  A-posteriori probability (APP) receivers operating over multiple-input,
multiple-output channels provide enhanced bit error rate (BER) performance at
the cost of increased complexity. However, employing full APP processing over
favorable transmission environments, where less efficient approaches may
already provide the required performance at a reduced complexity, results in
unnecessary processing. For slowly varying channel statistics substantial
complexity savings can be achieved by simple adaptive schemes. Such schemes
track the BER performance and adjust the complexity of the soft output sphere
decoder by adaptively setting the related log-likelihood ratio (LLR) clipping
value.
