JP5722407B2 - Optimal weights for MMSE space time equalizers in multicode CDMA systems - Google Patents

Description

  The present invention relates generally to CDMA communication systems, and more particularly to linear MMSE space-time equalizers for MIMO multicode CDMA systems.

  In wireless communication systems, several users share one channel in the common spectrum. In order to avoid collisions that result from several users sending information over the communication channel simultaneously, some rules are required when assigning available channel capacity to users. The rules for user access to the communication channel are achieved by various forms of multiple access protocols. One form of protocol is known as Code Division Multiple Access (CDMA). In addition to providing multiple access allocation to channels of limited capacity, the protocol can provide other functions. For example, the protocol can provide security by isolating users from each other, limiting interference between users, or performing interception and decoding that is difficult for unintended receivers, also known as low probability interception. .

  In a CDMA system, each signal is separated from signals from other users by encoding the signal. An information signal is uniquely encoded into a transmission signal. By knowing the user's code sequence, the intended receiver can decode the transmitted signal and receive the information. Since the information signal spectrum is spread by the code, the bandwidth of the encoded signal is much wider than the original bandwidth of the information signal. For this reason, CDMA is a form of “spread spectrum” coding. Since each user's signal energy is spread across the channel bandwidth, each user's signal appears to other users as noise. As long as the decoding process can obtain an appropriate signal-to-noise ratio, the information in the signal can be recovered (separation of the desired user's signal from the “noise” of other users' signals). Other factors that affect the recovery of the user's signal information are different conditions in the environment for each subscriber, such as fading, shadowing, and multipath. Shadowing is interference caused by an object that blocks a signal transmission path between a transmitter and a receiver, such as a large building. Multipath is signal distortion that results from a signal crossing multiple paths of different lengths and arriving at a receiver at different times. Multipath is also called “time dispersion” of communication channels. Signals received in phase strengthen each other and produce a stronger signal at the receiver, while signals received with phase mismatch produce a weak or fading signal. Multipath fading may also change over time. For example, in a communication unit carried in a moving car, the amount of multipath fading can change rapidly.

  Multiple transmit and receive antennas may be used to provide diversity against adverse path effects and to improve performance. If the transmission path between the transmit and receive antennas is linearly independent (ie, transmission on one path is not formed as a linear combination of transmissions on the other path, this is generally to some extent The truth is that the likelihood of correctly receiving a transmitted signal increases as the number of antennas increases. In general, diversity increases and performance improves as the number of transmit and receive antennas increases. The use of multiple antennas at the transmitter and receiver is used in multiple input multiple output (MIMO) systems.

  If multiple antennas are available at the transmitter and receiver, peak throughput can be increased using techniques such as spatial multiplexing and code reuse. With code reuse, each channel allocated for transmission can be modulated up to M different data streams. Where M is the number of transmission antennas. Data streams that share the same code are distinguished based on their spatial characteristics and require a receiver with at least M antennas. In principle, the peak throughput with code reuse is M times the rate achievable with a single antenna.

  In a MIMO multicode CDMA system, reuse of the same spreading code at different transmit antennas can be achieved if the space-time equalizer uses a minimum mean square error (MMSE) weighting vector that minimizes the mean square error of the equalizer output chip sequence. Reduce the equalization performance. The CDMA despreader distorts the inter-stream interference component unlike the multipath interference and background noise components. This degrades the performance of prior art MIMO systems.

  Therefore, there is a technical need for an enhanced chip-level linear space time equalizer for multiple-input multiple-output (MIMO) multicode CDMA systems that can reuse spreading codes in different transmit antennas. .

  In one aspect, a CDMA receiver includes a space time equalizer operably connected to a receive antenna, the space time equalizer applying a weighting vector consisting of coefficients that are a function of a spreading factor.

  In another aspect, a CDMA receiver includes a space time equalizer having a coefficient of equalization and a despreader, where the equalization coefficient is at least partially a function of the spreading factor.

  In yet another aspect, the method comprises receiving a plurality of signals via a plurality of receive antennas, wherein the received signal from each receive antenna is a combination of one or more signals transmitted from a transmission unit. Is provided. The method also includes processing the signal using a weighting vector having coefficients to generate a plurality of bitstreams. However, the coefficient is a function of at least partially spreading code reuse.

  In a further aspect, the CDMA receiver comprises equalizing means operatively connected to the receive antenna. The means for equalizing applies a weighting vector with coefficients that are a function of the spreading factor. The CDMA receiver further comprises a despreading means operatively connected to the equalizing means. A means for despreading separates the equalized metric sequence into a plurality of modulation symbol sequences.

