Performance Analysis of OTFS-NOMA System with Fractional Doppler

TL;DR

This paper analyzes how fractional Doppler affects OTFS-NOMA systems with users of varying mobility, revealing that inter-Doppler interference significantly impacts high-mobility user performance, especially under different NOMA configurations.

Contribution

It introduces a detailed performance analysis of OTFS-NOMA systems considering fractional Doppler effects and proposes a scheme distinguishing users by mobility profiles in a downlink scenario.

Findings

Fractional Doppler causes notable inter-Doppler interference.

High-mobility users' performance is sensitive to NOMA parameters.

System spectral efficiency varies with Doppler and NOMA configurations.

Abstract

In this work, we investigate the effect of fractional Doppler on the performance of a system using orthogonal time frequency space (OTFS) modulation and non-orthogonal multiple access (NOMA) where users have different mobility profiles. Fractional Doppler results in inter-Doppler interference (IDI) and degrades the quality of OTFS-modulated signals. We consider a downlink (DL) communication scenario where multiple users are distinguished based on their mobility profiles into a single high-mobility (HM) user and multiple low-mobility (LM) users. OTFS modulation is implemented for the HM user by embedding its information symbols in the delay-Doppler domain, while LM users' symbols are represented in the time-frequency (TF) domain. The LM users' signals are kept orthogonal to each other in the frequency domain by accessing disjoint subcarriers. Further, NOMA spectrum sharing is implemented between the HM user and the KM users to achieve higher spectral efficiency. Performance analysis in terms of DL spectral efficiency and outage probability is conducted for different system parameters. The numerical results show that IDI has a noticeable performance impact on the HM user, depending on the NOMA parameters.