INTRODUCTIONThe growing adoption of rich multimedia serviceshas resulted in explosive demand for wireless datacapacity. Recent studies indicate that wirelesstraffic has grown at a rate that is approximatelyan order of magnitude higher than spectral effi-ciency enhancements available to meet therequired increase in capacity. This gap will onlyincrease further as the number of devices per per-son increases, and newer devices enable the con-sumption of even richer multimedia content [1].Despite the increase in traffic consumption, oper-ators are facing flattening revenue per bit due tolargely flat-rate data-centric plans. Hence, it is
imperative for the operators to add new capacityat a significantly lower cost per bit while also find-ing methods of enhancing revenue.
Advanced antenna techniques and real-timechannel-adaptive scheduling have pushed thespectral efficiency of fourth-generation (4G) airinterfaces closer to system capacity limits. Now,the focus of the wireless industry is shifting fromsolely increasing spectral efficiency to signifi-cantly improving network efficiency. Here, newheterogeneous networks (Het-Nets) are apromising paradigm that can cost-effectivelyimprove system coverage and capacity.
Recent work in the area of heterogeneous net-works has primarily focused on network overlaytechniques for offloading data traffic to smallercells. While the gains from this approach arepromising, they represent only a starting point.We envision that heterogeneous networks will playa central role in the future evolution of mobilewireless broadband, and serve as a platform andenabler for disruptive technology innovations.
The main idea of heterogeneous networks is tooverlay low power and low cost devices on cover-age holes or capacity- demanding hotspots to sup-plement conventional single-tier cellular networks.While large cells, covered by macro base stations(MBSs), for example, provide blanket coverageand seamless mobility, small cells served by deviceslike femto access points (FAPs), pico base stations(PBSs), and relay stations (RSs) help provide cov-erage extension and boost local capacity. In anovel enhancement, clients can also cooperatewith network infrastructure to form an additionaltier to boost signals to neighboring clients duringoutages. By shrinking the transmission range anddense spatial reuse of the spectrum, low-powernetwork elements like FAPs, PBSs, and RSs canachieve significant improvement in coverage andareal capacity gain. We expect these networks willbe self-organizing with interfaces to a self-organiz-ing network (SON) function that enables tightcoupling between different types of base stations,including macro and smaller cells. An overview ofa heterogeneous network is illustrated in Fig. 1.
In order to boost capacity further, the networkwill leverage spectrum across different radioaccess technologies (RATs). Consumer devices
ABSTRACTDisruptive innovations in mobile broadband sys-
tem design are required to help network providersmeet the exponential growth in mobile trafficdemand with relatively flat revenues per bit. Het-erogeneous network architecture is one of the mostpromising low-cost approaches to provide signifi-cant areal capacity gain and indoor coverageimprovement. In this introductory article, we pro-vide a brief overview of heterogeneous networkarchitectures comprising hierarchical multitier mul-tiple radio access technologies (RAT) deploymentsbased on newer infrastructure elements. We beginwith presenting possible deployment scenarios ofheterogeneous networks to better illustrate the con-cepts of multitier and multi-RAT. We then focuson multitier deployments with single RAT andinvestigate the challenges associated with enablingsingle frequency reuse across tiers. Based on thespectrum usage, heterogeneous networks can becategorized into single carrier usage, where alldevices within the network share the same spec-trum, and distinct carrier usage, where differenttypes of devices are allocated separate spectra. Forsingle carrier usage, we show that interference man-agement schemes are critical for reducing theresulting cross-tier interference, and present severaltechniques that provide significant capacity andcoverage improvements. The article also describesindustry trends, standardization efforts, and futureresearch directions in this rich area of investigation.
CAPACITY AND COVERAGE ENHANCEMENT INHETEROGENEOUS NETWORKS
HE TNE T S : A NEW PARAD IGM FOR
IN C R E A S I N G CELLULAR CA PA C I T Y A N D CO V E R A G E
The authors studythe deployment ofheterogeneous networks from theperspective of cooperative communications toimprove the performance of physical layer multicast.
