WO2008147107A1

WO2008147107A1 - Method and apparatus for space-time coding mode in mimo wireless communication system

Info

Publication number: WO2008147107A1
Application number: PCT/KR2008/002989
Authority: WO
Inventors: Byung-Chul Kim; Zheng Zi Li; Sang-Bae Ji
Original assignee: Posdata Co., Ltd.
Priority date: 2007-05-30
Filing date: 2008-05-28
Publication date: 2008-12-04
Also published as: KR100910020B1; KR20080105211A

Abstract

A method and apparatus for Space-Time Coding (STC) mode in Multiple Input Multiple Output (MIMO) wireless communication system. The method includes estimating a channel capacity for each STC mode from reception signals received via a plurality of reception antennas; and determining the STC mode using the estimated channel capacity.

Description

Description Method and apparatus for space-time coding mode in MIMO wireless communication system Technical Field

[1] The present invention relates generally to Multiple Input Multiple Output (MIMO) wireless communication system, and in particular, to a method and apparatus for Space-Time Coding (STC) mode in MIMO wireless communication system. Background Art

[2] Generally, the wireless channel environment suffers a loss of information due to occurrence of unavoidable errors, caused by several factors such as multipath interference, shadowing, propagation loss, time-varying noise, interference, fading, etc. The information loss brings a significant distortion on actual transmission signals, causing a reduction in the entire performance of the wireless communication system.

[3] Meanwhile, diversity schemes are used to remove the possible instability of communication due to fading. The diversity scheme additionally secures spatial areas for resource utilization by mounting a plurality of antennas in a transmitter/receiver, making it possible to increase reliability of communication links through diversity gain without an increase in the bandwidth, and to increase transmission capacity through spatial multiplexing-based parallel transmission.

[4] FIG. 1 is diagram illustrating the outlines of a Single Input Single Output (SISO) system and a MIMO system, respectively.

[5] As illustrated in FIG. 1, a SISO system 100 is a technology for performing SISO transmission through one channel H formed between one transmission antenna TxAnt and one reception antenna RxAnt. In wireless communication, when a transmitter and a receiver each perform communication using their one antenna, they may suffer from fading due to occurrence of the multipath phenomenon caused by obstacles to the propagation path, such as the hills and steel towers, and the fading causes a decrease in data rate and an increase in error rate in digital communication such as the wireless Internet.

[6] Compared with the SISO system, the MIMO system increases the number of antennas of both a base station and a mobile station to transmit data through several paths. In this system, a receiver can reduce interference by detecting signals received through the paths, and a transmitter can increase transmission efficiency through space-time diversity and spatial multiplexing. FIG. 1 illustrates an exemplary 2x2 MIMO system 150 having two transmission antennas and two reception antennas. As illustrated, four channels of a first channel Hl 1, a second channel H 12, a third channel H21 and a fourth channel H22 are formed between first and second transmission antennas TxAntl and TxAnt2, and first and second reception antennas RxAntl and RxAnt2.

[7] Such MIMO modes can be classified into a Space-Time Transmit Diversity (STTD) mode and a Spatial Multiplexing (SM) mode according to the scheme of allocating multiple antennas and symbols to be transmitted there through. In particular, since the IEEE 802.16-based Portable Internet system has provided that STTD and SM can be selectively transmitted, a study is being made of a scheme for selecting such an STC mode according to the channel state.

[8] Particularly, when the STC mode is simply selected taking into consideration only the very basic channel characteristics (e.g., strength of MIMO channels and a strength deviation of MIMO channels) for the channel state, an influence of an error caused by channel estimation may be disregarded, causing performance degradation, and since the STC modes have various specific decoding schemes and their characteristics are different from each other, their implemented results may not show the ideal characteristics.

Disclosure of Invention Technical Problem

[9] An aspect of the present invention is to address at least the problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide a method and apparatus for supporting Space-Time Coding (STC) mode in Multiple Input Multiple Output (MIMO) system.

[10] Another aspect of the present invention is to provide a method and apparatus for supporting STC mode taking into account a possible error occurring during channel estimation in MIMO system.

[11] Another aspect of the present invention is to provide a method and apparatus for supporting STC mode in which characteristics based on a decoding scheme for each STC mode are reflected, in MIMO system.

