LPI in pulsed laser space communications

A basic low-probability of intercept (LPI) study for a pulsed-laser pulse position modulated (PPM) optical link with a colocated direct detection intercepting receiver is presented. The latter is considered to be a form of optical radiometer using continuous threshold testing to detect the presence of the transmitting laser. The various design alternatives for the coded PPM link, and their effect on the communication and LPI performance, are examined. It is shown that the use of optical pulse power spreading via multiple frame symbol coding is vital for avoiding interception. It can be achieved by transmitting message symbols as repeated PPM symbols using a laser pulse of sufficiently low peak power so that the detected pulse energy is much less than the detected background energy. The possibility of the use of both wideband and narrowband radiometers by the interceptor further limits the communicator's code design choices. Some equations defining effective LIP gain parameters for the communication link are developed