WO2006030479A1

WO2006030479A1 - Turbo encoding bit assignment system and turbo encoding bit assignment retransmission method in mimo-ofdm system

Info

Publication number: WO2006030479A1
Application number: PCT/JP2004/013309
Authority: WO
Inventors: Lee Ying Loh; Choo Eng Yap
Original assignee: Matsushita Electric Industrial Co., Ltd.
Priority date: 2004-09-13
Filing date: 2004-09-13
Publication date: 2006-03-23
Also published as: JPWO2006030479A1

Abstract

A turbo encoding bit assignment system in a MIMO-OFDM system. In the system, a V-BLAST processing part (110) calculates, for each of received signal vectors, the norm in each pseudo inverse row of a channel matrix during V-BLAST detection. A vector information output part (116) of a receiver stores information of feedback vector indicative of antenna position of a first layer that is a row having the smallest norm. The vector information output part (116) of the receiver transmits the feedback vector information to a transmitter via an error-free channel (118) in a desirable period. A bit assignment part (106) assigns, based on the feedback vector information, a systematic bit and a parity bit to respective different antennas. A bit rearrangement part (114) rearranges the bits to the original positions in the turbo encoding.

Description

Specification

MIMO—Turbo Coded Bit Assignment System and Turbo Coded Bit Assignment Retransmission Method in OFDM System

Technical field

The present invention relates to a turbo code / bit allocation system and a turbo code / bit allocation retransmission method in a multiple-input multiple-output (MIMO) communication system using orthogonal wave frequency division multiplexing (OFDM).

Background art

[0002] Simultaneous transmission of multiple data streams involves multiple (N) transmit antennas and multiple (N) receive

T R

Implemented in MIMO communication systems using antennas. Depending on how it is used, the MIM O system can contribute to improved performance through spatial diversity, and can contribute to increased system capacity through spatial multiplexing. This improvement is possible because the wireless communication system has random fading and multipath delay spread.

[0003] A plurality of communication channels existing between a transmission antenna and a reception antenna usually have different link states while changing with time. A MIMO system with feedback provides channel state information (CSI) to the transmitter and allows the use of methods such as link adaptation and water filling to give higher levels of performance.

[0004] A well-known technique for increasing system capacity gain through spatial multiplexing is the BLAST technique. This technique is discussed in [1].

[0005] MIMO techniques were originally designed for narrowband wireless systems, ie, flat 'fading * channels. Therefore, it is difficult to obtain a high effect in the frequency selective channel. Therefore, OFDM has been used in conjunction with MIMO systems to overcome the frequency selective channel proposed in the wireless environment!

[0006] Using Inverse Fast Fourier Transform (IFFT), OFDM can transform a frequency selective channel into a set of independent sub-channels with parallel frequencies. Since the frequencies of these subchannels are orthogonal and overlap each other, spectrum efficiency is improved and intercarrier interference is minimized. Cyclic prefix ( Multipath effects are further reduced by attaching a Cyclic Prefix) to the OFDM symbol.

[0007] Adaptive bit loading in OFDM systems has been discussed in various technical papers. By changing the number of bits allocated to OFDM subcarriers, adaptive 'bit' loading has the objective of optimizing the data rate without degrading system quality. This technique works on the fact that each different subcarrier has a variable attenuation depending on the channel conditions. Allocation decisions are usually made using specific feedback information such as channel state information (CSI) and signal-to-noise ratio (SNR) for each subcarrier.

[0008] In many conventional communication systems, turbo code is used as an error correction method. The reason is that characteristics close to the Shannon limit can be obtained. The problem of unequal power allocation to systematic and parity bits has been discussed. In Non-Patent Document 2, when the protection of systematic bits is lower than that of parity check bits, the larger the interleaver length, the better the performance is obtained, and this effect increases as the block length increases. It has been shown to be stronger.

[0009] On the other hand, Non-Patent Document 3 suggests that if the SNR is very low, a larger amount of power should be allocated by systematic bits.

[0010] Non-Patent Literature 1: V-BLAST: an architecture for realising very high data rates over the rich-scattering wireless channel "by PW Wolniansky et al in the published papers of the 1998 URSI International Symposium on Signals, Systems and Electronics, Pisa, Italy, Sep. 29 to Oct. 2, 1998.

