Unmanned Systems Technology 001 | UAV Factory Penguin C | Real-time operating systems | Hirth S1218 two-stroke twin | Base stations | ASV C-Enduro | Composites | Datacomms

46 Insight | Airborne base stations The Google X-Titan project needs to avoid having to design new headsets, otherwise it faces the same problems as traditional satellite-based communications systems with expensive handsets (such as Iridium) or suitcase-sized portable broadband satellite terminals. The power limit of 20 dBm (100 mW) on the ground for the IEEE 802.11n wi-fi standard also limits the range to around 100 m outdoors, which is not enough to reach a high-altitude base station. However, with the signal source being well away from the population, there is scope for a higher power specification that could enable a far higher output, but that would need standardisation within the IEEE 802.11 standards groups, which could take several years. Current standards in progress include 80211.ah , which is a sub-1 GHz version of wi-fi at around 960 MHz in Europe and 860 MHz in the US. Using these lower frequencies rather than the existing 2.4 GHz band would give much more penetration and so support more users for a given power output at the base station. The 80211.ah technology is set to be approved next year, and will work alongside 2.4 GHz systems, but new terminals and phones that support both the IEEE 802.11n and 80211.ah standards would be needed. Instead, 4G or even 5G – which aims to unify cellular with wi-fi – could be used. A typical base station uses between 6 and 13 kW of power, which with optimisation would just about fit inside the Solara 50’s power envelope. One advantage that Solara has is that about 20% of the power consumed by a base station on the ground is used for air conditioning, which would not be needed for an airborne design. However, most of the power, around 65%, is used in the power amplifier that boosts the signal for transmission. The next generation of base station designs are also taking advantage of techniques such as digital pre-distortion, outphasing and envelope tracking. These three different techniques modify the signal to compensate for losses in the transmission chain, so that when the signal reaches the power amplifier of the base station it can operate in the most efficient region. This can boost the efficiency of the power amplifier from 65% to about 90%, allowing less power to be used to transmit a signal over the given distance. A further challenge is in the design of the basestation as a payload. Building the base station elements with light and flexible substrates that can be easily integrated into the body of the aircraft will be an important element in reducing the weight, as will having lightweight and flexible cables between control units. There will also need to be a high- bandwidth, directional backhaul link to connect to the internet on the ground. This would connect to the ground using wireless or to a geostationary satellite, but that would also require a heavy dish and a low noise block to make a connection. This presents significant weight and performance challenges, as the backhaul link determines how many simultaneous users can be supported at one time. Titan worked with another New Mexico start-up, TriLumia, on a novel semiconductor laser technology to use free space optical (FSO) for this backhaul. Here, a laser carries the signal via light rather than radio. TriLumia was spun out of Sandia National Laboratory, and it has developed a technology to build an array of Vertical Cavity Surface Emitting Lasers (VCSELs) in standard CMOS silicon. When pulsed, the array is powerful enough to send a signal at between 200 Mbit/s and 10 Gbit/s across 20 km with minimal beam spreading. Being built in silicon reduces the size, power consumption and cost of the system compared to solid-state lasers that are currently used for FSO links for 4G cellular networks, and makes the technology viable for an aerial base station link. The key for researchers to getting the low-power laser link has been When pulsed, the array is able to send a signal at 200 Mbit/s to 10 Gbit/s across 20 km with minimal beam spread Solid-state satellite comms laser technology from TriLumia is being proposed for aerial base stations November 2014 | Unmanned Systems Technology