In this paper, we investigate the adoption of frequency domain differential modulation to support mini-slot-assisted URLLC, which can eliminate the channel estimation burden and reduce the power and bandwidth overhead incurred in traditional coherent modulation schemes. Furthermore, we derive the block error rate (BLER) for frequency domain differential OFDM (FDDi-OFDM) using non-asymptotic information-theoretic bounds. Simulation results verify the accuracy of the analysis and show that, for a high Doppler environment, FDDi-OFDM can yield a much better result than the pilot-assisted coherent scheme.

One of the primary challenges in ultra-reliable and low-latency communications (URLLC) is to achieve accurate channel estimation and data detection while minimizing latency. Given the small packet size in URLLC, relying solely on pilot-assisted (PA) coherent detection is almost impossible to meet the seemingly contradictory requirements of high channel estimation accuracy, high reliability, low training overhead, and low latency. In this paper, we explore both frequency domain differential modulation (FDDM) and time domain differential modulation (TDDM), enabling non-coherent short packet URLLC with mini-slot structures. The minimum achievable block error rate and the maximum achievable rate for all three modes (i.e., FDDM, TDDM and PA modes) are derived using non-asymptotic information-theoretic bounds. Furthermore, we show that FDDM can more than compensate for the training overhead inadequacy and performance degradation of PA mode in medium-to-high-mobility scenarios, thereby improving the performance of short packet transmission with mini-slot by dynamically activating FDDM. Simulation results validate the feasibility and effectiveness of the proposed low overhead FDDM mini-slot transmission scheme.

Acquiring accurate channel state information (CSI) in finite blocklength (FBL) ultra-reliable and low-latency communications (URLLC) with millimetre wave (mmWave) massive multiple-input multiple-output (MIMO) necessitates a significant pilot overhead. To overcome this challenge, differential modulation (DM) is a promising solution for eliminating the costly pilot overheads in FBL and avoiding pilot contamination in mmWave massive MIMO systems. In this paper, we propose a combination of DM and hierarchical codebook-based beam training schemes as a feasible way to enable FBL URLLC with mmWave massive MIMO. Additionally, we derive the block error rate (BLER) of the joint DM and hierarchical codebook-based beam training scheme in the FBL regime by employing non-asymptotic information-theoretic bounds. The simulation results verify the accuracy of the analysis and show that DM does offer an advantage over the pilot-assisted coherent schemes under FBL with mmWave massive MIMO.

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.

Grant-free (GF) access is expected to support low-latency services in fifth-generation (5G) systems, while non-orthogonal multiple access (NOMA) has been proposed to enable massive connectivity in cellular networks. However, the performance analysis for the GF access mode based on NOMA is not trivial, especially for large-scale multi-cell networks due to the inherent random near-far phenomenon. In this paper, we exploit tools from stochastic geometry to develop a tractable framework for analysing uplink performance in large-scale multi-cell networks under GF NOMA and short packet transmission. To make the framework tractable, we further transform the intra- and inter-cell interference to an equivalent interference model. The URLLC performance of GF NOMA networks is derived under the assumption of perfect successive interference cancellation (SIC) and short packet transmission. Numerical results obtained from theoretical calculations and Monte Carlo simulations verify the correctness of our analysis.