WO2008130102A1

WO2008130102A1 - Close loop transmission method and apparatus

Info

Publication number: WO2008130102A1
Application number: PCT/KR2008/001666
Authority: WO
Inventors: Yong-Su Lee; Young-Il Kim; Jee-Hwan Ahn; Wenbo Wang; Zheng Kan
Original assignee: Electronics And Telecommunications Research Institute; Samsung Electronics Co., Ltd.
Priority date: 2007-04-23
Filing date: 2008-03-25
Publication date: 2008-10-30
Also published as: KR100840618B1

Abstract

The present invention relates to a closed-loop transmission method for performing closed-loop transmission based on a quasi-orthogonal space-time block code (QOSTBC), and a device thereof. An orthogonal frequency division multiple access (OFDMA) transmitter selects a code matrix corresponding to a code index received from an uplink sub-frame, performs space-time coding for a modulated symbol by using the selected code matrix, and generates a plurality of data symbols, and an OFDMA receiver performs space-time decoding for a demapped signal by using maximum likelihood (ML), linear detection of zero forcing (ZF), and minimum mean square error (MMSE), and therefore complexity at the receiver may be reduced and bit error rate performance may be improved.

Description

TITLE OF THE INVENTION CLOSE LOOP TRANSMISSION METHOD AND APPARATUS

CROSS-REFERENCE TO RELATED APPLICATION This application claims priority to and the benefit of Korean Patent

Application No. 10-2007-0039227 filed in the Korean Intellectual Property Office on April 23, 2007, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION (a) Field of the Invention

The present invention relates to a closed-loop transmission method and a device thereof. More particularly, the present invention relates to a closed-loop transmission method for performing closed-loop transmission based on a quasi-orthogonal space-time block code (QOSTBC) and a device thereof. (b) Description of the Related Art

In a like manner of a mobile phone, a wireless broadband (WϊBro) Internet system for using high speed Internet anytime anyplace while a user moves has been developed. A multi-input multi-output (MIMO) method of the WiBro system includes various space-time code techniques. Here, most of the space-time code techniques are orthogonal space-time block code (OSTBC) techniques, and a full diversity full rate (RDRF) may be achieved when the number of antennas is two, but the FDFR may not be achieved even in a closed-loop method when the number of antennas is more than two (e.g., four antennas according to the WiBro standards). Accordingly, an open loop QOSTBC scheme has been suggested, but interference caused by quasi-orthogonality between antennas is high, and decoding complexity is higher than the OSTBC.

Therefore, development of a system having lower complexity and better performance than the existing WiBro system is required. The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. SUMMARY OF THE INVENTION

The present invention has been made in an effort to provide a closed-loop transmission method and a device for performing closed-loop transmission based on a quasi-orthogonal space-time block code (QOSTBC).

In addition, the present invention relates to a method and device for performing closed-loop quasi-orthogonal space-time block coding in an orthogonal frequency division multiple access (OFDMA) system.

According to an exemplary embodiment of the present invention, a closed-loop transmission method and device based on the QOSTBC that has been suggested to improve bit error rate performance in a space-time coding mode in a multi-input multi-output (MIMO) scheme of a wireless broadband (WiBro) system are realized. In this case, since a transmitting terminal selects one code from 12 code matrixes according to channel state information and a receiving terminal performs simplified linear detection, receiving complexity is reduced compared to a conventional QOSTBC, and performance is improved compared to open loop

QOSTBC and other closed-loop MIMO schemes in conventional WiBro standards.

According to an exemplary embodiment of the present invention, in a closed-loop transmission method of an orthogonal frequency division multiple access (OFDMA) transmitter for demodulating an encoded bit sequence to a symbol, and performing subcarrier mapping, inverse fast Fourier transform (IFFT), CP adding, and digital to analog (D/A) converting for the demodulated symbol, a code matrix corresponding to a code index received from an uplink sub-frame is selected, the selected code matrix is used, space-time coding is performed for the demodulated symbol, a plurality of data symbols are generated, the subcarrier mapping, the IFFT, the CP adding, and the D/A converting are performed for the generated data symbols, and they are transmitted.

