Cell-free massive MIMO meets OTFS modulation

Mohammadali Mohammadi*, Hien-Quoc Ngo, Michalis Matthaiou

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

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Abstract

We provide the first-ever performance evaluation of orthogonal time frequency space (OTFS) modulation in cell-free massive multiple-input multiple-output (MIMO) systems. To investigate the trade-off between performance and overhead, we apply embedded pilot-aided and superimposed pilot-based channel estimation methods. We then derive a closed-form expression for the individual user downlink and uplink spectral efficiencies (SEs) as a function of the numbers of APs, users and delay-Doppler domain channel estimate parameters. Based on these analytical results, we also present new scaling laws that the AP’s and user’s transmit power should satisfy, to sustain a desirable quality of service. It is found that when the number of APs, M a , grows without bound, we can reduce the transmit power of each user and AP proportionally to 1/ M a and 1/ M 2 a , respectively, during the uplink and downlink phases. We compare the OTFS performance with that of orthogonal frequency division multiplexing (OFDM) at high-mobility conditions. Our findings reveal that, OTFS modulation with embedded pilot-based channel estimation provides up to 20-fold gain over the OFDM counterpart in terms of 95%-likely per-user downlink SE. Finally, with superimposed pilot-based channel estimation, the increase in the uplink sum SE is more pronounced when the channel delay spread is increased.
Original languageEnglish
JournalIEEE Transactions on Communications
Early online date30 Sept 2022
Publication statusEarly online date - 30 Sept 2022

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