Radio communication system and method for avoiding control channel interference

A radio communication system including a plurality of base stations which communicate with a plurality of radio terminals respectively is provided with a capability of avoiding occurrence of radio interference of control channel signals among a plurality of base stations even when a plurality of the base stations are located in the close vicinity of each other. The base station of the radio communication system is provided with a device for receiving, when the power is turned on, a control channel signal which is being transmitted at a certain interval by each of the other base stations in the system, a device for frame synchronizing each of TDMA frames of the control channel signal which is received, a device for extracting base station identification information of each of the other base stations from the TDMA frames which are frame synchronized, and a device for storing in a memory each of the base station identification information which is extracted. The base station further includes a device for determining a cycle of transmitting a superframe signal in accordance with the number of the other base stations using the control channel in the system, which is determined based upon the base station identification information stored in the memory.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a radio communication system, and more 
particularly to a radio communication system which includes a plurality of 
base stations which communicate with a plurality of radio terminals 
respectively over radio waves. 
Examples of such a radio communication system include a digital cordless 
telephone system, a wide area digital cordless telephone system which is 
called a personal handy phone system (PHS) in Japan and a personal 
communications services system (PCS) in the United States, a cellular 
phone system, a wireless printer system, and a wireless local area network 
system. 
2. Discussion of the Background 
Radio communications systems are increasingly used for wireless mobile 
communications. 
An example of a radio communication system is a cordless telephone system 
which is used by individuals in home or office. A cordless telephone 
system includes at least one telephone base set which communicates with a 
radio mobile telephone set over a limited distance, such as several tens 
of meters, for example, by use of radio waves. The base station is 
electrically connected to a conventional public or private wire telephone 
network. The base station allows the user of the mobile telephone set to 
access the wire network while moving from one place to another or from a 
place distant from the base station in home or office as long as the 
mobile telephone set is within an area approximately several tens of 
meters from the base station. 
In a cordless telephone system, a specific range of frequencies is assigned 
to each system and a mobile telephone set in a system can only communicate 
with a base station in the same system utilizing the same specified range 
of frequencies. 
As understood by those skilled in the art, because only a limited number of 
frequencies are available for radio communication, the same range of 
frequencies must be assigned to and used by a plurality of cordless 
telephone systems. If two systems having the same range of frequencies are 
placed and used in close proximity to each other, such as in an area 
within several tens of meters of each other, a radio wave from one base 
telephone set may interfere with a radio wave from the other base 
telephone set. As a result, various problems may occur when receiving and 
sending calls with each of the mobile telephone sets. For example, an 
incoming call arriving at base telephone set of one of the systems, 
besides reaching a mobile telephone set within its system, may also reach 
a mobile telephone set of another system. 
Therefore, when one system has a same range of frequencies as another 
system, the two systems need to be placed in sufficiently separated areas 
for avoiding such an interference problem. 
Recently, radio communication systems have been proposed which allow the 
user of a mobile telephone set of a cordless telephone system to access a 
wire network not only in home or office but also in public places, outside 
of the home and office, such as on streets in business and shopping 
quarters. 
Such wide area radio communication systems include a plurality of base 
stations. Each of the plurality of base stations is capable of 
communicating with a plurality of mobile radio terminals over a limited 
distance. 
Generally a certain spectrum of frequencies is allocated for use in a wide 
area radio communication system. Because of the limited number of radio 
frequencies which are available in the allocated spectrum, the same 
plurality of ranges of frequencies within the allocated spectrum are 
allocated to and used by each of a plurality of base stations, and each of 
the plurality of base stations is placed in an area separated from each 
other, thus covering a wide area. 
An example of such a wide area radio communication system is a system 
called DECT (Digital European Cordless Communications) in Europe. Another 
example is a personal handy phone system called PHS in Japan. Similar 
systems have been proposed in the United States and are referred to as 
personal communications services (PCS) systems. 
Recent wide-area radio communication systems, such as the personal handy 
phone system (PHS) and the personal communications services (PCS) systems, 
provide mobile radio voice, data, video and/or multimedia communications 
using mobile radio terminals, which include a radio telephone, such as a 
cellular telephone, and other components for voice, data, video and/or 
multimedia communications. 
In the personal handy phone system (PHS), as an example of such a wide area 
radio communication system including a plurality of base stations which 
communicate with a plurality of mobile radio terminals, each of the base 
stations is electrically connected to conventional public and private wire 
telephone networks and is capable of simultaneously communicating with a 
plurality of mobile radio terminals over a limited distance, such as for 
example, several tens of meters. In the PHS system, the base station is 
called a cell station and the mobile radio terminal is called a personal 
station, and are hereinafter called as such when referring to the PHS 
system. 
For accomplishing multiple radio access and transmission between a cell 
station and a plurality of personal stations, a Time-Division Multiple 
Access (TDMA) architecture and a Time Division Duplex (TDD) architecture 
are used in the PHS system. 
A digital signal carried by a radio wave which is radiated by each of the 
cell stations and the personal stations is divided into 5 ms time 
segments, and each segment is defined as a TDMA frame. 
Each frame is further divided into 8 time slots. Accordingly, the time 
allocated for each slot is 625 .mu.s. Of the 8 slots, four slots are 
assigned for downlink (transmission from a cell station to a personal 
station) channels and four slots are assigned for uplink (transmission 
from a personal station to a cell station) channels. 
One slot of each four slots is used as a slot for setting function channels 
for transmitting control information necessary for controlling the 
connection between the cell station and the personal station and is called 
a control channel. Function channels for transmitting control information 
include, for example, a broadcast control channel (BCCH) for broadcasting 
control information, and a common control channel (CCCH) for transmitting 
control information necessary for call connection between the cell station 
and the personal station and so forth. The control channel is commonly 
used by each of the personal stations. 
The other three slots each are used as slots for communication and are 
called communication channels. Each of the communication channels is 
allocated to and used by an individual user (a personal station). 
The cell station broadcasts to all personal stations, with the broadcast 
control channel (BCCH), control information related to a channel structure 
and system information such as information regarding the slots which are 
available for transmitting control information for obtaining a 
communication channel. A personal station receives such information and 
transmits, for example, information for informing its current location to 
the system which is necessary for receiving a call addressed to the 
personal station. 
