Considering that efficient use of the available radio spectrum is an important require- ment 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 propos- ing 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 horn an iterative near to optimum MUD and is able to support overloaded conditions under multipath fading channel. The BER results show LDS-:9FDMA gains a performance near to single user bound under 200% load condition, Also, for load of 1QO%, 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 6,65 dB gain over 16 QAM coded OFDMA at BER of 10-3, Furthermore, in terms of complexity Also 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 ¥UDs, Based on the iterative nature of the MUD, we also proposed a suitable design and analy- . sis 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, Us- ing the EXIT chart analysis wc were able to tunc the receiver in a way that the turbo MUDD elm approach its final performance using a smaller number of inner iterations, 2,21 dB gain at BER=1O-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 arc more suitable for practical systems. :Nlonte- 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 Cl, 2 dB ga.p 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 eo des the gap can be reduced significantly. Key words: Low Density Signature Structures, Chip Level Iterated Multiuser Detec- tion, Overloaded Conditions, Turbo Multiuser Detection/Decoding.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:581797 |
| Date | January 2012 |
| Creators | Razavi, Razieh |
| Publisher | University of Surrey |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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