Hybrid OTFS/OFDM Design in Massive MIMO

TL;DR

This paper proposes a hybrid OTFS/OFDM modulation scheme for massive MIMO systems with users of varying mobility, introducing new precoding and power control strategies to enhance spectral efficiency and fairness.

Contribution

It introduces a hybrid OTFS/OFDM design with novel precoding and power control methods tailored for high-mobility users in massive MIMO systems.

Findings

Hybrid OTFS/OFDM outperforms conventional OFDM for high-mobility users.

Weighted max-min power control improves fairness and performance trade-offs.

Closed-form spectral efficiency expressions validate the proposed schemes.

Abstract

We consider a downlink (DL) massive multiple-input multiple-output (MIMO) system, where different users have different mobility profiles. To support this system, we categorize the users into two disjoint groups according to their mobility profile and implement a hybrid orthogonal time frequency space (OTFS)/orthogonal frequency division multiplexing (OFDM) modulation scheme. Building upon this framework, two precoding designs, namely full-pilot zero-forcing (FZF) precoding and partial zero-forcing (PZF) precoding are considered. To shed light on the system performance, the spectral efficiency (SE) with a minimum-mean-square-error (MMSE)-successive interference cancellation (SIC) detector is investigated. Closed-form expressions for the SE are obtained using some tight mathematical approximations. To improve fairness among different users, we consider max-min power control for both precoding schemes based on the closed-form SE expression. However, by noting the large performance gap for different groups of users with PZF precoding, the per-user SE will be compromised when pursuing overall fairness. Therefore, we propose a weighted max-min power control scheme. By introducing a weighting coefficient, the trade-off between the per-user performance and fairness can be enhanced. Our numerical results confirm the theoretical analysis and reveal that with mobility-based grouping, the proposed hybrid OTFS/OFDM modulation significantly outperforms the conventional OFDM modulation for high-mobility users.