Performance improvement of non-orthogonal multiple access systems based on power allocation considering user mobility and spatial modulation

Abstract

This thesis aims to improve the performance of mobile communication system using non-orthogonal multiple access technique using beamforming. The key principle of non-orthogonal multiple access technology is to obtain non-orthogonal gain by allocating different users to the same frequency resource. By using this principle, we have studied a novel non-orthogonal multi-access system by combining it with various systems such as multi beams or spatial modulation.

The first study is to improve performance for non-orthogonal multiple access system in a multi-beam environment. The operating principle of the non-orthogonal multiple access scheme is that continuous interference cancellation schemes can be used to distinguish the various signals assigned to the same frequency. The conventional non-orthogonal multiple access system is based on the distance difference between users, and the proposed non-orthogonal multiple access system in the multi-beam environment makes use of the difference in gain between multi beams. In this thesis, we present power allocation scheme and grouping user scheme, which are suitable for multi beam environments. The capacity of the proposed non-orthogonal multiple access system using the power allocation scheme and the grouping user scheme is confirmed to have a higher capacity than that of the orthogonal multiple access system. The proposed grouping scheme has a lower complexity than the exhaustive search grouping scheme with similar performance. In addition, we confirmed that the proposed non-orthogonal multiple access system using multi beams can solve the mobility problem of the conventional non-orthogonal multiple access system.

Second, the performance of the non-orthogonal multiple access system in a spatial modulation environment based on beamforming is studied. The spatial modulation scheme selects only one or a group of antennas among many antennas, and the antenna selection scheme is determined according to bits to be transmitted, which is called as spatial bits. A disadvantage of the spatial modulation system is that the spectrum efficiency is lower compared to the MIMO system using all antennas in case that the same number of antennas is used. If a data modulation scheme for transmitting huge bits is used in spatial modulation system, the symbol error probability increases. In this thesis, we proposed a novel system that combines a spatial modulation scheme with a non-orthogonal multiple access. The proposed SM-NOMA system can improve the spectral efficiency and reduce the symbol error probability compared to conventional NOMA system. We also proposed a power allocation scheme and a beamforming scheme which are suitable in the proposed system. Using the proposed schemes, we can improve spectral efficiency in non-orthogonal multiple access systems based on spatial modulation and verify that symbol error probability can be lowered.

Lastly, we proposed a novel spatial modulation and non-orthogonal system based on massive MIMO environment. In proposed system, more antenna can be integrated in limited space within $\frac{\lambda}{2}$. With the integration gain of antenna, more sharp beamforming is possible and multi beam environment can be applied to this proposed system. Through simulations, we showed that proposed NOMA-SM system has better performance than other systems such as OMA-MIMO, OMA-SM, and NOMA-MIMO.