[12] Further another aspect of the present invention is to provide a method and apparatus for supporting a substantial STC mode through parameters in which characteristics based on a decoding scheme for each STC mode are reflected, taking into account a possible error occurring during channel estimation, and for feeding back the determined STC mode thereby responding to a signal transmitted from a transmitter, in MIMO system where the transmitter and a receiver constitute a closed- loop MIMO architecture. Technical Solution

[13] According to one aspect of the present invention, there is provided a method for supporting Space-Time Coding (STC) mode in a receiver of Multiple Input Multiple Output (MIMO) system. The method includes estimating a channel capacity for each STC mode from reception signals received via a plurality of reception antennas; and determining the STC mode using the estimated channel capacity.

[14] According to another aspect of the present invention, there is provided a transmitting method for Space-Time Coding (STC) mode based on a reception response from a receiver in Multiple Input Multiple Output (MIMO) system. The method includes decoding the reception response that includes STC mode information determined using a channel capacity estimated for each STC mode; selecting STC mode based on the reception response, wherein the STC mode includes Space-Time Transmit Diversity (STTD) and Spatial Multiplexing (SM); and encoding the transmission signal according to the selected STC mode.

[15] According to further another aspect of the present invention, there is provided an apparatus for Space-Time Coding (STC) mode in a receiver of Multiple Input Multiple Output (MIMO) system. The apparatus includes a calculator for calculating a channel capacity for each STC mode from reception signals received via a plurality of reception antennas; and a determiner for determining a Space-Time Coding (STC) mode using the calculated channel capacity for each STC mode from the reception signals.

[16] According to still another aspect of the present invention, there is provided a transmitting apparatus for Space-Time Coding (STC) mode based on a reception response from a receiver in Multiple Input Multiple Output (MIMO) system. The apparatus includes a decoder for decoding the reception response that includes STC mode information determined using a channel capacity estimated for each STC mode; a selector for selecting the STC mode based on the reception response, wherein the STC mode includes Space-Time Transmit Diversity (STTD) and Spatial Multiplexing (SM); and a encoder for encoding transmission signal according to the selected STC mode.

Advantageous Effects

[17] According to the present invention, the proposed method and apparatus determines a preferred Space-Time Coding (STC) mode taking into account a possible error occurring during channel estimation in a Multiple Input Multiple Output (MIMO) system, making it possible to determine an STC mode suitable for a channel condition, thereby contributing to maximization of transmission efficiency.

[18] Furthermore, according to the present invention, the proposed a method and apparatus determines a preferred STC mode in which characteristics based on a decoding scheme for each STC mode are reflected, in a MIMO system, thereby ensuring correct STC mode determination.

[19] In addition, according to the present invention, the proposed method and apparatus determines a preferred STC mode reflected in a decoding scheme for each STC mode, taking into account a possible error occurring during channel estimation, and feeds the determined STC mode back to a transmitter, thereby making it possible to determine an STC mode in which substantial and correct channel characteristics are reflected when the transmitter transmits the next frame, in a MIMO system where the transmitter and a receiver constitute a closed-loop MIMO architecture. Brief Description of the Drawings

[20] The above and other aspects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

[21] FIG. 1 is diagram illustrating the outlines of a SISO system and a MIMO system, respectively;

[22] FIG. 2 is a block diagram illustrating a transmitter of an NxM MIMO system according to the present invention;

[23] FIG. 3 is a block diagram illustrating a receiver of an NxM MIMO system according to the present invention;

[24] FIG. 4 is a diagram for a description of an STTD transmission/reception principle in a 2x2 MIMO system;

[25] FIG. 5 is a diagram for a description of an SM transmission/reception principle in a

2x2 MIMO system;

[26] FIG. 6 is a block diagram illustrating the preferred STC mode determiner of FIG. 3; and

[27] FIG. 7 is a flowchart illustrating a method for determining a preferred STC mode according to the present invention. Mode for the Invention

[28] Preferred embodiments of the present invention will now be described in detail with reference to the annexed drawings. In the following description, a detailed description of known functions and configurations incorporated herein has been omitted for clarity and conciseness.

[29] FIG. 2 is a block diagram illustrating a transmitter of an NxM Multiple Input

Multiple Output (MIMO) system according to the present invention, and FIG. 3 is a block diagram illustrating a receiver of an NxM MIMO system according to the present invention. Although the transmitter can serve as a Base Station (BS) and the receiver can serve as a Mobile Station (MS), by way of example, the transmitter and the receiver can serve as an MS and a BS, respectively. [30] Referring to FIG. 2, a transmitter 200 includes an encoder 210, an interleaver 220, a modulator 230, a multiplexing and spatial coding unit 240, a plurality of antennas, a feedback message decoder 250, a scheduler 260, and an Space-Time Coding (STC) mode selector 270. Referring to FIG. 3, a receiver 300 includes a plurality of antennas, a demultiplexing and spatial decoding unit 310, a demodulator 320, a deinterleaver 330, a decoder 340, a MIMO channel parameter estimator 350, and a preferred STC mode determiner 360.

