DU LAYOUT_Layout 1 4/19/13 3:36 PM Page 44 NEXT GENERATION COGNITIVE CELLULAR NETWORKS COGNITIVE FEMTOCELL NETWORKS: AN OPPORTUNISTIC SPECTRUM ACCESS FOR FUTURE INDOOR WIRELESS COVERAGE LI HUANG AND GUANGXI ZHU, HUAZHONG UNIVERSITY OF SCIENCE AND TECHNOLOGY XIAOJIANG DU, TEMPLE UNIVERSITY ABSTRACT Advanced) standard has been developed to sup- port higher throughput and better user experi- Femtocells have emerged as a promising solu- ence. Moreover, it is predicted that in the near tion to provide wireless broadband access cover- future a large amount of traffic (about 60 percent age in cellular dead zones and indoor of voice traffic and 90 percent of data traffic) will environments. Compared with other techniques originate from indoor environments (e.g., resi- for indoor coverage, femtocells achieve better dential home and office) [1]. However, mobile user experience with less capital expenditure and cellular networks have a reputation for poor maintenance cost. However, co-channel deploy- indoor coverage due to the penetration losses of ments of closed subscriber group femtocells cause walls. The attenuation is more prominent at coverage holes in macrocells due to co-channel higher frequency in LTE-Advanced. The tradi- interference. To address this problem, cognitive tional cell splitting with denser base station (BS) radio technology has been integrated with femto- deployments cannot improve network capacity cells. CR-enabled femtocells can actively sense and coverage much. This is because the cell split- their environment and exploit the network side ting gains are severely reduced by high intercell information obtained from sensing to adaptively interference. Furthermore, dense macro BS mitigate interference. We investigate three CR- deployments cause high capital expenditure and enabled interference mitigation techniques, operational cost. Hence, it is important to find an including opportunistic interference avoidance, alternative strategy to improve indoor coverage. interference cancellation, and interference align- Femtocells offer a promising solution for ment. Macrocell activities can be obtained with- indoor communications. A femtocell access point out significant overhead in femtocells. In this (FAP) is a low-power, low-cost, short-range, article, we present a joint opportunistic interfer- plug-and-play cellular BS deployed in a residen- ence avoidance scheme with Gale-Shapley spec- tial area or small office. FAPs enhance system trum sharing (GSOIA) based on the interweave capacity and coverage via improving link quality paradigm to mitigate both tier interferences in and enabling spectral reuse. In femtocells, higher macro/femto heterogeneous networks. In this data rates and better coverage are achieved by scheme, cognitive femtocells opportunistically shorter communication distance and less penetra- communicate over available spectrum with mini- tion loss. Furthermore, high cellular capacity is mal interference to macrocells; different femto- reached by efficient spatial spectrum reuse in a cells are assigned orthogonal spectrum resources smaller cell size. Femtocells not only provide bet- with a one-to-one matching policy to avoid intra- ter quality of service (QoS) to indoor users, but tier interference. Our simulations show consider- also diminish site expenditure, maintenance cost, able performance improvement of the GSOIA and power consumption of base stations [2]. scheme and validate the potential benefits of CR- Nowadays femtocells have attracted a lot of enabled femtocells for in-home coverage. attention from both academia and industry. The channel deployment of femtocells in a NTRODUCTION two-tier heterogeneous network has three I options: dedicated-channel deployment, partial- Mobile Internet service has spurred exponential channel-sharing deployment, and co-channel growth in cellular network usage, such as video deployment. Co-channel deployment is more sharing, game playing, and movie downloading. attractive to operators due to low cost and back- The explosion of data traffic volume has been ward compatibility. However, co-channel deploy- further fueled by smartphones and various ments of closed subscriber group (CSG) portable devices. To enhance mobile broadband femtocells create coverage holes in macrocells. access, the Third Generation Partnership Project In order to solve this problem, cognitive femto- (3GPP) Long Term Evolution Advanced (LTE- cells are developed to sense their surroundings 44 1536-1284/13/$25.00 © 2013 IEEE IEEE Wireless Communications • April 2013 DU LAYOUT_Layout 1 4/19/13 3:36 PM Page 45 Specifications Femtocell Picocell DAS Relay Wi-Fi Outdoor: 250 Outdoor: Typical power 10–100 mW mW–2W, indoor: 250 mW–2W, 100–200 mW < 100 mW indoor: < 100 mW Coverage Coverage