Digital mobile telephone system having overlay configuration

The digital mobile telephone system according to the present invention having an overlay configuration, allows a mobile terminal communicating with a base station to detect the field strength of a transmission from the base station covering a sector and an overlay region, notifies a switching station of the result of detection, and causes a frequency used in communication between the base station and the mobile terminal to be switched to the communication frequency for the overlay region, when the detected field strength exceeds a predetermined threshold level. When the detected field strength drops below the predetermined threshold level, the switching station switches the frequency used in communication between the base station and the terminal to the communication frequency for the sector.

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 apparatus 3b changes the setting of the 
frequency used by the mobile terminal so that the frequency transmitted 
from the base station and producing the greatest field strength may be set 
to be used. 
More specifically, in the case that the switching station 3 is connected to 
a radio station BS-D via a line CH, and the mobile terminal 1 receives 
messages on the public telephone line via a transmission TX1 from the base 
station BS-D, the mobile terminal 1 measures the field strength of the 
transmission TX1 and determines whether or not the measurement result 
exceeds a predetermined threshold level. If it is found that the field 
strength exceeds the predetermined level, the mobile terminal 1 maintains 
its line connection with the base station BS-D; if it is found that the 
field strength is below the threshold level, the terminal 1 measures the 
field strength of transmissions from other radio base stations BS-E-BS-H 
searching for the base station producing the greatest field strength. The 
identity of the base station determined to be used in this search is 
reported to the base station BS-D via a transmission line TX2; and the 
information thus reported 35 is forwarded from the base station BS-D to 
the control apparatus 3b of the switching station 3 via the line CH. 
Thereupon, the control 3b switches the line for use in connecting with the 
mobile terminal 1 from the line CH which runs by way of the base station 
BS-D to, for example, a line CH' which runs by way of the base station H, 
i.e., the base station producing the greatest field strength. By thus 
selecting the base station used in communication with the mobile terminal 
1 on the basis of the field strength such that the greatest field strength 
may be obtained, it is possible to maintain a stable line connection 
whichever sector in the service area the mobile terminal is in. 
There is proposed, for such a conventional digital mobile telephone system 
as shown in FIGS. 1(A), 1(B) and in FIG. 2, a so-called overlay 
configuration shown in FIG. 3 designed to further improve efficiency in 
the use of frequencies. In the overlay configuration, overlay regions Q, 
covering a range substantially smaller than the size of the sector, are 
formed to correspond to each of the base stations BS-D, BS-E, BS-F, etc. 
For each base station, the overlay region Q is covered by a small-power 
transmission at a frequency different from the frequency characterizing 
the sector, with the result that the frequency band of the system 
increases substantially. By forming the overlay regions Q using the 
frequency of the same frequency range for every station, the system's 
efficiency in the use of frequencies is greatly improved. Since the 
overlay regions Q are independent of each other, no interference arises 
even if the same frequency is used from one overlay region to another. 
When operating the mobile telephone system having such overlay regions Q, 
various problems needing a solution exist that are related, for example, 
to frequency switching control of the mobile terminal. For example, while 
the conventional cellular system enables selection of the base station 
producing the greatest field strength around the mobile terminal, as 
described earlier, the mobile terminal frequency switching control 
employed in the conventional cellular system is not effective in case the 
mobile terminal moves in and out of the overlay region, since the field 
strength in the overlay region is smaller than that in the sector covered 
by the same base station. There is a need, when operating the digital 
mobile telephone system having an overlay configuration, to establish a 
switching control method for selecting the frequency used in communication 
between the mobile terminal and the base station, which method is effected 
when the mobile terminal moves in and out of the overlay region. 
DISCLOSURE OF THE INVENTION 
Accordingly, a general object of the present invention is to provide a 
novel and useful digital mobile telephone system in which the above 
problems are eliminated. 
A more specific object of the present invention is to provide a digital 
mobile telephone system of a cellular type having an overlay configuration 
and enabling the frequency used in communication between the mobile 
terminal and the base station to be changed when the mobile terminal moves 
in and out of the overlay region. 
Still another object of the present invention is to provide, in a digital 
mobile telephone system of a cellular type having an overlay 
configuration, a method of controlling frequency switching necessitated 
particularly by the movement of the mobile terminal in and out of the 
overlay region. 
