IEEE SYSTEMS JOURNAL, VOL. 6, NO. 2, JUNE 2012 329 High-Capacity OFDMA Femtocells by Directional Antennas and Location Awareness Ang-Hsun Tsai, Student Member, IEEE, Li-Chun Wang, Fellow, IEEE, Jane-Hwa Huang, Member, IEEE, and Ruey-Bing Hwang, Senior Member, IEEE Abstract —In this paper, we propose a location-aware mecha- nism combined with a low-cost four-sector switched-beam direc- tional antenna to enhance the spectrum efﬁciency of orthogonal frequency-division multiple access (OFDMA)-based femtocell systems. The considered location-awareness capability is speciﬁed in the current IEEE 802.16m WiMAX standard, but has not been applied to avoid the interference between indoor femtocells and outdoor macrocells. With the knowledge of the locations of outdoor users, the proposed four-sector switched-beam antenna in a femtocell can effectively avoid the interference among femtocells and macrocells by adjusting the number of OFDMA subcarriers used at each femtocell. Numerical results show that the proposed approach can signiﬁcantly improve spectrum efﬁciency compared to the existing methods. Index Terms—Closed subscriber group (CSG), femtocell, four-sector switched-beam directional antenna, link reliability, location awareness, open subscriber group (OSG), orthogonal frequency-division multiple access (OFDMA), spectrum efﬁciency, two-tier interference. I. Introduction F EMTOCELLS can improve system capacity and indoor coverage with low power and low cost [1]–[4]. Un- like base stations in conventional cellular systems, femto- cells connect to the network center through the broadband wirelines inside the customers’ homes. Due to short trans- mission distance, femtocells require very low transmission power. From the operator’s viewpoint, deploying femtocells can also signiﬁcantly improve system capacity because the same spectrum can be repeatedly used by a huge number of other femtocells. Femtocells can also off-load the trafﬁc of the outdoor macrocell users toward the indoor femtocell base station (fBS). From the customers’ perspective, the fBS Manuscript received June 30, 2011; revised November 2, 2011; accepted November 7, 2011. Date of publication January 10, 2012; date of current version May 22, 2012. This work was supported in part by the National Science Council (NSC) of Taiwan, under Research Grant NSC 100-2221-E- 009-103-MY3, and in part by the Industrial Technology Research Institute, Information and Communication Research Laboratories, Taiwan. Part of this work was presented at the IEEE Wireless Communications and Networking Conference, 2010. A.-H. Tsai, L.-C. Wang, and R.-B. Hwang are with the Department of Electrical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan (e-mail: anghsun@gmail.com; lichun@cc.nctu.edu.tw; raybeam@ mail.nctu.edu.tw). J.-H. Huang is with the Department of Electrical Engineering, National Chi Nan University, Nantou 545, Taiwan (e-mail: jhhuang@ncnu.edu.tw). Color versions of one or more of the ﬁgures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/JSYST.2011.2178724 is much closer than the outdoor macrocell base station (mBS), thereby improving indoor signal quality. However, the serious two-tier interference in femtocells is the key challenge [2]–[5]. After femtocells are densely deployed, both the femto-to-femto and the macro-to-femto interference occur. In the meanwhile, the macrocell users undergo the interference from the femtocells and macrocells as well. The severe two-tier interference signiﬁcantly affects the system capacity because the subcarriers available for the femtocells are reduced. In the literature, most studies on femtocells considered the omnidirectional femtocells and can be categorized into three kinds: power control, access method, and spectrum allocation. In [1] and [6], the power control method for femtocells with omnidirectional antenna were discussed. In [1], an autoconﬁg- uration method of transmit power was proposed for the code division multiple access (CDMA)-based femtocells with the shared spectrum allocation scheme. In [6], the authors pro- posed a distributed femtocell uplink power control to achieve higher signal-to-interference plus noise ratio performance on the condition that the link quality of the macrocell users can be guaranteed. Access methods for femtocells were discussed in [3], [7], and [8]. In [3], the authors compared the closed subscriber group (CSG) and open subscriber group (OSG) femtocell systems, where the CSG fBSs only serve the authorized users, and the OSG fBSs can serve any users. Because the OSG users can select the base station with best signal quality among the femtocells and macrocells, the OSG scheme has better coverage than the CSG scheme. In [7], the authors studied different access methods for femtocells and proposed a hybrid access approach to improve the average throughput. In [8], the authors investigated the effects of access methods in the uplink femtocell systems. They suggested that the access method depend on the cellular user density in the time division multiple access/orthogonal frequency-division multiple access (OFDMA) network, and showed that open access is the optimal choice in the CDMA network. In [9]–[14], spectrum allocation schemes for femtocells with omnidirectional antenna were discussed. In [9], the channel selection issues in the WiMAX-based femtocell systems was investigated by taking into account of the femto-to-femto inter- ference. In [11], distributed channel selection schemes for fem- tocells were developed to improve the capacity by adjusting the number of used subcarriers. In [10], the authors analyzed 1932-8184/$31.00 c  2012 IEEE