In the PHS system, there are provided three communication protocol phases, 
a phase for establishing the radio interface handshake, which is called 
link channel establishment phase, a phase for connecting a call between 
the cell station and the personal station which established the handshake, 
which is called a service channel establishment phase and a phase for 
performing communication and data transmission, which is called a 
communication phase. 
The link channel establishment phase is defined as the stage for using 
control channel functions to select a channel (link channel) with the 
quality and capacity required for each call connection. 
Function channels used in the link establishment phase are called a logical 
control channel (LCCH). As downlink logical control (LCCH) elements, there 
are a broadcast control channel (BCCH), a paging channel (PCH) which is a 
one-way point-multipoint channel that simultaneously transmits identical 
information to individual cell stations and a signaling control channel 
(SCCH) which is a bi-directional point-point channel that transmits 
information needed for call connection between the cell station and a 
personal station. 
Each of the cell stations intermittently transmits the control channel 
signal which indicates a structure of the logical control channel (LCCH) 
transmitted by the cell station and the positions of the control channel 
with which transmission is possible for each of the personal stations. 
The minimum cycle of the downlink logical control channel (LCCH) specifying 
the slot position of all logical control channel (LCCH) elements is called 
a LCCH superframe. 
In a radio communication system including a plurality of base stations 
which communicate with a plurality of radio terminals, such as the 
above-described PHS system, when a base station is located in the vicinity 
of another base station and both base stations broadcast a control channel 
signal at the same time, for example, a problem occurs because the 
broadcasting radio waves interfere with each other, since a same range of 
frequencies is used by each of the base stations. If such interference 
occurs, radio terminals can not receive the broadcasting radio wave and 
consequently the radio terminals cannot communicate with the base station. 
As a result, a call addressed to a radio terminal and arriving from a 
corresponding base station may not be received by the radio terminal or a 
call originating from the radio terminal may not reach the base station. 
For avoiding occurrence of such interference of control channel signals 
among a plurality of base stations, a technology exists to randomly change 
a timing at which a control channel signal is broadcast and is disclosed 
in Japanese Patent publication Toku-kai-hei No.7-75166. However, even if 
the timing for broadcasting the control channel signal is changed at 
random, the occurrence of interference of the control channel signals is 
not completely avoided since the changed timing for broadcasting the 
control channel signal might still coincide with the timing for 
broadcasting the control channel signal by one of the other base stations. 
As the number of base stations in the system increases, the amount of 
traffic on the control channel signal increases and consequently the 
frequency of occurrence of interference of the control channel signals 
increases. This may result in a situation in which a base station can not 
broadcast the control channel signal due to busy traffic on the control 
channel signal. 
In an attempt to solve such a problem of interference of the control 
channel signals, a cordless telephone system which is used in an office, 
as an example of a radio communication system including a plurality of 
base stations which communicate with a plurality of radio terminals, can 
provide two control channel signals at different frequencies. Each base 
station is allocated one of the two control channel signals. 
However, in such a system utilizing two control channel signals, as the 
number of base stations increases, a situation may occur in which the 
traffic on at least one of the two control channel signals increases and 
becomes busy while the traffic on the other control channel signal is 
relatively light. This is because each base station is allocated one of 
the two control channel signal frequencies and such allocated control 
channel signal can not be changed. Therefore, even with such a system 
using two control channel signals, interference of control channel signals 
among a plurality of base stations still occurs when the plurality of the 
base stations are located in close proximity to each other. 
SUMMARY OF THE INVENTION 
The present invention has been made in view of such problems and to address 
and solve the problems. 
Accordingly, a first object of the present invention is to provide a novel 
radio communication system including a plurality of base stations which 
communicate with a plurality of radio terminals respectively and a method 
for communication capable of avoiding occurrence of radio interference of 
control channel signals among a plurality of base stations even when a 
plurality of the base stations are located in the close vicinity to each 
other. 
A second object of the present invention is to provide a radio 
communication system, which uses two control channel signals having 
different frequencies, with a capability of avoiding a problem in which 
traffic on one of the two control channel signals becomes concentrated 
while the traffic on the other control channel signal is relatively light. 
In order to achieve the above-mentioned first object, each base station of 
the radio communication system of the present invention includes a device 
for receiving, when the power is turned on, a control channel signal which 
is being transmitted at a certain interval by each of the other base 
stations in the system, a device for frame synchronizing each of the TDMA 
frames of the control channel signals which are received by the control 
channel signal receiving device, a device for extracting base station 
identification information of each of the other base stations from the 
TDMA frames which are frame synchronized by the TDMA frame synchronizing 
device, and a device for storing in a memory each of the base station 
identification information which is extracted by the base station 
identification information extracting device. 
The base station of the radio communication system of the present invention 
further includes a device for determining a cycle of transmitting a 
superframe signal in accordance with the number of base stations using the 
control channel in the system, which is determined based upon the base 
station identification information stored in the memory. 
The base station further includes a device for extracting information of a 
superframe signal transmission cycle of each of the other base stations 
from the synchronized TDMA frames, a device for determining a vacant time 
cycle for transmitting a superframe signal based upon the superframe 
signal transmission cycle information of each of the other base stations, 
which is extracted by the superframe signal transmission cycle information 
extracting device, and a device for transmitting the superframe signal 
based upon the vacant time cycle determined by the vacant time cycle 
determining device. 
Further, the base station may include a device for determining whether 
transmission of a superframe signal is possible or not if the superframe 
signal is transmitted with the vacant time cycle which is determined by 
the vacant time cycle determining device and a device for stopping 
transmission of the superframe signal when the transmission of the 
superframe signal is determined to be impossible by the determining 
device. 
Still further, the base station may include a device for indicating, when 
transmission of the superframe signal is stopped, that the superframe 
signal transmission has been stopped and a device for displaying a cause 
of the stoppage of the superframe signal transmission. 