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IEEE Wireless Communications • June 2011 33
like FAPs will integrate multiple radio technolo-gies within a single device and further increasethe achievable transmission rate by exploitingalternate spectrum (e.g., free unlicensed bands).
Besides the capacity advantage, the cost struc-ture of heterogeneous networks is much lower.Since low-power devices serve much smaller areaswith minimal functionality, these devices can bemade cheaper. Judicious use of free unlicensedspectrum can also help lower the cost of capacity.In addition, the site acquisition cost is reduced,and backhaul cost can be saved for devices likeFAPs and RSs. Deployment of low-power devicesis also more efficient because the operators or con-sumers can place these devices at locations thatmost require coverage or capacity improvement.
In this article, we provide an overview of Het-Net technology and then focus on a key problemof enabling single-frequency deployment ofmultitier networks. We show that the promisedcapacity and coverage gains with such deploy-ments can only be realized with proper interfer-ence management to mitigate cross-tierinterference. The article is organized as follows.We first outline the infrastructure elements anddeployment scenarios for a heterogeneous net-work. Then, we identify that although single-car-rier multitier networks offer extra capacity,interference across tiers is a serious problem anddiscuss some interference mitigation (IM) solu-tions. After that, we summarize the status ofstandards efforts on heterogeneous networks aswell as highlight potential future research direc-tions. Finally, we conclude the article.
DEPLOYMENT SCENARIOS
As mentioned, a Het-Net may consist of severaltypes of devices with different characteristics. Tocreate a complete picture of what a Het-Net will
look like, we start from an overview of all poten-tial elements within a heterogeneous network.These network components can be categorizedinto two groups: one represents the multitier sin-gle-RAT Het-Net, and the other representsdevices integrating multiRAT to further enhancethe whole network.
SINGLE-RAT MULTITIER NETWORK COMPONENTSIn future mobile networks, devices with differentfootprints and capability may be overlaid withinthe same geographical area to serve users withdifferent requirements. Typically, devices withinthe same network will operate with the sameradio technology and share the same sets ofspectra. The multitier structure enhances capaci-ty and coverage via enabling dense reuse of thespectrum and improving link quality. Startingfrom the most capable devices with the largestfootprint, then gradually moving toward simpler,smaller footprint devices (Fig. 2 [8]), we examinepossible deployment scenarios forming a multi-tier network and explain the role of each tier.
Macrocells/microcells: In current cellular net-works, BSs are deployed for wide area coverage.The footprint of a BS varies depending on trafficdemand. Macrocells with more than 500 m site-to-site distance cover rural or suburban areas, whileurban areas require microcells with smaller cellradii. In Het-Nets, existing macrocells and micro-cells remain, providing essential coverage. Addition-ally, large macrocells hold advantages in supportinghigh-mobility users for reduced handover frequency.
Picocells: Serving smaller areas than micro-cells, pico base stations (PBSs) can be deployedas hotspots in capacity starved locations likeshopping malls, airports, and stadiums. PBSs arebasically simplified MBSs with reduced powerand hence reduced cost. They are typically partof a general operator deployed public infra-
Figure 1. Overview of a heterogeneous network.
Wireless accessWireless backhaulWired backhaul
Femto-AP(indoor coverage and offload macro-BS)
Pico-BS(areal capacity)
Femto/WiFi-AP(offload macro-BS)
Client relayRelay
Distributed antenna system
Mobile hotspot
Coverage hole
Macro-BS
Consumer deviceslike FAPs will
integrate multipleradio technologies
within a singledevice and further
increase the achievable
transmission rate byexploiting alternate
spectrum (e.g., freeunlicensed bands).
