Portable radio telephone system.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates generally to a portable radio telephone system. More particularly, the present invention relates to a private radio telephone system, for use in a local area such as in a office building or in a factory, of the type wherein when its portable telephone set is taken out from the area, it can communicate with subscribers of public telephone network through a public cellular radio telephone system which has already been in service over urban areas.

(2) Description of the Prior Art

A public cellular radio telephone system has been known in the past as a urban cellular radio telephone system. When its portable telephone set is brought into an office building or in a factory and connected to a subscriber of the private branch exchange for communication, not only the quality of telephone voice drops due to radio wave propagation attenuation by the office building but also the telephone charge of the urban cellular telephone system must be payed.

As one of the prior art references of urban cellular radio telephone systems which improve the radio wave propagation attenuation of this kind, mention can be seen in Proceedings of IEEE-VT Conference, 1984, ^pp 162,

In the portable telephone system described in the prior art reference described above, introduction of a private branch exchange which has radio telephone subscriber circuits is not taken into consideration, so that direct communication cannot be made from the portable telephone set to the subscribers of the private branch branch telephone system.

On the other hand, if we introduce private branch radio telephone systems that have been commonly considered, the portable telephone set becomes expensive because it must have a radio frequency circuit of the private radio frequency channels in addion to the -radio frequency of the urban cellular radio telephone.

SUMMARY OF THE INVENTION

The object of the present.invention is to accomplish a portable radio telephone system wherein a portable telephone set can be used both for an urban public cellular radio telephone system and for a private rad telephone system which is build inside a local area. For-example, the present invention is directed to accomplish a portable radio telephone system wherein direct access can be made from a portable telephone set to the radio frequency circuit of a private branch exchange without causing any radio wave interference to a heretofore known urban cellular radio telephone system

To realize the present invention, a portable radio telephone system of the type wherein a given area is divided into a plularity of large areas, each of the large areas is further divided into a plularity of small cellular areas, wherein the reuse of the same frequency channel is permitted between the large areas, different frequency channels are allocated to the small cellular areas of each large area and public radio communicationis made by portable telephone sets by use of the radio waves of the frequency channels allocated to the small cellular areas, the object of the present invention described above can be accomplished by a private radio telephone system consisting of an antenna for transmitting and receiving a radio wave pair which doubly reuse the same frequency channel pair with the public cellular radio telephone system but do not use the frequency channels allocated to the small cellular area wherein the private telephone system exists, a private branch exchange and a transceiver both of which are connected to the antenna and private radio communication is made through private.branch exchange by use of the portable telephone sets.

Incidently, the power intensity of the radio wave radiated from the antenna is kept to a very low intensity level so that it does not affect the public cellular radio telephone system in the large areas, and also the frequency channel pair is selected so that they hardly interfere the public cellular radio telephone system of the large areas. To prevent the occurence of the wave interference, the antenna described above may have directivity.

When the portable radio telephone system of the present invention is disposed in a district where the density of subscribers of a public cellular radio telephone system is high, the radious of the small cellular area( cell or sector ) becomes little so that the service area of a local private system using a single antenna becomes narrow. In such a case, the local area is divided into a plularity of mini cells with each antenna and radio waves of different frequency channel pairs are reused among the mini cells to secure a required service area, as described, for example, in "Personal Communication", September, 1985, pp. 28.

The above and other objects and also novel features of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

<ul id="ul0001" list-style="none"><li>Figs. 1, 4, 6 and 7 show examples of cell disposition of a urban cellular radio telephone system of the present invention;</li><li>Fig. 2 is an explanatory view of the intensity of a radio wave in an embodiment of the present invention;</li><li>Figs. 3 and 5 are schematic views showing the construction of the portable radio telephone system of the present invention;</li><li>Fig. 8 is a block diagram showing the construction of a portable telephone set used for the portable radio telephone system in one embodiment of the present invention; and</li><li>Fig. 9 shows the output power levels at which the portable telephone set of Fig. 8 operates.</li></ul>

DESCRIPTION OF THE PREFERRED EMBODIMENT

Fig. 1 shows an example of cell disposition of a heretofore known public cellular radio telephone system in order to have the present invention more easily understood.