In addition, in the selecting of the code matrix, the code index for minimizing interference caused by channel information of the uplink sub-frame is received from the uplink sub-frame, and one code matrix is selected from among the code matrixes according to the code index.

In this case, the data symbol is generated in a quasi-orthogonal space-time block code (QOSTBC) format. In addition, the data symbol is generated D , a row index indicates a number of antennas, and a

column index indicates symbol times. Further, the data symbol has a channel matrix of H=[h-i,-ι hi,₂ hi₎₃ hi,₄], and hjj denotes channel information transmitted from a transmitting antenna j to a receiving antenna i. According to another exemplary embodiment of the present invention, in a closed-loop transmission method of an orthogonal frequency division multiple access (OFDMA) receiver for performing subcarrier demapping for a received signal through analog to digital (A/D) converting, CP removing, and fast Fourier transforming, and demodulating and decoding the demapped signal into a bit sequence, maximum likelihood (ML), linear detection of zero forcing (ZF), and minimum mean square error (MMSE) with respect to the demapped signal are used, space-time decoding is performed, a transmission signal of a transmitting terminal is detected by performing the space-time decoding, and the transmission signal is demodulated to a bit sequence. According to another exemplary embodiment of the present invention, a closed-loop transmitting device of an orthogonal frequency division multiple access (OFDMA) transmitter for demodulating an encoded bit sequence to a symbol, and performing subcarrier mapping, inverse fast Fourier transform (IFFT), CP adding, and digital to analog (D/A) converting for the demodulated symbol includes a space-time coding unit. The space time coding unit selects one from various types of code matrixes, space-time encodes the modulated symbol according to a format of the selected code matrix, separates it into a plurality of streams, and transmits the streams to a unit for performing the subcarrier mapping.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram representing a configuration of an orthogonal frequency division multiple access (OFDMA) downlink sub-frame in a wireless broadband (WiBro) system according to an exemplary embodiment of the present invention.

FIG. 2 is a diagram representing simplified function blocks of an OFDMA transmitter according to the exemplary embodiment of the present invention. FIG. 3 is a diagram representing simplified function blocks of an OFDMA receiver according to the exemplary embodiment of the present invention.

FIG. 4 is a flowchart representing a closed-loop transmission method according to the exemplary embodiment of the present invention. FIG. 5 is a graph comparing performance of receivers in a pedestrian A

(PA) channel environment according to the exemplary embodiment of the present invention.

FIG. 6 is a graph comparing performance of receivers in a vehicular A (VA) channel environment according to the exemplary embodiment of the present invention.

FIG. 7 is a graph comparing realization complexities of respective decoding types of a space-time code according to the exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification.

In addition, unless explicitly described to the contrary, the word "comprise" and variations such as "comprises" and "comprising" will be understood to imply the inclusion of stated elements but not the exclusion of any other elements. The terms "unit", "module", and "block" used herein mean one unit that processes a specific function or operation, and may be implemented by hardware, software, or a combination thereof.

A closed-loop transmission method and a device based on a quasi-orthogonal space-time block code (QOSTBC) according to an exemplary embodiment of the present invention will be described.

As shown in FIG. 1 , the OFDMA downlink sub-frame of the WiBro system according to the exemplary embodiment of the present invention includes 27

OFDMA symbols. The first three symbols 101 include a preamble and a frame control header (FCH)₁ and the twenty-four subsequent symbols 102 are data symbols including a data subcarrier and a pilot subcarrier.

FIG. 2 is a diagram representing simplified function blocks of an OFDMA transmitter according to the exemplary embodiment of the present invention.

