MICROCELLULAR PROPAGATION CHARACTERISTICS Howard

To enhance the spectral efficiency and to improve service quality for cellular radio systems, a new microcellular system has been proposed [1-5] that operate over a relatively short radio path (on the order of 200 m to 1000 m), using relatively low antennas (about the same height as lamp posts), and transmitting at relatively low power (typically on the order of 10 m W). This new concept of microcells utilizes some of the special properties of radio propagation for small cells to reduce the severe multipath fading and shadow fading in the land mobile propagation environments. Over a relatively short propagation path, it is possible to arrange the radio link between the transmitter and receiver to be a clear line-ofthe-sight (LOS) path, so that the microcell can operate in a Rician channel, which has significantly less multipath fading than the Rayleigh channel of conventional cellular systems. On the other hand, the relatively low antenna can be located above the local vehicular traffic but below the surrounding buildings. This benefits the microcellular systems in two ways: first, the shadow fading due to the local traffic can be eliminated; and secondly the radio signal can be confined and directed into a limited size microcell. Signal delay due to multipath reflection may cause intersymbole interference (lSI) in digital radio systems. The lower microcellular base station antenna reduces excess delay spread since the distant reflectors are blocked [6]. LOS paths are expected to playa major role in providing signal coverage for microcell systems. However, any provident cellular system design must consider not only the signal coverage, but also potential co channel and adjacent channel interference. Unlike the conventional macrocellular systems where the mechanisms of carrier and interference signal propagation are essentially the same, e.g. propagation over buildings, the interference in a microcellular system can come from radio paths other than the LOS path used for the carrier signal.