[31] For convenience sake, an operation of the transmitter will first be described, and then

STC related to the present invention will be described in brief. Thereafter, an operation of the receiver will be described.

[32] In the transmitter 200, the encoder 210 channel-encodes an n"¹ bit u_n containing desired transmission information using an encoding scheme such as Forward Error Correction (FEC), and outputs encoded bits C₁. For example, the encoder 210 uses con- volutional encoding, Convolutional Turbo Codes (CTC), etc. as an encoding scheme. The interleaver 220 rearranges the encoded bits C₁ according to a rule determined such that consecutive bits C₁ having a high correlation are transmitted through channels having a low correlation. For example, the rule can be defined to map consecutive bits C₁ to non-adjacent subcarriers, or to alternately map consecutive bits C₁ to lower- significant bits and higher- significant bits on the constellation. For the interleaved bits b₁₅ the modulator 230 maps one or more bits to one symbol using a modulation scheme such as Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM). The multiplexing and spatial coding unit 240 multiplexes the modulated symbol with a modulation scheme such as Orthogonal Frequency Division Multiplexing (OFDM) and Code Division Multiplexing (CDM), and then can transmit some or all of the total symbols to the receiver 300 using multiple antennas according to the selected STC mode.

[33] In this case, the mapping between symbols and antennas is called 'space-Time

Coding (STC)', and STC can be classified into Space-Time Transmit Diversity (STTD) and Spatial Multiplexing (SM) according to the encoding scheme.

[34] STTD is a scheme for repeatedly transmitting two different symbols by changing a phase and a transmission antenna for each of the symbols, and the typical STTD includes an Alamouti scheme, and secures a stable and improved transfer rate by easing the spatial fading. Meanwhile, SM is a scheme for simultaneously transmitting and receiving multiple different symbols via multiple antennas, and the typical SM includes a Bell Labs Layered Space-Time (BLAST) scheme, and increases a data rate. Particularly, since the IEEE 802.16-based Portable Internet system has provided that STTD and SM can be selectively transmitted, there is a need for a technology capable of transmitting STTD and SM after making a proper performance comparison between them.

[35] Since relative performances of STTD and SM are different according to the channel state, it is necessary to properly select them, as stated above. To this end, the receiver 300 analyzes channel characteristics from the signals transmitted from the transmitter 200, determines STTD or SM according to a predetermined criterion, encodes information thereon, and transmits, i.e., feeds back, the encoded information to the transmitter 200. In this case, the encoded information includes the STC mode determined by the receiver 300, an Effective Carrier to Interference and Noise Ratio (Effective CINR), etc. and is transmitted to the transmitter 200 at a predetermined time.

[36] The information being transmitted after undergoing encoding will be referred to herein as a 'feedback message', and the STC mode determined by the receiver 300 will be referred to as a 'preferred STC mode'. A detailed description of STTD and SM will be given below with reference to FIGs. 4 and 5.

[37] Referring back to FIG. 2, the feedback message decoder 250 decodes the information transmitted from the receiver 300, and provides the decoded information to the STC mode selector 270, and the scheduler 260 provides resource status and its scheduling policy to the STC mode selector 270. Then the STC mode selector 270 can finally determine the STC mode that it will transmit to the receiver 300, taking the resource status and scheduling policy into account.

[38] Referring to FIG. 3, the demultiplexing and spatial decoding unit 310 of the receiver

300 performs demultiplexing on the signals received at multiple antennas on a symbol- by-symbol basis, and performs STC decoding thereon. For example, when the STC mode is STTD, the receiver 300 performs the decoding using at least two symbol indexes, and when the STC mode is SM, the receiver 300 performs decoding on each received symbol. The demodulator 320 demodulates the demultiplexed/decoded symbols and calculates a bit matrix, and the deinterleaver 330 deinterleaves the demodulated symbols using a deinterleaving scheme opposed as the interleaving scheme applied in the transmitter 200. The decoder 340 outputs an information bit estimate originally transmitted by the transmitter 200, based on the deinterleaved bit stream.

