Method and apparatus for selecting retransmission mode in a MIMO communication system

A method and apparatus for selecting a retransmission mode in a MIMO communication system are provided wherein a receiver receives m initial transmission streams from a transmitter, m being an integer of 2 or larger, channel-decodes the m initial transmission streams and checking errors in the m channel-decoded streams, selects, if at least two channel-decoded streams have errors among the m channel-decoded streams, a retransmission mode to be used for the at least two streams having errors in the transmitter, and transmits information including the selected retransmission mode to the transmitter.

The present application claims the benefit under 35 U.S.C. §119(a) of a Korean Patent Application filed in the Korean Intellectual Property Office on Mar. 20, 2008 and assigned Serial No. 10-2008-0026071, the entire disclosure of which is hereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and apparatus for selecting a retransmission mode in a Multiple Input Multiple Output (MIMO) communication system.

BACKGROUND OF THE INVENTION

Retransmission schemes have been proposed to increase the efficiency of Hybrid Automatic Repeat Request (HARQ) in a MIMO system.

One of the retransmission schemes is basis hopping that artificially provides time diversity to a channel in order to solve the problem of the decrease of HARQ gain on a slow fading channel.

Another retransmission scheme is Trellis-Coded Modulation (TCM) reallocation that reallocates a TCM code to multiple antennas at every retransmission.

A third retransmission scheme is antenna permutation that changes the mapping relation between a transmission stream and an antenna at every retransmission. The second and third retransmission schemes commonly provide time diversity to a slow fading channel.

The other retransmission scheme is to form an Alamouti Space Time Code (STC) by combining retransmission information with initial transmission information.

The first, second and third retransmission schemes aim to provide time diversity in a slow fading channel environment. Therefore, a desired performance improvement cannot be achieved in a fast fading channel environment already having time diversity.

The last retransmission scheme is based on the premise that a retransmission channel is almost constant to form an Alamouti STC with initial transmission information and retransmission information. A desired performance improvement can be achieved only in the slow fading channel environment.

However, in a real wireless communication environment, the channel environment mostly changes at every retransmission because of a time delay involved in signal processing and feedback. Moreover, if a channel is in deep fading status, it may be better to delay retransmission of erroneous information until the deep fading channel status is released. Accordingly, there exists a need for a new retransmission scheme adapted to various real channel environments in order to increase the efficiency of HARQ.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to address at least the problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of exemplary embodiments of the present invention is to provide a method and apparatus for selecting a retransmission mode in a MIMO communication system.

In accordance with an aspect of exemplary embodiments of the present invention, there is provided a method for selecting a retransmission mode in a receiver of a MIMO communication system wherein the receiver receives m initial transmission streams from a transmitter, m being an integer of 2 or larger, channel-decodes the m initial transmission streams and checking errors in the m channel-decoded streams, selects, if at least two channel-decoded streams have errors among the m channel-decoded streams, a retransmission mode to be used for the at least two streams having errors in the transmitter, and transmits information including the selected retransmission mode to the transmitter.

In accordance with another aspect of exemplary embodiments of the present invention, there is provided a method for decoding a retransmission stream in a receiver of a MIMO communication system wherein the receiver receives m retransmission streams from a transmitter, m being an integer of 2 or larger, checks errors in the m retransmission streams, space-time-decodes one stream, if the one stream has errors among the m retransmission streams, and checks retransmission-related information fed back to the transmitter, if at least two streams have errors among the m streams. The retransmission-related information includes an ACK/NACK signal indicating whether initial transmission was failed or successful for initial transmission streams, retransmission mode selection information indicating a retransmission mode for retransmission of an initial transmission stream, and retransmission stream selection information indicating an initial transmission stream to be retransmitted.

In accordance with a further aspect of exemplary embodiments of the present invention, there is provided a retransmission method of a transmitter in a MIMO communication system wherein the transmitter transmits m initial transmission streams to a receiver, m being an integer of 2 or larger, receives retransmission-related information about the m streams from the receiver, and retransmits, if the retransmission-related information includes NACK information indicating initial transmission failure for at least two initial transmission streams, the at least two initial transmission streams in a retransmission mode indicated by retransmission mode information included in the retransmission-related information.

