A Modified Viterbi Decoder for Joint Data-Recovery and Cycle-Slip Correction

James T. Curran,

Abstract:	This paper presents a novel modification of the traditional Viterbi decoder which performs a joint recovery of the encoded navigation data and the identification and correction of carrier cycle-slips. Many modern GNSS signals employ a BPSK modulation of a convolutional-encoded navigation message, offering appreciable error-correcting capability in the presence of additive noise. However, in many instances, the sources of error experienced by a GNSS receiver are not additive, and include multiplicative factors such as high phase dynamics and cycle-slips. Under such conditions the performance of the traditional Viterbi is sub-optimal. This paper presents a novel modification which provides improved performance under cycle-slippage by introducing a non-binary decoding trellis. Rather than a binary forking at each node, a quaternary decision is made between 0, 1, inverted-0 and inverted-1. The decoder returns both the most likely un-encoded data and a sequence representing the most likely locations of phase-inversions. The result is that the effective coding rate is reduced, providing slightly lower decoding performance under nominal conditions, but with a significant improvement in the presence of sporadic cycle-slips. Moreover, knowledge of the location of likely cycle slips offers a novel means of slip-correction in the carrier phase observable. A detailed description of the algorithm is provided along with performance results of Monte-Carlo simulation and experiments using live Galileo E1B signals.
Published in:	Proceedings of the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2016) September 12 - 16, 2016 Oregon Convention Center Portland, Oregon
Pages:	257 - 264
Cite this article:	Curran, James T., "A Modified Viterbi Decoder for Joint Data-Recovery and Cycle-Slip Correction," Proceedings of the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2016), Portland, Oregon, September 2016, pp. 257-264. https://doi.org/10.33012/2016.14671
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