OFDM-IDMA wireless communication systems

Abstract

Orthogonal frequency-division multiplexing code-division multiple-access (OFDMCDMA) also referred as multi-carrier CDMA (MC-CDMA) is a spread-spectrum transmission technique introduced in 1993. Since then, numerous technical papers about OFDM-CDMA have been published. OFDM-CDMA is considered to be a promising candidate for the air interface of the fourth generation (4G) wireless communication systems. Interleave-division multiple-access (IDMA) is a recently proposed multiple access scheme, which employs random interleavers as the only method for user separation. As a particular case of CDMA, IDMA inherits many distinguished features of well-studied CDMA. Furthermore, it allows a low-cost turbo-type multiuser detection (MUD) algorithm applicable to the system with large numbers of users, which is crucial for high-rate multiple access communication. The work presented in this thesis is completely devoted to the investigation of an OFDM-IDMA scheme. The proposed scheme employs IDMA instead of CDMA in the OFDM-CDMA scheme and combines advantages of both OFDM and IDMA. With sufficient guard intervals, OFDM can completely remove intersymbol interference (ISI). IDMA with a chip-by-chip (CBC) iterative multi-user detector can overcome both crosscell and intra-cell multiple access interference (MAI) problems efficiently. We first present the basic principles of OFDM-IDMA transmitter and receiver. Comparative studies between OFDM-IDMA and OFDM-CDMA are carried out. We also compare OFDM-IDMA with IDMA with LLRC algorithm. Thanks to the independent processing of ISI and MAI, OFDM-IDMA offers better performance than plain IDMA with LLRC in terms of both bit error rate (BER) and complexity. Then we derive a simple and effective performance analysis method for OFDMIDMA based on SNR evolution. To support large number of users and achieve high rates, we optimize the transmitted power profiles with the aid of the SNR evolution technique. Several power allocation methods are presented, which are selected according to the system configurations and channel conditions. The comparisons between OFDM-IDMA and OFDM-TDMA based on bit-interleaved coded modulation (BICM) are carried out, which demonstrates a spectral-power efficiency advantage of non-orthogonal multiple access approaches over orthogonal ones. The proposed OFDM-IDMA scheme provides a competitive alternative to the conventional OFDM-CDMA scheme for future high-rate multiuser communications over multipath fading channels.