Guest editorial: New advances in non-orthogonal multiple access

Multiple access is one of the core technologies in wireless communications, which enables wireless base stations to support a large number of terminal users and serve them simultaneously under stringent spectrum constraints. Orthogonal multiple access (OMA), as one multiple access method, has been prevailing since the first generation (1G) cellular system. However, the number of active users allowed access to the OMA system is strictly limited by the number of available orthogonal resources, since as required by OMA the communication resources allocated to different users are orthogonal in at least one radio resource dimension, e.g., frequency, time, code, etc. Nowadays, with the rapid growth of mobile network and Internet of Things (IoT) this problem becomes more and more critical.