OFDM-ISAC Beyond CP Limit: Performance Analysis and Mitigation Algorithms

TL;DR

This paper analyzes the limitations of OFDM-ISAC beyond the cyclic prefix (CP) limit, providing a unified model, performance metrics, and proposing interference cancellation algorithms that significantly improve sensing accuracy.

Contribution

It introduces a comprehensive echo model for OFDM-ISAC beyond the CP limit, derives key performance metrics, and proposes two effective interference mitigation algorithms.

Findings

SINR and RDM metrics deteriorate linearly with excess delay beyond CP

Proposed SIC-DFT and SIC-ESPRIT algorithms improve SINR by over 4 dB

SIC-ESPRIT reduces range/velocity RMSE by about an order of magnitude.

Abstract

Orthogonal frequency division multiplexing (OFDM) is well-suited for integrated sensing and communications (ISAC), yet its cyclic prefix (CP) is dimensioned for communications-grade multipath and is generally insufficient for sensing. When echoes exceed the CP duration, inter-symbol and inter-carrier interference (ISI/ICI) break subcarrier orthogonality and degrade sensing. This paper presents a unified analytical and algorithmic framework for OFDM-ISAC beyond the CP limit. We first develop a general echo model that explicitly captures the structured coupling of ISI and ICI caused by CP insufficiency. Building on this model, we derive closed-form signal-to-interference-plus-noise ratio (SINR) and range-Doppler Map (RDM) second-order moment, together with an approximate peak sidelobe level ratio (PSLR), both of which are shown to deteriorate approximately linearly with the normalized excess delay beyond the CP. To mitigate these effects, we propose two standard-compatible successive interference cancellation (SIC) methods: SIC-DFT, a low-complexity DFT-based scheme, and SIC-ESPRIT, a super-resolution subspace approach. Simulations corroborate the analysis and demonstrate consistent gains over representative benchmarks. Both algorithms provide more than $4$~dB SINR improvement under CP-insufficient conditions, while SIC-ESPRIT reduces range/velocity root-mean-square-errors (RMSE) by about one order of magnitude, approaching the performance achievable with a sufficiently long CP. These results offer both theoretical insight and practical solutions for reliable long-range OFDM-ISAC sensing beyond the CP limit.