Design considerations for next-generation airborne tactical networks

IEEE Communications Magazine • May 2014138 0163-6804/14/$25.00 © 2014 IEEE

This work is sponsored bythe Assistant Secretary ofDefense Research andEngineering (ASD-R&E)through Air Force Con-tract #FA8721-05-C-0002. Opinions,interpretations, recom-mendations and conclu-sions are those of theauthors and are not nec-essarily endorsed by theUnited States Govern-ment.

Bow-Nan Cheng, Freder-ick J. Block, B. RussHamilton, David Rip-plinger, Chayil Timmer-man, Leonid Veytser, andAradhana Narula-Tamare with MIT LincolnLaboratory.

INTRODUCTION

Airborne tactical networks (ATN) have been acritical communications capability for a numberof decades, enabling information sharingbetween both manned and unmanned militaryaircraft as well as surface and ground plat-forms. Similar to the commercial world, mili-tary communications needs have evolved andgrown immensely, and there is a desire to sup-port an ever-increasing number of users andemerging applications. To enable the promiseof net-centric operations for the warfighter,technologies for the next generation airbornetactical network must evolve to provide morecapacity, higher robustness, increased flexibility,better connectivity, improved interoperability,and faster response times for airborne andground users.

Challenges in designing ATNs arise from thehigh cost of platform integration, limited avail-

ability of spectrum, the need to support largernumbers of users in an infrastructure-less envi-ronment, the desire to provide much higher datarates and lower latencies, and the complexity inmanaging the numerous potential networkrequirements. Many of today’s ATNs meet thesediverse and stringent requirements through avertically integrated solution that tightly couplesall layers of the network stack from the applica-tion layer down to the physical layer, whereincommunications is essentially an integratedextension of the platform applications. Althoughthe concept of modularity is a foundation of thecurrent Internet, the air tactical domain hasembraced a tightly coupled, vertically integratedarchitecture in hopes of increasing efficiency.Unfortunately, this has led to a lack of interop-erability and the inability to upgrade layers inde-pendently.

Future ATNs will need a hierarchical, modu-lar architecture similar to that of the Internet toknit together the various heterogeneous systemsat a common convergence layer and improveinteroperability. By converging on the networklayer, waveform/radio developers can focus onbuilding link and physical layer technologiesindependent of application developers, andapplications can be defined to work over theconverged network, thereby allowing seamlessoperation over heterogeneous systems. Networklayer convergence also enables the coordinatedand collaborative utilization of the multiple het-erogeneous communications links available onmost platforms, thereby providing increasedrobustness, improved performance, and higheravailability. Modular designs enable easier tech-nology refresh and decoupled evolution of algo-rithms and technologies.

Although improved antenna and radio fre-quency (RF) technologies can increase potentialdata rates and performance, the cost of platformintegration of these new components can be pro-hibitive, and the ability to procure spectrum inalternate bands is becoming more difficult. As aresult, we consider signal processing and net-

ABSTRACT

Airborne tactical networks (ATNs) have pro-vided protected air-to-air communications formilitary aircraft for several decades. To supportemerging and future warfighter needs, the nextgeneration of systems will require significantimprovements to provide higher capacity, longerrange, greater flexibility, and increased interop-erability. Governed by domain characteristicssuch as long transmission ranges, low-to-mediumdata rates, latency constraints, and link protec-tion needs, the air tactical domain poses severalunique requirements on link and network design.Developing next-generation ATNs requires anunderstanding of the airborne tactical domain,including the design constraints and challengesat various layers of the network stack. In thisarticle, we provide an overview of the uniquedomain characteristics of ATNs and highlightthe key design challenges and research areasassociated with the physical, link, and networklayers.

ENABLING NEXT-GENERATION AIRBORNECOMMUNICATIONS

Bow-Nan Cheng, Frederick J. Block, B. Russ Hamilton, David Ripplinger, Chayil Timmerman,

Leonid Veytser, and Aradhana Narula-Tam

Design Considerations for Next-Generation Airborne Tactical Networks

CHENG_LAYOUT_Layout 5/7/14 1:12 PM Page 138

IEEE Communications Magazine • May 2014 139

working technology approaches that may reuseexisting or planned platform RF antenna andhardware technologies with or without substan-tial RF hardware upgrades. We focus on threekey areas of research that have the potential toprovide large improvements in network connec-tivity, capacity, and interoperability for ATNs:physical layer design, medium access control(MAC) layer design, and network layer design.Enhanced designs of the physical layer to incor-porate modern signaling and coding techniquescan increase capacity and reliability. Develop-ment of efficient MAC protocols can improvebandwidth efficiency and reduce latency byreducing guard times and allowing dynamic con-figurability. Modular network layer architecturessuch as those used in the commercial Internetcommunity can enable interoperability throughIP convergence, multi-hop reachability for longerrange communication, ease of configuration andmanagement, and design simplicity reducingredundant and sometimes conflicting efforts atmultiple coupled layers.

