Millimeter-wave Stealth Radio for Special Operations Forces

The continuation of worldwide peace-keeping operations by the United Kingdom, United States and coalition allies has placed a continued reliance upon the use of Special Operations Forces to perform information-harvesting and security-based activities such as covert reconnaissance and surveillance, and directed counter-terrorism strikes. Key to the success of these operations is the ability of the Special Forces to work undetected well behind enemy lines. While visual concealment can often be achieved through the cover of darkness, masking of other potential sources of detection such as soldier-to-soldier radio communications is not as straightforward. In fact, given the recent influx of technology at the disposal of today's soldier, such as miniaturized Global Positioning System (GPS)-based navigational aids and multi-megapixel video cameras, and the need to share data for improved situational awareness, this is a task that will become increasingly difficult. It is clear that mission success could benefit quite significantly from a 'stealth radio'.

Designing wireless devices that are capable of meeting the stringent demands of the special operations soldier is a challenging task. Soldier-mounted radios are expected to be extremely power efficient and ultra-reliable, mechanically robust and easy to operate, non-inhibitive to movement and capable of providing high data bandwidths. At the same time, they must be compact, lightweight and have a stealth mode of operation. With the widespread use of wireless technology, they must also be resilient to interference from both co-located spectrum users and malicious jamming by enemy forces. These are formidable challenges, but may be surmountable using both recent developments in millimeter-wave (mm-wave) transceiver technology,^1,2 and the 5 to 7 GHz of contiguous bandwidth currently being made available throughout the world in the 60 GHz mm-wave band.³

Benefits of 60 Ghz Technology
To achieve optimal network-centric operations, tactical information must be effectively distributed among soldiers while maintaining a low probability of detection and intercept. This places two distinct requirements on the air interface technology used: it must be capable of high data rates (when required) and have desirable propagation characteristics. Although there are a number of candidate air interface technologies that could be used to implement soldier-to-soldier communications such as ultra-wideband (UWB) in the 3.1 to 10.6 GHz band and Wi-Fi in the 2.45 and 5.2 GHz bands, this article focuses on 60 GHz communications. Operating soldier-mounted radios at this frequency will offer a number of key benefits compared to the other competing lower frequency technologies. For example, 60 GHz millimeter- wave communications will operate in currently under-utilized spectrum space and will provide high data rates of up to several gigabits per second for short-range applications.³ Furthermore, factors that would generally be considered to hinder traditional radio communications can be exploited to provide the desirable signal propagation characteristics required for short-range military communications. These include: increased covertness, high frequency reuse and reduced risk of interference, which may be attributed to higher path loss, increased atmospheric oxygen (O₂) absorption and the narrow antenna beamwidth inherent with high-gain arrays.

Shorter wavelength mm-wave frequencies are also subject to much greater losses caused by electromagnetic (EM) interactions with everyday objects (e.g., building structures and personnel) when compared to longer wavelength microwave frequencies. This effect can be observed in Figure 1, which shows the power received by a square grid (resolution 1 m²) of isotropic antennas placed at a height of 1 m above ground level, in a Computer Aided Design (CAD) model of a Middle Eastern compound, from a wireless node operating at 2.45 GHz positioned on the protective helmet of a soldier. For this simulation, the antenna associated with the node had an omnidirectional radiation pattern and was configured to operate with a transmit power of +20 dBm. The results were obtained using a ray tracing EM simulation tool, simple material models of the building structures, and human body models generated using the Poser 7 animation software as described in the literature.⁴

It can be seen quite clearly that the signal transmitted from this node illuminates the majority of the outdoor environment with a received power generally greater than -70 dBm. This high degree of signal coverage is clearly undesirable for the proposed stealth mode of radio operation. In contrast, Figure 2 shows a much less extensive coverage pattern for the same node when operated with an identical transmit power at 60 GHz. Here, the level of signal illumination is significantly reduced. For a considerable region of the outdoor environment, particularly within and in the immediate shadow of the buildings, virtually no signal is received at all, as shown by the white squares indicating levels below -100 dBm.

Another important feature of mm-wave frequencies is the small-size of product that may be achieved. At 60 GHz, it is possible to construct a cylindrical antenna array with 32 elements placed half-wavelength apart, all within a radius of 13 mm (close to the size of a US quarter). This will permit the development of compact, wearable smart antenna technology that could use technologies such as adaptive beam steering to dynamically adjust the array pattern by altering the amplitude and phase of a feed network. A system level view of the smart antenna hardware operation is shown in Figure 2c, where information passed down through the network layer is used to control the phase at the input of an antenna array. This will allow antenna gain to be focused in the required directions, helping to counteract eavesdropping, improving resilience to jamming and provide a lower probability of detection by enemy forces.