As shown in FIG. 2, the OFDMA transmitter according to the exemplary embodiment of the present invention includes a channel encoding unit 201, a modulating unit 202, a space-time coding (STC) unit 203, a plurality of subcarrier mapping units 204, a plurality of inverse Fourier transform (IFFT) units 205, a plurality of cyclic prefix (CP) adding units 206, and a plurality of digital to analog (D/A) converting units 207. The channel encoding unit 201 encodes source bit sequences. In this case, channel encoding processes include randomization, coding, and bit interleaving processes. In addition, the channel encoding unit 201 supports various codes including a convolutional code (CC)₁ a turbo code (CTC), a low density parity-check code (LDPC). The modulating unit 202 modulates the bit sequences encoded in the channel encoding unit 201 into quadrature phase shift keying (QPSK), sixteen-phase quadrature amplitude modulation (16QAM), or 64QAM data symbols.

The STC unit 203 performs space-time encoding for the data symbols modulated in the modulating unit 202 according to a code form in Table 1 , and separates the data symbols into four streams to output them to the respective subcarrier mapping units 204. In this case, the STC unit 203 uses a code (shown in Table 1) that is appropriate for a code index previously received from an uplink sub-frame, so as to perform the space-time encoding for the data symbols modulated in the modulating unit 202. Here, the code index H is a channel frequency response (CFR) in each subcarrier, and it is determined to a value that minimizes interference caused by channel state information (CSI) of a current uplink sub-frame in an OFDMA receiver according to the exemplary embodiment of the present invention.

The subcarrier mapping units 204 respectively receive the four streams from the STC unit 203, add pilot signals to them, perform sub-channelization, and apply the sub-channelized signals to the IFFT units 205. Here, methods for sub-channelizing includes a full usage of sub-channel (FUSC) method, a partial usage of sub-channel (PUSC) method, and an adaptive modulation and coding (AMC) method.

The IFFT unit 205 receives the signal sub-channelized in the subcarrier mapping unit 204 (i.e., a subcarrier mapped signal), converts the frequency domain data symbols into time domain data symbols, and applies them to the CP adding unit 206.

The CP adding unit 206 receives the signal converted in the IFFT unit 205, adds a cyclic prefix (CP) to the signal to reduce inter-symbol-interference (ISI), and applies the CP-added digital signal to the D/A converting unit 207. The D/A converting unit 207 converts the digital signal to which the CP is added by the CP adding unit 206 into an analog signal to transmit it through an antenna.

FIG. 3 is a diagram representing simplified function blocks of the OFDMA system according to the exemplary embodiment of the present invention. As shown in FIG. 3, the OFDMA receiver according to the exemplary embodiment of the present invention includes an analog to digital (AID) converter 301 , a CP removing unit 302, a fast Fourier transform (FFT) unit 303, a subcarrier demapping unit 304, an STC de-coding unit 305, a demodulation unit 306, and a channel decoding unit 307. The A/D converter 301 converts the analog signal received through the antenna into a digital signal and applies it to the CP removing unit 302.

The CP removing unit 302 removes the CP from the signal received from the A/D converter 301 to apply it to the FFT unit 303.

The FFT unit 303 converts time domain data symbols into frequency domain data symbols with respect to the signal in which the CP is removed by the CP removing unit 302, and applies the frequency domain data symbols to the subcarrier demapping unit 304.

The subcarrier demapping unit 304 receives the signal converted by the FFT unit 303 (i.e., the frequency domain data symbol), separates data, performs de-channelization, and applies the de-channelized signal to the space-time decoding unit 305.

The space-time decoding unit 305 receives the signal that is de-channelized in the subcarrier demapping unit 304 to perform space-time decoding. Here, the space-time decoding unit 305 uses a maximum likelihood (ML) method, a linear detection method, and a simplified algorithm with respect to the open loop QOSTBC, and applies the space-time decoded signal to the demodulation unit 306. The demodulation unit 306 receives the signal that is space-time decoded by the space-time decoding unit 305, demodulates the space-time decoded signal to a bit sequence, and applies the bit sequence to the channel decoding unit 307.