FIG. 1A is a diagram of a communication system that supports multiple users and can implement at least some aspects and embodiments of the present invention. FIG. 1B is a block diagram of one embodiment of a MIMO multicode CDMA system. FIG. 2A is a block diagram of another embodiment of a MIMO multicode CDMA system. FIG. 2B is a block diagram of one embodiment of an MMSE space time equalizer. FIG. 3 is a flowchart illustrating the operation of one embodiment of a multicode CDMA system. FIG. 4 is a block error rate graph for various chip-SNR values for one embodiment of the present invention using one code reuse and a 3 km / h vehicle A multipath channel model.

  The term “exemplary” is used herein to mean “serving as an example, instance, illustration”. Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.

  FIG. 1A is a diagram of a communication system 10 that supports a large number of users and is capable of implementing at least some aspects and embodiments of the present invention. The system 10 provides communication for a number of cells 2a-2g. Each of the multiple cells is served by a corresponding base station 4. The cells are organized in a way that obtains a coverage area for the desired area. The service area may be defined as an area where a user of the terminal 6 can obtain a specific grade of service (GOS), for example. Terminals 6 in the service area may be fixed or mobile, and are generally served by major base stations. For each active terminal, transmissions from other base stations and terminals represent potential interference.

  As shown in FIG. 1A, various terminals 6 are distributed throughout the system. The terminal 6 includes a processing device 8. Examples of processing device 8 include, but are not limited to, a processor, program logic, or other substrate configuration that represents data and instructions. In other embodiments, the processor may comprise a controller circuit, a processor circuit, a processor, a general purpose single or multi-chip microprocessor, a digital signal processor, a built-in microprocessor, a microcontroller, and the like.

  Each terminal 6 depends on, for example, whether “soft handoff” is employed or whether the terminal is designed and operated to receive multiple transmissions from multiple base stations simultaneously or sequentially. Anyway, it communicates with at least one and possibly one or more base stations 4 on the downlink and uplink. The downlink refers to transmission from the base station to the terminal, and the uplink refers to transmission from the terminal to the base station.

  In FIG. 1A, base station 4a transmits data to terminals 6a and 6j on the downlink, base station 4b transmits data to terminals 6b and 6j, base station 4c transmits data to terminals 6c, and so on. . In FIG. 1A, a solid line with an arrow indicates data transmission from the base station to the terminal. A broken line with an arrow indicates that the terminal is receiving a pilot signal but there is no data transmission from the base station. For simplicity, uplink communication is not shown in FIG. 1A.

  System 10 is a US patent application serial number entitled “HIGH EFFICIENCY, HIGH PERFORMANCE COMMUNICATIONS SYSTEM EMPLOYING MULTI-CARRIER MODULATION” filed on March 22, 2000. No. 09 / 532,492 or US Patent Application Serial No. 08/963, entitled “Method and Apparatus for High Rate Packet Data Transmission”. It may be designed based on the system disclosed in 386. Both of the above patent applications are assigned to the assignee of the present invention and incorporated herein by reference. The system 10 may also be designed as a CDMA system that supports one or more CDMA standards, such as IS-95 standard, W-CDMA standard, other standards, or combinations thereof.

  In the system 10, multiple terminals share a common resource, that is, the total operating bandwidth W. In order to obtain a desired level of performance at a particular terminal, interference from other transmissions needs to be reduced to an acceptable level. Also, it is necessary to operate at a certain carrier-to-noise plus interference (C / I) level or higher to ensure transmission at a high data rate for a given operating bandwidth. Reduction in interference and acquisition of required C / I is usually obtained by dividing the total available resources into fractions, each of which is assigned to a characteristic cell.

  For example, the total operating bandwidth W can be divided into N equivalent operating frequency bands (ie, B = W / N) and can be assigned to one of the frequency bands of each cell N. The frequency band is periodically reused to obtain higher spectral efficiency. As supported by FIG. 1A, for a 7-cell reuse pattern, cell 2a may be assigned a first frequency band, cell 2b may be assigned a second frequency band, and so on.

  Communication systems are typically designed to meet a number of system requirements that may include, for example, quality of service (QOS), service area, and performance requirements. Quality of service is typically defined as that all terminals within a service area can obtain a specified minimum average bit rate for a predetermined percentage of time.

  Recent advances in multiple-input multiple-output (MIMO) transmission technology promises a very large throughput gain for future wireless communication systems by using multiple antennas for both transmitter and receiver. MIMO technology can be incorporated into various modulation and multiple access schemes such as, for example, MIMO-CDMA, MIMO-OFDM, and the like.

