Sequence codes

Abstract

The objective of this work was the support of efficient and effective transportation of minimally delayed digital data, primarily via geostationary satellite, between two users. The applicable communication constraints are primarily the media effects related to the orbital distance of approximately 40000 kilometres, to and from the satellite. The media effects are largely related to the free space path loss of approximately 200dB and the satellite link time delay of approximately 0.27 seconds which is barely acceptable for speech. To achieve this objective a short, efficient, error correction system has been designed. The effort was approached using QPSK modulation as a BPSK modulation sequence code similar, in principle, to Ungerboeck allowing optimization of the signal's Euclidean distance. Due to transmission delay constraints, no iterative techniques were considered. Instead a logically based short phase sequence was generated which, on reception, was operated on by multiple integrated decoders rather than the usual one shot correction techniques of most non iterative systems. The decoder/decodulator consists of an integrated serial/parallel concatenated Viterbi with a new phase sequence decoder (PSD) designed to recover the transmission to a state. This output is further concatenated with a newly developed Metric Analysis Optimise \& Control Unit (MAOC) designed to recover to a bit. Separating the multiple integrated decoders to state from the bit correction unit simplifies the integration of the integrated decoder pair. This system is further concatenated with a short Reed Solomon coder. The results achieved are impressive and improvements are considered to be possible with further development.