Can drones be used to extend wireless coverage to remote areas?

With Goggle and Facebook both vying to use drone technology to increase Wi-Fi access to remote parts of the world, Dr Dave Sloggett examines the issues involved and the challenges to be overcome

Can drones be used to extend wireless coverage to remote areas?

Pick up any national newspaper and on a daily basis there will be a headline-grabbing article about so-called drone technology. If they are not being employed to target and kill people thought to be involved in terrorism in faraway countries, they are being a nuisance flying close to civilian airliners or over-sensitive installations such as nuclear power plants. From its nascent beginnings it seems that drone technology is now becoming ubiquitous. It is invading every part of our daily lives.

The telecommunications industry is not isolated from the developments in these unmanned aircraft technologies. Both Google and Facebook have been carrying out work to explore how such technologies might be applied to the problem of providing Wi-Fi coverage in remote parts of the world.

The obvious aim of such a service would be to open up new markets and services where terrestrial-based services may not be delivered cost-effectively to nascent local markets with limited spending power, and where ground-based or satellite-based communications infrastructure installation costs would be prohibitive.

Obvious potential markets for such ideas exist in some remote areas of South America, Africa and South East Asia; areas of the world that experience tropical climates. At the right price the concept could also find a favourable market position in some parts of the United States and Canada.

Engineering challenges
However, delivering services based on drone technology is not straightforward. A number of engineering challenges have to be overcome, one of which is the issue of the amount of energy that can be generated on the drone to run all of its avionics and systems and host a communications package that can provide the Wi-Fi services that users may wish to exploit.

Of the two organisations, Google’s interest in exploiting the 28GHz part of the radio spectrum has received the most coverage in the mainstream media. Its acquisition of Titan Aerospace from under the noses of Facebook allowed Google to add drone technology as an option for service delivery alongside its initial experimentation using high-altitude balloons in the stratosphere under what Google has called Project Loon.

Rather than being deterred by Google’s last-minute acquisition of the technologies from Titan, Facebook quickly turned its focus to purchasing the UK-based company Ascenta, which is building its own high-altitude drone technologies for a reported $20 million.

The obvious advantage of operating in the 28GHz part of the radio spectrum is that the bandwidths that can be available are huge; over 40 times the capacity of contemporary terrestrial-based 4G LTE networks. These have become rapidly overcrowded and this issue is forcing telecommunications companies to think very differently about how to develop and deliver new 5G services.

To enable the drones to stay aloft for years and operate economically, they have to be solar-powered and fly at altitude above the main weather systems at around 50,000 to 60,000 feet. This is close to 18km in altitude, which creates a line of sight of 480km to the horizon if the view is not obstructed by terrain features.

Theoretically, this provides access to an area of just over 720,000 square kilometres. This is an area around one two hundredth of the total surface area of the world, or just bigger than the total land area of Afghanistan or three times the total land mass of the UK.

Antenna design
Providing a service over this theoretical area creates a huge challenge for the design of the antenna systems and radio systems. The range variation for the service is nearly 30 from its shortest range to its longest; a factor of 28db that has to be added to the power budget considerations.

In situations where energy budgets on the drone may be limited, this is a technically challenging situation. Where power budgets are limited, additional platforms would have to be deployed. This all has an impact on the business model, which will impact its viability.

But the freespace path loss over the ranges involved is just one consideration that engineers have to address. The other is the impact of rainfall, as all users of satellite television are all too aware. This varies greatly across the globe and the International Telecommunication Union (ITU) has developed models of the impact rainfall has as a function of range and the rate at which the rain is falling.

While pathway losses in the atmosphere are reduced by exploiting the natural window that occurs at 28GHz, rainfall can create problems for those trying to calculate the link budgets and ensure an effective operating margin to ensure service reliability and availability.

To operate such services in tropical regions, where rainfall rates can vary significantly from more temperate climates, significant additional losses as the signals transit the atmosphere will occur; albeit briefly for some occasions.

Link budget
To achieve accessibility of 99.9%, irrespective of where in the world the service is operated, the link budget has to account for a further difference of up to 36 in the rates of rainfall across the globe. This adds an important variance in the pathway losses that occur when heavy rainfall is falling.

Using the ITU analysis of rainfall fade, and assuming a rain height of 5km and an angle of 45 degrees for the radio waves to transit the area raining, the link budget can be impacted by an additional 65db of signal loss. This, plus the 28db of freespace loss, are not inconsiderable factors that will drive the number of drones that have to be deployed in an operational system.

These losses in the transmission of data over such links can be made up in part by increasing the gain of the antenna on the ground and in the air using adaptive phased array technologies or by increasing the transmitter power on the drone to compensate for the air-to-ground pathway losses. But achieving the link-budget margins required to maintain a highly accessible service is difficult under serious atmospheric conditions.

Either way, the somewhat simplistic idea implied in some of the mainstream media press coverage, that a mobile phone network can communicate directly via a drone is unlikely. Getting sufficient power from a mobile phone to generate a signal to overcome all of those losses is very unlikely and would probably have major safety considerations for users.

New research
To address these issues from a practical viewpoint, Google has established a ground-based research facility in New Mexico under a project known as Skybender. Here, Google is trialling what it refers to as phased-array technologies, which are able to create ‘focused’ transmissions.

One illustration of the budget factors being addressed is the 8-metre size of one of the ground antennas currently being used experimentally on the test range. Clearly, Google is considering using large antennas on the ground and phased-array systems on the drone.

While Google’s research work and presumably that of Facebook offers some promise of yet another demonstration of the ubiquity of drone technologies, the practical fielding of such services is still some way off.

What is likely to emerge is an architecture where the drone communications systems provide a backbone through which large data rates can be transmitted. Once on the ground the data would be routed through low-cost repeater cells to and from their end users.

The technical challenges to be overcome to develop such a service are not insignificant. But a start has been made and it may not be that long before the shadow cast by a drone on the ground is not seen to indicate menace but to provide services that can help remote communities play their part in the globalised world in which we all live.

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