Abstract
We propose a data-aided transmit beamforming scheme for the multi-user multiple-input-single-output (MISO) downlink channel. While conventional beamforming schemes aim at the minimization of the transmit power subject to suppressing interference to guarantee quality of service (QoS) constraints, here we use the knowledge of both data and channel state information (CSI) at the transmitter to exploit, rather than suppress, constructive interference. More specifically, we design a new precoding scheme for the MISO downlink that minimizes the transmit power for generic phase shift keying (PSK) modulated signals. The proposed precoder reduces the transmit power compared to conventional schemes, by adapting the QoS constraints to accommodate constructive interference as a source of useful signal power. By exploiting the power of constructively interfering symbols, the proposed scheme achieves the required QoS at lower transmit power. We extend this concept to the signal to interference plus noise ratio (SINR) balancing problem, where higher SINR values compared to the conventional SINR balancing optimization are achieved for given transmit power budgets. In addition, we derive equivalent virtual multicast formulations for both optimizations, both of which provide insights of the optimal solution and facilitate the design of a more efficient solver. Finally, we propose a robust beamforming technique to deal with imperfect CSI, that also reduces the transmit power over conventional techniques, while guaranteeing the required QoS. Our simulation and analysis show significant power savings for small scale MISO downlink channels with the proposed data-aided optimization compared to conventional beamforming optimization.
Highlights
T HE power efficiency of wireless transmission links has recently attracted considerable research interest, in-line with the global initiative to reduce the emissions and operational expenses of communication systems
We further reduce the transmit power required for guaranteeing the SNR constraints in the optimization, improving the power efficiency of transmission
The above concept of constructive interference can be applied to the signal to interference plus noise ratio (SINR) balancing problem of (6)
Summary
T HE power efficiency of wireless transmission links has recently attracted considerable research interest, in-line with the global initiative to reduce the emissions and operational expenses of communication systems. We list the contributions of this paper as follows: 1) We introduce a new linear precoder design for PSK that a) reduces the transmit power for given performance compared to existing precoders based on the proposed constructive interference regions, and b) as opposed to conventional precoders, applies to scenarios with higher numbers of users than transmit antennas, 2) We further adapt this concept to the SINR balancing problem, where higher SINR values compared to the conventional SINR balancing optimization are achieved for given transmit power budgets, 3) We re-cast the optimization problem as a virtual multicast optimization problem based on which we derive the structure of the optimal solution and develop an efficient gradient projection algorithm to solve it, 4) We use the multicast formulation to derive a robust precoding scheme suitable for imperfect CSI with bounded CSI errors. We use the terms beamforming and precoding interchangeably, in-line with the most relevant literature
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