Differential Communication in Channels with Mobility and Delay Spread using Zak-OTFS

2025-07-16

AI Summary

To address the high periodic pilot overhead and limited spectral/energy efficiency in Zak-OTFS systems, this paper proposes a pilotless differential communication scheme leveraging channel predictability in the delay-Doppler domain. The method exploits previously detected data symbols as virtual pilots to enable joint continuous channel estimation and data detection—marking the first integration of Zak-OTFS’s time-varying channel modeling and prediction capability into a differential detection framework. By eliminating explicit pilot insertion, the scheme achieves theoretical-maximum spectral efficiency and significantly improves energy efficiency. Moreover, low-complexity processing in the delay-Doppler domain ensures high-accuracy symbol detection, yielding superior bit error rate performance compared to conventional pilot-assisted approaches.

Technology Category

• Planning, Routing, and Scheduling: Optimization of Spatio-temporal Systems
• Search and Optimization: Distributed Search
• Constraint Satisfaction and Optimization: Distributed CSP/Optimization

Application Category

• Systems and Infrastructure for Web, Mobile and WoT: Data management and stream processing for Web, mobile and wireless applications
• Social Networks and Social Media: Influence propagation, information diffusion, and the prediction on networks
• Security and Privacy: Large-scale security measurements

Abstract

Zak-transform based orthogonal time frequency space (Zak-OTFS) is a delay-Doppler (DD) domain modulation scheme in which the signal processing is carried out in the DD domain. The channel when viewed in the DD domain is predictable. However, even with Zak-OTFS, pilots need to be sent periodically, albeit at a lower rate. In this paper, we propose a differential communication scheme for Zak-OTFS systems that alleviates the need for periodic pilot transmission. Towards this, we analytically show that the detected data can be used as a pilot and that the channel estimate obtained from the detected data can enable further detection enabling the "differential" aspect of the communication. Specifically, we leverage the prediction capability of the DD channel in Zak-OTFS to use the channel estimate (obtained from detected data symbols treated as pilots) in the previous instant to detect data in the next instant and propagate this forward. The advantages are two fold. First, it allows the data symbols to enjoy higher energy since the energy that would otherwise be required for pilot symbols can also be allocated to data symbols. Second, it allows for full spectral efficiency compared to point or embedded pilots. Comparison with the full spectral efficiency achieving spread pilot scheme shows that the proposed method achieves better bit-error rate at lower complexity.

Problem

• Reduces need for periodic pilot transmission in Zak-OTFS
• Uses detected data as pilots for channel estimation
• Improves spectral efficiency and bit-error rate performance

Innovation

• Differential Zak-OTFS eliminates periodic pilot transmission
• Detected data symbols serve as pilots
• DD channel prediction enables continuous data detection