WO2008096308A1

WO2008096308A1 - Method and apparatus for hybrid automatic repeat request in multiple antenna system

Info

Publication number: WO2008096308A1
Application number: PCT/IB2008/050394
Authority: WO
Inventors: Xun Fan; Suoming Pu; Ni Ma; Gang Wu; Qi Zhou; Xiaobo Zhang
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-02-06
Filing date: 2008-02-04
Publication date: 2008-08-14
Also published as: TW200835212A

Abstract

The present invention proposes a method of hybrid automatic repeat request for a multiple antenna system and an apparatus thereof, where the method and the apparatus thereof only retransmit two symbols that constitute an Almouti code word together with a 5 part of initial transmitted symbols when performing symbol retransmission, and thus retransmission power consumption is reduced and receiving complexity is decreased due to the retransmitted symbols decreasing.

Description

METHOD AND APPARATUS FOR HYBRID AUTOMATIC REPEAT REQUEST

IN MULTIPLE ANTENNA SYSTEM

Field of the Invention

The invention relates generally to a radio communication technology, and more particularly, to a method and apparatus for hybrid automatic repeat request in a multiple antenna system.

Background of the Invention

Compared with Single Input Single Output (SISO) antenna wireless communication system, Multiple Input Multiple Output (MIMO) wireless communication system can provide a greater channel capacity. Recently, spatial multiplexing transmission solutions concerning MIMO systems with four transmit antennas have been proposed in many wireless communication standards. In such solutions four independent data streams are transmitted simultaneously via four transmit antennas, so as to provide a higher data rate in Full-Rank MIMO channels (i.e. the rank of the channel matrix of the system is 4.). However, such solutions are sensitive to the environment of the rank of the MIMO channel matrix. In a spatial multiplexing system, to keep an acceptable performance, the number of data steams transmitted simultaneously should not exceed the rank of the MIMO channel matrix. When the rank of the MIMO channel matrix decreases, i.e. when Rank-Deficient appears in the channel, numerous transmission errors may occur.

To solve the above problem, one of the current solutions is to introduce Automatic Repeat Request (ARQ) into the MIMO system. Table 1 shows an example of such an MIMO-ARQ solution for use in a four-transmit-antenna system.

Talbe 1

As shown in Table 1, based on the condition of the rank of the MIMO channel matrix, such an MIMO-ARQ solution retransmits the data packets (including four symbols), which have been initially transmitted, through multiple time slots, and recovers those symbols based on corresponding transmission time slots, instead of using the data from the initial transmission. Such an MIMO-ARQ solution may lead to a lower Packet Error Rate (PER) after several retransmissions, but its retransmission delay is also longer, since every retransmission needs more than one time slot.

Therefore, another solution which introduces Hybrid Automatic Repeat Request (HARQ) into an MIMO system is proposed. Compared with ARQ, HARQ is able to reuse the data from the initial transmission instead of discarding them, and therefore it is more efficient. For a two-transmit-antenna system, an HARQ solution of Space-Time Block Code (STBC-HARQ) retransmits the initial transmitted symbols according to the Alamouti rule. Symbols transmitted by a corresponding transmit antenna are shown in Table 2 as follows.

Table 2

Such an STBC-HARQ solution using the Alamouti rule exhibits a better performance than those non- Alamouti rule sulotions.

Based on a similar principle, for a four-transmit-antenna system, a solution called Double Space-Time Transmit Diversity (Double-STTD) HARQ may be used. In this solution, the initial transmitted symbols are retransmitted according to the Double-STTD rule, and symbols transmitted by respective transmit antennas are shown in Table 3 as follows.

Table 3

For the transmitters used in the above two types of HARQ solutions, a general Packet Combination method may be used at the receiver to obtain a high reception performance in various channel environments.

Although the Double-STTD HARQ takes less retransmission time and exhibits a better detection performance, and thus has an efficiency higher than that of the

MIMO-ARQ solution, it increases the power consumption of the transmitter and the complexity of the receiver to perform packet combination, because four symbols need to be retransmitted in the same time slot.

Summary of the Invention

An object of the invention is to provide an HARQ method and apparatus for a multiple antenna system, which decreases power consumption and receiving complexity by decreasing retransmitted symbols.

A method of hybrid automatic repeat request for a multiple antenna system transmitter according to the invention, comprising the steps of: a) transmitting symbols to be transmitted through a corresponding antenna channel; b) obtaining corresponding feedback information; and c) if the feedback information indicates that a retransmission is necessary, retransmitting only two symbols, so as to construct an Alamouti code word together with a part of the symbols transmitted in step a).

