International Journal of Computer Applications (0975 - 8887) Volume 124 - No.14, August 2015 Capacity Enhancement with Joint Subchannel and Power Allocation Scheme for OFDMA Femtocell Networks Arathy Varghese Electronics and Communication Engineering NSS College of Engineering Palakkad, Kerala, India Sudha T Electronics and Communication Engineering NSS College of Engineering Palakkad, Kerala, India ABSTRACT Efficient power and subchannel allocation methods are required for orthogonal frequency division multiple access (OFDMA) based femtocell networks to improve the capacity of the system. This pa- per considers a joint subchannel and power allocation algorithm with capacity maximization for downlink of an OFDMA based femtocell networks. In the proposed algorithm subchannel alloca- tion is first performed based on signal to interference plus noise ratio (SINR) of the channel with equal power distribution. Then for enhancing capacity with optimal power allocation, successive convex approximation (SCA) based power optimization is adopted. The effect of Arithmetic geometric mean (AGM) approximation with SCA on power optimization is also investigated. The optimal power is subsequently distributed by water-filling algorithm. Keywords Capacity enhancement,Convex optimization, Femtocell, Power al- location, SINR, Subchannel assignment, Successive convex ap- proximation 1. INTRODUCTION One of the major challenges for next generation wireless commu- nication systems is to improve the indoor coverage and provide high data rate services to the users in a cost effective manner and at the same time, to enhance network capacity [1]. Since 70% of data traffic and 50% of voice traffic are orginated from the mobile users in the indoor regions, serving those users by indoor systems is necessary which will effectively extend the coverage of mobile communication systems with low cost. The surest way to increase the system capacity of a wireless link is by getting the transmitter and receiver closer to each other, which creates the dual benefits of higher quality links and more spatial reuse. Femtocells are the cost effective alternate solution for the above problem. Femtocells or home base stations are data access points which are installed by home users for getting high data services to indoor regions. Femtocells or Femto Access Points (FAPs) are small, short-ranged (10-30 m) low powered (10-100 mW) access points developed to provide cost effective and high-bandwidth ser- vices in next generation wireless communication systems. One of the efficient method for multiple access is orthogonal frequency division multiple access(OFDMA)[2] which is used in many of the current communication standards. It is also a strong candidate for next generation cellular systems. The most important feature of OFDMA systems is that it can incorporate dynamic RA, which allocates subcarriers, power, and bits to multiple users when the transmitter has the channel state information to all users available. So the concept of OFDMA based femtocell network exploit the benefits of both. Thus an efficient subcarrier and power allocation will improve the performance or throughput of the system. In OFDMA-based femtocell networks, due to flexibility for dy- namic RA orthogonal sub-carriers can be assigned to femtocells and macrocells and OFDMA has been used as the air interface technology by Long Term Evolution (LTE) femtocells, i.e., home evolved Node Bs (HeNBs). The co-tier interference of the net- work is completely avoided with the use of OFDMA scheme [3]. Nonetheless, cochannel deployment implies that an OFDM sub- channel can be shared by UEs from different cells, giving rise to the intercell interference. Further more, there is another source of tech- nical difficulty is that the subchannel assignment problem which involves the allocation of the limited radio frequencies to different UEs in multiple cells. To directly solve this combinatorial problem, approaches with an exponential complexity are required. There- fore, the successful development of any resource allocation scheme for OFDMA based femtocell networks certainly relies upon how one can effectively overcome such a difficult problem. In wireless cellular or ad hoc networks where a variety of subchannel alloca- tion and power control problems can be formulated as optimization problems with a system wide objective, e.g., maximizing the total system capacity, subject to QoS constraints ,e.g., on data rate. The work in [4] solves the resource allocation problem for mul- ticell OFDMA-based network using a noncooperative game theory approach. The study in [5] considers game approach to solve the problem of joint power allocation and subchannel assignment in the downlink of a multicell OFDMA network. Another scheme is an improved iterative waterfilling algorithm, adapted to this mul- ticell scenario. It is noted that all the solutions developed in [6] depend on a central unit to collect and process the complete chan- nel state information. To alleviate the high complexity required by such solutions, [7] proposes a distributed low complexity scheme based on the concept of a reference user. Considering the downlink of an OFDMA network, [8] addresses the problem of maximizing the weighted sum of the minimal UE rates of coordinated cells. A 1