Method and apparatus for utilizing channel state information in a wireless communication system

ES 2 396 563 T3

DESCRIPTION

Procedure and apparatus for using channel status information in a wireless communication system

Background

Field

The present invention relates generally to data communication, and more specifically to a novel and improved method and apparatus for utilizing (in whole or in part) channel status information to provide improved performance for a wireless communication system.

Background

Wireless communication systems are widely used to provide various types of communication such as voice, data, etc. These systems can be based on code division multiple access (CDMA), time division multiple access (TDMA), orthogonal frequency division modulation (OFDM), or some other modulation techniques. OFDM systems can provide high performance for some channel environments.

In a terrestrial communication system (for example, a cellular system, a broadcasting system, a multipoint multi-channel distribution system (MMDs), and others), an RF modulated signal from a transmitter unit can reach a receiver unit through a series of transmission paths. The characteristics of transmission paths typically vary over time due to a number of factors such as fading and multipath.

To provide diversity against damaging path effects and improve performance, multiple transmit and receive antennas can be used. If the transmission paths between the transmitting and receiving antennas are linearly independent (that is, a transmission on one path is not formed as a linear combination of the transmissions on other paths), which is generally true to some extent, then the probability of correctly receiving a transmitted signal increases as the number of antennas increases. In general, diversity increases and performance improves as the number of transmitting and receiving antennas increases.

A multiple input multiple output (MIMO) communication system uses multiple (Nt) transmit antennas and multiple (Nr) receive antennas for data transmission. A MIMO channel can be decomposed into independent Nc channels, with Nc <min {Nt, Nr}. Each of the independent Nc channels is also referred to as a spatial subchannel of the MIMO channel and corresponds to one dimension. The MIMO system can provide improved performance by utilizing the additional dimensionalities created by the multiple transmit and receive antennas.

Therefore there is a need in the art for techniques to use channel state information (CSI) to take advantage of the additional dimensionalities created by a MIMO system to provide improved system performance.

WO98 / 09381 discloses a MIMO communication system that provides feedback of channel status information.

Summary

Aspects of the invention as set forth in the appended claims provide techniques for processing received signals in a multiple input multiple output (MIMO) communication system to recover transmitted signals, and to estimate the characteristics of a MIMO channel. Various receiver processing schemes can be used to obtain channel status information (CSI) indicative of the characteristics of the transmission channels used for data transmission. The cSI is then presented back to the transmitting system and used to adjust signal processing (eg, encoding, modulation, etc.). In this way, a high performance is achieved based on certain channel conditions.

A specific embodiment of the invention provides a method for transmitting data from a transmitter unit to a receiver unit in a MIMO communication system. According to the method, at the receiving unit, a number of signals are received through a series of receiving antennas, the signal received from each receiving antenna comprising a combination of one or more signals transmitted from the transmitting unit. The received signals are processed (for example, through a channel correlation matrix inversion scheme (CCMI), an unbiased least mean square error (UMMSE) scheme, or some other receiver processing scheme) to obtain the CSI indicative of characteristics of a series of transmission channels used for data transmission. The CSI is encoded and transmitted back to the transmitter unit. At the transmitting unit, the CSI is received from the receiving unit and the data is processed for transmission to the receiving unit based on the received CSI.

ES 2 396 563 T3

The submitted CSI can include a full CSI or a partial CSI. The total CSI includes sufficient characterization of the total bandwidth (eg, the amplitude and phase through the useful bandwidth) of the propagation path between all the transmitting and receiving antenna pairs. The partial CSI can include, for example, the signal-to-noise ratio plus interference (SNR) of the transmission channels. In the transmitter unit, the data for each transmission channel can be encoded based on the estimate of the SNR for the transmission channel, and the encoded data for each transmission channel can be modulated according to a selected modulation scheme based on the estimate of the SNR. For total CSI processing, modulation symbols are also pre-processed before transmission according to the received CSI.

The invention provides a method and apparatus as described in the appended claims.

Brief description of the drawings

The features, nature and advantages of the present invention will be more apparent from the detailed description set forth below when taken in conjunction with the drawings in which the same reference characters identify correspondingly throughout the document and in which :

FIG. 1 is a diagram of a multiple input multiple output (MIMO) communication system that can implement various aspects and embodiments of the invention;

Figures 2A and 2B are block diagrams of an embodiment of a MIMO transmitter system that can perform partial CSI processing and full CSI processing, respectively;

- Figure 3 is a block diagram of an embodiment of a MIMO transmitter system using orthogonal frequency division modulation (OFDM);

Figure 4 is a block diagram of a part of a MIMO transmitter system that can provide different processing for different types of transmission and that also employs OFDM;

- Figures 5 and 6 are block diagrams of two embodiments of a receiving system that has multiple (NR) receiving antennas and that can process a data transmission based on a channel correlation matrix inversion technique (CCMI) and in an unbiased least mean square error (UMMSE), respectively;

- Figure 7A shows the average performance for the MIMO system for three receiver processing techniques and for different SNR values; Y

- Figure 7B shows the cumulative probability distribution functions (CDF) for the three receiver processing techniques generated based on the histogram of the data.

Detailed description

FIG. 1 is a diagram of a multiple-input, multiple-output (MIMO) communication system 100 that can implement various aspects and embodiments of the invention. System 100 includes a first system 110 in communication with a second system 150. System 100 may function to employ a combination of time, frequency, and antenna diversity, (described below) to increase spectral efficiency, improve performance, and increase flexibility. In one aspect, the system 150 can function to determine the characteristics of the communication link and present the channel status information (CSI) back to the system 110, and the system 110 can function to adjust the processing (e.g., coding and modulation) of the data to be transmitted based on the presented CSI.

In system 110, a data source 112 provides data (i.e., bits of information) to a transmission (TX) data processor 114, which encodes the data according to a particular encoding scheme, interleaves (i.e., reorders) encoded data based on a particular interleaving scheme, and assigns the interleaved bits to modulation symbols for one or more transmission channels used to transmit the data. Encryption increases the reliability of data transmission. Interleaving provides temporal diversity for the encoded bits, allows data to be transmitted based on an average signal-to-noise ratio plus interference (SNR) for the transmission channels used for data transmission, combats fading, and also suppresses correlation between the coded bits used to form each modulation symbol. Interleaving can further provide frequency diversity if the coded bits are transmitted over multiple frequency subchannels. In accordance with one aspect of the invention, symbol encoding, interleaving, and mapping (or a combination thereof) are performed based on the total or partial CSI available to the system 110, as indicated in FIG. 1.

Coding, interleaving, and symbol assignment in the transmitter system 110 can be performed based on numerous schemes. A specific scheme is described in US Patent Application No. 09 / 776,073 entitled "CODING SCHEME FOR A WIRELESS COMMUNICATION SYSTEM", filed February 1, 2001 ..

The MIMO system 100 uses multiple antennas at both the transmitting and receiving end of the communication link. These transmitting and receiving antennas can be used to provide various

ES 2 396 563 T3 forms of spatial diversity, including transmit diversity and receive diversity. Spatial diversity is characterized by the use of multiple transmitting antennas and one or more receiving antennas. Transmission diversity is characterized by the transmission of data over multiple transmitting antennas. Typically, additional processing is performed on the data transmitted from the transmitting antennas to achieve the desired diversity. For example, data transmitted from different transmitting antennas can be delayed or reordered in time, encoded and interlaced through the transmitting antennas, and so on. Receive diversity is characterized by the reception of the transmitted signals at multiple receive antennas, and diversity is achieved simply by receiving the signals through different signal paths.