A given are is divided into a plurality of large areas ( which are referred to as "cluster" ) represented by thick line and each of the clusters is divided into twelve cells ( small areas ) 1 - 12. An antenna for transmission and reception ( represented by black dot "." ) is disposed at the center of each cell.

Radio frequency channels are allocated to each cell in the number corresponding to the traffic of the radio telephone. These radio waves will be called "f" in the cell 1, "f₂" in the cell 2, "f₃" in the cell 3, and so forth. The f₁, f₂, ....radio waves have an allocated bandwidth, and a plurality of frequency channels exist in each bandwidth related to the radio telephone traffic. Additionally, the position of the cell 1 in the cluster 13 is selected so that the intensity of the f wave interference from outside the cluster becomes minimal.

In the portable radio telephone system of the present invention, radio communication is made between a private branch radio telephone system in one cell and a portable telephone set by use of frequencies different from the frequencies allocated to the cell.

Incidentally, the frequency and intensity of the radio waves used for the private branch radio telephone system are selected in the following manner lest radio wave interference occurs with the other cells in the cluster. Namely, the frequency channels allocated to the cell 1 must be the channels which are used in the most apart from the cell 1. Thus, the f₂ wave of the cell 2 exists'for the cell 1. The cell 4 is the most remote from the cell 1 inside the cluster 13, but the interference intensity of the f₄ wave from outside the cluster 13 is greater than that of the f₂ wave. The f₃ wave used in the cell 3 also satisfies the condition described above.

Fig. 2 shows the intensity of radio wave used by the portable radio telephone system of the present invention. In other words, when the private radio telephone system 0 of the present invention is disposed inside the cell 1 as is shown in Fig. 1, the electric field intensity E₁ of the radio wave radiated from the cell 1 can be expressed by the following equation if the irregular radio wave propagation does not exist:<maths id="math0001" num=""><img file="EP0225607A2_D0001.tif" /></maths>where R is the distance from a transmitting antenna to a reception antenna, a is a proportional coefficient and P₁ is the transmitting power of the antenna.

In the public cellular radio telephone system, the radious of the cell are determined at a boundary where the ratio S₁/I₁ of the intensity S₁ of the received f_l wave to the intensity I₁ of the f₁ wave interference from outside the cluster 13, so that the the quality of telephone voice is kept a predetermined level.

The transmitting power P₂of the private radio telephone system 0 of the present invention is kept to a value so that the ratio of the intensity S₂ of the transmitting f₂ wave to the intensity I₂ of the f₂ wave interference at the boundary of the local service area satisfies the conditions below:<maths id="math0002" num=""><img file="EP0225607A2_D0002.tif" /></maths>

Fig. 3 is a structural view of the portable radio telephone system in accordance with one embodiment of the present invention. A private branch exchange ( PBX ) 15 for wired telephone subscribers is disposed in the private radio telephone system 0 ( corresponding to 0 in Fig. 1 ) and radio telephone subscriber circuit are added thereto. The radio telephone subscriber circuits are connected to transmission/reception antennas 18, 19 through transceivers ( TRX ) 16 and a booster 17, and the f2 wave having the intensity that can supress the f₂ wave interference of the public cellular radio telephone system, is transmitted from the antennas and enables the portable telephone set 20 to communicate by receiving the f₂ wave from the antennas.

As will be described elswhere, the portable telephone set 20 can communicate with the f_l wave from the cell 1 of the public cellular radio telephone system. The transmitting power of the portable telephone set 20 is also determined to a value that satisfies the condition of eq. (2). This controlling is made automatically by a radio frequency controller disposed inside the the private branch exchange (PBX) 15. The method controlling the transmission power for the private radio telephone system is the same as the method used for the heretofore known public cellular radio telephone system.