60 GHz Stealth Radio: The PHY Layer
Transmission Schemes: There are a number of different transmission schemes that could be adopted for soldier-to-soldier communications. These include the single carrier (SC) and orthogonal frequency-division multiplexing (OFDM) schemes specified in the IEEE 802.15.3c standard for high rate wireless personal area networks.⁵

OFDM is well known for its ability to mitigate against frequency selective fading due to multipath, by turning the transmission channel into a series of suitably modulated (e.g., quadrature amplitude modulation) orthogonal sub-carriers. This has the effect of greatly reducing the complexity of transceiver design through the use of IFFT and FFT signal processing stages for signal transmission and reception 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 amplifier³ directly impacting upon battery life. The complexity of time-domain channel equalization in wideband SC systems 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 inter-block interference. Signal recovery at the receiver is then performed through FFT processing with equalization followed by an IFFT stage. SC-FDE will then deliver performance similar to OFDM, with essentially the same overall complexity,⁶ but because SC modulation uses a single carrier it has the added advantages of lower PAPR and less sensitivity to both phase noise and carrier offset.⁷

RF Front-end Technology: The choice of 60 GHz RF front-end technology for a soldier-mounted radio will introduce a tradeoff between performance and cost. Traditionally group III-IV semiconductor technologies such as GaAs and InP have been used for mm-wave radios. While they offer superior noise characteristics and high gain at mm-wave frequencies, they also suffer from a high cost per unit, poor integration and low power efficiency. CMOS technology on the other hand will offer lower-cost mass production, improved integration and increased power efficiency; however, CMOS front-end circuits will also have to address issues in power amplifier output, local oscillator phase noise and low-noise amplifier design as discussed in the literature.⁷

As a compromise, more recent advances in silicon germanium (SiGe) technology have now made it possible to build miniaturized, low-cost mm-wave radio devices, such as the 60 GHz, 0.13 µm SiGe BiCMOS double-conversion superhetrodyne receiver (Rx) and transmitter (Tx) chipset recently developed by IBM.¹ Here, data rates of up to 630 Mbps have already been demonstrated for this chipset over a 10 m indoor Line of Sight (LOS) link using folded-dipole antennas for both Tx and Rx modules. Based upon link budget calculations, the IBM authors also state that increasing the receiver gain by 12 dBi (e.g., using smart antenna technology) could increase the range by a factor of four assuming free space propagation. Undoubtedly, even greater operating distances may be attained by sacrificing bandwidth and data rates or improving overall system gain.

In Reference 2 a single chip multigigabit transceiver on CMOS is described; the architecture of this device is illustrated in Figure 3a, while Figure 3b shows the device with an integrated antenna. In Figure 4, a 4 × 4 phased array transceiver implemented on 65 nm CMOS is shown.⁸ The measured receiver noise figure is 5.5 dB and the output P1dB of each transmit chain is 7 dBm. This device, including 4 transmit and receive chains, consumes a total of 650 mW.

mm-Wave Soldier-to-Soldier Communications
One of the greatest challenges to ensuring the success of a 60 GHz-based special operations radio will be the system's ability to cope with the unpredictable nature of its operating environment. Everyday obstacles like buildings, cars, vegetation and even humans, which can limit the propagation of microwaves, will have a much greater impact on mm-wave systems. For example, in Reference 10 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 by the authors investigating human body shadowing events on indoor point-to-point links found similar results (attenuations of 20 to 25 dB), with the greatest shadowing events occurring when a person moved in close proximity to a 60 GHz node, blocking the LOS. In military operations, the continual movement of soldiers in high-tempo urban (i.e. cluttered) environments is likely to lead to frequent loss of LOS links between two soldiers. Hence, the wireless link will become dependent upon multipath contributions from signals scattered, reflected and diffracted from the surrounding environment.

