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Femtocell Networks: A Survey
Vikram Chandrasekhar, Jeffrey Andrews, Alan Gatherer
TL;DR
Femtocells address the need for higher wireless data rates without the expense of deploying more infrastructure. This survey reviews their technical and business foundations, challenges, and research opportunities, concluding that they can provide low-cost, high-quality indoor access while reducing system burden.
Problem
Rising wireless data-rate demand and the expense of additional infrastructure motivate examining femtocells as a lower-cost alternative.
Method
The article surveys femtocell technical and business arguments, state of the art, technological challenges, and research opportunities.
Results
Femtocells can provide high-quality indoor network access at low cost while reducing the burden on the whole system.
Takeaways & Limitations
The femtocell approach offers subscribers higher data rates and reliability while reducing operator burden.
Takeaways & Limitations
Technical challenges include achieving low cost, mitigating RF interference, providing QoS over IP backhaul, and maintaining scalability.
Abstract
from arXiv · showhide
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. In a network with nomadic users, this inevitably involves deploying more infrastructure, typically in the form of microcells, hotspots, distributed antennas, or relays. A less expensive alternative is the recent concept of femtocells, also called home base-stations, which are data access points installed by home users get better indoor voice and data coverage. In this article, we overview the technical and business arguments for femtocells, and describe the state-of-the-art on each front. We also describe the technical challenges facing femtocell networks, and give some preliminary ideas for how to overcome them.
1 Introduction
Femtocells address costly cellular network micro-ization by providing consumer-installed, short-range, low-power base stations that improve indoor coverage and capacity. The survey reviews their technical and business benefits, challenges, and possible solutions.
- Introduction: Femtocells are consumer-installed, short-range, low-power base stations that provide better indoor voice and data reception through broadband or RF backhaul.They communicate with the cellular network over DSL, cable modem, or a separate RF backhaul channel.
- Introduction: The article surveys end-user-deployed infrastructure enhancements, explains their business and technical benefits, and discusses femtocell challenges and possible solutions.It also identifies fixed mobile convergence with existing handsets as an advantage of in-home femtocell deployment.
- Introduction: Femtocells offer operators cost benefits because they require little upfront service-provider investment and can reduce operating expenses from $60K per year per macrocell to $200 per year per femtocell.Their deployment also reduces the need for additional macrocell towers.
- Introduction: Indoor traffic can be absorbed by femtocell networks over the IP backbone, allowing macrocell base stations to redirect resources toward mobile users.More than 50% of voice calls and more than 70% of data traffic originate indoors.
- Introduction: Femtocells improve coverage and capacity by reducing transmit distance, power, and interference while increasing SINR, handset battery life, and spatial reuse.These effects support higher area spectral efficiency and improved reception.
2 Technical Aspects of Femtocells
Femtocells increase capacity through shorter transmitter–receiver distances, lower transmit power and interference, and more efficient resource sharing. Their simulations show substantial user and system throughput gains, though inadequate backhaul QoS can limit indoor capacity improvements.
- Capacity benefits: Nearly 34 dB of transmit power saving occurs with L=1000 meters, N=50 femtocells, and α=β=4; choosing β=2 raises savings to nearly 77 dB.The savings follow the stated transmit-power reduction expression under a fixed receive-power target and no-fading path-loss model.
- Capacity benefits: Femtocells improve capacity by reducing user distance, increasing received signal strength, and lowering interference through propagation and penetration losses.These mechanisms provide simultaneous signal-strength and interference improvements.
- Capacity benefits: Serving only around 1-4 users lets each femtocell allocate more transmit power and bandwidth per subscriber than a larger macrocell.Macrocells cover roughly a 500m-1 km radius and serve more users, making QoS for data users more difficult.
- Limitations: Insufficient wired-broadband backhaul QoS can reduce the indoor capacity gains provided by femtocells.The capacity benefits assume that the operator provides sufficient QoS over the backhaul.
- Simulation results: Nearly 0.6 b/s/Hz higher normalized median user throughput is observed for voice-only femtocell deployments.The example compares one macrocell serving 100 users with 50 femtocells serving two users each.
- Simulation results: 1.8 b/s/Hz normalized user throughput gain and nearly 250 b/s/Hz system-wide median sum-throughput gain occur with femtocell deployment under data traffic.The comparison uses a macrocell scheduling its 20 strongest users versus femtocells transmitting simultaneously across the entire bandwidth.
3 Business Aspects of Femtocells
Femtocells can reduce operators’ site lease, backhaul, and electricity costs, but require strategic investment, aggressive pricing, and cost-driven technical choices. Standardization efforts support lower-cost solutions, while deployment forecasts indicate substantial future adoption.
- Business Aspects of Femtocells: Femtocells reduce operator site lease, backhaul, and electricity costs but require strategic investment and aggressive pricing to compete with ubiquitous Wi-Fi.Sprint subsidizes an Airave femtocell at $49.99 with a $30/month family plan.