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structure, and are open and accessible to all cel-lular users. The deployment of PBSs is usuallycarefully planned by the operators. Given thatMBSs and PBSs are all controlled by operators,research on horizontal optimization across tierswill be beneficial here.
Relays: Relay stations serve similar sizes offootprints as PBSs. They provide coverage exten-sion and throughput enhancement by forwardingan enhanced version of the received signal fromBSs to mobile stations. Relays use wireless back-haul, so no landline resource is required; howev-er, this reduces the amount of spectrum availablefor access. Operators may choose to implementinfrastructural relays over PBSs at coverageholes where wired backhaul is unavailable or dif-ficult to implement.
Femtocells: A FAP covers even smaller area(10–50 m), such as a house or an apartment.Unlike MBSs or PBSs connected to the networkthrough operator-owned backhaul, FAPs saveinfrastructural cost by utilizing existing residentialbackhaul links such as digital subscriber line (DSL)or cable. In addition, FAPs are usually privatelyowned and more efficiently deployed based onusers’ needs. Typically, FAPs are under closed sub-scriber group (CSG) operation, where only restrict-ed users are granted permission to access a FAP.CSG FAPs may cause excess interference to thenetwork when sharing the same spectrum withother tiers. The interference issue is to beaddressed later. Another research challenge isensuring network scalability to support large num-bers of FAPs. The concept of device-centric systemdesign helps enable autonomously configurableFAPs and reduces required network loads.
Client relay: Client cooperation (i.e., clientrelay) creates yet another tier in the wirelessmulti-tier network. This tier is between clientsand therefore comprises very short-range links.Client cooperation (CC) utilizes the good linkbetween a cooperating client and a BS to “but-tress” the link of an end client whose link to thatBS is poor, thereby increasing its probability ofsuccessful transmission. In effect, CC improvesthe ‘virtual link quality” of users in poor locations(i.e., cell edge users), resulting in a significantreduction in the amount of channel resources and
battery power they consume, and the amount ofinterference they cause to other cells. Studiesshow that CC can improve average networkthroughput anywhere from 80–200 percent [2].
Besides the potential network elements in amultitier network discussed above, a novel wirelesstechnology called Distributed Antenna System(DAS) can also be applied in single-RAT multitiernetworks. By spatially separating the antennas of aconventional BS and connecting them to a com-mon processing unit via a fast transport mediasuch as optical fiber, small cells can be virtuallycreated in a macro network. This allows a set ofcentralized antennas radiating at high power to bereplaced by a group of low-power antenna ele-ments that cover the same cell area. The advan-tage of a DAS is that less power is needed toovercome penetration and shadowing losses, sincea line-of-sight channel is often present, leading toimproved link reliability and coverage. Differentvariants of DASs have been deployed recently byseveral service providers in many areas worldwide.An example deployment is along the high-speedtrain tracks in Taiwan, which allows broadbandwireless access inside fast trains while reducing thenumber of BSs and the need for frequent handoffsbetween successive BSs [3].
Another concept becoming popular amongoperators is China Mobile’s cloud of radio accessnetwork (C-RAN) [4]. Through optical fiberconnections, a C-RAN BS concentrates thebaseband signals from several hundreds of sec-tors/cells to a server platform for centralized sig-nal processing. This architecture creates a superBS with distributed antennas that can supportmultiple protocols and dynamically allocate itssignal processing resources to follow the varyingtraffic load within its geographical coverage.
MULTI-RAT NETWORK COMPONENTSGiven that an increasing number of clients in thenetwork are equipped with multiple radio inter-faces (e.g., WiFi in addition to 4G), an operatorcan also exploit the different radio networks toadd low-cost capacity, and improve coverage andquality of service (QoS) in the network. In addi-tion to the multitier aspect of Het-Nets, multi-RAT network components contribute extraperformance enhancement. Figure 3 [8] illus-trates some multi-RAT usage scenarios, and wepresent some of them here.