The channel decoding unit 307 decodes the bit sequence that is demodulated in the demodulation unit 306, so that an information bit is finally decoded.

A closed-loop transmission method according to the exemplary embodiment of the present invention will now be described with reference to FIG. 4.

FIG. 4 is a flowchart representing the closed-loop transmission method based on the QOSTBC that is suggested to improve a bit error rate in a space-time coding mode among the MIMO techniques of the OFDMA system according to the exemplary embodiment of the present invention.

Here, in the exemplary embodiment of the present invention, it is assumed that flat fading is provided within the respective sub-carriers since the number of transmitting antennas is four and the transmitting method is based on the OFDM. The OFDMA transmitter according to the exemplary embodiment of the present invention receives the code index from the uplink sub-frame and stores it in step S401. In this case, the OFDMA receiver according to the exemplary embodiment of the present invention determines the code index for minimizing interference caused by the channel state information (CSI) of the current uplink sub-frame among the code indexes, and transmits it.

Subsequently, the STC unit 203 receives the symbols modulated by the modulating unit 202 in step S402, performs the space-time encoding according to the code forms shown in Table 1 , separates the symbols into four data streams, and outputs them to the respective subcarrier mapping units 204. In this case, the STC unit 203 selects a code (shown in Table 1) corresponding to the code index previously received from the uplink sub-frame in step S403.

That is, the STC unit 203 selects one code matrix from among twelve code matrixes shown in Table 1. in this case, a method for selecting the corresponding code matrix is determined by the uplink transmitted code index.

In addition, the STC unit 203 uses the code matrix selected in step S403 to perform the space-time encoding in step S404. In this case, the four data symbols are formed in a format of the QOSTBC as given in Equation 1 in step S405. Here, a row index indicates the number of antennas, and a column index indicates symbol times.

(Equation 1)

The four data symbols formed in step S405 are transmitted through the respective subcarrier mapping units 204, the respective IFFT units 205, the respective CP adding units 206, the respective D/A converting units 207, and the respective transmitting antennas, in step S406.

In this case, channel matrixes of the data symbols transmitted through the respective transmitting antennas are given as Equation 2. Here, hg indicates CSI transmitted from a transmitting antenna j (j=1...4) to a receiving antenna i (i=1). (Equation 2)

Accordingly, the OFDMA receiver according to the exemplary embodiment of the present invention receives a signal given as Equation 3 through the receiving antenna in step S407. Here, r\_\ indicates additive white Gaussian noise (AWGN) having an average of 0 and a variance of s². In addition, r=[n r₂ r₃ r₄] and n=[ni n₂ n₃ n₄].

(Equation 3) r = H D + n Here, in the exemplary embodiment of the present invention, it is assumed that the number of receiving antennas is one to simplify the equations. However, it is not limited thereto, and the number of receiving antennas may be more than one.

The OFDMA receiver according to the exemplary embodiment of the present invention uses the ML method, the linear detection method, and the simplified algorithm with respect to the open loop QOSTBC to perform the space-time decoding. In this case, the space-time decoding unit 305 receives the de-channelized signal from the subcarrier demapping unit 304 and uses the linear ML detection method to perform the space-time decoding, in step S408.

That is, Equation 3 may be changed to Equation 4 according to the equivalent shown in Table 1. In this case, the space-time decoding unit 305 detects a transmission signal of the OFDMA transmitter according to the exemplary embodiment of the present invention based on Equation 4, in step S409. To detect the transmission signal, the space-time decoding unit 305 uses the ML method, a zero forcing method that is a linear detection method, and a minimum mean square error (MMSE) method.

