Research On Some Key Technologies In Non-Orthogonal Multiple Access

The multiple access technology of cellular wireless communications undergo the evolution from frequency division multiple access (FDMA) to time division multiple access (TDMA), and then to code division multiple access (CDMA) and orthogonal frequency division multiple access (OFDMA). The orthogonal multiple access (OMA) technology is popular based on the low-complexity detection algorithm. Modern information theory-demonstrates that Non-orthogonal Multiple Access (NOMA) can achieve higher capacity than OMA. As the improvement of the computational ability of hardware, NOMA has attracted researchers’ significant interesting.In single input single output (SISO) degraded broadcast channel (DBC) system, the capacity region can be achieved by superposition transmission with power allocation and successive interference cancellation (SIC) detection algorithm. In multiple input multiple output (MIMO) system, dirty paper coding (DPC) can achieve the capacity limit. However, it is limited by the needs of perfect channel state information (CSI) which make it impractical in cellular wireless communication system. In this paper, a NOMA scheme that joint designed in space-time and power domain is proposed. For uplink multiple access channel, (1) A modified linear minimum mean square error based component-level soft interference cancellation (CLSC-LMMSE) detection algorithm is proposed to enhance the performance. In addition, non-binary LDPC code over low order Galois field (GF) is employed to aid that detector, thus reduce the complexity of the LDPC decoder and achieve peforance and complexity tradeoffs. (2) To meet the increasing demands users in future internet of things, the spectrum efficient overloaded NOMA scheme is researched. We focus on the design of spreading spectrum pattern of the transmitter and on the advanced belief propagation (BP) detection algorithm. Overall, the main contributions of the paper are listed as follows:1 In MIMO broadcast communication system, some attentions are paid to complexity constrained and good performance transmission scheme and proposed a NOMA transmission scheme that joint quasi-orthogonal space-time coding (QO-STBC) and power allocation. By reasonable power allocation, we can distinguish one user from the others, and by diversity order, we can distinguish between data layers belongs to a specific user. The detailed steps are, firstly, inter-user data can be detected by successive interference cancellation (SIC) detector, then the diversity order of each layer is adjusted to make the post-detection diversity order of each layer be the same. In addition, a code-rate optimization rule is proposed to balance the post-decoding performance of all data layers.2 Multiuser detection (MUD) algorithm is one of the key technologies in uplink multiple access system. Based on traditional linear detection based algorithm such as LMMSE-SIC, a modified LMMSE based component-level soft interference cancellation (CLSC-LMMSE) iterative detection algorithm is proposed. In addition, both of extrinsic information transfer (EXIT) chart and numerical simulations demonstrate the proposed CLSC-LMMSE based iterative detection and decoding (IDD) scheme outperform the convention SLSC-LMMSE based one regarding convergence characteristic with different code rate and modulation scheme. Last, non-binary LDPC code over low-order Galois field is employed to aid the CLSC-LMMSE detector for high order modulation system. Compared to the SLSC-LMMSE scheme, both of average mutual information (AMI) chart and Mento Carlo simulation demonstrate the proposed scheme obtains comparable performance but with lower computational complexly, which means it achieves good performance and complexity tradeoffs.3 Considering OFDM based overloaded uplink NOMA system, sparse spreading spectrum modulation patterns with unequal diversity order are proposed. The characteristics of that scheme are (1) a low complexity suboptimal BP algorithm which has ML approaching performance can be employed as MUD algorithm, (2) Sparse spreading spectrum pattern is designed for each user to reduce the computational complexity of the BP based MUD algorithm, (3) Multiuser superposition scheme with unequal diversity order can facilities the convergence of the BP detection algorithm. In addition, IDD algorithm which exchange extrinsic information between the BP detector and the channel decoder is used to enhance the performance of the BP detection algorithm. For LDPC coded uplink NOMA sytem, an iterative linear programing algorithm is proposed to optimize the degree distribution of the concatenated LDPC code, which can further improve the IDD performance.