Sensing Trouble

The advent of 5G promises to revolutionise cellular communications. Networks will host more subscribers, handle larger bandwidths and boast lower latency than possible with today’s 4G services.

A new research initiative will examine both countermeasures against 5G-based passive sensing and general applications of 5G passive sensing technology.

Dr. Christos Ilioudis is a research fellow at the University of Strathclyde, southwest Scotland, specialising in electronic and electrical engineering. He is one of nine researchers awarded grants in late 2021 by the UK Government’s Office for Science. Although awarded by the government, these UK Intelligence Community (UKIC) postdoctoral research fellowships are administered by the Royal Academy of Engineering. Dr. Ilioudis’ research is examining 5G-based passive sensing and associated countermeasures.

Fifth-generation or ‘5G’ wireless communications protocols are being implemented around the world. These new protocols will supersede existing 4G standards. The United Nations’ International Telecommunications Union (ITU) is the global custodian of the radio spectrum and prescribes the specifications for 5G protocols. 5G uses low-band (400 megahertz/MHz to 3.4 gigahertz/GHz), mid-band (2.4GHz to 4.2GHz) and Millimetric Wave (MMW/24GHz to 72GHz) frequencies. The technology promises a major increase in data carriage for cellular communications. According to the website this will see speeds of 100 megabits-per-second/mbps up to two gigabits-per-second. This compares favourably with 4G speeds of between 50 to 70mbps. Latencies will also reduce. With 4G latencies of 50 to 100 milliseconds are common. 5G could reduce this to five milliseconds. Many more subscribers will be hosted on individual 5G network nodes than currently possible with 4G. Nodes are the individual constituent parts of a network. A 4G network can support circa 100,000 individual devices per square kilometre (0.39 square miles) with one cellphone tower. This increases to one million devices with 5G.

Alongside communications 5G could be used for passive sensing. A 2019 paper published by the Institute of Electrical and Electronics Engineers, a New York-based professional association, outlined how 5G could support passive sensing. The paper articulated that 5G signals could be used as signals of opportunity by passive radars. Whereas a conventional radar must generate, transmit, receive and process its own Radio Frequency (RF) emissions, a passive radar uses RF already in its locale. These sources can include cellphone coverage, radio or television broadcasting for example. A passive radar continuously monitors these transmissions checking for any disturbances which might be caused by an object moving through them like an aircraft.

How it works

5G sensing is elegantly simple. Consider a city or town. 5G nodes housing the cellular network’s transmitters and receivers will be positioned throughout the conurbation. They will be arranged to provide the city with a thick blanket of RF energy. Objects between the cellphone towers like buildings, people or vehicles, will create disturbances in this blanket of RF energy. Hence, they will cause changes to the characteristics of the RF energy produced by the network. As buildings and other static objects do not move, the disturbances they create will not change. Humans, animals and vehicles are another matter. When they are moving they create Doppler Shift disturbances to the frequencies of the 5G network’s signals. Doppler Shift is the subtle change in the frequency of a transmitted radio wave when it hits a moving object and is reflected to a receiver. This is a fundamental principle of radar. If the object is moving away from the transmitter, the frequency of the echo will be progressively lower. If the target is moving towards the transmitter, the echo’s frequency will increase.

A 5G network could provide a way of monitoring how many people and vehicles are moving around a city. It could help provide information on traffic congestion or the size of a crowd. Moreover, a 5G transmitter and receiver placed in a room could determine how many people are there. It could tell whether these people are static, whether they are moving and how much they are moving. Another paper examining 5G sensing suggests the technology could be used for healthcare. 5G sensors in a care home could immediately determine if a resident has fallen in their room. It could even help monitor a patient’s vital signs measuring breathing patterns by detecting the movement of their chest. In the home, the technology could be used to detect burglars. The 5G MMW frequencies could even be used for through-wall sensing, the paper adds. Such applications would be useful for hostage situations or determining if people are trapped in rubble.

This diagram illustrates some of the applications 5G passive sensing will support. A range of uses from public safety to healthcare are envisaged for the technology.


Dr. Ilioudis says the UKIC’s postdoctoral research fellowships will focus on identifying the principles of 5G signal processing, along with developing advanced signal processing algorithms. The work will also look at 5G sensing countermeasures. Like many technologies, 5G sensing could be used for nefarious as well as beneficial purposes. Intelligence agencies could use it to like covertly monitor movements in a building. Likewise, it might be used to identify congested areas to maximise casualties during acts of political violence. This makes it imperative that countermeasures are developed and will form a major part of the research.

Nonetheless, “during the progress of the project it is expected that useful applications for 5G sensing will be identified. Indeed, other research such as scholarships could potentially spin out from this fellowship.” Dr. Ilioudis says such areas could include 5G sensing for automotive applications like autonomous vehicles.

5G-based passive sensing is an interesting emerging technology promising a range of benefits from healthcare to public safety. Yet it also has the potential to be misused. The research of Dr. Ilioudis and his colleagues will help ensure the technology can be used beneficially and be countered when used nefariously.

by Dr. Thomas Withington