Tandem Spreading Multiple Access with Successive Interference Cancellation

Grant-free non-orthogonal multiple access (NOMA) is adopted to serve massive machine-type devices in massive machine-type communications (mMTC). One major challenge of the grant-free NOMA for mMTC is the collision induced by nonorthogonal resource allocation. A grant-free NOMA technique named tandem spreading multiple access (TSMA) is proposed to address this challenge. However, the system performance of TSMA could degrade due to multiple access interference when the active user number exceeds required threshold. In this paper, successive interference cancellation (SIC) is introduced in TSMA to mitigate the corresponding interference. The advantage of the combination is to enable more active users decodable by SIC. The simulation results show that TSMA maintains a high collision-resolution probability by adopting SIC even if the predefined condition of successful detection is violated.