Millimeter-Wave Soldier-to-Soldier Communications …wireless communications will be required to relay information on situational awareness, tac-tical instructions, and covert surveillance

IEEE Communications Magazine • October 200972 0163-6804/09/$25.00 © 2009 IEEE

INTRODUCTION

The infantry soldier of tomorrow promises to beone of the most technologically advanced mod-ern warfare has ever seen. Around the world,various research programs are currently beingconducted, such as the United States’ Future

Force Warrior (FFW) and the United King-dom’s Future Infantry Soldier Technology(FIST), with the aim of creating fully integratedcombat systems. Alongside vast improvementsin protective and weaponry subsystems, anothermajor aspect of this technology will be the abili-ty to provide information superiority at theoperational edge of military networks by equip-ping the dismounted soldier with advanced visu-al, voice, and data communications. Helmetmounted visors, capable of displaying maps andreal-time video from other squad members,ranges of physiological sensors monitoring heartrate, core body temperature, and mobility,arrays of biochemical sensors detecting noxiousgasses, as well as a range of night-vision andheat-sensing cameras will all become standardissue. These devices will improve situationalawareness, not only for the host, but also for co-located military personnel who will exchangeinformation using mobile ad hoc (wireless) net-works (MANETs).

The integration of body-worn wireless sys-tems into the dismounted combat soldier plat-form will present a unique set of challenges toscientists and engineers alike. Wireless deviceswill be expected to operate in a range of envi-ronments much more diverse than thoseencountered in civilian applications, yet stillmaintain an ultrahigh level of performance interms of reliability and efficiency. From a mate-rial perspective, these wireless devices must becompact, lightweight, unobtrusive to soldiermovements, and ideally mounted conformal tothe body surface. As soldiers may go for daysbetween opportunities for battery recharge orreplacement, power saving will be concurrentthroughout all layers of the protocol stack.Physical layer (PHY) technologies must beresilient to jamming, able to operate in marginalconditions while using the minimal amount ofenergy conserving hardware. The medium accesscontrol (MAC) layer must also be designed toact in a power-saving bandwidth-efficient man-ner while maintaining excellent quality of ser-vice (QoS). Wireless security will be critical tomaintaining a tactical edge as message intercep-

ABSTRACT

Mobile ad hoc networking of dismountedcombat personnel is expected to play an impor-tant role in the future of network-centric opera-tions. High-speed, short-range, soldier-to-soldierwireless communications will be required torelay information on situational awareness, tac-tical instructions, and covert surveillance relateddata during special operations reconnaissanceand other missions. This article presents someof the work commissioned by the U.K. Ministryof Defence to assess the feasibility of using 60GHz millimeter-wave smart antenna technologyto provide covert communications capable ofmeeting these stringent networking needs.Recent advances in RF front-end technology,alongside physical layer transmission schemesthat could be employed in millimeter-wave sol-dier-mounted radio, are discussed. The intro-duction of covert communications betweensoldiers will require the development of abespoke directive medium access layer. A num-ber of adjustments to the IEEE 802.11 distribu-tion coordination function that will enabledirectional communications are suggested. Thesuccessful implementation of future smartantenna technologies and direction of arrival-based protocols will be highly dependent onthorough knowledge of transmission channelcharacteristics prior to deployment. A novelapproach to simulating dynamic soldier-to-sol-dier signal propagation using state-of-the-artanimation-based technology developed for com-puter game design is described, and importantchannel metrics such as root mean square angleand delay spread for a team of four networkedinfantry soldiers over a range of indoor and out-door environments is reported.

MILITARY COMMUNICATIONS

Simon L. Cotton and William G. Scanlon, Queen’s University of Belfast

Bhopinder K. Madahar, Defence Science and Technology Laboratory

Millimeter-Wave Soldier-to-SoldierCommunications for Covert Battlefield Operations

SCANLON LAYOUT 9/21/09 3:59 PM Page 72

IEEE Communications Magazine • October 2009 73

tion and decryption could lead to compromiseof the mission. When combined with covertcommunications (by covert we mean that signaltransmissions remain hidden from the enemy),we have the prospect of achieving secure androbust wireless MANETs while maintaining theelement of surprise. These are formidable chal-lenges, but may be surmountable using bothrecent developments in millimeter-wave (mm-wave) transceiver technology [1], and the 5–7GHz of contiguous bandwidth currently beingmade available throughout the world in the 60GHz mm-wave band [2].

In this article we present some of the workundertaken in conjunction with the U.K. Min-istry of Defence (MOD) to investigate the feasi-bility of using mm-wave body-worn antennaarrays to provide covert mobile ad hoc wirelessnetworking for dismounted combat personnel.The objective of this article is twofold. First, webegin by introducing the concept of a soldier-to-soldier MANET and briefly discuss some of thecompeting air interface technologies that couldbe used to provide high-speed wireless network-ing for dismounted combat personnel. We thendiscuss some of the potential issues at the PHYand MAC layers in relation to the implementa-tion of stealthy, high-data-rate, mm-wave sol-dier-to-soldier communications. One of the keychallenges that remain for military hardware andnetwork designers is the simulation of the wire-less transmission channel. A good approximationof channel characteristics is fundamental to test-ing the performance of newly designed proto-cols, and understanding the required operatingmargins for front-end radio design. Therefore,the second section of this article takes a novelapproach to the simulation of the wireless trans-mission channel by exploiting state-of-the-artanimation-based technology developed for com-puter game design to accurately encapsulate thedynamics and mobility of soldier movement inthe simulation of signal propagation within sol-dier-to-soldier MANETs.

