WO2008117207A1 - Method and apparatus for transmitting signals in a multi-antenna system - Google Patents

Method and apparatus for transmitting signals in a multi-antenna system Download PDF

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Publication number
WO2008117207A1
WO2008117207A1 PCT/IB2008/051029 IB2008051029W WO2008117207A1 WO 2008117207 A1 WO2008117207 A1 WO 2008117207A1 IB 2008051029 W IB2008051029 W IB 2008051029W WO 2008117207 A1 WO2008117207 A1 WO 2008117207A1
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symbols
retransmission
detection
subset
post
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PCT/IB2008/051029
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French (fr)
Inventor
Xun Fan
Gang Wu
Ni Ma
Qi Zhou
Xiaobo Zhang
Yan Li
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Koninklijke Philips Electronics N.V.
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Publication of WO2008117207A1 publication Critical patent/WO2008117207A1/en

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/12—Arrangements for detecting or preventing errors in the information received by using return channel
    • H04L1/16—Arrangements for detecting or preventing errors in the information received by using return channel in which the return channel carries supervisory signals, e.g. repetition request signals
    • H04L1/18—Automatic repetition systems, e.g. Van Duuren systems
    • H04L1/1812—Hybrid protocols; Hybrid automatic repeat request [HARQ]
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/0001—Systems modifying transmission characteristics according to link quality, e.g. power backoff
    • H04L1/0023—Systems modifying transmission characteristics according to link quality, e.g. power backoff characterised by the signalling
    • H04L1/0025—Transmission of mode-switching indication
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/02—Arrangements for detecting or preventing errors in the information received by diversity reception
    • H04L1/06—Arrangements for detecting or preventing errors in the information received by diversity reception using space diversity
    • H04L1/0618—Space-time coding
    • H04L1/0625—Transmitter arrangements
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/02—Arrangements for detecting or preventing errors in the information received by diversity reception
    • H04L1/06—Arrangements for detecting or preventing errors in the information received by diversity reception using space diversity
    • H04L1/0618—Space-time coding
    • H04L1/0631—Receiver arrangements
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/02—Arrangements for detecting or preventing errors in the information received by diversity reception
    • H04L1/06—Arrangements for detecting or preventing errors in the information received by diversity reception using space diversity
    • H04L1/0618—Space-time coding
    • H04L1/0637—Properties of the code
    • H04L1/0668—Orthogonal systems, e.g. using Alamouti codes
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/12—Arrangements for detecting or preventing errors in the information received by using return channel
    • H04L1/16—Arrangements for detecting or preventing errors in the information received by using return channel in which the return channel carries supervisory signals, e.g. repetition request signals
    • H04L1/1607—Details of the supervisory signal
    • H04L1/1671—Details of the supervisory signal the supervisory signal being transmitted together with control information
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/12—Arrangements for detecting or preventing errors in the information received by using return channel
    • H04L1/16—Arrangements for detecting or preventing errors in the information received by using return channel in which the return channel carries supervisory signals, e.g. repetition request signals
    • H04L1/18—Automatic repetition systems, e.g. Van Duuren systems
    • H04L1/1825—Adaptation of specific ARQ protocol parameters according to transmission conditions

Definitions

  • the invention relates to a multi-antenna wireless communication system, and more particularly, to a method and apparatus for transmitting signals based on Transmit- Antenna-Selection in a multi-antenna system.
  • a multi-antenna system typically refers to a wireless communication system having multiple transmit antennas at the transmitter and at least one receive antenna at the receiver, for example, a MIMO (Multiple-Input Multiple- Output) system having 4 transmit antennas as shown in Fig.2.
  • MIMO Multiple-Input Multiple- Output
  • the MIMO system allows parallel transmission of different data streams over multiple spatial channels established between the transmitter and the receiver, and thus provides a higher channel capacity. Accordingly, research on the MIMO system has recently become a main focus.
  • a MIMO HARQ Hybrid Automatic Retransmission reQuest method for retransmission of a portion of symbols, abbreviated as PSM-HARQ (Partial-symbol retransmitted MIMO-HARQ)
  • PSM-HARQ Partial-symbol retransmitted MIMO-HARQ
  • the transmitter first transmits multiple symbols S x in a transmission slot via the corresponding transmit antennas to the receiver. If any transmission error occurs, i.e., the receiver returns a negative acknowledgment (NACK), the transmitter selects a portion of symbols from the transmitted multiple symbols and retransmits the selected portion of symbols via the corresponding transmit antennas in the retransmission slot.
  • NACK negative acknowledgment
  • a 4-transmit antenna system only retransmits two symbols corresponding to the first and second transmit antennas, as illustrated in Table 1 and Table 2. In Table 1 and Table 2, "0" means that no symbol is transmitted in the corresponding slot and antenna, and S x represents the transmitted symbol.
  • the two retransmitted symbols in Table 1 and the transmitted symbols over the corresponding transmit antennas form an Alamouti codeword (a codeword formed by Space-Time Block Coding), referred to as PSM-HARQ in STBC format whose transmission procedure is shown in Fig.l.
  • the retransmitted symbols in Table 2 have the same format as the transmitted symbols, referred to as PSM-HARQ in non-STBC format.
  • PSM-HARQ for the PSM-HARQ shown in Table 1 and Table 2, only a small amount of transmitted symbols (a portion of the originally transmitted symbols) are retransmitted so as to decrease power consumption and complexity for retransmission while improving the transmission performance by retransmission.
  • retransmission symbols are selected in an arbitrary manner, in other words, a transmit antenna for retransmission is selected in an arbitrary manner.
  • the wireless channel condition associated with the transmit antenna for retransmission will produce an impact on the retransmission. If the channel condition is very poor, the retransmission will not necessarily achieve the purpose of ideally decreasing the transmission error.
  • HSS-MIMO Hybrid SM-STBC MIMO
  • multiple symbols to be transmitted are transmitted to the wireless space simultaneously via multiple transmit antennas of the transmitter during a transmission period, using SM or STBC transmission.
  • SM single transmit antennas
  • the transmitter may select the transmit antennas with better channel conditions as the transmit antennas for transmission of SM symbols by using a transmit antenna selection method based on the channel state information without considering the complexity of the receiver. Therefore, the proposed HSS-MIMO system is able to select suitable transmit antennas for transmission of the SM symbols among the transmission symbols, so as to improve the transmission performance for the SM symbols, which in turn improves the transmission performance of the whole system.
  • the HSS-MIMO system is proposed by Walter Freitas et al and does not take into consideration the complexity of the receiver.
  • the transmit antenna selection is independent of the detection method used by the receiver.
  • the detection method used by the receiver is directly related with the system's capacity for tolerating code errors.
  • the detection method used by the receiver is likely to affect the success rate of the transmission.
  • TAS Transmit Antenna Selection
  • An object of the invention is to provide a PSM-HARQ method and apparatus for facilitating an increase in the retransmission efficiency and a decrease in the system transmission BER while reducing the retransmission symbols.
  • Another object of the invention is to provide a HSS-MIMO transmission method and apparatus for performing transmit antenna selection for SM symbols taking into consideration the detection method used by the receiver, so as to improve the transmission performance for the SM symbols and to further improve the transmission performance of the whole system.
  • a PSM-HARQ method based on TAS is provided in an embodiment of the invention.
  • the receiver when the receiver cannot correctly restore a symbol transmitted by the transmitter in a transmission slot, the receiver returns a feedback message including NACK to the transmitter, which feedback message indicates that a retransmission is required.
  • a newly added TAS module selects a portion of symbols from the transmitted symbols to form a subset of retransmission symbols and obtains all possible subsets of retransmission symbols.