Furthermore, the base station may include a device for measuring electric 
field intensity of the control channel signal for each of the other base 
stations based upon the synchronized TDMA frames, a device for storing 
information of the electric field intensity of the control channel signal 
of each of the other base stations in the memory associating the electric 
field intensity information with the corresponding base station 
identification information, and a device for displaying the base station 
identification information and the electric field intensity information of 
each of the other base stations which are stored in the memory by the 
storing device. 
Furthermore, the base station may include a device for displaying the 
superframe signal transmission cycle information of each of the other base 
stations, which is extracted by the superframe signal transmission cycle 
information extracting device, and the vacant time cycle in each of the 
superframe signal transmission cycles of the other base stations, and a 
device for designating a timing for transmitting a superframe signal based 
upon the vacant time cycle which is displayed by the displaying device. 
In order to achieve the aforementioned first object, a radio terminal of 
the radio communication system according to the present invention includes 
a device for monitoring, when the power is turned on, a control channel 
signal and receiving a superframe signal which is being transmitted by 
each of the base stations in the system, a device for frame synchronizing 
each of TDMA frames of received superframe signals, a device for 
extracting base station identification information of each of the base 
stations from the synchronized superframe signals, a device for measuring 
electric field intensity of the control channel signal of each of the base 
stations based upon the synchronized superframe signals, a device for 
storing the electric field intensity information in a memory associating 
the electric field intensity information with the corresponding base 
station identification information, a device for displaying the base 
station identification information and the electric field intensity 
information which are stored in the memory by said storing device, a 
device for selecting a base station for connection based upon the electric 
field intensity information displayed by the displaying device, a device 
for connecting only to the base station selected by the base station 
selecting device and receiving a superframe signal therefrom, a device for 
monitoring electric field intensity of the control channel signal of the 
base station being connected, monitoring the control channel signal again 
when the electric field intensity of the control channel signal being 
received reaches a level below a predetermined level, receiving a 
superframe signal being transmitted by each of the other base stations, 
and updating the base station identification information and the electric 
field intensity information stored in the memory based upon the base 
station identification information and the electric field intensity 
information extracted from the received superframe signals, a device for 
displaying a message asking if the base station for connection to be 
changed, a device for changing the base station for connection, and a 
device for connecting to the base station having the control channel 
signal with a strongest electric field intensity, which is determined 
based upon the base station identification information and the electric 
field intensity information updated by the updating device, when the base 
station for connection is not changed by the base station changing device. 
In order to achieve the above-mentioned second object of the present 
invention, the base station of the radio communication system of the 
present invention, when two control channel signals with different 
frequencies are provided in the system, further includes a device for 
receiving, when the power is turned on, one of the two control channel 
signals which is being transmitted at a certain interval by each of the 
other base stations in the system, a device for measuring a traffic of the 
control channel signal which is received, a device for receiving, when the 
traffic of the control channel signal which is received first reaches a 
predetermined level, the other control channel signal of the two control 
channel signals, a device for measuring a traffic of the second control 
channel signal, a device for comparing the traffics of the two control 
channel signals to determine which traffic of the two control channel 
signals is lighter, and a device for transmitting a superframe signal with 
the control channel signal having lighter traffic, of the two control 
channel signals. 
Alternatively, the base station may include, when two control channel 
signals with different frequencies are provided in the system, a device 
for transmitting a superframe signal with one of the two control channel 
signals, a device for measuring a time for transmitting a superframe 
signal when the superframe signal transmission is started with one of the 
two control channel signals, a device for stopping the superframe signal 
transmission with the control channel signal when the superframe signal 
transmitting time reaches a predetermined period of time, a device for 
transmitting the superframe signal with the other control channel signal 
of the two control channel signals when the superframe signal transmission 
with the first control channel signal is stopped, a device for measuring a 
superframe signal transmitting time with the second control channel signal 
when the superframe signal transmission is started with the second control 
channel signal and stopping the superframe signal transmission when the 
superframe signal transmitting time with the second control channel signal 
reaches the predetermined period of time, a device for transmitting the 
superframe signal with the first control channel signal again when the 
superframe signal transmission with the second control channel signal is 
stopped. 
Further alternatively, when two control channel signals are used in the 
system, the base station may include a device for stopping superframe 
signal transmission with one of the two control channel signals when a 
request for connection is received from a terminal station, and a device 
for transmitting the superframe signal, after stopping the superframe 
signal transmission with the first control channel signal, with the second 
control channel signal of the two control channel signals. 
In order to achieve the aforementioned second object of the present 
invention, the terminal station of the radio communication system of the 
present invention, when two control channel signals with different 
frequencies are provided in the system, includes a device for monitoring 
one of the two control channel signals, a device for connecting, when a 
superframe signal is received from a base station with the control channel 
signal which is monitored, to the base station, a device for monitoring 
the other control channel signal of the two control channel signals when a 
superframe signal is not received in a predetermined of time from a base 
station with the control channel signal which is monitored first, a device 
for connecting, when a superframe signal is received from a base station 
with the other control channel signal of the two control channel signals, 
to the base station, and a device for monitoring the control channel 
signal which is monitored first again, when a superframe signal is not 
received with the other control channel signal, in the predetermined 
period of time, from a base station.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts throughout the several views, embodiments 
of the present invention are explained hereinbelow. 
FIG. 1 is a schematic drawing illustrating an example of a structure of a 
radio communication system according to the present invention. 
The radio communication system shown in FIG. 1 employs a TDMA/TDD 
architecture, and includes a plurality of radio mobile terminals (personal 
station PS), which can be carried by individual users for communicating 
with a distant party while moving from one place to another, and a 
plurality of base stations (cell station CS), each of which is connected 
to a wire telephone network and is capable of communicating with a 
plurality of the radio terminals at one time over radio waves when the 
radio terminals are within a range of each of the base stations. 
The radio communication system in this example includes three sub-systems, 
as shown in FIG. 1, a first sub-system including a cell station CS1 and a 
personal station PS1, a second sub-system including a cell station CS2 and 
a personal station PS2 and a third sub-system including a cell station CS3 
and a personal station PS3. 
FIG. 2 is a schematic drawing illustrating an example of a block diagram of 
the cell stations CS1, CS2 and CS3. 