[39] The receiver 300 further includes the MIMO channel parameter estimator 350 for analyzing MIMO channel characteristics, e.g., MIMO channel estimate, noise power, etc. from the signals transmitted from the transmitter 200, and the preferred STC mode determiner 360 for determining a preferred STC mode using the estimated MIMO channel and noise power. The MIMO channel parameter estimator 350, if necessary, can be included in the preferred STC mode determiner 360.

[40] With reference to FIGs. 4 and 5, a description will now be made of a process of calculating each STC mode and a Signal to Interference and Noise Ratio (SINR) for each STC mode. Thereafter, with reference to FIGs. 6 and 7, a description will be made of an STC mode determining apparatus and method according to the present invention.

[41] FIG. 4 is a diagram for a description of an STTD transmission/reception principle in a 2x2 MIMO system.

[42] Referring to FIG. 4, if two desired transmission symbols or Orthogonal Frequency

Division Multiple Access (OFDMA) sub-symbols are defined as S₁ and S₂, and each row can be expressed as a vector corresponding to each antenna, k"¹ and (k+l)"¹ transmission symbol vectors s(k) and s(k+l) can be expressed as Equation (1). Herein, k is assumed to be an even number among the STTD symbol indexes, and k = 0, 2, 4,

[43] [Equation 1]

¹⁴⁴¹ s(k) = [s_k s_k+l]^J

[45] If transmission symbol vectors s(k), s(k+l), s(k+2), ... of transmission signals are transmitted from antennas of the transmitter 200, received sample vectors r(k), r(k+l), r(k+2), ... are obtained from reception signals at antennas of the receiver 300. If a channel gain between an n"¹ (where n=l,2) antenna of the transmitter 200 and an m"¹ (where m=l,2) antenna of the receiver 300 is defined as h_{n m}, and a noise component of a k"¹ received sample from a reception signal of an m"¹ reception antenna is defined as v m(k), the received sample vectors can be expressed as Equation (2).

[46] [Equation 2]

[47] r(k)=[r,(k) r₂(k)\^τ=[h_h,s_k+ h_2Λs_h+,+v_x{k) h_h2s_k+ h₂,₂s_k+i+v₂(k)\^τ r(Λ+l) =[r₁(&+l)r₂(Λ+l)]^r=[-A₁,₁/_A+1+A₂,₁s/+v₁(Λ+l)-Λ₁₎₂s^* _/t+1+/i₂,₂S_A ^;5+V₂(Λ+l)]^r

[48] For STTD, its STC decoding for restoring symbols s_k and s_k+1 transmitted by the transmitter 200 from the received sample vectors r(k) and r(k+l) is relatively simple, and STTD is possible even with one antenna, and can obtain higher receive diversity gain when two antennas are used (2x2 MIMO). Therefore, when noise power at an m"¹ antenna of the receiver 300 is defined as σ_m ², the restored symbols s_k and

S k+i can be expressed as Equation (3). [49] [Equation 3]

[50]

[51] In Equation (3), if at least one of four channel gains h_nm is not zero (0), the restored symbols

and

^S k+1 can be calculated. If symbol energy transmitted from the transmitter 200 is E/N = 1/2, and noise components are stationary, having no mutual correlation in terms of space (different antennas) and time (different time indexes), a Signal to Noise Ratio SNR_STTD of the estimated symbols can be expressed as Equation (4).

[52] [Equation 4] [53] k

SNR_STTD 1,1 ' + h '2,1 h '1,2 * + h '2,2

^■ + ^■

2σ; 2σ^" ₂

[54] FIG. 5 is a diagram for a description of an SM transmission/reception principle in a 2x2 MIMO system. [55] Referring to FIG. 5, in SM, all transmission symbol vectors are independent in terms of the time index k. Therefore, while s(k) is decoded from two consecutive received sample vectors r(k) and r(k+l) of reception signals in STTD, s(k) should be decoded only with r(k) in SM.

[56] It is known that the decoding method optimal for SM is Maximum Likelihood Decoding (MLD). Aside from it, Zero-Forcing (ZF) and Minimum Mean Squared Error (MMSE) have also been proposed. An SM decoding method based on MMSE will be described herein, by way of example.

[57] An MMSE-based SM decoding method linearly combines received samples obtained from each antenna, thereby estimating the symbols s_k and s_k+1 transmitted from the transmitter 200. This can be mathematically expressed as Equation (5), in which a superscript '^H' means a matrix transpose conjugate.

