When Cell-Free Massive MIMO Meets OTFS Modulation: The Downlink Case

TL;DR

This paper evaluates the performance of OTFS modulation in cell-free massive MIMO systems, showing significant throughput gains over OFDM in high-mobility scenarios by leveraging delay-Doppler channel sparsity.

Contribution

It introduces a channel estimation method using delay-Doppler sparsity and derives a closed-form expression for downlink throughput in cell-free massive MIMO with OTFS.

Findings

OTFS achieves up to 35% throughput gain over OFDM in high-mobility conditions.

Cell-free massive MIMO with OTFS improves median user throughput under correlated shadowing.

The proposed channel estimation method reduces guard intervals and enhances performance.

Abstract

We provide a performance evaluation of orthogonal time frequency space (OTFS) modulation in cell-free massive MIMO (multiple-input multiple-output) systems. By leveraging the inherent sparsity of the delay-Doppler (DD) representation of time-varying channels, we apply the embedded pilot-aided channel estimation method with reduced guard intervals and derive the minimum mean-square error estimate of the channel gains from received uplink pilots at the access points (APs). Each AP applies conjugate beamforming to transmit data to the users. We derive a closed-form expression for the individual user downlink throughput as a function of the numbers of APs, users and DD channel estimate parameters. We compare the OTFS performance with that of orthogonal frequency division multiplexing (OFDM) at high-mobility conditions. Our findings reveal that with uncorrelated shadowing, cell-free massive MIMO with OTFS modulation achieves up to 35% gain in 95%-likely per-user throughput, compared with the OFDM counterpart. Finally, the increase in the per user throughput is more pronounced at the median rates over the correlated shadowing scenarios.