Military Applications of Ultra-wideband Communications

Background

The principle of UWB technology is not new and may be traced back to some of Marconi’s early experiments in 1901 using spark gap transmitters. These produced very short signals, spread over a wide frequency band and were the basis of the first transatlantic wireless communications. However, this system was not adopted for radio communications as each sending station occupied too much of the available bandwidth and there was little scope to allow multiple use. Instead, for much of the past 100 years, radio communications have adopted some form of channel allocation by frequency. Each network of transmitter and receiver(s) operated at a limited number of centre frequencies, each associated with a bandwidth relevant to its particular application. Through these means many different networks could occupy the electromagnetic spectrum simultaneously; this is the traditional way of dividing up the limited space in the spectrum.

Research into short-pulse electromagnetics began in the post-war period but the main focus was not initially associated with communications. Work in this domain was assisted by the development of electronic systems that could cope with the transmission and receipt of the very short amplitude rise times associated with short pulses. This also meant that receivers could be gated, limiting the receive time, and therefore able to accept only the wanted signal and ignore unwanted reflections. In communication systems, gating may also be used to limit the receive time of a particular network and hence provide channels in the time domain. The wide bandwidth associated with a short pulse means that the signal power is spread over a wide frequency range, thus limiting the pollution of the electromagnetic spectrum. Much of the early work was associated with radar applications over short ranges and potential for the technology to form the basis of various collision-avoidance systems, such as in spaceship docking, harbour collision avoidance, and road vehicle braking was noted [1].

The development of short-pulse technology to form the basis of UWB radar continues and a number of commercial applications are now available. These include short-range systems such as ‘Soldiervision’ which claim to be able to detect moving targets at a range of 20 m through various building materials including 20 cm solid concrete [2].

Although recognised early as a potential application, the development of short-pulse technology as a communications bearer has only become the focus of significant commercial interest within the past 10 years. Such applications will be the focus of discussion in this paper, but in outline UWB offers some clear advantages over more traditional methods of communication and channel allocation. Very high data rates, of the order of Gbps, are possible over short ranges. In addition the low power density of short-pulse communications means they are less susceptible to intercept and analysis than narrowband systems with equivalent capacity. The time-domain nature of short-pulse systems allows them to operate more effectively in dense multipath environments than narrowband systems which would be subject to considerable fading. This means that in either the indoor or urban environments, UWB may offer a greater guarantee of a communications link than a narrowband system, albeit over a short range. Furthermore, additional advantages such as position location to a CEP (Circular Error Probable) of 3 cm, are offered by short-pulse communication systems.

Regulation and standards

Despite the fact that regulation of the frequencies associated with UWB has only been clearly defined in the US recently and remains the subject of debate within the EU, some short-pulse systems have entered the marketplace. Historically, the Federal Communications Commission (FCC) position within the US has been that UWB systems should radiate at levels below those defined for intentional emitters and therefore fall into the bracket of unintentional emitters. The European Telecommunications Standards Institute (ETSI), and by implication the UK Office of Communications (OFCOM), has been less accepting of this evasion of the issue and this may, in part, explain why the majority of commercially fielded systems are of US origin.

The FCC definition of a UWB system is one where either the absolute bandwidth is greater than 500 MHz or the fractional bandwidth is greater than 20% of the centre frequency [3]:

$B_f=2\frac{(f_H-f_L)}{(f_H+f_L)}$, B_f > 0.2 or B_f > 500 MHz (1)

-80-75-70-65-60-55-50-45-401357911FCC MaskETSI MaskFrequency (GHz)EIRP (dBmW/MHz)-80-75-70-65-60-55-50-45-401357911FCC MaskETSI MaskFrequency (GHz)EIRP (dBmW/MHz)

The upper and lower bounding frequencies (f_H and f_L respectively) are defined as the frequencies where the emitted power is 10 dB below the maximum power. It should be noted that this definition of UWB is in terms of its frequency range and bandwidth and not the technology used. This has meant that although short-pulse technology has traditionally been associated with UWB, other technologies, which conform to the FCC definition, may compete as the commercial bearers of UWB communication. The main commercial driver is to use UWB as the basis for home and office wireless networking, a situation where there is no clear need for some of the facilities such as precision location offered by short-pulse systems. As a result more-developed technologies look set to provide the medium for commercial UWB systems.

The IEEE 802.15.3a task group is currently developing a UWB standard, which will conform to the FCC regulations and provide data rates of between 110 and 480 Mbps at ranges of less than 10 m [4]. Consultation continues within the IEEE to determine the exact standard: two proposals are being considered, one based on Orthogonal Frequency Division Multiplexing (OFDM) [5], the other on Direct Sequence (DS) [6]. Both will conform to the FCC definition of UWB, and either could become the basis for high bandwidth, short-range networking. Currently it appears that the consortium backing the OFDM approach has the upper hand and is likely to win the competition within the IEEE. Despite the fact that some militarily advantageous facilities may be lost, either approach could be of considerable use in a military environment, particularly in ad hoc networking; however the very short range of these systems may limit their military utility.

Both proposals make use of well-established technology, geared to the newly available frequency band. Whichever is chosen for the IEEE 802.15.3a standard will become an important and widespread bearer and, as such, the unit cost of this technology will decrease. This technology is likely to challenge commercially the position currently occupied by Bluetooth, which only offers a data rate of 721 kbps at 10 m against the 110 Mbps envisaged for UWB. For applications which simply need the availability of high data rates this may well prove the most cost-effective solution in the military environment. However, other applications, both civilian and military, may yet need the additional functionality offered by short-pulse UWB and research in this technology is likely to continue.

While OFDM and DS may describe the physical layer of the respective submissions to the FCC, all other relevant layers for networking the technology are also defined. This means that whichever is chosen for the 802.15.3a standard will be marketed in a network-ready format and allow potential access to wider networks. From a military perspective this is advantageous as UWB will have the capability to link seamlessly to other IP-based networks.

Emission levels

Recent FCC legislation [3] has allocated the frequency band of 3.1–10.6 GHz to UWB systems. Emissions outside this specified band must be at least 20 dB below the allowable emissions in band. The FCC Effective Isotropic Radiated Power (EIRP) limits, which relate to average transmitted power over time, are outlined in Table 1. In general this places a legislative mask over the average EIRP from a UWB system operating in the 3.1–10.6 GHz band of –41.3 dBmW/MHz.

In the EU, there is concern about interference between UWB and a number of licensed users on or near the 3.1-10.6 GHz band. For example, Bluetooth and IEEE 802.11 both operate at 2.4 GHz. UK/EU legislation remains the subject of consultation between ETSI/OFCOM and industry [7] but is likely to apply a slightly more stringent mask than that of the FCC. The proposed ETSI/OFCOM and FCC masks are shown graphically in Figure 1.

The EIRP limits placed by the FCC and proposed ETSI/OFCOM masks significantly curtail the potential applications of UWB devices to ones with very short ranges. Furthermore, the propagation characteristics of the 3.1–10.6 GHz band mean that even if higher-powered emissions were authorised, communication would be difficult beyond the line of sight without an adequate reflector or relay. Some early investigation of the potential of UWB [8] demonstrated systems operating in the VHF/UHF band with data rates of over 1.5 Mbps achievable over ranges in excess of 1 km. The peak radiated power in this case, however, was 10 W (10 dBW).

Although there is no physical reason why UWB cannot be used to transmit at HF and VHF frequencies as well as the specified UHF/SHF band, the masks placed by regulatory bodies are legally binding and systems operating outside these limits will have to gain specific authorisation where the masks apply. In addition, UWB may well increase the background noise levels and hence interfere with existing systems.

Recently marketed short-pulse UWB systems [9]

Despite the current focus on developing UWB as the physical layer for personal area networks, some work continues to investigate applications for short-pulse UWB systems. Much of this work is being conducted within the US as a result of a military demand for Low Probability of Intercept (LPI), high-data-rate communication links over militarily useful ranges. This section contains some examples of such systems and their potential applications.

Draco UWB network transceiver

Draco is a technology demonstrator designed to prove the utility of UWB networking within a military environment. Working in the VHF/UHF band (that is, outside the limits placed by the FCC), Draco demonstrated an operational range of 1–2 km depending on the terrain. Encrypted data rates of 12–15 kbps or unencrypted rates of 115 kbps to 1.544 Mbps were achievable at these ranges. Field trials in 2002 in the US demonstrated that a set of eight Draco terminals could form a full ad hoc wireless network at distances of over 1 km between stations. It is assumed that the terminals were all static in this case. A system of this nature would have utility as a communications node, a sensor node, or as a destination terminal.

Orion UWB network and ground-wave non-LOS transceiver

Orion was designed as a communication system with short-range (1 km) and long range (50–60 km) facilities. The short range component operates in the 1-2 GHz band with a bandwidth of 500 MHz. At a peak output power of 0.8 W (–0.97 dBW) and a packet burst rate of 2 Mbps the average power is 4 mW (–24.0 dBW), or 8 pW/Hz (–51.0 dBW/MHz). Line of sight ranges of 1 km have been demonstrated over full duplex digital voice and data links of up to 1 Mbps. The long-range component operates in the lower portion of the military VHF band (30–50 MHz) with a peak power of 120 W (20.8 dBW). In this case ranges of around 13 km were achievable over land and more than 109 km over water. Orion works on a star topology with a single master unit controlling a number of slaves. This means that the maximum distance between slaves is approximately 2 km, sufficient for infantry section and some platoon operations.

Aircraft wireless intercommunication systems (AWICS) [10]

The Department of the Navy in the US conducted work in the mid 1990s to replace the dangerous tethered intercommunication cables on board its helicopters with wireless communication links. Initially a conventional spread-spectrum system was trialled, but this was prone to multipath interference within the helicopters, particularly when the engines were running, and proved unsuitable for the task. A UWB system operating in the 1–2 GHz band with a bandwidth of 400 MHz and an EIRP of 26 dBW was trialled. The trial proved that the communications link could operate throughout the aircraft giving a 64 kbps digital voice transmission which worked up to 60 m from the aircraft when the rear door was lowered.

Military drivers for UWB communications

The UK vision of the achievement of Network-Enabled Capability (NEC) [11–12] envisages development along several themes. Included amongst these are the development towards of a shared understanding whereby each user generates an understanding appropriate to their role within the operation and the task they have been given. Full information accessibility envisages users being able to search, manipulate and exchange relevant information derived from sources both internal and external to the battlespace. Dynamic collaborative interworking should enable all entities to work together towards the simultaneous planning and execution of operations. Effects synchronisation should achieve a desired outcome through the synchronised interworking within and between mission groups. Agile mission groups envisages the dynamic creation and configuration of mission groups orientated to a particular task.

In parallel, the US is developing a Network Centric Warfare (NCW) approach, the main difference between the two is that the US envisages implementing NCW as a complete package whereas the UK will undergo gradual development towards NEC. One key area of development for both NEC and NCW is the decoupling of sensors, deciders and actors, those entities which actually bring about an effect in the battlespace. Hitherto, sensors and actors have been either placed on the same platform, such as a tank, or formed components of a bounded system, such as for the delivery of indirect fire. The decoupling of sensors from their associated actors will allow for greater flexibility in both the choice of information on which to base a decision to act and the choice of the most appropriate actor to bring about the desired effect.

Underpinning all of these themes is a requirement for a communications network which is universally accessible, through which to exchange and pass relevant information in time for it to be of use. Owing to the restricted bandwidth available and differing requirements in terms of range and data rate for different elements of the network, one single communications systems is unlikely to fulfil all the required tasks. It is more likely that the network will comprise a number of different means of communication with appropriate linkages; a network of networks. This will help to ensure that components of the network do not interfere with each other so, for example, long-range communications provided by either HF or SATCOM are not compromised by the operation of short-range or medium-range systems. While the development of software-defined radio (SDR) may mean that one physical radio could ease the situation by performing a variety of tasks, the limited amount of available bandwidth will still figure as a factor in the form of the network.

A short-term problem that may be encountered in efforts to establish an all-pervasive network of this nature is the limited ability of many current communication systems to interlink seamlessly. Connection between the current tactical and infrastructure communication systems can only be made at a gateway point, meaning that data must be routed to that point and use some of the limited available bandwidth in the process. Equally, current tactical systems are not yet configured to exchange information in a seamless manner with external systems at callsign/node level. Therefore, alterations will have to be made to current tactical communication systems if NEC is to be fully realised.

UWB communication systems have the potential to provide high data rates over short ranges. As such, these systems are best employed in a military environment where there is a concentration of entities which require to exchange information, such as at the tactical level and below and within headquarters. The EIRP limits placed on these systems by the FCC, and the more stringent masks likely to be placed by ETSI, considerably limit the effective communication ranges by constraining both the emitted power and the bands in which UWB may operate. UWB systems are better suited to linking groups of battlefield entities over a small geographic area and if a niche exists for this technology it is at a level below tactical. While these limits remain extant, therefore, UWB will not be able to support the sort of medium or long-range systems necessary for either infrastructure communications or combat net radio.