Non-Patent Document 2: "Unequal power allocation to the turbo-encoder output bits with application to CDMA systems" by Atousa H. S. Mohammadiand Amir K. Khandani in the transaction letter of IEEE Transactions on Communication, Vol. 47, No. 11 , Nov 1999.

Non-Patent Literature 3: "Optimizing the energy of different Dit streams of Turbo codes" by J. Hokfelt and T. Maseng in Proc. Turbo Coding Seminar, Lund, Sweden, Aug. 1996, pp. 59-63. Disclosure of the invention

Problems to be solved by the invention

[0011] However, until now, a technique related to a method for allocating turbo coded bits in a MIMO-OFDM system has been disclosed. /.

An object of the present invention is to provide a turbo code / bit allocation system and a turbo code / bit allocation / retransmission method capable of performing optimal turbo code bit allocation in a MIMO-OFDM system! Is to provide. Another object of the present invention is to achieve improved system performance through information bit allocation based on information without overly complicating the system.

Means for solving the problem

[0013] The present invention allocates systematic bits and parity bits to OFDM subcarriers transmitted by different transmission antennas based on feedback information provided by the V-BLAST processing unit on the receiver side.

The invention's effect

[0014] According to the present invention, it is possible to perform optimal turbo code allocation without excessively complicating the system, and it is possible to achieve improvement in system performance.

Brief Description of Drawings

[0015] FIG. 1 is a block diagram showing an overview of the system of the present invention.

[Figure 2] Transmitter block diagram of MIMO—OFDM communication system

[Fig.3] MIMO—Block diagram of receiver in OFDM communication system

FIG. 4 is a flowchart showing a V-BLAST signal processing method and information acquisition in an embodiment of the present invention.

[Fig.5] Diagram showing the detection processing performance of each transmission layer on the assumption that error propagation does not occur

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a diagram showing a system of the present invention including a bit allocation unit 106 on the transmitter side and a bit rearrangement unit 114 on the receiver side. [0017] V-BLAST processing section 110 performs V-BLAST processing for separating a received signal into a data stream corresponding to a plurality of antennas of a transmitter. The V—BLAST processing unit 110 calculates the norm at each row of the pseudo inverse of the channel matrix for each received signal vector during V—BLAST detection. Since it has the best post-detection SNR with the smallest norm, the first detection target layer (hereinafter referred to as the “first layer”) is used to obtain better performance as a whole system. This feedback vector information indicating the antenna position of the first layer is stored in the vector information output unit 116 of the receiver.

[0018] The vector information output unit 116 of the receiver desires feedback vector information to the transmitter via the error-free channel 118 and periodically transmits it.

The input data is turbo-coded by the turbo code key unit 102. The systematic bits and parity bits generated by the turbo code are separated by the bitstream separation unit 104. The bit allocation unit 106 allocates systematic bits and parity bits to different antennas based on the feedback vector information.

On the other hand, the bit reordering unit 114 rearranges each bit demapped by the demapping unit 112 to the original position in the turbo code.

[0021] Here, in the V-BLAST detection, the diversity level increases because the layer that is detected later has more signals that are erased in the previous stage. The first layer with the highest post-detection SNR is detected first in V—BLAST detection, so the diversity level (gradient in the BER characteristics with the horizontal axis being E ZN) is the lowest and the highest error. Become a rate.

b o

[0022] In one embodiment of the present invention, the Nority bits are better protected than the Systematic bits by assigning Systematic bits to antennas that are likely to have the highest post-detection SNR. This allocation method is expected to improve performance as the interleaver length increases.

[0023] In another embodiment of the present invention, systematic bits are better protected than parity bits by assigning parity bits to the antenna with the highest SNR after detection in the previous transmission. This allocation method helps improve performance when the SNR is very low (eg, below a given threshold).

[0024] In one variation of the embodiment of the present invention, V-BLAST force is also obtained, and the detection is performed by SNR. The tenor order is transmitted to the transmitter. By assigning a larger number of bits to subcarriers transmitted by a lower SNR antenna at bit allocation section 106, adaptive bit loading can be performed. This is because a higher order modulation scheme can be used if a higher diversity effect is obtained during detection.

In another modification of the embodiment of the present invention, adaptive power allocation is executed by the power allocation unit 108 based on long-term statistics obtained from feedback information power. Allocate more power to the antenna that becomes the first layer. It has become clear that such allocations result in improved performance.

