This paper introduces a new two-dimensional modulation technique called Orthogonal Time Frequency Space (OTFS) modulation. OTFS is designed in the delay-Doppler domain, which allows it to exploit full channel diversity over both time and frequency. By converting the fading, time-varying wireless channel into a time-independent channel with a constant complex channel gain, OTFS eliminates the need for transmitter adaptation and simplifies system operation. The paper discusses the basic operating principles of OTFS and its potential implementation as an overlay to current or anticipated standardized systems. OTFS is shown to provide significant performance improvements in systems with high Doppler, short packets, and/or large antenna arrays, with simulation results indicating at least several dB of block error rate performance improvement over OFDM in these settings. The paper also covers the delay-Doppler representation of channels and signals, the Heisenberg transform, and the symplectic Fourier transform, which are crucial for understanding the mathematical framework of OTFS. Finally, the paper explores the interpretation and implementation of OTFS, including its multiplexing capabilities and the benefits of its almost-constant channel gain.This paper introduces a new two-dimensional modulation technique called Orthogonal Time Frequency Space (OTFS) modulation. OTFS is designed in the delay-Doppler domain, which allows it to exploit full channel diversity over both time and frequency. By converting the fading, time-varying wireless channel into a time-independent channel with a constant complex channel gain, OTFS eliminates the need for transmitter adaptation and simplifies system operation. The paper discusses the basic operating principles of OTFS and its potential implementation as an overlay to current or anticipated standardized systems. OTFS is shown to provide significant performance improvements in systems with high Doppler, short packets, and/or large antenna arrays, with simulation results indicating at least several dB of block error rate performance improvement over OFDM in these settings. The paper also covers the delay-Doppler representation of channels and signals, the Heisenberg transform, and the symplectic Fourier transform, which are crucial for understanding the mathematical framework of OTFS. Finally, the paper explores the interpretation and implementation of OTFS, including its multiplexing capabilities and the benefits of its almost-constant channel gain.