System 100 can operate in a number of different communication modes, each communication mode employing time, antenna, or frequency diversity, or a combination thereof. The communication modes may include, for example, a "diversity" communication mode and a "MIMO" communication mode. The diversity communication mode uses diversity to improve the reliability of the communication link. In a common application of diversity communication mode, which is also referred to as "pure" diversity communication mode, data is transmitted from all available transmitting antennas to a receiving receiving system. The pure diversity communications mode can be used in cases where the data rate needs are low or when the SNR is low, or when both are true. The MIMO communication mode employs antenna diversity at both ends of the communication link (i.e. multiple transmit antennas and multiple receive antennas) and is generally used both to improve reliability and to increase the capacity of the communication link. . The MIMO communication mode may further employ frequency and / or temporal diversity in combination with antenna diversity.

System 100 may further utilize orthogonal frequency division modulation (OFDM), which efficiently divides the operating frequency band into a number of (L) frequency subchannels (ie, frequency bins). In each time slot (i.e. a particular time slot that may depend on the bandwidth of the frequency subchannel), a modulation symbol may be transmitted on each of the L frequency subchannels.

System 100 can operate to transmit data over a series of transmission channels. As noted above, a MIMO channel can be decomposed into Nc independent channels, with Nc <min {Nt, Nr_}. Each of the independent Nc channels is also referred to as a spatial subchannel of the MIMO channel. For a MIMO system that does not use OFDM modulation, there may be only one frequency subchannel and each spatial subchannel can be referred to as a "transmission channel". For a MIMO system using OFDM modulation, each spatial subchannel of each frequency subchannel can be referred to as a transmission channel. And for an OFDM system that does not operate in MIMO communication mode, there is only one spatial subchannel and each frequency subchannel can be referred to as a transmission channel.

A MIMO system can provide improved performance by utilizing the additional dimensionalities created by multiple transmit and receive antennas. Although it is not necessarily required to know the cSI in the transmitter, an increase in system efficiency and performance is possible when the transmitter is equipped with cSI, which describes the transmission characteristics from the transmitting antennas to the receiving antennas. The cSI can be classified as "full cSI" or "partial cSI".

Total cSI includes sufficient characterization (e.g., amplitude and phase) across the entire system bandwidth (i.e., each frequency subchannel) for the propagation path between each transmit-receive antenna pair in the MIMO NtxNr matrix. The total CSI processing implies that (1) the channel characterization is available in both the transmitter and the receiver, (2) the transmitter calculates eigenmodes for the MIMO channel (described later), determines modulation symbols that will transmitted in the eigenmodes, preconditions (filters) the modulation symbols linearly, and transmits the preconditioned modulation symbols, and (3) the receiver performs supplementary processing (e.g., spatial matched filter) of the linear transmission processing based on channel characterization to calculate the NC spatial matched filter coefficients required for each transmission channel (i.e. each eigenmode). Total CSI processing further involves data processing (eg, selecting the appropriate modulation and coding schemes) for each transmission channel based on the channel's eigenvalue (described later) to obtain the modulation symbols.

The partial CSI may include, for example, the signal-to-noise ratio plus interference (SNR) of the transmission channels (i.e. the SNR ratio for each spatial subchannel for a MIMO system without OFDM modulation, or the SNR ratio for each subchannel frequency of each spatial subchannel for a MIMO system with OFDM modulation). Partial CSI processing may involve data processing (eg, selecting the appropriate modulation and coding schemes) for each transmission channel based on the SNR of the channel.

Referring to FIG. 1, a MIMO TX processor 120 receives and processes the modulation symbols from the TX data processor 114 to provide symbols suitable for transmission on the MIMO channel. The

ES 2 396 563 T3 processing performed by the MIMO TX processor 120 depends on whether full or partial CSI processing is used, and is described in more detail below.

For total CSI processing, the MIMO TX processor 120 can demultiplex and precondition the modulation symbols. And for partial CSI processing, the MIMO TX processor 120 can simply demultiplex the modulation symbols. The MIMO processing of full and partial CSI is described in more detail later. For a MIMO system that uses full CSI processing but no OFDM modulation, the MIMO TX processor 120 provides a stream of preconditioned modulation symbols for each transmitting antenna, one preconditioned modulation symbol per time slot. Each preconditioned modulation symbol is a linear (and weighted) combination of NC modulation symbols in a given time slot for the NC spatial subchannels, as described in more detail below. For a MIMO system employing total CSI processing and OFDM modulation, the 120 MIMO TX processor provides a stream of preconditioned modulation symbol vectors for each transmitting antenna, each vector including preconditioned modulation symbols L for the frequency L subchannels. for a given time slot. For a MIMO system that employs partial CSI processing but no OFDM modulation, the MIMO TX processor 120 provides a stream of modulation symbols for each transmitting antenna, one modulation symbol per time slot. And for a MIMO system employing partial CSI processing and OFDM modulation, the 120 MIMO Tx processor provides a stream of modulation symbol vectors for each transmitting antenna, each vector including L modulation symbols for the L frequency subchannels for a given time slot. For all the cases described above, each stream of modulation symbols (unconditioned or preconditioned) or vectors of modulation symbols is received and modulated by a respective modulator (MOD) 122, and transmitted through an associated antenna 124.

In the embodiment shown in FIG. 1, receiver system 150 includes an array of receiver antennas 152 that receive the transmitted signals and provide the received signals to respective demodulators (DEMODs) 154. Each demodulator 154 performs processing complementary to that performed in modulator 122. The demodulated symbols from all demodulators 154 are provided to a receive MIMO (RX) processor 156 and processed in a manner described later. The received modulation symbols for the transmission channels are then provided to an RX data processor 158, which performs processing complementary to that performed in the TX data processor 114. In a specific design, the RX data processor 158 provides bit values ​​indicative of the received modulation symbols, deinterleaves the bit values, and decodes the deinterleaved values ​​to generate decoded bits, which are then provided to the data collector 160. The deallocation, deinterleaving, and decoding of received symbols are complementary to the allocation, interleaving, and encoding of symbols performed in the transmitting system 110. Processing by receiver system 150 is described in greater detail later.

The spatial subchannels of a MIMO system (or more generally, the transmission channels in a MIMO system with or without OFDM modulation) typically experience different link conditions (for example, different fading and multipath effects) and can achieve a different SNR ratio. Consequently, the capacity of the transmission channels may be different from channel to channel. This capacity can be quantized by the information bit rate (ie, the number of information bits per modulation symbol) that can be transmitted on each transmission channel for a particular level of performance. In addition, the binding conditions typically vary with respect to time. As a result, the information bit rates supported for the transmission channels also vary with time. To more fully utilize the capacity of the transmission channels, the CSI describing the link conditions can be determined (usually at the receiving unit) and provided to the transmitting unit so that the processing can be adjusted (or adapted) appropriately. Aspects of the invention provide techniques for determining and using CSI (full or partial) to provide improved system performance.