[0026] The above embodiment aims to improve system performance with a low level of complexity.

The present invention uses the feedback information that is already provided in the system and has obtained the V-BLAST technique. Furthermore, adaptive allocation and bit loading are performed based on the SNR ranking between antennas. Unlike conventional methods that require the SNR to be intensively calculated for each symbol, the present invention simplifies the overall processing procedure, thereby reducing the complexity of the processing mechanism and reducing feedback delay. Make it possible.

FIG. 2 is a block diagram of a transmitter 200 in a multiple-input multiple-output communication system (that is, a MIMO-OFDM system) that uses orthogonal frequency division multiplexing. FIG. 3 is a block diagram of the receiver 300 of the system. Both figures show the power of a system that uses two transmit antennas and two receive antennas. The present invention uses multiple (N) transmit antennas and multiple (N

T

) Can be extended to a system using a receiving antenna. In the following explanation, as an example

R

1Z2 turbo code rate is used.

In transmitter 200, first, a cyclic redundancy check (CR C) code is added to input data by CRC adding section 202. Thereafter, turbo coding is performed by the turbo code key unit 204. At the output of the turbo code unit 204, the systematic bits and parity bits are divided into the same kind of individual streams 206 and 208, respectively. Each stream of encoded data is separately interleaved by interleaver 210 to reduce burst errors in the data. The separated systematic bits and parity bits are then input to the bit allocation unit 212. Based on the information provided from the receiver, the bit allocation unit 212 allocates coded bits to each allocated transmission antenna. For details of the allocation procedure It will be described later. Next, multilevel modulation constellation symbol mapping is performed in the mapping unit 214 for the data directed to each transmission antenna. A pilot signal is inserted in pilot insertion section 216 for the mapped signal. Inserting a no-lot signal facilitates channel evaluation at the receiver.

[0029] Before performing OFDM modulation, the SZP converter 218 converts a serial data stream into a parallel data stream. IFFT section 220 makes generated subcarriers orthogonal to each other. After the parallel data is converted to serial data by the PZS converter 222, a cyclic prefix for reducing the multipath effect is added to the OFD M symbol by the CP adder 224. Prior to transmission, the digital signal is converted into an analog signal by the DZ A converter 226. After various processing in each transmitter chain, the signal is ready for transmission by its assigned transmit antenna 228.

[0030] In receiver 300, the reception signal from reception antenna 302 is processed in the reverse manner, that is, conversion from analog to digital (AZD conversion unit 304), cyclic prefix removal (CP Processing such as a removal unit 306), serial / parallel conversion (SZP conversion unit 308), fast Fourier transform / acquisition unit 310), and parallel / serial conversion (PZS conversion unit 312) are performed. Since the received signal has a signal strength that overlaps multiple transmit antenna forces, it is necessary to separate this signal into separate streams. In this case, zero-forcing (Z F) or minimum mean square error (MMSE) t, a V—BLAST decoder 314 utilizing the technique is used to perform this function.

[0031] After demapping is performed by the demapping unit 316, each demapped bit is rearranged at its original position. Such rearrangement is performed according to the allocation pattern stored in the receiver. This is the same pattern used for bit allocation on the transmitter side. The output from the bit reordering unit 318 also constitutes a separate stream power of systematic bits and parity bits. In addition, dating (Dinterleaver 320) and decoding (Decoding unit 322) are executed. Subsequently, a cyclic redundancy check (CRC processing unit 324) is executed for each frame to confirm that the data is correct. If the inspected frame is determined to be error free, an acknowledgment (ACK) is sent to the transmitter and the transmitter does not retransmit the packet. If there is an error, reject to request retransmission A fixed response (NACK) is transmitted to the transmitter 200.

FIG. 4 is a flowchart showing a V-BLAST signal processing method and information acquisition in an embodiment of the present invention. Based on the information obtained from the V—BLAST process, the antenna SNRs are ranked. The V-BLAST technique using the ZF criterion and the antenna ranking procedure are described below.

[0033] The signal received by each receiving antenna is composed of a mixture of signals from each transmitting antenna. Therefore, V-BLAST aims to detect hybrids and separate them into valid data streams. The V-BLAST technique is performed for each sequence of symbols obtained from all received antenna power (this is called the received vector). The channel matrix obtained from the channel evaluation corresponding to each received vector is also required for V-BLAST processing.