In accordance with still another aspect of exemplary embodiments of the present invention, there is provided a retransmission method of a transmitter in a MIMO communication system wherein the transmitter transmits m initial transmission streams to a receiver, m being an integer of 2 or larger, retransmits one stream to the receiver in an STC mode, upon receipt of a NACK signal indicating initial transmission failure for the one stream, and retransmits two streams to the receiver in an SM mode, upon receipt of NACK signals indicating initial transmission failure for the two streams among the m streams.

In accordance with yet another aspect of exemplary embodiments of the present invention, there is provided an apparatus for selecting a retransmission mode in a MIMO communication system wherein m receive antennas receive m initial transmission streams from a transmitter, m being an integer of 2 or larger, and a retransmission information generator channel-decodes the m initial transmission streams, checking errors in the m channel-decoded streams, selects, if at least two channel-decoded streams have errors among the m channel-decoded streams, a retransmission mode to be used for the at least two streams having errors in the transmitter, and controls the m receive antennas to transmit information indicating the selected retransmission mode to the transmitter.

In accordance with yet still another aspect of exemplary embodiments of the present invention, there is provided an apparatus for decoding a retransmission stream in a receiver of a MIMO communication system wherein a receive antenna receives m retransmission streams from a transmitter, m being an integer of 2 or larger, and a channel decoder checks errors in the m retransmission streams, space-time-decodes one stream, if the one stream has errors among the m retransmission streams, and checks retransmission-related information fed back to the transmitter, if at least two streams have errors among the m streams. The retransmission-related information includes an ACK/NACK signal indicating whether initial transmission was failed or successful for initial transmission streams, retransmission mode selection information indicating a retransmission mode for retransmission of an initial transmission stream, and retransmission stream selection information indicating an initial transmission stream to be retransmitted.

In accordance with still another aspect of exemplary embodiments of the present invention, there is provided a retransmission apparatus in a MIMO communication system wherein m transmit antennas transmit m initial transmission streams to a receiver, m being an integer of 2 or larger, and a retransmission information controller receives retransmission-related information about the m streams from the receiver and if the retransmission-related information includes NACK information indicating initial transmission failure for at least two initial transmission streams, controls the m transmit antennas to transmit the at least two initial transmission streams in a retransmission mode indicated by retransmission mode information included in the retransmission-related information.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of the present invention provide a method and apparatus.

Further, exemplary embodiments of the present invention propose a method and apparatus for selecting a retransmission mode adaptively according to a channel status in a MIMO communication system. That is, a retransmission scheme most suitable for correcting errors in initial transmission may vary at every retransmission and, under some circumstances, a retransmission scheme different from an initial transmission scheme can be more effective.

Accordingly, the present invention proposes “estimation-based retransmission scheme selection” as a retransmission and combining method wherein a receiver monitors the previous transmission channel environment, selects an optimum retransmission scheme, and notifies a transmitter of the selected retransmission scheme. The present invention is based on the assumption that a Spatial Multiplexing (SM) mode and an STC mode are available as retransmission schemes. In the SM mode, individual transmission antennas transmit independent information to increase data rate, whereas, in the STC mode, every transmission antenna transmits a copy of the same information to increase transmission reliability.

To be more specific, the receiver selects the SM mode at a retransmission time and notifies the transmitter of the SM mode. Then the transmitter retransmits all streams having errors in the SM mode. If the receiver selects the STC mode at a retransmission time and indicates the STC mode to the transmitter, the transmitter retransmits only part of all streams having errors in the STC mode. Despite partial transmission of the streams in the STC mode, the transmitter can correct errors in the non-retransmitted streams with use of interference cancellation.

FIG. 1is a block diagram of a transmitter and a receiver in a MIMO communication system according to an exemplary embodiment of the present invention. While the transmitter and the receiver each have two antennas in the MIMO system, any other case could be used.

Referring toFIG. 1, a transmitter100includes a divider102, a buffer104, a channel encoder106, a modulator108, a transmission MIMO processor110, a retransmission information controller112, and two transmit antennas114and116. A receiver120includes two receive antennas122and124, a reception MIMO processor126, a channel decoder128, a buffer130, and a retransmission information generator132.

