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1.INTRODUCTION
The infantry soldier of tomorrow promises to be one of the most technologically advanced mod¬ern warfare has ever seen. Around the world, various research programs are currently being conducted, 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 integrated combat systems. Alongside vast improvements in protective and weaponry subsystems, another major aspect of this technology will be the abili¬ty to provide information superiority at the operational edge of military networks by equip¬ping the dismounted soldier with advanced visu¬al, voice, and data communications. Helmet mounted visors, capable of displaying maps and real-time video from other squad members, ranges of physiological sensors monitoring heart rate, core body temperature, and mobility, arrays of biochemical sensors detecting noxious gasses, as well as a range of night-vision and heat-sensing cameras will all become standard issue. These devices will improve situational awareness, not only for the host, but also for co-located military personnel who will exchange information 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 to scientists and engineers alike. Wireless devices will be expected to operate in a range of envi¬ronments much more diverse than those encountered in civilian applications, yet still maintain an ultrahigh level of performance in terms of reliability and efficiency. From a mate¬rial perspective, these wireless devices must be compact, lightweight, unobtrusive to soldier movements, and ideally mounted conformal to the body surface. As soldiers may go for days between opportunities for battery recharge or replacement, power saving will be concurrent throughout all layers of the protocol stack. Physical layer (PHY) technologies must be resilient to jamming, able to operate in marginal conditions while using the minimal amount of energy conserving hardware. The medium access control (MAC) layer must also be designed to act in a power-saving bandwidth-efficient man¬ner while maintaining excellent quality of ser¬vice (QoS). Wireless security will be critical to maintaining a tactical edge as message interception and decryption could lead to compromise of the mission. When combined with covert communications (by covert we mean that signal transmissions remain hidden from the enemy),we have the prospect of achieving secure and robust wireless MANETs while maintaining the element of surprise. These are formidable chal¬lenges, but may be surmountable using both recent developments in millimeter-wave (mm¬-wave) transceiver technology, and the 5-7 GHz of contiguous bandwidth currently being made available throughout the world in the 60 GHz mm-wave band .
In this article we present some of the work t:ndertaken in conjunction with the U.K. Min¬istry of Defence (MOD) to investigate the feasi¬bility of using mm-wave body-worn antenna arrays to provide covert mobile ad hoc wireless networking for dismounted combat personnel. The objective of this article is twofold. First, we begin by introducing the concept of a soldier-to-¬soldier MANET and briefly discuss some of the competing air interface technologies that could he used to provide high-speed wireless network¬ing for dismounted combat personnel. We then discuss some of the potential issues at the PHY and MAC layers in relation to the implementa¬tion of stealthy, high-data-rate, mm-wave sol¬dier-to-soldier communications. One of the key challenges that remain for military hardware and network designers is the simulation of the wire¬less transmission channel. A good approximation of channel characteristics is fundamental to test¬ing the performance of newly designed proto¬cols, and understanding the required operating 'margins for front-end radio design. Therefore, the second section of this article takes a novel approach to the simulation of the wireless trans¬mission channel by exploiting state-of-the-art animation-based technology developed for com¬puter game design to accurately encapsulate the dynamics and mobility of soldier movement in the simulation of signal propagation within sol¬dier-to-soldier MANETs.
2.SOLDIER-TO-SOLDIER MANET CONCEPT
The concept of a short-range soldier-to-soldier MANET is illustrated in Fig. 1. In this exam¬ple a small team of co-located infantry troops are wirelessly networked to facilitate high¬speed communications within a cluttered urban warfare environment. As the combat team progress through the environment their com¬munications requirements will be extremely varied, with needs ranging from short message text (e.g., spoken by the receiving terminal or displayed on a helmet mounted visor) and peer-to-peer voice (avoiding the need for shouting or hand movements), through to real-time streaming video. What is fundamental, however, 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 (i.e., inherently stealthy). This is especially true for special operations forces where knowledge by enemy forces of increased activity in any region of the radio spectrum may lead to dis¬covery, capture of transmitted data, and/or interference with it. Such intelligence of, or inference of intent from, communications may compromise operations, for example, by revealing the movements of the forces or loss of the element of surprise.
Figure1
3.SHORT-RANGE COVERT AIR INTERFACE TECHNOLOGIES
To achieve optimal network-centric opera¬tions, tactical information must be effectively distributed among soldiers while maintaining a low probability of detection and intercept. Ultra-wideband (UWB) is an air interface technology that could supply sufficient bandwidth to meet the high data rate requirements of future body-worn military communications systems. UWB radios operate by employing very short duration signal pulses that result in large transmission bandwidths. The U.S. Fed¬eral Communications Commission (FCC) defines a UWB device as any device where the fractional bandwidth is in excess of 0.2 of the arithmetic center frequency or greater than 500 MHz, whichever is less. The FCC have granted permission for unlicensed UWB devices 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 reduce interference with other co-located wireless sys¬tems operating within the same spectrum space. The stringent transmit power limita¬tions placed on UWB devices have been cho¬sen so that they minimize the risk of interference to authorized radio services by operating close to the noise floor. This is a feature that many in the military community will find attractive as it introduces a lower probability of detection compared to conven¬tional wireless systems. UWB could provide the dismounted soldier with a maximum data rate of 100 Mb/s for transmitter-receiver separations less than 10 m and as much as 480 Mb/s at very small separation distances (typi¬cally less than a few meters). Improvements to operating distance and channel capacity could be realized by raising the spectral density emission limit; however, such a move could prove unpopular as it will lead to increased interference with licensed radio users and remove the stealth mode of operation, leading to easier detection by the enemy.