To overcome channel impairments and exploit multipath propagation, 60 GHz soldier-mounted radios will have to make innovative use of beam steering hardware, time of arrival (TOA) and direction of arrival (DOA) information, digital navigation aids such as GPS (e.g. soldier's digital assistant) and inertial navigation systems as well as smarter routing tables and strategies. The positional information needs to be readily shared among squad members during communications packet exchange so that internal calculations may be performed to estimate relative geometries (these methods of DOA estimation may not be as effective in indoor environments or when the direct LOS is obstructed). All of this information will form the basis of an internal positioning table used to administer and manage connections between soldiers.⁴ The success of the system will also depend on a bespoke directive medium access layer (MAC). As the focus of this article is on the PHY technologies, required MAC operation is not discussed. Instead the interested reader is referred to Reference 4 and the references therein for a full description of its functionality. However, the reader should note two important points, vital to the understanding of the proposed system operation. Firstly, when a node is in idle mode, or during random back-off intervals in contention periods, it should listen to the channel omnidirectionally, that is, without beam steering. Secondly, all other operations associated with channel access, set up and data transfer (both transmit and receive) should be performed directionally.

To illustrate how 60 GHz communications could be achieved, consider single hop communications between soldiers B and C, as shown in Figure 5. If it is assumed that successful communications between these two soldiers have occurred very recently and hence they have good estimates of their relative locations or DOAs of significant multipath components, soldier C uses the last known 'good' directional entry for soldier B in his internal positioning table to initiate communications (not necessarily the LOS link). All nodes that may overhear the transmission (e.g., soldiers A and B who are in idle states) then update their internal positioning tables with the incoming signal's DOA and adjust the elapsed time of arrival information. This will include tracking and storing all major multipath components as well as the most significant path, as shown in Figure 2.⁴

Assuming unhindered channel set up, soldier B then uses the information stored in his positioning table to beam steer in the direction of soldier C. Meanwhile, soldier C also beam steers in the direction of soldier B and begins the directional transmission of data. Throughout this process, all nodes that can hear the exchange, continuously update their positioning tables. In the case of soldiers B and C, this will provide the maximum opportunity of re-establishing the link should it unexpectedly go into outage, before abandoning transmission and handing the problem to the network layer for routing as outlined in the 'packet transmission' flowchart (see Figure 2).⁴ Link sustainability can also be guaranteed by dropping repeater nodes as the team progress through the theater of operations. As these nodes simply capture and repeat packet transmissions, they carry no information on encryption methods used and therefore can be safely discarded.

To further enhance the stealth mode of operation, the system could also use adaptive power control. Here, radio transmit power is adjusted on a packet-by-packet basis to the minimum level required for operation with a given capacity and error probability. These schemes are often desirable in mobile wireless systems for the purposes of reducing interference and prolonging battery life.

Overall, this is only a brief overview of how directional 60 GHz communications could work between soldiers. However, it will be particularly susceptible to many of the common issues associated with wireless networking such as the hidden node problem, deafness and gain asymmetry.

Conclusion and Future Work
Previous sections outlined the innovative developments that are taking place at mm-wave frequencies that can help realize 'Stealth Radio' for the benefit of covert applications such as Special Operations Forces. By fusing a 60 GHz operating frequency with smart antenna technology it will be possible to build short-range body-centric networks that are virtually undetectable to the enemy. Not only will these systems provide covert communications, but they will also provide the bandwidths required to simultaneously transmit real-time streaming video, voice, health and location related data.

Current work is focused on developing the low cost, power efficient integrated beam steering transceivers needed for these systems and characterizing their performance in realistic scenarios and environments that represent the difficult propagation conditions expected for these systems. While today's state-of-the-art systems, implemented on 65 nm CMOS, consume approximately 650 mW, next generation 60 GHz systems, implemented on 45 nm and 32 nm CMOS, will reduce power consumption to less than 300 mW and substantially improve in receive sensitivity by incorporating better beam steering as well as MIMO receivers. Future work is aimed at the engineering and integration of a wearable prototype research system. This will be used for an assessment of mobile ad hoc networking between dismounted combat troops and channel performance using a combination of representative real and virtual environments. n

Acknowledgments
This work was partially supported by the UK Ministry of Defence through the 'Competition of Ideas' Program, under project reference RT/COM/5/001, the Royal Academy of Engineering and the UK Engineering and Physical Sciences Research Council (EPSRC) under grant EP/H044191/1 and the NICTA Victorian Research Laboratory. NICTA is funded by the Australian Government as represented by the Department of Broadband, Communications and the Digital Economy and the Australian Research Council through the ICT Centre of Excellence Program.

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