This chapter explains the context for femtocells, including some of the historical background, the alternative small-cell solutions, which have been available for addressing indoor coverage and capacity needs,1 and the unique combination of factors which make femtocells technically viable and commercially attractive at the present time.
First, we need to examine why small cells are increasingly needed in mobile systems.
The fundamental characteristic of cellular radio systems is that an unlimited amount of traffic can be served by a limited bandwidth of spectrum. This is achieved by reusing spectrum increasingly densely as the number of users in a given area increases, and by ensuring that the interference so generated is controlled and managed to avoid detriment to the services experienced by the users. This principle was recognised as early as 1947 in internal work at Bell Laboratories (13).
The most basic method of achieving this spectrum reuse is simply to directly reuse frequencies at base stations which are sufficiently separated from each other for the interference to be minimal. This gives rise to the traditional image of the cellular industry showing hexagons which approximate the coverage area of multiple base stations, with the same numbers/shading representing base stations which share the same frequencies (Figure 2.1).