  High-speed packet data channels within the 3G CDMA standard, such as high-speed downlink shared channel (HS-DSCH) and forward-link packet data channel (F-PDCH) and the like, have a fixed spreading factor (SF), Walsh Multiple channelization codes, such as codes, are typically used to send and receive large amounts of information in the short frame interval. Depending on the data rate of the current packet, the base station (BS) can select a number of codes from the available channelization codes to provide a corresponding number of modulation symbols. Since MIMO-CDMA systems support multiple transmit streams via multiple transmit antennas, corresponding BSs often reuse the same channelization code for different antennas. Unless designed within a MIMO-CDMA context, code reuse in transmit antennas may result in significant impairment to the mobile station (MS) space time equalizer.

Multicode CDMA System Model [0029] FIG. 1B is a block diagram of one embodiment of a MIMO multicode CDMA system 100 that includes a transmitter portion 102 and a receiver portion 104. The spreading factor is denoted as SF in the following description. The transmitter portion 102 includes an encoder 106, a mapper 108, a demultiplexer 110, a plurality of spreaders 112, and a plurality of transmit antennas 114. The number of transmission antennas 114 is M, and the number of orthogonal spreading codes assigned to each transmission antenna 114 is J. (J ≦ SF)
The receiver portion 104 includes a plurality of receive antennas 116, a minimum mean square error (MMSE) space time equalizer 118, a plurality of despreaders 120, a multiplexer 122, a demapper 124, and a decoder 126. The number of receiving antennas 116 is N, and the number of despreaders 120 assigned to each receiving antenna 116 is J. That corresponds to the number of spreaders 112 assigned to each transmit antenna 114. Those skilled in the art will appreciate that the space-time equalizer described herein can be applied to general purpose MIMO-CDMA systems.

  Encoders, decoders, rate matchers, interleavers, deinterleavers, mappers, demappers, spreaders, despreaders, and space time equalizers are broad terms intended to have their usual meaning. Furthermore, an encoder is a device or method used to encode a signal or data (such as a bitstream) from one form into another form such as a form suitable for transmission, storage or processing. obtain. The encoder can generally be implemented in software or hardware by, for example, a program, algorithm, method or circuit. A decoder can be a device that can perform the reverse of the encoder, undo the encoding, and thereby retrieve the original information.

  A rate matcher can be an apparatus or method that adjusts the rate or bit rate of a data stream to a desired rate. For example, at the transmitter, the rate matcher can adjust the bit rate to match the capabilities of the transmitter. At the receiver, the rate matcher can perform the reverse process.

  An interleaver can be an apparatus or method that arranges data in a discontinuous manner to increase performance. The deinterleaver generally performs the reverse of the interleaver and can arrange the interleaved data in a continuous manner so that it can be processed more easily.

  A mapper can be an apparatus or method that collects groups of bits and modulates them into a single symbol. A demapper can be an apparatus or method that generally performs the inverse of a mapper, such as converting a single modulation symbol into a group of bits.

  A spreader can be an apparatus or method that increases the bandwidth of a transmitted signal by a factor that exceeds the information signal bandwidth. A despreader can generally be an apparatus or method that reverses the spreader to reduce the bandwidth of the received signal. For example, the despreader can reduce the bandwidth of the received signal to the information bandwidth.

  A space-time equalizer can be an apparatus or method that provides space and time scale and couples to a signal. For example, a space time equalizer can scale and combine received signals spatially and temporally to recover the original signal.

  Referring to FIG. 1B, encoder 106 receives a source bit sequence 128. The source bit sequence 128 in each frame is encoded, rate matched (ie, punctured or repeated), interleaved in the encoder 106, and modulated symbol sequence (eg, QPSK, 16QAM, etc.) in the mapper 108. Is mapped to The modulation symbol sequence is then demultiplexed into M groups of the J stream at the demultiplexer 110. In this case, the m th group is transmitted via the m th transmit antenna 114. The J streams in each group are spread by the J spreading code in the spreader 112. In this case, the j-th spreading code is equivalent to the product of the j-th channelization code such as an orthogonal code, a quasi-orthogonal code or a Walsh code with a spreading factor SF and a pseudo random scrambling code of the BS. Each group typically reuses the same set of J spreading codes, and each transmit antenna 114 typically uses the same transmit power. However, the invention is not limited to these specific cases.

  After passing through a multidimensional multipath fading channel, the transmitted signal arrives at N receive antennas 116. In this case, the MMSE space time chip equalizer 118 separates the received signal into M groups of equalized soft metric sequences corresponding to the M transmit antennas 114.

