Copyright Transfer Agreement: “The authors represent that the work is original and they are the author or authors of the work, except for material quoted and referenced as text passages. Authors acknowledge that they are willing to transfer the copyright of the abstract and the completed paper to the SDR Forum for purposes of publication in the SDR Forum Conference Proceedings, on associated CD ROMS, on SDR Forum Web pages, and compilations and derivative works related to this conference, should the paper be accepted for the conference. Authors are permitted to reproduce their work, and to reuse material in whole or in part from their work; for derivative works, however, such authors may not grant third party requests for reprints or republishing.” Government employees whose work is not subject to copyright should so certify. For work performed under a U.S. Government contract, the U.S. Government has royalty-free permission to reproduce the author's work for official U.S. Government purposes. SDR Forum Technical Conference 2007 Proceeding of the SDR 07 Technical Conference and Product Exposition. Copyright © 2007 SDR Forum. All Rights Reserved A SURVEY OF MULTIPLE ANTENNA TX-RX METHODS FOR WIBRO/WIMAX Seokho Kang (The School of Electrical, Computer and Communication Engineering, Hanyang University, Seoul, Korea; [email protected]); Jaeho Chung (Network Infra Laboratory, KT, Seoul, Korea; [email protected]); Seungwon Choi (The School of Electrical, Computer and Communication Engineering, Hanyang University, Seoul, Korea; [email protected]) ABSTRACT which compared to single input single output (SISO). On the other hand, because SM scheme transmits different WiBro/WiMAX is an OFDMA-based multiple access signals from each transmit antenna, data-rate is increased service for wireless broadband multimedia environment. proportionally to number of transmit antennas. But SM Multiple antenna system has been considered to increase scheme does not provide a diversity gain. the capacity, service area, and data rate for WiBro/WiMAX In this paper, the performance of many MIMO base station system. In this paper, we compare the methods schemes and algorithms is compared and analyzed in for multiple antenna system of WiBro/WiMAX and show OFDMA-based wireless communication standard, the performance benefits of various multiple antenna WiBro/WiMAX downlink environment. Based on IEEE methods. We examine the space time transmit diversity 802.16-2004 [2] and 802.16e-2005 [3], WiBro (Wireless (STTD) and spatial multiplexing scheme for Broadband) is the next generation wireless communication WiBro/WiMAX. We first describe the signal modeling and standard that has been adopted in Korea. operations for STTD, spatial multiplexing. Then we This paper is organized as follows. In section 2, we explain advantages and disadvantages in each method, to examine the signal modeling of STTD, SM schemes and summarize the differences among the methods. We also algorithms. In section 3, we examine the structure of demonstrate various simulation results to show the WiBro/WiMAX downlink system. Simulation results to performances of STTD and spatial multiplexing which are apply many MIMO schemes are shown in section 4. Section compared in various communication circumstances. 5 concludes this paper. 2. MIMO SCHEMES 1. INTRODUCTION 2.1. Space Time Transmit Diversity Wireless communication using multiple input multiple output (MIMO) has recently emerged as one of the most 2.1.1. 2×1 Space Time Code significant technical issues to increase the system capacity There is MIMO system that has two transmit antennas and with limited resources in modern communications. MIMO a single receive antenna in figure 1. If two symbols to be system has many benefits that increase the system transmitted are s1 and s2 , STC sends the following for reliability, increase the data-rate and enlarge the wireless two symbol times: service coverage. However, all MIMO systems have not Antenna 1 2 these benefits. Each MIMO scheme has advantages and Time 0 disadvantages compared to one another. s1 s2 * * There are largely two algorithms of