Space-Time Block Codes for Virtual Antenna Arrays Mischa Dohler, Emeline Lefranc, Hamid Aghvami Centre for Telecommunications Research King’s College London mischa@ieee.org Abstract —A novel telecommunication system, termed Vir- tual Antenna Arrays, was recently presented that allows the application of Multiple-Input-Multiple-Output (MIMO) ca- pacity enhancement techniques to mobile terminals with a limited number of antenna elements [1]. This paper intro- duces a novel approach extending the possible applicability of recently emerged Space-Time Block Codes (STBC) to such Virtual Antenna Arrays. The main idea is to emu- late the (2 transmit antennas, 2 receive antennas) diversity scheme developed by Alamouti [2]. This approach success- fully deals with the problem of just one receiving antenna in a handset. Simulations results indicate an improvement in error performance over the simple (2,1) Alamouti scheme. I. Introduction I N the past few years significant progress in code design using transmit diversity over wireless channels has been made. This new coding technique is known as Space-Time Coding. Space-time Codes (STC) have been shown to be a good way for providing high data rate wireless transmis- sion [3]. They are designed to increase data rate and relia- bility of communication over fading channels using multi- ple transmit and/or receive antennas. STC can be viewed as a combined coding and modulation scheme for MIMO channels. Full antenna diversity and good coding gain is achieved via an efficient way to allocate different sym- bols to different antennas with added coding redundancy. Space-Time Block Codes (STBC) operate on a block of inputs symbols producing a matrix output over antennas and time [2], [4]. Full rate STBC do not provide coding gain. However, full diversity is achieved with minimum encoder and decoder complexity. For this reason, we will concentrate on Alamouti’s STBC. The STBC are applied to a recently proposed system of Virtual Antenna Arrays (VAA), [1], [5]. In the context of VAA, the application of STBC forms a subset of pos- sible encoding techniques to enhance the performance for the proposed type of ad-hoc networks. Unlike typical ad- hoc networks, VAA guarantees mutual support and inter- connectivity not only at higher layers but also at PHY and MAC layers. This makes VAA an attractive candidate for ad-hoc oriented 4G networks. As such, VAA could be de- ployed utilising the direct mode of HiperLAN2. The work reported in this paper has formed part of the Wireless AccessareaoftheCore2ResearchProgrammeoftheVirtualCentre of Excellence in Mobile & Personal Communications, Mobile VCE, www.mobilevce.co.uk, whose funding support is gratefully acknowl- edged. More detailed information on this research is available to Industrial Members of Mobile VCE. The concept of VAA mainly bases on relaying with a clever synchronisation and encoding technique. Relaying is a widely used technique for radio packet data transmis- sion both in commercial and military systems but it has so far not been widely used in Cellular systems. Benefits of relaying include extension of high data coverage, reduction of transmitting power, overcoming dead-spots and ad-hoc networks (private and uncoordinated systems). Relaying can be used to combat the difficulties of high data rate transmission over large distances. The idea of relaying was used for the concept of Opportunity Driven Multiple Ac- cess (ODMA), which was one of the proposals for UMTS Terrestrial Radio Access (UTRA). The main idea was to use mobile stations (MSs) as repeaters. In order to achieve performance improvement, an idea based upon adaptation and intelligent combination of existing sophisticated ca- pacity enhancement techniques is considered. The main objective of this work is to develop a novel concept based on the recently emerged Space-Time Coding techniques us- ing MSs as transparent repeaters, providing diversity gain. This study is based on the very simple Space-Time Block Coding techniques developed by Alamouti [2]. The concept of VAA is briefly summarised in Section II. For completeness, the mathematical model for two transmit antennas is provided in Section III. In Section IV, the new relaying scheme is analysed. Simulation results are presented in Section V. Finally, conclusions are drawn in Section VI. II. Concept of Virtual Antenna Arrays For completeness the concept of Virtual Antenna Arrays is revised here; more details can be found in [1]. Fig. 1. Virtual Antenna Array cell within an overlay cell [1].