Delivering wireless services from the sky with USRP & LabVIEW Communications

Author : Professor David Grace | Head of Communication Technologies Research Group | University of York

01 February 2019

Stratobus High Altitude Platform [image credit: Thales Alenia Space - Wikimedia Commons (CC BY-SA 4.0)]

Researchers at the University of York needed to implement a cost-effective, low-altitude aerial testbed that can verify novel wireless communications applications between airborne nodes and ground terminals, while meeting tight constraints for payload weight, volume and power consumption.

This article was originally featured in the February 2019 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy.

The team, led by Professor David Grace, Head of the University of York’s Communication Technologies Research Group, along with Research Associate, Dr Yi Chu, combined the rapid prototyping capabilities of National Instruments’ LabVIEW Communications System Design Suite with the processing power of its USRP RIO to directly drive tailored antenna elements in a highly flexible wireless testbed. The testbed is carried onboard a tethered aerial platform, Helikite, that can be deployed for hours, at up to 400 meters altitude, which allowed the team to trial multiple applications.

Today, many people regard access to broadband multimedia wireless services as an essential utility. At the beginning of 2017, 98% of the UK population had mobile coverage, driven by regulatory requirements that operators cover at least 95% of the population. However, only 70% of the UK landmass is covered, as Figure 1 illustrates.

Figure 1. Mobile coverage across the Northern United Kingdom & North York Moors National Park

Remote areas of Scotland, British National Parks and areas of outstanding natural beauty (AONBs) are underserved, with poor quality or no internet access and, in many cases, no mobile coverage. This occurs for two main reasons: the cost of delivery, and restrictions, such as planning. The UK government has committed £440M to provide broadband to 600,000 rural homes (approximately £750 per home). Extending 800 MHz wireless coverage to 99% of rural areas was anticipated to cost £270M in 2012. Clearly, our present approaches are costly.

Meanwhile, the lack of suitable wireless infrastructure creates a technological divide. Smart cities are increasingly receiving funding, with creative ideas given for the Internet of Things (IoT), monitoring and control. In rural areas with limited coverage, smart villages and farms are still a long way off, along with applications such as soil mapping, fertiliser and pest control, fleet management, and precision livestock farming, including sheep tracking, smart feeding and milking of cows.

High-altitude platforms (HAPs) can solve the civil planning and cost issues of serving such areas, by filling coverage gaps and working alongside existing and future terrestrial infrastructures in the cities. HAPs are unmanned aircraft, airships or balloons, situated in the stratosphere (at an altitude of 17 to 22 km), to provide applications over regional areas of coverage. They are poised to become a reality within the next few years, given several well-funded activities and advancements in the key enabling technologies of materials, battery and energy capture.

Figure 2. Helikite, trailer and payload

Tackling the challenges
Building wireless base stations on HAPs, compared with terrestrial deployments, presents many new challenges, including:
•  Maintaining long-term operations in the stratosphere without constant power
•  Ensuring aerial terminals coexist with terrestrial networks, without causing interference
•  Utilising the limited wireless backhaul bandwidth between aerial access points and ground core networks
•  Planning the cell coverage of the aerial base stations

The Communication Technologies Research Group at the University of York has been conducting research to tackle suchchallenges since 1999 through theoretical and practical work. In 2016, the university launched its Centre for High Altitude Platform Applications (CHAPA), to capitalise on this new generation of delivery platforms. It is important to have a readily accessible test facility for early prototyping and trials. CHAPA started conducting wireless experiments using a USRP-based solution that is attached to a 21m3   Helikite – a tethered helium balloon that can carry a 10kg payload at up to 400 meters altitude. The team used USRP (Universal Software Radio Peripheral) devices to drive its bespoke antenna elements for its various trials, programmed with LabVIEW Communications Systems Design Suite. 

This floating testbed can be used to efficiently prove novel designs. The designs can then be extended to deliver proof-of-concept payloads that can be deployed and tested on HAPs themselves, which currently have restricted availability and significantly higher running costs.

Prior to the announcement of LabVIEW Communications, the team used the open source software package, GNURadio, with USRP for several months. It started with zero experience with either software. However, the graphical user interface of LabVIEW Communications, comprehensive tutorials on, and NI technical support channels, provided a faster learning curve to implement applications. Using LabVIEW Communications, the team could also program the onboard FPGA without prior experience of VHDL or Verilog, helping it easily implement advanced baseband signal processing on the FPGA. By offloading intensive processing tasks, such as fast Fourier transform (FFT) and modulation/coding, to the USRP’s built-in FPGA, the team increased the determinism, signal integrity and reliability of the system, while freeing up the host processor for data logging and simpler processing tasks,  such as visualising power spectrum and constellation diagrams.

Figure 3. Helikite operating at 400m altitude

Operating USRP from above
The main software defined radio (SDR) kits used were the NI USRP-2943R and the  Ettus Research USRP-N210. Both kits are connected to the ground-based host PCs via Ethernet (the USRP-2943R needs an SFP-Ethernet adapter). To ensure the USRP devices operated correctly while airborne, the team had to account for the weight of the payload, power consumption and the connectivity to the host processor. It measured the voltage/current of each USRP device while running applications in the lab, and equipped each with an appropriate battery while on the Helikite. To reduce weight and ensure the Helikite could fly safely, the team removed the outer cases of some USRP devices.

