Abstract
The need for ubiquitous coverage and the increasing demand for high data rate services, keeps constant pressure on the cellular network infrastructure. There has been intense research to improve the system spectral efficiency and coverage, and a significant part of this effort focused on developing and optimizing the multiple access techniques. One such technique that been recently proposed is the Low Density Spreading (LDS), which manages the multiple access interference to offer efficient and low complexity multiuser detection. The LDS technique has shown a promising performance as a multiple access technique for single-carrier system. The objective of this thesis is to apply the LDS scheme on a multicarrier transmission technique to form an efficient non-orthogonal multiple access technique for uplink cellular systems. The new multiple access technique is referred to it as Multicarrier-Low Density Spreading Multiple Access (MC-LDSMA). MC-LDSMA combines the benefits of the low complexity receiver from LDS and the robustness against multipath channel effect offered by multicarrier transmission. The first part of the thesis focuses on extending the LDS scheme to the multicarrier transmission. The features of MC-LDSMA such as receiver complexity and frequency diversity are discussed, and its performance is evaluated and compared with existing multiple access techniques. The second part of the thesis focuses on the envelope fluctuations problem of the MC-LDSMA signals. The impact of subcarriers allocation schemes and phases of the LDS codes on the envelope fluctuations of MC-LDSMA signals are investigated. Accordingly, phasing schemes have been applied for the LDS codes, which resulted in significant envelope fluctuations reduction. Furthermore, Discrete Fourier Transform (DFT) pre-coding has been proposed with resource block based allocation, which was able to further reduce the system envelope fluctuations. The third part of the thesis deals with radio resource allocation for single-cell MC-LDSMA system. The LDS structure imposes constraints on the number of users share the same subcarrier and the number of subcarriers used for spreading each symbol, which represent new challenges for radio resource allocation in the MC-LDSMA context. Three main areas of the radio resource allocation in the MC-LDSMA studied in this thesis: Firstly, an optimal power allocation and subcarriers partitioning schemes for single-user with LDS are derived. Secondly, it is shown that MC-LDSMA can be a special case of the generic Multiple Access Channel (MAC) channel with a condition on the users’ weights. Suboptimal subcarrier and power allocation algorithms are proposed, and performance analysis is provided to investigate the effect of the sub carrier loading and effective spreading factor on the system performance. Thirdly, for practical considerations, the radio resource allocation is considered with finite symbol alphabet inputs. An upper bound on the mutual information loss due to suboptimal power allocation is derived and numerically evaluated. A low complexity subcarrier and power allocation algorithm is developed, and the performance of the algorithm is analysed and it is shown that it achieves spectral efficiency very close to the system upper bound.