WiFi offload: For example, as shown in Fig. 3,an operator can judiciously offload best effort traf-fic to WiFi hotspots to add capacity at much lowercost, without compromising the QoS requirements.This scenario requires the operators to have a cer-tain level of access control on the WiFi-AP.
Virtual carrier: This usage model calls for syn-ergistic use of licensed (4G connection, e.g.,802.16, Third Generation Partnership ProjectLong Term Evolution [3GPP LTE]) and unli-censed spectrum (WiFi) to improve networkcapacity and user . Different levels of synergy arepossible depending on whether a multiprotocolclient connects to distinct 4G and WiFi accesspoints or an integrated 4G/WiFi device is used forthe connection. New network devices, such as theintegrated WiFi/4G FAP shown, can implementmore synergistic utilization of the spectrum avail-able across both licensed and unlicensed bands.
Figure 2. Multitier architecture.
Relay
Macro
Micro
Pico
Femto
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In this virtual carrier usage scenario, an addi-tional or virtual carrier is available for use, andmay be utilized in several different ways toimprove network and user performance. Severaltechniques are applicable for utilizing the addi-tional carrier to improve multiple metrics, suchas handoff to multitier WiFi, interference avoid-ance, carrier aggregation, diversity/redundancytransmission, QoS/load balancing, and reducedoverhead with unified control. Further details inthe context of multiradio interworking between802.16 and 802.11 are available in [5].
Mobile hotspots (personal area networks): Amobile hotspot is a multiprotocol portable device,with both cellular and local area network(LAN)/personal area network (PAN) connectivi-ty, that routes traffic from devices within shortrange to the cellular network. Examples includeMiFi from Verizon Wireless, iSpot from Clear-wire, Overdrive from Sprint, and others. Thesedevices create a WiFi hotspot, connecting con-sumer devices such as a netbooks, cameras, orprinters to the Internet via cellular backhaul.Hence, consumer devices that have only WiFiconnectivity can now access the Internet wherev-er cellular service is available. We envision thathotspot capability will become more integrated inmobile devices of the future (e.g., smart phones,laptops), and this in turn will open the door to avast number of machine-to-machine (M2M) ser-vices that could be offered to cellular subscribers.
SINGLE-RAT INTERFERENCE MITIGATIONIn the previous section, we overview Het-Netdeployment scenarios and establish a general pic-ture of a heterogeneous network. We identify themultitier structure and multi-RAT cooperation astwo key features of Het-Nets to achieve significantcapacity and coverage enhancement. For multi-
RAT, we basically take advantage of additionalfree spectrum, and the challenge lies in coopera-tion between protocols. On the other hand, a mul-titier network utilizing single-RAT is capacityconstrained by the scarce spectrum available. Intel-ligent bandwidth allocation among tiers and inter-ference management are essential for the successof multitier networks. In this section, we addressthe frequency usage in single-RAT multitier net-works and focus on the interference mitigationproblem, which is critical in multitier networks.
Devices on different tiers can operate on thesame or different frequency bands, depending onthe amount of frequency resources an operatorowns and how s/he allocates the resources. Whilesharing the same spectrum achieves the highestfrequency reuse, orthogonal frequency accessavoids the cross-tier interference problem.
For operators that own more than one fre-quency bands, the simplest spectrum planning isto assign different operating frequencies to usersserved by first-tier devices like MBSs and usersserved by second-tier devices like PBSs andFAPs. No cross-tier interference will arise insuch a simple scheme. In addition, for operatorswith more available spectra, each second-tierdevice like a PBS or FAP can choose from mul-tiple accessible frequency bands for the one atwhich it will transmit. As a result, interferencefrom same-tier devices is further mitigated.
However, most network operators are capaci-ty constrained due to limited spectrum, andtherefore cannot afford to deploy separate carri-ers across tiers. Additionally, to pursue the high-est reuse of spectrum, operators with morespectra can consider carrier aggregation toimprove channel utilization efficiency. Hence,“intelligent” co-channel multitier deployments,where shared carrier(s) is/are judiciously used
Figure 3. Multi-RAT usage scenarios.