  • the TAS module establishes a corresponding equivalent signal model for each of the possible subsets of retransmission symbols and calculates a post-detection SNR for each of the transmitted symbols or retransmission symbols.
  • a post-detection SNR minimum value is searched from the post-detection SNRs of the various symbols.
  • a subset of retransmission symbols having the maximum post-detection SNR minimum value is selected from all the possible subsets of retransmission symbols, so as to instruct the transmitter to transmit the selected subset of retransmission symbols via the corresponding transmit antennas.
  • the TAS module may be placed in the receiver or the transmitter.
  • Fig.l illustrates a symbol transmission procedure in a prior-art PSM-HARQ method
  • Fig.2 illustrates a block diagram of a TAS-PSM-HARQ system, where the TAS module is implemented in the receiver according to an embodiment of the present invention
  • Fig.3 illustrates a block diagram of a TAS-PSM-HARQ system, where the TAS module is implemented in the transmitter according to an embodiment of the present invention
  • Fig.4 shows an overall flow chart of the TAS module according to an embodiment of the present invention
  • Fig.5 schematically illustrates the symbol transmission by using a TAS-PSM-HARQ method in a 4-transmit antenna system according to an embodiment of the present invention
  • Fig.6 illustrates a detailed flow chart of the TAS module when the receiver uses a general packet combination method according to an embodiment of the present invention
  • Fig.7 illustrates a detailed flow chart of the TAS module when the receiver uses an interference cancellation method according to another embodiment of the present invention
  • Fig.8 illustrates a block diagram of the TAS module according to an embodiment of the present invention.
  • Fig.9 and Fig.10 schematically illustrate simulation results when symbol transmission is carried out by using the TAS-PSM-HARQ method proposed in the present invention.
  • transmit antenna selection is made for a portion of transmission symbols based on the channel state information from the transmit antennas to the receive antennas and the detection method used by the receiver, so as to select transmit antennas suitable for transmission of the portion of transmission symbols, thus allowing the transmission symbols to have the minimum detection BER.
  • a TAS (Transmit Antenna Selection) module is added to the current PSM-HARQ system, so as to form a PSM-HARQ based on TAS, abbreviated as TAS-PSM-HARQ.
  • TAS-PSM-HARQ Transmit Antenna Selection
  • the added TAS module is used to select the transmit antennas that are most suitable for retransmission.
  • the transmitted symbols and the retransmitted symbols correspond to the transmit antennas in a one-to-one relationship, as shown in Table 1 and Table 2.
  • selection of the transmit antennas most suitable for retransmission is equivalent to selection of retransmission symbols from the transmitted symbols. Therefore, the function of the TAS module in embodiments of the invention is, in essence, to select suitable retransmission symbols according to the CSI (Channel State Information) and the packet combination method used by the receiver, so as to decrease the detection BER.
  • CSI Channel State Information
  • Fig.2 and Fig.3 illustrate two TAS-PSM-HARQ systems with N T transmit antennas and N R receive antennas, respectively.
  • the TAS module is implemented in the receiver.
  • the data to be transmitted is fed into a SM (Spatial Multiplexing) unit 215.
  • the SM unit 215 assigns the N T modulated symbols to the N T transmit antennas so as to transmit them in the same transmission slot to the wireless channel.
  • the receiver 250 receives the signals in the transmission slot via the N R receive antennas.
  • the received signals are fed into the CRC unit.
  • the CRC unit of the receiver 250 sends a feedback message back to the transmitter 210 according to the CRC result, the feedback message including an acknowledge information / a negative acknowledge information (ACK/NACK).
  • the feedback message is used to indicate whether data restoration is successful. If the feedback message includes a NACK, it means that data restoration fails and retransmission is required.
  • the TAS module 252 in the receiver selects a subset of transmit antennas suitable for retransmission, that is, a suitable subset of retransmission symbols, according to the CSI measured at the SM detection unit 255 and the packet combination method used by the packet combination unit 259, and generates a corresponding indication.
  • the indication is sent together with the feedback message via the feedback channel to the transmitter 210.
  • the packet retransmission unit 219 in the transmitter 210 selects the corresponding retransmission symbols according to the indication from the receiver 250 and sends the retransmission symbols in the retransmission slot via the corresponding transmit antennas to the receiver 250.
  • the receiver 250 then receives signals in the retransmission slot.
  • the received signals are combined and detected in the packet combination unit 259, and then sent to the subsequent processing units for data restoration.
  • the TAS module of the invention has to acquire the CSI from the transmitter to the receiver for selection of retransmission symbols and accordingly the CSI may be obtained easily if the TAS module is disposed in the receiver.
  • the TAS module may be disposed in the transmitter, for example disposed in a transmitter of a TDD system as shown in Fig.3.
  • the difference between the TAS-PSM-HARQ system of Fig.3 and that of Fig.2 is that a TAS module 312 is added to the TDD transmitter 310, and the receiver 350 is substantially similar to the existing TDD receiver. Similar to Fig.2, when the transmitter 310 receives a NACK transmitted via the feedback channel from the receiver 350, the TAS module 312 selects a suitable subset of retransmission symbols according to the estimated CSI and the predetermined packet combination method, and forms a corresponding indication.
  • the CSI from the transmitter to the receiver may be obtained through a reverse channel estimation based on the CSI from the receiver to the transmitter detected by the transmitter.
  • the reverse channel estimation is accurate because the TDD channel on a round trip in a short period may be considered constant and reciprocal.
  • the TAS module 312 needs to obtain the packet combination method used by the receiver in advance, for example, a general packet combination method or an interference cancellation (IC) method.
  • the indication generated by the TAS module 312 is fed into the packet retransmission unit 219 to indicate a subset of retransmission symbols, and on the other hand, the indication is sent via the control channel to the receiver 350 to indicate the packet combination unit 259 therein to bring about the corresponding combination.
  • Fig.4 is an overall flow chart showing the operation of the TAS module in the above system.
  • the TAS module obtains all the possible subsets of retransmission symbols in step S410.
  • step S420 for each possible subset of retransmission symbols p, the TAS module establishes a corresponding equivalent signal model according to the CSI and the packet combination method, and calculates the post-detection SNR for each symbol by using the equivalent signal model.
  • r (1) [r 1 (1) , r 2 (1) ,..., r ⁇ ) ] r represents N R -dimension received signals received by the N R receive antennas in the transmission slot, where r ⁇ ⁇ is the received signal at the mth receive antenna.
  • Equation (2) is the actual MIMO channel matrix in the transmission slot.
  • Equation (2) is the actual MIMO channel matrix in the transmission slot.
  • Equation (2) is the actual MIMO channel matrix in the transmission slot.
  • Equation (2) The element in Equation (2) is the channel gain between the nth transmit antenna and the mth receive antenna, which is obtained through channel estimation.
  • n (1) [n ⁇ ⁇ ,r ⁇ ⁇ ,...,r ⁇ R ⁇ f is the AWGN (Additive White Gaussian Noise) at the receiver in the transmission slot, where each element is an i.i.d. cyclically symmetric complex Gaussian random variable with mean of zero and variance of No.
  • AWGN Additional White Gaussian Noise
  • r p (2) ] represents N R -dimension received signals in the retransmission slot corresponding to the subset of retransmission symbols p and the element r ⁇ 2) represents the received signal at the mth receive antenna.
  • H p 2 is the actual MIMO channel matrix in the retransmission slot corresponding to the subset of retransmission symbols/?, which is given by Equation (4): h (2) h 'X (2 y ) (p)
  • H (2) n 2,x(p) n 2,y(p) (4) h n N (2 R ) ,x(p) h n N (2 R ) ,y(p)
  • ti m n * represents the channel gain between the mth receive antenna and the n ⁇ h retransmit antenna.