Still another object of the present invention is to provide a digital 
mobile telephone system comprising: a switching station connected to a 
public telephone line and connecting a mobile terminal to the public 
telephone line; one or a plurality of base stations connected to the 
switching station and each covering one or a plurality of sectors with 
transmission of a radio signal having a first frequency different from one 
sector to another, the covering being done in such a manner that the field 
strength of the radio signal exceeds a predetermined level in each sector, 
and the base station in each sector receiving a radio signal having a 
second frequency different from the first frequency and different from one 
sector to another; a mobile terminal receiving, in each sector, a radio 
signal having the first frequency and transmitting a radio signal having 
the second frequency in its communication with the base station 
corresponding to the sector; a control apparatus provided in the switching 
station and controlling the base station and the mobile terminal so as to 
control the first frequency and the second frequency to correspond to the 
one or the plurality of sectors, the digital mobile telephone system being 
characterized in that: 
each of the base stations covers, with transmission of a radio signal 
having a third frequency different from the first and second frequencies, 
an overlay region which is characterized by field strength smaller than 
that in the one or the plurality of sectors and has a range smaller than 
that of the one or the plurality of sectors; the mobile terminal monitors, 
in each sector, the field strength of the radio signal transmitted from 
the base station, and transmits a control signal indicative of the result 
of monitoring to the base station; the base station forwards the control 
signal transmitted from the mobile terminal to the control apparatus 
provided in the switching station; the control apparatus switches, on the 
basis of the control signal forwarded from the base station, the 
transmission frequency of the base station from the first frequency to the 
third frequency, when field strength of the radio signal transmitted from 
the base station and monitored by the mobile terminal exceeds a 
predetermined threshold field strength level, while, at the same time as 
this, switching the reception frequency from the second to the fourth 
frequency, switching the reception frequency of the mobile terminal from 
the first frequency to the third frequency; and switching the transmission 
frequency thereof from the second frequency to a fourth frequency. 
According to the present invention, it is possible to greatly improve 
efficiency in the use of frequencies in the digital mobile telephone 
system, by using an overlay region. Since the overlay regions are provided 
for each base station independently of each other, the same frequency can 
be used in all the stations as far as the third frequency and the fourth 
frequency are concerned and still no interference occurs. This contributes 
to a great improvement in efficiency of the use of frequencies in the 
system. Since the present invention allows the field strength in the 
overlay region to be monitored by the mobile terminal, making the result 
of monitoring available as a basis on which to switch the frequency, a 
smooth switching of the communication frequency is possible. It should be 
noted that, in the present invention, when switching the reception 
frequency of the mobile terminal to the reception frequency characterizing 
the overlay region, the third frequency, producing the field strength 
smaller than that produced by the frequency characterizing the sector, is 
selected. 
In a preferred embodiment of the present invention, the base station is 
notified of the result of monitoring by the mobile terminal when the field 
strength of reception as monitored by the mobile terminal drops below a 
predetermined level, whereupon the base station switches the frequency 
used in its transmission from the third frequency to the first frequency, 
switches the frequency used in its reception from the fourth frequency to 
the second frequency, and issues an instruction to the mobile terminal, by 
which instruction the frequencies used by the mobile terminal are switched 
from the third and fourth frequencies to the first and second frequencies. 
In another embodiment, the mobile terminal has a monitoring means for 
monitoring the field strength of the transmission of a radio signal having 
the first or third frequency; the field strength of the radio signal 
monitored by the monitoring means is forwarded to the control apparatus 
provided either in the base station or in the switching station; and the 
control apparatus instructs the base station to switch the frequency used 
in communication. 
In still another embodiment, the mobile terminal has a monitoring means for 
monitoring the field strength of all the transmissions from the base 
station; the data indicative of the field strength is forwarded to the 
control apparatus provided either in the base station or in the switching 
station; the control apparatus instructs, on the basis of the field 
strength of the radio signal having either the first or third frequency 
and contained in the data, the base station and the mobile terminal to 
switch the frequency used in communication. 
In still another embodiment, the base station adds, to the radio signal 
having the third frequency, either an identification signal indicating 
that the transmission is from the base station having the overlay region, 
or an identification signal indicating that the signal is intended for the 
overlay region. 
Other objects and further features of the present invention will be 
apparent from the following detailed descriptions when read in conjunction 
with the accompanied drawings.

BEST MODE OF CARRYING OUT THE INVENTION 
A description will be now given of the referred embodiments of the present 
invention, with reference to the figures. 
FIG. 4 illustrates the digital mobile telephone system according to the 
first embodiment of the present invention; 
In the figure, a base station 12 is provided in correspondence to a sector 
SC corresponding to the sector P of FIG. 3 and to an overlay region OL 
corresponding to the overlay region Q. The base station 12 is connected to 
the public telephone line via a switching apparatus 14 provided in a 
switching station 13. Further, a mobile terminal 11 is provided in such a 
manner that it can move within the sector SC including the overlay region 
OL and into the sectors of other base stations. When in operation inside 
the sector SC of the base station 12, the mobile terminal 11 is connected 
to the public telephone line via the base station 12; and a control 
apparatus 15 corresponding to the control apparatus 3b of FIG. 2 is 
provided in the switching station 13 for the purpose of effecting 
switching control of frequencies used in communication between the base 
station and the mobile terminal. The control apparatus 15 performs 
switching control of the communication frequency when the mobile terminal 
11 moves in and out of the sector SC, in the same manner as in FIG. 2. 
More specifically, when the mobile terminal 11 is located outside the 
overlay region OL but inside the sector SC, as shown by a broken line in 
FIG. 4, the base station 12 transmits to the mobile terminal 11 at a first 
frequency f.sub.A characterizing the sector SC, and receives transmissions 
from the mobile terminal 11 at a second frequency f.sub.B also 
characterizing the sector SC. The frequencies f.sub.A and f.sub.B will be 
called sector frequencies hereinafter in this specification. The mobile 
terminal 11 of FIG. 4 contains a field strength measuring device 11a for 
measuring the field strength of the received radio wave, and the device 
11a measures the field strength of the wave having the sector frequency 
f.sub.A. When the field strength measured drops below a predetermined 
threshold level, the device searches the service area for the base station 
producing the greatest field strength. The information, relating to the 
base station producing the greatest field strength thus determined by a 
search, is forwarded to the control apparatus 14 of the switching station 
13 via the base station 12; and the control apparatus 14 switches, via the 
switching station 13, the frequency used on a communication channel 
between the base station 12 and the mobile terminal 11 to the frequency, 
used in communication with the base station, which produces the greatest 
field strength. Such switching control of the frequency is performed in 
the conventional system, too. 
