Offset-Based Beamforming: A New Approach to Robust Downlink Transmission

Central Publication

• Mostafa Medra, Yongwei Huang and Timothy N. Davidson.
  Offset-Based Beamforming: A New Approach to Robust Downlink Transmission
  submitted

Abstract

• The design of a set of beamformers for the multiuser multiple-input single-output (MISO) downlink that provides the receivers with prespecified levels of quality-of-service (QoS) can be quite challenging when the channel state information is not perfectly known at the base station. The constraint of having the SINR meet or exceed a given threshold with high probability is intractable in general, which results in problems that are fundamentally hard to solve. In this paper, we will develop a highquality approximation of the SINR outage constraint that, along with a semidefinite relaxation, enables us to formulate the beamformer design problem as a convex optimization problem that can be efficiently solved. For systems in which the uncertainty size is small, a further approximation yields algorithms based on iterative evaluations of closed-form expressions that have substantially lower computational cost. Since finding the beamforming directions incurs most of the computational load of these algorithms, analogous power loading algorithms for predefined beamforming directions are developed and their performance is shown to be close to optimal. When the system contains a large number of antennas, the proposed power loading can be obtained at a computational cost that grows only linearly in the number of antennas. The proposed power loading algorithm provides an explicit relationship between the outage probability required and the power consumed, which allows us to precisely control the power consumption, and automatically identifies users who are consuming most of the power resources. The flexibility of the proposed approach is illustrated by developing a power loading technique that minimizes an average notion of outage.

Paper

• The preliminary version of the paper is available on arXiv

Other references

• This work was inspired, in part, by earlier work that generated the "constant-offset" approach

• A preliminary version of the ideas in this paper was developed for the case of power loading in a paper presented at the Asilomar Conference in 2015. A discussion of that paper is available here

• A preliminary extension of that power loading idea to the case of full beamformer design was presented at the 2016 IEEE Workshop on Sensor, Array and Multichannel Signal Processing. That paper is available on IEEEXplore: http://doi.org/10.1109/SAM.2016.7569674.

• Adaptations of the ideas in the paper to the case of "Massive MIMO" systems was presented at the Asilomar Conference in 2016. A discussion of that paper is available here.

• A variation on the ideas in this paper that enables the incorporation of per-antenna power constraints is available here.

• A preliminary extension of some of the ideas in this paper to the case of the multi-cell downlink is available here.

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