Next, in despreader 120, J despread codes equal to the conjugate of J spread codes separate each group of equalized soft metric sequences into J soft demodulated symbol sequences. Each of them corresponds to each orthogonal Walsh channel in the group. The resulting JXM demodulated symbol sequence is multiplexed into a single stream at multiplexer 122 and demapped at demapper 124 to a sequence such as, for example, a log likelihood ratio (LLR) sequence. The sequence is deinterleaved, reverse rate matched, and decoded at decoder 126 to recover the original bit sequence as decoded bits 130.

  FIG. 2A is a block diagram of one embodiment of a MIMO multicode CDMA system 200 that includes a transmitter portion 202 and a receiver portion 204. The spreading factor is denoted as SF in the following description.

  The transmitter portion 202 includes a plurality of encoders 206, a plurality of mappers 208, a plurality of demultiplexers 210, a plurality of spreaders 112 and a plurality of transmit antennas 114. The number of transmission antennas 114 is M, and the number of spreading codes assigned to each transmission antenna 114 is J.

  The receiver portion 204 includes a plurality of receive antennas 116, a minimum mean square error (MMSE) space time equalizer 118, a plurality of despreaders 120, a plurality of multiplexers 222, a plurality of demappers 224 and a plurality of decoders 226. The number of receiving antennas 116 is N, and the number of despreaders 120 assigned to each receiving antenna 116 is J. That corresponds to the number of spreaders 112 assigned to each transmit antenna 114.

  Each encoder 206 receives a source bit sequence 128 for the encoder 206. The source bit sequence 128 in each frame is encoded in its corresponding encoder 206, rate matched (ie, punctured or repeated), interleaved, and modulated symbol sequences (eg, QPSK, 16QAM) in its corresponding mapper 208. Etc.). The modulation symbol sequence is then demultiplexed into a group of J streams in its corresponding demultiplexer 210. In this case, the m th group is transmitted via the m th transmit antenna 114. The J streams in each group are spread by the J spreading code in the spreader 112. In this case, the j-th spreading code is equal to the product of, for example, the j-th channelization code such as spreading code SF orthogonal code, quasi-orthogonal code or Walsh code, and the pseudo random scrambling code of BS. Each group typically reuses the same set of J spreading codes and each transmit antenna 114 typically uses the same transmit power, although the invention is not limited to these particular cases.

  After passing through the multidimensional multipath fading channel, the transmitted signal arrives at N receive antennas 116. In this case, MMSE space time chip equalizer 118 separates the received signal into M groups of equalized soft metric sequences corresponding to M transmit antennas 114. Next, in despreader 120, J despread codes equal to the conjugate of J spread codes separate each group of equalized soft metric sequences into J soft demodulated symbol sequences. Each of them corresponds to each orthogonal Walsh channel in the group. Each of the M yielding J demodulated symbol sequences is multiplexed into a single stream at its corresponding multiplexer 222 and demapped to a sequence such as a log likelihood ratio (LLR) sequence at its corresponding demapper 224. . Each of the M sequences is deinterleaved, reverse rate matched, and decoded in its corresponding decoder 226 to recover the original source bit sequence as decoded bits 230.

  In one embodiment, the soft metric sequence of MIMO CDMA system 100, 200 after MMSE space time equalization includes five components: a desired signal; one or more on-times that reuse the same spreading code as the desired signal (on-time) inter-stream interference (or crosstalk in different transmit antenna signals); one or more on-time inter-stream interferences that do not reuse the same spreading code as the desired signal; one or more multipath interferences (Ie total serving cell signal component that is not on-time); and background noise (other cell interference, thermal noise, etc.).

  On-time inter-stream interference is left if the desired signal spreading code is reused by the despreading process, or disabled if the desired signal spreading code is not reused by the despreading process. The Multipath interference and background noise are roughly suppressed by the SF factor.

FIG. 2B is a block diagram of one embodiment of the space time equalizer 118. The space time equalizer 118 includes M equalizing banks 250 (bank m, where m = 0, 1,..., M−1) corresponding to the M transmitting antennas 114. Each bank 250 includes N filters 252 (filter n, where n = 0, 1,..., N−1) corresponding to N receive antennas 116 and adders 254. The filter 252 has a filter coefficient V ^H m, n OPT, where m = 0, 1, 2,. . . , M−1 and n = 0, 1, 2,. . . , N−1, and each filter 252 produces a filtered output signal. Each bank 250 receives a signal from each of the N receive antennas 116 and processes the signal in a corresponding filter 252. Adder 254 adds the filtered output signals from each filter 252 in each bank 250 to generate an equalized metric sequence 256.

