Patent GB-1472211-A (GB)
Title: Mobile communication system and method employing directional service area coverage
IPC: H04W16/24
Filed: 1974-05-13
Abstract: 1472211 Radio communication; aerials MARTIN MARIETTA CORP 13 May 1974 [15 May 1973] 21114/74 Headings H4L and H4A In a system providing communication between fixed stations and mobile units within a service area, at least one base station within the area directionally broadcasts modulatable carrier wave energy in a plurality of sectors of the service area extending radially outwardly from said base station. The energy is broadcast into adjacent sectors with different carrier frequencies, and into at least some sectors with the same carrier frequency. General description.-In one embodiment, Figs. 1 and 2, a central control unit 20 is coupled to a line telephone system 22 and controls the connection of incoming trunks IT to a base station 24 via lines 26 and also transmits signals via lines 28 to control the operation of the base station. The base station includes a plurality of transmitters, the outputs of certain of the transmitters being supplied to an omnidirectional aerial 30 located at the centre 24 of the sevice area 38 and that of the remaining transmitters being supplied to directional sections of an aerial array 32 also being located approximately at the centre of the service area. The omnidirectional aerial provides a mobile telephone service to zone C1 and each section of the directional array provides service to a different one of sectors A1 to A3 and B1 to B3 of the service area. The omnidirectional aerial 30 and directional array 32 also supply signals to a plurality of receivers at the base station 24, the outputs of the receivers being supplied via lines 27 to the control unit 20. To improve reception from mobile units a plurality of receivers 24 with omnidirectional aerials 36 are situated one in each sector A1 to A3 and B1 to B3 and coupled to the base station 24 by transmission lines. Twelve channels are available for use and are allotted as follows. A1 channels 1 to 4; B1 channels 5 to 8; A2 channels 1 to 4; B2 channels 5 to 8; A3 channels 1 to 4; B3 channels 5 to 8; C1 channels 9 to 12. However, the channels may be assigned on a dynamic or adaptive basis in accordance with the varying user density requirements if remote tuning facilities are provided for the transmitters and receivers, providing there is sufficient separation between zones using the same channels. One of the idle channels in each zone may be provided with a marker tone at a reduced power level relative to a normal transmission so that a mobile unit 40 can easily search for and lock on to an idle channel. A call from the telephone system 22 is transmitted from 24 as a four-digit number following a three-digit access code on the idle channel in each of the seven sectors. Upon receipt of its address the mobile unit returns a supervisory signal followed by an identifying code. The control unit 20 then designates and marks a new idle channel in that sector in which the idle channel has been seized by the mobile unit, causes the appropriate transmitter at the base station to increase the output power of the seized channel to its normal level and effects the appropriate connections for the transmitted and received signals in conventional manner. A call from a mobile unit to a subscriber on the telephone system 22 is carried out in normal manner over a seized idle line, the unit 20 effecting the required switching as before. In an alternative arrangement, Fig. 6 (not shown), the marked idle channel is broadcast from remotely tuned marker beacons in each of the zones. After the channel has been seized, communication is by way of the normal transmitters and receivers as in the previous embodiment. Larger areas may be covered by providing a base station centrally at each of a plurality of zones of the type described, which may overlap to some extent but ensuring that segments using the same channels are well spaced from each other. Means may be provided for maintaining the signal when the mobile unit moves from one sector to another by automatically switching over to another channel. Aerials.-The directional array 32 may utilize six aerials 210, Fig. 11 secured to a mast 212. Alternatively, Fig. 12, the array may comprise corner reflectors 218 in a cylindrical housing 216 of, for example, fibreglass, each reflector containing transmitting and receiving dipoles 224, 226 respectively.