An investigation into the Received Signal Strength Indica- tor (RSSI) dependency on receiver antenna height in UHF band is con- ducted. The results show a high correlation between RSSI and height on channels with high signal strength. There is approximately 2.5 dBm RSSI gain per 1 m increase in height above ground up to 8.5 m. From 8.5 m to 12 m, there is no consistent observable increase in RSSI. Further- more, the geolocation spectrum databases (GLSD) view of white space in the television band is compared with the ground truth. Results show signal presence on some of the channels indicated free by the spectrum database. These findings imply that an increase in transmission range of UHF links can be achieved by increasing receiver height. White space devices using A GLSD should additionally require spectrum scanning to determine clear channels

Hadzic, Dr Senka

Johnson, Dr David

Lysko, Dr Albert

Maliwatu, Mr Richard

UCT Computer Science Research Document Archive

A correlation between RSSI and height in UHFband and comparison of geolocation spectrumdatabase view of TVWS with ground truthRichard Maliwatu1, Albert Lysko2, David Johnson2 and Senka Hadzic11 University of Cape Town, Computer Science Department, South Africa,rmaliwatu@cs.uct.ac.za,shadzic@cs.uct.ac.za2 CSIR, Meraka Institute, South Africa,alysko@csir.co.za,djohnson@csir.co.zaAbstract. An investigation into the Received Signal Strength Indica-tor (RSSI) dependency on receiver antenna height in UHF band is con-ducted. The results show a high correlation between RSSI and heighton channels with high signal strength. There is approximately 2.5 dBmRSSI gain per 1 m increase in height above ground up to 8.5 m. From 8.5m to 12 m, there is no consistent observable increase in RSSI. Further-more, the geolocation spectrum databases (GLSD) view of white spacein the television band is compared with the ground truth. Results showsignal presence on some of the channels indicated free by the spectrumdatabase. These findings imply that an increase in transmission rangeof UHF links can be achieved by increasing receiver height. White spacedevices using A GLSD should additionally require spectrum scanning todetermine clear channels.Key words: UHF television white space, RSSI vs antenna Height, ge-olocation spectrum database, Received signal1 IntroductionThe 470-890 MHz frequency range has historically been used for television broad-cast. Traditionally protection of primary transmitters has been achieved throughstrict frequency assignment. However due to the increase in wireless applications,the spectrum assignment paradigm is shifting towards the adoption of technolo-gies such as smart antennas, cognitive radios, adaptive coding, etc. that allowefficient utilisation and reuse of spectrum [1]. While these ideas have been aroundfor quite some time, the technical details of dynamically allocating and sharingspectrum without interference among users are yet to be fully addressed.This study is motivated by the ever increasing relevance of wireless com-munication and the particular interest in television white space (TVWS) basedcommunication. The implementation of TVWS based communication might varyfrom application to application but, the general requirement is for the device toknow the free channels in the area and determine safe transmission power lev-els to use. There are two general approaches to meeting this requirement: the2 Richard Maliwatu, Albert Lysko, David Johnson and Senka Hadzicdevice can either (i) scan its spectral environment to determine free channelsor (ii) query a geolocation spectrum database (GLSD) that responds with a listof free channels based on the device’s location and parameters such as antennaheight, power, etc.Each of these approaches for protection of primary transmitters and coexis-tence with secondary users raises several questions. Firstly, at what height shouldthe scanning be done? Secondly, when the GLSD provides a list of free channels,are the channels really free?To answer these questions, which are essential for purposes of TVWS devicedeployment, this paper firstly looks at Received Signal Strength Indicator (RSSI)dependency on receiver height over the ultra-high frequencies (UHF). Using re-sults of RSSI dependency on height, the prospect of influencing the transmissionrange by varying the receiver antenna height is considered. Next, the GLSD’sview of TVWS is compared with measurements to assess the availability of chan-nels provided by the GLSD.The paper is organised as follows: Section 2 gives the related work; Section3 describes the method used in data collection; Section 4 discusses the