LPI, full-duplex voice communication using adaptive rate delta modulation

This paper proposes a novel modulation concept for robust, low-probability-of-intercept (LPI), full-duplex, wireless, digital voice communication system. The adaptive rate delta modulation (AR-DM) uses a tri-level threshold to produce delta-modulated pulses only when the magnitude change exceeds a predetermined threshold. This technique significantly reduces the average pulse rate required and lowers the noise generated by delta modulation under zero input conditions. The receiver is reduced to a simple pulse detector and a counter to recreate the original signal. This system does not require headers, packets or other synchronizing schemes. The reduced pulse rate and input-dependent nature of the adaptive rate implementation allows multiple users to use the same channel concurrently for full-duplex voice communication. The pauses and amplitude variations of natural conversation provide gaps in pulse transmission through which other user pulses can be received with acceptable loss due to pulse collisions. Simultaneous reception from multiple transmitters gives a result resembling natural group conversation. AR-DM can be combined with spread-spectrum techniques and/or transmission in the millimeter wave (MMW) 60 GHz oxygen absorption band to provide reliable LPI communication for forward operating platoons or other small groups. A reliable and compact communication system can be deployed to each soldier. This paper presents the essentials of AR-DM and characterizes a voice communication system using spread spectrum and/or 60 GHz transmission band for LPI.