  • n (2) [ « j 2) ,n 2 (2) ,...,n (2) f is the AWGN in the retransmission slot, where each element is an i.i.d. cyclically symmetric complex Gaussian random variable with mean of zero and variance of No.
  • the noise n (2) in the retransmission slot and the noise n (1) in the transmission slot are independent of each other.
  • a corresponding signal model may be established for each subset of retransmission symbols.
  • Two packet combination methods that is, the general packet combination method and the interference cancellation method, are provided in the embodiments of the invention. Detailed descriptions will be given below, in conjunction with Fig.6 and Fig.7, as regards the establishment procedure of the equivalent signal model in the case of the two packet combination methods.
  • the TAS module calculates the post-detection SNR for each symbol of each possible subset of retransmission symbols by using the established equivalent signal model according to the detection method used by the receiver.
  • a subset of retransmission symbols p whose detection BER is minimal is selected from all the possible subsets of retransmission symbols according to the calculated post-detection SNRs, and a corresponding indication is output.
  • the STBC retransmission format will be shown in Table 3 and its symbol transmission procedure will be shown in Fig.5. From Table 3 and Fig.5, it can be seen that the retransmission symbols (-S 4 *, S 2 *) and the transmitted symbols ( S 2 , S 4 ) form an Alamouti codeword to facilitate the detection of the receiver.
  • the retransmission symbols may be transmitted in the non-STBC mode and in this case the elements in Equation (3) will change accordingly in specific forms.
  • the TAS method in the case of the general packet combination method.
  • a corresponding equivalent signal model is established for each possible subset of retransmission symbols/? (step S610).
  • the TAS module uses the established equivalent signal model shown in Equation (5) to calculate the post-detection SNR for each of the total N T transmission symbols.
  • the packet combination unit in the receiver performs symbol detection according to a specific detection method, such as Zero-Forcing (ZF) and Minimum Mean Square Error (MMSE).
  • ZF Zero-Forcing
  • MMSE Minimum Mean Square Error
  • the post-detection SNRs for all the N ⁇ transmission symbols may be calculated according to Equations (7) and (8).
  • SNR (p> ,k l, 2,..., N T (ZF) (7)
  • step S630 for each subset of retransmission symbols p, a post-detection SNR minimum value is searched from the post-detection SNRs of all the N T transmission symbols, reflects the packet combination performance in the case of the pth subset of retransmission symbols.
  • the TAS method in the case of interference cancellation.
  • Fig.7 illustrates the operations of the TAS module when the receiver uses the interference cancellation method to perform packet combination.
  • the equivalent signal model indicates the relationship between the retransmission symbol s p in the subset of retransmission symbols p and the received signals in the retransmission slot. Therefore, a corresponding equivalent signal model is established for each possible subset of retransmission signals (step S710). Since the equivalent signal model in the interference cancellation mode only relates to the retransmission symbols and the corresponding received signal, the equivalent signal model is equivalent to the received signal model for the retransmission slot, as shown in Equation (3).
  • step S720 the equivalent signal model shown in Equation (3) is used to calculate the post-detection SNR for each of the retransmission symbols.
  • the packet combination method based on the interference cancellation first detects the retransmission symbols by using the calculated equivalent signal model according to a specific detection method, such as ZF or MMSE. Then, after cancelling the detected interference due to the retransmission symbols, detection continues for the remaining transmission symbols.
  • a specific detection method such as ZF or MMSE.
  • ZF or MMSE a specific detection method
  • the post-detection SNR is similar to the general packet combination method.
  • the post-detection SNR is calculated for each of the ⁇ retransmission symbols, as shown in Equations (9) and (10).
  • TAS method may reduce the impact of transmission errors on other symbols, and thus an overall better packet combination performance may be provided.
  • Fig.8 illustrates an exemplary structural block diagram of the TAS module used in the above embodiments of the present invention.
  • the TAS module comprises an acquisition unit 810, a channel model establishment unit 820, a calculation unit 830 and a determination unit 840.
  • the acquisition unit 810 is arranged to select a portion of symbols from the transmitted symbols to form a subset of retransmission symbols, and to obtain all possible subsets of retransmission symbols.
  • the channel model establishment unit 820 is arranged to establish an equivalent signal model for each of the possible subsets of retransmitted symbols based on the channel state information.
  • the equivalent signal model describes a relationship between symbols to be transmitted in the transmission slot and signals received in the transmission slot and the retransmission slot.
  • the equivalent signal model describes a relationship between symbols to be retransmitted in the retransmission slot and signals received in the retransmission slot.
  • the calculation unit 830 is arranged to calculate, for each of the possible subsets of retransmitted symbols, a post-detection SNR for each of the transmitted symbols or each of the retransmission symbols by using the equivalent signal model based on the detection method used.
  • the determination unit 840 is arranged to determine a subset of retransmission symbols having the minimum detection BER based on the calculated post-detection SNRs.
  • the proposed method to perform transmit antenna selection for a portion of transmission symbols according to the channel state information and the packet combination method used by the receiver can also be applied to HSS-MIMO systems.
  • a TAS-C module is added to select suitable transmit antennas for SM symbols.
  • the TAS-C module may be located in a transmitter or a receiver for the HSS-MIMO system.
  • the TAS-C module first selects a portion of transmit antennas from the known transmit antennas to form a subset of SM transmit antennas, and obtains all possible subsets of transmit antennas. Then, for each possible subset of SM transmit antennas, an equivalent signal model is established based on the detected channel state information and the SM-STBC coding format used in the transmitter.
  • the equivalent signal model describes a relationship between symbols to be transmitted during a transmission period and signals received in the corresponding detection period.
  • the TAS-C module calculates a post-detection SNR for each of the SM symbols by using the equivalent signal model in the case of each possible subset of SM transmit antennas. Thereafter, for each possible subset of SM transmit antennas, a post-detection SNR minimum value is searched from the calculated post-detection SNRs. Finally, the TAS-C module selects a subset having the maximum post-detection SNR minimum value from all the possible subsets of SM transmit antennas, so as to instruct the transmitter to transmit according to the indication about the subset of SM transmit antennas.
  • the method has taken into consideration the impact of the channel state on the SM symbol transmission, as well as the impact of the detection method used in the receiver on the SM symbol transmission.
  • the influence of channel conditions is considered in retransmission and a portion of retransmission symbols are selected by using the post-detection SNR based on the packet combination method used by the receiver. Accordingly, the method and apparatus yield a lower detection BER than the prior arts.
  • Fig.9 and Fig.10 schematically illustrate the simulation results of the existing PSM-HARQ system and the TAS-PSM-HARQ system proposed in the invention, both of which have 4 transmit antennas and 4 receive antennas, under the same MIMO channel condition and modulation and coding conditions.
  • Fig.9 shows the curve graph showing BER versus SNR of the above two systems
  • Fig.10 shows the curve graph showing BLER versus SNR. It can be seen from Fig.9 and Fig.10 that the TAS-PSM-HARQ system proposed in the invention can achieve a lower error rate and thus leads to a better performance.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Quality & Reliability (AREA)
  • Detection And Prevention Of Errors In Transmission (AREA)
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Abstract

The invention provides a multi-antenna transmission method. In this method, transmit antenna selection is performed for a portion of symbols to be transmitted based on channel state information from the transmit antennas to the receive antennas and a detection method used by the receiver, to select transmit antennas suitable for the portion of symbols, so that the transmitted symbols may have the lowest detection BER (Bit Error Rate). Based on the above idea, the invention proposes a PSM-HARQ method and apparatus based on transmit antenna selection, and it also proposes a method and apparatus for performing transmit antenna selection on spatial multiplexing signals in HSS-MIMO systems based on the post-detection SNR.

