This paper investigates a grant-free non-orthogonal multiple access (NOMA) system where massive number of users wake up and send data right away without performing a grant-based initial access procedure. Since no scheduling grant is used in this system, the base station (BS) does not know which user is transmitting on each resource. Also, due to the lack of an uplink (UL) timing control process, multiple user's signals are asynchronously received at the BS. This causes asynchronous interference, or multiuser inter-carrier and inter-symbol interference. We propose two ideas to address these two problems. We first propose an auxiliary preamble structure to successfully detect the user activity, even in the presence of a massive number of users. We, then, propose a modification to the interleave-division multiple access (IDMA) receiver to mitigate asynchronous interference. The simulation results show that the proposed scheme significantly improves the preamble detection performance and BER performance compared to the conventional schemes. Furthermore, we show that the proposed grant-free NOMA system can achieve much better performance than the grant-based NOMA system in terms of transmission time and signaling overhead.
All Science Journal Classification (ASJC) codes
- Computer Science Applications
- Electrical and Electronic Engineering
- Applied Mathematics