Beamformer Design With Smooth Constraint-Free Approximation in Downlink Cloud Radio Access Networks

Fehmi Emre Kadan,Ali Ozgur Yilmaz

doi:10.1109/access.2021.3063668

Abstract

It is known that data rates in standard cellular networks are limited due to inter-cell interference. An effective solution of this problem is to use the multi-cell cooperation idea. In Cloud Radio Access Network, which is a candidate solution in 5G and beyond, cooperation is applied by means of central processors (CPs) connected to simple remote radio heads with finite capacity fronthaul links. In this study, we consider a downlink scenario and aim to minimize total power spent by designing beamformers. We consider the case where perfect channel state information is not available in the CP. The original problem includes discontinuous terms with many constraints. We propose a novel method which transforms the problem into a smooth constraint-free form and a solution is found by the gradient descent approach. As a comparison, we consider the optimal method solving an extensive number of convex sub-problems, a known heuristic search algorithm and some sparse solution techniques. Heuristic search methods find a solution by solving a subset of all possible convex sub-problems. Sparse techniques apply some norm approximation (ℓ 0 /ℓ 1 ℓ 0 /ℓ 2 ) or convex approximation to make the objective function more tractable. We also derive a theoretical performance bound in order to observe how far the proposed method performs off the optimal method when running the optimal method is prohibitive due to computational complexity. Detailed simulations show that the performance of the proposed method is close to the optimal one, and it outperforms other methods analyzed.

Highlights

In new generation communication systems, the number of devices participating in the network grows exponentially
We consider a circular region in which remote radio heads (RRHs) and user equipments (UEs) are distributed uniformly
The results show that the power difference between SMOOTH CONSTRAINT-FREE APPROXIMATION (SCFA) and the bound is very low for small K values

Summary

Introduction

In new generation communication systems, the number of devices participating in the network grows exponentially. Data rate requirements become challenging to satisfy as the network density increases. Standard cellular systems where a set of user equipments (UEs) are served by a single central base station (BS) have a limited performance due to inter/intra-cell interference. In 5G and beyond, Cloud Radio Access Network (C-RAN) is a candidate solution which uses multi-cell cooperation idea. In C-RAN hierarchy, base stations are simple radio units called remote radio heads (RRHs) which only implement radio functionality such as RF conversions, filtering, and amplifying.

Objectives

Results

Conclusion