In the digital telephone system of this embodiment, the field strength 
measuring device 11a monitors whether or not the field strength of a 
transmission from the base station 12 using the first sector frequency 
f.sub.A exceeds a predetermined threshold level TH.sub.1, and, when it is 
found that the above strength exceeds the above level, sends a signal c 
indicating that fact to the control apparatus 15 of the switching station 
13 via the base station 12. The predetermined threshold level TH.sub.1 is 
preset to correspond to the boundary of the overlay region SC. That is, 
when the mobile terminal 11 moves into the overlay region OL within the 
sector SC as it approaches the base station 12, the field strength of the 
signal received by the terminal 11 and having the sector frequency f.sub.A 
exceeds the threshold level TH.sub.1, thus making it possible to determine 
whether or not the mobile terminal 11 is in operation inside the overlay 
region OL, by measuring field strength of the received signal having the 
sector frequency f.sub.A. 
In the system of FIG. 4, the control apparatus contains a control unit 15a, 
and the control unit 15a sends, in response to the signal c, a control 
signal d to the base station 12, by which signal d the transmission 
frequency and the reception frequency of the base station 12 are switched 
from the first frequency f.sub.A and the second frequency f.sub.B 
respectively, to the transmission frequency f.sub.A ' and the reception 
frequency f.sub.B ', respectively, for the overlay region. The frequencies 
f.sub.A ' and f.sub.B ' are called overlay frequencies hereinafter in this 
specification. 
FIG. 5 is a block diagram showing the concrete configuration of the mobile 
terminal 11 of FIG. 4. 
Referring to FIG. 5, the mobile terminal 11 comprises an antenna 101 and an 
antenna sharing unit 102 for sharing the antenna in the transmission 
system and reception system. First, the reception system will be 
described. A transmission at the sector frequency f.sub.A from the base 
station 12 received by the antenna 101 is sent from the antenna sharing 
unit 102 to a RF amplifier 103 and amplified thereby, after which the same 
signal is sent to a receiving unit 104 and converted to an IF signal. As 
in the ordinary heterodyne detection, the receiving unit 104 is supplied, 
by a local oscillator 113, with a local signal preset to selectively 
receive the frequency f.sub.A. The IF signal thus obtained is converted to 
a base band signal by a decoder 105, and is sent to a time division 
multiplexing access unit 106. The unit 106 extracts, under the control of 
a processor 12 and by means of time division multiplexing, a signal on a 
communication channel whose destination is the mobile terminal 11, for 
example, on the channel TCH.sub.1, sends the extracted signal to a CODEC 
107, and subjects the signal to D/A conversion in order to retrieve a 
speech signal. 
The transmission system shares the CODEC 107 and the time division 
multiplexing access unit 106 with the reception system, in which 
transmission system the CODEC 107 generates a digital speech signal by 
subjecting an input speech signal to A/D conversion, and the time division 
multiplexing access unit 106 generates a base band signal by subjecting 
such digital speech signal to time division multiplexing. The IF signal is 
modulated in accordance with the base band signal thus generated by means 
of a coder 108, and the modulated IF signal is converted to a transmitted 
signal having the sector frequency f.sub.B by a transmitting unit 109, and 
this signal is then sent to the antenna via the antenna sharing unit 102 
after being amplified by a power amplifier 110, and is transmitted to the 
base station 12. The local oscillator 113 supplies, to the transmitting 
unit, a local signal having a frequency preset such that the frequency of 
the transmitted signal may be f.sub.B. 
The mobile terminal 11 of FIG. 5 comprises a level detecting unit 111 for 
detecting the level of a received signal in collaboration with the RF 
amplifier 103 or the receiving unit 104, and the level detecting unit 111 
sends an output signal indicative of the detected level to the processor 
112. The level detecting unit 111 detects the field strength of a received 
signal, by means of, for example, an AGC signal. The processor 112 
compares the output signal from the unit 111 with a signal having a 
predetermined threshold level corresponding to the threshold level 
TH.sub.1, and, when the output signal exceeds the predetermined threshold 
value, in other words, when the field strength level of the received 
signal exceeds threshold value TH.sub.1, controls the time division 
multiplexing unit 106 and consigns, to the control channel CCH, the 
information indicating that the mobile terminal 11 has moved into the 
overlay region OL of the base station 12. Such information is consigned to 
a B channel constituting a part of the control channel CCH, as a single or 
multiple-bit binary code. 
FIG. 6 illustrates the configuration of the base station 12 and the 
switching station 13 shown in FIG. 4. 