In FIG. 2, an antenna 1 receives radio signals which are transmitted from 
the other stations. A radio wave processor 2 processes the received 
signal. For example, processor 2 down-converts the received signal from 
1.9 GHz to a lower frequency signal. A modem (modulator/demodulator) 3 
demodulates the received signal and sends the demodulated signal to a 
communication controller 4. Communication controller 4 includes a CPU for 
performing various control operations, a ROM for storing various programs, 
such as, for example, a protocol for controlling communication and so 
forth. The controller 4 applies TDMA baseband processing to the received 
signal and sends the voice signal to a voice coder/decoder 5. The voice 
coder/decoder 5 converts the voice signal to an analogue voice signal, 
which is then sent out to a wire line (e.g. a wire network) via a line 
interface 6. The communication controller 4 also performs various 
processes of this invention. 
The cell stations may optionally include a handset 11 with which a user 
talks with a destination party. The cell stations of this invention may 
further include an input key panel 10 for inputting various instructions 
and a display 9 which displays various information, such as, for example, 
communication traffic conditions and status of the cell stations, in 
accordance with instructions inputted by the user through the input key 
panel 10. The user manipulates the input key panel 10 for performing 
various inputting operations of the present invention, and via an 
interface 8 the display 9 displays various information according to such 
inputting operations. 
The cell stations of this invention further include an EEPROM 7 as an 
example of a memory for storing therein various information according to 
the present invention and an alarm LED 12 which lights and extinguishes 
for indicating occurrence of various events according to the present 
invention. 
FIG. 3 is a schematic drawing illustrating an example of an exterior 
appearance of the cell stations. 
FIG. 4 is a schematic drawing illustrating an example of a block diagram of 
the personal stations PS1, PS2, PS3 shown in FIG. 1. 
Like the cell stations, an antenna 21 receives a radio signal which is 
transmitted from the other stations and a radio wave processor 22 
processes the received signal. A modem (modulator/demodulator) 23 
demodulates the received signal and sends the demodulated signal to a 
communication controller 24, which includes a CPU, a ROM and so forth. The 
communication controller 24 then applies TDMA baseband processing to the 
received signals and sends a voice signal to a voice coder/decoder 25. The 
voice coder/decoder 25 then converts the voice signal to an analogue voice 
signal which is sent out to a speaker 26 to be reproduced as voice. When 
transmitting a voice signal which is inputted through a microphone 31, the 
inputted analogue voice signal is processed in the same manner in the 
reverse order and a radio wave carrying the signal is transmitted by the 
antenna 21. The communication controller 24 performs various processes of 
this invention also. 
The personal stations further include an input key panel 30 for inputting 
various instructions and a display 29 which displays various information, 
such as, for example, traffic conditions and communication status of the 
stations in accordance with the instructions inputted by the user through 
the input key panel 30. The user manipulates the input key panel 30 for 
performing various inputting operations of the present invention, and via 
an interface 28 the display 29 displays various information according to 
such inputting operations. 
The personal stations further include an EEPROM 27 as an example of a 
memory for storing therein various information according to the present 
invention and an alarm LED 32 which lights and extinguishes for indicating 
occurrence of various events according to the present invention. 
An exterior appearance of the personal stations PS1, PS2, PS3 is 
essentially the same as that of the cell stations, and therefore the 
drawing is omitted. 
Next, mapping of logical control channels on a TDMA frame in each of the 
cell stations CS1, CS2, CS3 is explained. 
FIG. 5 is a diagram illustrating an example of a structure of slots and 
logical control channels used by each of the cell stations CS1, CS2, CS3. 
Like a digital cordless telephone system employing a TDMA/TDD architecture, 
such as the PHS system, a digital signal carried by a radio wave which is 
radiated by each of the cell stations CS1, CS2, CS3 and the personal 
stations PS1, PS2, PS3 is divided into 5 ms time segments, each referred 
to as a TDMA frame. 
Each frame is further divided into 8 time slots, and therefore the time for 
each slot is 625 .mu.s. Of the 8 slots, four slots each are assigned for 
downlink (transmission from a cell station to a personal station) channels 
and uplink (transmission from a personal station to a cell station) 
channels, respectively. 
In this example, the first slot of each four slots is assigned as a control 
channel for setting function channels for transmitting control information 
necessary for controlling the connection between the cell station and the 
personal station, for example, a broadcast control channel (BCCH) for 
broadcasting control information. The other three slots each are assigned 
for communication channels. 
The cell station broadcasts to all personal stations, using the broadcast 
control channel (BCCH), control information related to a channel structure 
and system information such as information regarding which slots are 
available for transmitting control information for obtaining a 
communication slot. A personal station receives such information and 
transmits information for informing its current location to the system, 
which is necessary for receiving a call addressed to the personal station. 
Like the PHS system, there are provided three communication protocol phases 
in this example, a phase for establishing the radio interface handshake, 
which is called a link channel establishment phase, a phase for connecting 
a call between the cell station and the personal station which established 
the handshake, which is called a service channel establishment phase and a 
phase for performing communication and data transmission, which is called 
a communication phase. 
The link channel establishment phase is defined as the stage of using 
control channel functions to select a channel (link channel) with the 
quality and capacity required for each call connection. 
A control channel used in the link establishment phase is called a logical 
control channel (LCCH). As downlink logical control channel (LCCH) 
elements, there are a broadcast control channel (BCCH), a paging channel 
(PCH) which is a one-way point-multipoint channel that simultaneously 
transmits identical information to individual cell stations and a 
signaling control channel (SCCH) which is a bi-directional point-point 
channel that transmits information needed for call connection between the 
cell station and a personal station. 
Each of the cell stations CS1, CS2, CS3 intermittently transmits a control 
channel signal which indicates a structure of the logical control channel 
(LCCH) transmitted by the cell station and the positions of the control 
channel slot with which transmission is possible for each of the personal 
stations PS1, PS2, PS3 by the broadcasting message. 
Like the PHS system, an LCCH superframe, which is the minimum cycle of the 
downlink logical control channel (LCCH) slots specifying the slot position 
of all LCCH elements, is constructed of control channel signals which are 
intermittently transmitted. 