SOLDIER-TO-SOLDIERMANET CONCEPT

The concept of a short-range soldier-to-soldierMANET is illustrated in Fig. 1. In this exam-ple a small team of co-located infantry troopsare wirelessly networked to facilitate high-speed communications within a cluttered urbanwarfare environment. As the combat teamprogress through the environment their com-munications requirements will be extremelyvaried, with needs ranging from short messagetext (e.g., spoken by the receiving terminal ordisplayed on a helmet mounted visor) andpeer-to-peer voice (avoiding the need forshouting or hand movements), through to real-time streaming video. What is fundamental,however, and a key discrimination betweensoldier-to-soldier MANETs and otherMANETs, is that the communications aresecure and resilient, with a low probability ofdetection and low probability of intercept (i.e.,inherently stealthy). This is especially true forspecial operations forces where knowledge byenemy forces of increased activity in anyregion of the radio spectrum may lead to dis-covery, capture of transmitted data, and/orinterference with it. Such intelligence of, orinference of intent from, communications maycompromise operations, for example, byrevealing the movements of the forces or lossof the element of surprise.

SHORT-RANGE COVERTAIR INTERFACE TECHNOLOGIES

To achieve optimal network-centric opera-tions, tactical information must be effectivelydistributed among soldiers while maintaining alow probability of detection and intercept.Ultra-wideband (UWB) is an air interfacetechnology that could supply sufficient band-

Figure 1. Soldier-to-soldier MANET concept.

Urban multipath environment

Soldier-to-soldier MANET Repeater (optional)

Direct LOS link

A

B

D

C

Tracked NLOS reflection B to C

B to D link routed through node C

Uplink to operational command

What is fundamental

and a key

discrimination

between soldier-to-

soldier MANETs and

other MANETs is

that the

communications are

secure and resilient,

with a low

probability of

detection and low

probability of

intercept, that is,

inherently stealthy.


IEEE Communications Magazine • October 200974

width to meet the high data rate requirementsof future body-worn military communicationssystems. UWB radios operate by employingvery short duration signal pulses that result inlarge transmission bandwidths. The U.S. Fed-eral Communications Commission (FCC)defines a UWB device as any device where thefractional bandwidth is in excess of 0.2 of thearithmetic center frequency or greater than500 MHz, whichever is less [3]. The FCC havegranted permission for unlicensed UWBdevices to operate in the 3.1–10.6 GHz fre-quency range with the spectral density emis-sion limit set at –41.3 dBm/MHz to reduceinterference with other co-located wireless sys-tems operating within the same spectrumspace. The stringent transmit power limita-tions placed on UWB devices have been cho-sen so that they minimize the risk ofinterference to authorized radio services byoperating close to the noise floor. This is afeature that many in the military communitywill find attractive as it introduces a lowerprobability of detection compared to conven-tional wireless systems. UWB could providethe dismounted soldier with a maximum datarate of 100 Mb/s [2] for transmitter-receiverseparations less than 10 m and as much as 480Mb/s at very small separation distances (typi-cally less than a few meters). Improvements tooperating distance and channel capacity couldbe realized by raising the spectral densityemission limit; however, such a move couldprove unpopular as it will lead to increasedinterference with licensed radio users andremove the stealth mode of operation, leadingto easier detection by the enemy.

Compared to UWB, 60 GHz mill imeterwave communications will operate in current-ly under-utilized spectrum space and providehigh data rates of up to several gigabits persecond for short-range applications [2]. Oper-ating ad hoc network communications for dis-mounted combat personnel at 60 GHz willoffer a number of distinct advantages com-pared to the other competing lower-frequencytechnologies. Factors that would generally beconsidered to hinder traditional radio com-munications can be exploited to provide thedesirable signal propagation characteristicsrequired for short-range military communica-tions. These include increased covertness ,high frequency reuse , and reduced risk ofinterference (which may be attributed to high-er path loss), increased atmospheric oxygen(O2) absorpt ion, and narrow antennabeamwidth. Another important feature ofmm-wave frequencies i s the smal l s i ze ofproduct that may be achieved. Ultra-low formfactor transceiver design will become realitydue to the extremely short wavelength (λ ≈ 5mm), which will also facilitate the construc-tion of wearable smart antenna arrays capa-ble of electrical ly steering highly focusedbeams of electromagnetic energy in chosendirections. To realize the objective of direc-tional mm-wave communications, there arest i l l a number of hurdles at the PHY andMAC layers that need to be overcome, as dis-cussed below.

MM-WAVE SOLDIER-TO-SOLDIERCOMMUNICATIONS:

PHY LAYER CHALLENGES

CHANNEL CHARACTERISTICS

It is widely recognized that the successful devel-opment of hardware and wireless networkingprotocols is highly dependent on a thoroughknowledge of transmission channel characteris-tics relative to deployment. Much of the currentresearch involving mm-wave short-range commu-nications has been carried out considering arange of indoor environments for stationarytransmitter and receiver scenarios [4]. Here sta-tistical descriptors of the channel, such as pathloss exponent and root mean square (rms) delayspread, are found to be heavily influenced byantenna configuration and the local surround-ings. While these studies are useful for the devel-opment of indoor wireless networks, any attemptto apply this channel information to the designof mm-wave soldier-to-soldier networks wouldbe inappropriate, especially considering issuessuch as the scattering of signals from both usersand pedestrians, and the inherently dynamic andhighly mobile nature of military operations.