Referring to FIG. 6, the base station 12 comprises: an amplifying portion 
AMP connected to an antenna ANT; a coding/decoding portion MDE connected 
to the amplifier AMP; and a multiplexing interface portion MUX connected 
to the coding/decoding portion MDE and serving as an interface between: a 
multiplexed line linking the base station 12 and the switching station 13; 
and the base station. The amplifying portion AMP comprises: an antenna 
sharing unit 201 physically connected to the antenna; a transmission power 
amplifier 202 connected to the antenna via the unit 201 and generating a 
large-power transmitted signal having the frequency f.sub.A by being 
supplied with a signal having the sector frequency f.sub.A and amplifying 
the same signal; low-noise high-frequency amplifier 203 connected to the 
antenna also via the unit 201, amplifying a radio signal having the sector 
frequency f.sub.B received by the antenna, and raising the S/N ratio 
thereof. The coding/decoding portion MDE comprises: a transmitting unit 
204 being supplied with an intermediate frequency signal and converting 
the same to the radio signal having the frequency f.sub.A transmitted via 
the power amplifier 202; a receiving unit 205 being supplied with an 
output signal having the frequency f.sub.B from the low-noise 
high-frequency amplifier 203 and converting the same to an intermediate 
frequency signal; a modulator 206 being supplied with a digital base band 
signal which has undergone time division multiplexing and generating, by 
coding the same, an intermediate frequency signal transmitted by the 
transmitting unit 204; a demodulator 207 being supplied with the 
intermediate frequency signal output from the receiving unit 205 and, by 
decoding the same, generating the digital base band signal which has 
undergone time division multiplexing; and a time division multiplexing 
access unit 208 being supplied with the digital base band signal by the 
multiplexing interface portion MUX, subjecting the same to time division 
multiplexing, generating the time division multiplexed digital base band 
signal, supplying the same to the modulator 206, and reproducing the 
digital base band signal by subjecting the output signal from the 
demodulator 207 to time division multiplexing. The time division 
multiplexing access unit 208 performs time division multiplexing as shown 
in FIG. 1. 
The coding/decoding portion MDE further comprises: a local oscillator 209 
supplying a local signal having respective frequencies to both the 
transmitting unit 204 and the receiving unit 205 and controlling the 
operating frequencies f.sub.A and f.sub.B ; and a processor 210 for 
controlling the operating frequencies of the local oscillator 209. The 
time division multiplexing access unit 208 extracts the control signal 
transmitted from the switching station 13 via the multiplexing interface 
portion MUX, which control signal is derived from a time division 
multiplexing on the basis of the digital base band signal, and supplies 
the extracted signal to the processor 210. In other words, the operating 
frequencies of the transmitting unit 204 and the receiving unit 205 are 
switched, in accordance with the control signal sent from the switching 
station 13 via the multiplexing interface portion MUX, between a first 
pair (f.sub.A, f.sub.B) corresponding to the sector frequencies and a 
second pair (f.sub.A ', f.sub.B ') corresponding to the overlay 
frequencies. 
The multiplexing interface portion MUX comprises: a first interface unit 
211 connected to the time division multiplexing access unit 208; a 
transmission speed conversion unit 212 connected to the time division 
multiplexing access unit 208 via the first interface unit 211 and 
effecting transmission speed conversion; and a second interface unit 213 
connected to transmission channel 214 and serving as an interface between 
the unit 212 and the transmission channel 214. 
A description will now be given of the configuration of the switching 
station 13. 
Referring to FIG. 6 again, the switching station 13 comprises: the 
switching apparatus 14 described with reference to FIG. 4 and connected to 
the public telephone line; and the control apparatus 15 for controlling 
the switching apparatus 14. The switching apparatus 14 connects 
communication channels, for example, TCH.sub.1 or TCH.sub.2, which are 
contained in the signal transmitted along the line linking the switching 
apparatus and the mobile terminal 11, to the public telephone line under 
the control of the control apparatus 15. The control apparatus 15 
comprises the control unit 15a for effecting the above control, and the 
control unit 15a controls the exchange apparatus 14 via the interface unit 
15b. Thus, connection is established between a terminal on the public 
telephone line and the predetermined mobile terminal 11. 
The system of FIG. 6 further comprises another interface unit 15c that 
interfaces between the switching apparatus 14 and the control unit 15a, 
and the interface unit 15c extracts, from the signal transmitted along the 
control channel CCH described with reference to FIG. 1(B), the information 
indicating that it is found, through the monitoring by the mobile terminal 
11, that the field strength of the transmission from the base station 12 
at the sector frequency f.sub.A exceeds the predetermined threshold level, 
and forwards that information to the control unit 15a. The control unit 
15a sends, in response to such information relating to the field strength, 
an instruction, by which the transmission frequency f.sub.A and the 
reception frequency f.sub.B of the base station are switched to the 
overlay frequencies f.sub.A ', f.sub.B ', to the switching apparatus 14 
via the interface unit 15c. The switching apparatus 14 forwards that 
instruction to the base station 12 via the transmission channel 214. The 
base station 12 extracts the instruction by means of the time division 
multiplexing access unit 208, and sends it to the processor 210, the 
processor 210 then executing a change to the desired frequency by 
controlling the local oscillator 209. 