MIMO transmitter system with partial CSI processing

Figure 2A is a block diagram of one embodiment of a MIMO transmitter system 110a, which is an embodiment of the transmitting portion of system 110 in Figure 1. Transmitter system 110a (not using OFDM modulation) can adjust its processing based on in the partial CSI presented by the receiving system 150. System 110a includes (1) a TX data processor 114a that receives and processes bits of information to provide modulation symbols and (2) a MIMO TX processor 120a that demultiplexes modulation symbols for the Nt transmit antennas.

The TX data processor 114a is an embodiment of the TX data processor 114 in FIG. 1, and many other designs can also be used for the TX data processor 114 and are within the scope of the invention. In the specific embodiment shown in FIG. 2A, the TX data processor 114a includes an encoder 202, a channel interleaver device 204, a puncturer device 206, and a symbol assignment element 208. Encoder 202 receives and encodes the information bits according to a particular encoding scheme to provide encoded bits. Channel interleaving device 204 interleaves the coded bits based on a particular interleaving scheme to provide diversity. The selective deletion device 206 selectively removes zero or more of the interleaved coded bits to provide the

ES 2 396 563 T3 desired number of coded bits. And the symbol assignment element 208 assigns the unremoved coded bit to modulation symbols for one or more transmission channels used to transmit the data.

Although not shown in Figure 2A for simplicity, the pilot data (eg, data from a known model) can be encoded and multiplexed with the processed information bits. The processed pilot data may be transmitted (eg, in a time division multiplexed manner) on all or a subset of the transmission channels used to transmit the information bits. The pilot data can be used at the receiver to perform channel estimation, as is known in the art and is described in more detail below.

As shown in FIG. 2A, the coding and modulation can be adjusted based on the partial CSI presented by the receiver system 150. In one embodiment, adaptive encoding is achieved by using a fixed base code (e.g., a 1/3 turbo code rate) and adjusting the selective removal to achieve the desired code rate, supported by the SNR ratio of the transmission channel used. to transmit data. Alternatively, different coding schemes may be used based on the displayed partial CSI (as indicated by the dashed arrow in block 202). For example, each of the transmission channels can be encoded with a separate code. With this coding scheme, a successive "cancellation / equalization and interference cancellation" receiver processing scheme can be used to detect and decode the data streams to obtain a more reliable estimate of the transmitted data streams. Such a receiver processing scheme is described by PW Wolniansky, et al in a paper entitled "V-BLAST: An Architecture for Achieving Very High Data Rates over the Rich-Scattering Wireless Channel," Proc. ISSE-98, Pisa, Italy.

For each transmission channel, the symbol assignment element 208 can be designated to sets of groups of coded bits not removed to form non-binary symbols and to assign the non-binary symbols to points in a signal constellation corresponding to a particular modulation scheme. (eg, QPSK, M-PSK, M-QAM, or some other scheme) selected for that transmission channel. Each assigned point corresponds to a modulation symbol. The number of information bits that can be transmitted for each modulation symbol for a particular level of performance (eg, a one percent packet error rate) depends on the SNR of the transmission channel. Thus, the coding scheme and modulation scheme for each transmission channel can be selected based on the displayed partial CSI. Channel interleaving can also be adjusted based on the displayed partial CSI (as indicated by the dotted arrow at block 204).

Table 1 lists various combinations of code rate and modulation scheme that can be used for a number of SNR ranges. The supported bit rate for each transmission channel can be achieved using any of a number of possible combinations of code rate and modulation scheme. For example, one bit of information per symbol can be achieved using (1) a 1/2 code rate and a QPSK modulation, (2) a 1/3 code rate and an 8-PSK modulation, (3) a 1/4 and 16-QAM code rate, or some other combination of code rate and modulation scheme. Table 1 uses QPSK, 16-QAM, and 64-QAM for the listed SNR ranges. Other modulation schemes such as 8-PSK, 32-QAM, 128-QAM, etc., can also be used and are within the scope of the invention.

Table 1

<td>SNR interval</td><td># of bits / information symbol</td><td>Symbol of modulation</td><td># of bits / symbol encoded</td><td>Cup of coding</td>

<td> 1,5-4,4</td><td> 1</td><td>QPSK</td><td> 2</td><td> 1/2</td>

<td> 4,4-6,4</td><td> 1.5</td><td>QPSK</td><td> 2</td><td> 3/4</td>

<td> 6,4-8,35</td><td> 2</td><td>16-QAM</td><td> 4</td><td> 1/2</td>

<td> 8,35-10,4</td><td> 2.5</td><td>16-QAM</td><td> 4</td><td> 5/8</td>

<td> 10,4-12,3</td><td> 3</td><td>16-QAM</td><td> 4</td><td> 3/4</td>

<td> 12,3-14,15</td><td> 3.5</td><td>64-QAM</td><td> 6</td><td> 7/12</td>

<td> 14,15-15,55</td><td> 4</td><td>64-QAM</td><td> 6</td><td> 2/3</td>

<td> 15,55-17,35</td><td> 4.5</td><td>64-QAM</td><td> 6</td><td> 3/4</td>

<td> &gt;17,35</td><td> 5</td><td>64-QAM</td><td> 6</td><td> 5/6</td>

ES 2 396 563 T3

The modulation symbols from the TX data processor 114a are provided to a MIMO TX processor 120a, which is an embodiment of the MIMO TX processor 120 in FIG. 1. In the MIMO TX processor 120a, a demultiplexer 214 demultiplexes the received modulation symbols into a number of (Nt) demodulation symbol streams, one stream for each antenna used to transmit the modulation symbols. Each stream of modulation symbols is provided to a respective modulator 122. Each modulator 122 converts the modulation symbols to an analog signal and further amplifies, filters, quadrature modulates, and upconverts the signal to generate a modulated signal suitable for transmission over the wireless link.

If the number of spatial subchannels is less than the number of available transmit antennas (ie NC &lt;NT) then various schemes can be used for data transmission. In one scheme, NC modulation symbol streams are generated and transmitted on a subset (ie, NC) of the available transmitting antennas. The remaining transmit antennas (Nt-Nc) are not used for data transmission. In another scheme, the additional degrees of freedom provided by the additional transmit antennas (Nt-Nc) are used to improve the reliability of data transmission. For this scheme each one or more data streams, possibly interlaced, can be encoded and transmitted over multiple transmitting antennas. Using multiple transmit antennas for a data stream increases diversity and improves reliability against damaging path effects.

MIMO transmitter system with full CSI processing

Figure 2B is a block diagram of one embodiment of a MIMO transmitter system 110b (not using OFDM modulation) that can process data based on a total CSI presented by the receiving system 150. The information bits are encoded, interleaved, and assigned to symbols by a TX data processor 114 to generate modulation symbols. Coding and modulation can be adjusted based on the total available CSI presented by the receiving system, and can be performed as described above for the MIMO transmitter system 110a.

