Telecoms networks could provide next-gen GPS services without the need for satellites
Decimeter-level uncertainty, sub-nanosecond time synchronization – but what happens when there's no signal?
A recently published research paper proposes a system for terrestrial positioning that could give greater accuracy than the existing satellite-based systems, and could potentially be incorporated into future mobile networks.
Global positioning systems like GPS, Glonass and Galileo are widely used for applications such as navigation, but they do not always work well in dense urban environments due to buildings interfering with the direct line-of-sight to a satellite, and signals being reflected off buildings.
The paper, "A hybrid optical–wireless network for decimetre-level terrestrial positioning", published in science journal Nature, describes an alternative terrestrial positioning system that is independent of satellites and claimed to offer superior performance with centimeter-to-decimeter-level uncertainty plus sub-nanosecond time synchronization support.
Like satellite-based systems, the one proposed by the researchers relies on the accurate measurement of arrival times of radio signals, but it uses a time signal distributed to all the radio transmitters via an optical fiber network to ensure they are synchronized to a common reference clock.
According to the paper, the system was tested out at the campus of Delft University of Technology in the Netherlands, using six radio transmitters dispersed over an area of 660m2. The sub-nanosecond synchronization of the transmitters was achieved simply by the use of the White Rabbit (WR) protocol, an extension of the Ethernet standard that was originally developed at CERN to synchronize the measurement and control equipment of its particle accelerators.
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For the testbed, the researchers used a reference clock located at the Dutch metrology institute, VSL, which provides the Dutch realization of Coordinated Universal Time.
As with GPS receivers, time-of-arrival measurements in the proposed system are converted to ranges, and subsequently used to locate the receiver relative to the constellation of radio transmitters through multilateration, the paper states.
According to the authors, the sparse-band radio system used for their demonstration could be designed such that its spectral footprint matches that of the spectrum licensed to mobile network operators, while the OFDM (orthogonal frequency-division multiplexing) modulation format is similar to that used in 4G and 5G radio access networks. The paper suggests that the positioning, navigation and timing signals could therefore be transmitted by existing mobile base stations.
The authors also state that the WR protocol has been standardized, equipment using it has been commercially available for over a decade, and because it requires only a relatively modest bitrate, it can operate at wavelengths in fiber-optic networks that are typically not used for the high-bandwidth data transmissions.
The obvious implication is that this system could be added to mobile networks with little difficulty. However, it would most likely require updated radio access network kit at the base stations and support to be built into new handsets.
As with many other things, it will come down to whether the benefit of increased positioning accuracy is deemed to be worth the cost, and the operators already have plenty on their plates trying to bring the full spectrum of 5G capabilities to their networks. ®