In the commercial world, wireless capacityand capability leverages the proliferation of awired infrastructure. For example, much wirelesscommunication is via a single wireless hop to celltowers connected by optical fiber links. This typeof infrastructure is not available in the airbornetactical domain, and as such, the network thatneeds to be designed is a highly mobile ad hocnetwork (MANET). Much of the research onMANETs as well as lessons learned from groundnetwork implementations can be leveraged, but,as described below, the airborne tactical domaincan be quite different from various ground com-munications domains, with unique requirementsand environmental characteristics.

In this article, we first overview the air tacti-cal domain characteristics, identifying typicaltraffic requirements, communications ranges,communications channel characteristics, interfer-ence considerations, spectrum sharing and usagerequirements, and platform integrability con-straints. Next, we highlight the design considera-tions for next-generation ATNs at the physical,link, and network layers of the network stack,identifying:• The key characteristics of the environment

as they impact each layer.• The key design challenges.• Areas of research where contributions could

provide significant ATN performanceimprovements.

Although we propose a modular approach tointroduce and design ATNs, these sections high-light how choices at the physical layer clearlyimpact designs at the MAC layer and vice versa.A modular network layer approach enables inte-grating multiple and diverse physical/link layers.The goal is to help researchers and protocoldesigners understand key design considerationsfor improving ATN performance through physi-cal, link/MAC, and network layer technologies.

DOMAIN CHARACTERISTICSDeveloping next-generation ATNs requires anunderstanding of the communications require-ments, environmental conditions, and platform

integration constraints that comprise the air-borne tactical domain. The airborne tacticaldomain has several unique characteristics thatneed to be accounted for by protocol and systemdesigners. As shown in Fig. 1, ATNs are oftencharacterized by the following.

Low data rates: Many of today’s tacticalapplications require transmitting short messagesand as such, many communications needs can befulfilled with low data rates. Future applications,however, could potentially consume larger datarates. Due to limited spectrum and the need forinterference mitigation capabilities, ATNs typi-cally operate on the order of 10’s of Kbps to100’s of Kbps. Protocols that rely on floodingand network-wide synchronization of link infor-mation can consume too much bandwidth.

Significant multicast traffic: Much of thetraffic carried by ATNs is designed to bereceived by multiple participants. Networkdesign must prioritize one-to-many type trafficover traditional unicast approaches.

Latency constraints: ATNs carry time-sensi-tive information, thus much of the pipeline isoptimized for latency guarantees, and it is impor-tant to drop out-of-date messages to reduce inef-ficient resource utilization.

Long transmission ranges: Typical transmis-sion ranges can exceed 500 kilometers, depend-ing on altitude, modulation, code rate, andtransmit power. Additionally, because neighbor-ing aircraft can be anywhere from a few metersto hundreds of kilometers away, propagationdelays between nodes vary from nanoseconds tomilliseconds, and the received powers from dif-ferent transmitters can differ by several 10s ofdecibels. Such large differences in power cancause the weaker signal from the far node to becompletely lost due to interference from thestronger near node.

Body blockage effects: Although there is littleto no multipath fading in airborne links, aircraftbody blockage can add seconds to minutes worthof outages that need to be accounted for in pro-tocol design. Body blockage effects vary signifi-cantly due to operational frequencies, theplacement of the antenna(s), and orientation ofthe aircraft.

Figure 1. Unique domain characteristics of airborne tactical networks.

Latencyconstraints

Airborne tactical networks

One-to-manytraffic

100s of km

Signal aircraftbody blockage


IEEE Communications Magazine • May 2014140

Mobility patterns: Some tactical aircraftmaintain predictable orbits to form communica-tions relays while others fly attack runs. Whenaircraft are engaged, the mobility patterns canbe very sporadic. While many studies have beenperformed on MANET routing protocols withrandom waypoint mobility and trajectory-basedmobility, much less work has been done in pre-dictable orbit mobilities with realistic aircraftmaneuvering.

Spectrum considerations: Spectrum availabil-ity is limited, so new systems must coexist withexisting systems. Efficient use and sharing ofavailable spectrum is highly desired.

