A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering, School of Engineering and Technology

Thesis (M.Eng.) - Asian Institute of Technology, 2000

Multicarrier code division multiple access (MC-CDMA) is a technique that combines orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA). Since MC-CDMA system provides an efficient method of frequency diversity, it is suitable to a frequency selective rayleigh fading environment. This technique becomes a potential technique for future wireless networks. In addition, MC-CDMA is especially useful in the downlink transmission. However, similar to OFDM system, the transmitted signal from MC-CDMA system also exhibits a very high Peak to Average Power Ratio (PAPR) when using a large number of subcarriers. Therefore, the amplifiers should be operated in large linear ranges to avoid nonlinear distortion. This leads to very inefficient amplification and very expensive transmitters. Thus, P APR reduction techniques are very significant for multicarrier transmission systems. Consequently, several P APR reduction methods have been proposed, but most of techniques are considered with single user OFDM such as Clipping, Selected Mapping, Partial Transmit Sequence, and so on. In MC-CDMA systems, the P APR reduction technique is mostly based on the type of spreading sequences. Partial Transmit Sequence (PTS) technique is an efficient approach and a distortionless scheme for reducing P APR. Due to its flexibility, the PTS technique works with an arbitrary number of subcarriers. In addition, it works without restriction on the type of modulation scheme. Thus, the investigation of P APR in MC-CDMA system combined with PTS technique is very attractive. In this thesis, the P APR reduction in synchronous downlink MC-CDMA using PTS technique is investigated with a different number of active users. Moreover, WalshHadamard (WH) sequence and Orthogonal Gold (OG) sequence are used as the spreading sequences in MC-CDMA system. From the simulation results, when the number of active users is varied, P APR reduction of this system also varies depending on the P APR characteristics of spreading sequence. Although PTS technique is very efficient in reducing PAPR, high PAPR reduction is achieved at a cost of increased system complexity. Furthermore, to compromise between P APR reduction and system complexity, the suboptimal algorithms are used for combining partial transmit sequences. Based on simulation results, we can reduce system complexity by more than 90% at the expense of little P APR performance degradation.