Orthogonal time frequency space (OTFS) modulation is a two-dimensional signaling technique that has recently emerged in the literature to tackle the time-varying (TV) wireless channels. OTFS deploys the Doppler-delay plane to multiplex the transmit data where the time variations of the TV channel are integrated over time and hence the equivalent channel relating the input and output of the system boils down to a time-invariant one. This signaling technique can be implemented on the top o f a given multicarrier waveform with the addition of precodin g and post-processing units to the modulator and demodulator. In this paper, we present discrete-time formulation of an OFDM - based OTFS system. We argue against deployment of window functions at the OTFS transmitter in realistic scenarios an d thus limit any sort of windowing to the receiver side. We stud y the channel impact in discrete-time providing deeper insights into OTFS systems. Moreover, our derivations lead to simplified modulator and demodulator structures that are far simpler than those in the literature.