An example of the LCCH superframe shown in FIG. 5 is constructed of "m" 
number of LCCH slots (superframe signals) which are intermittently 
transmitted each cycle of "n" number of TDMA frames. In other words, the 
value "n" shows the cycle at which the cell station intermittently 
transmits an LCCH slot and is expressed by the number of TDMA frames 
within the intermittent transmission cycle. An "m" number of these 
intermittent transmission cycles form an LCCH superframe. An intermittent 
transmission cycle of a downlink LCCH slot is hereinafter called a 
superframe signal transmission cycle. 
FIG. 6 is a diagram illustrating an example of a structure of a control 
channel signal forming a superframe signal. As shown in FIG. 6, the 
control channel signal of the superframe signal includes a preamble (PR) 
41, a unique word (UW) 42, a channel identification (CI) 43, control data 
(CAC) 44 and error correction parity (CRC) 45. The control data (CAC) 44 
includes superframe signal transmission cycle information and cell station 
identification information (CS-ID) of each cell station. 
FIG. 7 is an example of a diagram illustrating a structure of the cell 
station identification information (CS-ID), which includes system 
identification information 51 consisting of 29 bits and additional 
identification information 52 consisting of 13 bits. 
Next, a process performed by each of the cell stations CS1, CS2, CS3 for 
monitoring the control channel signals and extracting the cell station 
identification information (CS-ID) from each of the received signals and 
for storing the information in a memory is explained. 
FIG. 8 is a flowchart illustrating an example of the above-mentioned 
process. The process is performed by the communication controller 4 of 
each the cell stations CS1, CS2, CS3. 
In the process, when the power of the cell station CS3, for example, is 
turned on in step S1, step S2 starts receiving, via the antenna 1, a 
control channel signal and receives a superframe signal being transmitted 
from each of the other cell stations CS1, CS2. Step S3 frame synchronizes 
each of the received TDMA frames using the unique word 42 contained in the 
broadcast control channel (BCCH) in each of the received superframe 
signals, and step S4 analyzes each of the received superframe signals to 
identify the control data (CAC) 44 in each of the received superframe 
signals. Then, step S5 extracts cell station identification information 
(CS-ID) from each of the control data (CAC) 44. Step S6 then tabulates the 
extracted cell station identification information (CS-ID) and stores the 
tabulated information in the EEPROM 7. 
Then, step S7 checks if a superframe signal is being transmitted from any 
of the other cell stations, and if the answer to the step S7 is YES, steps 
S3-S6 are repeated. When there are no superframe signals being transmitted 
from any of the other cell stations, the process proceeds to step S8 to 
stop receiving the control channel signals and end the process. 
The cell station CS3 repeats the above-described process intermittently 
when the power is on and updates the information stored in the EEPROM 7 
each time the process is performed. The cell stations CS1 and CS2 also 
perform the same process likewise. 
Thus, each of the cell stations CS1, CS2, CS3, when the power is on, 
automatically receive the cell station identification information (CS-ID) 
from each of the other cell stations and store the information in memory. 
Described next is a process to be performed by each of the cell stations 
CS1, CS2, CS3 for determining the superframe signal transmission cycle in 
accordance with the number of cell stations using the control channel 
signal in the system, which is determined based upon the cell station 
identification information (CS-ID) stored in the EEPROM 7. 
FIG. 9 is a timing chart illustrating an example of the superframe signal 
transmission cycle of each of the cell stations CS1, CS2, CS3. In the PHS 
system, it is specified that 8 slots or less are transmitted per second on 
one control carrier wave. Therefore, a superframe signal transmission 
cycle is required to be longer than 125 ms at the minimum. 
FIG. 10 is a flowchart illustrating a process performed by the cell 
stations CS1, CS2, CS3 for determining the superframe signal transmission 
cycle. The process is performed by the communication controller 4 of the 
cell stations CS1, CS2, CS3. 
After the power to the cell station CS1, for example, is turned on in step 
S1, step S12 performs a process for monitoring for control channel signals 
and extracting the. cell station identification information (CS-ID) from 
the received signals and for storing the information in the EEPROM 7, 
which corresponds to the process of steps S2-S8 of the flowchart shown in 
FIG. 8. Then, step S13 checks if any cell station is registered in the 
information stored in the EEPROM 7 and if the answer to the step S13 is 
NO, step S14 sets the superframe signal transmission cycle to the minimum 
value of 125 ms. Step S15 then transmits a superframe signal using the 125 
ms minimum transmission cycle and the process ends. 
If the answer to step S13 is YES, step S16 inputs a criterion value for the 
number of cell stations, which is inputted through the input key panel 10 
of the cell station CS1 by the user. For example, the inputted criterion 
value is based on the number of cell stations located in an area of cell 
station CS1. Step S17 compares the number of cell stations registered in 
the EEPROM 7 with the inputted criterion value. If the number of 
registered cell stations is larger than the inputted criterion value, the 
process proceeds to step S18 to set the superframe transmission cycle to a 
longer cycle, which is prescribed beforehand in the EEPROM 7. Then, step 
S15 transmits the superframe signal with the longer cycle and ends the 
process. 
A plurality of predetermined superframe transmission cycle values may be 
prestored in the EEPROM 7, so that one of the values is selected in 
accordance with the difference between the number of the cell stations 
registered in the EEPROM 7 and the inputted criterion value. For example, 
if the difference is 1, a cycle of 200 ms may be selected, if the 
difference is 2, a cycle of 300 ms may be selected and if the difference 
is more than 3, a cycle of 500 ms may be selected. Then, the communication 
controller 4 sets the superframe signal transmission cycle to the selected 
cycle and transmits the superframe signal with the selected cycle. 
If the number of cell stations registered in the information stored in 
EEPROM 7 is smaller than the criterion value, step S14 sets the cycle to 
the minimum value of 125 ms and ends the process after transmitting the 
superframe signal with the minimum cycle of 125 ms. 
Further, the superframe signal transmission cycle may be determined based 
upon the number of registered cell stations. For example, information 
specifying a superframe signal transmission cycle for each predetermined 
number of registered cell stations, such as, for example, 125 ms if the 
number of cell stations is within 1-3, 200 ms if the number of cell 
stations is 5 and 500 ms if the number of cell stations is more than 5, is 
stored in the EEPROM 7. Depending upon the number of cell stations 
registered in the EEPROM 7 the cycle is automatically determined. 
