Thomas, Howard John; (1990) Propagation measurements at 55GHz in an urban environment. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The work described in this Thesis is concerned with the study of the propagation characteristics of 55 GHz millimetre waves between a fixed transmitter site and a mobile receiver terminal in an urban environment. Transmitter-receiver ranges up to 400 m are used and measurements are obtained in the presence of high traffic densities. The results of both analogue and digital (15 Mb/s data rate) measurements conducted on the link are presented. The work has been motivated by the suggestion that a millimetre wave micro-cellular mobile radio system could form the basis of future urban mobile radio systems. The propagation of radio waves in random time variant media is discussed, and a product type model is developed which describes the non-stationary statistics of the received signal envelope over three different distance scale sizes. The variations on the different scale sizes describe: the general trend of the received signal to fall with increased receiver transmitter separation; the variation of the local average signal; and the "fast fading" of signal envelope. Results indicate that transmitted power is contained within the micro-cell by the buildings which surround it. A power law (r-3.6) describes the trend of the signal to fall with distance along the cell. Outside the micro-cell, the power law describing the reduction in signal power with distance is far steeper (approximately r-10). Comparison of experimental results with theoretical models of channel behaviour suggests that the received signal is dominated by line of sight propagation and specular reflections from principal scatterers. It is suggested that the prime cause of the variation of the statistical estimators of channel behaviour throughout the micro-cell is determined by the strength of the line of sight and specular component at each location. The variation of the first order statistics is comparable with results reported from measurements at 900 MHz in suburban environments. This suggests that the propagation mechanism is similar in both cases. The results show that the distribution of the "fast fading" envelope of the signal is seldom described by a Rayleigh distribution. The experimental results show an extreme range of coherence bandwidths from 17 MHz to 150 MHz throughout the micro-cell. The result is comparable with urban propagation measurements at 57 GHz. The potential for applying space, frequency, and polarization diversity reception to the channel is investigated. i) Auto-correlation measurements are used to estimate the antenna separation required to generate two un-correlated inputs for a space diversity system. The estimate varies from 2.5X (1.4 cm) up to 22X (12 cm). ii) Two frequency measurements are used to estimate the frequency separation required between two input channels to provide de-correlated inputs for a diversity receiver. The estimated frequency separation to produce a correlation of 0.5 varies from 17 MHz to 150 MHz. These values are considerably larger than values of 25 KHz and 840 KHz reported from measurements in urban and suburban environments at 900 MHz. This reflects the greater distances between major reflectors in the different environments. iii) The effect of the micro-cellular environment on transmission of orthogonal polarizations is measured. The two co-polar coefficients vv and HH are found to be de-correlated as are the two cross polar coefficients VH and HV. In addition, the value of HV is shown to be greater than VH. These results agree with those reported at 900 MHz. A simulation of two branch diversity reception, using experimental data, is undertaken. The results show a median signal level improvement of 1 dB and at the 99% reliability level, 9 dB. This compares with theoretical values assuming Rayleigh fading of 1.7 dB and 10.5 dB, This suggests that first order predictions of the performance of a diversity receiver operating in a micro-cell can be made by assuming Rayleigh fading input channels to a diversity receiver. Experimental results obtained from a digital experiment operating at 15 Mb/s across the mobile link are compared with theoretical predictions. The results indicate that errors are primarily due to fading and that inter-symbol interference is not important. This is in agreement with the coherence bandwidth measurements. A prediction is made of the performance which would be expected from an ideal digital receiver using bi-phase-shift-keying and employing two branch maximal ratio combination. The prediction suggests that an error rate better than 10 -3could be obtained at 15 Mb/s data rate operating over a 400 m range. The experimental apparatus used for both sets of experiments is described. The computer programs used for data analysis written by the author are included in the Appendix.

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