[58] [Equation 5] [59] ^" r(k) =

S(A) = Gr(A)

[60] Therefore, in MMSE-based STC decoding, after a symbol transmitted from an n"¹ transmission antenna undergoes decoding, SINR for the symbol can be expressed as Equation (6) or Equation (7). Herein, Equation (7) corresponds to the case where

in Equation (6), and in Equation (6) and Equation (7), R_n^ is a correlation matrix for received sample vectors represented by

R rr =E{r"r}

, H is a channel matrix, and h_n=[h_n i, ..., h_{n M}]^T. In addition, I_N is an NxN identity matrix.

[61] [Equation 6]

[63] [Equation 7] [64]

[65] As a result, it can be understood from Equation (4) and Equation (6) or Equation (7) that SINR after STTD decoding (hereinafter 'STTD Post-SINR') and SINR after SM decoding (hereinafter 'SM Post-SINR') vary not only according to the channel gain h_{n n} , but also account to the decoding method based on the STC mode and STC mode. Thus, it can be appreciated that the preferred STC mode determined by the receiver 300 cannot be determined simply depending only on the channel characteristics. Therefore, the present invention can determine a preferred STC mode by considering the decoding method based on the STC mode and deriving comparable parameters from the SINRs after STTD/SM decoding. For example, if a preferred STC mode is determined using channel capacity, a preferred STC mode suitable for a given channel condition can be selected, making it possible to maximize the transmission efficiency.

[66] FIG. 6 is a block diagram illustrating the preferred STC mode determiner of FIG. 3.

The preferred STC mode determiner 360, rather than determining an STC mode simply depending on the correct channel characteristics, estimates channel characteristics, calculates an STC Post-SINR for each STC mode, obtained from the channel estimate, and then determines a preferred STC mode based on the channel capacity obtained using the STC Post-SINR for each STC mode. In this way, most of errors occurring in actual implementation and most of characteristics based on the decoding schemes can be considered, thereby ensuring correct data detection. The term 'STC Post-SINR' as used herein refers to an SINR after corresponding symbols undergo decoding based on each STC mode.

[67] Referring to FIG. 6, the preferred STC mode determiner 360 includes an STC Post-

SINR estimation module 362 for estimating an STC Post-SINR from the channel and noise power estimate estimated by the MIMO channel parameter estimator 350 of FIG. 3, a channel capacity calculation module 363 for calculating a channel capacity for each STC mode from the STC Post-SINR estimate using a math expression or a predetermined look-up table, an accumulated average calculation module 364 for calculating an accumulated average for a difference in channel capacity between STC modes, and an STC mode determination module 365 for determining a preferred STC mode depending on the accumulated average. The STC Post-SINR estimation module 362 includes an STTD Post-SINR estimation module 362a for estimating SINR after STTD decoding, and an SM Post-SINR estimation module 362b for estimating SINR after SM decoding, and the channel capacity calculation module 363 includes an STTD channel capacity calculation module 363a and an SM channel capacity calculation module 363b.

[68] The STC Post-SINR estimation module 362 estimates an STC Post-SINR from the channel and noise power estimate estimated by the MIMO channel parameter estimator 350. The STC Post-SINR estimation module 362 is expressed as a k"¹ processing unit for an 1^th frame in an OFDM or OFDMA mode, and estimates the entire 1^th frame using a representative value from the k"¹ processing unit. The STC Post-SINR estimation module 362 is divided into the STTD Post-SINR estimation module 362a for estimating an SINR after STTD decoding and an SM Post-SINR estimation module 362b for estimating an SINR after SM decoding, according to the scheme of allocating the processing units to transmission antennas.

[69] The channel capacity calculation module 363 calculates a channel capacity for each

STC mode from the STC Post-SINR estimate using a math expression or a predetermined look-up table. For example, when the math expression is used, the channel capacity calculation module 363 calculates a channel capacity for each STC mode using Shannon's channel capacity, and when the look-up table is used, the channel capacity calculation module 363 previously stores a channel capacity based on the STC Post-SINR estimate, and reads a channel capacity based on a particular STC Post- SINR estimate from the look-up table. The channel capacity calculation module 363 is divided into the STTD channel capacity calculation module 363a and the SM channel capacity calculation module 363b according to the scheme of allocating the processing units to transmission antennas.

[70] The accumulated average calculation module 364 calculates an accumulated average for a difference in channel capacity between STC modes. For example, the accumulated average calculation module 364 calculates a difference between an STTD channel capacity and an SM channel capacity for every frame, and then accumulates the differences. Here, it should be noted that the SM channel capacity is accumulated according to the number of transmission antennas in a frame of a reception signal received from an n"¹ transmission antenna.