Upon this frequency change, the base station 12 sends an instruction to the 
mobile terminal via the control channel, by which instruction the 
reception frequency of the mobile terminal 11 is switched from the sector 
frequency f.sub.A to the overlay frequency f.sub.A ', and the transmission 
frequency thereof is switched from the sector frequency f.sub.B to the 
overlay frequency f.sub.B '. The mobile terminal 11 extracts that 
instruction from the control channel CCH by means of the time division 
multiplexing unit 106, and forwards the instruction to the processor 112. 
The processor 112 controls the local oscillator 113 and changes the 
frequency of the local signal supplied to the receiving unit 104 and the 
transmitting unit 109, such that the frequency received by the receiving 
unit 104 becomes f.sub.A ' and the frequency transmitted by the 
transmitting unit 109 becomes f.sub.B '. 
A summarized description of the switching control of the frequencies in the 
system of the present invention will be given with reference to the flow 
chart of FIG. 7. 
Referring to FIG. 7, in step S1, the mobile terminal 11 monitors, by means 
of the processor 112, whether or not the level of the received signal 
having the sector frequency f.sub.A, which signal is detected by the level 
detecting unit 111, exceeds the predetermined threshold level 
corresponding to the threshold field strength level TH.sub.1 described 
earlier. When it is found that the above level exceeds the above threshold 
level, step S2 is executed, and the base station 12 is notified of the 
fact that the received signal level has exceeded the predetermined 
threshold level, via the control channel CCH contained in a signal having 
the frequency f.sub.B. More specifically, the processor 112 of the mobile 
terminal 11 outputs, to the time division multiplexing access unit 106, 
the information indicating that the level of the received signal having 
the sector frequency f.sub.A has exceeded the predetermined threshold 
level. The unit 106 consigns that information to the control channel 
contained in the signal having the sector frequency f.sub.B. The base 
station 12 receives, in step S3, the information thus sent, and passes the 
same to the control unit 15a in the switching station 13 via the 
transmission channel 214 and the interface unit 15c. The control unit 15a, 
receiving same information in step S4, issues, in step S5, an instruction 
by which the transmission frequency is switched from the sector frequency 
f.sub.A to the overlay frequency f.sub.A ', and the reception frequency is 
switched from the sector frequency f.sub.B to the overlay frequency 
f.sub.B ', and transmits this instruction to the base station 12 via the 
interface unit 15c and the transmission channel 214. Moreover, in step S6, 
the base station 12 sends an instruction to the mobile terminal 11 via the 
control channel CCH, by which instruction the reception frequency of the 
mobile terminal is switched from the sector frequency f.sub.A to the 
overlay frequency f.sub.A ', and the transmission frequency thereof is 
switched from the sector frequency f.sub.B to the overlay frequency 
f.sub.B '. The mobile terminal 11 receives, in step S7, this instruction 
via the time division multiplexing access unit 106, and switches its 
transmission frequency and the reception frequency from the sector 
frequencies to the overlay frequencies. 
FIG. 8 represents the frequency control effected, when the mobile terminal 
11 moves out of the overlay region OL, in the first embodiment of the 
present invention, which embodiment has the configuration shown in FIGS. 5 
through 7. 
Referring to FIG. 8, in step S1' corresponding to step S1 of FIG. 7, the 
processor 112 of the mobile terminal 11 monitors the field strength of the 
signal having the overlay frequency f.sub.A ' and transmitted from the 
base station 12. When it is detected that the field strength of the 
transmitted signal drops below the level corresponding to the threshold 
field strength level TH.sub.2 characterizing the boundary of the overlay 
region, the processor 112 consigns, in step S2', the information 
indicative of that fact to the control channel contained in the 
transmitted signal having the frequency f.sub.B ', via the time division 
multiplexing access unit 106 so that the base station 12 may be notified. 
Since the overlay region is defined in accordance with transmission of a 
low-power radio signal having the frequency f.sub.A ', the threshold field 
strength level TH.sub.2 is generally lower than the threshold field 
strength level TH.sub.1 of the transmitted signal having the sector 
frequency f.sub.A. 
In step S3' corresponding to step S3 of FIG. 7, the base station 12 
forwards the information sent from the mobile terminal 11 to the switching 
station 13 via the transmission channel 214. The switching station 13 
receives the information in step S4' corresponding to step S4 and sends 
the same to the control unit 15a via the interface unit 15c, the control 
unit 15a then issuing an instruction by which the transmission frequency 
of the base station 12 is switched from f.sub.A ' to the vacant frequency 
f.sub.A and the reception frequency of the base station 12 is switched 
from f.sub.B ' to the vacant frequency f.sub.B. This instruction is 
returned, in step S6' corresponding to step S6, from the switching station 
13 to the base station 12 via the transmission channel 214, whereupon the 
base station 12 switches its transmission frequency from f.sub.A ' to 
f.sub.A and switches its reception frequency from f.sub.B ' to f.sub.B. At 
the same time as this, the base station 12 sends an instruction to the 
mobile terminal 11 via the control channel, by which instruction the 
reception frequency is switched from f.sub.A ' to f.sub.A and the 
transmission frequency is switched from f.sub.B ' to f.sub.B. The 
processor 112 in the mobile terminal 11 extracts, in step S7' 
corresponding to S7, this instruction via the time division multiplexing 
access unit 106, and executes the instructed frequency change. 