Description

Method and Apparatus for Transmitting Signals in a Multi-antenna System
Field of the Invention
The invention relates to a multi-antenna wireless communication system, and more particularly, to a method and apparatus for transmitting signals based on Transmit- Antenna-Selection in a multi-antenna system.
Background of the invention
A multi-antenna system typically refers to a wireless communication system having multiple transmit antennas at the transmitter and at least one receive antenna at the receiver, for example, a MIMO (Multiple-Input Multiple- Output) system having 4 transmit antennas as shown in Fig.2. By spatial multiplexing, the MIMO system allows parallel transmission of different data streams over multiple spatial channels established between the transmitter and the receiver, and thus provides a higher channel capacity. Accordingly, research on the MIMO system has recently become a main focus.
To take full advantage of the multiple parallel spatial channels in the MIMO system and decrease the BER (Bit Error Rate) for packet transmission as much as possible, many transmission schemes suitable for the MIMO system have been proposed recently. For example, a MIMO HARQ (Hybrid Automatic Retransmission reQuest) method for retransmission of a portion of symbols, abbreviated as PSM-HARQ (Partial-symbol retransmitted MIMO-HARQ), is proposed in patent application No. 200710001898.4, filed by the same applicant as the present invention on Feb. 6, 2007, and entitled "Method and Apparatus for Hybrid Automatic Repeat Request in Multiple Input Multiple Output System".
In this PSM-HARQ method, the transmitter first transmits multiple symbols Sx in a transmission slot via the corresponding transmit antennas to the receiver. If any transmission error occurs, i.e., the receiver returns a negative acknowledgment (NACK), the transmitter selects a portion of symbols from the transmitted multiple symbols and retransmits the selected portion of symbols via the corresponding transmit antennas in the retransmission slot. For example, a 4-transmit antenna system only retransmits two symbols corresponding to the first and second transmit antennas, as illustrated in Table 1 and Table 2. In Table 1 and Table 2, "0" means that no symbol is transmitted in the corresponding slot and antenna, and Sx represents the transmitted symbol. The two retransmitted symbols in Table 1 and the transmitted symbols over the corresponding transmit antennas form an Alamouti codeword (a codeword formed by Space-Time Block Coding), referred to as PSM-HARQ in STBC format whose transmission procedure is shown in Fig.l. The retransmitted symbols in Table 2 have the same format as the transmitted symbols, referred to as PSM-HARQ in non-STBC format. For the PSM-HARQ shown in Table 1 and Table 2, only a small amount of transmitted symbols (a portion of the originally transmitted symbols) are retransmitted so as to decrease power consumption and complexity for retransmission while improving the transmission performance by retransmission.
Table 1. PSM-HARQ in STBC format
Figure imgf000004_0001
Table 2. PSM-HARQ in non-STBC format
Figure imgf000004_0002
In the above PSM-HARQ method, however, retransmission symbols are selected in an arbitrary manner, in other words, a transmit antenna for retransmission is selected in an arbitrary manner. As such, the wireless channel condition associated with the transmit antenna for retransmission will produce an impact on the retransmission. If the channel condition is very poor, the retransmission will not necessarily achieve the purpose of ideally decreasing the transmission error.
There is, therefore, a need for a retransmission scheme to perform transmit antenna selection for a portion of symbols based on the channel state information.
In a reference entitled "Hybrid MIMO Transceiver Scheme with Antenna Allocation and Partial CSI at Transmitter Side"(IEEE/PIMRC-Personal Indoor and Mobile Radio Communications Conference) by Walter Freitas et al in 2006, another MIMO transmission scheme is proposed, that is SM (Spatial Multiplexing)-STBC (Space-Time Block Coding) hybrid MIMO system, abbreviated as HSS-MIMO (Hybrid SM-STBC MIMO) system.
In the proposed HSS-MIMO system, multiple symbols to be transmitted are transmitted to the wireless space simultaneously via multiple transmit antennas of the transmitter during a transmission period, using SM or STBC transmission. For example, in a HSS-MIMO system equipped with 4 transmit antennas, symbols over two transmit antennas are transmitted in STBC manner and symbols over the other two transmit antennas are transmitted in SM manner. Furthermore, in the proposed HSS-MIMO system, Walter Freitas et al proposed that the transmitter may select the transmit antennas with better channel conditions as the transmit antennas for transmission of SM symbols by using a transmit antenna selection method based on the channel state information without considering the complexity of the receiver. Therefore, the proposed HSS-MIMO system is able to select suitable transmit antennas for transmission of the SM symbols among the transmission symbols, so as to improve the transmission performance for the SM symbols, which in turn improves the transmission performance of the whole system.
As noted above, the HSS-MIMO system is proposed by Walter Freitas et al and does not take into consideration the complexity of the receiver. Thus, the transmit antenna selection is independent of the detection method used by the receiver. However, in a practical system, the detection method used by the receiver is directly related with the system's capacity for tolerating code errors. For the MIMO system especially, the detection method used by the receiver is likely to affect the success rate of the transmission.
There is, therefore, a further need to perform TAS (Transmit Antenna Selection) for a portion of symbols among the transmission symbols based on the channel state information and the detection method used by the receiver.
Summary of the Invention
An object of the invention is to provide a PSM-HARQ method and apparatus for facilitating an increase in the retransmission efficiency and a decrease in the system transmission BER while reducing the retransmission symbols.
Another object of the invention is to provide a HSS-MIMO transmission method and apparatus for performing transmit antenna selection for SM symbols taking into consideration the detection method used by the receiver, so as to improve the transmission performance for the SM symbols and to further improve the transmission performance of the whole system.
To fulfill the above objects, a PSM-HARQ method based on TAS is provided in an embodiment of the invention. In this method, when the receiver cannot correctly restore a symbol transmitted by the transmitter in a transmission slot, the receiver returns a feedback message including NACK to the transmitter, which feedback message indicates that a retransmission is required. Upon receipt of the NACK, a newly added TAS module selects a portion of symbols from the transmitted symbols to form a subset of retransmission symbols and obtains all possible subsets of retransmission symbols. Next, the TAS module establishes a corresponding equivalent signal model for each of the possible subsets of retransmission symbols and calculates a post-detection SNR for each of the transmitted symbols or retransmission symbols. Then, for each of the possible subsets of retransmission symbols, a post-detection SNR minimum value is searched from the post-detection SNRs of the various symbols. Finally, a subset of retransmission symbols having the maximum post-detection SNR minimum value is selected from all the possible subsets of retransmission symbols, so as to instruct the transmitter to transmit the selected subset of retransmission symbols via the corresponding transmit antennas. In this embodiment, the TAS module may be placed in the receiver or the transmitter.
Other objects and attainments together with a fuller 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
Detailed descriptions of the invention will be given below with reference to the accompanying drawings, in which,
Fig.l illustrates a symbol transmission procedure in a prior-art PSM-HARQ method;
Fig.2 illustrates a block diagram of a TAS-PSM-HARQ system, where the TAS module is implemented in the receiver according to an embodiment of the present invention;
Fig.3 illustrates a block diagram of a TAS-PSM-HARQ system, where the TAS module is implemented in the transmitter according to an embodiment of the present invention;
Fig.4 shows an overall flow chart of the TAS module according to an embodiment of the present invention; Fig.5 schematically illustrates the symbol transmission by using a TAS-PSM-HARQ method in a 4-transmit antenna system according to an embodiment of the present invention;
Fig.6 illustrates a detailed flow chart of the TAS module when the receiver uses a general packet combination method according to an embodiment of the present invention;
Fig.7 illustrates a detailed flow chart of the TAS module when the receiver uses an interference cancellation method according to another embodiment of the present invention;
Fig.8 illustrates a block diagram of the TAS module according to an embodiment of the present invention; and
Fig.9 and Fig.10 schematically illustrate simulation results when symbol transmission is carried out by using the TAS-PSM-HARQ method proposed in the present invention.