In a TX MIMO processor 120b, a channel MIMO processor 212 demultiplexes the received modulation symbols into a number of (NC) modulation symbol streams, one stream for each spatial subchannel (e.g., eigenmode) used to transmit the modulation symbols. For total CSI processing, channel MIMO processor 212 preconditions the Nc modulation symbols in each time slot to generate Nt preconditioned modulation symbols as follows:

<td>η Xj</td><td></td><td> [ _and^Gn *^and2i '</td><td>^β12’ ^22 ’</td><td> __________1</td><td></td><td>i -------------------- 1_____</td>

<td>M</td><td></td><td></td><td></td><td></td><td></td><td>M</td>

<td></td><td></td><td>^andN_Tiy</td><td></td><td></td><td></td><td>-----------------1 or OR ___</td>

Equation (1) in which b-ι, b2, ... and bNc are respectively the modulation symbols for the spatial subchannels 1, 2, ... Nnc, where each of the Nc modulation symbols can be generated using , for example, MPSK, M-QAM, or some other modulation scheme;

e, j are elements of an eigenvector matrix E related to the transmission characteristics from the transmitting antennas to the receiving antennas; Y

X1, X2 ,. Xnt are the preconditioned modulation symbols that can be expressed as:

= b_t + b₂ E₁₂ + ... 4b_Nc ,

<img file="ES2396563T3_D0001.tif" />

^Y ^ N_T ~ ‘ ^andNTl F ^ 2 ' ^andNT2 * ··· ^ N_c ' ^andN_TN_c ’

The eigenvector matrix E can be computed by the transmitter or is provided to the transmitter by the receiver.

ES 2 396 563 T3

For total CSI processing, each preconditioned modulation symbol, x, for a particular transmitting antenna represents a linear combination of modulation symbols (weighted) for up to N_Cspatial subchannels. The modulation scheme used for each modulation symbol x is based on the effective SNR ratio of that eigenmode and is proportional to an eigenvalue, λ, (described later). Each of the Nc modulation symbols used to generate each preconditioned modulation symbol may be associated with a different constellation of signals. For each time slot, the Nt preconditioned modulation symbols generated by channel MIMO processor 212 are demultiplexed by a demultiplexer 214 and provided to NT modulators 122.

Total CSI processing can be performed based on available CSI and selected transmit antennas. Total CSI processing can also be selectively and dynamically enabled and disabled. For example, Total CSI processing can be enabled for a particular data stream and disabled for some other data streams. Total CSI processing can be enabled under certain conditions, for example when the communication link has a suitable SNR ratio.

MIMO transmitter system with OFDM modulation

Figure 3 is a block diagram of one embodiment of a MIMO transmitter system 110c that uses OFDM modulation and that can adjust its processing based on a full or partial CSI. The information bits are encoded, interleaved, selectively stripped, and assigned to symbols by a TX data processor 114 to generate modulation symbols. Coding and modulation can be adjusted based on the available partial or full CSI presented by the receiving system. For a MIMO system with OFDM modulation, modulation symbols can be transmitted on multiple frequency subchannels and from multiple transmitting antennas. When operating in a pure MIMO communication mode, the transmission on each frequency subchannel and from each transmitting antenna represents unduplicated data.

In a MIMO processor 120c, a demultiplexer 310 (DEMUX) receives and demultiplexes the modulation symbols into a number of subchannel symbol streams, S1 through SL, a subchannel symbol stream for each frequency subchannel used to transmit the signals. symbols.

For total CSI processing, each subchannel symbol stream is then provided to a respective subchannel MIMO processor 312. Each sub-channel MIMO processor 312 demultiplexes the received sub-channel symbol stream into a number of (up to NC) symbol sub-streams, one symbol sub-stream for each spatial sub-channel used to transmit the modulation symbols. For total CSI processing in an OFDM system, eigenmodes are obtained and applied on a subchannel per frequency basis. Thus, each subchannel MIMO processor 312 preconditions up to Nc modulation symbols according to equation (1) to generate preconditioned modulation symbols. Each modulation symbol preconditioned for a particular transmitting antenna of a particular frequency subchannel represents a linear combination of modulation symbols (weighted) for up to NC spatial subchannels.

For total CSI processing, the (up to) NT preconditioned modulation symbols generated by each subchannel MIMO 312 processor for each time slot are demultiplexed by a respective 314 demultiplexer and provided to (up to) Nt combiners 316a to 316t of symbols. For example, the subchannel MIMO processor 312a assigned to frequency subchannel 1 can provide up to NT preconditioned modulation symbols for antenna frequency subchannel 1 to Nt. Similarly, the subchannel MIMO processor 3121 assigned to frequency subchannel L can provide up to Nt symbols for antenna frequency subchannel L 1 to Nt.

And for partial CSI processing, each subchannel symbol stream, S, is demultiplexed by a respective demultiplexer 314 and provided to (up to) Nt symbol combiners 316a to 316t. Processing by subchannel MIMO processor 312 is skipped for partial CSI processing.

Each combiner 316 receives the modulation symbols for up to L frequency subchannels, combines the symbols for each time slot into a vector V of modulation symbols, and provides the vector of modulation symbols to the next stage of processing (i.e., modulator 122).

The MIMO processor 120c thus receives and processes the modulation symbols to provide Nt modulation symbol vectors, V1 to Vt, a vector of modulation symbols for each transmitting antenna. Each modulation symbol vector V covers a single time slot, and each element of the modulation symbol vector V is associated with a specific frequency subchannel presenting a single subcarrier on which the modulation symbol is carried. If you are not operating in a "pure" MIMO communication mode, some of the modulation symbol vectors may present redundant or duplicate information on frequency subchannels for different transmitting antennas.

Figure 3 also shows one embodiment of modulator 122 for OFDM modulation. Modulation symbol vectors V1 to Vt from MIMO processor 120c are provided to modulators 122a to 122t,

ES 2 396 563 T3 respectively. In the embodiment shown in FIG. 3, each modulator 122 includes an inverse fast Fourier transform (IFFT) 320, a cyclic prefix generator 322, and an up converter 324.

The IFFT 320 converts each received modulation symbol vector into its time domain representation (which is referred to as an OFDM symbol) using the IFFT. The IFFT 320 can be designed to perform the IFFT on any number of frequency subchannels (eg, 8, 16, 32, etc.). In one embodiment, for each vector of modulation symbols converted to an OFDm symbol, the cyclic prefix generator 322 repeats a portion of the OFDM symbol time domain representation to form a transmit symbol for a specific transmit antenna. The cyclic prefix ensures that the transmit symbol retains its orthogonal properties in the presence of dispersion due to multipath delay, thereby improving performance against damaging path effects. The implementation of IFFT 320 and cyclic prefix generator 322 is known in the art and is not described in detail herein.

The time domain representations of each cyclic prefix generator 322 (that is, the transmit symbols for each antenna) are then processed (for example, converted to an analog signal, modulated, amplified, and filtered) by a converter 324 upstream to generate a modulated signal, which is then transmitted from the respective antenna 124.

OFDM modulation is described in greater detail in a document entitled "Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come", by John AC Bingham, IEEE Communications Magazine, May 1990.

A number of different types of transmission (eg voice, signaling, data, pilot, etc.) can be transmitted through a communication system. Each of these transmissions may require different processing.

Figure 4 is a block diagram of a part of a MIMO transmitter system 110d that can provide different processing for different types of transmission and that also employs OFDM modulation. The aggregated input data, which includes all bits of information to be transmitted by system 110d, is provided to a demultiplexer 408. The demultiplexer 408 demultiplexes the input data into a number of (K) channel data streams, B1 to BK. Each channel data stream can correspond to, for example, a signaling channel, a broadcast channel, a voice call or a packet data transmission. Each channel data stream is provided to a respective TX data processor 114 which encodes the data using a particular encoding scheme selected for that channel data stream, interleaves the encoded data based on a particular interleaving scheme, and allocates the bits. interleaved to modulation symbols for one or more transmission channels used to transmit that channel data stream.

