Inter-Numerology and Inter-Carrier Interference in Mixed-Numerology OFDM Systems

Abstract

Mixed-numerology has been adopted in the orthogonal frequency division multiplexing (OFDM)-based physical layer of 5G new radio (NR) to serve diverse use cases and services. The subcarriers of different numerologies have different bandwidths and symbol durations, and are non-overlapping. However, they still interfere with each other causing inter-numerology interference (INI). The Doppler spread, which depends on the speed and the carrier frequency, and the phase noise, which depends on the carrier frequency and the characteristics of the transmit and receive oscillators, make the channel vary with time. These variations affect INI and exacerbate inter-carrier interference (ICI). We present a unified analysis of the INI and ICI encountered by these systems.

We first focus on single-input-single-output systems. We analyze INI and ICI in wideband time-varying channels in the presence of phase noise. We derive novel expressions for the average INI and ICI powers at each subcarrier as a function of the channel’s power delay profile. These lead to insightful lower and upper bounds for the bandwidth-averaged INI and ICI powers, which bring out the combined impact of Doppler spread and phase noise statistics. The bounds for ICI power are tighter than those in the literature, which employ a tractable, but less accurate, continuous-time formulation and assume infinitely many subcarriers. Our results show that INI and ICI affect high-rate modulation and coding schemes (MCSs) of 5G NR. To mitigate this impact, we propose a novel statistical square root power allocation scheme that exploits the variation in the INI and ICI powers across the subcarriers. It achieves a lower block error rate than the uniform power allocation, which is used in 5G NR.

Next, we turn our attention to multiple-input-multiple-output (MIMO) systems. We derive novel insightful expressions for the INI and ICI covariances for a mixed-numerology, multiuser MIMO-OFDM system in a wideband spatially correlated time-varying channel with phase noise. With this analytical foundation, we propose a first-of-its-kind joint INI and ICI mitigation technique that is based on statistical information and incorporates fairness. We derive the precoder for each user that achieves the single-user ergodic capacity. We also determine a novel power allocation that maximizes the weighted sum rate. We solve this using an iterative algorithm based on the difference of convex programming framework. The proposed approach achieves a higher weighted sum rate than several benchmarks. Our results highlight the impact of INI and ICI and the influence of various system parameters. They show that INI and ICI can lead to novel power allocations in which the stronger spatial eigenmodes are assigned less power.