[71] The STC mode determination module 365 determines a preferred STC mode depending on the accumulated average. For example, as described above, an accumulated average is calculated from a difference obtained by subtracting the SM channel capacity from the STTD channel capacity, or from a difference obtained by subtracting the STTD channel capacity from the SM channel capacity, and the preferred STC mode is determined according to whether the accumulated average is positive or negative.

[72] Although not illustrated in FIG. 6, when the transmitter 200 and the receiver 300 constitute a closed-loop MIMO architecture, the receiver 300 can further include a feedback message transmitter (not shown) for transmitting the determined preferred STC mode to the transmitter 200 along with a feedback message, and can use it as an apparatus for responding to the signal transmitted from the transmitter 200.

[73] With reference to FIG. 7, a detailed description will now be made of the STC mode determination method according to an embodiment of the present invention.

[74] In a process where a signal received from each antenna is properly processed and quantized forming a baseband OFDMA symbol frame in a Radio Frequency (RF) unit (not shown), for a predetermined subcarrier sample group, adjacent subcarriers in each subcarrier or frequency-time (subcarrier-OFDMA symbol) two-dimensional (2D) region can be processed after being clustered into groups (clusters) having a predetermined scale. The subcarrier sample group can be some or all of the OFDMA symbols using STC in the corresponding frame. Herein, therefore, an index for the processing unit (subcarrier or cluster) in one frame will be defined as k and an index for the frame will be defined as 1. [75] The preferred STC mode determiner 360 estimates channel gain and noise power, which are parameters necessary for estimating an STC Post-SINR, in step S710. The channel gain and noise power are estimated for each processing unit (e.g., subcarrier or cluster) in every frame. Herein, for the estimation of a channel gain, a pilot can be used, and in this case, an interpolation scheme or an averaging scheme using a pilot can be applied.

[76] Thereafter, the preferred STC mode determiner 360 estimates an STC Post-SINR for each STC mode from the channel and noise power estimate in step S720.

[77] For STTD, the preferred STC mode determiner 360 estimates an STTD Post-SINR

ζsTTD \L -> fc) corresponding to a sample group of an 1^th frame from the channel gain and noise power estimate. For example, since OFDM or OFDMA uses multiple subcarriers, it considers multipath channels. In this case, as both the channel characteristics and the noise power, including interference, may have different values according to the foregoing processing unit (subcarrier or cluster), the STTD Post-SINR can be expressed as Equation (8), in which h_{n m}(l, k) denotes a channel gain for a k"¹ processing unit of an 1^th frame, and σ_m ²(l, k) denote interference and noise power for a k t^h processing unit of an 1^th frame received with an m"¹ antenna of the receiver 300.

[78] [Equation 8]

[79]

£ S ] J \ I h ^{~ "} _u1 , 1 ( V/, 7* ^{~ "}) / | I² + 1 A Z_2-.₁ I ( VU ' )/ II² I A_1-2 (/,*) I² + | A_2,2 (/,*) I

2σ_λ {l,k) 2σ₂ (l,k)

[80] The method of setting a representative value of the STTD Post-SINR for the entire 1^th frame can use at least one of a geometric average, a math average, and a center value, for an index k. The geometric average, the math average, and the center value can be expressed as Equation (9), Equation (10) and Equation (11), respectively. In Equation (11), median<x(l, k) | 1> is a center value for x(l, k) when 1 is given.

[81] [Equation 9]

[83] [Equation 10]

S b SSTTTTDD (A K)

[85] [Equation 11] [86]

£_SΓΓD(!) = median{ξ_STTD(l,k) | / [87] For SM, the preferred STC mode determiner 360 can calculate an SM Post-SINR

for a frame transmitted from an n"¹ transmission antenna of the transmitter 200 from the channel gain and noise power estimate according to Equation (6) or Equation (7). For example, since OFDM or OFDMA uses multiple subcarriers, it considers multipath channels. In this case, since both the channel characteristics and the noise power, including interference, may have different values according to the foregoing processing unit, an SM Post-SINR in a k"¹ processing unit of an 1^th frame transmitted from an n"¹ transmission antenna can be expressed as Equation (12) or Equation (13). Here, the processing unit represents a subcarrier or a cluster.