- Business Aspects of Femtocells: Femtocell features are more sophisticated than consumer-grade Wi-Fi, while vendors face mature Wi-Fi cost targets and operator demands for minimal subsidy.Cost issues consequently drive solution selection for each technical challenge.
- Business Aspects of Femtocells: After 1.5 years, operator investment is assumed to be recovered, allowing future profits.This estimate comes from a predictive femtocell network deployment cost breakup conducted by Airvana and Gartner.
- Business Aspects of Femtocells: The Femto-Forum, formed in 2007 by operators and vendors, develops open standards for product interoperability to help achieve low-cost solutions.Standardizing requirements across customers is presented as important under aggressive cost challenges.
- Business Aspects of Femtocells: 102 million users worldwide on more than 32 million femtocells were predicted by 2012.The forecast reflects expected adoption following operator trials and deployments in North America, Europe, and Japan.
4 Technical Challenges
The technical challenges facing femtocell networks span broadband operation, voice services, and network infrastructure. They include resource allocation, synchronization, backhaul, interference, access, mobility, emergency services, and secure operator-network connectivity.
- Broadband Femtocells: Broadband femtocells require resource allocation, timing and synchronization, and backhaul solutions.These challenges concern the operation and connectivity of broadband femtocells.
- Voice Femtocells: Voice femtocells must address interference management, access control, handoffs, mobility, and Emergency-911 services.The challenges cover both radio coordination and user-service requirements.
- Network Infrastructure: Femtocell network infrastructure must securely bridge femtocells with the operator network over IP.Secure IP connectivity is the central infrastructure challenge identified for femtocell networks.
4.1 Physical and Medium Access layers: Broadband Femtocells
Broadband femtocells face intertwined physical- and medium-access-layer challenges involving RF interference, decentralized spectrum allocation, synchronization, and IP-backhaul performance. These challenges arise from uncoordinated deployments, tight timing requirements, low-cost constraints, and shared backhaul capacity.
- Physical and Medium Access layers: Broadband Femtocells: Femtocell networks must mitigate macrocell-to-femtocell, femtocell-to-femtocell, and femtocell-to-macrocell interference despite potentially scaled-down signal-processing capabilities.The near-far effect mainly contributes to macrocell-to-femtocell and femtocell-to-macrocell interference, while femtocell-to-femtocell interference is relatively smaller because of low transmit power.
- Physical and Medium Access layers: Broadband Femtocells: Ad-hoc femtocell placement makes centralized frequency planning difficult and leaves decentralized spectrum allocation between macrocell and femtocell users as an open research problem.Key alternatives include bandwidth splitting or shared bandwidth with universal frequency reuse, with the appropriate allocation depending on configuration and femtocell density.
- Physical and Medium Access layers: Broadband Femtocells: IP backhaul must provide QoS, service parity, and sufficient capacity without becoming a bottleneck, while current backhaul networks lack the delay resiliency needed for femtocell traffic.Existing macrocell networks provide latency guarantees within 15 ms, but net-neutrality concerns can further constrain service delivery.
- Physical and Medium Access layers: Broadband Femtocells: Telefonica trials found that concurrent Wi-Fi use made femtocells struggle to transfer data, including low-bandwidth voice services.The issue is particularly important because improved voice coverage is expected to be a main femtocell objective.
4.2 Physical and Medium Access layers: Voice Femtocells
Voice femtocell deployments must balance spectral efficiency against cross-tier interference, while access policies, handover, mobility, and emergency calling introduce additional operational constraints. The section outlines interference failures and corresponding deployment and management challenges.
- Physical and Medium Access layers: Using the same bandwidth for macrocell and femtocell users improves area spectral efficiency but creates cross-tier interference challenges.Separate frequency bands eliminate cross-tier interference, whereas shared bandwidth is preferred because radio resources are scarce and deployment is easier.
- Physical and Medium Access layers: Femtocell power control produces dead-zones and asymmetric interference, with macrocell edge users disrupting femtocells and nearby femtocells disrupting macrocell users.Cell-edge femtocells experience higher interference than interior femtocells, while macrocell edge users suffer higher path-loss and are most affected by femtocell transmissions.
- Physical and Medium Access layers: -27.8 dB post-processing signal-to-interference ratio makes reverse-link reception infeasible for the example cell-edge macrocell user.The example assumes 500 m macrocell distance, 30 m femtocell distance, processing gain 128, and path-loss exponent 4; interference is 48.87 dB after 21.07 dB CDMA suppression.
- Access control: Open access can reduce macrocell load but may strain femtocell backhaul, raise privacy concerns, and risk starving paying home users.Operators are considering hybrid models that reserve some femtocell resources for registered family members while opening others to roamers.
- Handover and mobility: Conventional neighbor-list handover procedures do not scale to large numbers of femtocells, and open access can cause repeated handovers during channel fluctuations.Proposed mitigations include reducing pilot power when no calls are active and developing low-complexity dwell-time prediction algorithms.
- Emergency services: Emergency-911 provisioning requires femtocells to transmit location information and may require emergency communication for unsubscribed users.Location can be obtained through GPS, the backhaul, macrocell information, or mobile position estimates; closed-access femtocells should permit emergency access.