A method of hybrid automatic repeat request for a multiple antenna system receiver according to the invention, comprising the steps of: a) receiving transmitted symbols through a corresponding antenna channel; b) sending corresponding feedback information based on the transmitted symbols received; c) if the feedback information indicates that a retransmission is necessary, receiving two retransmitted symbols, wherein the two retransmitted symbols constitute an Alamouti code word together with a part of the symbols transmitted in step a); and d) recovering a corresponding data according to the transmitted symbols received and the retransmitted symbols.

A transmitter for a multiple antenna system according to the invention, comprising: a spatial multiplexing unit for transmitting symbols to be transmitted through a corresponding antenna channel; a packet retransmission unit for retransmitting two symbols that constitute an Alamouti code word together with a part of the transmitted symbols; and a switch unit for controlling the packet retransmission unit to retransmit the symbols according to a received corresponding feedback information.

A receiver for a multiple antenna system according to the invention, comprising: a spatial multiplexing detection unit for receiving transmitted symbols through corresponding antenna channels and generating corresponding feedback information according to the transmitted symbols received; a packet combination unit for receiving two retransmitted symbols and recovering corresponding data based on the transmitted symbols and the retransmitted symbols received, wherein the two retransmitted symbols constitute an Alamouti code word together with a part of the transmitted symbols; and a receiving switch unit for sending the retransmitted symbols received to the packet combination unit when the feedback information indicates that a retransmission is necessary.

Compared with the prior arts, the HARQ method and apparatus for the multiple antenna system according to the invention only need retransmit two symbols, therefore have lower complexity and lower retransmit power consumption.

Other objects and effects together with a more thorough understanding of the invention will become apparent and appreciated by referring to the following descriptions and claims taken in conjunction with the accompanying drawings.

Brief Description of the Drawings

A more detailed description will be made to the invention with reference to the drawings, in which:

Fig.l is a schematic diagram illustrating the basic operations of the Partially Space-Time Block Code (PSTBC) HARQ method of the present invention; Fig.2 illustrates a flowchart of the packet combination method performed by a receiver of the present invention;

Fig.3 is a block diagram of a transceiver according to an embodiment of the present invention.

Throughout the drawings, same labels refer to same, similar or corresponding features or functions.

Detailed Description of the Invention

Symbol transmission procedure using the PSTBC-HARQ method of the present invention is shown in Fig.l. It is assumed that the transmission symbol vector (i.e. a packet) in a certain time slot is S = [S₁ , S₂ , S₃ , S₄ ] , where S₁ J = 1, 2, ..., 4 , indicates the spatial multiplexed symbol transmitted through the ith transmit antenna. With reference to Fig. 1, firstly, symbol [S₁₉ S₂₉ S_{3 9} S₄ J is transmitted at the transmitter. If any error occurs during the receiver's reception, a negative acknowledgment(NACK) is returned to request the transmitter to retransmit data, and then the transmitter will transmit the symbols of

I _{* *} I

[- S₂ , S₁ J . The two symbols transmitted by the transmitter and the corresponding

[^₁ , .S₂ J in the initially transmitted symbols constitute a code word consistent with the Alamouti format, as shown in the dashed circle in Fig. 1.

The packet transmission format performed by the PSTBC-HARQ method of the invention can also be shown in the following Table 4.

Table 4

"0" in Table 4 indicates that no symbol is transmitted in the corresponding time slot via the corresponding antenna.

In the PSTBC-HARQ method of the invention, the current packet combination method may be used at the receiver, but the method is complicated to operate, and therefore, with respect to the packet transmission format of PSTBC-HARQ, the present invention proposes a less complicated method, i.e. an Interference Cancellation (IC) based packet combination method.

Without loss of generality, take the packet transmission format of PSTBC-HARQ shown in Table 4 as an example to explain the IC based packet combination method of the present invention. With regard to the solution illustrated in Table 4, the signal models in the initial transmission and in the retransmission slot may be represented respectively by the following equation (1) and (2):

r(2) _{= H}(2)_s(2) _{+ n}(2) ₍₂₎

where the superscripts i=l,2 respectively the signal models in the initial transmission and the retransmission slot, r ' = [T₁ ' , r₂ , ..., r_N ^l ] . 1 = 1, 2 indicates the received signal

vector of N_R dimensions, where the Entry T_m of the matrix indicates the signal received by the receive antenna m in the corresponding transmission time slot. s⁽¹⁾

indicate transmission symbol vectors respectively, where the Entry S J of the matrix indicates the signal transmitted by the transmit antenna n in the corresponding transmission time slot. A transmission symbol in a time slot is modeled as an independent and identically distributed (i.i.d) random variable with a zero mean and a variance <J_S . n ' = [H₁ ¹ , n₂ , ..., n^ ]^τ , i = 1, 2 indicates the additional white Gauss noise vector of the receiver in the corresponding transmission time slot, where all of the entries are modeled as i.i.d Cyclic Symmetry complex Gauss random variable with a zero mean and a variance C^ .