A description will next be given of the second embodiment of the present 
invention. 
FIG. 9 shows the configuration of a mobile terminal 21 used in the second 
embodiment of the present invention. 
Referring to FIG. 9, the mobile terminal 21 is provided, aside from the 
receiving unit 104, with a receiving unit 104' which unit is supplied with 
an output signal from the low-noise amplifier 103 and converts the same 
signal to an intermediate frequency signal. The receiving unit 104' is 
supplied, by the local oscillator 113, with a local signal having a 
frequency different from that of the local signal supplied to the 
receiving unit 104. As a result of this, the receiving unit 104' is tuned 
to a frequency different from that of the receiving unit 104. Further, the 
frequency of the local signal supplied to the receiving unit 104' is 
controlled by the processor 112. In this embodiment, the level of the 
signal transmitted from the base station 12 is detected by using an output 
signal from the receiving unit 104'. It is also to be noted that, in this 
embodiment, an identifying signal SIG1, indicating that the base station 
12 has an overlay region OL, is added to the signal transmitted from the 
base station 12 at the frequency f.sub.A ; and a second identifying signal 
SIG2 is added to the signal transmitted from the base station 12 at the 
frequency f.sub.A ', to indicate that the transmission covers the overlay 
region OL. 
FIG. 10 shows the frequency switching control according to this embodiment. 
Referring to the flow chart of FIG. 10, in step S20, the base station adds 
the identifying signal SIG1, indicating that the base station 12 has an 
overlay region, to its transmission signal having the frequency f.sub.A, 
adding also the identifying signal SIG2, indicating that the signal 
transmitted covers the overlay region, to the transmission signal having 
the frequency f.sub.A ' and covering the overlay region. These identifying 
signals SIG1, SIG2 are consigned to the control channels carried by 
respective frequencies, via, for example, the processor 210 or the time 
division multiplexing access unit 208 of the base station 12. 
In step S21, the mobile terminal 11 allows, upon entering into 
communication with the base station 12 at the frequencies f.sub.A and 
f.sub.B, the processor 112 to determine whether or not the identifying 
signal SIG1 is included in the signal transmitted from the base station 12 
at the frequency f.sub.A. If it is found that the identifying signal SIG1 
is included in the signal transmitted at the frequency f.sub.A, the mobile 
terminal 11 monitors, in step S21, the field strength of the transmission 
from the base station 12 to the overlay region at the frequency f.sub.A ' 
as received by the receiving unit 104', while at the same time maintaining 
the communication at the sector frequency f.sub.A, f.sub.B. When it is 
found that the level of the received signal exceeds a predetermined level 
corresponding to the threshold field strength level TH.sub.2, the 
processor 112 of the mobile terminal 11 consigns, via the time division 
multiplexing unit 106, the information indicative of that fact to the 
control channel contained in the signal transmitted at the frequency 
f.sub.B from the mobile terminal 11 to the base station 12. 
In response to the transmission from the mobile terminal 11 in step S23, 
the base station 12 executes step S24 corresponding to step S4 of FIG. 7, 
and forwards the information to the switching station 13. The switching 
station 13, upon receipt of this information in step S25, forwards the 
same to the control unit 15a of the switching apparatus 14 via the 
interface 15c. In step S26, the control unit 15a issues an instruction by 
which the transmission frequency of the base station 12 is switched from 
f.sub.A to f.sub.A ' and the reception frequency thereof is switched from 
f.sub.B to f.sub.B ', and forwards the instruction to the base station 12 
via the interface unit 15c and the transmission channel 214. In accordance 
with this instruction, the base station 12 switches, in step S27, the 
transmission frequency from f.sub.A to f.sub.A ' and switches the 
reception frequency from f.sub.B to f.sub.B '. Then, in step S28, the 
mobile terminal 11 switches, in accordance with the instruction sent via 
the base station, the reception frequency from f.sub.A to f.sub.A ' and 
switches the transmission frequency from f.sub.B to f.sub.B '. 
FIG. 11 shows the switching control of the frequency used in communication 
between the base station 12 and the mobile terminal 11 effected in this 
embodiment when the mobile terminal 11 moves out of the overlay region OL. 
Referring to FIG. 11, the base station 12 adds, in step S30, as in step 
S20, the identifying signal SIG1 to the signal transmitted at the sector 
frequency f.sub.A as well as adding the identifying signal SIG2 to the 
signal transmitted at the overlay frequency f.sub.A '. In step 32, the 
mobile terminal 11 monitors the field strength of the transmission from 
the base station at the sector frequency f.sub.A while communicating with 
the base station 12 at the overlay frequencies f.sub.A ', f.sub.B ' in 
step 31 corresponding to step 21. It is generally practiced that the 
transmission from the base station covering the sector SC shown in FIG. 4 
uses not only the frequency f.sub.A alone but a plurality of frequencies, 
and the level is monitored for each of these frequencies in step S32. 
