Performance Evaluation of Low-Complexity Algorithms for Orthogonal Time-Frequency Space Modulation

Abstract

Data transmission in wireless systems brings numerous challenges, especially when it involves propagating in a multipath scenario over rapidly time-varying channels. In this context, Orthogonal Time-Frequency Space (OTFS) modulation has been recently proposed to work with time-frequency selective channels with high Doppler. In this modulation, the symbols are first multiplexed in a delay-Doppler domain rather than in the time-frequency domain used by Orthogonal Frequency Division Multiplexing (OFDM). The studies point out advantages of OTFS performance over OFDM in many aspects, such as data rate increase in high mobility. Another advantage is the sparsity of the channel produced by OTFS that allows using lowcomplexity algorithms for the detection of the data. In this paper, the performance of OTFS modulation in a doubly dispersive channel is evaluated with several low-complexity variants of the message passing algorithm (MPA) in terms of complexity and Bit Error Rate (BER) performance. The results show that MPA and Approximate Message Passing simplified by Expectation Propagation (AMP-EP) algorithms achieve higher performance. However, when taking into account both complexity and BER performance, AMP simplified by First-Order (AMP-FO) achieves the best performance-complexity tradeoff.

A Generalized Two-Dimensional FFT Precoded Filter Bank Scheme With Low Complexity Equalizers in Time-Frequency Domain

“…Simulation results also were obtained and compared for OTFS systems using different detection techniques: MMSE delay-Doppler (MMSE-DD) domain, MMSE-TF, IB-DFE and AMP-FO. The last one operates iteratively with 12 iterations and a damping factor of 0.6 as used in [21]. AMP-FO and MMSE-DD were also applied to the 2D-FFT FB scheme in order to demonstrate its compatibility with OTFS.…”

“…For the IB-DFE equalizer, due to the iterative process the term ī is the maximum number of iterations used, while (LK log(L/β) + (L/β)K log K) represents the complexity of SFFT. In addition to the size of the delay-Doppler grid, the complexity of the AMP-FO receiver is calculated as a function of the constellation size Z-QAM, the number of the channel taps, P and also the number of iterations used in the algorithm [21]. Although the order of the AMP-FO and IB-DFE receivers are similar (linear), the computational implementation of AMP-FO has a much greater complexity due to the number of operations performed by the algorithm.…”