The matrixes H and H shown in equations (3) and (4) represent the MIMO channel matrixes respectively, where the Entry H₁^ _n of the matrixes indicate the channel gain between the transmit antenna n and the receive antenna m in a corresponding transmission time slot.

According to the signal models in the above equations (1) and (2) and with reference to Fig.2, the IC based packet combination method of the invention recovers the transmission packets with the following steps:

Firstly, the two retransmitted symbols are detected by the linear spatial multiplexing detection algorithm based on the signal received in the corresponding retransmission time slot (Step SlO).

As indicated by equation (2), the retransmission signal model in PSTBC-HARQ corresponds to a signal model of two-transmitting-multiple(N_R)-receiving spatial multiplexing system. Thus, the conventional linear spatial multiplexing detection algorithm, such as Zero Forcing (ZF) and Minimum Mean Square Error (MMSE) algorithms etc., may be used directly. Taking the MMSE algorithm as an example, two retransmitted symbols ^₁ and S₂ ^are detected using the following two equations (5) and (6).

where I_M is an M identity matrix, Q(-) indicates the demodulation operation corresponding to the modulation solution.

In Step SIl, interference cancellation is performed by using the detected two retransmitted symbols, that is, the effect caused by the two symbols detected in Step SlO is cancelled from the signal received in the initial transmission time slot by using the following equation (7):

:2}^S(1,2) (7) r⁽i⁾ Jh where H. _{{ }} indicates the resulting matrix formed by taking the p column through

J the Q ihn column of the matrix τ Hτ(i) . S_{(1 2)} = r [S ^Λ ₁, S ^Λ ₂ V J is a judgement symbol vector,

(i) whose matrix entries are from the output of equation (6) in Step SlO. ^r _/c₍i_,2) represents the received signal vector in the initial transmission time slot after the interference cancellation on S₁ and S₂ •

In Step S 12, the remaining symbols of the signal received in the initial transmission time slot are detected, except S₁ and S₂ • It is assumed that S₁ and S₂ detected in Step

SlO are correct, i.e. S₁ = S₁ and S₂ = S₂ , the signal model of the remaining two symbols

S₃ and S₄ after interference cancellation may be obtained with the following equation (8) directly: r ¹Z⁽C¹⁾(U) = H ^1J-:⁽,¹{⁾3:4r s(3,4) + ^n¹¹⁽¹⁾ m W

where S_{(3 4)} = [s₃ , S₄ ] . Equation (8) is also equivalent to a signal model of two-transmitting-multiple(N_R)-receiving spatial multiplexing system. Thus, the conventional linear spatial multiplexing detection algorithm may be used directly. Taking the MMSE algorithm as an example, the two retransmitted symbols S₁ and S₂ are obtained by the following two equations (9) and (10):

£ ^S(3,4) _ r L^Λ?₃ 5 ^Λ ?₄ ΛJ^τ

Then, in Step S 13, interference cancellation is performed by using the detected remaining symbols, that is, the effect caused by the two remaining symbols detected in Step S 12 is cancelled from the signal received in the initial transmission time slot by using the following equation (11): r£>_(3>4) = r⁽¹⁾ - Hj₃^_4J (11)

where ^s _(3;4) ⁼ [ S₃, S₄] is a judgement symbol vector, whose matrix entries are from the

output of equation (6) in Step SlO. ^r _/c₍i_,2) indicates the received signal vector in the

initial transmission time slot after interference cancellation on S₃ and S₄ .

Finally, in Step S 14, the two retransmitted symbols are detected again, so as to finally restore all the transmission symbols. It is assumed that S₃ and S₄ detected in step 13 are

correct, i.e. S₃ = S₃ and S₃ = S₄ , the signal model of the two retransmitted symbols S₁ and S₂ ^aft^er interference cancellation can be obtained with the following equation (12) directly: r(D _ ||(1) _{s + n}(D 1IC(IA) **:,{1,2}^S(1,2) ^τ " (12)

where S_{(1 2)} — [S₁ , S₂ J .

