Zaid Jaber - Department of Computer Engineering, University of Technology, Baghdad, Iraq
Dheyaa Kadhim - Department of Electrical Engineering, College of Engineering, University of Baghdad, Baghdad, Iraq
Ahmed Al-Araji - Department of Computer Engineering, University of Technology, Baghdad, Iraq

Different multiple access methods in wireless communications allow multiple contented stations to share the wireless medium. It can be either orthogonal or non-orthogonal. In orthogonal multiple access (OMA), the channel allocation between the contented stations is orthogonal (either by time, frequency, space or code). This is done to avoid interference between the wireless stations. However, it decreases the performance because the number of simultaneous connections is limited by the orthogonal channel resources, which minimizes the degree of freedom per station. On the other hand, non-orthogonal multiple access (NOMA) allows multiple stations to transmit and receive at the same channel resource time and frequency by using another domain. For example, different power levels for different stations or what is known as PD-NOMA. This paper shows that NOMA can improve the performance of wireless communications even with the presence of interference because each station has a full degree of freedom per resource. Besides, PD-NOMA and MIMO have a good combination by taking advantage of MIMO array gain with non-orthogonal access to improve spectral efficiency. By allocating different power to different stations, different optimization objectives are proposed to maximize the fairness, spectral efficiency, and energy efficiency in the downlink channel.

DoF, energy efficiency, Fairness, OMA, PD-NOMA, Spectral efficiency