Codebook design and link level performance of closed-loop 4×2 MIMO in HSDPA

The feedback quantization has a considerable impact on the performance of precoded four branch High Speed Downlink Packet Access (HSDPA) MIMO. As the number of transmit antennas is increased, constructing an efficient quantization scheme becomes more important, and compared to the case of two transmit antennas, more quantization loss needs to be tolerated. In this paper, the performance of four branch dual-codeword single-user MIMO is evaluated assuming two different types of compact codebooks. First of the studied codebooks is similar to the 4-Tx codebook used in Long Term Evolution (LTE) system and the second is a novel multi-granular codebook that tries to address the different short and long term correlation properties of the radio channel. An extension to the multi-granular codebook is also proposed in this paper to further improve the performance. Simulation results are presented assuming cross-polarized antenna configuration. The link performance is compared to Release 7 2×2 MIMO and 4×2 MIMO with singular value decomposition based unquantized feedback. The studies show link throughput gains up to 66% for the LTE codebook and up to 79% for the multi-granular codebook compared to the 2×2 MIMO. The relative gains are highest at low geometries where rank 1 transmission is dominant.