Cognitive radio-inspired NOMA in short-packet communications

Short-packet communication is an important feature of the internet of things (IoT). For allowing massive IoT devices to connect to a network through short packets, this paper designs a cognitive radio-inspired non-orthogonal multiple access (NOMA) scheme, where the IoT users will not necessarily occupy an exclusive spectrum. To further improve the connections in an IoT network, we propose a general NOMA scheme that serves multi- ple IoT users. Since the IoT devices are generally homogeneous, a user fairness-oriented power allocation scheme is designed for the proposed NOMA scheme. Besides, for comparison purposes, the benchmark orthogonal multiple access (OMA) scheme and its corresponding user fairness transmission scheme are also proposed. The closed-form expressions of average block error rates for secondary users are derived over Nakagami-m channels in both NOMA and benchmark OMA scheme. Simulation and analysis results show that a faraway primary network will benefit the spectrum sharing. In addition, shorter packets and more different channel conditions improve the NOMA gains over OMA. Besides, power allocation factors are found to be capable of achieving user fairness and the fairness transmission is achieved by the proposed scheme. Interestingly, when the channel differences are bigger enough, paring an extra NOMA user nearly does not affect the previous system performance.