results;and Section 5 concludes the paper and outlines follow on work.2 Related workStudies have been done to characterise UHF wave for point-to-multi-point prop-agation in outdoor and indoor environments e.g. [2, 6]. Substantial work hasbeen done in the area of propagation models used to predict path loss for pur-poses such as estimating transmission range, inter-system interference analysisand so forth. Propagation models use known parameters such as operating fre-quency, distance, transmit power, antenna and terrain characteristics, etc. topredict path loss.Results from studies [3, 4], conducted to evaluate performance of propagationmodels show that the accuracy of path loss variability prediction is dependenton environmental characteristics in and around the area in which the model isapplied. A propagation model is considered accurate if it obtains a root meansquare error (RMSE) that is below a set margin for urban and rural areas.Empirical propagation models derived from measurements in one area oftenhave to be modified to increase the accuracy when applied in a different area.Other studies have been done to establish dependency of propagation dis-tance on operating frequency [5], effect of distance and transmitter antennaheight on path loss [6]. Previous work [7] covering dependency of received poweron receiver height reported no significant dependency for height 1-3m however,this was study was carried out in 225-450 MHz frequency range. From the lit-erature surveyed, the RSSI dependency on receiver antenna height for the UHFtelevision band is a less well explored issue.RSSI vs Height Dependency 33 MethodologyThe study was done experimentally. The objective of the experiment was toestablish the relationship between RSSI and receiver antenna height in the UHFband. In absence of additional equipment and to avoid the need for a spectrumlicense, the local television broadcasting was used as a source of the referencesignals. The receiving hardware setup included a R&S FSH4 spectrum analyser,a 2.1 dBi gain R&S omnidirectional antenna and short cables, all mounted ona boom lifter. The spectrum analyser was controlled over a WiFi network fromwithin a vehicle. The setup is shown in Fig. 1.Fig. 1. Setup used to perform measurements. Visible are the boom lifter and antennamounted on it (covered with a low permittivity radome). The spectrum analyser ismounted under the antenna (not visible).The measurement procedure was to lift the arms of the boom lifter (withthe measurement setup), measure the actual height using a laser range finder,and perform a frequency scan (the scan was set and triggered remotely, over thewireless network.). The results of a scan would be saved into a data file. Thiswas repeated several times for each value of height.Each frequency scan was done from 451.25 MHz until 1,081.25 MHz withresolution bandwidth of 1 MHz and video bandwidth of 3 MHz. The choice offrequency and resolution bandwidth was dictated by two factors: (i) the needto capture the video carriers of analogue TV transmissions (which are locatedaround 1.25 MHz from the left edge of a TV band), and by the limitations on thenumber of points in a single sweep which can be used by the spectrum analyserused, i.e. fixed to 631 points.The measured values, in dBm, recorded in the data files reflect the powerentering the input of the spectrum analyser. These can be translated into thepower incident onto the antenna by taking into account the losses in the cablesand connectors (Lc, about 1dB) and applying the antenna gain ( GA=2.1dBi)4 Richard Maliwatu, Albert Lysko, David Johnson and Senka Hadzicfor the selected antenna. Thus, the power incident onto the antenna, Pinc, maybe estimated via the power entering the spectrum analyser, Psa, asPinc = Psa + Lc −GA = Psa − 1.1, (dB). (1)3.1 First set of spectrum scan measurementsThe first round of measurements was conducted at the location with GPS coordi-nates: 18.423980, -34.1398. This location had one floor houses and a large numberof very tall trees around the area. From the presence of tall trees, one may expectthat the dependence of the signal strength on the height may be weakened ascompared to more empty space. The measurements were performed at the fol-lowing antenna heights: 2m, 3.25m, 4.57m, 6.04m, 7.87m, 9.27m, 11.57m, overthe above-mentioned frequency range. Only one frequency scan was collected atthis location at each of the antenna heights.3.2 Second set of spectrum scan measurementsThis location was a residential area with one floor houses and only short trees.From this, one may expect the dependence on height to be composed of twodifferent types