed or mobile subscriber units, pagers, or any other type of device that may operate in a wireless environment. The term "access point" (AP) as referred to hereinafter includes, but is not limited to, a node B, a base station, a site controller, or any other type of interfacing device in a wireless environment.
The present invention will be described with reference to the drawing figures wherein like numerals represent like elements throughout. It is noted that the illustrations provided in this patent are high-level functional block diagrams, and the functions performed by these functional blocks may be implemented more or less by blocks. The features of the present invention may be incorporated into an Integrated Circuit (IC), or be configured in a circuit comprising a multitude of interconnecting components.
Embodiments of the present invention provide transmitter and receiver matched filters that can implement SFBC MIMO encoding. Embodiments also provide transmitter channel precoding and receiver antenna processing as well as channel decomposition functions.
The system operation has two modes: closed loop and open loop. The closed loop is used when Channel State Information (CSI) is available to the transmitter. The open loop is used when CSI is not available. The variation may be used for transmissions to native STAs that may provide diversity benefits.
In closed loop mode, CSI is used to create virtual independent channels by decomposing and diagonalizing the channel matrix and precoding at the transmitter. Spread out by the eigenvalues of the TGn channel given, the present invention adds robustness at the cost of reduced data rate using space-frequency orthogonal MIMO coding in the transmitter at the input of the channel precoder. Any coding scheme in MIMO must handle diversity versus multiplexing gain permutation. It is expected to have the permutation scheme best suited for the particular channel statistics. SFBC is selected for low mobility and long channel coherence time. This scheme allows for a simpler receiver implementation than an MMSE receiver. The combined solution may contribute to higher yield over a wide range. Embodiments of the present invention may allow for per-carrier power/bit loading and maintain a persistent strong link via closed loop operation of channel state feedback. Another potential benefit is that it can easily scale any number of antennas at the transmitter and receiver.
The CSI may be obtained at the transmitter through feedback from the receiver or via exploiting channel reciprocity. Channel reciprocity is used for the primary TDD infrastructure. In this case, the transmitter and receiver may independently estimate and resolve the channel. The channel update rate may be reduced when the signal-to-noise ratio is high, resulting in a reduced feedback bandwidth load. The latency requirement and the feedback data rate non-selectivity to the natural frequency of the natural value are not usually significant.
Closed loop mode requires calibration of the transmitter to compensate for the amplitude and phase differences of the estimated channel in the uplink and downlink directions. This is not often done, for example, during STA association or under application control, and channel reciprocity can be used to estimate the channels at both ends. In addition, the CQI (or SNR) per eigenbeam is fed back to the transmitter to support adaptive rate control.
Fig. 1 is a block diagram of an OFDM-MIMO system 100 implementing a closed loop mode. System 100 includes a transmitter 110 and a receiver 130. The transmitter 110 includes a channel encoder 112, a multiplexer 114, a power loading unit 116, a plurality of SFBC encoding units 118, a plurality of series-parallel (S/P) converters 120, a plurality of eigenbeamformers 122, a plurality of IFFT units 124, and a plurality of transmit antennas (not shown). The channel encoder 112 preferably encodes data according to a Channel Quality Indicator (CQI) transmitted from the receiver 130. The CQI is used to determine the coding rate and modulation scheme per carrier or group of subcarriers. The encoded data streams are multiplexed into two or more data streams by multiplexer 114.
The transmit power level of each data stream is adjusted by the power loading unit 116 based on feedback. The power loading unit 116 adjusts the power level for the data rate of each eigenbeam to balance the total transmit power on all eigenbeams (or subcarriers), as will be explained in detail below.
The SFBC encoding unit 118 performs SFBC encoding on a data stream. SFBC coding is achieved on the eigenbeams and subcarriers for each transmitted data rate. The eigenbeam and sub-carrier pairing is selected to ensure independent channels. The OFDM symbols are performed on K subcarriers. To accommodate SFBC, the subcarriers are partitioned into L pairs of subcarriers (or subcarrier groups). The bandwidth of each set of subcarriers should be less than the coherence bandwidth of the channel. However, when combining the natural beam forming, this limitation is relaxed due to the natural beam frequency dullness.
The set of sub-carrier groups used for block coding is considered separately. The following is an example of Alamouti type SFBC applied to OFDM symbols:
once the SFBC coder 118 constructs the OFDM symbols for all subcarriers, the code blocks are multiplexed by the S/P converter 120 and input to the eigenbeamformer 122. The eigenbeamformer 122 distributes the eigenbeams to the transmit antennas. The IFFT unit 124 converts data in the frequency domain into data in the time domain.
Receiver 130 includes a plurality of receiving antennas (not shown), a plurality of FFT sections 132, a fixed beamformer 134, an SFBC decoding section 136, a combiner 138, a channel decoder 144, a channel estimator 140, a CSI generator 142, and a CQI generator 146.