At present, very little is known about thecharacteristics of signal propagation betweenwearable wireless devices forming a humanbody-to-body network (BBN). Recent narrow-band studies at 2.45 GHz [5] have shown thatsignal propagation is dependent on the user’sphysical characteristics, including mobility, andmay be modeled using κ–μ fading statistics [6].The effect of human body shadowing on mm-wave wireless links has received some coveragein the literature. In [7] it is reported that humanbody shadowing can cause attenuations ofgreater than 20 dB on indoor 60 GHz device-to-device links. Field trials performed for this studyto investigate human body shadowing events onindoor point-to-point links have found similarresults (attenuations of 20–25 dB), with thegreatest shadowing events found to occur whenthe human body moved in the direct vicinity of a60 GHz node, blocking line of sight (LOS).

TRANSMISSION SCHEMESThere are a number of different transmissionschemes that could be adopted for soldier-to-sol-dier communications. These include the singlecarrier (SC) and orthogonal frequency-divisionmultiplexing (OFDM) schemes currently beinginvestigated by IEEE 802.15 TG3c [8]. OFDM iswell known for its ability to mitigate against fre-quency selective fading due to multipath, by turn-ing the transmission channel into a series ofsuitably modulated (e.g., quadrature amplitudemodulation) orthogonal subcarriers. This has theeffect of greatly reducing the complexity oftransceiver design through the use of inverse fastFourier transform (IFFT) and FFT signal pro-cessing stages for signal transmission and recep-tion, respectively, and negates the need forintricate wideband equalizers. While OFDM maybe resilient to multipath effects, it is prone to ahigh peak-to-average power ratio (PAPR), phasenoise, and carrier offset. High PAPR will be a

Operating ad hoc

network

communications for

dismounted combat

personnel at 60 GHz

will offer a number

of distinct

advantages

compared to the

other competing

lower frequency

technologies.



particular problem for soldier mounted radios, asit will cause nonlinear distortion and low powerefficiency in the power amplifier [2], directlyimpacting battery life. The complexity of time-domain channel equalization in wideband SC sys-tems is regarded as its main drawback for use inhigh-data-rate mobile radio channels. However,this challenge can be overcome through the useof frequency domain equalization (FDE). Singlecarrier systems with FDE (SC-FDE) typically usetransmission blocks with a cyclic prefix to preventinterblock interference. Signal recovery at thereceiver is then performed through FFT process-ing with equalization followed by an IFFT stage.SC-FDE will then deliver performance similar toOFDM, with essentially the same overall com-plexity [9], but because SC modulation uses a sin-gle carrier it has the added advantages of lowerPAPR and less sensitivity to both phase noiseand carrier offset [10].

RF FRONT-END TECHNOLOGYThe choice of 60 GHz RF front-end technologyfor soldier mounted radio will introduce a trade-off between performance and cost. Traditionallygroup III–IV semiconductor technologies such asgallium arsenide and indium phosphide havebeen used for mm-wave radios. While they offersuperior noise characteristics and high gain atmm-wave frequencies, they also suffer from ahigh cost per unit, poor integration, and lowpower efficiency. Complementary metal oxidesemiconductor (CMOS) technology, on the otherhand, will offer lower-cost mass production,improved integration, and increased power effi-ciency; however, CMOS front-end circuits willalso have to address issues in power amplifieroutput, local oscillator phase noise, and low-noiseamplifier design, as discussed in [10]. As a com-promise, more recent advances in silicon germani-um (SiGe) technology have now made it possibleto build miniaturized low-cost mm-wave radiodevices, such as the 60 GHz 0.13 mm SiGe BiC-MOS double-conversion superhetrodyne receiverand transmitter chipset recently developed byIBM [1]. Here, data rates of up to 630 Mb/s havealready been demonstrated for this chipset over a10 m indoor LOS link using folded-dipole anten-nas for both transmitter and receiver modules.Based on link budget calculations made in [1], theauthors also state that increasing the receiver gainby 12 dBi (e.g., using smart antenna technology)could increase the range by a factor of fourassuming free space propagation. Undoubtedly,even greater operating distances may be attainedby sacrificing bandwidth and data rates or improv-ing overall system gain. It is clear that multi-giga-bit short-range mm-wave devices will be readilyavailable in the short term.

MM-WAVE SOLDIER-TO-SOLDIERCOMMUNICATIONS:

MAC LAYER CHALLENGES

DIRECTIONAL MAC

The successful introduction of covert 60 GHzsoldier-to-soldier communications will requirethe development of a robust and efficient

bespoke directional MAC (DMAC), which willbe underpinned by future developments inbeamforming hardware, driven by thewidespread adoption of standards such asIEEE 802.15.3c and IEEE 802.11-very highthroughput (VHT) into the wireless consumermarket. At 60 GHz, it is possible to constructa cylindrical antenna array with 32 elementsplaced a half-wavelength apart, all within aradius of 13 mm. This will permit the develop-ment of compact wearable smart antenna tech-nology that uses adaptive beamforming todynamically adjust the array pattern by alter-ing the amplitude and phase of a feed net-work. Highly directive narrow-beamwidthinterpersonal communications coupled with a60 GHz operating frequency will help counter-act eavesdropping, improve resilience to jam-ming, and provide a lower probabil i ty ofdetection by enemy forces. However, it willintroduce a number of key design challengesat the MAC layer.

