Cell-Free Massive MIMO Meets OTFS Modulation

Abstract

We provide the first-ever performance evaluation of orthogonal time frequency space (OTFS) modulation in cellfree 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, Ma, grows without bound, we can reduce the transmit power of each user and AP proportionally to 1/Ma 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.

Low Complexity Detection of Spatial Modulation Aided OTFS in Doubly-Selective Channels

“…Let us consider a P -path time-varying multi-path Rayleigh fading channel, whose channel impulse response (CIR) between the n t th TA and n r th RA is expressed as [17], [18] h nr,nt (τ, ν)…”

Software-Defined Radio-Based 5G Physical Layer Experimental Platform for Highly Mobile Environments

“…To support the ever-evolving means of mobility in various fields of land, sea, and air, various new physical layer (PHY) technologies need to be developed for 6G, including the development of transmission and reception methods that enable communication at speeds exceeding 500 km/h, new communication methods to enable many users to conduct high-capacity communication, and methods to achieve lower latency and more reliable communication (e.g., non-orthogonal multiple access (NOMA) [18], [19], [20], in-band full-duplex (IBFD) [21], [22], [23], and new waveforms [24], [25], [26], [27], [28]). However, many studies have been limited to numerical and simulation evaluations based on theoretical formulas or computer simulations, and there are significant barriers to actual device demonstrations.…”

On the Spectral Efficiency of MMSE-based MIMO OTFS Systems

“…Several works have been conducted in the related field of MIMO-OTFS systems. For example, in [15], [16], the authors showed the tradeoff between the system performance and the signaling overhead for a cell-free massive MIMO system with OTFS modulation. In [17], the authors provided a channel estimation method for massive MIMO with OTFS transmissions.…”