Multi - Carrier CDMA Mobile Radio Systems - Analysis and Optimization of Detection , Decoding , and Channel Estimation ∗

Performance and flexibility of mobile radio systems essentially depend on the choice of the multiple access scheme. The novel MC-CDMA scheme which is the topic of this thesis is a promising alternative to conventional FDMA, TDMA, and CDMA schemes. With MC-CDMA, each data modulated chip of the user specific spreading code is mapped onto a separate subcarrier, so that the chips of the spreading code and, hence, the data symbol is transmitted in parallel on multiple subcarriers. The application of OFDM enables one to avoid ISI and ICI in the detection process, resulting in simple SD and MD techniques. In MC-CDMA mobile radio systems, SD can be realized by equalizing each subcarrier individually, yielding only one complex-valued multiplication per subcarrier and OFDM symbol. Due to the M -, Q-, and M&Q-Modification of the MC-CDMA scheme, even the optimum JD techniques MLSSE and MLSE can be applied with reasonable complexity. After introducing the MC-CDMA signal structure, SD and MD techniques are analyzed, optimized, and compared to each other in this thesis. Taking into account convolutional channel coding with soft decision decoding, the optimum soft decided values which can be delivered to a Viterbi decoder are derived for MC-CDMA receivers with SD and MD. Exploiting the optimum soft decided values, an iterative detection and decoding scheme referred to as soft IC is introduced. With soft IC, reliability information about the reconstructed interference is taken into account in the iterative IC scheme, reducing error propagation. The suitability of the recently proposed Turbo codes as channel codes for MC-CDMA mobile radio systems is also investigated. However, it is shown that Turbo codes are only of limited use in MCCDMA mobile radio systems when considering speech transmission applications. The trade-off between channel coding and spreading is determined and a bandwidth efficiency plan for MCCDMA, MC-FDMA, and MC-TDMA mobile radio systems is presented, which reveals that amongst these, MC-CDMA systems are the most bandwidth efficient systems. To enable coherent detection, channel estimation with filtering in frequency and time is investigated for an MC modulated system. A 2× 1-D channel estimation is analyzed and optimized by taking into account model mismatch between the correlation functions of the channel and those used for the filter design. The performance of the presented 2× 1-D FIR filtering closely approximates the performance of optimum 2-D Wiener filtering. It is shown that the proposed MC-CDMA mobile radio system can handle high velocities of the mobile user in various propagation areas. Guaranteeing a good transmission quality with a BER Pb equal to 10 , the presented MCCDMA mobile radio system achieves the high bandwidth efficiency of 0.81 bit/s/Hz at a value of 10 log10(γb) between 8 dB and 12 dB on the average.