Bad 4G link
Good 4G link
Good WiFilink
Mobile hotspot
Setup peer-to-peercooperation
Virtual carrier:use 4G and LANsimultaneously
Integrated APLAN
network
M2Mnetwork
Body areanetwork
Shortrangecomm.
Settop
Convergedgateway
Multimedianetwork
EKGHeartbeat
WiFi APOffload to LAN
We envision thathotspot capabilitywill become more
integrated in mobiledevices of the future(e.g., Smart phone,laptop), and this in
turn will open thedoor to vast number
of machine-to-machine (M2M) ser-
vices that could beoffered to the cellu-
lar subscriber.
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across tiers, are crucial to the success of multi-tier architectures.
When all MBSs and FAPs/PBSs/RSs areoperating simultaneously in the same spectrum,interference becomes a major issue that limitsthe performance of the network. The problem iseven more prominent in CSG femtocells, due tothe additional coverage holes created by FAPsfor users without access permission. As FAPdeployment density increases, a growing propor-tion of macro-users are driven into outage dueto the increased interference, leading to unsatis-factory macro performance. Therefore, interfer-ence mitigation techniques are critical to fullyenable co-channel deployments on a scale wherea reasonable density of FAPs may be supported.
Simulation results that demonstrate the poten-tial coverage and capacity improvement with fem-tocell overlay networks are shown in Table 1.Although FAPs provides great indoor coverageand over 100 times areal capacity gain, we can seethat a significant portion of outdoor users aredriven into outage due to co-channel interference.To address the cross-tier interference problem, wediscuss the key system requirements for enablinginterference mitigation (IM), techniques, potentialIM solutions, and related issues when applying IMin heterogeneous networks. Our interference dis-cussion focuses on CSG FAPs given they areexperiencing the most stringent interference. IMideas discussed below are also applicable to othertypes of heterogeneous networks, like picocelloverlay networks, as well. The performance of var-ious IM schemes is also compared.
Synchronization: Synchronization between net-work devices is critical for most interference man-agement schemes to work without incurringexcessive complexity and performance loss. In thecontext of orthogonal frequency-division multipleaccess (OFDMA) systems, this requirementimplies that an interfering signal must be receivedwithin the cyclic prefix of the OFDMA symbolboundary; otherwise, an asynchronous interferercan cause significant inter-carrier as well as inter-symbol interference, which are difficult to mitigatewith standard interference management schemes.
In [6], we show that femto networks may besynchronized with the overlay macro networkthrough existing over-the-air synchronizationcapabilities used in the overlay system. However,in [6] we also show that simply aligning the timingreference of each tier to a global reference isinsufficient to guarantee synchronous interferenceacross tiers. Here it is also important to maintainthe relative time alignment between devices suchthat transmissions from devices across tiers arereceived synchronously at the receiver. Thisrequirement is analogous to how uplink transmis-sions from devices in a single-tier network areadjusted based on their distance from the BS, sothat they may be aligned at the receiving base-sta-tion. Similarly, time alignment between devicesacross tiers must also be maintained to guaranteesynchronous interference in multi-tier networks.Further details are available in [6].
Power control: The transmit power level of aFAP affects its coverage range and the amountof interference it generates in the network.Although higher transmit power can providewider coverage and better signal quality, it canat the same time cause tremendous interferenceto surrounding users. Therefore, power controlcan be a candidate solution for mitigating inter-ference in Het-Nets.
Properly selecting the FAP transmit powerlevel can help manage the interference fromFAPs to macro-users, while maintaining femto-cell performance. In [7], three power settingschemes are developed and evaluated: fixedpower level operation, femto-QoS power con-trol, and macro-QoS power control. From simu-lation results shown in Table 1, we see thatalthough power control helps reduce interfer-ence from FAPs toward macro-users, degradedFAP signal quality causes substantial reductionin femto-user rates. Nevertheless, power controlcan still be applied for control channels wheredata rate is not a major concern.