Upon receipt of a packet from an upper layer, the divider102divides the received packet into streams. The buffer104stores the divided streams and provides streams to the channel encoder106under the control of the retransmission information controller112. The channel encoder106channel-encodes the received streams and the modulator108modulates the coded streams in a predetermined modulation scheme. The transmission MIMO processor110MIMO-processes the modulated streams for transmission to the transmit antennas114and116. Specifically, the MIMO process is carried out as follows. If the streams are initially transmitted, the SM mode is used. If the streams are retransmitted, the retransmission information controller112detects a retransmission scheme in feedback retransmission-related information received from the receiver120and controls the buffer104to output the streams to the channel encoder106according to the retransmission scheme.

The MIMO-processed streams are transmitted to the receiver120through the transmit antennas114and116.

Upon receipt of the streams through the receive antennas122and124, the reception MIMO processor126MIMO-processes the streams. The channel decoder128channel-decodes the MIMO-processed streams. The buffer130stores error-having streams among the channel-decoded streams in order to perform HARQ as indicated by the retransmission information generator132. The retransmission information generator132checks whether the channel-decoded streams have errors. If they have errors, the retransmission information generator132selects a retransmission scheme for the error-having streams and controls the buffer130to store the streams according to the selected retransmission scheme. The retransmission information generator132generates retransmission-related information indicating the selected retransmission scheme and provides it to the reception MIMO processor126. The reception MIMO processor126feeds back the retransmission-related information to the transmitter100. The retransmission-related information includes an Acknowledgment/Negative Acknowledgment (ACK/NACK) bit indicating whether the streams have been received successfully, a retransmission mode selection bit indicating the selected retransmission mode, and retransmission stream selection bits indicating a stream to be retransmitted.

FIG. 2is a flowchart illustrating an initial transmission operation of the transmitter100illustrated inFIG. 1according to an exemplary embodiment of the present invention.

Referring toFIG. 2, upon receipt of a packet from the upper layer, the transmitter100divides the packet into two streams in step200. The transmitter100channel-encodes the two streams in steps205aand205band modulates the channel-encoded streams in a predetermined modulation scheme in steps210aand210b. The transmitter100initially transmits the modulated streams in the SM mode to the receiver120in step215.

While initial transmission and retransmission mode selection for two streams are described with reference toFIGS. 3,4and5, to which the present invention is not limited, it is to be clearly understood that the present invention is applicable to three or more streams.

FIG. 3is a flowchart illustrating an operation of the receiver120illustrated inFIG. 1when the receiver120receives an initial transmission stream according to an exemplary embodiment of the present invention.

Referring toFIG. 3, the receiver120receives two initial transmission streams in step300, linearly filters the two initial transmission streams in step305, and channel-decodes them in step310. While not shown, one successfully decoded stream between the two channel-decoded streams is encoded again and interference is cancelled from the one encoded stream due to channel effects in step305. The interference-cancelled stream is subject to linear filtering in step305and decoded in step310.

The receiver120checks errors in the decoded streams in step315. If both decoded streams are normal, the receiver120generates ACK signals for them and transmits the ACK signals to the transmitter100in step320.

However, if both decoded streams are not normal, the receiver determines whether one stream has errors in step325. In the case of one erroneous stream, the receiver120generates retransmission-related information and transmits it to the transmitter100in step330. The retransmission-related information includes an ACK signal for the normal stream, a NACK signal for the erroneous stream, a retransmission mode selection bit indicating the STC mode, and retransmission stream selection bits indicating the erroneous stream.

If both decoded streams have errors in step325, the receiver120calculates retransmission error probabilities for the cases where the two streams are retransmitted in the SM and STC modes by Equation 1 to Equation 4 in step335. The retransmission error probability computation will be detailed later. In step340, the receiver120compares the retransmission error probability PSMexpected when the SM mode is used with the retransmission error probability PSTCexpected when the STC mode is used.

