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The distortion of transmission signal ,when passed through the channel of the Expendable Conductivity-Temperature-Depth (XCTD) profiler under the influence of the probing depth, transmission frequency, random noise and other factors, will be more serious. And it will affect the stability and accuracy of the DPSK transmission. This paper firstly establishes the model of XCTD channel and explains the theory of differential phase shift keying (DPSK) demodulation, which proposes that the selection of the demodulation carrier frequency is the key factor to affect the DPSK demodulation accuracy. Through specific analysis of the impact of the demodulation carrier frequency offset, we ascertain the best demodulation carrier transmission frequency satisfies the demand of the actual measurement within the range of detecting depth of 2000 meters. Meanwhile, we analyze that the increase of the probing depth and random noise based on the increase of transmission frequency has an influence on the size of bit error rate caused by the demodulation frequency offset. The results show that we can further optimize the design of the XCTD profiler measurement equipment, and improve the accuracy of the measurement data and the stability of the transmission signal.

The Analysis of the DPSK Transmission Carrier Frequency of Time-Varying Channel Model of XCTD Profiler