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Low Overhead and Scalable Time-Frequency Pilots Design for MIMO OTFS Channel Estimation

Kailong Wang, Athina Petropulu

Abstract

Orthogonal Time Frequency Space (OTFS) modulation has recently garnered attention for its robustness in high-mobility wireless communication environments. In OTFS, the data symbols are mapped to the Doppler-Delay (DD) domain. In this paper, we address low-overhead, scalable pilot-aided estimation of channel state information (CSI) for MIMO OTFS systems. Existing channel estimation techniques either require non-overlapping DD domain pilots with guard regions across multiple antennas, thus sacrificing significant communication rate as the number of transmit antennas increases, or allow pilots to overlap between antennas and rely on high-complexity methods to mitigate pilot pollution. We propose a novel pilot placement approach that embeds pilots within the time-frequency (TF) frame of each OTFS burst, along with a new use of TF and DD guard bins to preserve waveform orthogonality on the TF pilot bins and data integrity in the DD domain, respectively. The proposed pilot placement enables low-complexity coarse estimation of the channel parameters. Moreover, the pilot orthogonality allows the construction of a virtual array (VA), enabling the formulation of a sparse signal recovery (SSR) problem in which the coarse estimates are used to build a low-dimensional dictionary matrix. The SSR solution then yields high-resolution estimates of the channel parameters. Simulation results show that the proposed approach achieves good performance with very low overhead and is robust to pilot pollution. Importantly, the required overhead is independent of the number of transmit antennas, ensuring scalability to large MIMO arrays. The proposed approach accounts for practical rectangular transmit pulse shaping and receiver matched filtering, as well as fractional Doppler effects.

Low Overhead and Scalable Time-Frequency Pilots Design for MIMO OTFS Channel Estimation

Abstract

Orthogonal Time Frequency Space (OTFS) modulation has recently garnered attention for its robustness in high-mobility wireless communication environments. In OTFS, the data symbols are mapped to the Doppler-Delay (DD) domain. In this paper, we address low-overhead, scalable pilot-aided estimation of channel state information (CSI) for MIMO OTFS systems. Existing channel estimation techniques either require non-overlapping DD domain pilots with guard regions across multiple antennas, thus sacrificing significant communication rate as the number of transmit antennas increases, or allow pilots to overlap between antennas and rely on high-complexity methods to mitigate pilot pollution. We propose a novel pilot placement approach that embeds pilots within the time-frequency (TF) frame of each OTFS burst, along with a new use of TF and DD guard bins to preserve waveform orthogonality on the TF pilot bins and data integrity in the DD domain, respectively. The proposed pilot placement enables low-complexity coarse estimation of the channel parameters. Moreover, the pilot orthogonality allows the construction of a virtual array (VA), enabling the formulation of a sparse signal recovery (SSR) problem in which the coarse estimates are used to build a low-dimensional dictionary matrix. The SSR solution then yields high-resolution estimates of the channel parameters. Simulation results show that the proposed approach achieves good performance with very low overhead and is robust to pilot pollution. Importantly, the required overhead is independent of the number of transmit antennas, ensuring scalability to large MIMO arrays. The proposed approach accounts for practical rectangular transmit pulse shaping and receiver matched filtering, as well as fractional Doppler effects.

Paper Structure

This paper contains 16 sections, 2 theorems, 39 equations, 7 figures, 3 tables, 1 algorithm.

Table of Contents

  1. Introduction
  2. OTFS System
  3. SISO-OTFS Modulation and Demodulation
  4. Extension to MIMO-OTFS Systems
  5. TF Pilots Guarded by TF and DD Bins
  6. Low Complexity Coarse Channel Estimation
  7. Virtual Array and Sparse Signal Recovery
  8. Recovering the Communication Information
  9. Pilot Overhead Comparison
  10. Simulation Results
  11. Coarse Estimation Performance
  12. NMSE Performance
  13. Effect of Number of Pilots, Pilot Power, and Number of Scatterers
  14. BER Performance
  15. Compare with overlapped DD pilot schemes
  16. ...and 1 more sections

Key Result

Theorem 1

The TF I/O expression of the OTFS system can be expressed as where and $W[n,m]$ is TF domain AWGN.

Figures (7)

  • Figure 1: TF domain pilots (indicated by '$*$'s) and TF guard bins (indicated by '$0$'s) on TX$_1$ and TX$_2$. Each antenna has $20$ empty bins in the DD domain.
  • Figure 2: Bi-static radar geometry model with $3$ scatterers.
  • Figure 3: NMSE performance of the TF pilot vs. non-overlapped DD pilot.
  • Figure 4: NMSE performance of different numbers of pilots.
  • Figure 5: NMSE performance of different pilot power and numbers of scatterers.
  • ...and 2 more figures

Theorems & Definitions (2)

  • Theorem 1
  • Theorem 2