Sequential Decoding for Mobile Receivers

Currently many classical detection and decoding algorithms receive new attention in mobile communications. Amongst those classics are low density parity check codes [1], concatenated codes [2], weighted decoding with log-likelihood values as well as soft-in/soft-out decoding as performed by the BCJR algorithm and its derivatives [4], [5]. All four classics show up as key elements in the so-called turbo decoding which first was invented by Berrou et al. in [6] as an iterative scheme to decode parallel concatenated codes. Later it was recognized that this scheme, namely iterative decoding by exchanging extrinsic values between two or more decoders, is a general principle named the "Turbo Principle" [5]. In this talk we will revive another classical decoding method called sequential decoding in order to use it for iterative decoding in high memory situations. For iterative detection/decoding (turbo) schemes in mobile receivers we modify sequential decoding which contrary to APP (BCJR) decoding enjoys a complexity almost independent from the number of states. This novel LISt-Sequential (LISS) decoder avoids most of the drawbacks of the classical sequential decoders such as variable work load and erased frames when working within a turbo scheme. It uses are a metric containing a priori and channel values, a metric length bias term for speeding up the tree search, a soft extension of paths without increasing the stack size and soft weighting to obtain a soft output. We present several mobile turbo applications using the LISS including equalization, single antenna interference cancellation, multiuser and MIMO detection.