4.3 Network Infrastructure
Femtocell networks require a secure, scalable, and cost-conscious IP interface between femtocells and operators. Three architectures have been proposed, with IMS/SIP and UMA identified as preferred choices despite distinct scalability, cost, and handset constraints.
- Network interface requirements: Operators must provide a secure, scalable, reasonably priced IP interface connecting femtocells to the network.Traditional RNCs support tens to hundreds of macrocells, raising questions about equivalent service for femtocells over the internet.
- Candidate architectures: Three interfaces have been proposed, with IMS/SIP and UMA appearing to be the architectures of choice.The alternatives are Iu-b over IP, IMS/SIP, and RAN gateway-based UMA.
- Iu-b over IP: Iu-b over IP lowers Capex by reusing existing RNCs but lacks scalability and standardization.It connects femtocells through the standard Iu-CS and Iu-PS interfaces used in macrocell networks.
- IMS/SIP: IMS/SIP offers scalability and rapid standardization by converting subscriber traffic to IP packets over VoIP, but requires upgrade Capex and two core networks.The IMS core network resides between femtocells and the operator and coexists with the macrocell network.
- RAN gateway based UMA: UMA uses a RAN gateway and secure IP tunneling to aggregate femtocell traffic, while integrating the UMA client into femtocells could support legacy handsets.Current UMA-enabled services require dual-mode handsets to switch between in-home Wi-Fi and outdoor cellular access.
5 Research Directions
Research directions for femtocell networks emphasize interference avoidance over suppression because ad-hoc femtocell placement makes cancellation unreliable. They also explore low-complexity adaptive designs and MIMO techniques that balance capacity, robustness, coverage, interference, and cost.
- Interference Management: Interference avoidance is favored over suppression because ad-hoc femtocell locations limit suppression effectiveness and cancellation errors quickly degrade successive interference cancellation.Avoidance lets users prevent mutual interference rather than subtracting it after reception.
- Interference Management: Low-cost femtocells motivate low-complexity receivers and transmission schemes that sense nearby frequency channels to avoid collisions.Simple matched-filter processing is one example of the targeted receiver design.
- Interference Management: 7x improvement in system capacity is obtained through CDMA time-hopping and directional antennas when macrocell and femtocell users share bandwidth.The result applies to CDMA femtocell networks with universal frequency reuse.
- Interference Management: Random time-hopping reduces the average number of interfering users by a factor of Nhop while trading off processing gain.Each user randomly selects one of Nhop hopping slots and remains silent during the others.
- Interference Management: Adaptive power control can reduce macrocell interference by varying femtocell power with location, but reducing far-femtocell power decreases home coverage.Closed-access strategies adjust transmit or receive power targets according to femtocell location and user tier.
- MIMO: MIMO research targets adaptive switching between spatial multiplexing for high-SINR links and diversity schemes for robust low-SINR transmission.Open- and closed-loop diversity include space-time codes and beam forming.
- MIMO: MIMO femtocell research must address co-channel-interference-induced channel-state-information errors, receiver complexity and cost, and femtocell-specific channel models.Diversity characteristics may differ substantially from those of macrocells.
6 Conclusions
Femtocells can provide high-quality indoor network access at low cost while reducing the burden on the overall system. The article identifies their benefits, technological and business challenges, and research opportunities.
- Conclusions: Femtocells can provide high-quality network access to indoor users at low cost while reducing the burden on the whole system.These benefits motivate femtocell deployment.
- Conclusions: The article identifies femtocell benefits, technological and business challenges, and research opportunities.These areas structure the survey’s conclusions.
- Technical challenges: Technical challenges include delivering a low-cost solution, mitigating RF interference, providing QoS over the IP backhaul, and maintaining scalability.Operators must address these challenges together when developing femtocell networks.
- Business challenges: Business challenges include generating long-term revenue growth and overcoming initial end-user subsidies.Both issues are identified as key business obstacles.
Biographies
The biographies profile three wireless communications researchers spanning academia and industry. Their backgrounds include advanced electrical-engineering training, leadership roles, and research or product-development responsibilities in wireless networking.
- Vikram Chandrasekhar: Vikram Chandrasekhar is a Ph.D. candidate at UT Austin whose research addresses fundamental limits and algorithms for microcellular and hotspot-aided broadband cellular networks.He earned degrees from IIT Kharagpur and Rice University and previously held industry positions at National Instruments, Freescale, and Texas Instruments.
- Jeffrey G. Andrews: Jeffrey G. Andrews is an Associate Professor at UT Austin and directs its Wireless Networking and Communications Group.He holds electrical-engineering degrees from Stanford, authored Fundamentals of WiMAX, received the 2007 NSF CAREER award, and worked at Intel.
- Alan Gatherer: Alan Gatherer is Texas Instruments’ CTO for the Communications Infrastructure and Voice Business and a Distinguished Member of Technical Staff.He holds electrical-engineering degrees from Stanford and oversees strategic development of TI digital baseband modems for 3G and 4G wireless infrastructure.