FFT unit 132 converts the received samples to the frequency domain, and eigenbeamformer 134, SFBC decoding unit 136, and channel decoder 144 perform the reverse operations implemented at transmitter 110. The combiner 138 combines the SFBC decoding results using Maximal Ratio Combining (MRC).
The channel estimator 140 generates a channel matrix using a training sequence transmitted from a transmitter and decomposes the channel matrix into two beamforming single matrices U and V (U for transmission and V for reception) and a diagonal matrix D per carrier (or per carrier group) by Single Value Decomposition (SVD) or eigenvalue decomposition. The CSI generator 142 generates CSI from the channel estimation result, and the CQI generator generates CQI based on the decoding result. The CSI and CQI are sent back to the transmitter 110.
The channel matrix H between nT transmit antennas and nR receive antennas can be written as follows:
the channel matrix H is decomposed by SVD as follows:
H＝UDVH，
where U and V are single matrices and D is a diagonal matrix. U is belonged to CnR×nRAnd V ∈ CnT×nT. Then, for the transmit symbol vector s, transmit precoding is simply performed as follows:
x is Vs (transmitted signal).
The received signal becomes as follows:
y＝HVs+n，
where n is the noise introduced into the channel. The receiver accomplishes this decomposition by using a matched filter:
VHHH＝VHVDHUH＝DHUH。
after generalizing the channel gain of the eigenbeam, the estimate of the transmitted symbol s becomes
s can be detected without having to perform successive interference cancellation or MMSE type detectors. DHD is a diagonal matrix formed by the eigenvalues of H across the diagonals. Thus, the generalization factor α ═ D-2. U is HHHIs an eigenvector of, V is HHHAnd D is a single value diagonal matrix (HH) of HHThe square root of the eigenvalue of).
FIG. 2 is a block diagram of a system 200 for implementing open loop mode in accordance with the present invention. System 200 includes a transmitter 210 and a receiver 230. In open loop mode, the combination of space-frequency coding and spatial spreading in transmitter 210 provides diversity without CSI. Variations of this scheme may be used when operating native 802.11a/g stas.
Transmitter 210 includes a channel encoder 212, a multiplexer 214, a power loading unit 216, a plurality of SFBC encoding units 218, a plurality of series-parallel converters 220, a beam-former network (BFN)222, a plurality of IFFT units 224, and a plurality of transmit antennas 226. As in closed loop mode, the channel encoder 212 uses the CQI to determine the coding rate and modulation per carrier or group of subcarriers. The encoded data streams are multiplexed into two or more data streams by a multiplexer 214.
In the open loop, the native beamformer is replaced by a beamformer network (BFN) 222. BFN 222 forms N beams in space, where N is the number of antennas 226. The beam is constructed pseudo-randomly by BFN matrix operations. The independent groups of subcarriers used for SFBC coding are transmitted on individual beams.
For intrinsic support, SFBC encoding may not be performed. Alternative diversity via beam permutation is performed which may improve diversity and hence the performance of the native 802.11a/g device.
Receiver 230 includes receive antennas 231, FFT unit 232, BFN 234, SFBC decoding and combining unit 236, and channel decoder 238. The FFT unit 232 converts the received signal in the time domain into a signal in the frequency domain. The SFBC decode and combine unit 236 decodes and combines the symbols received from the sub-carrier groups/eigenbeams and converts them from parallel to serial using previous knowledge of constellation size. The symbols are combined using MRC. A channel decoder 238 decodes the combined symbols and generates CQI.
A first embodiment of the power load is explained as follows. Spatial processing is a combination of spatial frequency coding and eigen-beamforming. This is performed to give the best compromise between the redundant gain suffered by the SFBC and the spatial multiplexing provided by the native beamformer. The power loading scheme operates across the eigenmodes of the channel matrix. However, SFBC also introduces the limitation that the output of the encoder has the same power load regardless of the input power load due to the encoder internal interleaving operation.
Fig. 3 is a block diagram of the transmitter 110 depicting the power load. Fig. 3 illustrates a 4 × 4 case as an example, and the first embodiment of the power load scheme will be explained with reference to the 4 × 4 case. It should be noted, however, that the 4 x 4 case can be extended to any other case.
For a particular subcarrier k, four data streams are mapped to 2 pairs of power load/Adaptive Modulation and Coding (AMC) modes. That is, the modulation order of each pair of inputs is selected to be the same. This is later mapped to the eigenmode pairing. The output of the power loading unit 116 is applied to the dual 2 x 2SFBC encoding unit 118 and then passed to the eigenbeamformer 122. The eigenbeamformer 122 maps the inputs to the eigenmodes of the channel via pre-processing.