FIG. 11 is a flowchart illustrating an example of a process for determining 
the superframe signal transmission cycle based upon the number of cell 
stations. Step S122 is similar to step S12 in FIG. 10. Step S133 checks if 
any cell stations are registered in the EEPROM 7. If yes, step S177 then 
determines the superframe signal transmission cycle in accordance with the 
number of cell stations, based upon the information stored in the EEPROM 7 
as described above. 
Thus, the cell station CS3 determines the superframe signal transmission 
cycle depending upon the number of cell stations using the control channel 
signal. In other words, depending on the traffic of the control channel 
signals in the system. Likewise, the other cell stations CS1, CS2 also 
determine their superframe signal transmission cycle depending on the 
traffic of the control channel signals in the system. 
Each of the cell stations CS1, CS2, CS3, therefore, when the traffic of the 
control channel signals becomes busy, can prevent its own superframe 
signal from interfering with the superframe signal from the other cell 
stations by setting the superframe signal transmission cycle to a longer 
cycle. 
Next, a process of the cell stations CS1, CS2, CS3 for determining the 
superframe signal transmission cycle based upon the vacant time in the 
superframe signal transmission cycle of each of the other cell stations is 
explained by referring to FIG. 12, which is a flowchart illustrating an 
example of the above-mentioned process. The explanation will also be made 
by further reference to FIG. 9. The process is performed by the 
communication controller 4 in each of the cell stations CS1, CS2, CS3. 
In the process, when the power to the cell station CS3, for example, is 
turned on, step S21 performs the process of monitoring the control channel 
signal and extracting the cell station identification information (CS-ID) 
from each of the received signals for storing the information in the 
EEPROM 7, which corresponds to the process performed by steps S2-S8 shown 
in FIG. 8. 
Step S22 extracts the superframe signal transmission cycle information T1 
of the cell station CS1 and the superframe signal transmission cycle 
information T3 of the cell station CS2, each from the control data (CAC) 
44 in the received superframe signals. Step S22 then determines a time T2, 
which is a time from the time when the cell station CS1 has finished 
transmitting its superframe signal to the time when the cell station CS2 
begins transmitting its superframe signal. 
The time T2 may be determined by subtracting 625 As from a time, which is 
obtained, for example, by starting a timer when the unique word (UW) of 
the first control channel signal from the cell station CS1 is detected and 
stopping the timer when the unique word (UW) of the control channel signal 
from the cell station CS2 is detected. 
Then, step S23 calculates the vacant time cycle T4, which is a period of 
time in which a superframe signal is not transmitted by either cell 
station CS1 or cell station CS2, based upon the above-mentioned 
information T1 and T2. 
Now, in more detail by reference to FIG. 9, the superframe transmission 
cycle T1 of the cell station CS1 is defined as 5.times.n (ms), and the 
superframe signal transmission cycle T3 of the cell station CS2 is defined 
as 5.times.n' (ms), respectively, as examples in this embodiment. The time 
T2 from the end of the superframe signal transmission of the cell station 
CS1 and the start of the superframe signal transmission of the cell 
station CS2 is determined as described above by use of a timer. The vacant 
time cycle T4 in each of the superframe signal transmission cycles of the 
cell station CS1 and CS2, which is available for the cell station CS3 to 
transmit the superframe signal, is calculated as T4=T1-(625 
.mu.s.times.2)-T2. 
Step S24 then determines the superframe signal transmission cycle T5 of the 
cell station CS3 based upon the vacant time cycle T4 using a process as 
illustrated in FIG. 13, which will be explained next. The cell station CS3 
transmits the superframe signal in accordance with the superframe signal 
transmission cycle T5 in step S25 and the process ends. 
FIG. 13 is a flowchart illustrating an example of a process for determining 
the superframe signal transmitting cycle based upon the vacant time cycle 
in each of the superframe signal transmission cycles of the other cell 
stations. 
In FIG. 13, step S241 makes a list of start timings of the superframe 
signal transmission cycles of the cell station CS1 based upon the 
superframe signal transmission cycle information T1 of the cell station 
CS1, and step S242 makes a list of start timings of the superframe signal 
transmission cycles of the cell station CS2 based upon the superframe 
signal transmission cycle information T3 of the cell station CS2. Because 
the superframe signal transmission cycle T1 of the cell station CS1 is 
expressed as T1=5.times."n" (ms), as shown in FIG. 9, each start timing of 
the superframe signal transmission cycle T1 of the cell station CS1 is 
calculated by changing the value "n" in the equation 5.times."n" (ms), for 
example, from 1 to 100. Likewise, because the cell station CS2 transmits 
its superframe signal a time T2 after the transmission of the superframe 
signal transmitted by cell station CS1 as shown in FIG. 9, each start 
timing of the superframe signal transmission cycle T3 of the cell station 
CS2 is obtained by changing the value "n" in the equation (T2+625 
.mu.s)+5.times."n" (ms), for example, from 1 to 100. 
Then, step S243 sets a value of the time "a", which is a period of time 
after the transmission of the superframe signals by the other cell 
stations that cell station CS3 would begin transmitting its superframe 
signal, to an initial value of 1 (ms). 
Step S244 then determines if the vacant time cycle T4, which is a period of 
time in which a superframe signal is not transmitted by either cell 
station CS1 or cell station CS2, is longer than the time T2. 
If T4 is longer than T2 in step S244, step S245 determines that 
transmission of the superframe signal of the cell station CS3 starts 
during the time T4 and makes a list of start timings of the superframe 
signal transmission cycle T5 of the cell station CS3 by changing the value 
"n" from 1 to 100, for example, in the equation (T2+"a"+625 
.mu.s.times.2)+5.times."n" (ms), ("a" being set to 1 (ms) as the initial 
value). If the vacant time cycle T4 is less than the time T2, step S246 
determines that the superframe signal transmission of the cell station CS3 
starts during the time T2, and makes a list of start timings of the 
superframe signal transmission cycle T5 of the cell station CS3 by 
changing the value "n" from 1 to 100 in the equation 625 
.mu.s+"a"+5.times."n" (ms), "a" being set to 1 (ms) as the initial value. 