Compared to UWB, 60 GHz millimeter wave communications will operate in current¬ly under-utilized spectrum space and provide high data rates of up to several gigabits per second for short-range applications . Oper¬ating ad hoc network communications for dis¬mounted combat personnel at 60 GHz will offer a number of distinct advantages com¬pared to the other competing lower-frequency technologies. Factors that would generally be considered to hinder traditional radio com¬munications can be exploited to provide the desirable signal propagation characteristics required for short-range military communica¬tions. These include increased covertness, high frequency reuse, and reduced risk of interference (which may be attributed to high¬er path loss), increased atmospheric oxygen (02) absorption, and narrow antenna beam width. Another important feature of mm-wave frequencies is the small size of product that may be achieved. Ultra-low form factor transceiver design will become reality due to the extremely short wavelength (X, 5 mm), which will also facilitate the construc¬tion of wearable smart antenna arrays capa¬ble of electrically steering highly focused beams of electromagnetic energy in chosen directions. To realize the objective of direc¬tional mm-wave communications, there are still a number of hurdles at the PHY and MAC layers that need to be overcome, as dis¬cussed below.
4.MM-WAVE SOLDIER-TO-SOLDIER COMMUNICATIONSHY LAYER CHALLENGES CHANNEL CHARACTERISTICS
It is widely recognized that the successful devel¬opment of hardware and wireless networking protocols is highly dependent on a thorough knowledge of transmission channel characteris¬tics relative to deployment. Much of the current research involving mm-wave short-range commu¬nications has been carried out considering a range of indoor environments for stationary transmitter and receiver scenarios . Here sta¬tistical descriptors of the channel, such as path loss exponent and root mean square (rms) delay spread, are found to be heavily influenced by antenna configuration and the local surround¬ings. While these studies are useful for the devel¬opment of indoor wireless networks, any attempt to apply this channel information to the design of mm-wave soldier-to-soldier networks would be inappropriate, especially considering issues such as the scattering of signals from both users and pedestrians, and the inherently dynamic and highly mobile nature of military operations.
At present, very little is known about the characteristics of signal propagation between wearable wireless devices forming a human body-to-body network (BBN). Recent narrow-band studies at 2.45 GHz have shown that signal propagation is dependent on the user's physical characteristics, including mobility, and may be modeled using x—µ fading statistics. The effect of human body shadowing on mm-wave wireless links has received some coverage in the literature. In it is reported that human body shadowing can cause attenuations of greater than 20 dB on indoor 60 GHz device-to-¬device links. Field trials performed for this study to investigate human body shadowing events on indoor point-to-point links have found similar results (attenuations of 20-25 dB), with the greatest shadowing events found to occur when the human body moved in the direct vicinity of a 60 GHz node, blocking line of sight (LOS).
4.1.TRANSMISSION SCHEMES
There are a number of different transmission schemes that could be adopted for soldier-to-sol¬dier communications. These include the single carrier (SC) and orthogonal frequency-division multiplexing (OFDM) schemes currently being investigated by IEEE 802.15 TG3c . OFDM is well known for its ability to mitigate against fre¬quency selective fading due to multipath, by turn¬ing the transmission channel into a series of suitably modulated (e.g., quadrature amplitude modulation) orthogonal subcarriers. This has the effect of greatly reducing the complexity of transceiver design through the use of inverse fast Fourier transform (IFFT) and FFT signal pro¬cessing stages for signal transmission and recep¬tion, respectively, and negates the need for intricate wideband equalizers. While OFDM may be resilient to multipath effects, it is prone to a high peak-to-average power ratio (PAPR), phase noise, and carrier offset. High PAPR will be a particular problem for soldier mounted radios, as it will cause nonlinear distortion and low power efficiency in the power amplifier , directly impacting battery life. The complexity of time-domain channel equalization in wideband SC sys-tems is regarded as its main drawback for use in high-data-rate mobile radio channels. However, this challenge can be overcome through the use of frequency domain equalization (FDE). Single carrier systems with FDE (SC-FDE) typically use transmission blocks with a cyclic prefix to prevent interblock interference. Signal recovery at the receiver is then performed through FFT process¬ing with equalization followed by an IFFT stage. SC-FDE will then deliver performance similar to OFDM, with essentially the same overall com¬plexity [9], but because SC modulation uses a sin¬gle carrier it has the added advantages of lower PAPR and less sensitivity to both phase noise and carrier offset [10].