Many of the DMACs currently proposed inthe literature are directional variations of thesingle-channel carrier sense multiple accesswith collision avoidance (CSMA/CA) approach[11,12]. One of the most widely used and wellunderstood CSMA/CA-based protocols isIEEE 802.11. The adaptation of the currentIEEE 802.11 MAC for use in directional mili-tary systems will require a number of nontriv-ial adjustments, most notably to thedistribution coordination function (DCF). Vir-tual carrier sensing (VCS), which is performedby l istening to directional request to send(DRTS) and directional clear to send (DCTS)exchanges in the local vicinity, should be per-formed directionally. Nodes that detect aDRTS or DCTS control packet will update atable of directional network allocation vectors(DNAVs) [11]. When the channel is sensedidle, the DNAV will be used to determinewhether to defer transmission in a particulardirection.

For a node to perform directive VCS(DVCS) and transmit its RTS directionally itmust know, or have a good estimate of, thelocation of its intended receiver. This is proba-bly the single biggest challenge facing direc-tional protocols, but one military wirelesssystems designers may be well placed toresolve. RTS direction could be obtained froman internal positioning table that is compiledusing time and direction of arrival of signalcomponents. In [11] it is suggested that nodesshould listen for ongoing transmissions omnidi-rectionally while in idle mode and during ran-dom backoff intervals in contention periods,providing the opportunity to collect time anddirection of arrival information as illustrated inFig. 2. Directional information could also beretrieved during packet reception, using proac-tive training beacons [12], and from routinginformation obtained from relayed packets.Digital navigation aids that are often utilizedby the military such as GPS (e.g., soldier’s digi-tal assistant) and inertial navigation systemsmay also be used to provide useful informationon node locations by appending readouts tothe data payload of control packets. However,

Highly directive

narrow-beamwidth

interpersonal com-

munications coupled

with a 60 GHz

operating frequency

will help counteract

eavesdropping,

improve resilience to

jamming, and pro-

vide a lower

probability of

detection by enemy

forces. However, it

will introduce a

number of key

design challenges at

the MAC layer.



these methods of direction of arrival (DOA)estimation may not be as effective in indoorenvironments or when the direct LOS link isobstructed.

To illustrate the operation of a directiveMAC layer in a soldier-to-soldier MANET, wereturn to the scenario depicted in Fig. 1 andassume the soldiers are equipped with radiosthat make use of mm-wave adaptive beamform-ing technology, based on a directional derivativeof the IEEE 802.11 MAC. As an example, con-sider single-hop communications between sol-diers B and C, and assume that the team hasbeen tracking each other’s movements (e.g.,using proactive beaconing), and hence havegood estimates of their relative locations orDOAs of significant multipath components. Ifsoldier C wishes to initiate communications withsoldier B, s/he must transmit a DRTS using thelast known good directional entry for soldier Bin her/his internal positioning table. All nodesthat receive the DRTS transmission (e.g., sol-diers A and B who are in idle states) thenupdate their DNAV and internal positioningtables with the incoming signal’s DOA andadjust the elapsed time of arrival information.This will include tracking and storing all majormultipath components as well as the most sig-nificant path as shown in Fig. 2. If soldier B’sDNAV table permits transmission in the returndirection, the node uses the stored informationto beamform in the direction of soldier C andattempts to send a DCTS for a predefined peri-od before timing out. Meanwhile, soldier C also

beamforms in the direction of soldier B waitingfor the DCTS. If the DCTS is successfullyreceived, soldier C initiates the directionaltransmission of data. Throughout this process,all nodes that can hear the exchange continu-ously update their positioning tables. In the caseof soldiers B and C, this will provide the maxi-mum opportunity of re-establishing the linkshould it unexpectedly go into outage, beforeabandoning transmission and handing the prob-lem to the network layer for routing, as outlinedin the packet transmission flowchart in Fig. 2.This is a relatively simple overview of how direc-tional communications could work in soldier-to-soldier MANETs. However, DMACs will beparticularly susceptible to many of the commonissues associated with wireless networking suchas the hidden node problem, deafness, and gainasymmetry. Clearly, these are issues that alsoneed to be carefully considered and warrant fur-ther research.

ADAPTIVE POWER CONTROLAnother facility that would further enhance thestealth mode of wireless operation is judiciousadaptive power control. Here, radio transmitpower is adjusted on a packet-by-packet basis tothe minimum level required for operation with agiven capacity and error probability. Theseschemes are often desirable in mobile wirelesssystems for the purposes of reducing interfer-ence and prolonging battery life. In [13] a simplemethod of implementing power control withinthe RTS/CTS framework is proposed. This tech-

Figure 2. DOA estimation and packet transmission in an mm-wave soldier-to-soldier MANET. Soldiermodel (height, 1.83 m) generated using Poser 7 animation software.

Link establishedusing αk ?

No Yes

No

Transmit packet

Tag node as unreachable and let network layer handle routing

Is link layer retrynumber k > kmax?

Determine elapsed time, Δt, anddirection of arrival for αk

Set beamwidth (depends on Δtand retry number k) for phased

array

Packet transmission

Look up last α0 to αu fornode n, k = 0

Preamble Routing info

Determine DOA by beam scanning during IDLEmode: main lobe and u significant paths: α0 to αu.

