用于MIMO检测的sphere decoder程序-　 This function implements a soft output sphere decoder.

　 Based on the paper: C. Studer, M. Wenk, A. Burg, and H. Blocskei: "Soft-Output Sphere

　 Decoding: Performance and Implementation Aspects", Asilomar 2006



　 R ... is an upper triangular matrix obtained from the QR decomposition of the MIMO channel H

　 s ... received symbol (column) vector, s=Q^H*y

　 dist_ZF ... Distance for the zero forcing solution

　 symbols_ZF ... ZF solution

　 M ... number of bits encoded in every layer [1 x nTA]

　 symbol_alphabet ... for the demapping [nTA x 2^max(M)], filled with zeros for smaller symbol alphabets

　 bittable ... matrix containing the bits according to the symbol_alphabet [sum(M) x 2^max(M)]