[0034] The first step after setting the pair of received vectors (r) and the corresponding channel matrix (H) is to calculate the pseudo inverse of H to obtain the set of matrices G (step 404). ).

[0035] After obtaining the pseudo inverse of the channel matrix, the norm of each row of G is calculated (step 406).

The purpose of this step is to select the first layer. It has already been proven that better performance can be obtained if detection is performed first at the layer that exhibits the highest post-detection signal-to-noise ratio (SNR). This is equivalent to detecting the row of G with the minimum norm.

[0036] Therefore, the row of G having the minimum norm is selected as the zeroization vector (w) (step

i k

408). This vector zeros (disables) all signals except those transmitted at k ^th . The V-BLAST processing unit 110 internally generates a reception vector (r) and a weight vector (w).

i k

As a result, the symbols transmitted from the k ^th transmission antenna are detected (step 410).

[0037] The detected symbol is regenerated by slicing to the nearest value in the signal constellation (step 412). In this way, the evaluation for signal erasure performed in the next step is efficiently processed.

If one layer is detected, subsequent layer detection processing can be performed efficiently. By subtracting the vector force of the received signal from the detected signal part (step 414), the number of layers to be zeroed in the next step is reduced. This cancellation process interferes with the received signal vector The signal component is corrected to a reduced vector. It also reduces detection complexity.

[0039] Correspondingly, it is necessary to correct the channel matrix by deleting the k ^th column of H (step 416).

[0040] The entire process is repeated until all layers are successfully detected (step 418). Note that V-BLAST detection continues for other sets of received vectors.

[0041] Since the norm is an indication value of the SNR after detection of each transmission layer, the present invention uses this information for turbo code bit allocation in the transmitter. For each symbol in the frame, the receiver stores the antenna position of the first layer having the minimum norm (maximum SNR) as a vector. Subsequent layers need not be saved as long as only the first layer is saved. The receiver then feeds back the information vector to the transmitter via the error-free 'channel. The transmitter uses this vector for bit allocation of the next transmission frame. This storage and feedback process is repeated at the desired period. These two steps may be performed every time a frame is received at the receiver, or may be performed every few desired frames. As a general guideline, feedback is performed in a shorter cycle for radio channels with fast fluctuations. On the other hand, slow-changing channels should update information at longer intervals to conserve resources and keep complexity at a minimum level.

FIG. 5 is a diagram showing the performance of each detected transmission layer on the assumption that error propagation does not occur during V-BLAST detection. In Fig. 5, the horizontal axis is E ZN and the vertical axis is BER (b O

Bit Error Rate). In Fig. 5, communication is performed between a transmitter with four transmitting antennas and a receiver with four receiving antennas, and the signals of transmitting antenna 4 (Tx4) are separated in order from the signal of transmitting antenna 1 (Txl). Show the error curve for the case.

[0043] Theoretically, the diversity level is the lowest when the first layer is detected. For each detected layer, each received antenna is constant, but the detected layer is erased. As a result, the diversity level of the system increases as the layer progresses. This is clearly shown in FIG. The error curve of antenna 1 detected in the first decoding step gradually falls as the SNR increases (diversity level of antenna 1). The error curves for antennas 2, 3, and 4 are It descends much more rapidly than Na 1. By gaining the advantage that symbols detected before that are subtracted, the subsequent layers increase to the diversity leveler. Level 4 occurs when only one signal remains to be detected by the four receiving antennas. Therefore, this observational power can also be concluded that the first layer after ranking has the highest post-detection SNR power and its performance is lower than the other transmission layers detected thereafter. is there.

[0044] Research conducted so far has revealed that systematic bits obtained from turbo coding and parity bits have different protection levels and system performance changes. How these bits are protected to gain performance depends on system parameters and channel conditions.

[0045] Therefore, based on the above-described research results, the present invention aims at obtaining an optimal allocation of systematic bits and parity bits to the allocated transmission antennas.

[0046] As described above, the receiver sends feedback 'vector information including the first layer antenna position detected for each sequence of processed received symbols to the transmitter.

[0047] In one embodiment of the present invention, the transmitter bit allocation module allocates systematic bits to the antenna specified in the feedback 'vector. In other words, systematic bits are allocated to the antenna that is highly likely to have the highest SNR after detection, that is, the first layer. This means that systematic bits have a higher error probability than parity bits detected in subsequent layers. This agrees with research results that when the protection of systematic bits is lower than the parity check 'bit, better performance can be obtained by increasing the interleaver length. Therefore, better protection of NOTICE bits is achieved by using this allocation method. The larger the interleaver length, the better the performance.

