38 Focus | Radio and telemetry The redundancy of a MIMO antenna system results in a higher chance of being able to use one or more of the received copies to correctly decode the received signal. Space-time coding combines all copies of the received signal in an optimal way to extract as much information from each of them as possible. This allows system developers to trade off the range of the links, the data rate and power consumption, depending on the mission profile. Antenna diversity The ability to make use of multiple antennas in a MIMO system, each one at a slight angle, provides increased performance and resilience – this is commonly referred to as antenna diversity. Antenna diversity benefits a datalink by giving the antenna a different ‘viewpoint’ to its peers. While antenna 1 might be susceptible to interference because of its angle (polarisation), antennas 2 and 3 might not be as influenced by interference because of their slightly different polarisation. This is key for UAV radio designs as there are many sources of environmental RF interference operating in the same frequency space as the transmitter and receiver, over which the operator has no control. This interference is inevitable and not something that can be prevented easily. The RF spectrum in a particular airspace is rarely exclusive and has to be shared with other users, such as mobile phone network operators, TV broadcast operators and wi-fi hotspots. This interference is likely to vary in different scenarios. In some environments, vertically polarised antenna may be more susceptible to interference than horizontally polarised ones because, for example, there may be lots of other vertically polarised transmitters and receivers within the local vicinity. In this case, a MANET radio can be fitted with two vertically polarized antennas, with a central, horizontally polarized antenna. Spatial multiplexing and space-time block coding means the same data is sent three times (once per antenna) and re-assembled at the receiving end of the datalink. If there is interference on the horizontally polarised antenna, data sent via the vertically polarised antenna will still have a very good chance of being received, even if the UAV is moving around. In field tests, having an antenna combination such as this has proven to almost double the bandwidth of the datalink. Beam steering An alternative approach is to use a tracking antenna with beam steering. This technique uses multiple antenna elements to direct a signal to a specific fast-moving UAV in the air, and even swap from one UAV to another instantaneously. This boosts the range between two nodes when using a tracking antenna to over 400 km. A conventional, motorised tracking antenna system has a narrow antenna beam that is steered in the direction of interest, where it focuses energy. This increases operational range, but because it is locked with a motorised system, it can only operate as a pointto-point link. The motorised tracking antenna also has a limited rotation speed, and this problem is enhanced when tracking a fast-moving platform such as a UAV or USV on water. In contrast, an electronically steerable radio system can change the direction of the antenna at high speed, enabling a single ground antenna to operate several UAVs simultaneously. This pointto-multipoint functionality provides connectivity to every unit in a network, even if all the units are moving. The digitally steerable beam constantly optimises the direction of transmission and reception several thousand times per second. The electronic steering involves no moving parts and enables the ultranarrow beam to be steered instantly in the right direction to communicate with several fast-moving objects. This enables an ultra-long-range tactical data link with point-to-multipoint ability, together with high broadband capability, even when tracking multiple fast-moving platforms, such as UAVs in a swarm or USV on the water. The number of transceivers defines the antenna gain for the range and the narrowness of the beam. A tighter beam has more signal margin and therefore provides a higher data rate. Spiral antennas Customised spiral antennas are used in a wide range of demanding environments, providing almost frequency-independent performance over wide bandwidths with broad beam widths. The purity of circular polarisation creates outstanding left-hand circularly polarised or right-hand circularly February/March 2024 | Uncrewed Systems Technology A radio for a MANET network (Image courtesy of Steatite Communications)
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