Cell association: When power control isapplied, mobile stations may need to perform re-association after power back-off is performed attheir serving base stations. Here users may adopt
a For data, outage is defined as SINR < -0.8 dB (SE = 0.5)b Results are derived from the same simulation methodology described in [7].
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the same rule as what is used in the initial asso-ciation process like selecting from the maximumreceived signal level. Different rules, such asperforming re-association only when the user isin outage, may also be used to ensure sufficienttraffic is offloaded to low-power devices whileimproving coverage.
More advanced interference mitigation mayinvolve joint operation of power control and cellassociation. For example, given the user’s geograph-ical distribution, a clustering optimization processmay be performed so that the users are groupedbased on the potential to minimize interference withproper power settings at the base stations.
Low duty cycle: Even when low transmitpower is used, increasing device density can stillcause significant aggregated interference in Het-Nets. For example, in an urban environmentwith multistory buildings, there can be 1000FAPs installed per sector within a 500 m cell.Since the current standard requires all FAPs,both active and inactive, to transmit control mes-sages at the same time as the macro-BSs, intol-erable levels of interference on control signalingis inevitable at high FAP densities, which pre-vents wide adoption of FAPs in urban areas.
One possible approach is to have FAPs con-trol signals transmitted with low-duty cycle andto have neighboring FAPs take turns to transmittheir control signals. This approach “virtually”reduces the equivalent FAP density. The Femto-QoS power control scheme can also be usedtogether with low-duty cycle operation to achievesufficient coverage under reuse-1 setting.
Frequency planning: Frequency planning isanother possible approach for interference miti-gation in heterogeneous network. For example, infemtocell overlay networks, CSG FAPs can causesevere interference to macro-user and may drivea great potion of macro-users in outage. A simpleapproach of designating a time-frequency zonethat is free of FAP interference can easily recovermacro-user coverage. Other advance frequencyplanning approach like fractional frequency reuse(FFR) can be applied together with this femto-free zone (FFZ) approach. Simply assigningorthogonal bandwidth to macro-user and femto-user is another simple frequency planningmethod. The performance is essentially the sameas assigning distinct frequencies to different tiers.
From the simulation results shown in Table 1,we can see that via frequency planning, we canachieve huge areal capacity gain without degra-dation in macro-user coverage. Therefore, fre-quency planning can be the promising solutionto solve the interference problems in data chan-nels.
STANDARDIZATION ANDFUTURE RESEARCH DIRECTIONS
The potential benefit of heterogeneous networkhas drawn lots of attention from both academiaand industry. Wireless standard bodies areactively investigating enabling techniques andare working to expedite the deployment of het-erogeneous networks.
3GPP has made substantial progress on het-erogeneous networks in the past few years. Areas
including network architecture, security, andradio frequency (RF) requirements are exam-ined, and new features are added in order toenable femtocells, called H(e)NB in 3GPP ter-minology, deployment. Currently, there areworking items in Release 10 working on non-car-rier-aggregation based interference control inheterogeneous networks and H(e)NB mobilityenhancement. There are also activities regardingLTE relays and Wireless LAN offload.
IEEE 802.16 has also recently formed a newstudy group on hierarchical networks [8]. Thefinal study report will contain usage cases, net-work architecture, key features and require-ments, as well as the simulation modelingmethodology.
Heterogeneous network technologies are stillat an early stage of development. New applica-tions and services will continue to add newdesign requirements. Advanced signal processingtechnologies are expected to enable new featuresand architectural options for more cost-effectiveimplementations. Some potential HetNetresearch directions are described as follows.
VERTICAL AND HORIZONTAL OPTIMIZATIONWe expect future heterogeneous networks toevolve and to optimize in both the vertical (i.e.,across multiple protocol layers) and the horizon-tal (i.e., across multiple sectors/cells) directions.
