Patent GB-1461624-A (GB)
Title: Radio telephone system
IPC: H04B1/50
Filed: 1974-10-14
Abstract: 1461624 Radio telephone systems MOTOROLA Inc 14 Oct 1974 [17 Oot 1973] 44414/74 Heading H4L  The invention relates to a radio telephone system having a plurality of base station and portable units, each having a pre-determined coverage area and means for adjusting the operating frequencies of the portable units to provide the optimum communication path. GENERAL DESCRIPTION. The area to be covered is divided into a series of cells which are each divided into a series of sub-cells. A base station, such as 102, Fig. 2, is located in each cell and is provided with a base station receiver 110, 112, 114 in each sub-cell, for communication with portable receivers 132 etc. within the cell. A control centre 130 enables communication between telephone subscribers and portable units or between portable units. The transmission range of the portable units is made much less than that of the base station to avoid interference between portable units. This enables the same carrier frequencies to be used in cells which are widely separated geographically although, as described, a reduced frequency separation is used, a different set of incoming and outgoing frequencies being employed in adjacent cells. Each base station transmits all signalling information on a signalling channel and the receiver in each portable unit scans the signalling channels and stores an indication of which received signalling channel is the strongest to determine which base station will provide the best link. Transmission initiated by the portable unit, e.g. 132, is on a signalling frequency paired with the strongest received signalling frequency which is selected automatically and this is picked up by the base station receivers 110, 112, 114 and the receiver providing the strongest signal is selected. The control centre 130 then causes the corresponding base station, e.g. 102, to transmit a signal on a signalling frequency to cause the portable unit to retune its transmitter and receiver to the pair of speech communication carrier frequencies assigned to that sub-cell. A link is established at the same time between the selected receiver and its base station. Once communication has been established, the location of the portable unit is continuously monitored in order that new carrier frequencies may be assigned thereto as the unit moves between cells. This is provided by a group of receivers and directional aerials at each base station. Each directional aerial is connected to either a plurality of receivers or a single scanning receiver that may be rapidly tuned to each incoming communication frequency, each receiver having means to measure signal strength and being connected to the control centre 130. The control centre determines the location of each unit on the basis of signal strength and reassigns channels as the unit moves from one cell to another. BATTERY ECONOMY AND INTERFERENCE PREVENTION. This is achieved by automatically controlling the transmitter power output of the portable units. If the signal received by a base station receiver exceeds a predetermined level, the base station transmits a command to the portable unit to reduce its power until the signal received at the base is the minimum required for satisfactory communication. FREQUENCY ALLOCATION. The cells forming the area to be covered are divided into groups, as described, seven cells to a group. The total frequency spectrum available is 175 KHz giving 25 KHz separation between adjacent cells of each group, which is the normal requirement for satisfactory communication. There are three groups of seven cells each having a different set of seven frequencies and the separation between frequencies in different groups is 8À33 KHz which is stated to be satisfactory in practice.