  Soft-input soft-output (SISO) detection algorithms form the basis for
iterative decoding. The associated computational complexity often poses
significant challenges for practical receiver implementations, in particular in
the context of multiple-input multiple-output wireless systems. In this paper,
we present a low-complexity SISO sphere decoder which is based on the single
tree search paradigm, proposed originally for soft-output detection in Studer
et al., IEEE J-SAC, 2008. The algorithm incorporates clipping of the extrinsic
log-likelihood ratios in the tree search, which not only results in significant
complexity savings, but also allows to cover a large performance/complexity
trade-off region by adjusting a single parameter.
