Fast codeword selection for limited feedback beamforming multiple-input-multiple-output systems using breadth-first tree search

Codebooks with constant modulus entries are being considered for the limited feedback beamforming multiple-input-multiple-output (MIMO) system to improve the power amplifier efficiency at the transmitter. Assume that one such codebook is known to both the transmitter and receiver. The receiver must determine a target codeword from this codebook and send its index back to the transmitter. A receiver can be implemented with low cost if fast codeword selection algorithm is executed. In this study, the authors utilise the properties possessed by three such codebooks and propose a novel tree search algorithm for fast codeword selection. Compared with two previously reported codeword selection algorithms, our algorithm is of fixed complexity and does find the target codeword. Furthermore, our algorithm for codeword selection is similar to the tree search for MIMO detection. These two similar tree search algorithms for both codeword selection and MIMO detection can be implemented by the same hardware architecture. A baseband receiver with small chip area for the dual-mode (the spatial multiplexing and beamforming transmission modes) MIMO system is possible.