Development of Improved SOQPSK based Data Transmission over Aeronautical Telemetry Link

In aeronautical telemetry Alamouti encoded Shaped Offset Quadrature Phase Shift Keying - Telemetry Group (SOQPSK-TG) modulated signal is used to resolve “Two antenna problem” due to simultaneous transmission from the two onboard antennae. Detection of this signal at the receiver requires estimation of channel impairments (channel gains, time delays and frequency offset). The Maximum Likelihood Sequence Estimation (MLSE) based decoder of Space Time Coding (STC) encoded SOQPSK- TG signal requires 512 states, which is too complex for implementation. In this paper, pulse shaping is performed on SOQPSK- TG frequency pulse to reduce the pulse duration. Pulse of length 2 bit interval is found to be approximately matching the SOQPSK- TG characteristic while reducing the decoder complexity to 8 number of states. Subsequently parameter estimation is carried out for STC encoded SOQPSK-2T by Maximum Likelihood (ML) estimation method. The performances of proposed pulse shaping functions are compared with those of SOQPSK- TG and Feher's Quadrature Phase Shift Keying (FQPSK-JR), and are found to be superior for aeronautical telemetry display and level flight operations.