In this paper, we consider the problem of achieving max-min fairness amongst multiple co-channel multicast groups through transmit beamforming. We explicitly focus on overloaded scenarios in which the number of transmitting antennas is insufficient to neutralize all inter-group interference. Such scenarios are becoming increasingly relevant in the light of growing low-latency content delivery demands, and also commonly appear in multibeam satellite systems. We derive performance limits of classical beamforming strategies using degrees of freedom (DoF) analysis unveiling their limitations; for example, rates saturate in overloaded scenarios due to inter-group interference. To tackle interference, we propose a strategy based on degraded beamforming and successive interference cancellation. While the degraded strategy resolves the rate-saturation issue, this comes at a price of sacrificing all spatial multiplexing gains. This motivates the development of a unifying strategy that combines the benefits of the two previous strategies. We propose a beamforming strategy based on rate-splitting (RS), which divides the messages intended to each group into a degraded part and a designated part, and transmits a superposition of both degraded and designated beamformed streams. The superiority of the proposed strategy is demonstrated through DoF analysis. Finally, we solve the RS beamforming design problem and demonstrate significant performance gains through simulations.
Bibliographical noteFunding Information:
Manuscript received March 9, 2017; revised June 7, 2017 and August 10, 2017; accepted August 19, 2017. Date of publication August 29, 2017; date of current version November 9, 2017. This work was supported in part by UK EPSRC under Grant EP/N015312/1. This paper was presented in part at the 17th IEEE International Workshop on Signal Processing Advances in Wireless Communications, Edinburgh, U.K., July 2016. The associate editor coordinating the review of this paper and approving it for publication was L. Song. (Corresponding author: Bruno Clerckx.) The authors are with the Communications and Signal Processing Group, Department of Electrical and Electronic Engineering, Imperial College London, London SW7 2AZ, U.K. (e-mail: firstname.lastname@example.org; email@example.com).
© 2017 IEEE.
- Downlink beamforming
- Overloaded transmission
- Physical layer multicasting
- WMMSE approach
ASJC Scopus subject areas
- Computer Science Applications
- Electrical and Electronic Engineering
- Applied Mathematics