of dependencies: one for the heights below the average heightof houses, and another for greater heights. Measurements were conducted atantenna heights 2m, 2.1m, 2.66m, 3.2m, 3.75m, 4.3m, 4.9m, 5.46m, 6.3m, 7m,7.7m, 8.3m, 8.8m, 9.3m, 10.7m, 11.9m. Multiple samples were collected at eachheight.3.3 Data preparationThe frequency scans were conducted from 451.25 MHz to 1081.25 MHz and theresults presented focus on television channels 21 to 69 (470-862 MHz).In order to reduce the influence of random noise, the frequency points in eachfrequency scan have been grouped into channels 8MHz wide, corresponding toTV channels, i.e. a channel N starting at 470 + 8(N − 21) [MHz] and ending at478 + 8(N21) [MHz]. Each channel is represented by an average of all the valueswhich belong to this channel as well as by associated statistics. In order to reducethe effects of the noise further, the values from each channel were averaged overa number of different frequency scans at the same height. Most of the resultspresented in this paper are based on averages from 31 frequency scans at eachlevel.4 Discussion of results4.1 RSSI dependency on heightIn order to provide the overview of the spectrum scan, we consider the averageof samples at the main height. The results are shown in Fig. 2 (a). Fig. 2 (b)RSSI vs Height Dependency 5shows the Standard deviation of the samples collected, which was computedusing equation 2:Standard deviation =√√√√ 1NN∑i=1(xi − µ)2 (2)where N is the sample size, xi are the values and µ is the mean.21 25 29 33 37 41 45 49 53 57 61 65 69012ChannelStandarddeviation(b)21 25 29 33 37 41 45 49 53 57 61 65 6900.51ChannelRSSI vs heightcorrelation(c)21 25 29 33 37 41 45 49 53 57 61 65 69−82−72−62−52−42−32ChannelRSSI (dBm)(a)  GLSD view of TVWSActive TV channelsFig. 2. Overview of the spectrum scan.There are fluctuations in the observed standard deviation especially fromchannel 46 to channel 63. This could be due to changes in the spectral environ-6 Richard Maliwatu, Albert Lysko, David Johnson and Senka Hadzicment or occurrence of temporal obstructions caused by objects such as vehiclesparked in the far distance thereby affecting signal propagation during the time ofmeasurement. Spearman’s Rank correlation was computed using equation 3 todetermine statistical dependency between height and RSSI over channels 21-69.Rank correlation coefficient = 1−6n∑i=1d2in3 − n∑(3)where di is the difference between the ranks for RSSIi, heighti pairs and nis the number of pairs observed [8].The degree of correlation in RSSI dependency on height across channels21-69 is shown in Fig. 2 (c). Fig. 3 shows the degree of correlation in RSSIdependency on height plotted against RSSI. The results show that correlation ismost significant on channels with strongest signal. Therefore, to determine theRSSI dependency on height the peak channels are considered.In Fig. 2 (c) correlation of 0.96907, 0.94334, 0.92862, 0.8933, 0.88153, 0.82561is observed on channels 64, 36, 37, 28, 27 and 46 respectively. The plot of RSSSIvs height for these channels with the highest correlation is shown in Fig. 4.There is approximately 2.5 dBm RSSI gain per 1 meter increase in receiverheight from 2 m to 8.5 m across the observed channels. The dependency of RSSIon height implies that the transmission range of UHF-based wireless links can beextended by increasing the receiver antenna height. After increasing the receiverantenna height beyond 8.5 m the observed gain in RSSI plateaus and becomesless consistent.−81 −76 −71 −66 −61 −56 −51 −46 −41 −3600.51RSSI (dBm)CorrelationFig. 3. Correlation vs RSSI.RSSI vs Height Dependency 72 3 4 5 6 7 8 9 10 11 12−80−75−70−65−60−55−50−45−40−35−30Height (m)RSSI (dBm)  channel 27 channel 28 channel 36 channel 37 channel 46 channel 64Fig. 4. RSSI dependency on receiver height at selected channels.4.2 Comparing GLSD with ground truthThe results of the geolocation spectrum database (GLSD) query are indicatedby (*) marks in Fig. 2 (a). The CSIR GLSD [9] service was used to get a listof available channels or TVWS for a white space device (WSD). The GLSD’scriteria for determining TVWS is based on the ITWOM+ITU-R P.1546-4(grade-B) propagation model [10] for WSD 3-10 meters high and 4 W power. Details ofregistered television channels indicated by circular marks in Fig. 2 (a) is based oninformation provided by SENTECH [11], the national licensed broadcast signaldistributor in South Africa.According to the GLSD, the following channels are free: 21, 22, 23, 24, 25, 26,30, 31, 32, 33, 34, 35, 36, 41, 42, 43, 45, 47, 53, 60, 61, 64, 65, 66, 68. Consideringthe spectrum scan shown in Fig. 2 (a), there is a -50.27 