Focusing on equalizing bank 0 250a, the jth filter in bank 0 with filter coefficient V ^H 0, j OPT, where j = 0, 1,. . . , N−1, the input of the filter j is connected to the j-th receiving antenna, and the output of the filter j is connected to the input of the adder 254a.

For example, the input of filter 0 252a in equalizing bank 0 250a with filter coefficient V ^H 0,0 OPT is connected to receive antenna 0 116a, and the output of filter 0 252a is connected to the input of adder 254a. Similarly, the input of the filter N-1 252b having the filter coefficient V ^H 0, N-1 opt is connected to the receiving antenna N-1 116b, and the output of the filter N-1 252b is connected to the input of the adder 254a. .

  Filter n in block 0 250a, where n = 0, 1,. . . , N-1 are summed in adder 254a to produce an equalized metric sequence of sequence 0 256a.

  Similarly, each block m 250 where m = 0, 1,. . . , M−1, the N filtered outputs of N filters 252 are summed to produce M equalized metric sequences 256.

As further described in Equation 8, the channel coefficient hi and noise covariance Rn are calculated from the pilot signal. Filter coefficient V ^H , n OPT where m = 0, 1, 2,. . . , M-1 and n = 0, 1, 2,. . . , N−1 is calculated using the calculated channel coefficients h _i and noise covariance R _n .

  In other embodiments, the equalizer 118 is implemented as software within the processor 8.

  FIG. 3 is a flowchart 300 illustrating the operation of one embodiment of the multicode CDMA receiving system 104,204. In one embodiment, the multicode CDMA receiving system 104, 204 operates in a continuous loop that begins with a start block and ends with an end block. At block 310, equalizer 118 receives a pilot symbol sequence. At block 312, the equalizer 118 calculates equalizer coefficients using the pilot symbols.

  At block 314, the receiving system 104, 204 receives the signal via the antenna 116. At block 316, the received signal is equalized at the equalizer 118 using the equalizer coefficients. The equalizer 118 processes the received signal to generate an equalized metric sequence 256.

  At block 318, the equalized metric sequence 256 is processed by the despreader 120 to generate a demodulated symbol sequence.

  Since the conventional chip level MMSE equalizer does not consider despreading effects, the presence of on-time inter-stream interference makes the conventional chip level MMSE equalizer suboptimal. Conventional chip level MMSE weights are directed in the sub-optimal direction of noise space within MIMO CDMA applications. It degrades decoding performance. Furthermore, the despreading effect for MMSE weight optimization in SISO multicode CDMA does not change the weight (or steering direction) except for different scaling factors. Assuming that demappers 124, 224 rescale the soft demodulated symbols, decoding performance is not affected in SISO multicode CDMA.

  In general, since the number of spreading codes used for each stream is increased, the gap between the optimal MMSE weight (considering the despreading effect) and the sub-optimal MMSE weight decreases. This is because, as will be described below, the despread gain of on-time interstream interference will be roughly discounted by a factor of the number of spreading codes used.

Linear MMSE Equalizer Weights for MIMO Multicode CDMA Conventional Chip Level MMSE Weight Optimization in MIMO Multicode CDMA A conventional MMSE space time chip equalizer equalizes a received signal corresponding to M transmit antennas 114. Separate into M groups of soft metric sequences. The sequence is then processed by multiplexers 122, 222, demappers 124, 224, and decoders 126, 226 to produce decoded bits 130, 230, respectively.

In the following description of conventional chip level MMSE weight optimization, the multipath delay spread span is L chips long, the equalizer span is E chips long, and the receiver takes P samples per chip. (Ie, the oversampling factor is P). Further, h _{n, m, p} (l) (l = 0, 1,..., L−1; n = 0, 1,..., N−1; m = 0, 1,. M−1; p = 0, 1,..., P−1) is the relationship between the m th transmit antenna 114 and the n th receive antenna 116 corresponding to the l th chip delay and the p th sample of the chip Is the channel coefficient between. The chip signal of the mth transmitting antenna 114 at the chip time k is denoted by σ _x x _m (k). In this case, E [| x _m (k) | ² ] = 1,

Is the average chip energy of each transmit antenna 114.

x _m (k) ≡σ _x [x _m (k) x _m (k + 1). . . x _m (k + E + L−2)] ^T (1) is defined as the (E + L−1) -dimensional chip vector of the mth transmit antenna 114 ranging from index k to k + E + L−2. Also, _let y _{n, p} (k) and n _{n, p} (k) be the sample received at the n th receive antenna 116 on the p th sample of the k th chip and its background noise component. .

