Achieving the Welch bound with difference sets

Consider a codebook containing N unit-norm complex vectors in a K-dimensional space. In a number of applications, the codebook that minimizes the maximal cross-correlation amplitude (I_max) is often desirable. Relying on tools from combinatorial design theory, we construct analytically optimal codebooks meeting, in certain cases, Welch's lower bound. When analytical constructions are not available, we develop an efficient numerical search method based on Lloyd's algorithm, which leads to considerable improvement on the achieved I_max over existing alternatives. We also derive a composite lower bound on the minimum achievable I_max that is effective for any N.