Performance Evaluation of Space-Time and Harq Diversity in MIMO HSDPA

Multi-antenna techniques can be used to achieve improved system performance, including improved system capacity (more users per cell) and improved coverage (possibility for larger cells), as well as improved service provisioning, for example, higher per-user data rates. This study provides an overview of space-time diversity technique using Alamouti codes applied to the 3G evolution. The delays introduced by HARQ retransmission decrease the performance of the HSDPA system. Multi-antenna techniques are specifically applied to HSDPA to avoid HARQ retransmissions. The authors demonstrate that space-time Rake detector in MIMO 2x2 based on Alamouti code without HARQ process could give better performances than MIMO 2x1 taking into account the HARQ context. In the case of MIMO 2x2,they also observed that the limit of retransmissions affects the system performance and efficiency. Using the generalized Alamouti coding technique in MIMO 4x4 with Multipath Parallel Interference Canceller (MPIC) detector, we achieve notable improvement of performance under the same channel and signal to-Noise Ratio (SNR) conditions. DOI: 10.4018/jmcmc.2012070105 72 International Journal of Mobile Computing and Multimedia Communications, 4(3), 71-86, July-September 2012 Copyright © 2012, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited. In addition to HARQ diversity, the availability of multiple antennas at the transmitter and/or the receiver can be utilized, for example to provide additional diversity against fading on the radio channel (Lee, 2007). In this case, the channels experienced by the different antennas should have low mutual correlation, implying the need for a sufficiently large inter-antenna distance (spatial diversity), alternatively the use of different antenna polarization directions (Lee, 2007). The use of multiple antennas at the receiver side is often noted receive diversity or RX diversity. In case of mutually uncorrelated antennas, the channel gains are uncorrelated and the linear antenna combining provides diversity of order NR (Alamouti, 1998). Also, as an alternative or complement to multiple receive antennas, diversity and beamforming can also be achieved by applying multiple antennas at the transmitter side. With the transmitted signal propagating to the receiver via multiple, independently fading paths with different delays, provides the possibility for multi-path diversity or, equivalently, frequency diversity. Delay diversity is in essence invisible to the mobile terminal, which will simply see a single radio-channel subject to additional time dispersion. Delay diversity can thus straightforwardly be introduced in an existing mobile-communication system without requiring any specific support in a corresponding radio-interface standard. The objective of this paper is to investigate the performance (through simulation) of a MIMO HSDPA system that employs space-time transmit diversity using Alamouti codes through a Rayleigh fading channel. The remainder of this paper is organized as follows. In Section 2, we present the MIMO HSDPA transmitter model. Section 3 presents the MIMO HSDPA receiver model. The numerical results are discussed in Section 4, while the last section provides a discussion and concludes this paper. 2. MIMO HSDPA TRANSMITTER MODEL To support MIMO transmission, the High Speed-Downlink Shared Channel (HS-DSCH) is modified to support up to two antennas. Data are treated in the same manner like in the case of Single Input antenna (Base Station side) and Single antenna Output (Mobile Station side). Only the modified part is spreading and scrambling (Figure 1). We consider one user activated in the HSDPA system. The user has P physicals channels for transmitting the signal according to multicode technique. An individual orthogonalvariable-spreading-factor (OVSF) code cp(t) is assigned to each physical channel with length SF= 16. For each code, the bits are converted to complex symbols a0,p, a2,p,...,aM-1,p using QPSK modulation or 16-QAM modulation. After, the packet is space time coded and distributed in NT sub-frame a (i) 0,p, a (i) 2,p,...,a (i) M-1,p where i=1,...,NT. The NT sub-frames, for each code p, are spreaded with the corresponding spreading code (3 GPP TS 25.212, 2004). The order of MIMO block data symbols (a0,p, a2,p,...,aM-1,p) is denoted by z. For each physical channel code p, the transmitter works by sending the sequence (a2z,p, a*2z+1,p) on the first antenna and (a2z+1,p, a*2z1,p) on the other. The corresponding Matrix Cp (z) is given by Definition 2.1. Definition 2.1. Alamouti code matrix The Alamouti code matrix is: