Estimation of the Coherence Bandwidth for a Millimetre Wave Mobile Radio Link

Abstract

A bandwidth study of multipath propagation at 55 GHz in a heavily built up urban environment is presented. Estimates of coherence bandwidth between two phase coherent signals transmitted at various frequency separations are given. Results show that a reasonable estimate for the correlation bandwidth in an urban multipath environment is about 25 MHz. I -INTRODUCT IONPressures are growing rapidly on the capacity offered by the current cellular mobile radio systems particularly in dense inner city areas. Increasing demand has turned attention to the use of frequencies in the millimetre wave band where potentially wide bandwidths are available to be exploited. It has been suggested that frequencies in the region of the oxygen absorption band could be used for future mobile communication systems, particularly in metropolitan areas where most users are concentrated and congestion is already evident,Several preliminary studies at millimetre wave frequencies have been made in an urban environment with low traffic density, Violette et al [2] and Huish et al [3]. Other results have shown that on heavily urbanised paths up to about 400 m long, signal fades of less than 40 dB can be assumed at frequencies in the region of 55 GHz, Thomas et al [4]. Fades of this magnitude make these frequencies a viable proposition for a potential mobile radio system from the propagation point of view.Recently, a narrow band time division multiplexed, time division multiple access (TDMITDMA) system has been accepted as the European Standard for a digital mobile system at UHF. This system proposes 8 channels at 16 Kbit/s with additional channels for signalling etc. The total data rate is 150 Kbit/s. If this system is implemented at 60 GHz, link budget calculations have shown that this data rate can be accommodated with an adequate signal/noise ratio of 12 -14 dB for speech traffic assuming a 40 dB fade margin on the repeater to mobile link over a maximum range of 500 m, Norbury [5]. However, it is likely that a 60 GHz system would make use of the very large bandwidth to reduce multipath distortion by using a degree of spectrum spreading together with a modulation scheme such as BPSK which, although not very spectrally efficient, provides the best error rate for a given bit energy.The object of this study is to investigate the usable bandwidth in a microcell of 200 -300 m in extent and thus give an indication of the scope available for the use of these techniques in the millimetre wave region. Inevitably, delay spreads in the received signal due to reflections from the road surface, buildings, trees, lamp posts, other vehicles, etc. will limit the usable bandwidth.One method of evaluating channel bandwidth is via the coherence bandwidth of the channel. This is a derivative of the method presented by Masing [6]. Masing showed that, for a given range, receiver complexity and bit error rate, maximum channel bandwidth is dependent on the standard deviation of the multipath time delays (a). The coherence bandwidth (B ) of a fading channel is related...