Throughout all of the above drawings, like reference numerals will be understood to refer to like, similar or corresponding features or functions.
Detailed Description of the Invention
According to the multi-antenna transmission method proposed in the invention, transmit antenna selection is made for a portion of transmission symbols based on the channel state information from the transmit antennas to the receive antennas and the detection method used by the receiver, so as to select transmit antennas suitable for transmission of the portion of transmission symbols, thus allowing the transmission symbols to have the minimum detection BER.
The above idea proposed in the invention may be applied to current PSM-HARQ systems, as well as current HSS-MIMO systems. Detailed descriptions will be given in conjunction with different embodiments as to how to implement the above idea of the present invention in the two applications mentioned above.
The architecture of the TAS-PSM-HARQ system
To implement the above transmit antenna selection method as proposed in the present invention, a TAS (Transmit Antenna Selection) module is added to the current PSM-HARQ system, so as to form a PSM-HARQ based on TAS, abbreviated as TAS-PSM-HARQ. Here, the added TAS module is used to select the transmit antennas that are most suitable for retransmission.
For the PSM-HARQ method, the transmitted symbols and the retransmitted symbols correspond to the transmit antennas in a one-to-one relationship, as shown in Table 1 and Table 2. Thus, selection of the transmit antennas most suitable for retransmission is equivalent to selection of retransmission symbols from the transmitted symbols. Therefore, the function of the TAS module in embodiments of the invention is, in essence, to select suitable retransmission symbols according to the CSI (Channel State Information) and the packet combination method used by the receiver, so as to decrease the detection BER.
Fig.2 and Fig.3 illustrate two TAS-PSM-HARQ systems with NT transmit antennas and NR receive antennas, respectively.
In the TAS-PSM-HARQ system of Fig.2, the TAS module is implemented in the receiver. As shown in Fig.2, after being subjected to CRC, channel coding, interleaving and modulation in a sequential order, the data to be transmitted is fed into a SM (Spatial Multiplexing) unit 215. The SM unit 215 assigns the NT modulated symbols to the NT transmit antennas so as to transmit them in the same transmission slot to the wireless channel. Afterwards, the receiver 250 receives the signals in the transmission slot via the NR receive antennas. Next, after being subjected to detection at SM detection unit 255, demodulation, de-interleaving and channel decoding, the received signals are fed into the CRC unit. The CRC unit of the receiver 250 sends a feedback message back to the transmitter 210 according to the CRC result, the feedback message including an acknowledge information / a negative acknowledge information (ACK/NACK). The feedback message is used to indicate whether data restoration is successful. If the feedback message includes a NACK, it means that data restoration fails and retransmission is required.
When a NACK is included in the feedback message ( retransmission is required), the TAS module 252 in the receiver selects a subset of transmit antennas suitable for retransmission, that is, a suitable subset of retransmission symbols, according to the CSI measured at the SM detection unit 255 and the packet combination method used by the packet combination unit 259, and generates a corresponding indication. The indication is sent together with the feedback message via the feedback channel to the transmitter 210.
Upon receipt of the NACK, the packet retransmission unit 219 in the transmitter 210 selects the corresponding retransmission symbols according to the indication from the receiver 250 and sends the retransmission symbols in the retransmission slot via the corresponding transmit antennas to the receiver 250. The receiver 250 then receives signals in the retransmission slot. The received signals are combined and detected in the packet combination unit 259, and then sent to the subsequent processing units for data restoration.
The TAS module of the invention has to acquire the CSI from the transmitter to the receiver for selection of retransmission symbols and accordingly the CSI may be obtained easily if the TAS module is disposed in the receiver. Alternatively, the TAS module may be disposed in the transmitter, for example disposed in a transmitter of a TDD system as shown in Fig.3.
The difference between the TAS-PSM-HARQ system of Fig.3 and that of Fig.2 is that a TAS module 312 is added to the TDD transmitter 310, and the receiver 350 is substantially similar to the existing TDD receiver. Similar to Fig.2, when the transmitter 310 receives a NACK transmitted via the feedback channel from the receiver 350, the TAS module 312 selects a suitable subset of retransmission symbols according to the estimated CSI and the predetermined packet combination method, and forms a corresponding indication. Here, the CSI from the transmitter to the receiver may be obtained through a reverse channel estimation based on the CSI from the receiver to the transmitter detected by the transmitter. The reverse channel estimation is accurate because the TDD channel on a round trip in a short period may be considered constant and reciprocal. Furthermore, the TAS module 312 needs to obtain the packet combination method used by the receiver in advance, for example, a general packet combination method or an interference cancellation (IC) method. In Fig.3, on the one hand, the indication generated by the TAS module 312 is fed into the packet retransmission unit 219 to indicate a subset of retransmission symbols, and on the other hand, the indication is sent via the control channel to the receiver 350 to indicate the packet combination unit 259 therein to bring about the corresponding combination.
The TAS operation in the TAS-PSM-HARQ system
Since a TAS module is added to the systems of Fig.2 and Fig.3, the retransmission symbols are no longer selected in an arbitrary manner. Rather, a subset of retransmission symbols whose detection BER is minimal is selected according to the CSI and the packet combination method. Fig.4 is an overall flow chart showing the operation of the TAS module in the above system.
As shown in Fig.4, the TAS module obtains all the possible subsets of retransmission symbols in step S410. For example, in a TAS-PSM-HARQ system having N^A transmit antennas, the transmitter may transmit NT^A symbols (e.g., [.S1 , s2 , s3 , s4 \) simultaneously in the transmission slot over 4 transmit antennas. These symbols form a transmit symbol vectors = [S1, S2, S3, S4] . In the retransmission slot, the transmitter only needs to transmit Kj symbols of the Nr=A transmit symbols, where Kj<Nτ. For a system with 4 transmit antennas, assuming that Kr=I, there are CN T = C4 = 6 subsets of retransmission symbols p in total that may be used as the retransmission symbols Csx, sy) . Thus, all the possible subsets of retransmission symbols p = (sx(p),sy(p)) e P may be found, where
P = ( (S1 , S2 ), (S1 , S3 ), (S1 , S4 ), (S2 , S3 ), (S2 5 S4 ), (S3 , S4 ) }
In step S420, for each possible subset of retransmission symbols p, the TAS module establishes a corresponding equivalent signal model according to the CSI and the packet combination method, and calculates the post-detection SNR for each symbol by using the equivalent signal model.
Here, it is assumed that the received signal model in the transmission slot may be given by Equation (1): r(D = H(i)s(i) + n(i) ( 1 ) where the superscript 1 denotes the signal model for the transmission slot. r(1) = [r1 (1), r2 (1),..., r^)]r represents NR-dimension received signals received by the NR receive antennas in the transmission slot, where r^} is the received signal at the mth receive antenna.
H(1) is the actual MIMO channel matrix in the transmission slot. The matrix may be given by Equation (2):
• • ή "l,(,1N) T
Λ2 • • • n 2(1,N) T
Hw = (2)
KU *£, • nNR ,NT
The element in Equation (2) is the channel gain between the nth transmit antenna and the mth receive antenna, which is obtained through channel estimation. s(1) = [5'1 1) 55'2 1) 5...55'^) ]71 represents the transmit symbol vector in the transmission slot, where s(χ) is the symbol transmitted over the nth transmit antenna in the transmission slot and the symbol is assumed to be an i.i.d. (independent and identically distributed) random variable with mean of zero and variance of σ2. In the above system with 4 transmit antennas, the transmit symbol vector is s(1) =
Figure imgf000011_0001
]τ = [sl , s2, s3 , s4f . n(1) = [n^} ,rξ} ,...,rξR } f is the AWGN (Additive White Gaussian Noise) at the receiver in the transmission slot, where each element is an i.i.d. cyclically symmetric complex Gaussian random variable with mean of zero and variance of No.
