Zak-OTFS based Multiuser Uplink in Doubly-Spread Channels

TL;DR

This paper introduces a novel Zak-OTFS based multiuser uplink scheme that enables flexible, guard-band free spectrum sharing in doubly-spread channels, outperforming traditional OFDM in dynamic environments.

Contribution

The paper proposes a new DD domain pulse shaping method for Zak-OTFS that allows non-overlapping TF resource allocation without guard bands, enhancing multiuser uplink performance.

Findings

Effective multiuser Zak-OTFS uplink performance demonstrated in vehicular channels.

Theoretical interference measures developed for multiuser Zak-OTFS.

No guard bands needed between users in the proposed scheme.

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.