LPI communication in channels with absorption loss

This paper deals with covert communication in channels with and without absorption loss. Let R_b be the user communication rate, W the transmission bandwidth, and n the message duration in bits. The threshold input power to a radiometer interceptor is given as a function of its performance (P_F, P_D), the processing gain W/R_b, and the message size n. In order to analyze the LPI ability of the communication system, the quality factor K = r_c/r_l is derived, where r_c is the maximum communication range and r_l is the maximum interceptor range. The calculation is done for the following two types of propagation loss models: 1) Propagation loss that is only proportional to the power of the range. 2) Propagation loss that is proportional to the product of the power of the range and the exponent of the range. The first derivative of K with respect to A the ratio (in dB) between the threshold input power to the interceptor and the threshold input power to the communication system, is calculated at D = 0 and its meaning is discussed. Several numerical examples are given.