Low complexity quantization codebooks for CoMP

Coordinated multipoint (CoMP) is an interference mitigation technique in LTE release 10, which exploits base station (BS) cooperation to improve throughput for cell-edge users. An important new feature present in CoMP is that variable numbers of BSs can service a given user. This poses a new problem for beamforming that is not present in single-cell operation: the quantization codebook must support a variable dimension codebooks, with the dimension corresponding to the number of BSs employed. This is a problem that has not appeared in previous releases of LTE. In this paper, we propose a low complexity structured codebook that has linear complexity in both the codebook size and dimension. As such, the variable dimension codebooks are readily accommodated and the codebook can be constructed online as the number of transmitting BSs varies. We also propose a new method to store optimal structured codebooks-in the sense of the Grassmannian criterion-of variable dimension by exploiting properties of the combinatorial designs known as cyclic difference sets. Although the size of the optimal codebooks is limited, our method reduces storage requirements as subsets of the same parameters are used to construct the codebook for each dimension. We show via simulations that our low complexity codebook construction performs comparably with the standard Fourier codebook obtained using an exhaustive search, with only linear complexity in both size and dimension.