Sixth generation (6G) wireless networks are envisioned to provide global coverage for the intelligent digital society of the near future, ranging from traditional terrestrial to non-terrestrial networks, where reliable communications in high-mobility scenarios at high carrier frequencies would play a vital role. In such scenarios, the conventional orthogonal frequency division multiplexing (OFDM) modulation, which has been widely used in both the fourth generation (4G) and the emerging fifth generation (5G) cellular systems, is vulnerable to severe Doppler spread. A new two-dimensional modulation technique called Orthogonal Time Frequency Space (OTFS) modulation designed in the delay-Doppler domain is introduced. Through this design, which exploits full diversity over time and frequency, OTFS coupled with equalization converts the fading, time-varying wireless channel experienced by modulated signals such as OFDM into a time-independent channel with a complex channel gain that is roughly constant for all symbols. In this paper, we evaluate and analyze the performance of the OTFS receiver using four data detection algorithms: Single-tap frequency domain equalizer, Linear minimum mean-square error detection (LMMSE), Message passing detection (MP), and Maximum-ratio combining detection (MRC). Further, we extend our study to the variants of OTFS, where the waveform is transmitted with a cyclic prefix (CP) or zero padding (ZP) added to each OTFS frame or block. The simulation results show that the MRC detector can be improved the detection of the OTFS receiver.