Based on the equations (12) and (2), the equivalent model of the symbols ^₁ and

S₂ can be established after a simple conversion with the following equation (13):

^?(1,2) = ^H (l,2)^a(l,2) + n (1,2) (13)

where r_{(1 2)}

is an equivalent receiving vector including the

effect of ^₁ and S₂ in the initial transmission and retransmission time slots.

Equation (14) is the corresponding equivalent channel matrix. Moreover, n, _{j 2)} = [(n ) , — (n ) ] is the corresponding equivalent noise vector.

According to the equivalent signal model of equation (13), the following two methods may be used to detect ^₁ and S₂ :

The first detection method is a conventional detection method. A conventional detection method such as the conventional linear spatial multiplexing detection algorithm (ZF or MMSE algorithms etc., for example) may be directly used, since the matrix expression of the model of equation (13) is equivalent to a two-transmitting-multiple(N_R)-receiving spatial multiplexing system. Taking the MMSE algorithm as an example, ^₁ and S₂ are obtained by the following equation (15) and (16):

^S(l,2) ^~ (15)

S_n = Q(O_* « = 1, 2 (16)

The second detection method is a linear Alamouti decoding method. When the MIMO channel changes slowly enough, the initial transmission and retransmission channel gains may be considered as approximately the same, i.e.

n hm^{1,⁾n = ⁿhm^{2,n⁾ = h ⁿm,n ra "^l = 1 ^A' 2 ^' " ^*' N ^{L V} fl ' n ⁿ = \ ^A' 2 ^' ^{* * *} ' 4 ^ ( Uλ i '\J

The equivalent channel matrix shown in equation (13) has the same form with that of a two-transmitting-multiple(Ν_R)-receiving Alamouti STBC system. Therefore, a more ideal detection performance may be obtained with a very low complexity according to the following equations (18) and (19) using linear Alamouti decoding method.

^S _n = Q(KX ^ = ¹ _' ² (19)

where

K K₂

T Ύ slow

^H(U) - -K; K: (20)

-K; K:

Fig.3 is a block diagram of a transceiver which implements the PSTBC-HARQ method according to the present invention. For the sake of simplicity and clarity, the other modules of the transceiver that are not related directly to the present invention are not shown in the diagram. For the initial transmission, at the transmitter, the information bits of the input signal are processed by the Cycle Redundancy Check (CRC) protection/channel coder 110, interleaver 120 and the modulation unit 130 sequentially. The modulated signal is transmitted to the spatial multiplexing unit 150 via the switch 140 and is divided into 4 independent data streams by the spatial multiplexing unit 150, and the 4 independent data streams are transmitted through 4 independent transmit antennas respectively, that is, are spatial multiplexed.

At the receiver, it is assumed that the channel condition information has been obtained, the received spatial multiplexed signals are separated by the spatial multiplexing detection unit 220 (using algorithms such as ZF or MMSE), and then are processed by the demodulation unit 240, deinterleaver 250 and CRC verification/channel decoder 260 sequentially, thus the transmitted data are recovered. According to the CRC verification result, the corresponding acknowledgment information (ACK/NACK) is returned to the transmitter to decide whether the transmitter should retransmit the data.

When the rank of the channel decreases from 4 to 2 or 3 due to certain cause, such as rank deficiency in the channel, the receiver transmits NACK to the transmitter to indicate an initial transmission error. In this case, at the transmitter, the modulated retransmission signal is sent to the PSTBC packet retransmission unit 160 via the switch 140 and then is transmitted from the antenna after spatial multiplexed by the PSTBC packet retransmission unit 160. At the receiver, the receiving switch unit 160 sends the received retransmission signal to the PSTBC packet combination unit 230. The PSTBC packet combination unit

230 performs the IC based packet combination method or the conventional detection method described above based on the retransmission signal, so as to restore the initial transmitted data.

Compared with the prior arts, the multiple antenna system PSTBC-HARQ method of the present invention and the apparatus thereof are less complicated than that of the current

STBC-HARQ solution, esspecially the Alamouti detection method used in Step S 14 is obviously less complicated. When compared with non STBC-HARQ solutions, although the present invention is a little more complicated, the performance is improved more significantly. Moreover, in the current STBC-HARQ solutions, especially in the Double-STTD

HARQ solution, four symbols need to be retransmitted, while in the present invention, only two symbols should be retransmitted, so the power consumption of retransmission is reduced. Therefore, the present invention is more attractive for the wireless transmission system that requires high performance and low complexity.

