Citation
Abstract
A prototype hardware Big Viterbi Decoder (BVD) has been completed for an experiment with the Galileo spacecraft. This decoder resulted from six years of research and development by many members of the Communications Systems Research Section. Searches for new convolutional codes, studies of Viterbi decoder hardware designs and architectures, mathematical formulations and decompositions of the deBruijn graph into identical and hierarchical subgraphs, and VLSI chip design are just a few examples of tasks completed for this project. In this article, BVD bit error rates, measured from hardware and software simulations, are plotted as a function of bit signal-to-noise ratio E,/Ng on the additive white Gaussian noise channel. Using the constraint length 15, rate 1/4, experimental convolutional code for the Galileo mission, the BVD gains 1.5 dB over the NASA standard (7,1/2) Maximum-Likelihood Convolutional Decoder (MCD) at a bit error rate (BER) of 0.005. At this BER, the same gain results when the (255,223) NASA standard Reed-Solomon decoder is used, which yields a word error rate of 2.1 x 10-8 and a BER of 1.4 x 10-9. The (15,1/6) code to be used by the CRAF/Cassini missions yields 1.7 dB of coding gain. These gains are measured with respect to symbols input to the BVD and increase with decreasing BER. Also, 8-bit input symbol quantization makes the BVD resistant to demodulated signal-level variations which may cause 0.1-dB or more loss for the MCD. Since the new rate 1/4 or rate 1/6 codes require higher bandwidth than the NASA (7,1/2) code, these gains are offset by about 0.1 dB of expected additional receiver losses. Coding gains of several decibels are possible by compressing all spacecraft data.
Details
- Volume
- 42-106
- Published
- August 15, 1991
- Pages
- 170–174
- File Size
- 256.8 KB