(Equation 4) r eq ^= H eg X + n

Here, ¹W

tH

Equation 4 may be changed to Equation 5 by multiplying H_e? by the left side of Equation 4. Here, a superscript H denotes complex conjugate transpose, and 91 denotes a real part of a complex number. In addition, p in H_e(? shown in Equation 5 denotes signal power obtained through fading channel, and an absolute value of f_D indicates amplitude of interference between antennas. (Equation 5)

Here, H -

ID = Kx ^* Ki ^{+ 7}C ^• ^,4 + Ki ^' Kϊ ⁺ Ki ^' K*

= 2-SfKKl -K3 +K2 -KJ

In addition, the QOSTBC has various code matrix types. In this case, when the code matrix varies, f_D varies but p does not vary. Twelve code matrix types may be given as in Table 1.

Table 1 shows twelve code matrix types D according to the exemplary embodiment of the present invention, and corresponding interference f. Here, I denotes an imaginary part of a complex number. In addition, since p does not vary, an absolute value of the interference value f determines bit error rate performance. (Table 1)

In the closed-loop method according to the exemplary embodiment of the present invention, an interference value is calculated while assuming a transmitting terminal knows the ideal CSI, and one QOSTBC for minimizing the interference among the QOSTBCs is selected. In this case, a k-th code matrix is calculated by Equation 6, and || denotes absolute value.

(Equation 6)

* ^{= ar}s^mmj//|)

Accordingly, when assuming the transmitting terminal selects the k-th code matrix as shown in Equation 6, a received signal of a receiving terminal is given as Equation 7. Here, D_k has formats shown in Table 1 with respect to respective "k"s.

(Equation 7) r, = H D_A +n

Similarly to Equation 3, Equation 7 may be changed to Equation 8. Here, X is given as Equation 4. In addition, H_A is given as in Table 1. (Equation 8) r^ = H_fc . χ+n¹

Similarly to Equation 5, Equation 8 may be changed to Equation 9. Here,

H, has a unique configuration having eight non-zero values. (Equation 9)

In this case, diagonal constituent elements are p , and f_k that is the interference value varies according to a value of k. Since f_k is a minimum among the twelve code matrixes, it is considerably lower than p . Therefore, the interference is ignored, and simplified linear ML decoding as given in Equation 10 may be performed. (Equation 10)

FIG. 5 is a graph comparing performance of receivers in a pedestrian A

(PA) channel environment according to the exemplary embodiment of the present invention, and FIG. 6 is a graph comparing performance of receivers in a vehicular

A (VA) channel environment according to the exemplary embodiment of the present invention.

Here, the open loop QOSTBC using the MMSE detection method, a WiBro standard matrix A having the same code rate as the QOSTBC, and the closed-loop

QOSTBC according to the exemplary embodiment of the present invention are compared. In addition, Nr shown in FIG. 5 and FIG. 6 denotes the number of receiving antennas.

Basic parameters are given as in Table 2. Table 2 shows basic parameters of a WiBro downlink system in the FUSC.

(Table 2)

In the exemplary embodiment of the present invention, the number of transmitting antennas is 4, and the number of receiving antennas is 1 or 2. In addition, the FUSC method is used in a subcarrier allocation method, and it is assumed that channel information applied to a simulation is ideally known.

Therefore, according to a simulation result, the closed-loop QOSTBC according to the exemplary embodiment of the present invention has excellent performance compared to other methods.

FIG. 7 is a graph comparing realization complexities of respective decoding types of the space-time code according to the exemplary embodiment of the present invention. Here, every decoding scheme is calculated by the number of floating point number multiplications.

The open loop QOSTBC is not considered since the ML detection method is quite complicated, the linear MMSE detection method and a simplified MMSE are applied, and the linear ML detection method is used to compare the closed-loop QOSTBC and the matrix A.

Table 3 shows comparison results with respect to the number of floating point number multiplications in the respective detection methods. In addition, Table 3 shows complexities according to the respective detection method with respect to the number of floating point number multiplications.

(Table 3)

A preparing phase is additionally required to obtain a detection matrix in the MMSE detection method, but it is not required in the linear ML method. Therefore, the closed-loop QOSTBC according to the exemplary embodiment of the present invention has lesser complexity than the open loop QOSTBC.