Operation in adversarial environments: Air-borne tactical networks operate in environmentswhere adversaries might actively target the sys-tem. This may require employment of interfer-ence mitigation, jam resistance, and secureprotocols.

High cost of platform integration: Installingnew RF hardware and antennas on existing air-craft can incur significant integration costs. Evo-lution of new system technologies may befacilitated by reusing existing antennas and RFhardware. Due to the high cost of platform inte-gration, the life of many existing systems aredecades long. Deployments of new designs musthave an integration path where new and old sys-tems coexist.

The unique domain characteristics governingATNs help frame design constraints and consid-erations. In the following section, we will identifyhow these domain characteristics impact thedesign of the first three layers of the networkstack.

DESIGN CONSIDERATIONSInformed by the ATN domain characteristics,this section examines the key design challenges,potential solutions, and areas of future researchat the physical layer, the link/MAC layer, andthe network layer.

PHYSICAL LAYER DESIGN CONSIDERATIONSThe physical layer is responsible for variousencoding and signaling functions that enabletransmission and reception of bits over a wire-less medium. Two of the major challenges indesigning physical layers for next-generationATNs are:• The airborne wireless channel.• Interference resiliency.In this subsection, we detail these two issues andhow they affect the design of ATN physical lay-ers.

The channel in an airborne MANET differssignificantly from that in a terrestrial network. Inground networks, the channel between two nodestends to be dominated by fading. In ATNs, how-ever, two communicating aircraft will often haveline-of-sight (LOS) connectivity even if separat-ed by several hundred kilometers. Althoughthere may be limited multipath and fading dueto ground reflection [1] and aircraft antennablockage [2], which are highly dependent on air-craft geometry, antenna placement, and groundterrain, these effects are generally much lowerthan those experienced in ground-to-ground

communications. Another key difference fromterrestrial networks is the much higher relativevelocity between nodes, potentially causing sig-nificant Doppler frequency offsets for which thereceiver must compensate.

The large region over which an aircraft hasLOS coverage allows the communication links tospan large distances. Design of airborne commu-nications systems must balance the tradeoff ofusing omni-directional communications versusdirectional communications techniques. Omni-directional systems simplify connectivity by low-ering coordination overhead, but require highpower transmissions to close long-distance links.Additionally, omni-directional systems cause sig-nificantly higher interference to neighbors.Directional systems can provide additional gainand minimize interference, however, antennasize constraints may require the use of highercarrier frequencies which suffer more from theDoppler shift. Additionally, the use of direction-al antennas requires accurate pointing, whichmay be complicated by the high mobility of theaircraft.

In ATNs the path loss may vary by over 50 dBdue to the different ranges of neighboring air-craft. Link adaptation may be needed to handlesuch a wide range of channel conditions. Usinghigher code rates and more spectrally efficientmodulation when distances are small and pathloss is relatively low can improve spectral effi-ciency. Spatial reuse increases (and hence over-all network capacity improves) by reducingpower levels when transmitting to nearby nodes[3]. However, even with power and rate adapta-tion, the radio still needs to be able to discernnear and far users. Due to the extreme ranges insignal-to-noise ratio (SNR), the radios willrequire a sophisticated analog front end.

The LOS range of the aircraft may makethem susceptible to interference from otherground or airborne systems operating in thesame band, and the airborne network may sim-ilarly cause interference to these other systems(Fig. 2). As an example, Link 16 operates inthe aeronautical radio navigation bands(960–1215 MHz) for ground/aircraft and satel-lite and employs techniques to minimize inter-ference to these systems [4]. Attempts to reducethe potential for interference by limiting use ofparticular channels to certain geographic regionsmust account for both the large LOS distancesand the high mobility of the aircraft. A furthercomplication for tactical airborne networks isthat the interference may be hostile and notamenable to mitigation through coordination.Interference resiliency can be achieved throughwaveform design (e.g., spread spectrum tech-niques [5]). Multiuser detection (MUD) can alsobe beneficial if interference is due to other net-work nodes. Successful use of MUD brings manychallenges, such as the complexity of processinglarge numbers of users and, if ad hoc networkingis used, the potential lack of coordinationbetween nodes [6]. Although MUD is often con-sidered in conjunction with direct-sequence codedivision multiple access (CDMA), it can also beapplied to other signaling techniques such as fre-quency-hop spread spectrum. In this case, theMUD receiver may need to handle only a small

The physical layer is

responsible for vari-

ous encoding and

signaling functions

that enable transmis-

sion and reception of

bits over a wireless

medium. Two of the

major challenges in

designing physical

layers for next-gener-

ation ATNs are the

airborne wireless

channel and interfer-

ence resiliency.



number of users colliding on a hop, and addi-tional complexity may result from having to esti-mate the relevant channel parameters on ahop-by-hop basis [7]. Antenna directivity canalso provide interference mitigation.