Equally important to the successful functioning of the overall network is the ability of individual entities automatically and dynamically to form themselves into effective sub-networks. These ad hoc networks may operate over wide or narrow areas and will form the backbone for information exchange between entities. Robustness will be added to the system through the identification of a variety of communication paths so that the loss of one entity does not render the entire network ineffective. Ideally, the system should choose the most appropriate method of communication for each link in the path based on the path length and required channel capacity. For security, authentication procedures will need to be used to prevent unwanted entities joining the network.

The main problem with a network of this nature is that high-data-rate communication systems must operate at higher frequencies where there is appropriate available bandwidth. However, frequencies above the VHF band generally propagate on a line-of-sight basis and are therefore best used as data bearers over short ranges for ground-to-ground systems. Satellite systems provide a means of overcoming some of these shortcomings, but again there is a limit on the data rate of a satellite channel and an associated cost. Another approach may be to spread a large number of high-bandwidth systems across the envisaged battlespace so as to form a network through which entities can move; such an approach forms the basis of cellular telecommunications systems but is currently more difficult to achieve in unpredictable environments where the military is likely to operate. It is, therefore, conceivable that a greater data limit will be placed on medium-range communication systems than short-range systems and that some form of selection will be required to determine which information should be transmitted over lower-rate channels.

The high data rate and low pollution of the spectrum associated with UWB means that it offers considerable potential for inclusion as a short-range component of such a network. In line with the IEEE 802.15.3a standards, the ranges of these systems are likely to be limited to around 10 m. Greater ranges may be achievable, but at the expense of a lower data-rate. However, there are a number of potential applications which could make use of such a short-range, high data rate, LPI form of communication, whether directly linked to the network or forming an isolated system with indirect linkage.

Potential military applications of UWB

Given the framework within which future military communication systems will operate and the physical limitations imposed on UWB systems, UWB will be constrained to applications either within platforms or between entities which are within the operating range of the system.

Additional constraints that will be placed on any system that is used in the military sphere will be the size and weight of the equipment and its supportability, particularly in terms of battery usage and the positioning of the antenna. For equipments placed on AFVs, the power supply is unlikely to be limited and the equipment weight is unlikely to be detrimental to the vehicle performance. However, space is limited and equipments may have to be designed specifically for use on particular vehicle types. All four factors, however, must be considered when the equipment is carried by dismounted personnel, particularly when they need to be self-sufficient for a protracted period.

Linking of entities to form a headquarters

In line with the vision for NEC, the ability to conduct dynamic collaborative interworking in an environment of full information accessibility is viewed as one which will assist in the delivery of goals such as information superiority and synchronisation of effects. In order to plan and conduct such operations successfully, decision makers must have the support of a wide variety of information sources including databases of relevant information and staff with relevant experience. While the current method of bringing these supporting information resources together is a hard-wired headquarters, such an entity takes time to establish and places limits on the way that planning may be conducted. Bringing entities together on a wireless link would allow greater flexibility in its establishment. Furthermore, this would allow elements to join and leave the headquarters on an ad hoc basis without the need to wire themselves in.

One key concern when siting and establishing a headquarters at any level is that it has a considerable and recognisable electronic signature. Physical and electronic procedures are adopted to reduce the emission levels, particularly in the threat direction, yet it may remain an obvious target. Adding a further communication system to this already-busy formation runs the risk of making the headquarters yet more recognisable and may, therefore, increase the chance of compromise. The advantage of providing these wireless links on a UWB bearer is that the emission density is kept low and the risk of compromise limited.

Remote sensor applications

There are a number of applications in the military sphere where it may be desirable to position a series of sensors across a wide area and access the information derived from all of them from one or more positions. These sensors could be video cameras but may also be acoustic, seismic or infra-red devices, all of which will deliver some form of situation awareness. The current configuration of many such arrays means that they are usually hard-wired to a single central node and operated from there. However, it takes time to lay out and collapse a hard-wired array of this type, reducing its speed of deployment and recovery and hence its flexibility. With hard-wired systems the potential for the discovery of the connecting cables adds to the potential for compromise of the array as a whole. One alternative to a hard-wired sensor array is a series of wireless links; however current communications technology gives an obvious electromagnetic signature and is therefore also subject to compromise.

