Reduced complexity joint detection generalized RAKE receiver for WCDMA MIMO systems

We develop a technique for significantly reducing the complexity of soft-value generation for a joint detection generalized RAKE (JD-GRAKE) receiver. The complexity of the JD-GRAKE receiver, developed previously for multiple-input, multiple-output (MIMO) WCDMA systems, becomes unmanageable when combined with higher-order modulation (e.g., 16-QAM) and a large number of transmit antennas (four or more). The proposed technique for the simplified receiver is based upon the m-algorithm, which operates on a tree (rather than trellis), enumerating all possible values of the transmit symbol vector. It is shown that, with the m-algorithm, the number of retained branches may be significantly reduced compared to an exhaustive search. For example, with 16-QAM and 4 transmit antennas where the number of branches in the tree is 65,536, the number of retained branches may be as few as 32 while still achieving performance very close to the full complexity receiver.