Software Implementation of a PN Spread Spectrum Receiver to Accommodate Dynamics

To optimize the threshold of a pseudonoise (PN) spread spectrum modem for use over an aircraft/satellite communications link at SHF, the effects of Doppler must be taken into account. Reconstitution of carrier phase by a Costas loop to coherently demodulate the PSK data and also the delay-lock error voltage has typically been the practice in PN modems intended for ground applications. To accommodate the platform dynamics, the Costas loop must have a relatively wide bandwidth, and this implies a significant threshold degradation. An alternate implementation employs a noncoherent carrier tracking loop which maintains frequency lock rather than phase lock. Now, the delay-lock error voltage is noncoherently demodulated. For the airborne application, analysis and simulations show this implementation will extend the receiver's tracking threshold significantly (up to 6 dB) for the worst case dynamics profile. An experimental project was undertaken to modify an existing ground PN modem (AN/USC-28, ADM version) for flight test. A software implementation of the digital tracking algorithms was selected where a HP-2100A minicomputer controls carrier frequency and PN code phase via digital phase shifters. The Costas demodulator for extracting PSK data resides entirely in software, and is completely segregated from PN tracking. In laboratory testing of the receiver with simulated dynamics and in actual flight tests, the demonstrated performance was found to approach closely the goals established by the analyses and simulations.