Furthermore, y _n (k) ≡ [y _{n, 0} (k). . . y _{n, P-1} (k). . . y _{n, 0} (k + E−1). . . y _{n, P-1} (k + E-1)] ^T (2) and n _n (k) ≡ [n _{n, 0} (k). . . n _{n, p-1} (k). . . n _{n, 0} (k + E−1). . . n _{n, p−1} (k + E−1)] ^T (3) is defined as the PE-dimensional received sample vector at the n th receive antenna 116 and the corresponding background noise vector. Therefore,

It is. In Equation 4,

Denotes a PE × (E + L−1) multipath channel matrix between the m th transmit antenna 114 and the n th receive antenna 116;

As given.

further,

Is defined as the entire received sample vector of the NPE dimension. Also,

Is defined as the overall background noise vector of the NPE dimension. Also,

Is defined as an NPE × NPE noise covariance matrix. Also,

Is defined as the entire multipath channel matrix of NPE × M (E + L−1).

next,

Optimal chip level linear MMSE weight vector for m th transmit antenna chip with D chip target delay

Is

become. In this case, the channel matrix coefficients are calculated from the pilot signal as described above.

Applying the matrix inversion lemma, equation 7 can be rewritten as:

In this case, the equalizer output chip SNR is

It is.

In addition, the equalizer output soft chip metric is

become.

When the j-th spreading code (or the product of the j-th Walsh code and the common scrambling code) is represented by C _j (k) (where | C _j (k) | 2 = 1), the (spreading factor is The output soft symbol of the despreader 120 (which is SF) is

become. However, A ^* shows the complex conjugate of A. Demapper 124 rescales the output soft symbols and converts them to bit values for symbol index n, code index j, and transmit antenna index m.

  The MMSE weighting vector in equation (7) is not optimal in terms of decoders 126,226. This is because it has been optimized without considering the outstanding behavior of on-time inter-stream interference in the despreader.

Enhanced Chip Level MMSE Weighting Vector for MIMO Multicode CDMA A MIMO multicode CDMA system that equalizes the received signal before despreading is described below. The space time equalizer applies a weighting vector with a coefficient that is a function of the spreading factor.

Orthogonal channel components with transmit chip value x _m (k) of J, ie

However,

Is a chip component corresponding to the j-th spreading code of the m-th transmitting antenna 114 (provided that

) Despreader output symbol metric in equation (11)

Is

It may be.

  It should be noted that orthogonal despreading is to introduce a gain factor of SF and a loss factor of J for the chip SNR.

  However, the actual SNR of the despreader output symbols is lower than equation (13) in the multicode CDMA system 100 where the code is reused. This is because on-time interstream interference operates differently from multipath interference or background noise in the despreading process. Furthermore, the MMSE weighting vector in equation (7) is not optimal in terms of decoders 126, 226. This is because it has been optimized without considering the significant behavior of on-time stream interference in the despreader 120. Therefore, as described further below, the SNR in equation (13) is difficult to achieve in practice.

Referring to equations (4)-(6) and equations (10)-(12), software that is equalized with a weighting vector (for the mth transmit antenna stream) and despread with the jth despread code The demodulation symbol can be written as follows:

Where the first and second terms represent the signal and interference components, respectively. More specifically, the equation (14)

Represents the desired symbol component after despreading, the on-time interstream interference component using the j th spreading code, and the multipath interference component, respectively. On-time inter-stream interference components that do not use the jth spreading code disappear during the despreading process. Conversely, the on-time inter-stream interference component using the jth spreading code has a spreading gain of SF due to despreading as the desired signal component does. (Of equation (14)

The covariance of the multipath interference component and the background noise component (shown by) is not changed by the despreading operation.

In view of the decoders 126, 226, the optimal MMSE weighting vector

Is

Must be minimized (ie, minimization must be performed on the target symbol). Therefore,

become.

By applying the matrix inversion lemma, the MMSE weighting vector that depends on the spreading factor is

Can be rewritten as:

The despreader output symbol SNR of the j th code of the m th transmit antenna 114 is

become.

