Propagation characteristics of a narrowband microcellular radio channel

Green, Edward (1990) Propagation characteristics of a narrowband microcellular radio channel. University of Southampton, Doctoral Thesis.

Abstract

This thesis reports the results of radio propagation and 16 kbit/s bit error ratio measurements taken in a microcellular system. Microcells are different from the cells found in current cellular mobile radio systems in that they use both low radiated powers and low antenna heights. Microcells are intended to provide strategy coverage to an area which has a large teletraffic demand. It is believed that only via the use of such techniques will the vision of a low cost personal mobile communications system be possible. The propagation characteristics found in conventional type cells have already been well researched and documented. In comparison little work has been completed on microcells. The material presented in this thesis therefore represents one of the first contributions to what will be an area of growing importance. Measurements are reported which show how high quality radio coverage can be achieved along roads to distances of greater than 1 km in city centres and 2 km on highways, for transmitter powers as low as 16 mW. Within the coverage area of the microcell it is shown how the channel fading is greatly reduced, due to the presence of a line-of-sight propagation path, which allows virtual error free data transmission. Co-channel and cluster measurements, taken both on highways and in London, demonstrate how the surrounding environment, via either buildings or terrain, tends to attenuate co-channel signals. This implies that microcells will be able to operate with relatively small cluster sizes and will therefore be very spectrally efficient. The development of a number of theoretical propagation models for microcells is reported. These assume that all surfaces are smooth such that the propagation is characterised by a direct path and a number of reflected paths. Comparison with measured data shows that a two path model provides a good approximation to results taken in highway locations. In city centre locations it is found that a four path model provides a reasonable representation of the propagation process. A discrepancy is however noted in this latter model as probably being due to the clutter which exists in real environments compared to the ideal environment assumed in the model. It is found from the measurements, and partly supported by the theoretical models, that path loss can be characterisd as following a free space law to a breakpoint before changing to a faster signal decay rate which was always greater than an inverse 4th power law. Attempts are made to characterise the slow and fast fading. The slow fading is found to be approximated by a Log-normal distribution having a standard deviation of 2.8 dB. The fast fading when characterised by a Rician distribution are found to be very variable. This fading is however found to be less extreme than Rayleigh, especially in highway areas where little signal variability was present. City centre locations suffered from a greater level of fading but were still less than the extreme Rayleigh case. In conclusion, the measurement results and theoretical models presented provide a valuable contribution to the understanding of the radio propagation process within microcellular systems.

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