There is no perfect waveband. Armies principally rely on VHF and UHF transmissions to move voice and data traffic from point to point around the battlefield. HF and SATCOM are also used, although typically for carriage at beyond-line-of-sight (BLoS) range.
V/UHF has been in widespread use for tactical communications since the Second World War, but these wavebands have limitations – VHF has 270MHz of bandwidth compared to the 2.7GHz offered by UHF.
From a communications security perspective, this affords a large number of frequencies across which transmissions can hop in a pseudo-random fashion to reduce their chances of detection and interception by hostile COMINT gatherers, although this width will be contingent on how much of the band national telecoms regulators afford to their military’s use of VHF. The downside of VHF is that it is limited to LoS range.
To put this into perspective, even when using an antenna 3m above the ground on an AFV, the range achievable using VHF transmissions from one vehicle to another, assuming both are on flat ground at sea level, is going to be about 7km. Place an obstacle such as a tall building or hill between these vehicles and communications can be blocked altogether.
There is a general rule of thumb that the antenna to equip a radio using a specific frequency needs to be half the length of the frequency’s wavelength: VHF radios will therefore need antennas of between 5m and 50cm long, while this diminishes to 50cm to 4.5cm for UHF transceivers. One of the key attractions of the latter is that radios can have small antennas, making them particularly suitable for handheld and personal role form factors carried by soldiers.
Like VHF, UHF radios boast impressive megabits of bandwidth for data carriage, although they are similarly hampered by obstacles which can impede transmission ranges.
Terrain, buildings and other obstacles are not necessarily showstoppers for V/UHF. Both wavebands can exploit the phenomenon of diffraction. Picture a soldier equipped with a V/UHF radio standing at the bottom of a valley flanked by sharp ridges on either side of their position and their comrade on the other side of one of these ridges. The soldier activates their radio and makes a voice transmission. The radio begins to send out their voice as a transmission in a spherical pattern away from the antenna.
As well as radiating outwards away from the soldier, the transmission will hit the sharp edges of the ridge line, ‘trip up’, pivot around them and continue on its way. The second soldier can receive the transmission thanks to their diffraction around the sharp ridge line. Diffraction is a double-edged sword, however. On the one hand, it allows the two soldiers to communicate even if rugged terrain separates them, but the same phenomena can cause significant problems in urban environments.
Diffraction is a double-edged sword.
Taking a second example, an armoured vehicle is making voice transmissions down a long, straight road to another vehicle some distance away. The two might have an unimpeded LoS, but the radio transmissions do not exclusively move in one straight line. As they radiate outwards from the antenna, some of the transmissions will move directly towards the other vehicle, but others will bounce off walls, other vehicles and objects like a pinball.
The effect for the radio receiving the traffic is that it will pick up the direct LoS transmissions, but at the same time will receive ‘echoes’ of those same transmissions caused by the signal bouncing off solid obstacles. These will take slightly longer to arrive at their destination, as they have not followed a straight path. Such echoes can make voice communications unclear and seriously degrade data transmissions where streams of bits become jumbled, risking their integrity.
The situation for V/UHF becomes more problematic when these radios are being used inside buildings. Thick walls are difficult to penetrate, although thinner partitions and windows are narrow enough to let signals reach the outside world. A similar problem ensues. In the final example, a soldier is standing in a room with a single window wanting to talk to their comrade in the street below. Some of the transmissions will go straight through the window directly to the recipient in the street, others will ricochet off the walls and arrive at the receiver slightly later, causing the same problems with jumbled transmissions, lack of clarity and potential data loss.
Go forth and multiply
One way to mitigate LoS and diffraction phenomena is to use MIMO (multiple in/multiple out) technology. This is already in routine use for domestic Wi-Fi routers and smartphones: ‘You have two MIMO antennas on your fourth-generation cell phone,’ said Louis Sutherland, VP of business development at MIMO technology specialist Persistent Systems. ‘Your old flip phone might die in a tunnel, but your new 4G phone will not die as quickly.’ The technology is now entering military use and could herald a minor revolution in tactical communications, particularly for land forces.
