Analysis of macro — femtocell interference and implications for spectrum allocation

Deployment of femtocells represents a promising solution to increase cost-capacity benefits for network operators and provide higher data rates to end-users. Femtocells are conceived to provide indoor wireless access to a cellular network through a Home Base Station, which is connected via internet to the operator's core network, helping to improve coverage in indoors, offload the macrocell and reduce costs for operators. However, large scale deployment of femtocells can severely interfere with the existing macrocell within which they are deployed, particularly when operating in co-channel or in immediate adjacent channels with respect to the macrocell and when using a closed access policy. For instance, macrocell coverage holes in the downlink will appear, i.e. zones in the vicinity of a home base station where interference from home base station signals will prevent macrocell users to receive the desired service from the macrocell network. In this paper, we estimate macrocell coverage holes produced by closed access femtocells when both, macro and femto networks operate FDD WCDMA-based technologies. We investigate the case when the home base station transmits at constant power and when uses a power adjustment mechanism, and for different values of separation in the frequency domain. Results show that coverage holes at the edge of the macrocell can have a radius up to 30 meters when the same carrier is used and up to 5 m when carriers separated 5 MHz are used. Finally, we analyze and discuss how coverage and spectrum allocation are influenced by varying the carrier separation.