Abstract
Considering that efficient use of the available radio spectrum is an important requirement for future wireless communication, designing a multiple access technique which can support higher loaded conditions with affordable complexity and near single user performance is a challenge. This thesis aims to address this challenge by proposing an efficient multiple access technique called low density signature OFDMA (LDS-OFDMA). LDS-OFDMA is a special case of the multi-carrier CDMA (MC-CDMA) system where its spreading sequences have low density. This technique benefits from an iterative near to optimum MUD and is able to support overloaded conditions under multipath fading channel. The BER results show LDS-OFDMA gains a performance near to single user bound under 200% load condition. Also, for load of 100%, LDS-OFDM outperforms MC-CDMA with MMSE detector by 8.1 dB at BER= 10-3 which is a considerable gain. Comparing the BER performance with OFDMA, it is shown that the BPSK coded LDS-OFDMA system with 400% loading offers 0.65 dB gain over 16 QAM coded OFDMA at BER of 10-3. Furthermore, in terms of complexity it is shown that for LDS-OFDMA the raise in the complexity as the number of users increase is much less than similar techniques with optimum MUDs. Based on the iterative nature of the MUD, we also proposed a suitable design and analysis tool to obtain an efficient detection/decoding algorithm. Therefore, by applying the extrinsic information transfer (EXIT) chart, the convergence behaviour of the iterative detector is analysed. Evaluating the effect of different factors on the performance of MUD could help us find design guidelines for improving the LDS structure. Moreover, a tuned turbo multiuser detector/decoder (MUDD) for LDS-OFDMA is proposed. Using the EXIT chart analysis we were able to tune the receiver in a way that the turbo MUDD can approach its final performance using a smaller number of inner iterations. 2.21 dB gain at BER=10-4 is achieved by turbo MUDD with 3 turbo iterations over conventional LDS-OFDMA receiver. In order to gain insights on the performance of LDS schemes, an analytical model for the capacity of LDS based schemes for the uplink of multipath fading channels is proposed. The proposed model is then improved to take into account realistic assumptions, which are more suitable for practical systems. Monte Carlo simulation results are used to validate the proposed model by comparing the results with the sum rate of conventional multiple access channel. The simulation results show that there is a 2 dB gap between the sum rate of conventional LDS scheme and the one for multiple access channel which is the cost for complexity reduction. Also it is shown that for practical LDS schemes with modulation and convolutional coding the gap increases to 4.8 dB. However using capacity approach codes such as LDPC codes the gap can be reduced significantly.