In the exemplary embodiment of the present invention, since the closed-loop transmission method and device based on the QOSTBC that are suggested to improve the bit error rate performance in the space-time encoding mode among the MIMO techniques of the WiBro system are realized, the transmitting terminal selects one code from the 12 code matrixes according to the channel state information, and the receiving terminal performs the simplified linear detection.

The exemplary embodiment of the present invention that has been described above may be implemented by not only an apparatus and a method but also by a program capable of realizing a function corresponding to the structure according to the exemplary embodiment of the present invention and a recording medium having the program recorded therein. It can be understood by those skilled in the art that the implementation can be easily made from the above-described exemplary embodiment of the present invention.

While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

According to the exemplary embodiment of the present invention, since the closed-loop transmission method and device for performing the closed-loop transmission based on the QOSTBC in the OFDMA system are realized, the complexity at the receiving terminal is reduced, and the bit error rate performance is improved.

Claims

WHAT IS CLAIMED IS:

1. A closed-loop transmission method of an orthogonal frequency division multiple access (OFDMA) transmitter for demodulating an encoded bit sequence to a symbol, and performing subcarrier mapping, inverse fast Fourier transform (IFFT), CP adding, and digital to analog (D/A) converting for the demodulated symbol, the closed-loop transmission method comprising: selecting a code matrix corresponding to a code index received from an uplink sub-frame; using the selected code matrix, performing space-time coding for the demodulated symbol, and generating a plurality of data symbols; and performing the subcarrier mapping, the IFFT, the CP adding, and the D/A converting for the generated data symbols and transmitting them.

2. The closed-loop transmission method of claim 1 , wherein the selecting of the code matrix comprises: receiving the code index for minimizing interference caused by channel information of the uplink sub-frame from the uplink sub-frame; and selecting one code matrix from among the code matrixes according to the code index.

3. The closed-loop transmission method of claim 1 , wherein the data symbol is generated in a quasi-orthogonal space-time block code (QOSTBC) format.

4. The closed-loop transmission method of claim 3, wherein the data

symbol is generated by D , a row index indicates a number

of antennas, and a column index indicates symbol times.

5. The closed-loop transmission method of claim 3, wherein the data symbol has a channel matrix of H=[hi,i h-ι,₂ hi,₃ hi,₄], and hy denotes channel information transmitted from a transmitting antenna j to a receiving antenna i.

6. The closed-loop transmission method of claim 1 , wherein the k-th code matrix is calculated by k = arg mind/,1).

7. A closed-loop transmission method of an orthogonal frequency division multiple access (OFDMA) receiver for performing subcarrier demapping for a received signal through analog to digital (A/D) converting, CP removing, and fast Fourier transforming, and demodulating and decoding the demapped signal into a bit sequence, the closed-loop transmission method comprising: using maximum likelihood (ML), linear detection of zero forcing (ZF), and minimum mean square error (MMSE) with respect to the demapped signal, and performing space-time decoding; and detecting a transmission signal of a transmitting terminal by performing the space-time decoding, and demodulating the transmission signal to a bit sequence.

8. The closed-loop transmission method of claim 7, wherein the

transmission signal is detected by r = H - X + n^' , and here, ^r _eq -

9. A closed-loop transmitting device of an orthogonal frequency division multiple access (OFDMA) transmitter for demodulating an encoded bit sequence to a symbol, and performing subcarrier mapping, inverse fast Fourier transform (IFFT), CP adding, and digital to analog (D/A) converting for the demodulated symbol, the closed-loop transmitting device comprising a space-time coding unit for selecting one from various types of code matrixes, space-time coding the modulated symbol according to a format of the selected code matrix, separating it into a plurality of streams, and transmitting the streams to a unit for performing the subcarrier mapping.

10. The closed-loop transmitting device of claim 9, wherein the space-time coding unit determines the code matrix according to a code index for minimizing interference caused by channel state information of an uplink sub-frame.