In the civilian world, MIMO takes care of two common requirements: the need for wideband communications and to overcome potential LoS restrictions imposed by the urban environment and restrictions on data carriage. As already noted, ‘most radios which have one antenna are not effective in urban areas or underground’, said Sutherland – a serious tactical shortcoming.
Furthermore, the need for ‘big data’ on the battlefield to share C2 information and situational awareness makes the potential loss of signal fidelity due to diffraction all the more serious. ‘MIMO is unique as it is the only technology that enables you to avoid the limitations of the Shannon-Hartley theorem,’ said Jimi Henderson, VP of sales at Silvus Technologies, another MIMO systems provider.
The theorem specifies the maximum data rate which can be sustained over a channel of a specific bandwidth according to the signal-to-noise ratio. MIMO transmits multiple data streams and this helps to avoid the limitations inherent when transmitting a single data stream. ‘It’s sort of a loophole that allows us to transmit more data over a link than previously thought possible,’ Henderson stated.
Put simply, MIMO is a method of simultaneously sending the same communications traffic with several signals transmitted from several antennas. In a voice transmission, the soldier speaks into the radio and the analogue sound of their voice is converted into data. With a MIMO device, the data is split into several streams and then transmitted through multiple antennas, in this case three, which equip the radio.
These transmissions move through the ether, some going directly to the receiving radio, others bouncing off walls and solid objects in much the same way as they would from a conventional device. The transmissions are received by another MIMO radio. As normal, some of these transmissions will have taken slightly longer or shorter to arrive at their destination because of diffraction, but the radio will collect all the transmissions using its three antennas, even those which get corrupted and lose data during their journey. The radio will reassemble all the transmissions and convert the data back into an analogue sound which can be heard by the soldier.
Ultimately, when using MIMO, the signal is stronger, clearer and complete, whereas it might be badly degraded using a conventional SISO (single in/single out) radio.
The fact that MIMO can provide wideband communications and work in built-up areas explains its attraction in the civilian world and why a user can move around the house into different rooms to watch movies on a tablet or still use a mobile phone far into a tunnel.
MIMO radios typically have several different antenna configurations such as two transmitting (TX) and two receiving antennas (RX), three TX/three RX, four TX/four RX and eight TX/eight RX. As a rule of thumb, the more antennas a MIMO radio possesses, the more data streams can be sent simultaneously, the more traffic can be handled and the higher the clarity of the transmissions. However, the number of antennas can be limited by the physical size of the radio, ensuring that the set is still practical to use.
Radios using MIMO are finding their ways into armies. Initially, these are fulfilling what might be considered relatively niche applications, although this could herald wider adoption in the future. In January 2019, Persistent Systems was contracted to provide its MPU5 Wave Relay MIMO radios to equip American Aerospace Technologies’ Resolute Eagle Group 3 UAV.
US DoD stipulations state that Group 3 UAVs have a maximum take-off weight of 1,320lb (600kg) and will typically operate at altitudes of up to 18,000ft. MPU5 Wave Relay radios can be equipped with specific modules allowing them to use either S-band or L-band frequencies according to customer preference. The MPU5 has a three TX by three RX antenna configuration, providing channel bandwidths of 5MHz, 10MHz and 20MHz, which translates into data throughputs of up to 100Mb/s. From a security perspective, the radios are certified to the AES-256 data encryption standard.
It is not hard to see why a system like MPU5 might be attractive to UAV manufacturers. Unmanned aircraft can gather data-heavy intelligence such as live video pictures. This imagery consumes bandwidth, so anything that can widen the pipe to ensure smooth delivery of data will be warmly welcomed.