The primary concern for the implementationof an airborne tactical radio is its size, weight,and power (SWaP). The signal processingrequirements of such a radio are approximatelyon the same order as a modern mobile phone.However, the production volume which enablesinvestments in SWaP miniaturization in the com-mercial cellular world simply does not exist inthe airborne tactical market. Directional anten-nas, advanced receiver technologies (to capital-ize on spread spectrum and/or MUDs), andchallenges associated with fast-moving airborneplatforms further increase the SWaP of themodem. Additionally, large and relatively ineffi-cient high power amplifiers (HPAs) are requiredto establish and maintain the desired link rangesof several hundred kilometers.

Production of a cost-effective yet appropri-ately sized radio is difficult, and generally forcesmany technology tradeoffs. Multi-packet recep-tion (MPR) and MUD techniques may signifi-cantly reduce the effects of networkcongestion/interference and increase overallthroughput. However, these techniques come ata high cost to modem SWaP requirements. Inthe case of MPR, the modem must be designedwith multiple demodulators to process the addi-tional packets, dramatically increasing all threevariables in the SWaP equation. In some casesthe processing speed of the demodulators maybe increased to reduce the size and weight of themodem, at the cost of increased power. MPRsignificantly improves spectral efficiency byenabling improved sharing of the communica-tions channel, and as hardware capabilities con-tinue to improve, the increased SWaP is oftenjustified. Technologies such as staring receiversenabling the reception of multiple packets simul-taneously have been demonstrated. The numberof packets that can be received concurrently aswell as the number of packets that can be simul-taneously processed impacts hardware complexi-ty, and again there is a tradeoff betweencapability and SWaP. The effects of MUD pro-cessing are directly related to the method ofdetecting multiple transmissions and how thatinformation is exploited within the modem.From the myopic viewpoint of modem complexi-ty and SWaP, MUD processing is very costly;however, the increased throughput and protec-tion from certain types of interference may justi-fy those costs.

LINK/MAC LAYER DESIGN CONSIDERATIONSThe link and medium access control (MAC)layer is responsible for establishing RF linksbetween neighboring aircraft, informing the net-work layer of these links and their current condi-tions, and managing access to shared networkcommunication resources with some notion offairness between aircraft. ATNs are oftendesigned to operate over a wide bandwidth, forexample, by frequency hopping. In such a case,radios can tolerate a significant amount of multi-ple-access interference, and the MAC can be

designed to allow multiple simultaneous trans-missions and MPR. While these techniquesincrease the effective capacity of the network,they also complicate the MAC and physical layerdesign. The MAC must control the multipleaccess interference, ensure the receiver process-ing capabilities at each node are not exceeded,and limit the losses due to transmit-while-receiveinterference, as most radios will cause self-inter-ference to their receiving subsystem while trans-mitting. In designing link/MAC layers fornext-generation ATNs, the major design chal-lenges are suitably managing the interferenceassociated with a spread spectrum MPR systemunder:• Long and highly varying propagation delays.• Varying receive power levels.In this subsection, we detail these two issues andhow they may be addressed in the design ofATN link/MAC layers.

The problem of medium access control inATN mobile ad hoc environments is much morecomplicated than those of centralized or fixednetworks. Specifically in ATNs, the distancesbetween nodes can range from tens of meters tohundreds of kilometers, meaning that the propa-gation delay can range from tens of nanosecondsfor nearby nodes to several milliseconds atlonger ranges, as illustrated in Fig. 3. In mostWiFi networks and cellular networks, there is acentral access point that is wired into the Inter-net and provides one-hop wireless connectivityto client devices. This setup makes mediumaccess control straightforward, since the accesspoint can control which device transmits orreceives at any time in a centralized manner.Furthermore, since the central access point canact as a common receiver to all of the clientdevices, the devices can synchronize their trans-missions based on when their signals arrive atthe access point. Unfortunately, in a full meshMANET (such as an ATN) there is no computa-tionally efficient method to compensate for dif-

Figure 2. The altitude of the airborne network makes it susceptible to inter-ference from a variety of sources such as navigation systems, radar, etc.Link 16, for example, operates in the aeronautical radio navigation fre-quency band which is heavily occupied by a number of ground, airborne,and satellite systems.



ferences in propagation delay, especially sincemuch of the traffic is broadcast or multicast andhence must be received by multiple destinations.

