Currently around 70% of mobile usage is inside buildings and some analysts predict that in the next few years around 90% of mobile usage will take place indoors (Paolini, 2011), be it at home, in the office or in public buildings, with the majority of that being in an individual user's home and main office locations. Traditionally such services have been provided on an ‘outside-in’ basis from macrocells, which were originally deployed to provide voice coverage to vehicles travelling along major roads. This conventional architecture is increasingly limited in its ability to meet modern mobile users' needs for reliable indoor mobile service because:
- The increased volume of mobile data use puts a greater load on macrocell capacity to deliver a reliable service.
- User expectations of the minimum data rate that constitutes a viable service are continuing to increase and are dominated by the indoor locations in which they mostly consume services.
- Modern mobile devices have to support a wide range of frequency bands in a small-form factor, reducing their sensitivity and hence increasing the signal strength needed to achieve a given coverage.
- Improved thermal insulation properties of buildings lead to an increase in the use of denser and more conductive external construction materials, including metallized windows, increasing the losses that radio waves encounter in penetrating a building.
- Increased use of high-frequency bands at 2.1 GHz, 2.4 GHz, 2.6 GHz, ...