Introduction

In the past decade, the world has witnessed a dramatic increase in the traffic carried by the telecommunication networks. The increasing demand for high-speed Internet services (high-definition TV, video calls, and cloud-based computing) has underpinned the need for further innovation, research, and development in new emerging technologies capable of delivering ultra-high data rates to the end users. The existing radio frequency (RF) wireless spectrum is outstripping the supply, thus leading to spectrum congestion, which needs urgent attention. This is currently motivating what is known as the “tragedy of the commons” paradigm, a situation in which all users without any clear intention to do so will contribute to deplete a common resource, in this case, available spectrum. Such situations arise in high-density scenarios such as sport venues, concerts, airport, emergency situations, etc., where user demands may lead to the dramatic situation of limited access. Current RF-based communications suffer in particular from multipath propagation effects in dense urban environments, which reduce the link availability and its performance. The limited bandwidth of these systems together with the spectrum congestion means that relatively very few high-definition channels can be accommodated in a given area. This problem is more acute for indoor applications where there is a lack of adequate bandwidth to be shared among the large number of users who want a lion’s share of the channel capacity. It is estimated that more than 70% of the wireless traffic takes place in indoor environments (home, office, etc.). Therefore low-cost and highly reliable technologies are required to enable seamless indoor wireless communications. Squeezing more out of RF technologies or using an alternative such as optical technologies are the only two options available.