Bounds for Low Probability of Detection for Underwater Acoustic Communication

Most military underwater activities require stealth operations, and hydroacoustic transmissions might temper the mission. For this reason, military underwater acoustic transmission aims for low probability of detection (LPD). LPD communication systems are characterized by low transmission power and reception at low signal-to-noise power ratio (SNPR). As a side effect, LPD decreases acoustic noise pollution. In this paper, we study the LPD capability of underwater acoustic communication (UWAC) systems. We consider an interceptor with target false alarm and detection probabilities and a legitimate receiver with required detection and packet error probabilities. For the interceptor, we focus on an energy detector and calculate the minimal SNPR which satisfies its required performance. For the legitimate communication link, we consider spread-spectrum signaling with several modulation techniques, and calculate the minimal required SNPR to allow reliable detection and decoding of a data packet. Based on these minimal SNPR terms and using bounds on power attenuation in the channel, we quantify the LPD capability of the system and analyze the effect of channel and communication parameters. We also present results from a sea trial conducted in the Saanich Inlet off the coast of Vancouver Island, where we conducted LPD experiments for different transmission powers and carrier frequencies. We provide bounds for the minimal bandwidth and maximal transmission rate for LPD communication. Our results also show that LPD UWAC performs better in shallow and warm water, and for transmission at low carrier frequencies.