Fig. 4 shows the relation of the f_N wave ( N = 12 of the radio frequency channel with Fig. 1 when the private radio telephone system is disposed for each of the cells inside the cluster 13. For example, the cells 4, 2, 5 in Fig. 1 use the f₅, f₃ and f₆ waves, respectively.

When a plularity of private radio telephone systems are disposed for each of the cells inside one cluster, the total interference intensity must be determined below a predetermined level. However, the condition which prevents the interference to the nearest adjacent system of the present invention is far severer. As the radiation from an outdoor antenna shown in Fig. 3 is particulary severe, the interference is restricted by providing the antenna with directivity.

Furthermore, when a plurality of private radio telephone systems exist extrimely close to one another inside the small cell, the service area of the private radio telephone system of the present invention becomes extremely narrow in order to prevent their mutual interference. To solve this problem, other radio frequency channels ( e. g. alternate use of f₂ and f₃ waves in the cell 1 ) are used for the adjacent private branch telephone systems.

Fig. 5 shows the construction of the portable radio telephone system in accordance with another embodiment of the present invention. In this embodiment, a plurality of radio frequency channels are reused in one local area.

A transceiver ( TRX ) 21 having a radio wave transceiver and an antenna is disposed inside a building while a transceiver ( TRX ) 22 is disposed outside the building, and the radio frequency channels, the transmission power and the antenna directivity are selected so that⁼they do not interfere with each other. The private branch exchange ( PBX ) 15 is equipped with a radio frequency controllar, and when the portable telephone set 20 moves from the f₂ area to the area of f₃, it detects automatically the movement and switches the channels in the same way as the public cellular radio telephone system. If the radio frequency controller selects the empty channel(s) of both the public cellular radio telephone system and the other private radio telephone systems inside the cluster, the quality of the telephone voice is highly improved.

The district in which the portable radio telephone system of the present invention is disposed is not restricted to the cell disposition shown in Fig. 1.

Figs. 6 and 7 show an example of sector cell allotment. Each cell is divided into six sectors and four cells = 24 sectors form a unit cluster. A multibeam antenna having 60° directivity is disposed at the center of the hexagonal cell. The frequency channel_'- of each sector is reused among the clusters where a public cellular radio telephone system is in service. Fig. 7 shows the sector number of the frequency channel corresponding to Fig. 6 when the private radio telephone system of the present- invention is disposed inside each sector, and the system doubly reuses the radio frequency channel inside the sector. Other examples of sector numbers of the frequency channels can of course exist in the same way as the case of 12-cell unit sectors in Fig. 1. When the wave interference is taken into consideration for sector cells, the directivityof the antenna becomes an important factor, and the gain function G(6) must be introduced in the proportional coefficient a = a_oG(θ) of eq. (1).

In a.district where the number of subscribers of the public cellular radio telephone system becomes large, the traffic per unit area is increased by reducing the radius of the cell. The portable radio telephone system of the present invention is designed in consideration of this increase inside the respective area. Table 1 illustrates the radious of the service area of the private radio telephone system and the radious of the cell of the public cellular radio telephone system.

<tables id="tabl0001" num="0001"><img file="EP0225607A2_D0003.tif" /></tables>If a large number of mini cells must be disposed inside the local area, a plurality of radio frequency channels also must be reused in the private radio telephone system. In this case, the radius of the mini cell becomes much smaller than the muximum radius of the service area shown in Table 1.

In the actual cases, the shape of the oell is not so ideal as the ideal cell disposition shown in Fig. 1 and 6, and the power of the transmitted radio wave attenuates more rapidly than eq. (1). However, the application of the present invention is not essentially changed by these, factors.

Fig. 8 shows the construction of a telephone set used in the portable telephone system described above.