Secondly, the fact that MIMO offers multiple data streams provides a degree of redundancy for ground-to-air/air-to-ground communications: should one data stream experience interference, two streams are still heading to and from the aircraft. This is particularly important in a military context where the link between a UAV and its ground control station could be an attractive target for electronic attack – MPU5’s AES-256 encryption is an important defence against such jamming.
While threats like the Russian Protek 1L269 Krasukha-2 mobile EW system might be capable of jamming specific S-band frequencies, the frequencies of all three MPU5 data streams would need to be ascertained, as MIMO will treat the jammer as noise and use the multi-stream information to recreate the original signal, just as it does in a noisy urban environment.
Back on the ground, the US Army has shown interest in MIMO technology to equip land forces. In May 2019, the 3rd Brigade Combat Team of the 101st Airborne Division used MPU5 radios during its field training exercises (FTX) at Fort Polk, Louisiana. Army literature states that during the FTX, MPU5s were used to carry voice, data and positional information between ground units, aircraft and HQ at ranges of up to 25km.
Also involved in the exercise was the MPU5’s Cloud Relay technology, which can manage communications between radios at BLoS ranges. The FTX initiative was a chance for the army to evaluate MPU5 wand Cloud Relay. While the service did raise some concerns regarding the amount of hardware the radio and Cisco SPOKE router, also used in the exercise, required, together with the ruggedness of their construction and power consumption, FTX showed that the army is interested in this technology and sees clear potential for enhancing land forces communications.
As a result of this exercise, Persistent Systems will be reducing the requirement for an external SPOKE device and will integrate the capability as software internal to the MPU5.
Summing up its experience using the MPU5 during FTX, the army said it saw potential in extending the ranges of soldiers’ standard tactical radios by plugging these into the MPU5 and benefitting from the latter’s MIMO capabilities. ‘The consensus from the field usage is that the MPU5 radios provided reliable communications for infantry units in combat operations,’ the army’s assessment noted.
Another observed benefit was the radio’s use of the Android OS. End-user devices (EUDs) such as laptops and tablets can be plugged into the set for the carriage of C2 or situational awareness information from device to device across an MPU5 network.
Always quick to spot a technologically practical trend, SOF are early adopters of MIMO technology. ‘They tend to be forward-leaning regarding new technologies and act as the beachhead for other forces,’ noted Henderson. Silvus Technologies has witnessed significant interest in MIMO from the US SOF community. For example, the company is involved in the US Army Integrated Virtual Augmentation System (IVAS) programme. This is developing a head-up display for soldiers in the form of an augmented vision system which will be able to display navigation or target information, for example, transmitted from offboard sources in the field of vision – think ‘Google Glass’ for troops.
Henderson said that 1,000 Silvus MIMO radios have been purchased to support the IVAS pilot initiative to carry data between troops, with the potential for acquisition of over 40,000 devices should IVAS move ahead as a programme of record in the 2021 timeframe.
The firm’s radios are also providing the middle-tier backbone for the army’s Integrated Tactical Network. This will allow soldiers to plug EUDs into a network which can securely carry unclassified data around the battlefield.
Despite being in civilian use for several years, MIMO is still awaiting wider adoption by the military. One restraint may be that armed forces around the world retain legacy radios, procured over the past two decades, which could still have another ten to 20 years of useful life, Sutherland noted. This makes it unlikely that armies will rush out and buy new MIMO radios to replace existing sets.
Instead, it seems more plausible that MIMO will be adopted over the next decade in a piecemeal fashion, being acquired to support key capabilities such as unmanned vehicles or to augment existing communications systems to provide connectivity when dismounted troops are fighting in urban areas.
Another obstacle may lie in changing mindsets. MIMO radios need at least two TX/RX antennas to work. ‘Some might ask: “Why should I carry a radio which has two or more antennas? That’s dumb”,’ said Sutherland, ‘but they don’t know what they are talking about.’ He added that troops ‘may not believe the technology when they read about it, but they do believe it when they see it’. Henderson agrees: ‘It took a while for the market to understand the benefits and how things work. The question now is why would you not use MIMO?’