Nonuniform true time delay precoding in wideband MISO systems.

We focus on true time delay (TTD) precoding which is a reduced hardware complexity alternative to fully-digital frequency selective beamforming in wide band systems. It is challenging to select the delays for TTD beamforming architectures for a channel with multi-path and near-field effects because of the nonuniform delay variation across the array. We design the TTD beamformer by maximizing the total energy in the filtered channel response. Based on the idea of temporal focusing, we reformulate the TTD constraint by including a sparsity regularizer penalty term in the objective. We then propose a procedure to compute the delays using the precoder response obtained after solving a sparse eigenvalue problem for maximizing the total energy. Incorporating sparsity in the problem formulation enables us to obtain a response that closely resembles the TTD precoder response in time domain. The proposed approach outperforms the time-reversal precoder based TTD design and the phase-shifter implementation in terms of the wideband mutual information.