If PSM>PSTC, the receiver120compares the Signal-to-Noise Ratios (SNRs) of the two erroneous streams at the initial transmission and selects a stream having the smaller SNR as a retransmission stream in step345. The receiver120then generates retransmission-related information and transmits it to the transmitter100in step355. The retransmission-related information includes NACK signals for the initial transmission streams, a retransmission mode selection bit indicating the STC mode, and retransmission stream selection bits indicating the stream having the smaller SNR.

If PSM<PSTCin step340, the receiver120generates retransmission-related information and transmits it to the transmitter100in step350. The retransmission-related information includes NACK signals for the respective streams, a retransmission mode selection bit indicating the SM mode, and retransmission stream selection bits indicating the erroneous streams.

First, PSMis calculated by Equation 1. Let the two initial transmission streams be called first and second streams, respectively.

In Equation 1, β(1),1and β(1),2denote the SNRs of the first and second streams, at the initial transmission respectively. The first and second subscripts of each of β(1),1and β(1),2denote the number of transmission occurrences and an indicator indicating the stream, respectively, ρ denotes the average SNR of each antenna, and βframe( ) denotes a function determined by modulation and coding schemes. For example, if Mc-Quadrature Amplitude Modulation (Mc-QAM) is used without channel coding, Pframe( ) is given as defined in Equation 2:

When Mc-QAM is used as the modulation scheme and convolutional coding is used, which has adfreewrong paths having as many different bits as a free distance dfree, Pframe( ) is defined by Equation 3:

Equation 2 and Equation 3 are merely example applications for computing Pframe( ). Therefore, Pframe( ) can be computed in other various manners.

In Equation 4, γ(1),m denotes the SNR of a stream, the first subscript of γ(1),m denotes the number of transmission occurrences, and m denotes an indicator indicating a stream that will not be retransmitted. If the first stream is to be retransmitted, m=2 and if the second stream is to be retransmitted, m=1.

FIG. 4is a flowchart illustrating a retransmission operation of the transmitter100according to an exemplary embodiment of the present invention. It is assumed herein that the transmitter100has initially transmitted two streams in the procedure illustrated inFIG. 2.

Referring toFIG. 4, the transmitter100receives feedback retransmission-related information from the receiver120in step400. The retransmission-related information includes at least one NACK signal for the initial transmission streams, a retransmission mode selection bit, and retransmission stream selection bits. While not shown, if ACK signals are received for all of the initial transmission streams, retransmission is not needed. Thus the transmitter100receives the next packet from the upper layer and initially transmits new streams.

In step405, the transmitter100determines whether NACK signals have been received for both initial transmission streams. If a NACK signal has been received for one initial transmission stream, the transmitter100retransmits the initial transmission stream corresponding to the NACK signal in the STC mode in step410.

If NACK signals have been received for both initial transmission streams, the transmitter100determines whether the retransmission mode selection bit indicates the SM mode in step415. In the case of the SM mode, the transmitter100retransmits both initial transmission streams in the SM mode in step425.

If the retransmission mode selection bit indicates the STC mode in step415, the transmitter100retransmits a stream indicated by the retransmission stream selection bits in the STC mode in step420.

FIGS. 5A and 5Bare a flowchart illustrating an operation for receiving retransmission data in the receiver120according to an exemplary embodiment of the present invention.

Referring toFIG. 5A, the receiver120receives retransmission streams from the transmitter100according to its feedback retransmission-related information in step500and determines whether both streams received at a previous transmission had errors in step505. If one of the streams had errors, the transmitter100has transmitted the error-having stream by space-time coding. Therefore, the receiver120STC-decodes the error-having stream in step510and combines the STC-decoded stream with an initial transmission value of the error-having stream in step515.

After channel decoding of the combined stream in step520, the receiver120checks errors in the channel-decoded stream in step525. If the channel-decoded stream is normal, the receiver120transmits ACK signals for the retransmission streams to the transmitter100in step530. If the channel-decoded stream has errors in step525, the receiver120generates retransmission-related information for the channel-decoded stream and transmits it to the transmitter100in step535. The retransmission-related information includes a NACK signal for the channel-decoded stream, an ACK signal for the other normal stream, and a retransmission mode selection bit indicating the STC mode.