In ATNs, the duration of each transmissionneeds to be limited to several milliseconds, atmost, in order to meet the low-latency con-straints of tactical networks. Since the variationin the propagation delays is on the same orderas the transmission duration, transmissions thatare completely separated in time at one node inthe network could overlap (collide) at anothernode [8]. Carrier sensing is a technique, com-monly used in ad hoc WiFi networks, where aradio first listens to the channel for a short timebefore transmitting and then only transmits ifthe channel is idle. ATNs, however, cannotleverage this approach because the propagationdelay can be on the same order of magnitude asthe transmission duration.

Two viable ATN MAC options include thetraditional random access protocol (ALOHA),where a node, based on information gatheredabout the current local network conditions,waits a random amount of time (the backoff)between transmissions. In a scheduled MAC,either nodes or links in the network are explicit-ly assigned time to transmit. Scheduling has anadvantage over random access in that it can pre-cisely control which nodes or links in the net-work are active at any time. The result is higherthroughput per link. Random access requiresless node coordination and generally enableslower latency operations, but inherently lacksprecise coordination, which may lead to exces-sive collisions or underutilization of the medi-um. In a narrowband wireless local area network(WLAN), it is well known that the inefficienciesof random access result in a throughput capacityonly 1/e of scheduling if time is slotted (1/2e iftime is unslotted) [9]. However, with a wide-band ATN using frequency hopping, it has beenshown that the disparity in the throughputbetween scheduling and random access is notnearly so great [10].

A dynamically scheduled MAC protocolpotentially provides throughput benefits due tothe precise control of the multiple access inter-ference and the ability to limit node transmis-sions during scheduled receptions. In afrequency-hopped system with MPR, the sched-uler must assign multiple simultaneous transmis-sions and concurrent listeners across multiplefrequency bands in a consistent manner for allneighbors. Coordination and control of theschedule in a distributed manner trades off over-head, complexity, scheduling gains, and network

stability. One method to lower the overhead indynamic scheduling is to reduce the amount ofstate information exchanged by using implicitscheduling, such as Lyui’s algorithm [11], wherea single slot request can result in additionallyobtaining some previously unused slots in theschedule. Another inefficiency of scheduling isguard times. In order to ensure that a packet isboth transmitted and received within a designat-ed time slot, a guard time at least as long as themaximum propagation delay in the networktakes up the latter portion of every slot.

Similar to other wireless networks, ATNs suf-fer from the near-far problem [12]. This is theproblem of maintaining a far link while there issignificant nearby unintentional or intentionalinterference. The far transmitter can improvethe link by increasing power or lowering the datarate. Traditional power control techniquesapplied in cellular (centralized) networks are notas useful since a user that increases its power totransmit to a far node will cause additional inter-ference to nodes in its local neighborhood. In afrequency-hopped system, a source far from thereceiver can use additional coding to mitigatethe interference, as not all hops will generally belost. In some cases, it may be desirable to pro-vide a higher data rate to the far link. When theprimary source of interference is from neighbor-ing multiple-access nodes, an alternative solutionis for the nearby nodes to coordinate with thedisadvantaged receiver and transmit at a lowerpower or less frequently. Other mitigationsinclude MUD and prioritization of nodes/linkswithin the backoff algorithm. Finding the rightbalance between these techniques merits furtherresearch to determine both the optimal solutionand the required coordination.

NETWORK LAYER DESIGN CONSIDERATIONSFor future ATNs, an appropriately designed net-work layer architecture enables several desiredcapabilities. First, convergence at the networklayer provides a mechanism for carrying trafficover multiple heterogeneous links and systemsand is one of the key elements for enablinginteroperability. Second, the network layerenables the ability to autonomously add andremove nodes and links from the network. Third,providing multi-hop routing at the network layerenables end-to-end traffic delivery despite under-lying time-varying topology and link dynamics.

When systems tightly couple the design of theapplication, network, link, and physical layers,system interoperability is generally providedfrom one system to another through application-layer gateways. These gateways translate applica-tion-layer messages between each pair ofsystems, creating an n2 translation problem.Alternatively, by leveraging the hierarchicalmodular principles that made the Internet a suc-cess and converging on Internet Protocol (IP) asthe common addressing scheme (not necessarilythe full TCP/IP suite), future ATNs can enableeasier interoperability.

