Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Patent
1993-03-05
2000-05-23
Trost, William G.
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
455436, H04Q 720
Patent
active
060674555
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention generally relates to a digital mobile telephone system, and more particularly to a digital mobile telephone system of a so-called cellular type in which system the service area is divided into a plurality of cells or sectors, a base station being provided for each cell.
BACKGROUND ART
With automobile telephones or portable telephones becoming more and more widely used, a problem has been revealed in that the conventional analog mobile telephone system tends to be short on radio channels. While it is true that the conventional analog mobile telephone system can be easily configured, it has a disadvantage in that its allocation of one frequency to one channel entails inefficiency in the use of frequencies. On the other hand, with time division multiplexing effected in the digital mobile telephone system, a single frequency can be shared by a plurality of channels, thereby improving efficiency in usage of frequencies. It is also to be noted that the digital mobile telephone system can provide advanced services other than conventional voice call service.
FIG. 1(A) illustrates a service area of the conventional digital mobile telephone system of a cellular type.
Referring to FIG. 1(A), a service area of the mobile telephone system is divided into cells or sectors P having, for example, a hexagonal shape, each of a plurality of base stations D, E, F being provided to correspond to a plurality of sectors. In the illustrated example, a base station D is provided in the center of the three hatched sectors P, the base station D covering the three sectors in different frequencies. Likewise, each of the base stations E, F cover three sectors. In such a digital mobile telephone system of a cellular type, it is possible to avoid interference among the sectors by using different frequencies in different sectors. Typically, a sector has a scale of about 500 m-5 km.
FIG. 1(B) schematically illustrates a signal format used in the system of FIG. 1(A).
Referring to FIG. 1(B), a signal having a frequency f.sub.A transmitted from a base station--for example, the base station D--contains three channels TCH.sub.1, TCH.sub.2, CCH obtained as a result of time division multiplexing, of which three channels the channels TCH.sub.1, TCH.sub.2 are used as communication channels, and the channel CCH is used as a control channel for line connection control, etc. Likewise, a signal having a frequency f.sub.a received by the base station D contains channels obtained as a result of time division multiplexing: namely, communication channels TCH.sub.1, TCH.sub.2 and a control channel CCH. While the communication channels TCH.sub.1, TCH.sub.2 carry independent speech signals, etc., the control channel CCH is provided to be shared by the channels TCH.sub.1, TCH.sub.2. With such time division multiplexing, the digital mobile telephone system shown in FIG. 1(A) accomplishes more efficient use of frequencies than the analog system.
FIG. 2 illustrates the configuration of the digital mobile telephone system covering the service area of FIG. 1(A).
Referring to FIG. 2, the system comprises: a mobile terminal 1 moving about in the service area of FIG. 1(A); radio base stations 2 corresponding to the base stations D, E, F of FIG. 1 and communicating with the mobile terminal 1 using a radio channel; and a switching station 3 connected to the radio base stations 2 via wire or radio lines and connected further to the public telephone line, the mobile terminal being connected to the public telephone line via the switching station and the radio base station. In such a digital mobile telephone system, there is a need to change the frequency used in communication with the base station, when the mobile terminal 1 moves from one sector to another. For this purpose, the switching station 3 is equipped with a control apparatus 3b besides an ordinary switching apparatus 3a. On the basis of the field strength of the radio wave signal transmitted from the base station and detected by the mobile terminal 1, the control
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patent: 4989230 (1991-01-01), Gillig et al.
patent: 5243598 (1993-09-01), Lee
patent: 5278991 (1994-01-01), Ramsdale et al.
patent: 5327576 (1994-07-01), Uddenfeldt et al.
Endo Takashi
Matsukida Yuriko
Fujitsu Limited
Trost William G.
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