On uplink non-orthogonal multiple access for 5g: opportunities and challenges

In this paper, the concept of grantfree non-orthogonal multiple access (NOMA) for uplink data transmission is elaborated. NOMA in combination with grant-free can be applied to ultra reliability low latency communication (URLLC), massive machine type communication (mMTC), enhanced mobile broadband (eMBB) small packet and two-step random-access channel (RACH) scenarios. The advantages of grant-free NOMA are low latency and signaling overhead, high access capability and efficient resource utilization. Candidate uplink NOMA schemes are summarized and preliminary comparison among a subset of schemes are presented. Furthermore, design aspects for grant-free NOMA are discussed, with special notes on particular issues such as blind UE identification and transmitter/ receiver (Tx/Rx) impairments in realistic deployment.