Many platforms carry multiple radio systemsdesigned to operate either in different environ-ments or to transmit different message sets. Theability to seamlessly interconnect these heteroge-neous radio systems enables additional robust-

Figure 3. Addressing long propagation delays and near-far issues are twokey design considerations of the link/MAC layer in ATNs.

Large propagationdelay issue

100s of kmms prop. delay

10s of meters<µs prop. delay



ness in the face of typical airborne environmentswhich are susceptible to interference (uninten-tional or intentional), link outages due to anten-na blockages from aircraft maneuvering, andweather [2].

Future ATNs can also leverage MANETtechnologies to enable automatic node discoveryand route self-healing. As a result, network flexi-bility and range are increased significantly. Inorder to make this modular, IP-converged designapproach feasible for ATNs, several key compo-nents are needed:• Header compression.• Radio-to-router interface (R2RI) definition.• Multi-topology MANET routing.

In tactical networks, many of the messagesare short. By moving to IP, headers may domi-nate the traffic and waste precious resources. Avoice-over-IP (VoIP) transmission between twoaircraft might incur 60 bytes of header (40 bytesIPv6, 8 bytes UDP, 12 bytes RTP) for a 10-18byte message, depending on the codec used.Employing commercial header compressiontechniques can potentially reduce this to 4-6bytes. Additionally, pushing IP packets throughLink 16 is inefficient because IP headers con-sume much of the limited Link 16 capacity [13].In previous work [14], commercial IP headercompression protocols as well as a new proposedstateless header compression approach, MANETIP Header Compression (MIPHC), were evalu-ated for suitability in the MANET environment.It was shown that commercial header compres-sion techniques work fairly well in a MANETenvironment because of the ability to re-use pre-viously built state between compressor anddecompressor even though the topology is regu-larly changing. However, these techniques sufferfrom potential security vulnerabilities on statesetup and refresh. MIPHC did not provide asmuch compression gains as the commercialstateful protocols, however it provided lowerlatency operation and resilience toward attack.

Cross-layer optimization can be achievedthrough two main methods, as shown in Fig. 4:• Tightly coupling the network stack and

allowing multi-hop routing decisions toaccess link layer information directly.

• Modularizing key components by definingthe radio or link layer to provide the bestone-RF hop possible, using an externalrouter to perform multi-hop routing, anddefining a standard radio-to-router inter-face (R2RI) to pass relevant informationbetween the radio and router.

One of the advantages of a tightly coupled designapproach is the potential for cross-layer designand optimization to enable efficient utilization ofthe limited wireless resources. It is well knownthat by jointly optimizing routing and mediumaccess control, significant capacity gains can beachieved [15]. However, this approach limits thesystem to homogeneous nodes and links.

With a modular network architecture, effi-cient and informed use of the multiple heteroge-nous links can be enabled by passing cross-layerinformation between the radio and the router.Link quality, reliability, availability, SNR, andother statistics affect the usability of a link, andan informed network layer can leverage this

information to optimize and tailor routing deci-sions to traffic flows. Similarly, higher-layerinformation provided to the link layer allows thelink layer to optimize link resource allocationbased on traffic priority or end-to-end deliveryrequirements.

To enable the cross-layer optimization withphysically separate and diverse radio and routercomponents, a standardized radio-to-routerinterface (R2RI) is needed. Several R2RI tech-niques are explored in [16], and it was discov-ered that although access to instantaneous linkinformation speeds up reaction to link outages,modifying routes based on small changes to linkmetrics can yield significantly higher overheadand instability. Open areas of research includethe definition of clear and appropriate standard-ized metrics, design and optimization of routingprotocols to effectively use the link metrics, andflow control strategies between the router andradio. Abstracting radio systems to a set of linkswith associated link metrics through the R2RIenables research into multi-topology routingprotocols that leverage a heterogeneous set oflinks and link metrics to provide efficient, stable,and resilient routing over multiple radio paths.Although there are some schemes that havebeen proposed [17], a holistic test with represen-tative links is still needed.

ATN nodes must be able to relay informationto enable longer range and connectivity. Due totheir long transmission ranges (100s of kilome-

Figure 4. IP convergence, a common path to passing cross-layer informa-tion, and routing with heterogeneous link technologies are key aspects ofthe network layer design.