In the drawing, reference numeral 23 represents an antenna and 24 does an antenna distributor for sending the reception signal from the antenna to a receiver and applying a reception signal from the receiver to the antenna 23. Reference numerals 24 and 26 represents frequency synthesizers for operating a transmitter 27 and a receiver 25 on predetermined channels. A control part 28 receives, as its input signals, the reception signal from the receiver 25, a reception level signal of the reception signal by a reception signal level detector 30 and signals from a handset 29 having a microphone M, a speaker R and key pads K, and supplies predetermined signals to the receiver 27, a transmitting power controller 31 and the frequency synthesizer 26.

Since the construction of the portable telephone set is substantially the same as the construction of a mobile telephone set used in the heretofore known urban mobile telephone system (such as the telephone set described in "The Bell System Technical Journal", Vol. 58, No. 9, pp. 132), its detailed description is hereby omitted.

The first characterizing feature of this embodiment lies in that a small output level, which is used for the private radio telephone system, is set to the output power controller 31 of the portable telephone set so that it can be used commonly for both the public cellular radio telephone system and the private radio telephone system. This is because the reception sensitivity of the local base station can be made substantially equal to that of the large area cellular radio telephone system and the distance between the portable telephone set and the station is small.

Fig. 9 shows the levels of the output power that can be accepted by the output power controller 31 for North America and Japan. In the case of the public cellular radio telephone system of North America, the minimum power is stipulated to 6 mW of the level 7, but 2.5 mW of the level 8 and 1 mW of the level 9 are further added in order to use it for the system of the present invention. In the case of the public cellular radio telephone system of Japan, the minimum power level is 160 mW of the step 1, but 25 mW, 5 mW and 1 mW of the levels 2, 3 and 4 are further added for the system of the present invention.

In other words, the portable telephone set of Fig. 8 has a switch portion for the power levels necessary for communication in the cells (1 - 12) shown in Fig. 1 and the communication power levels necessary for the private radio telepone system (e.g. 0.14) inside the cell (1) described above.

The second characterizing feature of the portableteleph- one set shown in Fig. 8 lies in that it has switch means for changing over the modes of use as desired by an operator to a mode where communication can be made by utilizing the private radio telephone system when the telephone set enters the local service area of the private radio telephone system.

When the mode switch 32 is manually changed from the contact P to C, the frequency channel of the frequency synthesizer 26 and the level range of the transmitting power controller 31 are forcedly set to the state where the control signal of the private radio telephone system can be received. In other words, the switch 32 selects the combination of the frequency channels of the communication system of the cells (1 - 12) of Fig. 1 with the power levels and the frequency channels of the private radio telephone system (e.g. 0.14 in Fig. 1) with the transmitting power levels.

This mode switch 32 may be of an automatic type. Namely, a reception level judging detector 33 represented by dash line in Fig. 8 is disposed. In the-public cellular radio telephone system, when the telephone set is under the waiting state where it receives the control signal from its station, it receives the time slot for calling other subscribers for the most of the time. This waste time is utilized in order.to receive the control signal in the private radio telephone system and when the reception intensity is at a sufficiently high level, the mode switch can be changed over automatically.

As described above, the present invention provides the following effects.

<ul id="ul0002" list-style="none"><li>(1) In accordance with the present invention, the radio frequency channels used in the existing public cellular radio frequency system are reused when disposing a private radio telephone system. Accordingly, the use of new frequency resources is not necessary.</li><li>(2) In accordance with the present invention, when the portable telephone set is brought into an area where the public cellular radio telephone system is in service, direct access can be made from the telephone set.. Accordingly, radio wave attenuation due to buildings or the like does not occur and the toll for the public cellular radio telephone system is not necessary.</li><li>(3) In accordance with the present invention, the mobile telephone set that can make direct access to both the public cellular radio telephone system and the. private radio telephone system can be accomplished without increasing a radio frequency circuit at the portable telephone set. Therefore, the portable set can be made compact in size and simple in construction and can be produced economically.</li></ul>