Codebooks in Flag Manifolds for Limited Feedback MIMO Precoding

We consider unitary codebook design problems for various precoded MIMO scenarios, and interprete them as generalized discretization problems of flag manifolds. As a concrete example, we consider codebooks for MIMO transmission with linear receivers. In this case, the problem reduces to discretizing permutation-invariant flag manifolds. A corresponding Lloyd algorithm is given, providing low-distortion codebooks. The achievable rate of the system applying the generated codebooks is compared by simulations. We find that for a N-by-N full-rank MIMO system with N > 2 and a linear receiver, the gain of precoding with a small number of feedback bits is small. This differs from the behavior of low-rank transmissions, where it is known that a small number of feedback bits allows near-optimal channel adaptation.