During the last few decades, mobile communications have significantly contributed to the economic and social developments of both developed and developing countries. Mobile communications from an indispensable part of the daily lives of millions of people in the world; a situation which is expected to continue and become even more undeniable in the future. In the history of wireless communications from the first generation (1G) to 4G, the multiple access schemes have been the key technology to distinguish different wireless systems. It is well known that frequency-division multiple access (FDMA) for 1G, time-division multiple access (TDMA) mostly for 2G, code-division multiple access (CDMA) for 3G, and orthogonal frequency division multiple access (OFDMA) for 4G are primarily orthogonal multiple access (OMA) schemes. The fifth generation of mobile communications (5G), which is expected to be commercialized towards year 2020 and beyond, has become a focal point for global research and development.