Based on the received signal model in Equation (1), for each possible subset of retransmission symbols p = (sx(p),sy(p)) e P , the received signal model in the retransmission slot may be obtained correspondingly, as expressed in Equation (3):
Figure imgf000011_0002
where rp (2) =
Figure imgf000011_0003
] represents NR-dimension received signals in the retransmission slot corresponding to the subset of retransmission symbols p and the element r^2) represents the received signal at the mth receive antenna.
Hp 2) is the actual MIMO channel matrix in the retransmission slot corresponding to the subset of retransmission symbols/?, which is given by Equation (4): h(2) h 'X(2y)(p)
H(2) = n2,x(p) n2,y(p) (4) h nN(2 R ),x(p) h nN(2 R ),y(p)
where the element tim n* represents the channel gain between the mth receive antenna and the n\h retransmit antenna. sp2) = [51X(P) >4o>) J7" = [~sy*<.p)> sχ*(.p) ]r rePresents the retransmission symbol vector for the subset of retransmission symbols p in the STBC mode, where the element sn (2) is the retransmission symbol from the n\h transmit antenna in the retransmission slot. n(2) = [«j 2) ,n2 (2) ,...,n(2)f is the AWGN in the retransmission slot, where each element is an i.i.d. cyclically symmetric complex Gaussian random variable with mean of zero and variance of No. The noise n(2) in the retransmission slot and the noise n(1) in the transmission slot are independent of each other. On the basis of the received signal models in the transmission slot and the retransmission slot given by Equation (1) and Equation (3) in combination with the packet combination method used by the receiver, a corresponding signal model may be established for each subset of retransmission symbols. Two packet combination methods, that is, the general packet combination method and the interference cancellation method, are provided in the embodiments of the invention. Detailed descriptions will be given below, in conjunction with Fig.6 and Fig.7, as regards the establishment procedure of the equivalent signal model in the case of the two packet combination methods.
After the equivalent signal model is established, the TAS module calculates the post-detection SNR for each symbol of each possible subset of retransmission symbols by using the established equivalent signal model according to the detection method used by the receiver.
Finally in step S430, a subset of retransmission symbols p whose detection BER is minimal is selected from all the possible subsets of retransmission symbols according to the calculated post-detection SNRs, and a corresponding indication is output. For example, in the above system with 4 transmit antennas, assuming that the TAS module has selected a subset of retransmission symbols (^. si) through the above calculation, the STBC retransmission format will be shown in Table 3 and its symbol transmission procedure will be shown in Fig.5. From Table 3 and Fig.5, it can be seen that the retransmission symbols (-S4*, S2*) and the transmitted symbols ( S2, S4) form an Alamouti codeword to facilitate the detection of the receiver. Alternatively, the retransmission symbols may be transmitted in the non-STBC mode and in this case the elements in Equation (3) will change accordingly in specific forms.
Table 3: TAS-PSM-HARQ in STBC format
Figure imgf000012_0001
The TAS method in the case of the general packet combination method.
Descriptions regarding the overall flow chart of the TAS module as proposed in the invention are given above, with reference to the accompanying drawings. In the above overall flow chart, the establishment of the equivalent signal model depends on the packet combination method used by the receiver. Separate descriptions will be given below, with reference to Fig.6 and Fig.7, regarding the specific processes of the TAS module in the case of the general packet combination method and the interference cancellation method.
As illustrated in Fig.6, when the packet combination unit of the receiver uses the general packet combination method, the equivalent signal model fp = Hps + n describes the relationship between the symbols s to be transmitted in the subset of retransmission symbols p and the received signals fp in the transmission slot and the retransmission slot. As such, a corresponding equivalent signal model is established for each possible subset of retransmission symbols/? (step S610).
For example, for a subset of retransmission symbols p = 5 , i.e., the subset (s2, s4) , the corresponding equivalent signal model is:
f5 = H5S + ή (5)
where f5 = [(r (1) f , -(r5 (2) )H f , s = [S1 , s2 , s3 , s4 f n = [(n(1) f , -(n(2) )H f and
Ki
Λg Kl z(1) nNR ,4
Hc = (6)
O -C O h nϊ!-,22r
Figure imgf000013_0001
O -h (2) * 'NR A O h (2) * W»,2
Thereafter, in step S620, for each subset of retransmission symbols p, the TAS module uses the established equivalent signal model shown in Equation (5) to calculate the post-detection SNR for each of the total NT transmission symbols. Typically, the packet combination unit in the receiver performs symbol detection according to a specific detection method, such as Zero-Forcing (ZF) and Minimum Mean Square Error (MMSE). For the two detection methods of ZF and MMSE, the post-detection SNRs for all the Nτ transmission symbols may be calculated according to Equations (7) and (8). SNR(p> ,k = l, 2,..., NT (ZF) (7)
Figure imgf000014_0001
SNRk (p) , k = 1, 2, ..., Nτ (MMSE) ( 8 )
Figure imgf000014_0002
where I^ represents an identity matrix of size N7. , and {X}k k represents the (k,k) th element in the matrix X.
Thereafter, in step S630, for each subset of retransmission symbols p, a post-detection SNR minimum value is searched from the post-detection SNRs of all the NT transmission symbols,
Figure imgf000014_0003
reflects the packet combination performance in the case of the pth subset of retransmission symbols.
Finally, in step S640, the TAS module selects a subset of retransmission symbols having the maximum SNR^ from all the subsets of retransmission symbols P, that is p(sd) = arg max SNR^ . Since the selected subset of retransmission symbols p(sd> has the peP maximum post-detection SNR minimum value, the detection BER may be minimized, thus providing a better packet combination performance.
The TAS method in the case of interference cancellation.
Fig.7 illustrates the operations of the TAS module when the receiver uses the interference cancellation method to perform packet combination. As illustrated in Fig.7, when the receiver employs the interference cancellation method, the equivalent signal model indicates the relationship between the retransmission symbol sp in the subset of retransmission symbols p and the received signals in the retransmission slot. Therefore, a corresponding equivalent signal model is established for each possible subset of retransmission signals (step S710). Since the equivalent signal model in the interference cancellation mode only relates to the retransmission symbols and the corresponding received signal, the equivalent signal model is equivalent to the received signal model for the retransmission slot, as shown in Equation (3).
Thereafter, in step S720, the equivalent signal model shown in Equation (3) is used to calculate the post-detection SNR for each of the retransmission symbols. Typically, the packet combination method based on the interference cancellation first detects the retransmission symbols by using the calculated equivalent signal model according to a specific detection method, such as ZF or MMSE. Then, after cancelling the detected interference due to the retransmission symbols, detection continues for the remaining transmission symbols. Thus, calculation of the post-detection SNR is similar to the general packet combination method. Based on the equivalent signal model shown in Equation (3), the post-detection SNR is calculated for each of the ^retransmission symbols, as shown in Equations (9) and (10).
,k = l,2,...,Kτ (ZF ) (9)
Figure imgf000015_0001
2
SNR[P) = ■,■ „ °s : 1 ,k = l,2,...,KT (MMSE) ( 10)
Thereafter, similar to the general packet combination method, for each subset of retransmission symbols p, a post-detection SNR minimum value is searched in step S730, that is, SNR^ = argmin SNRk (p) . Since the detection result of the retransmission symbols will be
£=1,2, ,KT used for the detection of the remaining symbols immediately,
Figure imgf000015_0002
at this time reflects the detection performance of the retransmission symbols.