When the level of the highest-level signal transmitted from the base 
station 12, which level is monitored in step S32, drops below the level 
corresponding to the threshold field strength level TH.sub.1, the mobile 
terminal 11 sends, in step S38, the information indicating that fact to 
the base station 12 via the control channel carried by the transmission 
frequency f.sub.B '; and the base station 12 forwards, in step S34, the 
same information to the switching station 13 via the transmission channel 
214. After receiving the information from the base station 12 in step S35, 
the switching station 13 forwards the same to the control unit 15a via the 
interface 15c; the control unit 15a issues an instruction by which the 
frequency used in transmission from the base station 12 is switched from 
the overlay frequency f.sub.A ' to the sector frequency f.sub.A and the 
frequency used in reception by the base station 12 is switched from the 
overlay frequency f.sub.B ' to the sector frequency f.sub.B ', and sends 
the instruction to the base station 12, as in the aforementioned 
embodiment. The base station 12 switches, in step S37, the frequencies in 
accordance with this instruction; and, in step S38, the mobile terminal 11 
switches, in correspondence to the frequency switching in the base station 
12, the reception frequency from the overlay frequency f.sub.A ' to the 
sector frequency f.sub.A and switches the transmission frequency from the 
overlay frequency f.sub.B ' to the sector frequency f.sub.B. 
A description will now be given of the third embodiment of the present 
invention, with reference to FIGS. 12 and 13. In this embodiment of the 
present invention, the monitoring of the level of the signal transmitted 
from the base station 12 and received by the mobile terminal 11 is 
effected not by the processor 112 of the mobile terminal 11 but by the 
control unit 15a of the switching station 13. 
FIG. 12 shows the frequency switching operation effected when the mobile 
terminal 11, in communication with the base station 12 at the sector 
frequencies f.sub.A, f.sub.B, moves into the overlay region OL. In step 
41, the mobile terminal 11 monitors the reception level at all the 
reception frequencies f.sub.A, f.sub.A '. Specifically, when there are 
actually a plurality of frequencies being used for each of the reception 
frequencies f.sub.A, f.sub.A ' above, the monitoring is performed for all 
of them. The mobile terminal 11 sends the level data, indicating the 
reception level and obtained as a result of the monitoring, to the base 
station 12 via the control channel provided in the signal transmitted at 
the frequency f.sub.B ; and the base station 12 forwards the data to the 
switching station 13 in step S42. The switching station 13 forwards the 
level data sent from the base station 12 to the control unit 15a via the 
interface unit 15c; and the control unit 15a determines, in step S43, 
whether or not the transmission at the overlay frequency f.sub.A ' exceeds 
a predetermined threshold level corresponding to the threshold field 
strength level TH.sub.2. If an affirmative answer results from the step 
S43 determination, the control unit 15a issues, in step S44, an 
instruction by which the transmission frequency of the base station 12 is 
switched from the sector frequency f.sub.A to the overlay frequency 
f.sub.A ' and the reception frequency thereof is switched from the sector 
frequency f.sub.B to the overlay frequency f.sub.B ', and sends the 
instruction to the switching apparatus 14 via the interface unit 15c. The 
switching apparatus 14 forwards the same to the base station 12 via the 
transmission channel 214. The base station 12 executes, in step S45, the 
frequency switching from the sector frequency to the overlay frequency in 
accordance with the instruction from the switching station 13, as well as 
giving an instruction relating to the frequency switching in the mobile 
terminal 11 via the control channel carried by the sector frequency 
f.sub.A. In response to this, the mobile terminal 11 switches, in step 
S46, its transmission frequency from the sector frequency f.sub.B to the 
overlay frequency f.sub.B ' and switches the reception frequency from the 
sector frequency f.sub.A to the overlay frequency f.sub.A '. 
When the mobile terminal 11 moves out of the overlay region OL, the mobile 
terminal 11 measures, in step S51, the level of all the received signals, 
and sends the data indicative of the result of the measurement to the base 
station 12 via the control channel provided in the signal transmitted at 
the overlay frequency f.sub.B '. The base station 12 forwards, in step 
S52, the same data to the switching station 13 via the transmission 
channel 214; the data thus transmitted is forwarded to the control unit 
15a of the control apparatus 15 via the interface unit 15c. 
In step S53, the control unit 15a monitors whether or not the level of the 
signal, transmitted from the base station 12 at the frequency f.sub.A, is 
below the predetermined threshold level corresponding to the threshold 
field strength level TH.sub.1 of the overlay region OL, and, when it is 
found that the above signal level is below the threshold level, issues, in 
step S54, an instruction by which the transmission signal of the base 
station 12 is switched from the overlay frequency f.sub.A ' to the sector 
frequency f.sub.A and the reception signal thereof is switched from the 
overlay frequency f.sub.B ' to the sector frequency f.sub.B, and forwards 
the instruction to the switching apparatus 14 via the interface unit 15c. 
The switching apparatus 14 transmits the instruction thus forwarded to the 
base station 12 via the transmission channel 214; and the base station 12 
executes, in step S55, the frequency switching in accordance with the 
above instruction. Further, the base station 12 transmits an instruction, 
by which instruction the transmission frequency of the mobile terminal 11 
is switched from f.sub.B ' to f.sub.B and the reception frequency thereof 
is switched from the overlay frequency f.sub.A ' to the sector frequency 
f.sub.A, to the mobile terminal 11 by consigning the same instruction to 
the control channel carried by the transmission frequency f.sub.A '. In 
response to this, the mobile terminal 11 executes, in step S56, the 
corresponding frequency switching. As described in the earlier embodiment, 
there are generally a plurality of frequencies used as each of the sector 
frequencies f.sub.A, f.sub.B. It is determined, in step S53, whether or 
not the level of the transmitted signal producing the maximum level is 
below the predetermined threshold level. In step S54, the control unit 15a 
allocates the vacant sector frequency. 
