Hybrid multiple access system based on non orthogonality and sparse code

Abstract

In this thesis, a hybrid multiple access system based on Non-Orthogonal Multiple Access (NOMA) and Sparse Code Multiple Access (SCMA) is proposed, which are promising techniques for next generation communication system. NOMA applied to communication network could exploit higher spectral efficiency than conventional Orthogonal Multiple Access(OMA) scheme. In the NOMA system, the signals for near and far user are superimposed in one resource. The decoding method of NOMA is successive interference cancellation (SIC). Far user signal will be decoded first by using channel gain difference. In the view of near user, decoded far user signal is canceled from original received signal, after that, which symbol is transmitted is decided. By using this method, a transmitter can transmit more information. SCMA scheme could give massive connectivity and performance similar to maximum a posteriori (MAP) detector. The SCMA codebook which is designed by sub-optimal method could give higher diversity effect to communication network. Many signals of users in the cell of SCMA system, are spread and summed by generated SCMA codebook in the certain number of resources. In the decoding process, Message Passing Algorithm (MPA) is used. The diversity effect can be obtained from MPA which operates as iteratively updating symbol probability. In this point, we have a purpose of combination of two systems to use following two properties. First, the capacity of wireless communication system can be increased by using channel gain difference. Second, a hybrid access system can support many users simultaneously, which means that proposed access system could be a new multiple access scheme for coming Internet of Things era. A scheduler of transmitter can separate users into two groups, i.e., near users and far users. In this case, SCMA scheme will support near users by using multi-dimensional complex constellation and near users and far users are linked by NOMA scheme. We proposed two hybrid access systems and another switching system to achieve better performance. The first system is simple combining of NOMA and SCMA. This hybrid access system will have moderate BER performance in low SNR region. The second system is the hybrid access system with far user spreading code. This hybrid system will show best performance by making up first hybrid access system’s fault. The switching system is based on the switching between SCMA and hybrid system without far user spreading. The advantage of the switching system is also explained. An analysis on BER performance of hybrid access systems are conducted and optimization problem for power allocation between near users and far users is also considered. The power allocation method considered in this thesis is a fractional transmission power allocation (FTPA). By using FTPA method based on channel gain difference, a transmitter can determine power allocation factors to achieve best performance. A theoretical BER for far users and approximate BER for near user derived in this study provide a framework to predict performance of hybrid access systems. Far user uses more simple code than near user, so we can derive exact formula with a little assumption. However, in case of the symbol of near user which uses complicated SCMA codebook and MPA detection, it is difficult to derive BER equation. Therefore we approximate BER performance by using the property of SCMA, diversity and interference. Moreover, to demonstrate superiority, Bit Error Rate (BER) of hybrid access systems are derived from simulation and compared with OMA, NOMA and SCMA.