For all K subcarriers, the eigenvalues of the channel matrix are known to be at the transmitter. The channel energy of each eigenmode is defined as follows:
wherein λi，kThe ith eigenvalue of the channel for the kth subcarrier. The two-signal-to-noise-plus-interference ratio (SNIR) is defined for two coupled eigenmodes as follows:
and
where M is the number of eigenmodes. That is, the eigenmodes are grouped such that half of the eigenmodes with the largest channel energy (or SNIR) are in one group and the other half with the weakest channel energy are in the other group. Thus, the harmonic SNIR represents the total channel energy of the stronger and weaker eigenmodes. Channel energy is an indication of how strong the eigenmodes will be and therefore how strong the signal carried on these eigenmodes will be. This information is used to apply different Adaptive Modulation and Coding (AMC) and/or different power loading to the halves as explained in more detail later. The separation of the coupled SNIRs is defined as follows:
Δβ＝βmod1-βmod2
during closed loop operation, the transmitter 110 has knowledge of the current CSI from which it can extract the eigenvalues and the pre-processing matrix. The transmitter 110 also deduces from the CSI that the data rate in the link, Rb, can be supported. The power load for a given, acceptable CQI is then an optimization between the number of bits that can be transmitted per OFDM symbol and the modulation type to be used by each mode.
As described above, using the channel energy calculated for eigenmode i, the maximum bit rate at which channel conditions can be supported is determined. Thus, using the above-described mode independent calculation, it can be determined how the bit rate must be allocated between the two pairs of modes. Fig. 4 is a diagram of an exemplary power load and an adaptive modulation and coding mapping between two pairs of modes. In this example, the bit rate that a particular subcarrier may be supported is 24 bits per OFDM symbol. The lowest modulation order that satisfies the bit rate is found in fig. 4 as indicated by the dashed arrow. In this example, the first and second modes (first pair coupled mode) would use 16 Quadrature Amplitude Modulation (QAM) and the third and fourth modes (second pair coupled mode) would use 256 QAM.
Note that this mapping is illustrated for one acceptable CQI and for one secondary carrier. In an example of an alternative MIMO configuration, such as 2 x 4, 2 x 2, etc., the same power loading scheme is acceptable except that the total number of bits in the table entry is plotted down to represent the transmit capability and the power loading can be achieved in a single pair mode.
The power load scheme according to the second embodiment is explained as follows. Intrinsic value of each sub-carrier (λ)1(k)＞λ2(k)＞...＞λnT(k) Are ordered, and the eigenbeams (E)1，E2，...，EnT) The eigenvalues of this same ordering are created by grouping for all subcarriers as follows:
Ei＝{λi(1)，λi(2)，...，λi(k)}，i＝1，2，...，nT，
where K is the number of subcarriers, nT is the number of transmit antennas, and λi(j) Is the ith eigenvalue of the jth subcarrier. NT is an even number.
The average of the eigenvalues per eigenbeam is calculated as follows:
i＝1，2，...，nT。
the eigenbeams are paired to create an Almouti spatial frequency block, e.g. { E }1，E2}1，{E3，E4}2，...，{E2i-1，E2i}i...{EnT-1，EnT}nT/2. However, if a pair of SNRs is greater than the SNRmaxThen the pair of second pair of eigen-waveforms is replaced by an eigen-beam having the next lowest eigenvalue average until its SNR is less than or equal to the SNRminUntil now.
WhereinFor noise variation, SNRminThe minimum required SNR for the highest data rate required for quality of service. This step is repeated until all the eigenbeams are paired. Fig. 5 shows an example of power/bit load secondary carrier group pairing.
The data rate for each pair of eigenbeams is determined by mapping a pair of SNRs to the data rate for a given quality. The required SNR can be adjusted for all intrinsic beam pairs to compensate for measurement errors and make the total transmit power constant.
The weight vector for each pair of eigenbeams per carrier can be calculated as follows:
where i is the ith pair of eigenbeams and j is the jth subcarrier.
In addition to the first or second embodiments, according to a third embodiment, another power load is applied across the subcarriers or subcarrier groups for weak eigenmodes. That is, instead of being applied to the power loads of all the natural modes, the power loads may be applied only to the weaker one, and thus the most profit may be obtained from the power loads. In this example, these intrinsic modes not loaded by power may still have SFBC or other coding, or may have different AMC settings individually, while these intrinsic modes loaded by power share the same AMC settings. At the same time, the eigenmodes of the channel are always ordered power from strongest to weakest. The power loading of the channel can be improved by pairing eigenmodes like power.
The spatial processing scheme may be configured to any receive and transmit antenna combination. Depending on the number of antennas on each side, SFBC and eigen-beamforming selection combinations are used. The following table summarizes the various configurations supported and the states of spatial processing and power loads applicable to each case.
TABLE 1
Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention.