Phase-Shifted Pilot Design for NOMA-Empowered Uplink ISAC Systems

Abstract

The deployment of multiple transmitters (TXs) in integrated sensing and communication (ISAC) networks necessitates efficient resource sharing to overcome the limitations of orthogonal allocation. While conventional interleaved (CI) pilots combined with non-orthogonal multiple access (NOMA) improve spectral efficiency (SE), they inherently compromise sensing resolution due to spectral sparsity, rendering the CI nulling (CIN) extension a strictly limited remedy. This paper proposes a phase-shifted (PS) pilot design and its novel PS nulling (PSN) variant to integrate a communication TX (CTX) over the PS-ISAC framework. The PSN variant strategically punctures sensing signals at CTX pilot locations to preserve initial channel estimates, enabling a dense data overlay. To resolve the resulting multi-TX interference, joint iterative interference cancellation (IIC) is adapted for non-nulling configurations and sequential IIC is adapted for nulling variants, optimizing for both detection robustness and convergence speed. Simulation results across varying STX densities and modulation orders demonstrate that the phase-shifted frameworks maintain sensing integrity while explicitly reducing receiver-side computational complexities by $18.8\%$ and $21.0\%$ against their respective interleaved baselines.

Figures (8)

• Figure 1: System model of the proposed UL ISAC-NOMA scheme.
• Figure 2: Frequency-domain resource allocation for UL ISAC-NOMA with communication signal as the base layer. Sensing pilots are overlaid via: (a) CI: Interleaved with communication pilots; (b) CIN: Interleaved with nulling at communication pilot locations; (c) PS: Superimposed across the frame using cyclic phase shifts; and (d) PSN: Phase-shifted with spectral nulling at communication pilot locations.
• Figure 3: Outlier analysis of the IIC architectures evaluating the sensing NMSE versus the SNR, specifically for a network density of $U=4$ scheduled STX operating under a fixed modulation order of $M=4$.
• Figure 4: Convergence analysis of the IIC architectures for $U=4$ and $M=4$: explicitly subfigure (a) illustrating sensing NMSE convergence, subfigure (b) depicting communication NMSE convergence, and subfigure (c) showing BER convergence. The iteration parameters $Q_1 = 3$, $Q_2 = 4$, and $Q = 7$ are selected based on the common maximum iterations required to satisfy a relative performance improvement threshold of $10^{-4}$ across all evaluated schemes.
• Figure 5: Robustness analysis of the ISAC-NOMA frameworks against the CTX modulation order for $M \in \{2, 4, 16\}$ and $U=4$: subfigure (a) illustrating sensing NMSE, subfigure (b) depicting communication NMSE, and subfigure (c) showing the BER. The theoretical limits, explicitly defined as the CO LB, PS LB, and CI LB, represent the communication-only scenario as well as the phase-shifted sensing-only, denoted as PS-ISAC, and interleaved sensing-only, denoted as CI-ISAC, baseline configurations.