If both of the retransmission streams have errors in step505, the receiver120checks the previous feedback retransmission mode selection bit in step540. If the previous feedback retransmission mode selection bit indicates the STC mode, the receiver120considers that the transmitter100transmitted only a stream indicated by the previous feedback retransmission mode selection bit. Hence, the receiver120STC-decodes the stream indicated by the previous feedback retransmission mode selection bit in step545, cancels the STC-decoded stream from an initial transmission value of the indicated stream in step550, and goes to step560.

If the previous retransmission mode selection bit indicates the SM mode in step540, the receiver120performs pre-combining and linear filtering on all of the retransmission streams and their stored initial transmission streams in step555and proceeds to step560.

After channel decoding in step560, the receiver120checks errors in the channel-decoded streams in step565. If one stream has errors, the receiver120returns to step535.

If all of the channel-decoded streams have errors in step565, the receiver120goes to step580.

Referring toFIG. 5B, the receiver120calculates the retransmission error probabilities PSMand PSTCof the SM mode and the STC mode by Equation 1 to Equation 4 in step580.

In step585, the receiver120compares PSMwith PSTC. If PSMexceeds PSTC, the receiver120compares the SNRs of the two streams at the initial transmission and selects a stream with the smaller SNR as a stream to be retransmitted in step590. In step595, the receiver120generates retransmission-related information and transmits it to the transmitter100. That is, the retransmission-related information includes NACK signals for the initial transmission streams, a retransmission mode selection bit indicating the STC mode, and retransmission stream selection bits indicating the stream with the smaller SNR.

If PSMis below PSTC, the receiver120generates retransmission-related information and transmits it to the transmitter100in step600. The retransmission-related information includes NACK signals for the initial transmission streams, a retransmission mode selection bit indicating the SM mode, and retransmission stream selection bits indicating the error-having stream.

When a further retransmission needed as a result of performing the procedure illustrated inFIGS. 5A and 5B, the procedure is repeated.

It can be further contemplated as another exemplary embodiment of the present invention that the receiver120does not transmit retransmission-related information to the transmitter100and the transmitter100selects a transmission mode for a stream upon receipt of an ACK/NACK signal for the stream from the receiver120.

The receiver120transmits an ACK signal selectively according to the decoding result of two initial transmission or retransmission streams from the transmitter100. That is, if the decoding result indicates that the two streams are normal, the receiver120transmits ACK signals for the respective streams to the transmitter100.

If the decoding result indicates one erroneous stream, the receiver120transmits a NACK signal for the erroneous stream and an ACK signal for the other normal stream to the transmitter100.

If the decoding result indicates that both of the streams have errors, the receiver120selects a retransmission mode according to Equation 1 to Equation 4. When selecting the SM mode, the receiver120transmits NACK signals for all of the erroneous streams. If the receiver120selects the STC mode, it compares the SNRs of the erroneous streams. The receiver120transmits a NACK signal for a stream with the smaller SNR and an ACK signal for the other stream.

Upon receipt of ACK signals for both of the streams, the transmitter100initially transmits new streams out of a new packet.

Upon receipt of a NACK signal for one of the two streams, the transmitter100retransmits the stream corresponding to the NACK signal through the two transmit antennas in the STC mode.

Upon receipt of NACK signals for both of the streams, the transmitter100retransmits the two streams in the SM mode.

FIG. 6is a graph illustrating effects of the exemplary embodiments of the present invention.

Referring toFIG. 6, Frame Error Rates (FERs) are measured after a retransmission, while an average SNR is changed under 16-QAM. No channel coding is used and each frame is 40 bits long. Herein, the terms “frame” and “stream” are interchangeably used in the same sense.

That is, the graph illustrates the rates of frames having errors with respect to total transmitted frames, after one retransmission. Compared to Spatial Multiplexing-Fixed Retransmission Mode (SM-FRM) or STC-FRM, the proposed estimation-based RMS offers a 2-dB SNR gain at a target FER of 10−4. This SNR gain becomes more apparent as the SNR increases.

As is apparent from the above description, the exemplary embodiments of the present invention can improve the error correction capability of HARQ for a fast fading channel by adaptively changing a retransmission mode according to a channel status in a MIMO communication system.