Encoding can be done on a per-stream basis (ie, on each channel data stream, as shown in Figure 4). However, encoding can also be performed on the aggregated input data (as shown in Figure 1), on a number of channel data streams, on a part of a channel data stream, through a set of frequency subchannels, through a set of spatial subchannels, through a set of frequency subchannels and spatial subchannels, through each frequency subchannel, on each modulation symbol or on some other unit of time, space and frequency.

The modulation symbol stream from each TX data processor 114 may be transmitted over one or more frequency subchannels and through one or more spatial subchannels of each frequency subchannel. A MIMO TMX processor 120d receives the modulation symbol streams from the TX data processors 114. Depending on the communication mode to be used for each modulation symbol stream, the MIMO TX processor 120d may demultiplex the modulation symbol stream into a number of subchannel symbol streams. In the embodiment shown in Fig. 4, the modulation symbol stream S1 is transmitted on one frequency subchannel and the modulation symbol stream SK is transmitted on L frequency subchannels. The modulation stream for each frequency subchannel is processed by a respective subchannel MIMO processor 412, demultiplexed by demultiplexer 414, and combined by combiner 416 (for example, similar to that described in Figure 3) to form a vector of modulation symbols for each transmitting antenna.

In general, a transmitter system encodes and modulates data for each transmission channel based on information that describes the transmission capacity of that channel. This information is usually in the form of the full CSI or the partial CSI described above. The full / partial CSI for the transmission channels used for data transmission is normally determined in the receiving system and reported back to the transmitting system, which then uses the information to adjust the coding and modulation appropriately. The techniques described herein can be applied to multiple parallel transmission channels supported by MIMO, OFDM modulation, or any other communication scheme (eg, a CDMA scheme) that can support multiple parallel transmission channels.

MIMO processing is described in greater detail in US Patent Application Serial No. 09 / 532,492, entitled "HIGH EFFICIENCY, HIGH PERFORMANCE COMMUNICATIONS SYSTEM EMPLOYING MULTI-CARRIER MODULATION," filed March 22, 2000.

ES 2 396 563 T3

MIMO receiver system

Aspects of the invention provide techniques for processing received signals in a MIMO system to recover transmission data and to estimate the characteristics of the MIMO channel. The estimated channel characteristics can then be presented back to the transmitting system and used to adjust signal processing (eg, coding, modulation, etc.). In this way, high performance is achieved based on the determined channel conditions. The receiver processing techniques described herein include a channel correlation matrix inversion (CCMI) technique, an unbiased mean square error (UMMSE) technique, and a CSI-total technique, all described in greater detail below. Other receiver processing techniques can also be used and are within the scope of the invention.

FIG. 1 shows a receiver system 150 that has multiple receive antennas (NR) and is capable of processing a data transmission. The signals transmitted from up to NT transmitting antennas are received by each of the NR antennas 152a to 152r and routed to a respective demodulator (DEMOD) 154 (which is also referred to as a front end processor). For example, the receiving antenna 152a can receive a number of transmitted signals from a number of transmitting antennas, and the receiving antenna 152r can similarly receive multiple transmitted signals. Each demodulator 154 conditions (eg, filters and amplifies) the received signal, downconverts the conditioned signal to an intermediate frequency or baseband, and digitizes the downconverted signal. Each demodulator 154 may further demodulate the digitized samples with a received pilot signal to generate the received modulation symbols, which are provided to the MIMO RX processor 156.

If OFDM modulation is used for data transmission, each demodulator 154 additionally performs complementary processing to that performed by modulator 122 shown in Figure 3. In this case, each demodulator 154 includes an FFT processor (not shown) that generates transformed representations. of the samples and provides a stream of modulation symbol vectors, each vector including L modulation symbols for L frequency subchannels. The modulation symbol vector streams from the FFT processors of all demodulators are then provided to a demultiplexer / combiner (not shown in Figure 5), which first "funnels" the modulation symbol vector stream from each FFT processor. in a number of (up to L) subchannel symbol streams. Each of the (up to) L subchannel symbol streams can then be provided to a MIMO RX processor 156.

For a MIMO system that does not use OFDM modulation, a 156 MIMO RX processor can be used to perform MIMO processing for the modulation symbols of the Nr receiving antennas. And for a MIMO system using OFDM modulation, a 156 MIMO RX processor can be used to perform MIMO processing for the modulation symbols of the Nr receiving antennas for each of the L frequency subchannels used for data transmission.

In a MIMO system with Nt transmitting antennas and Nr receiving antennas, the signals received at the output of the Nr receiving antennas can be expressed as:

r = Hx + n, Equation (2) in which r_is the vector of received symbols (that is, the output of the vector N_R x 1 of the MIMO channel, as measured in the receiving antennas), H is the channel coefficient matrix Nr x Nt that gives the channel response for the Nt transmitting antennas and the Nr receiving antennas in a specific time, x is the vector of transmitted symbols (that is, the vector input Nt x 1 to the MIMO channel), and n_ is a vector Nr x 1 representing noise plus interference. The received symbol vector r includes Nr modulation symbols of Nr signals received through Nr receive antennas at a specified time. Similarly, the vector x_ of transmitted symbols includes Nt modulation symbols in Nt signals transmitted through Nt transmitting antennas at a specified time.

MIMO receiver using CCMI technique

For the CCMI technique, the receiving system first performs a channel matched filter operation on the vector r of received symbols and the filtered output can be expressed as:

H^WT = +, Equation (3) in which the exponent ^H represents a complex and transposed conjugate. An R-squared matrix can be used to denote the product of the channel coefficient matrix H with its transposed conjugate H ^ (i.e., R = H^h H).

The channel coefficient matrix H can be obtained, for example, from the pilot symbols transmitted along with the data. To perform optimal reception and estimate the SNR of the transmission channels, it is often convenient to insert some known symbols into the transmission data stream and transmit the symbols

ES 2 396 563 T3 known on one or more transmission channels. Such known symbols are also referred to as pilot symbols or pilot signals. Procedures for estimating a single transmission channel based on a pilot signal or data transmission can be found in a number of documents available in the art. Such a channel estimation procedure is described by F. Ling in a document entitled "Optimal Reception, Performance Bound, and Cutoff-Rate Analysis of References-Assisted Coherent CDMA Communications with Applications" IEEE Transaction on Communication, October 1999. This or some other channel estimation procedure can be extended to a matrix to obtain the channel coefficient matrix H.

An estimate of the vector of transmitted symbols, x ', can be obtained by multiplying the vector H^wr of signals with the inverse (or pseudoinverse) of R, which can be expressed as:

x '= R^_1H^wr = X + R¹H^{/ Í}n Equation (4) = x + n '.

From the above equations it can be seen that the vector x of transmitted symbols can be covered by adaptive filtering (i.e., multiplying with the matrix H^h), from the vector r of received symbols and then multiplying the filtered result with the matrix R^-1 inverse square.