Radiosystem 1

Net(custom)

Network layer(modular)

Router

Link(custom)

Link

PHY

Radio/waveform

Link

PHY

Radio/waveform

Link

Pass crosslayer info

Commoninterface

PHY

Radio/waveform

Phy(custom)

Radiosystem 2

Net(custom)

Link(custom)

Phy(custom)

Radiosystem 3

Legacy systems

Net(custom)

Link(custom)

Phy(custom)

Next-generation A

TNs



ters), the number of RF hops for area coveragemay be only 3 to 4. Although aircraft speeds aremuch higher than ground speeds, the airbornetransmission range to mobility speed ratio yieldssignificantly less frequent topology changes dueto mobility than in a ground environment. Fur-thermore, the airborne environment does notsuffer from much fading. In an airborne environ-ment, the primary cause for more frequent topol-ogy changes may be due to link outages fromairframe blockage and interference. As a result,robust unicast and multicast MANET routingprotocols are needed to dynamically elect relaysand re-route information under varying link con-ditions and topology changes. Link outages are afunction of the transmission frequency as well asthe location of the antennas on the aircraft. Insome scenarios, the routing overhead associatedwith maintaining routes in ATNs may be rela-tively insignificant compared to available rates[18]. Additional research is needed to fully char-acterize airborne links as a function of frequen-cy, antenna placement, aircraft, and mobility.

Additional higher-layer research challengesinclude techniques to handle efficient multicastdissemination in a heterogeneous environment,congestion control and admission control to pro-vide suitable network performance, and the abil-ity to interoperate with legacy non-IP systems.There is also a need to develop a frameworkthat enables rapid and flexible development ofIP-based airborne applications.

SUMMARY

Table 1 highlights the key domain characteris-tics, design considerations, and research chal-lenges associated with each layer of the networkstack as it pertains to ATNs.

CONCLUSIONThe current generation of airborne tactical net-works (ATNs) are a set of links, networks, andapplications that allow communication betweenmilitary aircraft. To support growing warfighterneeds, the next generation of airborne tacticaldata links will need to provide higher capacity,longer range, greater flexibility, and increasedinteroperability between diverse systems. Thisarticle provides an overview of the ATN domaincharacteristics. These domain characteristics,such as high mobility, long transmission ranges,low data rates, constrained bandwidths, interfer-ence and blockage, latency requirements, etc.,frame the environment under which ATNs mustoperate.

Informed by these domain characteristics,some critical design considerations for next-gen-eration ATNs were presented as well as areasfor future research. At the physical layer, inter-ference resilience and spectral reuse considera-tions lead to potential research in spectrumsharing, multiuser detection, and interferencemitigation techniques. At the link layer, the longpropagation delays and near-far problem domi-nate medium access control designs and researchin developing efficient and flexible channel shar-ing mechanisms is needed. At the network layer,converging on IP and separating the radio capa-bility (one RF hop) from router functionality(multi-hop) enables greater interoperability andheterogeneity. Heterogeneous systems have thepotential to provide increased robustnessthrough frequency diversity, path diversity, andcapability diversity. Efficiently leveraging multi-ple heterogeneous radio systems is also a signifi-cant research challenge. Some of the techniquesand considerations described here may alsoapply to next-generation commercial airborneMANETs.

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[2] B.-N. Cheng et al., “Evaluation of a Multi-hop AirborneIP Backbone with Heterogeneous Radio Technologies,”IEEE Trans. Mobile Computing, vol. 13, no. 2, Feb.2014, pp. 299–310.

[3] P. Gupta and P. R. Kumar, “The Capacity of WirelessNetworks,” IEEE Trans. Inf. Theory, vol. 46, no. 2, Mar.2000, pp. 388–404.

[4] W. J. Wilson, “Applying Layering Principles to LegacySystems: Link 16 as a Case Study,” IEEE Military Com-mun. Conf., MILCOM 2001.

[5] M. B. Pursley, “The Role of Spread Spectrum in PacketRadio Networks,” Proc. IEEE, vol. 75, no. 1, Jan. 1987,pp. 116–34.

[6] J. Zhang et al., “Multiuser Detection based MAC Designfor Ad Hoc Networks,” IEEE Trans. Wireless Commun.,vol. 8, no. 4, Apr. 2009, pp. 1836–46.

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Table 1. Summary of airborne tactical network design considerations.

Design Consideration Research Challenges

Spectral reuse (PhysicalLayer)

–Techniques to share spectrum with long-range omni-directional transmissions.

–Link adaptation (power and rate) to provide spectral efficiency.

Interference resilience(Physical Layer)

–Techniques to overcome interference fromintentional or unintentional sources (spread spectrum, directional antennas, MUD, etc.).

Long propagation delays(Link Layer)

–Efficient medium access control in spread spectrum systems with long propagation delays.

–Random access MAC backoff to manage node and channel resources.