Then, step S247 checks if any start timing of the superframe signal 
transmission cycle T5 will collide with any start timing of the superframe 
signal transmission cycles T1 and T3. If, for example, the start timing of 
any cycle is within 1 ms of the start timing of any other cycle, it is 
determined that a collision will occur. 
If step S247 determines that collision will occur, step S248 changes the 
value of "a", for example, to 2 (ms) by adding one to the initial value of 
l(ms), and the process returns to step S244. 
If step S247 determines that collision will not occur, step S249 sets the 
superframe signal transmission cycle, which has been determined as not 
causing a collision, as the superframe signal transmission cycle of the 
cell station CS3 and transmits the superframe signal with the transmission 
cycle as determined. 
Thus, even when a plurality of cell stations, for example, cell stations 
CS1, CS2 as described above, are transmitting the superframe signal, the 
cell station CS3 can transmit the superframe signal without causing 
interference with the superframe signal from the other cell stations CS1, 
CS2 by determining the superframe signal transmission cycle based upon the 
vacant time in the superframe signal transmission cycles of each of the 
other cell stations CS1, CS2. 
Although the above process is explained for the case of the cell station 
CS3, the other cell stations CS1, CS2 perform the same process likewise. 
Next, another example of a process for determining the superframe signal 
transmission cycle based upon the vacant time cycle in each of the 
superframe signal transmission cycles of the other cell stations is 
explained. 
In this example, each of the cell stations CS1, CS2, CS3 determines whether 
or not transmission of the superframe signal is possible with the 
transmission cycles as determined based upon the vacant times cycle which 
is determined based upon the information of the superframe signal 
transmission cycle of each of the other cell stations. If it is determined 
that transmission is not possible the transmission of the superframe 
signal is discontinued and a message informing the user that the 
transmission has been discontinued is displayed. 
FIG. 14 is a flowchart illustrating an example of the above-mentioned 
process, which is performed by the communication controller 4 of each of 
the cell stations CS1, CS2, CS3. 
When the power to the cell station CS1, for example, is turned on, step S31 
performs the process of monitoring the control channel signal and 
extracting the cell station identification information (CS-ID) from the 
received signal for storing the information in the EEPROM 7, which 
corresponds to steps S2-S8 shown in FIG. 8. 
Step S32 performs the process of determining the vacant time cycle, which 
corresponds to steps S22-S23 of FIG. 12. Step S33 then determines if the 
vacant time cycle is longer than a predetermined period of time, for 
example, 1 ms, which is necessary at the minimum for transmitting one slot 
of 625 .mu.s. If the answer to step S33 is NO, the process proceeds to 
step S34 to discontinue transmission of the superframe signal. Step S35 
then displays in display 9 a message informing the user of the 
discontinuance of the transmission and a cause of the discontinuance and 
step S36 lights the alarm LED 12 to indicate the discontinuance of the 
transmission to the user. 
If it is determined that there is a sufficient vacant time cycle in step 
S33, then step S37 checks whether or not the superframe signal will 
collide with any of the other superframe signals being transmitted by the 
other cell stations if the superframe signal is transmitted during the 
vacant time cycle as determined. If it is determined that collision will 
occur in step S37, the process proceeds to step S34 to discontinue 
transmission of the superframe signal. Step S35 then displays a message 
informing the user of the discontinuance of the transmission and the cause 
of the discontinuance in the display 9 and step S36 lights the alarm LED 
12 to indicate the discontinuance of the transmission to the user. 
If it is determined that a collision will not occur in step S37, the 
process proceeds to step S38 to determine the superframe signal 
transmission cycle based upon the vacant time cycle, and step S39 
transmits the superframe signal with the cycle as determined. Step S40 
then displays in the display 9 a message informing the user that the 
superframe signal is being transmitted and step S41 extinguishes the alarm 
LED 12 (if on) to inform the user that the superframe signal is being 
transmitted without trouble. 
Whether or not the superframe signal will collide with any of the other 
superframe signals being transmitted by the other cell stations if the 
superframe signal is transmitted during the determined vacant time cycle, 
is determined by the process which is explained above by reference to FIG. 
13. 
Even when the transmission of the superframe signal is discontinued due to 
the traffic conditions, when the other cell stations discontinue the 
transmission of the superframe signal or change the superframe signal 
transmission cycle and the transmission of the superframe signal during 
the vacant time cycle becomes possible, the display 9 displays a message 
informing the user that the superframe signal transmission is now possible 
and the alarm LED 12 is extinguished. 
Thus, when a plurality of cell stations, for example CS1, CS2, are 
transmitting the superframe signal respectively as explained above, the 
cell station CS3 performs the above-mentioned process, so that collision 
of the superframe signals is avoided. 
Further, because each of the cell stations displays a message informing of 
discontinuance of transmission of the superframe signal and a message 
informing of the cause of the discontinuance, the user can recognize that 
the cell station is not broken and is working properly. Further, the user 
can identify the cause of the discontinuance and, for example, can 
re-arrange the positions of the cell stations if the discontinuance is due 
to the traffic conditions of the control signals in the system. 
Next, another example of a process for monitoring the control channel 
signal and extracting the cell station identification information (CS-ID) 
from each of the received signals for storing the information in memory is 
explained. 
In this example, each of the cell stations measures the electric field 
intensity of the control signal of each of the other cell stations based 
upon the TDMA frames received from each of the other cell stations. The 
information of the electric field intensity of the control signal of each 
cell station is stored in the EEPROM 7 corresponding to the cell station 
identification information (CS-ID) of each cell station. 
FIG. 15 is a flowchart illustrating an example of the above-mentioned 
process. The process is performed by the communication controller 4 of 
each of the cell stations, CS1, CS2, CS3. 
In the process, when the power of the cell station CS1, for example, is 
turned on in step S51, step S52 starts receiving the control channel 
signals and receives a superframe signal being transmitted by each of the 
other cell stations CS2, CS3. Step S53 frame synchronizes each of the 
received frames. 
