Differential Modulation for Short Packet Transmission in URLLC

TL;DR

This paper proposes differential modulation combined with multi-connectivity to enhance short packet transmission in URLLC, reducing overhead and improving reliability in high Doppler environments.

Contribution

It introduces a differential modulation scheme with multi-connectivity for URLLC, deriving theoretical bounds and demonstrating performance gains over traditional schemes.

Findings

Differential modulation reduces pilot overhead in SPT.

Multi-connectivity enhances diversity gain.

Significant performance improvement in high Doppler environments.

Abstract

One key feature of ultra-reliable low-latency communications (URLLC) in 5G is to support short packet transmission (SPT). However, the pilot overhead in SPT for channel estimation is relatively high, especially in high Doppler environments. In this paper, we advocate the adoption of differential modulation to support ultra-low latency services, which can ease the channel estimation burden and reduce the power and bandwidth overhead incurred in traditional coherent modulation schemes. Specifically, we consider a multi-connectivity (MC) scheme employing differential modulation to enable URLLC services. The popular selection combining and maximal ratio combining schemes are respectively applied to explore the diversity gain in the MC scheme. A first-order autoregressive model is further utilized to characterize the time-varying nature of the channel. Theoretically, the maximum achievable rate and minimum achievable block error rate under ergodic fading channels with PSK inputs and perfect CSI are first derived by using the non-asymptotic information-theoretic bounds. The performance of SPT with differential modulation and MC schemes is then analysed by characterizing the effect of differential modulation and time-varying channels as a reduction in the effective SNR. Simulation results show that differential modulation does offer a significant advantage over the pilot-assisted coherent scheme for SPT, especially in high Doppler environments.