APP processing for high performance MIMO systems [receiver symbol detector]

One of the most demanding components in a multiple-input multiple-output (MIMO) receiver is the symbol detector. While linear cancellation techniques are typically used, the optimum maximum likelihood (ML) search algorithm combined with soft output processing, e.g. a posteriori probability (APP), offers superior performance and is feasible in current technology. An ASIC can be built for a 4×4 QPSK detector for HSDPA in less than 1.68 mm² in a 0.18 μm process. An MLAPP search grows exponentially as the constellation size is increased, but it is still feasible to achieve near ML performance for 4×4 16QAM using "spherical" techniques in a complexity that remains a minor portion of the overall MIMO receiver.