Waveform for Next Generation Communication Systems: Comparing Zak-OTFS with OFDM

TL;DR

The paper addresses waveform choice for 6G in doubly-spread channels by comparing CP-OFDM and Zak-OTFS across realistic propagation scenarios. It analyzes how the input-output (I/O) relation can be acquired and how overhead behaves when preventing versus embracing inter-carrier interference, highlighting the crystallization condition $\nu_p >$ Doppler spread and $\tau_p >$ delay spread that enables Zak-OTFS I/O predictability. Through Veh-A-channel simulations with realistic MCS optimization, it shows Zak-OTFS achieving more than double the effective spectral efficiency in high mobility/large-cell and NTN scenarios, while CP-OFDM and Zak-OTFS perform similarly in low-delay/Doppler conditions; CP overhead grows with Doppler, while Zak-OTFS overhead can be kept small with appropriate pilot/guard design. The findings provide system-level guidance for architectural decisions in 6G and motivate exploring alternative operating points within standards to balance reliability, SE, and complexity.

Abstract

Across the world, there is growing interest in new waveforms, Zak-OTFS in particular, and over-the-air implementations are starting to appear. The choice between OFDM and Zak-OTFS is not so much a choice between waveforms as it is an architectural choice between preventing inter-carrier interference (ICI) and embracing ICI. In OFDM, once the Input-Output (I/O) relation is known, equalization is relatively simple, at least when there is no ICI. However, in the presence of ICI the I/O relation is non-predictable and its acquisition is non-trivial. In contrast, equalization is more involved in Zak-OTFS due to inter-symbol-interference (ISI), however the I/O relation is predictable and its acquisition is simple. {Zak-OTFS exhibits superior performance in doubly-spread 6G use cases with high delay/Doppler channel spreads (i.e., high mobility and/or large cells), but architectural choice is governed by the typical use case, today and in the future. What is typical depends to some degree on geography, since large delay spread is a characteristic of large cells which are the rule rather than the exception in many important wireless markets.} This paper provides a comprehensive performance comparison of cyclic prefix OFDM (CP-OFDM) and Zak-OTFS across the full range of 6G propagation environments. The performance results provide insights into the fundamental architectural choice.

Figures (6)

• Figure 1: Orthogonal allocation of TF resources.
• Figure 2: Zak-OTFS (Embracing ICI) vs. CP-OFDM (avoiding ICI).
• Figure 3: (a) Effect of channel Doppler spread on overhead in CP-OFDM. (b) Effect of channel delay spread on overhead in CP-OFDM.
• Figure 4: A typical Zak-OTFS frame with pilot, guard and data regions.
• Figure 5: Ratio of effective SE achieved by Zak-OTFS to that achieved by CP-OFDM.