Finally, in step S740, the TAS module selects a subset of retransmission symbols having the maximum SNR^ , that is p(sel) = argmax SNR^ . Since the selected subset of
retransmission symbols p(sel) has the maximum post-detection SNR minimum value, the
TAS method may reduce the impact of transmission errors on other symbols, and thus an overall better packet combination performance may be provided.
The above method executed in the TAS module of the invention may be implemented in software or hardware, such as a large scale integrated circuit, DSP, a programmable logic gate circuit or the like, or in a combination of both. Fig.8 illustrates an exemplary structural block diagram of the TAS module used in the above embodiments of the present invention. As shown in Fig.8, the TAS module comprises an acquisition unit 810, a channel model establishment unit 820, a calculation unit 830 and a determination unit 840. The acquisition unit 810 is arranged to select a portion of symbols from the transmitted symbols to form a subset of retransmission symbols, and to obtain all possible subsets of retransmission symbols. The channel model establishment unit 820 is arranged to establish an equivalent signal model for each of the possible subsets of retransmitted symbols based on the channel state information. As described above, when the receiver uses the general packet combination method as its packet combination method, the equivalent signal model describes a relationship between symbols to be transmitted in the transmission slot and signals received in the transmission slot and the retransmission slot. When the receiver uses the interference cancellation method as its packet combination method, the equivalent signal model describes a relationship between symbols to be retransmitted in the retransmission slot and signals received in the retransmission slot. The calculation unit 830 is arranged to calculate, for each of the possible subsets of retransmitted symbols, a post-detection SNR for each of the transmitted symbols or each of the retransmission symbols by using the equivalent signal model based on the detection method used. The determination unit 840 is arranged to determine a subset of retransmission symbols having the minimum detection BER based on the calculated post-detection SNRs. With the TAS module shown in Fig.8, it would be easy to select the retransmission symbols based on the channel state information and the packet combination method used by the receiver.
Descriptions are given above, with reference to Figs. 2-7, regarding the TAS-PSM-HARQ method and apparatus proposed according to the idea of the invention. The proposed method to perform transmit antenna selection for a portion of transmission symbols according to the channel state information and the packet combination method used by the receiver can also be applied to HSS-MIMO systems.
In a HSS-MIMO system according to an embodiment of the invention, a TAS-C module is added to select suitable transmit antennas for SM symbols. The TAS-C module may be located in a transmitter or a receiver for the HSS-MIMO system. The TAS-C module first selects a portion of transmit antennas from the known transmit antennas to form a subset of SM transmit antennas, and obtains all possible subsets of transmit antennas. Then, for each possible subset of SM transmit antennas, an equivalent signal model is established based on the detected channel state information and the SM-STBC coding format used in the transmitter. The equivalent signal model describes a relationship between symbols to be transmitted during a transmission period and signals received in the corresponding detection period. Then, the TAS-C module calculates a post-detection SNR for each of the SM symbols by using the equivalent signal model in the case of each possible subset of SM transmit antennas. Thereafter, for each possible subset of SM transmit antennas, a post-detection SNR minimum value is searched from the calculated post-detection SNRs. Finally, the TAS-C module selects a subset having the maximum post-detection SNR minimum value from all the possible subsets of SM transmit antennas, so as to instruct the transmitter to transmit according to the indication about the subset of SM transmit antennas. With the TAS-C module selecting SM transmit antennas based on the post-detection SNR, the method has taken into consideration the impact of the channel state on the SM symbol transmission, as well as the impact of the detection method used in the receiver on the SM symbol transmission.
Advantageous Effects
According to the TAS-PSM-HARQ method and apparatus proposed on the basis of the idea of the present invention, the influence of channel conditions is considered in retransmission and a portion of retransmission symbols are selected by using the post-detection SNR based on the packet combination method used by the receiver. Accordingly, the method and apparatus yield a lower detection BER than the prior arts.
Fig.9 and Fig.10 schematically illustrate the simulation results of the existing PSM-HARQ system and the TAS-PSM-HARQ system proposed in the invention, both of which have 4 transmit antennas and 4 receive antennas, under the same MIMO channel condition and modulation and coding conditions. Fig.9 shows the curve graph showing BER versus SNR of the above two systems, while Fig.10 shows the curve graph showing BLER versus SNR. It can be seen from Fig.9 and Fig.10 that the TAS-PSM-HARQ system proposed in the invention can achieve a lower error rate and thus leads to a better performance.
It is to be noted that the above embodiments are intended to be illustrative of, rather than limiting, the present invention. Furthermore, it is to be understood by those skilled in the art that various modifications may be made to the method and apparatus as proposed in the invention without departing from the scope of the invention. Therefore, the scope of the present invention is to be defined by the attached claims. Additionally, any reference numerals in the claims shall not be construed as limiting the scope of the present invention.

Claims

What is claimed is:
1. A method for hybrid automatic repeat request in a receiver of a multi-antenna system, comprising the steps of:
Al. receiving multiple symbols transmitted simultaneously in a transmission slot;
A2. sending a feedback message over a feedback channel if the received symbols cannot be restored successfully, the feedback message indicating that a retransmission is required;
A3. receiving symbols retransmitted in a retransmission slot, the retransmitted symbols being a subset of the transmitted multiple symbols, the subset being selected based on channel state information and a packet combination method that the receiver uses for receiving signals; and
A4. restoring corresponding data based on signals received in the transmission slot and the retransmission slot.
2. The method as set forth in claim 1, wherein step A2 comprises: selecting a portion of symbols from the transmitted multiple symbols to form a subset of retransmission symbols, and obtaining all possible subsets of retransmission symbols; for each of the possible subsets of retransmission symbols, establishing a corresponding equivalent signal model based on the channel state information and the packet combination method, and calculating a post-detection SNR for each symbol; selecting a subset of retransmission symbols having the minimum detection BER based on the calculated post-detection SNRs; and sending an indication about the selected subset of retransmission symbols over the feedback channel.
3. The method as set forth in claim 2, wherein when the packet combination method is a general packet combination method, the equivalent signal model describes a relationship between symbols to be transmitted in the transmission slot and signals received in the transmission slot and the retransmission slot; and the step of establishing the equivalent signal model and calculating the post-detection SNR further comprising: for each of the possible subsets of retransmission symbols, calculating a post-detection SNR for each of the transmitted symbols by using the equivalent signal model based on a detection method used.
4. The method as set forth in claim 2, wherein when the packet combination method is an interference cancellation method, the equivalent signal model describes a relationship between symbols to be retransmitted in the retransmission slot and signals received in the retransmission slot; and the step of establishing the equivalent signal model and calculating the post-detection SNR further comprising: for each of the possible subsets of retransmission symbols, calculating a post-detection SNR for each of the retransmission symbols by using the equivalent signal model based on a detection method used.
5. The method as set forth in claim 3 or 4, wherein the step of selecting a subset of retransmission symbols having the minimum detection BER comprises: for each of the possible subsets of retransmission symbols, searching a post-detection SNR minimum value from the calculated post-detection SNRs; and selecting a subset of retransmission symbols having the maximum post-detection SNR minimum value from the possible subsets of retransmission symbols, to achieve a minimum detection BER.
6. The method as set forth in claim 5, wherein the detection method is ZF detection or MMSE detection.
7. The method as set forth in claim 1, wherein step A3 comprises: receiving an indication, the indication indicating the selected subset of retransmission symbols; and receiving the retransmission symbols transmitted in the retransmission slot according to the indication.
8. The method as set forth in claim 1, wherein the transmitted multiple symbols include 4 symbols and the subset of retransmission symbols contains two retransmission symbols, which form an Alamouti codeword with the transmitted symbols over the corresponding antennas in the transmission slot.
