RADIO TELEPHONE SYSTEM of the radio frequency spectrum than systems hereto
_ fore developed.
BACKGROUND FIELD OF INVENTION u fa yfit objec( of the ... u> ... y
This invention relates generally to communications fully automatic portable telephone system.
systems, and more particularly to organized radio tele- 5 In accordance with a preferred embodiment of the
phone systems having a plurality of base station and invention, the geographic area over which communiea
portable units, each having a predetermined coverage tions is to be provided is divided into a series of base
area, and means for adjusting the operating frequencies station cells, and each station cell is further subdivided
of the portable units to provide the optimum communi- into a series of sub-cells. A base station transmitter is
cations path. 10 located within each cell and transmits to portable re
ccivers within the cell. The transmission ranee of the
portable transmitters is deliberately reduced to allow Organized communications systems are known, one less precise location of the portable units without causvariety of which is commonly known as a cell system. ing interference between the portable units. A network In such a system, the geographic area to be covered is 15 of satellite base station receivers, one base station redivided into a group of cells, each cell having a base cciver being located in each sub-cell, is employed to restation transmitter and a base station receiver. The ceive signals from the portable transmitters. A different ranges of the base and portable or mobile units are set of incoming and outgoing frequencies are employed made substantially equal, and the mobile unit covers in each cell to avoid interference between units in adjathe entire geographic area covered by the base station 20 cent or closely spaced cells. The same frequencies may transmitter. The base and mobile frequencies of adjoin- be reused in cells that are sufficiently geographically ing cells are selected to be different to avoid interfer- separated from each other to prevent interference ence between cells, and the same frequencies may be therebetween.
reused in cells that are sufficently spaced so as to pre- Each base transmitting station radiates at least one vent interference therebetween. Location means are 25 out-going signalling frequency to the sub-cells within its provided to determine the cell in which the portable coverage area. The receiver in each of the portable unit is operating, and to adjust the operating frequency units scans the signalling frequencies of all of the base thereof to the frequency designated for the cell in station transmitters within its area of operation and which the portable is located. The location function stores an indication of which of the received signalling may be accomplished by base station receivers located 30 signals is the strongest to determine the base station in the corners of the cell which have directional anten- transmitter that will provide the best communications nas looking inwardly into the cell and a computer con- link therewith. Transmissions by the portable unit are nected to the base receivers for determining the made on an incoming signalling frequency that is strength of the signal received from the portable unit by paired or associated with the strongest outgoing signalthe corner located receivers. ^ ling frequency received. The transmission from the Whereas this technique provides a way to achieve portable unit is received by the receivers in the nearest reasonably good communications, because the trans- sub-cells and a comparison is made between the signal mission range of a portable or mobile unit is equal to strength received by the various satellite receivers to the coverage range of a base station, the location of the determine which satellite receiver provides the best portable unit must be determined very accurately, and 40 communications with the portable unit. After the optithe assignment of the operating frequency of the porta- mum base station transmitter and satellite receiver blc must be based on the geographic location of the have been determined, the base station transmitter sigunit to avoid interference with portables in other cells nals the portable unit, on the outgoing signalling freoperating on the same frequency. The aforementioned quency, to retune to a communications channel cornrequirement requires complex and expensive location 4^ prising a pair of frequencies assigned to the selected equipment, docs not provide optimum spectrum utili- base station transmitter and satellite receiver over zation, and does not assure that the portable unit is re- which communication will he established, ceiving the best signal since the assignment of opcrat- Other scanning base station receivers are employed ing frequency is based on location and not on the to monitor all active communications channels, and strength of the signal received thereby. Furthermore," means are provided to compare the signal strengths rethe fixed, relatively high power of the portable unit ceived by each of the scanning receivers. Automatic causes interference to other units in the system when switching circuitry is provided to cause the portable the portable unit is operated at a high location, such as unit to change operating frequency unci to make the the upper floors of a high rise building. This occurs be- necessary wire line switching as a portable proceeds cause the increased coverage area resulting from the' from one cell to another.
improved propagation characteristics of a high antenna Because the range of each portable unit is less than
cause the portable unit to radiate into areas in which the range of a base station transmitter, the frequencies
other portable units may be operating on the same fre- at w hich the portable unit operates may be chosen to
quency. assure that the portable unit is receiving the best signal,
regardless of whether it is actually operating within the particular cell to which those frequencies have been as
It is an object of the present invention to provide an signed, without causing interference to the rest of the
improved organized communications system that pro- system. The aforementioned feature assures that the
vides improved communications and reduced interfer- best possible communications link is provided, elimi
ence between units operating on the same frequency. nates the need for precise geographic location of each
It is a further object of this invention to provide a individual portable unit and makes more efficient use
communications system that makes more efficient use of the radio frequency spectrum.