[88] [Equation 12] [89]

[90] [Equation 13]

[92] The method of setting a representative value of the SM Post-SINR for the entire 1^th frame of an n"¹ transmission antenna can use at least one of a geometric average, a math average, and a center value, for an index k. The geometric average, the math average, and the center value can be expressed as Equation (14), Equation (15) and Equation (16), respectively. In Equation (16), median<x(l, k) | 1> is a center value for x(l, k) when 1 is given.

[93] [Equation 14]

[95] [Equation 15] [96]

[97] [Equation 16]

[99] Next, the preferred STC mode determiner 360 calculates a channel capacity for each

STC mode from the STC Post-SINR estimate for each STC mode using a math expression or a predetermined look-up table in step S730. For example, when the math expression is used, the preferred STC mode determiner 360 calculates a channel capacity for STTD from Equation (17), and calculates a channel capacity for SM from Equation (18). Here, Equation (18) means an accumulated channel capacity for N transmission antennas in SM.

[100] [Equation 17]

[101]

^CSTΓD (0 = ^1Og(I + ξsTTD (θ

[102] [Equation 18]

n=\

[104] Such channel capacities are basically derived from Equation (19) and Equation (20). That is, in wireless channels, frequency efficiency is defined as a data rate per unit frequency, and when its unit is bps/Hz, the maximum frequency efficiency trans- mittable without error is defined by the Shannon's channel capacity. For the SISO system having one transmission/reception antenna, the Shannon's channel capacity can be expressed as Equation (19), where p denotes SNR.

[105] [Equation 19]

[106] C = log₂(l+p)

[107] If the Shannon's channel capacity is applied to the MIMO system with multiple antennas (especially, to a 2x2 MIMO system), the channel capacity can be expressed as Equation (20).

[108] [Equation 20]

[109] C = lo_g2(Det(I+pHH^H/2))

[110] When the look-up table is used, the preferred STC mode determiner 360 previously writes a channel capacity for each Modulation and Coding Scheme (MCS) level associated with the channel characteristics in the look-up table, and can search the look- up table every time it determines a preferred STC mode. [I l l] Thereafter, the preferred STC mode determiner 360 calculates an accumulated average for a difference in channel capacity between STC modes in step S740. That is, the preferred STC mode determiner 360 accumulates/averages a difference in channel capacity for each STC mode in every frame, as shown in Equation (23), using Equation (21) or Equation (22). Here, α(l) is a variable for adjusting a ratio to which

Δ(/) contributes during calculation of

[112] [Equation 21]

¹¹¹³¹ A(l) = C_SM (l) - C_STTD (l)

[114] [Equation 22]

¹¹¹⁵¹ A(l) = C_STTD(!) - C_SM(l)

[116] [Equation 23]

"^17I A_Avg (/) = «(/) • Δ<7) + (1 - «(/)) • Δ^ (/ - 1)

[118] The preferred STC mode determiner 360 determines a preferred STC mode depending on the accumulated average in step S750. That is, the preferred STC mode determiner 360 determines an STC mode according to whether the accumulated average is positive or negative. For example, if Equation (21) is used in the process of calculating an accumulated average, the preferred STC mode determiner 360 selects SM for

> 0, and STTD for

<0. However, if Equation (22) is used in the process of calculating an accumulated average, the preferred STC mode determiner 360 selects STTD for

Δ,_w(0 >0, and SM for

<0.

[119] In this manner, by determining a preferred STC mode using the channel capacity as described above, it is possible to select a preferred STC mode suitable for the given channel condition, thus contributing to maximization of the transmission efficiency.

[120] Although not illustrated in FIG. 7, when the transmitter 200 and the receiver 300 constitute a closed-loop MIMO architecture, the receiver 300 can further include a step (not shown) of transmitting the determined preferred STC mode to the transmitter 200 along with a feedback message, and can use it as a method for responding to the signal transmitted from the transmitter 200.

[121] While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

[1] A method for supporting Space-Time Coding (STC) mode in a receiver of

Multiple Input Multiple Output (MIMO) system, the method comprising: estimating a channel capacity for each STC mode from reception signals received via a plurality of reception antennas; and determining the STC mode using the estimated channel capacity.

[2] The method of claim 1, wherein the STC mode includes Space-Time Transmit

Diversity (STTD) and Spatial Multiplexing (SM).

[3] The method of claim 1, wherein the channel capacity is calculated using a Signal to Interference and Noise Ratio (SINR) based on the reception signals.

