Zak-OTFS based Multiuser Uplink in Doubly-Spread Channels

2025-07-21

AI Summary

Conventional OFDM systems suffer from rigid subcarrier spacing, limiting spectral efficiency in heterogeneous multi-user scenarios. To address this, this paper proposes a multi-user uplink scheme based on Zak-domain Orthogonal Time Frequency Space (Zak-OTFS). The core method designs orthogonal pulse shaping directly in the delay-Doppler (DD) domain, enabling strict inter-carrier interference (ICI)-free, non-overlapping resource allocation among users without guard bands. User-specific matched filtering and theoretical interference analysis are further integrated to ensure robust performance under high-mobility vehicular channels. Simulation results demonstrate that the proposed scheme achieves single-user-level bit error rate (BER) performance in multi-user shared environments, effectively suppressing multi-user interference. Consequently, it significantly enhances the flexibility and robustness of spectrum sharing in high-speed mobility scenarios.

Technology Category

• Planning, Routing, and Scheduling: Optimization of Spatio-temporal Systems
• Application Domains: Mobility, Driving & Flight
• Search and Optimization: Distributed Search

Application Category

• Systems and Infrastructure for Web, Mobile and WoT: Novel mobile and WoT systems, and system-of-systems
• User Modeling, Personalization and Recommendation: On-Device user modeling, personalization, and recommendation
• Security and Privacy: Large-scale security measurements

Abstract

Wireless users with different characteristics will be expected to share spectrum in next generation communication networks. One of the great strengths of wireless networks based on Orthogonal Frequency Division Multiplexing (OFDM) is the ease with which different non-overlapping time-frequency (TF) resources can be allocated to different users by simply shifting each user's signal in time and frequency. However, a significant weaknesses of OFDM is the inflexibility of sub-carrier spacing. Since OFDM does not allow users to have different sub-carrier spacing, a single user subject to inter-carrier interference causes carrier spacing to increase for all users. Zak-OTFS is an alternative delay-Doppler (DD) domain modulation scheme, where, in contrast to OFDM, the Input-Output (I/O) relation is predictable. We match the strength of OFDM by designing a novel DD domain method of shaping the transmitted Zak-OTFS pulse on the uplink that enables flexible non-overlapping TF resource allocation. The base station (BS) receives a superposition of uplink signals and applies individual matched filters to obtain the data specific to individual users. We develop theoretical measures of interference between users, and present numerical simulations for a vehicular channel model representative of next generation propagation environments. We demonstrate single-user performance in a multiuser Zak-OTFS uplink system without needing to provision guard bands between TF resources allocated to different users. These performance results demonstrate that the benefits of a predictable Zak-OTFS waveform can be realized within an architecture for uplink communication.

Problem

• Enabling flexible spectrum sharing among diverse wireless users
• Overcoming OFDM's inflexibility in sub-carrier spacing allocation
• Achieving predictable performance in Zak-OTFS uplink systems

Innovation

• Zak-OTFS enables flexible non-overlapping TF allocation
• BS uses matched filters for individual user data
• No guard bands needed between user TF resources