Store direction and time of arrival.

CRC Data

Soldier Entry SINR (dB)

DOA (deg)

Elapsed time since arrival

(ns) Packet type

Routing info

GPS (optional)

INS (optional)

C α0 20 110 5 DRTS - N 54.625660* W -5.889453*

-

C α1 10 105 7 DRTS - - -

C αu X Y Z DRTS ... ... ...

... ... ... ... .. ... ... ... ...

...

Identify link node n(not source node)

α0 typically LOS orstrong dominantsignal component

DOA estimation during IDLE mode

Tracked signal components arriving during IDLE mode

Power

Time

α0 α1α1

α2

αu

α2αu

k++

DMACs will be

particularly suscepti-

ble to many of the

common issues asso-

ciated with wireless

networking such as

the hidden node

problem, deafness,

and gain asymmetry.

Clearly, these are

issues that also need

to be carefully con-

sidered and warrant

further research.



nique could readily be adapted for use in direc-tional systems by including the beamforminggain within the link power control budget. TheDRTS is always transmitted at a predeterminedpower (e.g., maximum power, PTX, measured indecibels) and the beamforming gain (BTX) usedto transmit the DRTS stored in memory. Uponsuccessful DRTS reception, the receiver calcu-lates the difference between the instantaneousreceived power and its received power threshold,and adds this to its future intended beamforminggain (BRX) to calculate the power difference,denoted Pd. This value is then added to the datapayload and transmitted with the DCTS. Whenthe sender node begins transmitting data, it doesso using the predetermined power and previousbeamforming gain, less the difference (i.e., PTX+ BTX – Pd). An optional safety margin (Ps),which takes into account fading and node mobil-ity, may also be added to the adjusted outputpower level chosen by the sender (i.e., PTX +BTX + Ps – Pd). Including adaptive power con-trol in future soldier-to-soldier MANETs willnot only aid in reducing interference and batterylongevity, but it will also improve the covertnature of communications.

SIMULATING SOLDIER-TO-SOLDIERSIGNAL PROPAGATION

A range of electromagnetic solver tools areavailable for simulating signal propagation inwireless channels. Of these, the finite-differencetime-domain (FDTD) and ray launching meth-ods are of particular interest. FDTD modelingworks by solving Maxwell’s equations in the timedomain. The FDTD method becomes computa-tionally intractable over large distances at mm-wave frequencies due to the high simulation gridresolution required to achieve an accurate result.Another approach, which is based on geometri-cal optics (GO) and the uniform theory ofdiffraction (UTD), is ray launching. In raylaunching the transmit antenna launches N raysover a selected spatial angle. The ray launchingalgorithm then tracks each ray until it illumi-nates the area of interest or the power of the rayfalls below a preselected threshold level. Becauseray launching is based on GO, as the frequencyof the carrier increases, the approximation of raylaunching to signal propagation improves. A par-ticular strength of using ray launching simulationmethods to make channel predictions is thatthey allow infinite resolution of transmitted andreceived signal contributions in both time andspace. This feature will make them inherentlysuitable for generating transmission channelinformation to be used in conjunction with net-work simulators (OPNET, NS2, etc.), allowingtime/direction of arrival based protocols to berigorously tested. To illustrate the use of raylaunching in the simulation of signal propagationin soldier-to-soldier MANETs, we now describethe steps taken to simulate the mm-wave trans-mission channel between a team of four dis-mounted combat personnel performing ahypothetical counter-insurgency cordon andsweep operation (this is the fictitious operationdepicted in Fig. 3).

DYNAMIC HUMAN BODY ANDENVIRONMENTAL MODEL

GENERATION

A particular problem with achieving realistictransmission channel predictions in scenariosthat involve the human body is the encapsula-tion of movement. One possible solution to thisproblem is the use of animation software togenerate the required dynamics. These pro-grams typically allow the straightforward manip-ulation of user-created 3D polymesh humanfigures with the ability to export the animationsequence to a native file format readily inter-preted by computer aided design (CAD) soft-ware (e.g., drawing exchange format). Figure 2shows an example snapshot of the soldier modelused in this study generated using the walkdesigner feature of the Poser 71 animation soft-ware. The lifelike model of the U.S. infantrysoldier includes the improved outer tactical vest(IOTV) and lightweight helmet, backpack,pouches, and weaponry. Also shown in Fig. 2 isa single 60 GHz wireless node positioned on theright shoulder which, for simulation purposes,was fitted with a vertically polarized (when thesoldier is standing upright) dipole antenna, cho-sen because of its favorable omnidirectionalradiation characteristics in the azimuth. Thecomputer generated environmental model creat-ed for this study was designed using the Auto-CAD software package from Autodesk.2 It wasbased on a small compound as encountered bycoalition troops in the Middle East, and isshown in Fig. 3.

A SIMULATEDCOUNTER-INSURGENCY CORDON

AND SWEEP OPERATION

In this section we present a selection of simula-tion results from the hypothetical military opera-tion shown in Fig. 3. The simulations followedthree distinct stages of a cordon and sweep oper-ation, and modeled bidirectional signal propaga-tion between a squad of four U.S. infantrytroops.

