Coded Tandem Spreading Multiple Access for Massive Machine-Type Communications

Abstract

Future IoT applications provoke the connectivity and reliability requirements of machine-type communications, which yield mMTC and URLLC, respectively. In mMTC, grant-free NOMA is adopted to serve massive machine-type devices with low signaling overhead. The challenge of grantfree NOMA for mMTC is the collision induced by non-orthogonal resource allocation. Various grantfree NOMA techniques have been proposed to address this challenge, aiming at high connectivity. In this article, we introduce a novel grant-free NOMA approach, namely coded tandem spreading multiple access (CTSMA). The basic idea is that the data packet of each user is divided into segments, and redundancy segments are generated by coding through the data segments, and multiple orthogonal spreading codes are utilized to tandemly spread the data symbols of different segments from one user. Thereby, collision only occurs on the segments using identical spreading codes, which can be resolved by the redundancy segments to achieve reliable access. In addition to the transmitter and receiver designs, a closedform expression of the collision resolution capability is derived in this article to reveal the trade-off among the connectivity, the reliability, and the user data rate when using CTSMA. Theoretical analysis shows that CTSMA achieves both high connectivity and ultra-reliability at the expense of user data rate.