The above embodiments are described only for an MIMO system with four transmit antennas of the present invention, however, it is obviously that the present invention is also applicable to the MIMO system with more than four transmit antennas. It should be noted that the above embodiments are intended to be illustrative rather than limiting, and it is to be understood by those skilled in the art that various improvements and modifications may be made to the disclosed multiple antenna system PSTBC-HARQ method and the apparatus thereof without departing from the basis of the invention. Therefore, the scope of the present invention is to be defined by the attached claims herein. Moreover, the reference numerals in the claims should not be understood as limiting the scope of the claims.

Claims

CLAIMS:

1. A method of hybrid automatic repeat request for a multiple antenna system transmitter, comprising: a) transmitting symbols to be transmitted through a corresponding antenna channel; b) obtaining corresponding feedback information; and c) if the feedback information indicates that a retransmission is necessary, retransmitting only two symbols, so as to construct an Alamouti code word together with a part of the symbols transmitted in step a).

2. The method according to claim 1, wherein the multiple antenna system is a four-transmit-antenna system, and the transmission in step a) is to transmit four symbols simultaneously in a time slot through four transmit antennas respectively.

3. The method according to claim 2, wherein the retransmission in step c) is to transmit two symbols in a time slot through two transmit antennas thereof respectively.

4. The method according to claim 1, wherein the feedback information includes acknowledgement/negative acknowledgement signal to indicate whether the symbols transmitted in step a) are received successfully.

5. A method of hybrid automatic repeat request for a multiple antenna system receiver, comprising: a) receiving transmitted symbols through a corresponding antenna channel; b) sending corresponding feedback information based on the transmitted symbols received; c) if the feedback information indicates that a retransmission is necessary, receiving two retransmitted symbols, wherein the two retransmitted symbols constitute an Alamouti code word together with a part of the symbols transmitted in step a); and d) recovering a corresponding data according to the transmitted symbols received and the retransmitted symbols.

6. The method according to claim 5, wherein the multiple antenna system is a four-transmit-antenna system, and the transmitted symbols include four symbols transmitted in a time slot.

7. The method according to claim 6, wherein the retransmitted symbols are two symbols transmitted in a time slot.

8. The method according to claim 5, wherein the feedback information includes acknowledgement/negative acknowledgement signal to indicate whether the transmitted symbols are received successfully.

9. The method according to claim 5, wherein the recovering of the corresponding data in step d) further comprising: dl) detecting the two retransmitted symbols; d2) performing interference cancellation on the transmitted symbols using the detected two retransmitted symbols; d3) detecting the remaining symbols except the two symbols corresponding to the two retransmitted symbols, based on the transmitted symbols after interference cancellation; d4) performing interference cancellation on the transmitted symbols processed in step d2) by using the detected remaining symbols; and d5) redetecting the two retransmitted symbols by using packet combination of the transmitted symbols and the retransmitted symbols after interference cancellation.

10. The method according to claim 9, wherein redetecting the two retransmitted symbols in step d5) is performed by using linear Alamouti decoding method.

11. A transmitter for a multiple antenna system, comprising: a spatial multiplexing unit for transmitting symbols to be transmitted through a corresponding antenna channel; a packet retransmission unit for retransmitting the two symbols that constitute an Alamouti code word together with a part of the transmitted symbols; and a switch unit for controlling the packet retransmission unit to retransmit the symbols according to obtained a received corresponding feedback information.

12. The transmitter according to claim 11, comprising four transmit antennas, wherein the spatial multiplexing unit transmits four symbols simultaneously through the four transmit antennas respectively.

13. The transmitter according to claim 12, wherein the packet retransmission unit retransmits the two retransmission symbols in a time slot through two transmit antennas thereof respectively.

14. A receiver for a multiple antenna system, comprising: a spatial multiplexing detection unit for receiving the transmitted symbols through corresponding antenna channels and generating corresponding feedback information according to the transmitted symbols received; a packet combination unit for receiving two retransmitted symbols and recovering corresponding data based on the transmitted symbols and the retransmitted symbols received, wherein the two retransmitted symbols constitute an Alamouti code word together with a part of the transmitted symbols; and a receiving switch unit for sending the retransmitted symbols received to the packet combination unit when the feedback information indicates that a retransmission is necessary.

15. The receiver according to claim 14, wherein the multiple antenna system is a four-transmit-antenna system, the initially transmitted symbols include four symbols transmitted in a time slot.

16. The receiver according to claim 15, wherein the retransmitted symbols are two symbols transmitted in a time slot.

17. The receiver according to claim 14, wherein the feedback information includes acknowledgement/negative acknowledgement signal to indicate whether the transmitted symbols are received successfully.