Branch Processing of Search Tree in a Sphere Decoder

Sphere decoding of signals for MIMO detection involves a first distance processor arranged to determine a distance between symbols of a received symbol vector and possible transmit symbols, in a search sequence according to a search tree. A line from root to leaf of the search tree represents a possible transmit symbol vector and the sequence of processing root level symbols can be ordered with respect to increasing distance without explicit sorting of the root level symbols based on distance calculations. A next symbol in the sequence is determined, based on a symbol currently being processed by the first distance processor, and a second distance processor determines the distance in respect of the next symbol in the search sequence for the same possible transmit symbol vector, in parallel with the determining of the distance in respect of the current symbol by the first distance processor. By looking ahead to the next symbol in the sequence and determining its distance in parallel, without waiting to see if the sequence jumps away from that next symbol, the processing can be speeded up or use less resources such as silicon area on an integrated circuit.