extension Coverage extension of 20–50 m 150 m 100–200 m range of macro macro Real-time voice and Real-time voice Real-time voice and Real-time voice and Primarily data and Services data and data data data VOIP Deployment Hot spot/office/ In-home/office Hot spot/office Indoor extension In-home/hot spot scenarios tunnel, high-speed train Access mode Closed/open/hybrid Open access Open access Open access Closed/open access DSL/cable/optical Optical fiber or RF Wireless in-band or DSL/cable/optical Backhaul X2 interface fiber links to macrocell out-of-band fiber Peak data rate LTE-Advanced ( 3GPP R10): 1G b/s (DL) 300M b/s(UL) 600 Mb/s (802.11n) Table 1. Comparison of various indoor wireless access technologies. and flexibly adapt their operations to minimize tive alternatives to macrocells, they are still too interference. In the cognitive paradigm, interfer- expensive to be deployed in a house or small ence mitigation approaches can be classified as office. As an energy-saving, cost-efficient small opportunistic interference avoidance, interfer- cellular base station, FAP is suitable for a resi- ence cancellation, and interference alignment. dential home that connects with the service The goal of this article is to study how cogni- provider’s network via home broadband. tive radio (CR)-enabled femtocells mitigate both Wi-Fi is another popular wireless broadband inter-tier and intra-tier interference to improve access technology. A Wi-Fi router is also a plug- indoor coverage in a two-tier heterogeneous net- and-play access point without time-consuming work. We first compare femtocells with other network planning. Wireless LAN (WLAN) main- existing technologies for indoor coverage. Next, ly provides data service and voice over IP (VoIP), we investigate the problem of macrocell coverage while a femtocell realizes any call in real time. holes caused by co-channel CSG femtocells. Then Although the carrier sense multiple access with we study three CR-enabled interference mitiga- collision avoidance (CSMA/CA) mechanism in tion approaches in co-channel deployment. A WLAN is a simple distributed coexistence solu- joint opportunistic interference avoidance scheme tion, it is not robust due to the static resource with Gale-Shapley spectrum sharing (GSOIA) is allocation nature. Because of the selfish coexis- presented to mitigate both intra- and inter-tier tence strategy, their mutual interference acutely interference. Finally, simulations are conducted to increases with widespread deployments of evaluate performance improvement of the pro- WLAN. However, low-cost WLAN is still very posed scheme on interference mitigation. attractive for free spectrum license on the indus- trial, scientific, and medical (ISM) band. In order to compete with Wi-Fi technology, FAP should INDOOR COVERAGE: improve performance at a reasonable price. EXISTING TECHNOLOGIES Various technologies for indoor coverage are FEMTOCELL DEPLOYMENT AND summarized in Table 1. In LTE-Advanced, fem- TECHNICAL CHALLENGES tocells, picocells, relays, and distributed antenna systems (DASs) are low-power nodes in a het- FEMTOCELL DEPLOYMENT SCENARIOS erogeneous network [3]. Picocells are located inside hotspots like airports, shopping malls, and Femtocells are overlaid within macrocells in a stadiums. Picocells work as simplified macrocells two-tier heterogeneous network. Allocating with low power and reduced cost. DASs com- spectrum resources between femtocells and prise many separate antenna elements (AEs) macrocells is a very important issue. There are connecting to macro BSs via dedicated fiber three possible strategies for femtocell resource cables or radoi frequency (RF) links to extend deployment: dedicated-channel deployment, par- macro coverage. A set of low-power distributed tial-channel-sharing deployment, and co-channel AEs replace high-power centralized antennas to deployment [3]. cover the same cell area. Relays are installed in 1) Dedicated-channel deployment: Femtocells cellular dead zones without wired backhaul to are allocated a dedicated carrier frequency dif- compensate for the attenuation loss of barriers, ferent from those of macrocells. This deploy- such as buildings, tunnels, or high-speed trains. ment is a simple solution to avoid mutual Although picocells/DASs/ relays are cost-effec- interference between the two tiers, but the spec- IEEE Wireless Communications • April 2013 45 DU LAYOUT_Layout 1 4/19/13 3:36 PM Page 46 Inter-tier interference (or cross-tier interfer- 1# 6# CFAP E# CFAP C# L1 MBS ence) and intra-tier L6 5# interference coexist in this network. For 3# inter-tier interference, L2 L10 2# L5 CFAP D# the aggressor and L7 the victim of interfer- L3 L9 ence belong to FeUE L4 L11 L8 different tiers, while MacUE 4# CFAP A# CFAP B# they