  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.