  Equations (13) and (17) show that the variance of the on-time interference component of equation (17) is greater than that of equation (13) by a factor of SF / J. Therefore, unless a separate SF code is assigned for data transmission and the transmit antenna 114 completely reuses them (ie, J = SF), the achievable SNR in equation (17) is the expectation of equation (13). Lower than the SNR made. In practice, the number of codes assigned and reused is a spreading code allocation that depends on the data rate (e.g., a smaller number of codes for lower data rates, and a higher number for higher data rates). Due to the presence of large numbers of codes), control channels, voice channels, etc., they are often smaller than SF. Equations (8) and (16) show that the conventional chip-level optimized MMSE weighting vector is determined by the demappers 124 and 224 and the decoders 126 and 226 due to the mismatch of the power factor SF / J of the on-time inter-stream interference component. It shows that it is not optimal at the soft symbol level used. The conventional chip-level MMSE weighting vector underestimates the on-time inter-stream interference component because it does not take into account the effects of despreading and is therefore oriented in a sub-optimal direction. As a result, in one embodiment, using the weighting vector of equation (8), the actual symbol SNR is even lower than equation (17). It is far from the upper limit of equation (13). Since the number of spreading codes reused by multiple antennas is reduced, between the MIMO-CDMA optimized MMSE weighting vector in equation (16) and the conventional weighting vector in equation (8) The performance gap becomes larger.

  In deriving the enhanced chip level equalizer 118, the system model of FIGS. 1 and 2 is used. In this case, a plurality of antennas 114 reuse the same spreading code, and all antennas 114 and codes use substantially the same transmission power.

  Referring to equations (8) and (16), the component that changes the steering direction of the weighting vector is on-time interstream interference. Therefore, in a SISO multicode CDMA system where there is no inter-stream interference, the conventional chip-level MMSE weighted vector and the enhanced MMSE weighted vector point in the same direction (ie they are signal In a straight line in space). However, the weighting vector scaling can be different. The scaling factor is a function of the SNR and is enhanced with the conventional chip-level MMSE weighting vector if the demapper 124, 224 can accurately rescale the input soft symbols and yield an unbiased estimate. The MMSE weighting vectors have approximately the same decoding performance.

Enhanced Equalizer Generalization for Arbitrary Power and Code Placement Cases in MIMO Multicode CDMA In Equations (12)-(17), enhanced chip level for MIMO code CDMA receivers 104, 204 In generating the MMSE weighting vector, all M transmit antennas 114 reuse the same J spreading code,

Of the total transmit chip energy was assumed to be evenly divided and allocated to the JM stream and spreading code spread by the transmit antenna 114. Equivalently, each of the JM streams

It was assumed to have a chip energy of In this section, any code and power allocation case considers the existence of practical code division multiplexed pilots, control and voice channels, and unequal power allocation.

For this purpose, the m th transmit antenna 114 (m = 0, 1,..., M−1) and the j th of the spreading factor SF (j = 0, 1,. As chip energy assigned to the code

Define It contains the sum of chip energies assigned to all possible subcode trees of the jth code if they are used for the mth antenna 114. If the jth code is not used by the mth transmit antenna 114,

Is equal to 0. As mentioned above, the enhanced chip-level MMSE weighting vector results for MIMO multicode CDMA are

The transmission power is allocated to data transmission. In one embodiment, the control channel or pilot channel does not share transmit power with the MIMO data stream at the same time. Total transmit chip energy for the mth transmit antenna 114, including all channels such as data, pilot, control, etc.

Indicated by

Define the optimal MMSE weighting vector for the j th code and the m th transmission stream

Can be derived by the method used for equation (15). that is,

become.

Furthermore, applying the matrix inversion lemma, the equivalent weighting vector is

become. In this case, the despread output symbol SNR of the j th code of the m th transmit antenna 114 is

become.

  As illustrated in FIG. 4, a block error rate (BLER) simulation between a conventional equalizer (legacy EQ) and an enhanced equalizer (enhanced EQ) can be obtained with different chip SNR values, Ec. / No is compared. The simulation was performed for four transmit (ie, M = 4) antennas and four receive (ie, N = 4) antennas 116. The coding, rate matching, interleaving, constellation mapping, and receiver counterpart are configured according to 3GPP HSDPA HS-DSCH usage. In HS-DSCH, the chip rate is 3.84 Mcps, the frame length (or block length) is 2 ms, the SF is 16, and the number of modulation symbols per spreading code per frame is set to each antenna 114. On the other hand, 480. The modulation constellation is fixed to QPSK in the simulation. Thus, the total number of coded bits transmitted in the frame via the four antennas 114 using J spreading codes is 3840J. The four transmit antennas 114 are set to use the same amount of transmit chip energy as the same set of J spreading codes, and Ec / M is evenly divided and assigned to the J code channel of each antenna 114.