SIMULATOR OPERATION AND SETTINGSThe dynamic soldier-to-soldier transmissionchannel simulations used a full 3D ray launch-ing algorithm3 that was set to calculate allreflections, penetrations, and diffractions. Alibrary of purposely written executables wereresponsible for the amalgamation of the ani-mation sequence and CAD environmentmodel, assignment of dielectric properties tomaterial layers, transmit power, and otherrelated channel simulation properties as wellas controll ing the automation of the raylaunching algorithm. Proprietary software alsotracked the changes in antenna orientationcaused by soldier movements in 3D vectorspace. The soldiers were given movements andspeeds relative to their role within the opera-tion. The animations were performed at a rateof 50 frames/s, which for the purposes of ray

1

http://my.smithmicro.com/win/poser/index.html

2 http://usa.autodesk.com/

3 Algorithm formed aspart of RPS softwarepackage: http://www.actix.com/radioplan_rps/

Including adaptive

power control in

future soldier-to-

soldier MANETs will

not only aid in

reducing interference

and battery

longevity, but it will

also improve the

covert nature of

communications.



launching simulation is analogous to 50 sam-ples of the mm-wave channel per second. The60 GHz wireless nodes were set to transmit ata power level of +20 dBm.

SIMULATION DESCRIPTIONThe overall simulation scenario was designed toencompass a wide range of channel types:indoor, outdoor, and indoor to outdoor. Stage 1involved movement of the team from a dropoffzone beside the Abrams M1 tank (Fig. 3) to theentrance of an enemy command center andanalyzed outdoor soldier-to-soldier signal prop-agation. This stage took approximately 5 s tocomplete. Stage 2, duration 4.5 s, investigatedboth indoor and outdoor-to-indoor signal prop-agation as three members of the team sweptthe building, while the remaining soldier main-tained guard at the entrance. Finally, stage 3lasted for 5.2 s and studied the movement ofthe team within a large multiroom buildingstructure.

DIRECTION OF ARRIVAL

In this study we are particularly interested in thedirection of arrival of significant signal compo-nents and, more important, their angular distri-bution in space. This knowledge will be crucialfor proving the covert aspect of communicationsat mm-wave frequencies and the usefulness ofbeamforming in future soldier-to-soldierMANETs. The key question here: is the distribu-tion of DOA suitably constricted to provide astealth mode of operation and warrant the use ofbeamforming arrays but not too narrow as to limitthe system’s ability to overcome channel impair-ments? The azimuth (or elevation) rms anglespread of the channel impulse response is ameasure of the angular dispersiveness of thechannel [14]. A transmission channel in whichmajor signal components arrive from significant-ly different spatial orientations is characterizedby a large rms angle spread and vice versa.

The azimuth and elevation rms angle spreads

Figure 3. Upper image: illustration of all three stages of the cordon and sweep operation simulation within a2258 m2 Middle Eastern enclosure. Lower image: expanded picture of simulated mm-wave signal propaga-tion between soldiers A and B as the team prepares to raid the enemy command center.

Drop off zone

Soldier B Soldier D

Soldier A

Soldier C

Colored rays show simulated mm-wave signal propagation between soldier A and soldier B

Counter insurgency cordon and sweep operation

Stage 3 - Sweep of insurgency living quarters

Stage 2 - Targeted raid of enemy command center

Stage 1 - Movement to enemy command

center

we are particularly

interested in the

direction of arrival of

significant signal

components and

more importantly

their angular distri-

bution in space. This

knowledge will be

crucial for proving

the covert aspect of

communications at

mm-wave frequen-

cies and the useful-

ness of beamforming

in future soldier-to-

soldier MANETs.



were calculated at each simulation time step(animation frame) for individual soldier-to-sol-dier links. Figure 4 summarizes these results bypresenting the cumulative distribution functions(CDFs) of rms angle spread over all the simula-tions (all three stages) and for all bidirectionallinks. For stage 1, the azimuth rms angle spreadhad a 90 percent probability of being less than90°. The corresponding figure for the elevationrms angle spread was lower at 20°, an observa-tion most likely to have been caused by the factthat each of the soldiers were of the same heightand were vertically upright for the duration ofstage 1. Most important, these results providesolid evidence of the directional characteristicsof the outdoor transmission channel and thepotential for beamforming arrays to provide agood degree of covertness.

For stage 2, rms angle spread for bothazimuth and elevation planes were increasedcompared to stage 1 (Fig. 4). The increase inrms angle spread, especially elevation, withinindoor environments was to be expected, andmay be explained by the larger number of multi-path components caused by the close proximityof scattering and reflecting objects such as walls,ceilings, and furniture. In stage 3 of the opera-tion, the rms angle spread also increased com-pared to stage 1. The results from stages 2 and 3show quite clearly that rms angle spread increas-es as the soldiers move from a mostly unclut-tered LOS outdoor communications scenario toan often obstructed indoor communications one.The rich multipath conditions observed withinindoor environments at 60 GHz should generateenough signal components with sufficient angu-lar separation to sustain short-range soldier-to-soldier communications should the main signalpath become unexpectedly shadowed or blocked.It is worth noting that while larger rms anglespread within indoor environments may appeardetrimental to the proposed stealth mode ofoperation, mm-wave propagation characteristicswill mean that supporting structures shouldinhibit signal propagation beyond the perimeterboundaries.

