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The topology and architecture of cellular networks are undergoing a major paradigm shift from voice- centric, circuit switched and centrally optimized for coverage towards data-centric, packet switched and organically deployed for capacity. Current cellular systems in the world, especially in Pakistan, are homogenous in the sense that there is a base station that serves a specified larger area (in kilometers) and then an adjacent base station serves the adjacent area. This macro-cellular system is deployed in most parts of the world since 3 last decades. However, a disadvantage of the macro-cellular system is that in remote areas or some typical terrains where the base-station’s signal cannot reach the end-user, the probability of outage (i.e., a user without signal coverage) increases. This outage behavior calls for an immediate solution to serve the remote areas and users without coverage. A foremost solution is to deploy another base station to solve the problem. However, deploying the new base station would require a lot of infrastructure and cost, which is undesirable. An alternative to the problem is the introduction of femtocells.
Femtocells are small indoor base stations, which are used for enhancement of wireless coverage. This technology is in the developmental stage and is relatively new. Femtocells connect the mobile station with the mobile operator by using the internet. In this way it helps in capacity offload from the macro base stations as well as coverage enhancement of wireless signals. Specific applications of femtocells are in the scenarios where the user is in outage and does not receiver the appropriate power from the macro base station. Another advantage of femtocells is the enhancement of data rates received by mobile user inside the buildings.
An inherent problem in the femtocells is the interference. This interference for a femto-user from a macro base station or for a macro-user from a femto base station is unavoidable in various scenarios. The introduction of femtocells fundamentally alters the cellular topology by creating an underlay of small cells, with largely random placements and possible restrictions on access to certain base stations. Precise characterizations of the interference conditions in such heterogeneous and multi-tier networks have been the subject of extensive study. Two types of femtocells are commonly deployed: a) closed-access and b) open-access. In closed-access mode, an external user is not allowed to connect to a femtocells, while the opposite holds true in the open-access mode. This proposal deals with the interference alignment/avoidance/mitigation in downlink and uplink of femtocells, which are operating in either the closed mode or open mode.
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