The SNR of the transmission channels can be determined as follows. The matrix φ_nn The autocorrelation of the noise vector n is first calculated from the received signal. In general, φ nn is a Hermitian matrix, that is, it is complex, conjugate, and symmetric. If the components of the noise channel are not correlated and are also independent and identically distributed (iid), the autocorrelation matrix φ nn of the noise vector n can be expressed as:

&lt;L = &lt;7.²I, and Equation (5) you.

where I is the identity matrix (that is, ones along the diagonal and zeros otherwise) and is the noise variance of the received signals. The autocorrelation matrix φ nn- of the noise vector n 'processed later (that is, after the adapted filtering and previous multiplication with the matrix R^-1) can be expressed as:

<img file="ES2396563T3_D0002.tif" />

<img file="ES2396563T3_D0003.tif" />

Equation (6)

From equation (6), the variance ^ση noise of the element of order i of the vector n 'of processed noise cr²k, I. ₁ subsequently is equal to where is the element of the diagonal of R of order i. For a MIMO system that does not use OFDM modulation, the element of order i is representative of the receiving antenna of order i. And if OFDM modulation is used, then the subscript "i" can be decomposed into a subscript "jk", where "j" represents the frequency subchannel of order j and "k" represents the spatial subchannel of order k corresponding to the antenna of receipt of order k.

For the CCMI technique, the SNR relation of the element of the vector of symbols received of order i after processing (that is, the element of order i of x ') can be expressed as:

ES 2 396 563 T3

<img file="ES2396563T3_D0004.tif" />

Equation (7)

X

If the variance of the transmitted symbol I '1 of order i is equal to one (1.0) over the mean, the SRN ratio of the vector of received symbols can be expressed as:

a

The noise variance can be normalized by scaling the element of order i of the vector of received symbols

<img file="ES2396563T3_D0005.tif" />

The scaled signals from the receiving NR antennas can be summed together to form a combined signal, which can be expressed as

<img file="ES2396563T3_D0006.tif" />

Equation (8)

The SNR of the combined signal, SNRtotal, would then have a maximum combined SNR that is equal to the sum of the SNR of the signals from the receiving NR antennas. The combined SNR relationship can be expressed as:

swr, „„, == 4-Σγ ^AND™ w ί = 1 ^ση i = l Oí

Figure 5 shows an embodiment of a MIMO RX processor 156a, which can implement the CCMI processing described above. In the MIMO RX processor 156a, the modulation symbols from the Nr receive antennas are demultiplexed by a multiplexer 512 to form a stream of vectors r of received modulation symbols. The channel coefficient matrix H can be estimated based on pilot signals similar to conventional pilot-assisted single and multi-carrier systems, as is known in the art. The matrix R is then calculated according to R = H^h H as shown above. The received modulation symbol vectors r are then filtered by a fitting filter 514 that previously multiplies each vector r with the matrix H^h of conjugate and transposed channel coefficient, as shown in equation (3). The filtered vectors are previously multiplied further by a multiplier 516 with the matrix R^-1 inverse square to form an estimate x 'of the vector x of transmitted modulation symbols, as shown above in equation (4).

For certain communication modes, the sub-channel symbol streams from all antennas used for transmission of the channel data stream may be provided to a combiner 518, which combines redundant information across time, space, and frequency. The combined modulation symbols x "are then provided to an RX data processor 158. For some other communication modes, the estimated modulation symbols x 'may be provided directly to the RX data processor 158 (not shown in FIG. 5).

The MIMO RX processor 156a thus generates a series of independent symbol streams corresponding to the series of transmission channels used in the transmitter system. Each symbol stream includes post-processed modulation symbols, which correspond to modulation symbols prior to full / partial CSI processing in the transmitting system. The symbol streams (post-processed) are then provided to the RX data processor 158.

In RX data processor 158, each modulation symbol post-processed symbol stream is provided to a respective demodulation element that implements a demodulation scheme (e.g., M-PSK, M-QAM) that is complementary to the modulation used in the transmitter system for the transmission channel being processed. For MIMO communication mode, the demodulated data from all assigned demodulators can then be decoded independently or multiplexed into one stream.

ES 2 396 563 T3 of channel data and then decoded, depending on the coding and modulation procedure used in the transmitter unit. Each channel data stream can then be provided to a respective decoder that implements a decoding scheme complementary to that used in the transmitter unit for the channel data stream. The decoded data from each decoder represents an estimate of the transmitted data for that channel data stream.

The estimated modulation symbols x 'and / or the combined modulation symbols x' are also provided to a 520 CSI processor, which determines the total or partial CSI for the transmission channels and provides the total / partial CSI to be displayed accordingly. back to the transmitter system 110. For example, the 520 CSI processor can estimate the matrix φ_nn transmission channel noise covariance of order i, based on the received pilot signal, and then calculating the SNR based on equations (7) and (9). The SNR ratio can be estimated similarly to conventional pilot-assisted single and multi-carrier systems, as is known in the art. The SNR for the transmission channels comprises the partial CSI that is presented back to the transmitting system. The modulation symbols are additionally provided to a channel estimator 522 and a matrix processor 524 which respectively estimate the channel coefficient matrix H and derive the square R matrix. A controller 530 couples to MIMO RX processor 156a and RX data processor 158 and directs the operation of these units.

MIMO receiver using the UMMSE technique

For the UMMSE technique, the receiving system performs a multiplication of the vector r of received symbols with a

V matrix M to obtain an estimate - initial MMSE of the vector x of transmitted symbols, which can be expressed as:

Equation (10)

The matrix M is selected such that the mean square error of the error vector e between the initial AA estimate 5 MMSE and the vector x of transmitted symbols is minimized (ie, e = X - x).

To determine M, a cost function ε can be initially expressed as:

<img file="ES2396563T3_D0007.tif" />

To minimize the cost function ε, a derivative of the cost function with respect to M can be taken, and the result can be set to zero, as follows:

<img file="ES2396563T3_D0008.tif" />

Using the equalities E {xx^H} = I, E {rr ^H} = HH^h + φ _nn and E {rx^H} = H, the following is obtained:

<img file="ES2396563T3_D0009.tif" />

Therefore, the matrix M can be expressed as:

-i

Equation (11)

V

Based on equations (10) and (11), the initial MMSE estimate of the vector x of transmitted symbols can be determined as:

ES 2 396 563 T3 χ = Mr

Equation (12) = H^H(HH + ¿,,) - 'r.

To determine the SNR ratio of the transmission channels for the UMMSE technique, the signal component

Y can be initially determined based on the mean of - given x, mean calculated over the additive noise, which can be expressed as:

E [x | x] = £ [Mr | x]₌ h^h(hh + o^_1£ [! ·] = ^ (^ + ^) ¾ = Vx, where the matrix V is defined as:

v = {v, J = MH = H^h(H H^h +^)-^ .