9. A method for hybrid automatic repeat request in a transmitter of a multi-antenna system, comprising steps of:
Bl. transmitting a multiple symbol to a wireless channel over corresponding multiple antennas in a transmission slot;
B2. receiving a corresponding feedback message from a receiver, the feedback message indicating whether a retransmission is required;
B3. determining retransmission symbols if the feedback message indicates that a retransmission is required, the retransmission symbols being a subset of the transmitted multiple symbols, the subset being selected based on channel state information of the wireless channel and a predetermined packet combination method that the receiver uses for receiving signals; and
B4. transmitting the determined retransmission symbols in a retransmission slot over corresponding transmit antennas.
10. The method as set forth in claim 9, wherein step B3 comprises: selecting a portion of symbols from the transmitted multiple symbols to form a subset of retransmission symbols, and obtaining all possible subsets of retransmission symbols; for each of the possible subsets of retransmission symbols, establishing a corresponding equivalent signal model based on the channel state information and the packet combination method, and calculating a post-detection SNR for each symbol; selecting a subset of retransmission symbols having the minimum detection BER based on the calculated post-detection SNRs; and sending an indication about the selected subset of retransmission symbols over a control channel.
11. The method as set forth in claim 10, wherein when the packet combination method is a general packet combination method, the equivalent signal model describes a relationship between symbols to be transmitted in the transmission slot and signals received in the transmission slot and the retransmission slot; and the step of establishing the equivalent signal model and calculating the post-detection SNR further comprising: for each of the possible subsets of retransmission symbols, calculating the post-detection SNR for each of the transmitted symbols by using the equivalent signal model based on a predetermined detection method used.
12. The method as set forth in claim 10, wherein when the packet combination method is an interference cancellation method, the equivalent signal model describes a relationship between symbols to be retransmitted and signals received in the retransmission slot; and the step of establishing the equivalent signal model and calculating the post-detection SNR further comprising: for each of the possible subsets of retransmission symbols, calculating the post-detection SNR for each of the retransmission symbols by using the equivalent signal model based on a predetermined detection method used.
13. The method as set forth in claim 11 or 12, wherein the step of determining a subset of retransmission symbols having the minimum detection BER comprises: for each of the possible subsets of retransmission symbols, searching a post-detection SNR minimum value from the calculated post-detection SNRs; and selecting a subset of retransmission symbols having the maximum post-detection SNR minimum value from the possible subsets of retransmission symbols.
14. The method as set forth in claim 13, wherein the detection method is ZF detection or MMSE detection.
15. The method as set forth in claim 10, wherein the channel state information is obtained via reverse channel estimation based on the channel state information of a reverse wireless channel.
16. The method as set forth in claim 9, wherein the feedback message further comprises an indication about the selected subset of retransmission symbols, and the step of B3 further comprising: determining the retransmission symbols according to the received indication about the selected subset of retransmission symbols.
17. The method as set forth in claim 9, wherein the multi-antenna system is a system with 4 transmit antennas, and the retransmission symbols include two retransmission symbols, which form an Alamouti codeword with the transmitted symbols over the corresponding transmit antennas in the transmission slot.
18. A receiver for a multi-antenna system, comprising: a spatial multiplexing detection unit, for receiving multiple symbols transmitted in a transmission slot and generating a corresponding feedback message according to the received signals; a packet combination unit, for receiving symbols retransmitted in a retransmission slot when the feedback message indicates that a retransmission is required, and restoring the corresponding data based on signals received in the transmission slot and the retransmission slot; a transmit antenna selection unit, for selecting symbols to be retransmitted from the transmitted multiple symbols, based on channel state information and a packet combination method used by the packet combination unit; and a transmission unit, for transmitting an indication about the selected symbols to be retransmitted and the feedback message from the transmit antenna selection unit to a wireless channel.
19. The receiver as set forth in claim 18, wherein when the packet combination unit uses a general packet combination method, the transmit antenna selection unit comprises: an acquisition unit, for selecting a portion of symbols from the transmitted multiple symbols to form a subset of retransmission symbols, and obtaining all possible subsets of retransmission symbols; a channel model establishment unit, for establishing an equivalent signal model for each of the possible subsets of retransmission symbols based on the channel state information, wherein the equivalent signal model describes a relationship between symbols to be transmitted in the transmission slot and signals received in the transmission slot and the retransmission slot; a calculation unit, for each of the possible subsets of retransmission symbols, for calculating a post-detection SNR for each of the transmitted symbols by using the equivalent signal model based on a detection method used; and a determination unit, for determining a subset of retransmission symbols having the minimum detection BER based on the calculated post-detection SNRs.
20. The receiver as set forth in claim 18, wherein when the packet combination unit uses an interference cancellation method, the transmit antenna selection unit comprises: an acquisition unit, for selecting a portion of symbols from the transmitted multiple symbols to form a subset of retransmission symbols, and obtaining all possible subsets of retransmission symbols; a channel model establishment unit, for establishing an equivalent signal model for each of the possible subsets of retransmission symbols based on the channel state information, wherein the equivalent signal model describes a relationship between symbols to be retransmitted in the retransmission slot and signals received in the retransmission slot; a calculation unit, for each of the possible subsets of retransmission symbols, for calculating a post-detection SNR for each of the retransmission symbols by using the equivalent signal model based on a detection method used; and a determination unit, for determining a subset of retransmission symbols having the minimum detection BER based on the calculated post-detection SNRs.
21. A transmitter for a multi-antenna system, comprising: a spatial multiplexing unit, for transmitting multiple symbols to a wireless channel over corresponding transmit antennas in a transmission slot; a transmit antenna selection unit, for selecting a portion of symbols as a subset of retransmission symbols from the transmitted multiple symbols based on an estimated channel state information of the wireless channel and a predetermined packet combination method, when an obtained feedback message indicates that a retransmission is required; a packet retransmission unit, for retransmitting the subset of retransmission symbols selected by the transmit antenna selection unit over corresponding transmit antennas; and a transmission unit, for transmitting an indication about the selected subset of retransmission symbols over a control channel.
22. The transmitter as set forth in claim 21, wherein when a general packet combination method is used, the transmit antenna selection unit comprises: an acquisition unit, for selecting a portion of symbols from the transmitted multiple symbols to form a subset of retransmission symbols, and obtaining all possible subsets of retransmission symbols; a channel model establishment unit, for establishing an equivalent signal model for each of the possible subsets of retransmission symbols based on the channel state information, wherein the equivalent signal model describes a relationship between symbols transmitted in the transmission slot and signals received in the transmission slot and the retransmission slot; a calculation unit, for each of the possible subsets of retransmission symbols, for calculating a post-detection SNR for each of the transmitted symbols by using the equivalent signal model based on a detection method used; and a determination unit, for determining a subset of retransmission symbols having the minimum detection BER based on the calculated post-detection SNRs.
23. The transmitter as set forth in claim 21, wherein when an interference cancellation method is used, the transmit antenna selection unit comprises: an acquisition unit, for selecting a portion of symbols from the transmitted multiple symbols to form a subset of retransmission symbols, and obtaining all possible subsets of retransmission symbols; a channel model establishment unit, for establishing an equivalent signal model for each of the possible subsets of retransmission symbols based on the channel state information, wherein the equivalent signal model describes a relationship between symbols retransmitted in the retransmission slot and signals received in the retransmission slot; a calculation unit, for each of the possible subsets of retransmission symbols, for calculating a post-detection SNR for each of the retransmission symbols by using the equivalent signal model based on a detection method used; and a determination unit, for determining a subset of retransmission symbols having the minimum detection BER based on the calculated post-detection SNRs.
PCT/IB2008/051029 2007-03-23 2008-03-19 Method and apparatus for transmitting signals in a multi-antenna system WO2008117207A1 (en)

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