A Bit-serial Viterbi processor
Bree, Michael Arthur
The Viterbi algorithm is used for Forward Error Control (FEC) in systems such as satellite communication. Smaller networks are now utilizing satellite technology, which has created a demand for low cost, moderate speed Viterbi Decoders. Many low cost VLSI Viterbi decoders use bit parallel, sequential node processing techniques. In this thesis, bit-serial techniques are applied which reduce circuit size and allow for a parallel node processing implementation. The use of bit-serial communication paths between circuits also reduces wiring area requirements when compared with bit-parallel busses. Off-chip wiring of the processing trellis allows multiple chips to be cascaded, thus increasing decoder constraint length and bit-error correction capability. A technique is presented which pairs the node processing circuits and further reduces the number of I/O pins and wires. VLSI chips were designed using the QUISC silicon compiler and associated standard cell library. A single chip can implement constraint length K=4 and will support eight level soft decision and code rates of R=1/2 and R=1/3. A Viterbi algorithm simulator, VSIM, was written to aid in the design and debugging of the chip. The simulator can also be used to predict the performance of various decoder configurations of cascaded chips. Fabricated chips were found to operate as expected at decoded data rates up to 287 kbps. Simulations indicate coding gains ranging from 3.8 dB to 5.2 dB at a decoded bit-error-rate of 10-5.