[4] The method of claim 1, wherein estimating a channel capacity for each STC mode comprises: estimating channel gain and noise power from the reception signals; estimating an SINR for each STC mode using the channel gain and noise power; and calculating a channel capacity for each STC mode using the estimated SINR.

[5] The method of claim 1, wherein determining the STC mode comprises: calculating an accumulated average for a difference in channel capacity between

STC modes; and determining the STC mode using the accumulated average.

[6] The method of claim 1, wherein the channel capacity is estimated in units of sub- carriers or clusters when the receiver supports Orthogonal Frequency Division Multiplexing (OFDM) or Orthogonal Frequency Division Multiple Access (OFDMA).

[7] The method of claim 1, further comprising: transmitting the determined STC mode to the transmitter along with a feedback message.

[8] The method of claim 7, wherein the feedback message further includes an

Effective Carrier to Interference and Noise Ratio (Effective CINR) of the receiver, and is transmitted to the transmitter at a predetermined time.

[9] An apparatus for Space-Time Coding (STC) mode in a receiver of Multiple Input

Multiple Output (MIMO) system, the apparatus comprising: a calculator for calculating a channel capacity for each STC mode from reception signals received via a plurality of reception antennas; and a determiner for determining a Space-Time Coding (STC) mode using the calculated channel capacity for each STC mode from the reception signals.

[10] The apparatus of claim 9, wherein the STC mode includes Space-Time Transmit Diversity (STTD) and Spatial Multiplexing (SM).

[11] The apparatus of claim 9, wherein the calculator calculates the channel capacity using a Signal to Interference and Noise Ratio (SINR) based on the reception signals.

[12] The apparatus of claim 9, wherein the calculator calculates the channel capacity channel in units of subcarrier or clusters when the receiver supports Orthogonal Frequency Division Multiplexing (OFDM) or Orthogonal Frequency Division Multiple Access (OFDMA).

[13] The apparatus of claim 9, wherein the calculator comprises: a first estimator for estimating channel gain and noise power from the reception signals; a second estimator for estimating a Signal to Interference and Noise Ratio

(SINR) for each STC mode using the estimated channel gain and noise power; and a calculation module for calculating a channel capacity for each STC mode using the estimated SINR.

[14] The apparatus of claim 13, wherein the second estimator comprises: a first estimation module for estimating a Space-Time Transmit Diversity (STTD) SINR after performing STTD decoding on the reception signals; and a second estimation module for estimating a Spatial Multiplexing (SM) SINR after performing SM decoding on the reception signals.

[15] The apparatus of claim 14, wherein the first estimation module sets a representative value for a particular frame of the reception signals using at least one of a geometric average, a math average, and a center value, for the STTD SINR.

[16] The apparatus of claim 14, wherein the second estimation module sets a representative value for a particular frame of the reception signals using at least one of a geometric average, a math average, and a center value, for the SM SINR.

[17] The apparatus of claim 13, wherein the calculation module comprises: a first calculation module for calculating a channel capacity for STTD from the estimated STTD SINR; and a second calculation module for calculating a channel capacity for SM from the estimated SM SINR.

[18] The apparatus of claim 9, wherein the determiner comprises: an average calculator for calculating an accumulated average for a difference in channel capacity between STC modes; and a determination module for determining the STC mode using the accumulated average.

[19] The apparatus of claim 9, further comprising a feedback message transmitter for transmitting the determined STC mode to the transmitter along with a feedback message. [20] The apparatus of claim 19, wherein the feedback message further includes an

Effective Carrier to Interference and Noise Ratio (Effective CINR) of the receiver, and is transmitted to the transmitter at a predetermined time. [21] A transmitting apparatus for Space-Time Coding (STC) mode based on a reception response from a receiver in Multiple Input Multiple Output (MIMO) system, the transmitting apparatus comprising: a decoder for decoding the reception response that includes STC mode information determined using a channel capacity estimated for each STC mode; a selector for selecting the STC mode based on the reception response, wherein the STC mode includes Space-Time Transmit Diversity (STTD) and Spatial

Multiplexing (SM); and a encoder for encoding transmission signal according to the selected STC mode. [22] A transmitting method for Space-Time Coding (STC) mode based on a reception response from a receiver in Multiple Input Multiple Output (MIMO) system, the transmitting method comprising: decoding the reception response that includes STC mode information determined using a channel capacity estimated for each STC mode; selecting STC mode based on the reception response, wherein the STC mode includes Space-Time Transmit Diversity (STTD) and Spatial Multiplexing (SM); and encoding the transmission signal according to the selected STC mode.