Introduction

Nowadays, 50% of phone calls and 70% of data services are carried out indoors [1]. For this reason, one may expect that operators' networks are optimized to provide good indoor coverage and capacity for voice, video, and high-speed data services. However, surveys have shown that 45% of households and 30% of businesses experience poor indoor coverage [2]. This poor indoor coverage may lead to reduced subscriber loyalty and increased subscriber churn, which may significantly affect operators' revenues. As a consequence, vendors and operators are developing new solutions to address the indoor coverage problem.

A straightforward solution to enhance indoor coverage would be to increase the number of outdoor macrocell base stations (MBSs). Deploying a larger number of MBSs with a reduced cell radius may provide improved network coverage and capacity, but this approach is too expensive due to the high cost associated with MBSs. Moreover, this approach presents challenges in terms of site acquisition due to municipality and people's concerns about MBS towers [3]. It is also very difficult to achieve high indoor signal quality when providing coverage from outdoors due to wall attenuation losses. Therefore, providing indoor coverage from outdoors is not the best solution.

As a result, indoor solutions such as distributed antenna systems (DASs) and picocells have become attractive alternatives to provide services in indoor hotspots, e.g., shopping malls and office buildings. These operator-deployed solutions improve in-building coverage, enhance signal quality, offload traffic from outdoor MBSs, and allow high-data-rate services due to the fact that transmitters are closer to receivers.