Orthogonal Time Frequency Space Modulation

Abstract

This paper introduces a new two-dimensional modulation technique called Orthogonal Time Frequency Space (OTFS) modulation. OTFS has the novel and important feature of being designed in the delay-Doppler domain. When coupled with a suitable equalizer, OTFS modulation is able to exploit the full channel diversity over both time and frequency. Moreover, it 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 essentially constant for all symbols.This design obviates the need for transmitter adaptation, and greatly simplifies system operation. The paper describes the basic operating principles of OTFS as well as a possible implementation as an overlay to current or anticipated standardized systems. OTFS is shown to provide significant performance improvement in systems with high Doppler, short packets, and/or large antenna array. In particular, simulation results indicate at least several dB of block error rate performance improvement for OTFS over OFDM in all of these settings.

Low complexity LMMSE Receiver for OTFS

“…Doppler-delay domain data d(k, l) is mapped to time-frequency domain data X(n, m) on lattice Λ ⊥ = {(nT, m∆f )}, n ∈ N[0 N − 1] and m ∈ N[0 M − 1] by using inverse symplectic fast Fourier transform (ISFFT). X(n, m) can be given as [2], Next, X(n, m) is converted to a time domain signal s(t) through a Heisenberg transform as,…”

“…Although subcarrier bandwidth in 5G-NR can be increased to combat Doppler spread, the provision of proportional decrement of CP length to retain OFDM symbol efficiency induces interference when both delay and Doppler spreads are significant. Orthogonal time frequency space modulation (OTFS) has been recently proposed in [2] to efficiently transfer data in such channel conditions. In OTFS, data symbols are spread across available time-frequency resources which can be exploited to extract diversity gain.…”

OTFS: Interleaved OFDM with Block CP

“…This is due to its high sensitivity to inter carrier interference (ICI), caused by Doppler spread and phase noise. Orthogonal time frequency space (OTFS) [2] is shown to be superior to OFDM in such high mobility environments, which is an integral part of 5G's operating scenarios.…”

“…The data is then spread across the time-frequency grid using a unitary transform. This is followed by an OFDM [2] or block OFDM [3] modulator. Cyclic prefix (CP) [2] or block CP [3] is added to absorb the channel delay spread.…”

“…This is followed by an OFDM [2] or block OFDM [3] modulator. Cyclic prefix (CP) [2] or block CP [3] is added to absorb the channel delay spread. When OFDM modulation is used with CP, it is called as CP-OTFS whereas, OTFS with block OFDM modulation and block CP is termed as reduced CP OTFS (RCP-OTFS) [4].…”