dBm, -49.85 dBm, -59.83dBm, and -72.95 dBm signal present at channels 36, 47, 53 and 60 respectively,which are among the list of ”free” channels provided by the GLSD.Model-driven GLSDs use information about registered primary transmittersand compute white space at a given location based on propagation models suchas ITU-R P.1546 [12]. The GLSD is required to have up-to-date informationabout primary transmitters but, the spectrum scan shows that some of the free8 Richard Maliwatu, Albert Lysko, David Johnson and Senka Hadzicchannels as determined by the GLSD are in fact not free. They are tainted byunregistered sources to which a model-driven GLSD is agnostic. Furthermore,from the spectrum scan, we find (at the time) other free channels not providedby the GLSD. These findings expose the pitfalls of deploying ”sense-less” whitespace devises (WSD) i.e. white space Devices with no inbuilt spectrum scanningcapability. We recommend that WSDs using the GLSD for white space detectionand incumbent protection supplement the query results with localised real-timespectrum scans for increased spectrum reuse and determining white space witha ”finer grain”.5 Conclusion and future workThe correlation of RSSI dependency on antenna height is most significant overchannels with strongest signal. RSSI increases as the height increases up to 8.5m, similar to model prediction.The differences observed in the GLSD’s view of TVWS and the ground truthhighlights the complex dynamics of the wireless communication medium and thelimitations of propagation models. Considering the noise levels in some of thechannels provided by the GLSD, the suitability of these channels will dependon the application specifics. However, it is evident that the spectrum databasealone is inadequate in determining perfectly clear white space to use. Althoughthis observation was made with TVWS GLSD, similar results can be expectedwith model-driven GLSD for other spectrum bands.One limitation of this study is that measurements at different antenna heightswere done one height level at a time. The observed variation in RSSI could bedue to changes in the height or changes in the spectral environment over themeasurement period. With this observation, future work will include a setupwhere measurements at different heights may be taken simultaneously.References1. Struzak, R., Wiecek, D.: TV White Spaces: a pragmatic approach. ICTP -TheAbdus Salam International Center for Theoretical Physics, (2013)2. Porrat, D., Cox, D. C.: UHF propagation in indoor hallways. In IEEE Transactionson Wireless Communications, vol. 3, no. 4, pp. 1188-119 (2004)3. Sridhar, B., Ali Khan, M. Z.: RMSE comparison of path loss models for UHF/VHFbands in India. Region 10 IEEE Symposium, pp. 330-335. IEEE, Kuala Lumpur(2014)4. Ogbeide, O. K., Edeko, F. O.: Modification of the Hata empirical propagation modelfor application in VHF band in Edo State, Nigeria. International Journal of Engi-neering Science Invention, vol. 2, no. 8, pp. 35-39 (2013)5. Xiaohu, Z., Burres, T. W., Albers, K. B., Kuhn, W. B.: Propagation comparisonsat VHF and UHF frequencies. IEEE Radio and Wireless Symposium, pp. 244-247.IEEE, San Diego, CA (2009)RSSI vs Height Dependency 96. Perez-Vega, C., Zamanillo, J. M.: Path-loss model for broadcasting applications andoutdoor communication systems in the VHF and UHF bands. In IEEE Transactionson Broadcasting, vol. 48, no. 2, pp. 91-96 (2002)7. Hampton, J. R., Merheb, N.M., Lain, W. L., Paunil, D. E., Shuford, R. M., Kasch,W. T.: Urban propagation measurements for ground based communication in themilitary UHF band. In IEEE Transactions on Antennas and Propagation, vol. 54,no. 2, pp. 644-654 (2006)8. Jerrold H. Zar: Biostatistical Analysis: Pearson New International Edition, 5th edi-tion, Pearson Education Limited (2013)9. http://whitespaces.meraka.csir.co.za10. Luzango, M., Fisseha, M., Mjumo, M.: Geo-location White Space SpectrumDatabases: Models and Design of South Africas First Dynamic Spectrum AccessCoexistence Manager, KSII Transaction on Internet and Information Systems vol.8, no. 11, pp. 3810-3836 (2014)11. http://www.sentech.co.za/television12. ITU-R P.1546-5: Method for point-to-area predictions for terrestrial services in thefrequency range 30 MHz to 3 000 MHz. ITU (2013).

A Correlation between RSSI and Height in UHF Band and Comparison of Geolocation Spectrum Database View of TVWS with Ground Truth.

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A Correlation between RSSI and Height in UHF Band and Comparison of Geolocation Spectrum Database View of TVWS with Ground Truth.

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