MIMO schemes: 1 - s2 s1 one is space time transmit diversity (STTD) and the other is During two symbol times, transmitted symbols in each spatial multiplexing (SM). STTD scheme transmits the antenna are orthogonal with each other. Using matrix orthogonal signals in each transmit antenna and is often expression, received symbol vector is expressed in called by Alamouti code or space time code [1]. This following (1). An assumption is that the channel h , h scheme can obtain a diversity gain like the maximum ratio 1 2 remain constant over two symbol times. combining in receive antenna, but data-rate is not increased SDR Forum Technical Conference 2007 Proceeding of the SDR 07 Technical Conference and Product Exposition. Copyright © 2007 SDR Forum. All Rights Reserved ant 1 h1 * - s2 s1 s s 2 Data 2 1 h ˆ ˆ STC y2 y1 STC s2 s1 Data Modulation Encoder ant 2 Decoder Demodulation * s1 s2 space time Fig. 1. 2×1 space time code ant 1 h1 * - s s1 2 h2 y3 y1 STC s s 2 1 STC Decoder ant 2 sˆ2 sˆ1 Encoder h3 Combining * y4 y2 s1 s2 h4 STC Decoder space time Fig. 2. 2×2 space time code * 2 2 é y1 ù é s s ùéh1 ù én1 ù és ù éh h ùéy ù é(h + h ) × s + N ù = 1 2 + (1) 1 3 4 3 3 4 1 3 (4) ê ú ê * * úê ú ê ú ê ú = ê * úê *ú = ê 2 2 ú y - s s h n s h - h y ë 2 û ë 2 1 ûë 2 û ë 2 û ë 2 û ë 4 3ûë 4 û ëê(h3 + h4 )× s2 + N4 ûú where n , n are additive white Gaussian noise. ~ 2 2 2 2 1 2 sˆ s s é ù é 1 ù é 1 + 1 ù ( h1 + h2 + h3 + h 4 ) × s1 + N1 + N3 (5) Received symbols are decoded as below. ê ú = ê~ ú = ê 2 2 2 2 ú ësˆ2 û ës2 + s2 û ê( h + h + h + h ) × s + N + N ú * é 2 2 ù ë 1 2 3 4 2 2 4 û ésˆ1 ù éh1 h2 ùé y1 ù ( h1 + h2 )×s1 + N1 (2) We can see that data-rate of 2×2 STC is the same with ê ú = ê * úê * ú = ê 2 2 ú sˆ h - h y ë 2 û ë 2 1 ûë 2 û ëê( h1 + h2 )×s2 + N2 ûú 2×1 STC. As (2), we can see that 2×1 STC obtains a diversity gain like the maximum ratio combining (MRC) of 1×2 2.2. Spatial Multiplexing receive diversity system. In summary, the 2×1 STC achieves the same diversity order and data-rate as a 1×2 SM is a scheme that a set of independent symbols are receive diversity system with MRC but with 3dB penalty, transmitted in each transmit antenna, which make the data- owing to the redundant transmission required to remove the rate be doubled. Mathematical expression is as follows. spatial interference at the receiver. y = Hx + n , i.e. é y ù é h h L h ù é x ù é n ù 2.1.2. 2×2 Space Time Code 1 11 12 1N 1 1 (6) ê y ú ê h h L h ú ê x ú ê n ú Figure 2 is 2×2 STC and STC encoding is operated as ê 2 ú = ê 21 22 2N ú ê 2 ú + ê 2 ú follow. ê M ú ê M M O M ú ê M ú ê M ú ê ú ê ú ê ú ê ú é y y ù é s s ùéh h ù én n ù ëyM û ëhM1 hM 2 L hMN û ëxN û ënM û 1 3 = 1 2 1 3 + 1 3 (3) ê ú ê * *úê ú ê ú where M is the number of receive antennas, N is the ëy2 y4 û ë- s2 s1 ûëh2 h4 û ën2 n4û number of transmit antennas and 2 . · term 2×2 STC decoding is operated like (2) in each receive n ~ Nc (0,s I) antenna and then, decoded results are combined as below. indicates a matrix, · term indicates a vector. ~s é * ù y é 2 2 ù é 1 ù h1 h2 é 1 ù ( h1 + h2 ) × s1 + N1 , Figure 3 is the structure of 2×2 SM. In SM scheme, to ê ú = ê * úê * ú = ê 2 2 ú ~s h - h y detect received symbols, the detection processing is ë 2 û ë 2 1ûë 2 û ëê( h1 + h2 ) × s2 + N2 ûú required. SDR Forum Technical Conference 2007 Proceeding of the SDR 07 Technical Conference and Product Exposition. Copyright © 2007 SDR Forum. All Rights Reserved ant 1 ant 1 h11 h x1 21 y1 2 h1 yˆ yˆ Data x2 x1 MIMO 2 1 Data S / P ant 2 ant 2 Modulation h22 Detection Demodulation x 2 y2 Fig. 3. 2×2 spatial multiplexing There are many algorithms that are used to detection ~ 2 H H H -1 W = arg min Eé Wy - x ù = H (HH + E[nn ]) processing and in this paper we examine maximum W ëê ûú (9) likelihood (ML), zero forcing (ZF), minimum mean square ~ H H H -1 error (MMSE), and successive interference cancellation xˆ = W ×y = H (HH + E[nn ]) y (SIC), ordered SIC (OSIC). Because noise effect is considered, its performance is better than ZF. But the computational complexity is larger 2.2.1. Maximum Likelihood than ZF. The following equation is criterion for ML detection.