Hybrid frequency allocation scheme for capacity improvement in densely deployed small cells

Zhang et al. EURASIP Journal on Wireless Communications and Networking (2015) 2015:152 DOI 10.1186/s13638-015-0387-1 R ESEA R CH Open Access Hybrid frequency allocation scheme for capacity improvement in densely deployed small cells Qixun Zhang* , Zhiyong Feng, Yue Zhang and Tuo Yang Abstract Driven by the demands for better user experience and high data-rate service, there is an ever increasing trend for capacity enhancement for wireless networks. The densely deployed small cells is a promising solution which can provide a huge capacity gain and improve the user experience with high data-rate services. Due to the random deployed manner of the customers, small cells should have automatic parameters an tuning and optimization abilities to realize the self-deployment in heterogeneous networks. Therefore, a novel dynamic hybrid frequency allocation scheme is proposed by applying both inner and outer circle regions for different frequency allocation schemes for small cells, which can improve the capacity and minimize interference among different hierarchical networks. Closed-form solutions are achieved for proposed downlink capacity model and key parameters affecting the capacity are also analyzed thoroughly. Furthermore, an optimal geographic region division scheme is designed and the optimal square zone length is theoretically obtained for an efficient information delivery among small cells. Numerical results verify that the capacity can be improved with the appropriate small cell deployment using proposed novel schemes. Keywords: Small cell; Self-deployment; Femtocell; Heterogeneous networks 1 Introduction According to recent studies, 50 % of phone calls and 70 % of data services will take place indoors in the coming years [1]. Moreover, over two thirds of voice services and about 90 % of data services will take place indoors [2]. Studies also show that more than 45 % of households and 30 % of businesses users experienced inadequate indoor coverage [3], leading to the poor quality-of-service (QoS). Furthermore, Cisco forecasts that the global mobile data traffic grew 81 % in 2013 and smarter mobile devices are increasing fast in [4]. Moreover, the uneven traffic distribution leads to new challenges for the coverage optimization and capacity enhancement. But traditional wireless network planning and optimization techniques cannot guarantee both a wide range coverage and an effective service quality indoors, leading to the expansion of network capacity as a fundamental problem for mobile network operators. *Correspondence: Key Laboratory of Universal Wireless Communications, Ministry of Education, Information and Telecommunication Engineering of Beijing University of Posts and Telecommunications (BUPT), Xitucheng Road, Beijing, Haidian District, 100876, P.R. China Traditional network capacity improvement techniques such as cell splitting and adding more spectrum cannot meet the demands of capacity enhancement, uneven traffic distribution, and dynamic changing services. Besides, due to the building blocking, signal propagation loss, and reflection effect, capacity holes in hot spots and deteriorated user experiences are still big challenges [5]. Therefore, how to make full use of heterogeneous networks resources, how to decrease the complexity of resource management, and how to improve network capacity and user experiences indoors are still problems unsolved. Traditional network planning and optimization techniques face the problems of high implementation cost, long deployment period, and complex optimization process. Therefore, the self-deployed small cells, such as femtocell, picocell, and microcell, for indoor capacity enhancement and coverage optimization are proposed recently and considered as one of the efficient solutions in [6, 7]. As one of the efficient capacity enhancement techniques, small cells are designed to operate on the licensed bands for both indoor and outdoor scenarios in [3] and [8]. Moreover, small cells can provide a fast, flexible, and © 2015 Zhang et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. Zhang et al. EURASIP Journal on Wireless Communications and Networking (2015) 2015:152 cost-efficient solution for existing cellular networks in [9]. Small cells include femtocells, picocells, microcells, and metrocells [10]. Small cells can be easily deployed in hotspots and indoor scenarios to improve the capacity and user experience. However, challenges of small cell deployment still exist, including interference analysis among different heterogeneous networks, frequency allocation scheme, and access mode selection for small cells. In the literature, existing research works on capacity analysis of small cells are studied by considering different frequency allocation schemes and interference issues. In terms of uplink capacity analysis in hierarchical networks, both the closed subscriber group (CSG) access mode and open access mode are considered in orthogonal frequency division multiple access (OFDMA) and time division multiple access (TDMA)-based femtocell networks in [8]. Furthermore, the macrocell users’ density is proposed as a key factor to the optimal selection of CSG and open access modes, where the open access mode is preferred when macrocell users’ density is small. Moreover, open access mode is applied in code division multiple access (CDMA)-based femtocell networks in [11]. A distributed orthogonal frequency allocation scheme is proposed by using the optimal frequency allocation ratio between femtocell and macrocell networks in [12] to minimize the interference among different layers in hierarchical networks. In [13], both the joint and disjoint subchannel allocation schemes are proposed for two-tier networks with quality-of-service constraints in terms of success probabilities and per-tier minimum rates. Considering the frequency allocation in hierarchical networks, a hybrid orthogonal frequency allocation scheme is proposed in [14], which considers about the distance between femtocell and macrocell and the interference constraint area (ILCA) factor. In terms of interference problems among OFDMAbased hierarchical networks, an intelligent self-organized femtocell network based on real-time multi-agent reinforce learning technology is proposed in [15] by using the accumulated interference from different femtocell networks to solve the interference problems. Moreover, a docition-based real-time scheme is also proposed in [15] to improve the learning ability and accuracy of Q-learning process, which solves the unstable decisionmaking problem and the drawbacks of complex learning process and slow learning speed. Considering the uplink capacity and interference cancelation problems in CDMA-based hierarchic (...truncated)