RMS DELAY SPREADWhen high-data-rate wireless systems operateunder dispersive channel conditions, they can besubject to intersymbol interference (ISI), whichmay significantly degrade their performance.The rms delay spread is considered one of themost important parameters for defining the timeextent of a time-dispersive radio channel [15]and assessing the potential impact of ISI. Whenthe modulation symbol time is on the order ofthe rms delay spread, the wireless link is general-ly considered to be at risk from ISI. Therefore,rms delay spread provides important informationfor ISI mitigation techniques such as equalizerdesign. In this work we calculate the rms delayspread from the discrete time power delay pro-file (PDP), obtained from the complex channelimpulse response acquired at each individualsimulation time step. Figure 5 shows an examplePDP for the wireless link from soldier A to sol-dier B during stage 1. It can be seen quite clear-ly that the vast majority of the energy is

contained within the direct path, which typicallyarrives within 25 ns. Figure 5 also shows theexistence of occasionally significant multipathcomponents that arrive between 100 and 150 ns.

Table 1 provides a summary of the rms delayspread statistics for all bidirectional soldier-to-soldier links for all three stages. In stage 2,where the majority of soldier-to-soldier linkswere confined within the small dimensions of theenemy command center, the median rms spreadwas much lower than in stage 1 due to muchshorter contributing path lengths. Interestingly,stage 3 had comparable median rms delay spreadto Stage 1 despite considering a different envi-ronmental scenario. The maximum rms delayspread for stage 3 was 160 ns (Table 1), which,

Figure 4. CDFs of rms angle spread over the three stages of simulation for allbidirectional links.

rms angle spread (°)

200

0.1

0

Prob

abili

ty t

hat

ms

angl

e sp

read

≤ a

bsci

ssa

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

40 60 80 100 120 140 160 180

Stage 1 azimuthStage 1 elevationStage 2 azimuthStage 2 elevationStage 3 azimuthStage 3 elevation

Figure 5. PDPs for wireless link from soldier A to soldier B for the duration ofstage 1.

Time (s)

–55 dBm

–60 dBm

–65 dBm

–70 dBm

–75 dBm

–80 dBm

–85 dBm

–90 dBm

–95 dBm

–50 dBm

–90Rece

ived

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er (

dBm

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–95

–100

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–80

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–70

–65

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Delay (ns)

50 100 150 200

1 0

2 3

4



as Fig. 6 indicates, is a relatively rare event. Thesimilar shapes of the rms delay spread CDFs inFig. 6 also suggest that a similar type of wide-band equalizer may be used for all wireless chan-nels in this scenario. This trend was alsorepeated for stages 1 and 2, and this in turnmeans that the complexity of the design of mm-wave transceivers for soldier-centric communica-tions design is reduced.

CONCLUSIONSCovert high-speed wireless networks are now afunctional requirement of modern infantry andspecial operations warfare. In this article wehave discussed a wide range of issues associatedwith the application of directive mm-wave sys-tems to meet future soldier-centric networkingneeds. The concept of a soldier-to-soldierMANET has been introduced and used as anexample of a military scenario that could benefitfrom a stealth mode of operation. To realizecovert ad hoc networking, considerable chal-lenges relating to hardware, directive protocolsand power control must be addressed. OFDMand SC-FDE transmission schemes coupled withrecent advances in SiGe technology look promis-ing for providing high-bandwidth mm-wave sol-dier mounted radio. The short electromagnetic

wavelength at 60 GHz will allow the constructionof compact adaptive beamforming arrays. Adirective MAC layer will be required and expect-ed to provide seamless connectivity as well effec-tive management of resources including powercontrol. To this end, possible strategies for creat-ing a directive MAC layer based on the IEEE802.11 MAC have been described.

A fundamental understanding of the soldier-to-soldier transmission channel will be necessaryto rigorously test future protocols and hardware.With the aim of gaining greater insight into sig-nal propagation within soldier-to-soldier trans-mission channels, we have combined computergenerated dynamic human body models and acommercial ray-launching engine to performchannel prediction. It is worth noting that thisplatform has the ability to simulate dynamicmilitary scenarios beyond soldier-to-soldier sig-nal propagation such as soldier-to-vehicle,including unmanned aerial vehicles (UAVs),and UAV-to-UAV, and is applicable in the fre-quency range 300 MHz–300 GHz. Simulationsperformed for this study were based on threedistinct stages of a military cordon and sweepoperation. For outdoor operations, it wasobserved that signal DOA was adequately con-stricted to provide a good degree of covertness.When the team moved indoors, the signal DOAwas observed to increase, although the favorablepropagation characteristics at mm-wave fre-quencies mean that signal propagation is likelyto be contained within the structure. Delay dis-persion was also dependent on local surround-ings with the greatest rms delay spreadsobserved outdoors and within large buildingstructures.

ACKNOWLEDGMENTSThis work was supported by the U.K. Ministry ofDefence through the “Competition of Ideas” Pro-gram under project reference RT/COM/5/001.

REFERENCES[1] S. K. Reynolds et al., “A Silicon 60 GHz Receiver and

Transmitter Chipset for Broadband Communications,”IEEE J. Solid-State Circuits, vol. 41, no. 12, Dec. 2006,pp. 2820–31.

[2] C. Park and T. S. Rappaport, “Short-Range WirelessCommunications for Next-Generation Networks: UWB,60 GHz Millimeter-Wave WPAN, and ZigBee,” IEEECommun. Mag., Aug. 2007, pp. 70–78.

