MIMO transmit beamforming under uniform elemental power constraint

We consider multi-input multi-output (MIMO) transmit beamforming under the uniform elemental power constraint. This is a non-convex optimization problem, and it is usually difficult to find the optimal transmit beamformer. First, we show that for the multi-input single-output (MISO) case, the optimal solution has a closed-form expression. Then we propose a cyclic algorithm for the MIMO case which uses the closed- form MISO optimal solution iteratively. The cyclic algorithm has a low computational complexity and is locally convergent under mild conditions. Moreover, we consider finite-rate feedback methods needed for transmit beamforming. We propose a novel vector quantization method, where the codebook is constructed under the uniform elemental power constraint and the method is referred as VQ-UEP. Numerical examples are provided to demonstrate the effectiveness of our proposed transmit beamformer designs and the finite-rate feedback technique.