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Hybrid OTFS/OFDM Design in Massive MIMO

Ruoxi Chong, Mohammadali Mohammadi, Hien Quoc Ngo, Simon L. Cotton, Michail Matthaiou

TL;DR

This work tackles downlink massive MIMO under heterogeneous mobility by introducing a mobility-aware hybrid OTFS/OFDM framework, assigning OTFS to high-mobility users and OFDM to low-mobility users. It develops two precoding schemes, FZF and PZF, and derives MMSE-SIC-based spectral-efficiency expressions with closed-form approximations, enabling efficient power-control design. Max-min and weighted max-min power allocation strategies are proposed to balance fairness and per-user performance, with user scheduling explored to further boost LM-UE SE. Numerical results confirm theoretical insights and demonstrate notable HM gains from OTFS alongside practical complexity-performance trade-offs for the hybrid system.

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.

Hybrid OTFS/OFDM Design in Massive MIMO

TL;DR

This work tackles downlink massive MIMO under heterogeneous mobility by introducing a mobility-aware hybrid OTFS/OFDM framework, assigning OTFS to high-mobility users and OFDM to low-mobility users. It develops two precoding schemes, FZF and PZF, and derives MMSE-SIC-based spectral-efficiency expressions with closed-form approximations, enabling efficient power-control design. Max-min and weighted max-min power allocation strategies are proposed to balance fairness and per-user performance, with user scheduling explored to further boost LM-UE SE. Numerical results confirm theoretical insights and demonstrate notable HM gains from OTFS alongside practical complexity-performance trade-offs for the hybrid system.

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.
Paper Structure (21 sections, 5 theorems, 90 equations, 9 figures, 2 tables, 2 algorithms)

This paper contains 21 sections, 5 theorems, 90 equations, 9 figures, 2 tables, 2 algorithms.

Table of Contents

  1. Introduction
  2. System Model
  3. Preliminaries on OTFS Transmitters
  4. Input-Output Relationship
  5. Performance Analysis
  6. FZF precoding
  7. PZF Precoding
  8. Complexity Analysis
  9. Power Allocation
  10. FZF Precoding
  11. PZF Precoding
  12. Max-min power control
  13. Weighted max-min power control
  14. Numerical Results
  15. Large-scale Fading Model
  16. ...and 6 more sections

Key Result

Proposition 1

With FZF, the SE for the $k_h$-th HM and the $k_l$-th LM-UE can be obtained in closed-form as

Figures (9)

  • Figure 1: Frame comparison between OFDM and OTFS.
  • Figure 2: Illustration of the hybrid OTFS/OFDM modulation design.
  • Figure 3: NMSE as in \ref{['NMSE']}.
  • Figure 4: Theoretical and numerical per-user SE with $M=8$.
  • Figure 5: Theoretical per-user SE with different $M$ values.
  • ...and 4 more figures

Theorems & Definitions (8)

  • Proposition 1
  • proof
  • Proposition 2
  • proof
  • Corollary 1
  • Proposition 3
  • proof
  • Corollary 2