[3] FCC, “First Report and Order: Revision of Part 15 of theCommissions Rules Regarding Ultra-Wideband Trans-mission Systems,” ET Docket 98-153, 2002.

[4] H. Yang, P. F. M. Smulders, and M. H. A. J. Herben,“Channel Characteristics and Transmission Performancefor Various Channel Configurations at 60 GHz,”EURASIP J. Wireless Commun. Net., vol. 2007, no. 1,Jan. 2007, pp. 1–15.

[5] S. L. Cotton and W. G. Scanlon, “Channel Characteriza-tion for Single and Multiple Antenna Wearable Systemsused for Indoor Body to Body Communications,” IEEETrans. Antennas & Propagation, Special Issue on Anten-nas & Propagation for Body-Centric Wireless Communi-cations, vol. 57, no. 4, Apr. 2009.

[6] S. L. Cotton and W. G. Scanlon, “Higher Order Statisticsfor the κ–μ Distribution,” Elect. Lett., vol. 43, no. 22,2007, pp. 1215–17.

[7] S. Collonge, G. Zaharia, and G. E. Zein, “Influence ofthe Human Activity on Wide-Band Characteristics of the60 GHz Indoor Radio Channel,” IEEE Trans. WirelessCommun., vol. 3, no. 6, Nov. 2004, pp. 2396–2406.

[8] IEEE 802.15 WPAN Millimeter Wave Alternative PHYTask Group 3c (TG3c); http: / /www.ieee802.org/15/pub/TG3c.html

Table 1. Summary statistics for rms delay spread over all three stages.

StageMedianrms delayspread (ns)

Inter-quartilerange of rmsdelay spread (ns)

Max rmsdelay spread(ns)

Min rmsdelay spread(ns)

1 9.1 17.9 162.5 ~0.0

2 5.3 6.5 96.1 ~0.0

3 10.1 12.4 160.0 ~0.0

Figure 6. CDFs of all rms delay spreads calculated for each bidirectional link over the duration of Stage 3.

rms delay spread (ns)

200

0.1

0

Prob

abili

ty t

hat

rms

dela

y sp

read

≤ a

bsci

ssa

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

40 60 80 100 120 140 160

Soldier A to BSoldier A to CSoldier A to DSoldier B to CSoldier B to DSoldier C to D



[9] D. Falconer et al., “Frequency Domain Equalization forSingle-Carrier Broadband Wireless Systems,” IEEE Com-mun. Mag., vol. 4, no. 40, Apr. 2002, pp. 58–66.

[10] C. H. Doan et al., “Design Considerations for 60 GHzCMOS Radios,” IEEE Commun. Mag., vol. 42, no. 12,Dec. 2004, pp. 132–40.

[11] R. R. Choudhury et al., “On Designing MAC Protocols forWireless Networks Using Directional Antennas,” IEEE Trans.Mobile Comp., vol. 5, no. 5, May 2006, pp. 477–91.

[12] R. Ramanathan et al., “Ad Hoc Networking with Direc-tional Antennas: A Complete System Solution,” IEEEJSAC, vol. 23, no. 3, Mar. 2005, pp. 496–506.

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[14] J. C. Liberti and T. S. Rappaport, Smart Antennas forWireless Communications: IS-95 and Third GenerationCDMA Applications, Prentice Hall PTR, 1999.

[15] K. Witrisal, “On Estimating the rms Delay Spread fromthe Frequency-Domain Level Crossing Rate,” IEEE Com-mun. Lett., vol. 5, no. 7, July 2001, pp. 287–89.

BIOGRAPHIESSIMON L. COTTON [S’04, M‘07] ([email protected])has been a research fellow at the ECIT Institute of Electron-ics, Communications and Information Technology, Queen’sUniversity of Belfast, United Kingdom, since 2007. Hereceived his B.Eng. degree in electronics and software fromthe University of Ulster, United Kingdom, in 2004 and hisPh.D. degree in electrical and electronic engineering from

Queen’s University of Belfast in 2007. His recent work hasincluded the investigation of millimeter-wave technologiesfor personal communications and body-to-body networks.His other research interests include radio channel modelingfor wireless body and personal area networks, and the sim-ulation of wireless channels.

WILLIAM G. SCANLON [M‘98] ([email protected]) receivedhis B.Eng. degree in electrical engineering and Ph.D.degree in electronics from the University of Ulster in 1994and 1997, respectively. He is currently holds the chair inwireless communications and leads the Radio Communica-tions research group at Queen’s University of Belfast, andhe is also a professor of short-range radio at the Universityof Twente, Netherlands. His current research interestsinclude personal communications, wearable antennas, RFand microwave propagation, channel modeling and char-acterization, wireless networking and protocols, and wire-less networked control systems.

BHOPINDER K. MADAHAR ([email protected]) is ChiefTechnologist of the Information Management Departmentof the Defence Science and Technology Laboratory, UK andhe holds a visiting chair at Queen’s University Belfast. He isan experimental Physicist, with a B.Sc. in Physics, a Ph.D. inSpace Physics. His research work over the last threedecades, for both industry and government, has beendominated by the advanced engineering and integration ofdistributed high performance C4ISR systems and system ofsystems. He is also the UK member of the NATO Informa-tion Systems Technology panel.


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Millimeter-Wave Soldier-to-Soldier Communications …wireless communications will be required to relay information on situational awareness, tac-tical instructions, and covert surveillance