Interference Mitigation in Cognitive-Radio-Based Femtocells

Femtocell architecture involves the use of two separate layers-the macrocell and femtocell layers. In this architecture, the former is the conventional cellular network, whereas the latter incorporates a range of shorter range cells. Femtocells are designed to coexist alongside macrocells, providing spatial frequency reuse, higher spectrum efficiency, and cover areas where macrocells cannot. Femtocells positioned in the macrocell considerably improve the indoor coverage and provide better user experience. However, interference between the two layers is imminent; therefore, ways to manage it must be employed to efficiently avoid problems such as coverage holes in the macrocells. Essential limits of capacity and attainable data rates also mainly depend on the interference faced by a femtocell network. Recently, cognitive radio (CR), which has the ability to sense its environment and accordingly alter its characteristics, e.g., transmission parameters, has been merged with femtocells to exploit the capabilities of the former in the latter. CR-enabled femtocells in a two-tier network can sense the environment and opportunistically allocate both licensed and unlicensed frequency bands to user equipments to avoid interference. This paper examines interference mitigation in femtocells using CR and provides comprehensive survey of different CR-enabled interference mitigation schemes. Presented schemes such as power control, spectrum access, antenna, and joint schemes are classified before they are compared for pros and cons. Finally, tradeoffs and cost of using CR in femtocells are highlighted with some insight into future research issues and challenges.