Research On Spectrum Sharing Techniques For Femtocell-Enhanced Cellular Networks

Femtocell technique can solve the problems of indoor coverage and low capacity faced by the existing cellular communication systems through installing an access point (AP) sharing the same spectrum resource with Macrocell in subscribers' homes. However, spectrum sharing may induce serious mutural interference between Femtolls and Macrocell as well as among Femtocells, hence the performances of both Femtocells and Macrocell are restricted.This thesis studies the spectrum sharing techniques between Femtocell and Macrocell so as to achieve a good system performance.The thesis analyzes the relationships between overall average system capacity, average Femtocell capacity as well as average Macrocell capacity and the number of the Femtocells that have been accessed. Analysis results show that through proper channel selection, Femtocell can improve the overall system capacity significantly without an obvious impact on the uplink and downlink capacities of Macrocell.For the Femtocells to be introduced into the existing cellular communication systems, the spectrum sharing techniques should be compatible with the existing standards. A novel algorithm is proposed for AP to estimate independently the path loss between itself and WiMAX base station as well as users. This algorithm does not need any extra information or information exchange except those already in IEEE 802.16e standard, and will not produce any influence on the existing communications in Macrocell. Simulation results show that the path loss estimation algorithm has a very good performance.A set of mechanisms compatible with IEEE 802.16e standard is proposed, including channel selection, power allocation and access control. Utilizing the path loss estimates, AP assesses the interference intensities it may produce to the Macrocell users respectively. An access probability is then calculated for each occupied channel based on the assessed intensity, so that AP tends to access into a channel that the Femtocell has little influence on the co-channel Macrocell user. According to the channel quality information sent by Macrocell users, base station (BS) estimates the interference intensity suffered by each Macrocell user, and then decides either to accept or refuse further access requests from Femtocells.In the future cellular communication systems accommodating Femtocell as a basic component, certain amount of information exchange is permitted in order to achieve a better overall system performance. The spectrum sharing problem is formulated in this thesis as an optimization problem, which is simplified after exploiting the characteristics of Femtocell network and removing the coupling between Femtocells and Macrocell. A low complexity dynamic spectrum access and power allocation algorithm for Femtocells is then proposed by solving this simplified problem with the Lagrangian method. The algorithm only requires a little information exchange between Macrocell users and BS as well as BS and Femtocells.