Robust Beamforming Design for Ultra-Dense User-Centric C-RAN in the Face of Realistic Pilot Contamination and Limited Feedback

Cunhua Pan,Maged Elkashlan,Hong Ren,Arumugam Nallanathan,Lajos Hanzo

doi:10.1109/twc.2018.2882442

Abstract

The ultra-dense cloud radio access network (UD-CRAN), in which remote radio heads are densely deployed in the network, is considered. To reduce the channel estimation overhead, we focus on the design of robust transmit beamforming for user-centric frequency division duplex UD-CRANs, where only limited channel state information (CSI) is available. Specifically, we conceive a complete procedure for acquiring the CSI that includes two key steps: channel estimation and channel quantization. The phase ambiguity (PA) is also quantized for coherent cooperative transmission. Based on the imperfect CSI, we aim to optimize the beamforming vectors in order to minimize the total transmit power subject to the users' rate requirements and fronthaul capacity constraints. We derive the closed-form expression of the achievable data rate by exploiting the statistical properties of multiple uncertain terms. Then, we propose a low-complexity iterative algorithm for solving this problem based on the successive convex approximation technique. In each iteration, the Lagrange dual-decomposition method is employed for obtaining the optimal beamforming vector. Furthermore, a pair of low-complexity user selection algorithms is provided to guarantee the feasibility of the problem. The simulation results confirm the accuracy of our robust algorithm in terms of meeting the rate requirements. Finally, our simulation results verify that using a single bit for quantizing the PA achieves good performance.

Highlights

U LTRA dense networks (UDNs), where more and more small base stations (BSs) are deployed within a given area, have been widely regarded as one of the most promisingManuscript received October 25, 2017; revised September 17, 2018; accepted November 15, 2018
2) To address the feasibility issue, we provide a pair of low-complexity user selection algorithms, namely the successive UE deletion method having a complexity order of O(K) and a bisection based search method having a complexity order of O(log2(K)), where K is the total number of users
The channels are generated according to the LTE specifications [38], which are composed of three elements: 1) the large-scale path loss given by P L = 148.1 + 37.6log10d, where d is the distance between a RRH and a UE in km; 2) the log-normal shadowing fading having a zero mean and 8 dB standard deviation; 3) small-scale Rayleigh fading with zero mean and unit variance

Summary

Introduction

U LTRA dense networks (UDNs), where more and more small base stations (BSs) are deployed within a given area, have been widely regarded as one of the most promising. Manuscript received October 25, 2017; revised September 17, 2018; accepted November 15, 2018. Date of publication December 17, 2018; date of current version February 11, 2019.

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Results

Conclusion