Ideally a sensor array could be deployed quickly, using LPI wireless links across an ad hoc network as the data bearer. One further advantage of this system would be that the information could be accessible across the array and not just from the position where the cables end. Furthermore, the use of short-pulse technology for the physical layer of such a system would allow recovery of hidden sensors by individuals who had not been involved in the initial deployment.

The utility of UWB as the bearer for these applications may be compromised by the range that is achievable for given data and error rates. For remote sensor applications a path-length of greater than 100 m is useful and greater than 1 km is highly desirable.

Wireless links around the body

Some dismounted soldiers, particularly commanders, currently have to carry a variety of electronic equipment, mainly for communication. In many cases, this equipment comprises a number of components joined by wired links. These links are uncomfortable, restrict movement and are prone to breakage through wear and pinching. Some links, such as between the battery, amplifier and antenna on a radio need to be wired owing to the power passed between them. Other links, such as between the earpiece and the modulator, pass only data and could be replaced by a wireless alternative. UWB, with its low spectral power and high capacity may be a suitable technology to provide that wireless network around the body. In this case, the maximum achievable path length is unlikely to constrain the utility of UWB as a bearer. However, emission levels should be kept as low as possible to avoid possible adverse health effects.

The fielding of technologies such as the Future Infantry Soldier Technology (FIST) will add to the number of components that need to be linked around the body, so considerable spare capacity must be built into this sort of system. The system would have to support voice and some data communications. The estimated capacity required would be in the order of 500 kbps at a maximum range of 2 m.

Logistic tracking

Logistics is an area of military consideration that stands to become increasingly efficient as NEC is realised. The ability to predict logistic need and communicate this information automatically through the network will facilitate a move from the current supply-based system to one based on demand. Some logistic requirements are predictable, for example rations are delivered in 24-hour boxes so consumption, and hence resupply rates can be determined externally. However, many logistic demands are made on an ad hoc basis; resupply of ammunition and clothing must be based on need and although usage models exist, these needs cannot yet be accurately predicted. Efforts are, however, being made to develop devices that can predict logistic need. For example, monitoring the level of impurities in the engine oil of AFVs can help to determine when that AFV will need a new power pack.

Another area of current research is the development of ‘smart’ clothes which detect wear rates and damage and can communicate this information to external devices. Some clothing, such as NBC protection suits, only have a limited use time, with performance degrading depending on the environment they are exposed to. Automatically monitoring this and communicating the information externally would help to predict when suit performance had degraded and it required replacement. Aside from clothing, other sensors may help determine logistic need. The ability to count automatically the quantity of ammunition used by a particular individual would help to ensure correct resupply. In the future it may even be possible to monitor the health of an individual either for preventative medical treatment or to cue the right sort of emergency treatment.

While sensors placed around the body may be able to communicate logistic and medical demands, some form of communications network will be needed to pass this information into the logistic chain. Although communication bearers such as Bowman and FIST may be able to carry these demands over a wide area, what would be needed is a network around the body to consolidate the information from the variety of sensors before passing it into the longer-range bearer.

One other logistic application of short-pulse UWB technology that has been marketed allows precise location of tagged assets over relatively short ranges. The Multispectral Solution PAL 650 system [13] consists of asset tags with a four-year life which can be located using UWB equipment. A trial in the US found that assets could be located to a few cm² over an area of 4 km². Such a system may be useful when locating a particular piece of equipment from a large storage depot.

Conclusions

This paper has outlined recent investigations into UWB technology and the legislative constraints placed on signal EIRP and bandwidth. A number of applications of this technology already exist, and others have been considered in the paper, which has considered applications that are of particular benefit to the military. The specific advantages of UWB for military use are its high capacity, the low intercept probability and the resistance to multipath fading. The very large bandwidth also means that transmitters can be precisely located, offering a position-reporting capability. A limiting factor for some of the applications considered is one of range—the benefits of UWB can only be harnessed if the distance between units is of the order of 100 m or less. When range is not a limitation however, then the benefits of using UWB are appreciable.

References

Authors

Major James McCulloch QDG is studying at the Defence Academy of the United Kingdom, Shrivenham, Swindon. He has recently completed the MSc in Defence Technology.

Dr Bob Walters is a Senior Lecturer in Communications, Department of Power, Aerospace and Sensors, Cranfield University. E-mail: c.r.walters@cranfield.ac.uk.