Vertical optimization consists of cross-layerprotocol optimization and/or the further flatten-ing of network architecture. This approach isessential for reducing network latency andimproving end-to-end QoS.
Horizontal optimization involves cooperativesignal processing among multiple sectors/cells ofthe same network or among the different layersof a heterogeneous network. In the case of superbase stations, they can serve as an interferencemanagement and coordination point for differ-ent network layers. Since the X2 interface [9]between base stations can become an internalconnection within the super base station, it maybe more cost-effective to implement Coordinat-ed multipoint transmission (CoMP) and carrieraggregation features in future Het-Nets.
SERVICE ON NETWORK EDGEFuture Het-Net research also needs to fullyleverage new technologies such as virtualizationand multicore processor platforms. Virtualiza-tion enables multicore processors to support dif-ferent operating environments at the same time,and allows dynamic computing resource alloca-tion according to the ever changing servicerequirements.
While latency reduction continues to be a pri-ority for future wireless broadband systems, weexpect services to continue moving toward thenetwork edge. The significance of this trend istwofold. First, it offers service providers revenueopportunities in a heterogeneous network envi-ronment (e.g., local content, localized radioresource management for user experience, andvalue-added data processing at network edge).Second, it may significantly reduce network con-gestion in the core network, which is increasinglya risk due to the proliferation of media-intensiveapplications.
The potential benefitof heterogeneous
networks has drawnlots of attention
from both academiaand industry.
Wireless standardbodies are actively
investigatingenabling techniquesand are working to
expedite thedeployment ofheterogeneous
networks.
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DEVICE-CENTRIC SYSTEM DESIGN
One continuing challenge for heterogeneous net-works is the coordination and management ofmultiple layers of networks. Traditional cellularnetworks employed a network-centric model,where extensive intersystem interface standardswere developed to ensure mostly seamless oper-ation of terminal devices across different net-works. The main limitation of this approach isthat it does not scale well in a heterogeneousnetwork of multiple technologies.
We expect future Het-Nets to employ a device-centric model. Intelligent multimode devices withsophisticated connection management will be akey technology direction, and we expect moreradio link management decisions are made at theterminal devices to simultaneously minimize net-work complexity and improve user experience.
HETEROGENEOUS NETWORKS FORM2M COMMUNICATIONS
M2M stands for the information exchangeamong connected intelligent devices like sensors.Unlike conventional human-centric network,machines automatically communicate withoutthe need for human intervention. Examples ofM2M applications include: smart metering foroptimizing household gas and electric usage,vehicular communication for safety and trafficcontrol, or sensors for earthquake monitoring.
As WiFi access points and femto/pico stationsbecome widely deployed, the system converge of aheterogeneous network will continue to improve,particularly inside buildings. This creates opportu-nities for new services such as M2M to penetratethese traditionally difficult-to-reach locations.
Furthermore, existing M2M networks mayalso be integrated into future Het-Nets. In fact,the aggregation points in a M2M network canbecome a node in such a network [10].
REGULATORY CONSIDERATIONSAs heterogeneous networks become part of themainstream wireless communication infrastructure,special attention must be paid to address existingand future regulatory requirements. Lawful inter-cept support will become a more complex issue ina heterogeneous networking environment. Also,new services (e.g., m-health) carried by the net-work may increase service providers’ liability.Future research and development on Het-Nettechnologies must address these new requirements.
