940 M. KAWSER, M. ISLAM, K. AHMED, ET AL., EFFICIENT RESOURCE ALLOCATION AND SECTORIZATION FOR FFR IN LTE … DOI: 10.13164/re.2015.0940 APPLICATIONS OF WIRELESS COMMUNICATIONS Efficient Resource Allocation and Sectorization for Fractional Frequency Reuse (FFR) in LTE Femtocell Systems Mohammad T. KAWSER, Mohammad R. ISLAM, Kazi I. AHMED, Mir R. KARIM, Jaeed B. SAIF Dept. of Electrical and Electronic Engineering, Islamic University of Technology, Board Bazar, Gazipur-1704, Bangladesh mkawser@hotmail.com, rakibultowhid@yahoo.com, {ishfaq06, mrejaul, saifsiam}@iut-dhaka.edu Abstract. The Fractional Frequency Reuse (FFR) is a resource allocation technique that can effectively mitigate inter-cell interference (ICI) in LTE based HetNets and it is a promising solution. Various FFR schemes have been suggested to address the challenge of interference in femtocell systems. In this paper, we study the scopes of interference mitigation and capacity improvement. We propose a resource allocation scheme that gradually varies frequency resource share with distance from the eNodeB for both macrocells and femtocells in order to attain better utilization of the resources. This is performed effectively using three layers in the cell. The proposal also employs high number sectors in a cell, low interference and good frequency reuse. Monte-Carlo simulations are performed, which show that the proposed scheme achieves signifi- cantly better throughput compared to the existing FFR schemes. Keywords LTE, Fractional Frequency Reuse (FFR), femtocells, resource allocation 1. Introduction Long term evolution (LTE), based on 3GPP stand- ards, is the latest step in the evolution of cellular services. The ever increasing demand for higher data rates and better quality of services has motivated the consideration of het- erogeneous networks (HetNets) in LTE. We consider Het- Nets that comprise macrocells and femtocells. The femto- cells provide high quality indoor communications with low transmit power and it improves the spectral efficiency at lower cost [1], [2]. The femtocell base station or Home eNodeB (HeNB) can be operated in either open or closed access mode. In open access mode, the HeNB provides services to all subscribers. On the other hand, in closed access mode, the HeNB controls the access of users and provides services only to the users who belong to a closed subscriber group (CSG). Like most literature addressing the interference issues in femtocell systems, we assume that a good number of femtocells are deployed where significant macro coverage already exists. Since femtocells use the same spectrum as do the macrocells, the co-channel interference (CCI) be- tween macrocells and femtocells must be tolerable [3]. Apart from the CCI among neighboring macrocells, there are, in fact, two types of interference concerns in femtocell systems: co-tier interference, which occurs between neigh- boring femtocells and cross-tier interference, which occurs between femtocells and macrocells. These interferences can arise in six possible scenarios as shown below. 1. Femto DL is affected by macro eNB. 2. Femto UL is affected by macro-users. 3. Macro DL is affected by HeNB. 4. Macro UL is affected by femto-users. 5. Femto DL is affected by another HeNB 6. Femto UL is affected by femto-users. Scenario #1 can have substantial interference because macro eNB typically uses much higher power than HeNB. Scenarios #2 and #3 can have substantial interference when the macro-users stay close to the HeNB. Scenario #4 offers substantial interference when there are a large number of femtocells deployed. Then the aggregate UL interference from the large number of femto-users becomes high. Sce- narios #5 and #6 exhibit substantial interference when the femtocells are randomly located, which is more likely in residential deployments. In public or enterprise deploy- ments, femtocells may be carefully located at predesigned places limiting the interference [4]. To mitigate the interference in femtocell systems, various inter-cell interference coordination (ICIC) tech- niques have been suggested. One of the major ICIC tech- niques is fractional frequency reuse (FFR), which also yields better frequency reuse [5–8]. In case of FFR, the cell is divided into different regions and the available frequency band is also divided into several sub-bands. Then different sub-bands are allocated to different regions. Also, different power levels may be used for different sub-bands in differ- ent regions. A FFR deployment mitigating interference in femtocell systems successfully can achieve high spectral efficiency. To address the interference issues effectively, a number of FFR deployment schemes have been sug- gested. The strict FFR [7], soft FFR [9], FFR-3 [6], [10], optimal static FFR (OSFFR) [11] and 3-layer/3-sector FFR