Time Division Duplex MC-CDMA for Next Generation Mobile Radio Systems

Over the past several years there has been increased interest in wireless mobile communication systems that use code division multiple access (CDMA). The systems such us IS-95 (Direct Sequence CDMA, DS-CDMA) and UMTS (Wideband CDMA, W-CDMA) already use CDMA based techniques. On the other hand, thanks to excellent multi carrier (MC) modulation properties in the frequency selective fading radio channels, MC techniques gained a lot of popularity lately. The Orthogonal Frequency Division Multiplexing (OFDM) as the most common MC technique has been chosen as the European standard for digital audio broadcasting (DAB) and as the digital terrestrial video standard (DVB-T) (OFDM in DAB and DVB-T). Following this, a lot of research was devoted to the usage of hybrid schemes that use the benefits of both CDMA and MC. This led to the development of MC-CDMA, MC-DS-CDMA and similar techniques that are suited for applications in future mobile communications [1]. The MC-CDMA proved to be a suitable technique for the downlink transmission [2]. Uplink transmission, due to the more complex propagation conditions, introduces additional problems which result in harder applicability of MC-CDMA in uplink. The goal of this work is to explain the problems involved in uplink transmission and to offer some solutions which enable us to use modified MC-CDMA as an uplink transmission technique. Moreover, time division duplex (TDD) MC-CDMA as a promising transmission technique for next generation mobile radio systems is presented. The remaining part of the paper is organized as follows. In Section II the basic principles of CDMA, MC and hybrid techniques will be reviewed. Then in Section III the principle of the channel estimation in the downlink MC-CDMA is presented. In Section IV main problems that occur in the uplink transmission are discussed. Section V introduces TDD MC-CDMA, that uses pre-compensation algorithm in order to overcome some of the problems that occur in uplink transmission. Possible TDD MC-CDMA strategies are presented in Section VI. Finally conclusions are given in Section VII. II. BASIC PRINCIPLES OF CDMA, MC AND HYBRID TECHNIQUES