VLSI Architectures for SISO-APP Decoders

Very large scale integration (VLSI) design methodology and implementation complexities of high-speed, low-power soft-input soft-output (SISO) a posteriori probability (APP) decoders are considered. These decoders are used in iterative algorithms based on turbo codes and related concatenated codes and have shown significant advantage in error correction capability compared to conventional maximum likelihood decoders. This advantage, however, comes at the expense of increased computational complexity, decoding delay, and substantial memory overhead, all of which hinge primarily on the well-known recursion bottleneck of the SISO-APP algorithm. This paper provides a rigorous analysis of the requirements for computational hardware and memory at the architectural level based on a tile-graph approach that models the resource-time scheduling of the recursions of the algorithm. The problem of constructing the decoder architecture and optimizing it for high speed and low power is formulated in terms of the individual recursion patterns which together form a tile graph according to a tiling scheme. Using the tile-graph approach, optimized architectures are derived for the various forms of the sliding-window and parallel-window algorithms known in the literature. A proposed tiling scheme of the recursion patterns, called hybrid tiling, is shown to be particularly effective in reducing memory overhead of high-speed SISO-APP architectures. Simulations demonstrate that the proposed approach achieves savings in area and power in the range of 4.2%-53.1% over state of the art.