Using identity

<img file="ES2396563T3_D0010.tif" />

matrix V can be expressed as:

<img file="ES2396563T3_D0011.tif" />

The i-order element of the initial MMSE x estimate, X can be expressed as:

Equation (13)

V

If all elements of - are uncorrelated and have a mean of zero, the expected value of the element

And of order i of - can be expressed as:

Former₍. [χ] = ν ^ ·

Equation (14)

As shown in equation (14), X, · is a biased estimate of x. This bias can be removed for improved receiver performance according to the UMMSE technique. An unbiased estimate of x can be obtained by dividing x by v,. Thus, the estimation of the unbiased least mean square error of x, X, can be

D¹ be obtained by previously multiplying the unbiased estimate by a diagonal V matrix, as follows:

ES 2 396 563 T3

<img file="ES2396563T3_D0012.tif" />

in which

<img file="ES2396563T3_D0013.tif" />

jr

To determine noise plus interference, the error e between the unbiased estimate and the vector x of transmitted symbols can be expressed as:

<img file="ES2396563T3_D0014.tif" />

The autocorrelation matrix of the error vector can be expressed as:

<img file="ES2396563T3_D0015.tif" />

The variance of the element of order i of the error vector e is equal to u ,,. The elements of the error vector é are correlated. However, sufficient interleaving can be used so that the correlation between the elements of the error vector can be ignored and only the variance affects the performance of the system.

If the channel noise components are uncorrelated and iid, the channel noise correlation matrix can be expressed as shown in equation (5). In this case, the autocorrelation matrix of the error vector é can be expressed as:

<img file="ES2396563T3_D0016.tif" />

And if the noise components of the channel are uncorrelated, then

Equation (17)

The SNR ratio of the demodulator output corresponding to the transmitted symbol of order i can be expressed as:

<img file="ES2396563T3_D0017.tif" />

<img file="ES2396563T3_D0018.tif" />

If the variance,, of the processed received symbols, x ,, is equal to one (1.0) about the mean, the SNR ratio of the vector of received symbols can be expressed as:

ES 2 396 563 T3

Figure 6 shows an embodiment of a 156b MIMO RX processor, which can implement the UMMSE processing described above. Similar to the CCMI procedure, the matrices H and φ_nn they can be initially estimated based on received pilot signals and / or data transmissions. The weighting coefficient matrix M is then calculated according to equation (11). In the MIMO RX processor 156b, the modulation symbols from the Nr receive antennas are multiplexed by a multiplexer 612 to form a stream of vectors r of received modulation symbols. Then the received modulation symbol vectors r are previously multiplied by a multiplier 614 with the matrix M to form an estimate 5 of the transmitted symbol vector x, as shown above in equation (10). The estimate ϊ is also multiplied previously

D¹ using a multiplier 616 with the diagonal matrix V to form an unbiased estimate of the vector x of transmitted symbols, as shown above in equation (15).

Again, depending on the particular communication mode being implemented, the subchannel symbol streams from all antennas used for transmission of the channel data stream may be provided to a combiner 618, which combines redundant information across time, space and frequency. The combined modulation symbols 5 "are then provided to RX data processor 158. And for some other modes of communication, the estimated modulation symbols 5 can be provided directly to an RX data processor 158.

The unbiased estimated modulation symbols * and / or the combined modulation S''s are also provided to a 620 CSI processor, which determines the full or partial CSI for the transmit channels and provides the full / partial CSI to be returned. to the transmitter system 110. For example, the 620 CSI processor can estimate the i-th transmission channel SNR according to equations (16) to (18). The SNR for the transmission channels comprises the partial CSI that is presented back to the transmitting system. The optimal matrix M as calculated in equation (11) should already minimize the norm of the error vector. Dv is calculated according to equation (16).

MIMO receiver using total CSI technique

For the total CSI technique, the signals received at the output of the Nr receiving antennas can be expressed as indicated above in equation (2), which is:

r = Hx + n.

The decomposition of the eigenvector of the Hermitian matrix formed by the product of the channel matrix with its conjugate-transpose can be expressed as:

<img file="ES2396563T3_D0019.tif" />

in which E is the matrix of eigenvectors, and Λ is a diagonal matrix of eigenvalues, both of dimensions Nt x Nt. The transmitter preconditions a set of Nt modulation symbols b using the eigenvector matrix E, as shown above in equation (1). The modulation symbols (preconditioned) transmitted from the Nt transmitting antennas can therefore be expressed as:

x = Eb.

Since the matrix H^h H is Hermitian, the matrix of eigenvectors is unitary. Therefore, if the elements of b have the same power, the elements of x also have the same power. The received signal can then be expressed as:

Equation (19)

The receiver performs a channel matched filter operation, followed by multiplication by the correct eigenvectors. The result of the channel matched filter and the multiplication operations is a vector z that can be expressed as:

ES 2 396 563 T3

<img file="ES2396563T3_D0020.tif" />

where the new noise term has a covariance that can be expressed as:

E (ññ^w) = £ (E^HH^HnnHE) = E^rtH^HHE = A, Equation (21) that is, the noise components are independent with a variance given by the eigenvectors. The SRN relation of the component of order i of z is λ the element of order i diagonal of Λ.

The processing of total CSI is described in greater detail in the aforementioned US Patent Application Serial No. 09 / 532,492.

The receiver embodiment shown in figure 5 can also be used to implement the full CSI technique. The received modulation symbol vectors r are filtered by the adjustment filter 514 that previously multiplies each vector r with the matrix H^h conjugate-transpose channel coefficient, as shown above in equation (20). The filtered vectors are also multiplied previously by a multiplier 516 with the eigenvectors E^H correct to form an estimate z of the modulation symbol vector b, as shown above in equation (20). For the total CSI technique, the matrix processor 524 is configured to provide the eigenvectors E^H correct. Post processing (eg, by combiner 518 and RX data processor 158) can be accomplished as described above.

For the total CSI technique, the transmitter unit can select a coding scheme and a modulation scheme (ie, a constellation of signals) for each of the eigenvectors based on the SRN relationship that is given by the eigenvalue. As long as the channel conditions do not change significantly in the interval between the time the CSI is measured at the receiver and the time it is presented and used to precondition transmission at the transmitter, the performance of the communications system can be equivalent to that of the of a set of independent AWGN channels with known SNR relationships.

Present full or partial CSI back to the transmitting system

By using either the partial CSI technique (eg CCMI or UMMSE) or the full CSI technique described herein, the SNR of each transmission channel can be obtained for the received signals. The SNR ratio determined for the transmission channels can then be presented back to the transmitter system via a reverse channel. By feeding back the SNR values ​​of the transmitted modulation symbols for the transmission channels (that is, for each spatial subchannel and possibly for each frequency subchannel if OFDM modulation is used), it is possible to implement adaptive processing (for example, coding and adaptive modulation) to improve the utilization of the MIMO channel. For partial CSI feedback techniques, adaptive processing can be achieved without full CSI. For total CSI feedback techniques, sufficient information (and not necessarily explicit eigenmodes and values) is fed back to the transmitter to facilitate the calculation of eigenvalues ​​and eigenmodes for each frequency subchannel used.

For the CCMI technique, the SNR values ​​of

SNR ^ l / σ ^ _l or '' ⁿ “For the feed back to the transmitter. For the UMMSE technique, example, SNR, = E [| x¡ \²] / u¡¡ó SNR, = 1 / u¡¡ for the symbol received in the transmission channel of order i, calculating u¡¡ as shown previously in equations (16) and (17)), they are fed back to the transmitter. And for the total CSI technique, the SNR values ​​of the received modulation symbols (for example

<img file="ES2396563T3_D0021.tif" />

the received modulation symbols (for example, the i-order transmission channel) are received symbol at the SNR values ​​of the received modulation symbols (for

<img file="ES2396563T3_D0022.tif" />

or '' ' ⁿ for the symbol received in the transmission channel of order i, in which λ ¡¡is the eigenvalue of the matrix R squared) they can be fed back to the transmitter. For the total CSI technique, the E eigenmodes can be further determined and fed back to the transmitter. For the partial and total CSI techniques, the SNR ratio is used in the transmitting system to fine-tune the data processing. And for the total CSI technique, the eigenmodes are additionally used to precondition the modulation symbols before transmission.