[0048] In other embodiments of the present invention, systematic bits are better protected than parity bits. This is in contrast to the previous examples. In this embodiment, the normality bit that is not allocated to the systematic bit in the first layer is transmitted in this layer. This allocation method is used when the SNR is very low, i.e. in severe channel conditions. [0049] In one variation of the above embodiment, the receiver feeds back not only the antenna position of the first layer but also the antenna position of the subsequent layer. Therefore, after the detection of each antenna, the ranking between the antennas based on the SNR can be obtained at the transmitter. Based on this ranking, some form of adaptive 'bit' loading can be performed. For example, a larger number of bits may be allocated to the subcarrier transmitted by the antenna having the lowest SNR rank. This antenna will probably have the highest diversity when detecting V-BLAST, so that it can maintain a given error rate even with higher order modulation schemes.

[0050] In another variation of the above embodiment, adaptive power allocation can be performed based on long-term statistics of feedback information. Since the feedback information includes the antenna position of the first layer for each row of received symbols, the number of times each antenna becomes the first layer can be calculated. To improve system performance, transmitters have been shown to be more likely to allocate higher power to earlier detection stages. Therefore, the overall error rate can be reduced by giving larger power to antennas that have a greater number of times they become the first layer in this long-term statistics.

Note that the above description is considered as a preferred embodiment of the present invention. The present invention is not limited to the disclosed embodiment, and can be implemented in various forms and embodiments. The scope is determined with reference to the following claims and their equivalents.

Industrial applicability

[0052] The present invention is suitable for use in a multiple-input multiple-output (MIMO) communication system using orthogonal wave frequency division multiplexing (OFDM).

Claims

The scope of the claims

[1] A turbo code bit allocation system in a multiple-input multiple-output (MIMO) communication system using orthogonal wave frequency division multiplexing (OFDM),

The receiver separates the received signal into data streams corresponding to the multiple antennas of the transmitter V-BLAST signal processing unit that performs B-BLAST processing, and feedback “vector information obtained by V-BLAST processing is transmitted. A vector information output unit to be sent to a machine, wherein the transmitter generates a systematic bit and a parity bit by a turbo code, and the feedback 'the systematic bit based on the vector information And a bit allocation unit that allocates the separated stream of the NOR bits to individual antennas.

[2] The system according to claim 1,

The transmitter further includes a separate 'interleaver corresponding to each stream of the systematic bits and the NORMAL bits and preventing the systematic bits and the parity bits from being mixed before the bits are allocated.

[3] The system according to claim 1,

The receiver further includes a bit rearrangement unit that rearranges each demapped bit to its original position based on the allocation pattern at the previous transmission.

[4] A turbo code bit allocation method in a multiple-input multiple-output (MIMO) communication system using orthogonal wave frequency division multiplexing (OFDM),

In the receiver, performing a V-BLAST process that separates the received signal into data streams corresponding to the multiple antennas of the transmitter; sending feedback vector information obtained by the V-BLAST process to the transmitter; Comprising

The transmitter generates a systematic bit and a parity bit by a turbo code, and a separated stream of the systematic bit and the parity bit based on the feedback vector information. Assigning to individual antennas.

[5] The method of claim 4,

In the receiver, the best post-detection SNR as the feedback 'vector information Sending information indicating the antenna position having to the transmitter,

In the transmitter, systematic bits are allocated to the antenna of the feedback vector information.

[6] The method of claim 4,

In the receiver, information indicating the antenna position having the highest post-detection SNR as the feedback vector information is sent to the transmitter,

When the SNR is lower than a predetermined threshold, the transmitter assigns a NORITY bit to the antenna of the feedback vector information.

[7] The method of claim 4,

In the receiver, information indicating the SNR order of the antenna is sent to the transmitter as the feedback vector information.

The transmitter is assigned a lower number of bits to subcarriers transmitted by an antenna having a low SNR order, while a smaller number of bits is assigned to subcarriers transmitted by an antenna having a higher SNR order. .

[8] The method of claim 4,

In the transmitter, the number of times each antenna has the highest post-detection SNR is calculated, and the larger the number of antennas, the greater the power.