Investigation into joint use of UHF and VHF bands for future Internet of Things: field test platform and measurement campaigns

The UK telecommunications regulator, Ofcom, decided in 2016 to repurpose parts of the VHF spectrum between 55MHz and 81.5MHz for use in Internet of Things applications. It is believed by Ofcom that the VHF band can provide more reliable long range communications when compared to currently commercially operated UHF spectrum, due to the more favourable propagation characteristics of lower frequencies. The Internet of Things is an important, large and growing sector of communications technology with many important applications, so investigation of this re-purposed spectrum could prove to be beneficial. Studies could facilitate the design of highly reliable systems by supplying detailed knowledge of the characteristics of the new spectrum across multiple different environments. In order to conduct a propagation and spectrum measurement study a very expensive commercially produced spectrum analyser would normally be required. This thesis takes a different approach by developing an instrument using Software Defined Radio techniques with Commercial Off the Shelf hardware, this produces a very low cost instrument. It is shown that the Software Defined Radio instrument is capable of performing a propagation study to a high degree of agreement with a commercial spectrum analyser, thus validating the approach. Readings of received power taken by the instrument are shown to agree with readings taken at the same locations with a commercial spectrum analyser to within an average of 1.4dB at 71MHz and 1.1dB at 869.525MHz. These readings of received power along with GPS locations in relation to a known transmit power allow propagation and shadowing to be calculated, background noise present in the measurements is also recorded. The developed instrument is used to conduct a propagation study in a number of different environments (rural, suburban, urban and dense urban), with measurement equipment deployed in a manner suitable for a portable, short range (up to 2km) IoT use case with receiving antennas placed close to the ground. Results are presented in comparison to other propagation studies available in the literature and widely used propagation models such as the Hata model. Shadowing and noise are also measured and examined. It is found that current propagation models do not provide adequate predictions of path loss within the considered use case, with Root Mean Squared Errors of up to 72.4dB between measured and predicted path loss. The collected data is used to calculate log-distance based path loss propagation models that provide good predictions, with Root Mean Squared Errors of between 5.5dB and 9.0dB when comparing measured path loss to the calculated models path loss predictions. Path loss is found to be constantly lower, by between 20dB and 33dB, at VHF than UHF, but RF noise is consistently higher by between 8dB and 18dB at VHF. The instrument is then further developed, so it may be used remotely to make long-term observations of the VHF and UHF spectrum at a static location in order to observe how the spectrum behaves over a number of days. Initial testing was performed in a sub urban environment for 6 days, with the intention of longer future deployments to rural, sub urban, urban and dense urban environments once the testing confirms the changes to the instrument function properly. In the studied area RF noise was again found to be consistently higher at VHF, by an average of 16dB, over the whole 6 day timespan of the measurements. Waterfall plots were produced that show qualitatively that more RF interference was measured in the considered environment at the VHF band. The number of users in each band was assessed by manual examination of the waterfall plots, with the UHF band found to be much more heavily used than the VHF band (up to 12 users in the UHF band and 3 to 5 in the VHF band), with some areas seemingly congested. Work is continuing to produce better spectrum sensing algorithms to allow quantitative measurements of interference and users. Overall, the newly released spectrum is found to compare favourably with the currently well used Short Range Devices band in all the examined environments and therefore be suitable for IoT deployments. Different strengths and weaknesses were identified in each band with VHF having lower path loss and less congested spectrum, UHF having lower noise and less interference. IoT communications could be provided by either band or a combination of both to allow the strengths of each to be exploited, increasing reliability, such as by the use of VHF spectrum to avoid congestion in the UHF band.