Pilot Distortion Design for ToA Obfuscation in Uplink OFDM Communication

TL;DR

This paper proposes a method to deliberately distort OFDM pilots to obscure ToA estimation at a base station, balancing obfuscation effectiveness with minimal impact on communication capacity.

Contribution

It introduces a novel pilot distortion design that increases ambiguity function sidelobes to hide ToA, while maintaining communication performance within power and dissimilarity constraints.

Findings

Increases MAF sidelobes to degrade delay estimation.

Degrades ToA estimation accuracy with minimal capacity loss.

No protocol changes needed for implementation.

Abstract

We study uplink orthogonal frequency-division multiplexing (OFDM) pilot distortion to deliberately obfuscate time-of-arrival (ToA) estimation at a single base station while preserving communication performance. We design a complex per-subcarrier distortion vector that increases sidelobes of the mismatched ambiguity function (MAF) relative to its mainlobe, using two objectives: the sidelobe-to-peak level ratio and the integrated sidelobe level. The design is subject to a transmit-power budget and a proximity (dissimilarity) constraint around the communication-optimal pilot. Communication impact is quantied by a capacity-motivated lower bound obtained from the linear minimum mean-squared error error covariance with a mismatched channel estimate. The resulting generalized fractional program is solved with Dinkelbach's transform and a difference-of-convex update that yields a closed-form Karush-Kuhn-Tucker step. Simulations on a single-input single-output OFDM link show that the optimized distortions raise MAF sidelobes and degrade delay estimation, as validated by a mismatched maximum-likelihood ToA estimator, while incurring only marginal capacity loss over a broad signal-to-noise ratio range. The method requires no protocol changes or artificial path injection and provides a signal-level mechanism to control ToA observability under communication constraints.