Interference mitigation in cognitive femtocell networks

“A thesis submitted to the University of Bedfordshire, in partial fulfilment of the requirements for the degree of Doctor of Philosophy”.Femtocells have been introduced as a solution to poor indoor coverage in cellular communication which has hugely attracted network operators and stakeholders. However, femtocells are designed to co-exist alongside macrocells providing improved spatial frequency reuse and higher spectrum efficiency to name a few. Therefore, when deployed in the two-tier architecture with macrocells, it is necessary to mitigate the inherent co-tier and cross-tier interference. The integration of cognitive radio (CR) in femtocells introduces the ability of femtocells to dynamically adapt to varying network conditions through learning and reasoning. This research work focuses on the exploitation of cognitive radio in femtocells to mitigate the mutual interference caused in the two-tier architecture. The research work presents original contributions in mitigating interference in femtocells by introducing practical approaches which comprises a power control scheme where femtocells adaptively controls its transmit power levels to reduce the interference it causes in a network. This is especially useful since femtocells are user deployed as this seeks to mitigate interference based on their blind placement in an indoor environment. Hybrid interference mitigation schemes which combine power control and resource/scheduling are also implemented. In a joint threshold power based admittance and contention free resource allocation scheme, the mutual interference between a Femtocell Access Point (FAP) and close-by User Equipments (UE) is mitigated based on admittance. Also, a hybrid scheme where FAPs opportunistically use Resource Blocks (RB) of Macrocell User Equipments (MUE) based on its traffic load use is also employed. Simulation analysis present improvements when these schemes are applied with emphasis in Long Term Evolution (LTE) networks especially in terms of Signal to Interference plus Noise Ratio (SINR)

A serious gaming approach to managing interference in ad hoc femtocell wireless networks

The aim of this paper is to optimize femtocell performance by managing interference between femtocell devices and between a femtocell and a macrocell. It achieves this using a three-phase approach that involves deployment of femtocells and control of resulting connections through consideration and management of path loss, transmission power, signal strength and coverage area. Simulation experiments of the proposed three-phase approach at a local college that experiences a poor service from the macrocell predict significant improvements in femtocell performance in terms of managing both types of interference: co-tier and cross-tier, number of users who experience good service, coverage, and mitigating outage probability. The overall and individual complexity of each phase has also been considered. Our approach has been compared with some existing techniques chosen from the literature that has been reviewed and its predicted performance is significantly improved in comparison to these

Efficacy of coverage radius-based power control scheme for interference mitigation in femtocells

A novel coverage radius-based downlink power control scheme to mitigate interference in densely deployed femtocells is presented. A femtocell access point (FAP) self-update algorithm is implemented, which determines the coverage radius of the femtocell with respect to its farthest served femtocell user equipment (FUE). Based on varying coverage radii, a max/min function is used to adjust the downlink transmit power value of a FAP. System-level simulations are performed to compare the performance of the presented scheme with the existing fixed coverage radius schemes. Even though the proposed scheme results in better cross-tier signal to interference plus noise ratio (SINR) values, due to a low co-tier SINR it is found that the efficacy of adaptive power control schemes based on the pilot power of a FAP is less significant if FUEs are located close to the neighbouring FAPs in densely deployed urban femtocells

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 co-exist 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

Threshold power based UE admittance and contention free resource allocation for interference mitigation in cognitive femtocells

Femtocells are aimed at providing strong coverage in the indoor area where typical macrocell coverage is very poor. It has hugely attracted network operators and stakeholders mainly due to its simple plug and play operation and low cost. Femtocells operate on a much lower power compared to macrocell and thus provide a number of benefits including energy efficiency and frequent spatial reuse of the spectrum. Femtocells are overlaid on macrocells and designed to co-exist with them sharing the same spectrum pool. However, since they are deployed by the end user, no pre-deployment resource planning is possible. So, interference among the femtocells as well as between femtocells and macrocells remain a major bottleneck for successful operation of femtocell networks. This paper proposes a novel threshold power based admittance and contention free resource allocation for interference mitigation in cognitive femtocell networks. In our proposed scheme, a Femtocell Access Point with Cognitive radio capability known as Cognitive Femtocells (CF), sets a threshold value on the mutual interference between itself and a close by macrocell user equipment (MUE). To mitigate cross-tier interference, a CF classifies MUEs with higher than threshold interference value as Undesirable MUEs (UMUEs) and subsequently admits it as one of its user equipment. MUEs with lower than threshold interference values are classified as Desirable MUEs (DMUEs). To mitigate co-tier interference, proposed scheme introduces a scheduling engine which employs matching policy attributes and assigns resource blocks (RBs) of unique DMUEs to CFs to avoid any possible contention problems, thus providing improved co-tier interference. System level simulations have been performed to demonstrate effectiveness of scheme and significant performance improvement in terms of SINR, throughput and spectrum efficiency