For simplicity, overhead channels (eg, common pilot channel, control channel, voice channel, etc.) were not modeled in the simulation. Therefore, the total BS transmission chip energy lor is equal to the HS-DSCH chip energy Ec. The turbo code in the 3GPP HSDPA specification is used for encoding and the code rate is maintained to be about 1/3 through simulation. The carrier frequency is set to 2 GHz. The background noise components of the four receive antennas 116 were modeled by a spatially uncorrelated white noise random process with a power spectral density of N ₀ . A chip space equalizer 118 (ie, the oversampling factor P is set to 1) with perfect synchronization and perfect estimation of channel coefficients and noise covariance was used in the simulation. When the multipath delay spans L chips, the space time equalizer time span E and target delay D were set to 3L chips and 2L-1 chips.

Figure 4 shows a standard SCM link level description 3km / h vehicle A model (6 paths, 2 degree BS angular spread, 35 degree MS angular spread, 10 wavelength BS antenna spacing, 0.5 wavelength Fig. 6 shows BLER performance for single code reuse in MS antenna spacing. The corresponding information data rate is set to 640 kbps and the number of encoded bits is 3840. As illustrated in FIG. 4, in a single code case, there is approximately 3 dB gain at a block error rate (BLER) of 10 ⁻² .

  It is observed that the gain from the enhanced equalizer decreases as the number of codes increases. As the number of codes approaches SF, the power balance between the on-time inter-stream interference part of (16) and the multipath interference and background noise part approaches that of the conventional equalizer of (8). Thus, there is a smaller improvement for 15 codes than for a single code.

  The conventional chip level MMSE weighting vector (8) provides a smaller signal-to-noise ratio than the enhanced MMSE weighting vector (16) for MIMO multicode CDMA yields the same code on different transmit antennas 114. . Looking at the comparison between (8) and (16), the two weighting vectors point in different directions even after compensating for the scaling factor.

In one embodiment, on-time inter-stream interference is a critical component.

Therefore, an enhanced MMSE weighting vector that takes into account the despreading effect is preferred.

  Those of skill in the art will understand that information and signals may be represented using any of a variety of techniques and techniques. For example, data, instructions, commands, information, signals, bits, symbols and chips that can be referred to throughout the above description are voltages, currents, electromagnetic waves, magnetic fields or magnetic particles, optical fields or optical particles, or any combination thereof It may be expressed by

  Those skilled in the art will further recognize that the various exemplary logic blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein are electronic hardware, computer software, or a combination of both. You will understand that it may be realized as. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Those skilled in the art can implement the described functionality in a variety of ways for each particular application, but such implementation decisions are interpreted as causing deviations from the scope of the present invention. Should not.

  Various exemplary logic blocks, modules, and circuits described in connection with the embodiments disclosed herein can be general purpose processors, digital signal processors (DSPs), application specific integrated circuits (ASICs), fields, and the like. Implementation or execution on a programmable gate array (FPGA), or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein can do. A general purpose processor may be a microprocessor, conventional processor, controller, microcontroller, state machine or the like. A processor may also be implemented as a combination of computing devices, eg, a combination of DSP and microprocessor, multiple microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. .

  The method or algorithm steps described in connection with the embodiments disclosed herein may be implemented directly in hardware, in software modules executed by a processor, or in a combination of the two. Can do. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, removable disk, CD-ROM, or any other form of storage medium known in the art. it can. A storage medium is coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor.

The processor and the storage medium can reside in the ASIC. The ASIC can reside in the user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal.

  Modules can include, but are not limited to, any of the following: Software object-oriented components (software components, class components, and task components), processes, methods, functions, attributes, procedures, subroutines, program code segments, drivers, firmware, microcode, circuits, data, databases, data structures, Software or hardware components such as tables, arrays, and variables.

  The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. May be. Accordingly, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (7)

Receiving a plurality of signals via a plurality of receiving antennas, wherein the received signal from each receiving antenna is one or more signals transmitted from a transmitting unit using a plurality of transmitting antennas; With a combination of
Processing the signal with a weighting vector having coefficients to generate a plurality of bitstreams , wherein the coefficients are at least partly a function of the spreading factor and obtained according to reuse of the spreading code by the transmitter; Processing the signal comprises processing the signal with a weighting vector having coefficients for generating a plurality of groups of equalized metric sequences corresponding to the plurality of transmit antennas;
A method comprising: The method of claim 1, wherein processing the signal comprises generating a plurality of chip sequences with a space-time equalizer.   The method of claim 1, wherein the weighting vector is a minimum mean square error (MMSE) weighting vector. 4. The method of claim 3 , further comprising separating the equalized metric sequence for each group into a plurality of modulation symbol sequences.   An apparatus comprising a receiver for receiving information according to the method of claim 1. A program for causing a computer to execute the method according to one of claims 1 to 4 . A computer readable storage device having a program stored therein, wherein the program causes a computer to perform the method according to one of claims 1 to 4 .

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