An O(log2 N)-latency SISO with application to broadband turbo decoding

The standard algorithm for computing the soft-inverse of a finite-state machine (i.e., the Soft-in/Soft-out or SISO) module, is the forward-backward algorithm. These forward and backward recursions can be computed in parallel, yielding an architecture with latency omicron (N), where N is the block size. We demonstrate that the standard SISO computation may be formulated using a combination of a prefix and suffix operations. Based on well-known tree-structures for fast parallel prefix computations in the Very Large Scale Integration literature (e.g., tree adders), we propose a tree-structured SISO that has latency omicron (log(2) N). The decrease in latency comes primarily at a cost of area, with, in some cases, only a marginal increase in computation. We discuss how this structure could be used to design a very high throughput turbo decoder, or more generally an iterative detector. Various sub-windowing and tiling schemes are also consider to further improve latency.