Extended GFDM Framework: OTFS and GFDM Comparison

TL;DR

This paper compares OTFS and GFDM modulation schemes, showing that OTFS can be viewed as a permuted GFDM signal, which explains its superior performance in time-varying channels and offers efficient implementation insights.

Contribution

The paper presents an extended GFDM representation revealing OTFS as a permuted GFDM signal, enhancing understanding and implementation of OTFS in LTV channels.

Findings

OTFS can be modeled as a permuted GFDM signal.

Permutation explains OTFS's robustness in LTV channels.

GFDM framework facilitates efficient OTFS implementation.

Abstract

Orthogonal time frequency space modulation (OTFS) has been recently proposed to achieve time and frequency diversity, especially in linear time-variant (LTV) channels with large Doppler frequencies. The idea is based on the precoding of the data symbols using symplectic finite Fourier transform (SFFT) then transmitting them by mean of orthogonal frequency division multiplexing (OFDM) waveform. Consequently, the demodulator and channel equalization can be coupled in one processing step. As a distinguished feature, the demodulated data symbols have roughly equal gain independent of the channel selectivity. On the other hand, generalized frequency division multiplexing (GFDM) modulation also employs the spreading over the time and frequency domains using circular filtering. Accordingly, the data symbols are implicitly precoded in a similar way as applying SFFT in OTFS. In this paper, we present an extended representation of GFDM which shows that OTFS can be processed as a GFDM signal with simple permutation. Nevertheless, this permutation is the key factor behind the outstanding performance of OTFS in LTV channels, as demonstrated in this work. Furthermore, the representation of OTFS in the GFDM framework provides an efficient implementation, that has been intensively investigated for GFDM, and facilitates the understanding of the OTFS distinct features.