Cell selection in two-tier microcellular/macrocellular systems

Because hand-off limitations restrict the deployment of microcells to slow-moving mobiles, a two-tier cellular system is considered to cover areas with mobiles moving at different speeds. Such systems consist of microcells embedded within macrocells. The microcells cover the slow-moving mobiles and the macrocells the rest. The selection of a cell to serve a call must now be based not only on the received signal strength, but also on the speed of the mobile. A simple procedure for cell selection had been proposed, which prevents the microcellular system from carrying the full call traffic load it is capable of. We calculate this capacity loss and find it to be 43 per cent for pedestrian-held mobiles moving at an average speed of 5 km/hr inside a 600-meter microcell. The capacity loss is higher for faster moving mobiles. We propose an alternative procedure for cell selection, which preserves the simplicity of the original procedure but causes lower capacity losses. The case of a mobile changing speed is also addressed.