OTFS vs. OFDM in the Presence of Sparsity: A Fair Comparison

Abstract

Many recent works in the literature declare that Orthogonal Time-Frequency-Space (OTFS) modulation is a promising candidate technology for high mobility communication scenarios. However, a truly fair comparison with its direct concurrent and widely used Orthogonal Frequency-Division Multiplexing (OFDM) modulation has not yet been provided. In this paper, we present such a fair comparison between the two digital modulation formats in terms of achievable communication rate. In this context, we explicitly address the problem of channel estimation by considering, for each modulation, a pilot scheme and the associated channel estimation algorithm specifically adapted to sparse channels in the Doppler-delay domain, targeting the optimization of the pilot overhead to maximize the overall achievable rate. In our achievable rate analysis we consider also the presence of a guard interval or cyclic prefix. The results are supported by numerical simulations, for different time-frequency selective channels including multiple scattering components and under non-perfect channel state information resulting from the considered pilot schemes. This work does not claim to establish in a fully definitive way which is the best modulation format, since such choice depends on many other features which are outside the scope of this work (e.g., legacy, intellectual property, ease and know-how for implementation, and many other criteria). Nevertheless, we provide the foundations to properly compare multi-carrier communication systems in terms of their information theoretic achievable rate potential, within meaningful and sensible assumptions on the channel models and on the receiver complexity (both in terms of channel estimation and in terms of soft-output symbol detection).

Orthogonal Time Frequency Space Modulation—Part II: Transceiver Designs

“…In this case, it has been shown that OTFS yields an overall achievable rate gain compared to OFDM if practical successive interference cancelation (SIC) detection is employed at the receiver [21]. An alternative, more practical performance metric is the pragmatic capacity, defined as the achievable rate of the channel induced by the signal constellation and the soft-output of the detector [22]. The authors in [22] showed that OTFS transmission enjoys a better pragmatic capacity performance compared to OFDM over static channels with practical channel estimation and detection schemes, thanks to the smaller signaling overhead.…”

“…An alternative, more practical performance metric is the pragmatic capacity, defined as the achievable rate of the channel induced by the signal constellation and the soft-output of the detector [22]. The authors in [22] showed that OTFS transmission enjoys a better pragmatic capacity performance compared to OFDM over static channels with practical channel estimation and detection schemes, thanks to the smaller signaling overhead. Furthermore, the pragmatic capacity of OFDM is very sensitive to the Doppler effect [22], such that OTFS has a clear advantage in high-mobility channels.…”

“…The authors in [22] showed that OTFS transmission enjoys a better pragmatic capacity performance compared to OFDM over static channels with practical channel estimation and detection schemes, thanks to the smaller signaling overhead. Furthermore, the pragmatic capacity of OFDM is very sensitive to the Doppler effect [22], such that OTFS has a clear advantage in high-mobility channels.…”

Cell-Free Massive MIMO with OTFS Modulation: Power Control and Resource Allocation

“…and transformed to the TF domain via two-dimensional (2D) orthogonal basis functions, covering the entire TF domain. Therefore, OTFS exploits both time and frequency diversity, providing a significant improvement over OFDM by combating the Doppler effect [9]. Moreover, OTFS transforms a timevariant channel into an effective fairly time-invariant channel in the DD domain, which has sparse and stable characteristics.…”

Joint Channel Estimation and Data Detection for Hybrid RIS aided Millimeter Wave OTFS Systems

“…In the mean time, pilot and data symbols can also be mapped and demapped over this newly constructed two-dimensional signal space [25]- [27]. Compared with OFDM system, it has been illustrated that OTFS is indeed more insensitive to the magnitude of Doppler frequency shifts [28]. Thus, signal processing techniques can be implemented over delay-Doppler domain for better performance with low complexity in high mobility scenario.…”