Moreover, the iterative processing may include using soft output of a first SISO as soft input to another SISO, and/or may include performing marginalization-combining operations which form a semi-ring over the soft-information. [0012] The tree structure used by at least one SISO may be a tree structure that results in the SISO having a latency of O(log2 N), where N is a block size, a Brent-Kung t