The CSI presented back to the transmitter can be sent entirely, differentially, or a combination thereof. In one embodiment, the full or partial CSI is presented periodically and the differential updates are sent based on the previously transmitted CSI. As an example for the total CSI, the updates can be corrections (based on an error signal) to the presented eigenmodes. Eigenvalues ​​do not normally change as quickly as eigenmodes, so they can be updated to a lower

ES 2 396 563 T3 speed. In another embodiment, the CSI is sent only when there is a change (eg, if the change exceeds a particular threshold) that can lower the effective speed of the feedback channel. As an example for partial CSI, SNR relationships can be sent back (eg differentially) only when they change. For an OFDM system (with or without MIMO), the frequency domain correlation can be exploited to allow reduction in the amount of CSI to be fed back. As an example for an OFDM system using partial CSI, if the SNR ratio corresponding to a particular spatial subchannel for M frequency subchannels is the same, the SNR ratio and the first and last frequency subchannels for which this condition is true can introduce oneself. Other compression and feedback channel error recovery techniques to reduce the amount of data to be fed back to CSI can also be used and are within the scope of the invention.

Referring back to Figure 1, the full or partial CSI (e.g., channel SNR ratio) determined by the MIMO RX processor 156 is provided to a TX data processor 162 which processes the CSI and provides processed data to one or more. more modulators 154. Modulators 154 further condition the processed data and transmit the CSI back to the transmitting system 110 via a reverse channel.

In system 110, the transmitted feedback signal is received by antennas 124, demodulated by demodulators 122, and provided to RX data processor 132. The RX data processor 132 performs supplemental processing to that performed by the TX data processor 162 and retrieves the displayed full / partial CSI, which is then provided, and is used to adjust the processing by, the TX data processor 114 and the processor. 120 MIMO TX.

The transmitting system 110 may adjust (ie, tailor) its processing based on the full / partial CSI (eg, SNR information) of the receiving system 150. For example, the encoding for each transmission channel can be adjusted so that the information bit rate matches the transmission capacity supported by the channel SNR ratio. Furthermore, the modulation scheme for the transmission channel can be selected based on the channel SNR ratio. Other processing (eg, interlacing) can also be adjusted and are within the scope of the invention. Adjusting the processing for each transmission channel based on the SNR ratio determined for the channel allows the MIMO system to achieve high performance (ie high overall throughput or bit rate for a particular level of performance). Adaptive processing can be applied to a single carrier MIMO system or a multi-carrier based MIMO system (eg, a MIMO system using OFDM modulation).

Adjustment in coding and selection of the modulation scheme in the transmitter system can be accomplished based on numerous techniques, one of which is described in the aforementioned US Patent Application Serial No. 09 / 776,073.

The partial (for example, CCMI and UMMSE) and full CSI techniques are receiver processing techniques that allow a MIMO system to utilize the additional dimensionalities created by using multiple transmit and receive antennas, which is a major advantage to use. MIME. CCMI and UMMSE techniques can allow the same number of modulation symbols to be transmitted, for each time slot, as for a MIMO system using full CSI. However, other receptor processing techniques can also be used in conjunction with the full / partial CSI feedback techniques described herein and are within the scope of the invention. Similarly, Figures 5 and 6 depict two embodiments of a receiver system that can process a MIMO transmission, which determines the characteristics of the transmission channels (i.e., the SNR ratio) and display the total or partial CSI back to the transmitter system. Other designs based on the techniques presented herein and other receiver processing techniques may be contemplated and are within the scope of the invention.

The partial CSI technique (eg CCMI and UMMSE techniques) can also be used in a simple way without adaptive processing at the transmitter when only the SNR ratio of the total received signal or the overall achievable performance estimated based on such SNR is fed back. In one implementation, a modulation format is determined based on the received SNR estimate or the estimated overall throughput, and the same modulation format is used for all transmission channels. This procedure can reduce the overall performance of the entire system, but it can also significantly reduce the amount of information that is sent back over the reverse link.

An improvement in system performance can be made using the full / partial CSI feedback techniques of the invention. The overall performance of the system with partial CSI feedback can be calculated and compared to the overall performance with feedback of the total CSI. The overall performance of the system can be defined as:

N_c &lt;7 = Σ^1Ο&amp;2(^{1 +} ^) 'í = l

ES 2 396 563 T3 in which γ i is the SNR ratio of each received modulation symbol for partial CSI techniques or the SNR ratio of each transmission channel for the total CSI technique. The SNR relationship for various processing techniques can be summarized as follows:

for CCMI technique

<img file="ES2396563T3_D0023.tif" />

for the UMMSE technique, and

<img file="ES2396563T3_D0024.tif" />

for total CSI technique

Figures 7A and 7B show the performance of a 4 x 4 MIMO system employing partial CSI and full CSI feedback techniques. The results are obtained from a computer simulation. In the simulation, the elements of each channel coefficient matrix H are modeled as independent Gaussian random variables with zero mean and unit variance. For each calculation, a number of random matrix realizations are generated and the average of the overall performance calculated for the realization is calculated to generate the average of the overall performance.

Figure 7A shows the mean overall performance for the MIMO system for the techniques of Total CSI, CCMI of Partial CSI and UMMSE of Partial CSI for different SNR values. It can be seen from Figure 7A that the overall performance of the partial CSI UMMSE technique is approximately 75% of the overall performance of the total CSI at high SNR values, and is close to the overall performance of the total CSI at SNR values. low. The overall performance of the CCMI technique of partial CSI is approximately 75% to 90% of the overall performance of the UMMSE technique of partial CSI at high SNR values ​​and is approximately less than 30% of the overall performance of the UMMSE at values Low SnR.

Figure 7B shows the cumulative probability distribution functions (CDF) for the three techniques generated based on the histogram of the data. Figure 7B shows that at an average SNR of 16 dB per transmission channel, there are approximately 5% of the cases where the overall throughput is less than 2 bps / Hz for the CCMI technique. On the other hand, the overall performance for the UMMSE technique is above 7.5 bps / Hz for all cases in the same SNR ratio. Thus, the UMMSE technique is likely to have a lower stopping probability than the CCMI technique.

The elements of the transmitter and receiver systems can be implemented with one or more digital signal processors (DSP), application-specific integrated circuits (ASIC), processors, microprocessors, controllers, microcontrollers, programmable field gate assemblies (FPGAs), devices programmable logic, other electronic units, or any combination thereof. Some of the functions and processing described in this document can also be implemented with software running on a processor.

Aspects of the invention can be implemented with a combination of software and hardware. For example, calculations for symbol estimates for CCMI and UMMSE techniques and obtaining the channel SNR ratio can be performed based on program codes executed in a processor (controllers 530 and 650 in Figures 5 and 6, respectively) .

The preceding description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Therefore, the present invention is not intended to be limited to the embodiments shown herein.