To ensure fast, secure connectivity between airborne USRP devices and the host PCs running LabVIEW Communications on the ground, the team used fibre Ethernet. It developed a second winch (separate from the tether winch) to store the fibre, tensioning the fibre loosely on the tether, using a clutch and preventing the fragile fibre from stretching. The fibre Ethernet can achieve 1 Gb/s throughput, which can support multiple USRP devices operating at full duplex.

With the testbed in place, a wide variety of experiments and application trials could be operated. Here are three examples:

Figure 4. Aerial-terrestrial propagation measurement

1. Measuring elevation angle dependent attenuation
It’s important to know the quality of radio coverage before deploying a base station. In contrast to the well-established terrestrial propagation models, aerial-terrestrial propagation still requires further exploration. The team carried out a field trial to measure the signal propagation between aerial and terrestrial terminals.

The Helikite carried a USRP-N210 as the receiver while, on the ground, a trolley carried a USRP-2943R acting as a mobile transmitter (a transmitter-based experiment on the ground causes less potential interference to other users on the same band). The team measured several propagation scenarios, at different elevation angles and distances, including line-of-sight (LOS), non-LOS (NLOS) shadowed by buildings, partial LOS through trees, and rich reflection NLOS in residential areas. Further field trials are planned to collect more data to generate appropriate propagation models for each scenario.

2. Improving spectral efficiency by physical layer network coding
Given the nature of HAPs (unlike the Helikite test platform), cabled connections to the terrestrial core network are unfeasible. So, the technologies that improve the efficiency of the limited wireless spectrum are particularly beneficial to aerial-terrestrial communications. The team’s EPSRC-funded NetCoM project has investigated improving backhaul/access link spectral efficiency by using physical layer network coding (PNC). PNC exploits the additive nature of the RF wave to compress the received signals from multiple users into coded data and uses appropriate side information to ensure desired data is recoverable at the destinations. The team tested a simple two-way-relay (TWR) channel using three USRP devices and the Helikite testbed.

Figure 5. TWR channel

The TWR channel simulates the scenario where two users exchange data through a relay, because the direct link between them does not exist. In the field trial, we have two USRP devices on the ground (as users), transmitting different pilot data simultaneously on the same carrier frequency to a USRP (as relay) on the Helikite. We calculate the bit-error-rate (BER) performance directly from the PNC-encoded superimposed signal. The results show that the aerial experiments have achieved similar BER performance as the indoor experiments (LOS existed in both scenarios), which indicate the possibility of applying PNC technology on aerial platforms.

3. Avoiding interference  by beamforming
Delivering services using shared HAP/terrestrial spectrum will require highly dynamic HAP coverage, along with the ability to specifically protect areas from HAP-generated interference. Phased array antennas on the HAP can help achieve tight control of interference, and delivering electronically steerable beams can also track the mobility of the users to provide consistent quality of service (QoS).

Inspired by a research project and NI case study from Imperial College, which discussed direction finding and beamforming, the team extended the phased array antenna testbed to use the array to transmit steerable beams, and tested the system, both in the lab and on the Helikite.

Figure 6. This LabVIEW Communications GUI displays the superimposed constellation at the relay node

The team implemented a similar approach to calibrate the phases of the transmitted signals as Imperial College. As Figure 7 illustrates, it connected the TX/RX port of each USRP daughterboard to one antenna array element, and then connected the RX2 ports to the phase synchronisation tone generated by the same USRP, through splitters. However, unlike Imperial College, the team operated the USRP devices in full duplex mode, by setting the TX/RX ports to transmit, and the RX2 ports to receive. In this setup, the signal transmitted by TX/RX port couples into the RX2 port, so the RX2 ports receive both the synchronisation tone, and the tone sent by the TX/RX port. The two tones are on different frequencies, so they can be separated by a digital filter. The team will first correct the phase ambiguities across the RX2 ports of all daughterboards, and then calibrate the signals of TX/RX ports, based on the observed signals at the phase synchronised RX2 ports. All the USRP devices driving the antenna array elements are connected to the common 10 MHz and 1 PPS reference signals.

The team tested a four-element phased array with the fibre Ethernet backhaul on the Helikite, using two USRP-2943R to drive the four array elements, and one USRP-2943R to source the reference signals. One USRP-N210 generates the phase synchronisation tone, to be distributed to four daughterboards through a splitter. To make sure the payload mass does not exceed the capability of the Helikite, the team removed the case of the USRP-N210, keeping the payload weight just below 10 kg. Each USRP device is powered by one battery, and all USRP devices are connected to one Ethernet adapter, to allow the host PC on the ground to control them through the fibre backhaul, controlled by LabVIEW Communications.

During the field trial, the team clearly observed the phased array tracking the arriving signal direction from a moving source (one USRP-B210) on the ground, and the received signal power changes while steering the TX beam of the phased array to different directions.

Figure 7. Four-element phased array on the Helikite

Future plans
The team plans to expand its Helikite testbed by investing in a larger, 100m3  Helikite that can carry a 30kg payload, so it can complete larger scale experiments. The performances of its current applications are limited by the need to process data on the ground-based PC, rather than the FPGA. The recently announced standalone USRP-2974 is equipped with an onboard real-time processor, which would allow the team to take full advantage of the built-in FPGA for baseband signal processing, even when the device isn’t tethered to a PC. This would significantly improve the throughput of applications and allow the team to operate its testbed at higher altitudes. The team plans to test this out soon.

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