–Dynamically scheduled MAC algorithms and coordination.

Near-far problem (LinkLayer)

–Near-far interference mitigation via coordinated power control, adaptive coding, MUD.

Radio interoperability(Network Layer)

–IP Convergence layer.–Separate radio (1 RF hop) functionality fromrouter (multi-hop) functionality.

–Radio-to-router interface definition and appropriate standard metrics to pass from radio to router.

Unicast/Multicast MANETrouting over heteroge-neous radios (NetworkLayer)

–Leveraging link metrics in routing decisions with network stability.

–Multi-topology routing over heterogeneous links.–Effective QoS and admission control in tactical network.



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BIOGRAPHIESBOW-NAN CHENG ([email protected]) is a member of Tech-nical Staff in the Airborne Networks Group at MIT LincolnLaboratory. His research interests include design, develop-ment, prototyping, and test and evaluation of next genera-tion routing and information disseminations solutions forairborne backbone and tactical networks. Recent work hasfocused heavily on radio-aware routing, which leverageslink layer information at the network layer to enhancemulti-hop MANET routing. He received M.S. and Ph.D.degrees in computer systems engineering from RensselaerPolytechnic Institute and holds a B.S. degree in electricalengineering from the University of Illinois at Urbana-Cham-paign.

FREDERICK J. BLOCK ([email protected]) is a member of theTechnical Staff in the Advanced Satcom Systems and Oper-ations Group at MIT Lincoln Laboratory. He received theB.S. (summa cum laude), M.S., and Ph.D. degrees in Electri-

cal Engineering from Clemson University, Clemson, SC. Hisresearch interests include spread-spectrum communica-tions, multiuser techniques, and packet radio networks.

BENJAMIN “RUSS” HAMILTON ([email protected])received his B.S. in Electrical Engineering from Auburn Uni-versity in 2005, his M.S. and Ph.D. in Electrical and Com-puter Engineering from the Georgia Institute of Technologyin 2007 and 2012. While at Georgia Tech, he performedresearch on synchronization and channel estimation inwireless systems and focused on distributed estimation inwireless networks for his dissertation. After receiving hisPh.D., he joined the Communications division of MIT Lin-coln Laboratory as a member of the Technical Staff. Hisresearch focuses primarily on communications and net-working in the airborne tactical domain.

DAVID RIPPLINGER ([email protected]) is an Associ-ate Staff member in the Airborne Networks Group at MITLincoln Laboratory. His research interests include theoreti-cal optimization, design, simulation, and testing of newprotocols for wireless networks, with an emphasis onmedium access control (MAC) design. Recent work hasfocused on characterization of the behaviors of randomaccess and scheduling MACs in a frequency hopping envi-ronment, as well as the design of low-overhead, distribut-ed random access protocols. He received an M.S. inComputer Science and a B.S. in Physics and Spanish Trans-lation from Brigham Young University.

CHAYIL TIMMERMAN ([email protected]) is a member ofTechnical Staff in the Advanced Satcom Systems and Oper-ations Group at MIT Lincoln Laboratory. His research inter-ests include design, development, and implementation ofadvanced communications systems. Recent work hasfocused on tactical communications systems, high band-width satcom systems, and dynamic code rate adaptation.He holds a M.S. from Villanova University and a B.S.E.E.from Drexel University.

LEONID VEYTSER ([email protected]) is a member of the Air-borne Networks Group at MIT Lincoln Laboratory. Hisresearch interests include routing and distributed comput-ing in disadvantaged wireless networks. His recent workhas focused on enhancing application performance at thetactical edge as well as radio-aware routing in airborneand tactical networks. He received his B.A. and M.A.degrees in Computer Science at Boston University.

ARADHANA NARULA-TAM ([email protected]) is an AssistantLeader of the Airborne Networks Group at MIT Lincoln Lab-oratory. Her research interests include topology design,resource scheduling, MANET capacity, networking proto-cols, and Quality of Service. She has worked on a variety ofsystems including optical networks, satellite communica-tion systems, and tactical wireless communication systems.She holds a B.S.E. degree from the University of Pennsylva-nia and S.M. and Ph.D. degrees from the MassachusettsInstitute of Technology, all in electrical engineering.

Efficiently leveraging

multiple heteroge-

neous radio systems

is a significant chal-

lenge. By abstracting

radios to a set of

links and associated

link metrics, efficient

multi-topology

routing mechanisms

can be employed

to route over

heterogeneous links

simultaneously.


Documents

Design considerations for next-generation airborne tactical networks