CONCLUSION
Wireless broadband data traffic driven by con-sumer demand for rich mobile Internet services isnow the primary driver for both consumer purchas-es and network operator deployments. Mobile sub-scribers desire the same rich content available ontheir fixed broadband Internet connections —including streaming video — to be available ontheir mobile devices. Mobile M2M connections arealso expected to increase wireless data consump-tion. As wireless broadband continues to be rolledout worldwide, the use of wireless data shows allsigns of accelerating demand. Heterogeneous net-works hold great promise for meeting consumer
demand, while providing optimum total cost ofownership for network operators. However, thesenetworks have many technical challenges at the airinterface and network layers the wireless communi-ty is working to address in standards forums. Weexpect that heterogeneous networks will be the keyto enabling the cost effective deployment of highperformance networks in order to bring wirelessbroadband to every corner of the globe.
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Data Traffic Forecast Update, 2009-2014,” Feb. 2010.[2] K. Johnsson et al., “Cooperative HARQ,” IEEE C802.16m-
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[4] China Mobile, “C-RAN — Road towards Green RadioAccess Network,” C-RAN Int’l. Wksp., http://labs.chi-namobile.com/focus/C-RAN, Beijing, Apr. 2010.
[5] N. Himayat et al., “Heterogeneous Networking forFuture Wireless Broadband Networks,” IEEE C80216-10_0003r1, Jan. 2010.
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[7] S. Yeh et al., “Power Control Based Interference Mitiga-tion in Multi-tier Network,” IEEE 1st Wksp. FemtocellNetworks, Globecomm Dec, 2010.
[8] R. Kim and N. Himayat, “Study Report on HierarchicalNetworks,” IEEE C802.16ppc-10/0008, July, 2010.
[9] 3GPP TS36.420, “X2 general aspects and principles”[10] G. Wu et al., “M2M: From Mobile to Embedded Inter-
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BIOGRAPHIESSHU-PING YEH ([email protected]) is a research scien-tist in the Wireless Communications Laboratory at Intel.She received her M.S. and Ph.D. from Stanford Universityin 2005 and 2010, respectively, and her B.S. from NationalTaiwan University in 2003, all in electrical engineering. Hercurrent research focus includes interference mitigation inmultitier networks utilizing multi-antenna techniques,machine-to-machine communications, and interworking ofmultiple radio access technologies within a network.
SHILPA TALWAR ([email protected]) is a principal engi-neer in the Wireless Communications Laboratory at Intel,where she is conducting research on mobile broadbandtechnologies. She has over 15 years of experience in wire-less. Prior to Intel, she held several senior technical posi-tions in wireless industry. She graduated from StanfordUniversity in 1996 with a Ph.D. in applied mathematics andan M.S. in electrical engineering. She is the author ofnumerous technical publications and patents.
GENG WU ([email protected]) is the chief architect anddirector of Wireless Standards of the Wireless TechnologyDivision at Intel Corporation. He has 20 years of experiencein the wireless industry. Prior to Intel, he was the directorof Wireless Architecture and Standards at Nortel Networks,with extensive experience in 3G/4G technology develop-ments. He obtained his B.Sc. in electrical engineering fromTianjin University, China, and his Ph.D. in telecommunica-tions from Université Laval, Canada.
NAGEEN HIMAYAT ([email protected]) is a seniorresearch scientist with Intel Labs, where she performs researchon broadband wireless systems, including heterogeneous net-works, cross-layer radio resource management, MIMO-OFDMtechniques, and optimizations for M2M communications. Shehas over 15 years of research and development experience inthe telecom industry. She obtained her B.S.E.E from Rice Uni-versity and her Ph.D. in electrical engineering from the Univer-sity of Pennsylvania in 1989 and 1994, respectively.
KERSTIN JOHNSSON ([email protected]) is a seniorresearch scientist in the Wireless Communications Laboratoryat Intel, where she conducts research on network, MAC, andPHY optimizations that improve wireless network cost, cover-age, and capacity. She graduated from Stanford with a Ph.D.in electrical engineering and has almost 10 years’ experiencein wireless industry. She is the author of numerous publica-tions and patents in the field of wireless communication.
We expect that heterogeneous networks will be thekey to enabling thecost effective deployment of highperformance networks in order tobring wireless broadband to everycorner of the globe.