A description will now be given of the fourth embodiment of the present 
invention, with reference to the block diagram of FIG. 14 and the flow 
charts of FIGS. 15 and 16. 
Referring to FIG. 14, this diagram shows the configuration of the base 
station 12' and the switching station 13' used in this embodiment. In the 
figure, parts that are common to FIG. 6 are given the same reference 
numerals, and the descriptions thereof are omitted. 
In this embodiment, a control unit 15a' is provided to correspond to the 
control unit 15a of FIG. 6 and is connected to the time division 
multiplexing access unit 208 via an interface unit 15c'. The interface 
unit 15c' extracts a B channel signal from the control channel as does the 
unit 15c, and forwards the extracted signal to the control unit 15a' as 
well as forwarding the output signal from the control unit 15a' to the 
time division multiplexing access unit 208 using the B channel. 
FIG. 15 shows the frequency switching control of the mobile terminal 11 and 
the base station 12, which control is effected when the mobile terminal 11 
moves into the overlay region. 
Referring to FIG. 15, in step S61, the mobile terminal 11 is communicating 
with the base station 12 at the sector frequency, monitoring the level of 
transmission from the base station 12 at all the sector frequencies 
f.sub.A and the overlay frequencies f.sub.A '. The mobile terminal 11 
forwards the monitored data thus obtained to the base station 12 via the 
control channel carried by the frequency f.sub.B ; and the base station 12 
sends the monitored data to the control unit 15a' via the interface unit 
15c'. In step S62, the control unit 15a' of the base station 12 determines 
whether or not the level of the signal having the overlay frequency 
f.sub.A ' exceeds the threshold field strength level TH.sub.2. If an 
affirmative answer results from this determination, the control unit 
transmits, in step S63, the information indicating the fact to the 
switching station 13. The control unit 15a of the switching station 13 
issues, in step S64, an instruction by which the frequency used in 
communication between the base station 12 and the mobile terminal 11 is 
switched from the sector frequencies to the overlay frequencies, and this 
instruction is forwarded to the base station 12 via the transmission 
channel 214. In step S65, the base station 12 switches, in accordance with 
the instruction from the switching station 13, the transmission frequency 
and the reception frequency from the sector frequencies f.sub.A, f.sub.B 
to the overlay frequencies f.sub.A ', f.sub.B ', respectively. In 
correspondence to this, the mobile terminal 11 switches, in step 66, the 
reception frequency and the transmission frequency from the sector 
frequencies f.sub.A, f.sub.B to the overlay frequencies f.sub.A ', f.sub.B 
'. 
A description will now be given, with reference to FIG. 16, of the 
frequency switching control of the mobile terminal 11 and the base station 
12, which control is effected when the mobile terminal 11 moves out of the 
overlay region OL. 
Referring to FIG. 16, the mobile terminal 11 is communicating with the base 
station 12 at the overlay frequency in step 71, while at the same time 
monitoring the level of the signal transmitted from the base station 12, 
at the sector frequency f.sub.A and the overlay frequency f.sub.A '. 
Further, the mobile terminal 11 transmits the monitored data indicative of 
the result of monitoring to the base station 12 via the control channel 
carried by the frequency f.sub.B '. The base station 12 extracts the 
monitored data transmitted from the mobile terminal 11, via the time 
division multiplexing access unit 208, and forwards the extracted data to 
the control unit 15a'. The control unit 15a' monitors, in step S72, 
whether or not the level of the signal in the monitored data, transmitted 
at the sector frequency f.sub.A, is below the predetermined threshold 
level, and when the above signal level is below the threshold level, 
sends, in step S73, the switching station 13 the information indicative of 
that fact. In step S74, the control unit 15a of the switching station 13 
issues, in accordance with the information sent from the base station 12, 
an instruction by which the frequency used in communication between the 
base station 12 and the mobile terminal 11 is switched from the overlay 
frequency to the sector frequency, and forwards the instruction to the 
base station 12. In response to this, the base station 12 switches the 
transmission frequency from the overlay frequency f.sub.A ' to the sector 
frequency f.sub.A and switches the reception frequency from the overlay 
frequency f.sub.B ' to the sector frequency f.sub.B. 
Possible Industrial Application 
The present invention enables a stable frequency switching control in a 
digital mobile telephone system in which an overlay region is formed 
within a sector to correspond to a base station, and the sector and the 
overlay region are covered by signals transmitted at different 
frequencies, the stable control being enabled because the frequency used 
in communication between a mobile terminal and the base station is 
switched in accordance with the field strength level of the signal 
transmitted from the base station and received by the mobile terminal, 
when the mobile terminal moves from the sector into the overlay region or 
when the mobile terminal moves from the overlay region into the sector. 
Thus, it is possible to smoothly operate the digital mobile telephone 
system having an overlay configuration, and to greatly improve efficiency 
in the use of frequencies thereof.