On Covertness of DSSS Against Energy and Cycle Detectors Under Link QoS Requirements

We consider a three party wireless network- Alice is a legitimate transmitter, Bob is a legitimate receiver, and Willie is a malicious adversary. Alice's goal is to communicate with Bob with a certain minimum Quality of Service (QoS) while evading detection from Willie. Alice transmits a Direct Sequence Spread Spectrum (DSSS) signal where the processing gain and transmit power are selected with the aim of evading detection by spreading the bit energy over a larger bandwidth while maintaining required QoS. Concurrently, Willie tries to detect the legitimate transmission by using either an energy or a cycle detector. We study this scenario within an adversarial optimization framework that aims at achieving a robust max-min covertness under link performance constraints. The framework is common to both detectors and hence allows for a comparative performance analysis that identifies common key performance parameters. The adversarial signal-to-noise ratio (SNR) detection gain enables direct trading of the DSSS processing gain for the (squared) channel quality ratio when aiming to improve link covertness, regardless of which detector Willie uses. The DSSS processing gain and SNR gain (and, hence, covertness) are limited by the bit rate and bit error rate link requirements, respectively. While both detectors' performance is limited by the SNR at Willie, the cycle detectors benefit significantly more from longer observation time of the legitimate transmission.