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International Journal of Antennas and Propagation
Volume 2015, Article ID 840142, 11 pages
Research Article

Outage-Constrained Beamforming for Two-Tier Massive MIMO Downlink with Pilot Reuse

1The State Key Laboratory of Advanced Optical Communication Systems and Networks, Peking University, Beijing 100871, China
2Department of Electrical and Computer Engineering, Hong Kong University of Science and Technology, Hong Kong

Received 20 August 2014; Revised 14 February 2015; Accepted 22 February 2015

Academic Editor: Ahmed T. Mobashsher

Copyright © 2015 Guozhen Xu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Massive multiple-input multiple-output (MIMO) systems and small cell networks are both regarded as promising candidates to meet the exponential growth of mobile data traffic for the next generation (5G) wireless communications. Hence, a new kind of multitier networks which combine massive MIMO macro cells with a secondary tier of small cells is proposed to resolve the contradiction of large network coverage and high data rate. In such multitier networks, it is inevitable to allocate nonorthogonal uplink pilot sequences to user equipment (UE) due to the large number of users. We propose a pilot reuse scheme by exploiting the unique architecture of this networks and analyse the special mixed channel state information (CSI) yielded by the pilot reuse scheme. Based on the mixed CSI, we formulate a downlink transmit beamforming problem of minimizing the total power consumption while satisfying the quality of service (QoS) requirements with outage constraints. After decomposing the original problem into simpler subproblems, we provide an efficient algorithm to combine these subproblems and solve them iteratively for generating the beamforming vectors. Monte Carlo simulations show that the average power consumption of the proposed pilot reuse scheme and its associated beamforming algorithm is close to that of the perfect CSI case.