Efficient MIMO-OFDM Schemes for Future Terrestrial Digital TV with Unequal Received Powers

Abstract- This article investigates the effect of equal and unequal received powers on the performances of different MIMO-OFDM schemes for terrestrial digital TV. More precisely, we focus on three types of non- orthogonal schemes: the BLAST scheme, the Linear Dispersion (LD) code and the Golden code, and we compare their performances to that of Alamouti scheme. Using two receiving antennas, we show that for moderate attenuation on the second antenna and high spectral efficiency, Golden code outperforms other schemes. However, Alamouti scheme presents the best performance for low spectral efficiency and equal received powers or when one antenna is dramatically damaged. When three antennas are used, we show that Golden code offers the highest robustness to power unbalance at the receiving side. Keywords- OFDM, MIMO, Space Time codes. I. INTRODUCTION The potential advantages of digital television broadcasting over conventional analogue broadcasting are numerous and well known. For broadcasters, digital technology offers significantly improved operational flexibility, providing the means for new services which go beyond the scope of conventional television programmes. Since its inauguration in 1993, digital video broadcast (DVB) project for terrestrial (DVB-T) transmission has fully responded to the objectives of its designers, delivering wireless digital TV services in almost every continent [1]. In fact, there is no single DVB standard, but rather a collection of standards, technical recommendations and guidelines. In Spring 2006, DVB community was asked to provide technical specifications and studies for a future second generation of DVB-T called DVB-T2. It is expected that the first profile of DVB-T2 specification, for fixed reception of high definition television (HDTV) services, will be completed as soon as possible, with a second profile offering improved mobile performance completed around the end of 2008. Against this background, a new European CELTIC project called Broadcast for 21st Century (B21C) was launched [2]. It constitutes a contribution task force to the reflections engaged by the DVB project and should give a real support for the conclusions and decisions within DVB project, particularly on multiple input multiple output (MIMO) with orthogonal frequency division multiplexing (OFDM) transmission for HDTV services.
The work presented in this paper has been carried out within the framework of B21C project. The contribution of this work is twofold. First, a generalized framework is proposed for modelling the effect of unequal received powers on different receiving antennas. Therefore, we analyze and compare some of the most promising MIMO- OFDM systems in the context of broadcasting for future terrestrial digital TV with equal but also unequal received powers i.e. with unequal received signal to noise ratio (SNR) per antenna. In the literature, most of the works consider equal received powers for the performance comparison of MIMO-OFDM schemes [3][4]. The assumption of unequal received powers could be seen in different communications contexts like in a broadcast link where two different antennas are used at the receiving side or in a mobile link. Indeed, the call for technology within DVB-T2 consortium moves towards an expectation of such situations where one outdoor antenna (roof antenna for example) and one or two indoor antennas are used. Eventually, we note that for complexity reasons the analysis of different MIMO- OFDM systems is not achieved with the optimal maximum likelihood (ML) receiver. Instead, we use a sub-optimal iterative receiver with few iterations. This paper is structured as follows. Section 2 describes the system model for MIMO-OFDM. In section 3 we discuss the choice of different MIMO schemes considered in this paper. Section 4 presents the iterative receiver with a detailed description of its blocks. Simulation results are drawn in section 5. Section 6 concludes the paper. II. SYSTEM MODEL WITH UNEQUAL RECEIVED POWERS Consider an OFDM communication system using MT transmit antennas (Tx) and MR receive antennas (Rx) for a downlink communication. Such a system could be implemented for the MT transmit antennas using a space- time (ST) encoder which takes Q data complex symbols and transforms them to a (MT,T) output matrix according to the ST block coding (STBC) scheme. The ST STBC coding rate is then defined by L=Q/T. Figure 1 depicts the transmitter modules. Information bits bk are first channel encoded with a convolutional encoder of coding rate

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