Frequency-Selective Hybrid Beamforming Based on Implicit CSI for Millimeter Wave Systems.

Hybrid beamforming is a promising concept to achieve high data ratetransmission at millimeter waves. To implement it in a transceiver, manyreferences optimally adapt to a high-dimensional multi-antenna channel but moreor less ignore the complexity of the channel estimation. Realizing thatreceived coupling coefficients of the channel and pairs of possible analogbeamforming vectors can be used for analog beam selection, we further propose alow-complexity scheme that exploits the coupling coefficients to implementhybrid beamforming. Essentially, the coupling coefficients can be regarded asimplicit channel state information (CSI), and the estimates of these couplingcoefficients yield alternatives of effective channel matrices of much lowerdimension. After calculating the Frobenius norm of these effective channelmatrices, it turns out that the effective channel having the largest value ofthe Frobenius norm provides the solution to hybrid beamforming problem.