NOMA with Index modulation for uplink URLLC through grant-free access

This paper proposes non-orthogonal sharing of available resources between latency-critical and latency-tolerant communication for fulfilling tight requirements of ultra-reliable low-latency communication (URLLC) as well as avoiding inefficient spectrum utilization of grant-based (GB) access for sporadic URLLC traffic. In the proposed system, grant-free (GF) access is adopted for URLLC to reduce transmission latency, while GB access is used for latency-tolerant communication. Due to GF access, collision emerges between the communications, and use of OFDM technology for both communications leads to wideband interference (WB-I) on URLLC. Therefore, a novel non-orthogonal multiple accessing (NOMA) scheme based on orthogonal frequency division multiplexing (OFDM) and OFDM with index modulation (OFDM-IM) is proposed in order to reduce the impact of the collision on URLLC, that requires 99.999% success probability within 1ms. OFDM-IM technology is used for latency-tolerant communication since WB-I is converted to either narrowband dominant interference (NB-DI) or narrowband interference (NB-I) by fractional subcarrier activation in OFDM-IM. In this way, URLLC is partially affected by latency-tolerant communication. It is shown that the proposed NOMA scheme significantly reduces the latency in comparison to classical NOMA scheme based on pure OFDM while guaranteeing <formula><tex>$10^{-5}$</tex></formula> reliability for URLLC, via both computer-based simulations and theoretical analysis.