Step S54 then analyzes each of the received control signals and step S55 
extracts the cell station identification information (CS-ID) from the 
control data 44 contained in each of the control signals. Then, step S56 
measures the electric field intensity of the control signal received from 
the cell station which corresponds to the extracted cell station 
identification information (CS-ID). Various methods of measuring electric 
field intensity are known in the art and may be used by the present 
invention. Step S57 then tabulates the information of the electric field 
intensity of the control signal and the corresponding cell station 
identification information (CS-ID) of each of the cell stations, and step 
S58 stores the tabulated information in the EEPROM 7. 
The process then proceeds to step S59 to check if there is any superframe 
signal being transmitted from the other cell stations. If the answer to 
step S59 is YES, steps S53-S58 are repeated, and the information of the 
cell station identification (CS-ID) and the electric field intensity of 
the control signal of each of the cell stations, which is stored in the 
EEPROM 7, is updated. If there is no superframe signal being received from 
another cell station, the process proceeds to step S60 to discontinue 
receiving the control channel signal. Step S61 displays the tabulated 
information of the cell station identification (CS-ID) and the electric 
field intensity of the control signal of each cell station in the display 
9 and ends the process. 
Each of the cell stations CS1, CS2, CS3, when the power is on, periodically 
performs the above-mentioned process and updates the information of the 
cell station identification (CS-ID) and the electric field intensity of 
the control signal of each cell station, which is stored in the EEPROM 7. 
Further, the stored information is displayed in the display 9. 
The user therefore can confirm, with each of the cell stations, the radio 
wave conditions surrounding each of the cell stations based upon the 
information of the electric field intensity of the control signals of the 
other cell stations, which is stored in the EEPROM 7. When the user 
desires to add a new cell station in the system for improving the traffic 
conditions, for example, the user can determine the most appropriate 
location for placing the new cell station based upon the information of 
the radio wave conditions surrounding each of the cell stations. 
Next, a process performed by the cell stations for determining a timing for 
starting the superframe signal transmission cycle in accordance with a 
timing inputted by the user is explained. 
FIG. 16 is a flowchart illustrating an example of the above-mentioned 
process. The process is performed by the communication controller 4 of 
each of the cell stations CS1, CS2, CS3. 
In the process, when the power to cell station CS1, for example, is turned 
on, step S71 performs the process of monitoring the control channel signal 
and extracting the cell station identification information (CS-ID) from 
each of the received control signals for storing the information in the 
EEPROM 7, which corresponds to steps S2-S8 of FIG. 8. 
Step S72 then performs the process of determining the vacant time cycle, 
which corresponds to the steps S22-S23 of FIG. 12. 
Step S73 displays in the display 9 the vacant times cycle, together with 
the superframe signal transmission cycle of each of the other cell 
stations. 
Step S74 then checks if any key input has been made via input key panel 10 
for setting the start time of the superframe signal transmission cycle. If 
such a key input has been made, the process proceeds to step S75 to 
determine the start timings for multiple superframe signal transmission 
cycles using the inputted start time. Step S76 then checks if a collision 
will occur if the superframe signal is transmitted with the timing as 
inputted. 
Whether or not a collision will occur is determined by comparing the start 
time of each of the superframe signal transmission cycles, which is 
determined as inputted as described above, with the start time of each of 
the superframe signal transmission cycles of the other cell stations, as 
explained previously referring to FIG. 13. 
If the answer to the step S76 is YES, the process returns to the step S72 
and if the answer to the step S76 is NO, the process proceeds to step S77 
to transmit the superframe signal with the timing as determined. 
If there is no key input in step S74, the process proceeds to step S78 to 
determine if a mode of automatically setting the timing to a predetermined 
timing is prescribed. For example, the cell station can be programmed to 
automatically set the start time to a predetermined value if a key input 
is not made within a predetermined amount of time. If the answer to step 
S78 is NO, the process returns to steps S73 and S74 to wait for a key 
input. If the answer to step S78 is YES, the process proceeds to step S75 
to determine the start time for each cycle using the initial start time as 
automatically set. Step 76 then determines if a collision will occur. If 
yes, the process returns to step S72. If no, step S77 transmits the 
superframe signal with the timing as automatically set. 
FIG. 17 illustrates an example of a diagram displayed in the display 9 for 
inputting the start time of the superframe signal transmission cycle based 
upon the superframe signal transmission cycle of each of the other cell 
stations and the vacant time cycle during which the superframe signal is 
not transmitted by the other cell stations. 
As illustrated in the drawing, the cell station identification (CS-ID) of 
the cell station CS1 along with a signal waveform W1 illustrating the 
superframe signal transmission cycle of the cell station CS1, and the cell 
station identification (CS-ID) of the cell station CS2 along with a signal 
waveform W2 illustrating the superframe signal transmission cycle of the 
cell station CS2 are displayed. In addition, a waveform W3 illustrating 
the vacant time cycle, during which the superframe signal is not 
transmitted by either cell station CS1 or cell station CS2 and which is 
determined based upon the superframe signal transmission cycle information 
of each of the cell stations CS1, CS2, is displayed. 
The user may move a positioning curser P1, which illustrates a start time 
of the superframe signal transmission cycle of the cell station CS3, which 
the user is using, to a desired position, such as for example, to a 
position shown by one of the dotted lines. 
If the user then selects the timing, as displayed, through manipulation of 
the key input panel 10, for determining the timing, the communication 
controller 4 determines the timing of starting the superframe signal 
transmission cycle as inputted. 
A determination is then made whether transmission is possible using the 
input start time by determining if a collision will occur with the other 
signals using one of the methods described above. If the transmission is 
possible with the timing as determined, the controller 4 starts the 
transmission and if the transmission is not possible, the display 9 
displays a message requesting the user to set the timing again. 
Thus, the user can input the start time of the superframe signal 
transmission cycle while viewing in the display 9 the superframe signal 
transmission cycle of each of the other cell stations. Further, the user 
can confirm visually whether or not the superframe signal will collide 
with the superframe signal being transmitted by each of the other cell 
stations if the superframe signal is transmitted with the timing inputted 
by the user. Thus, the user can determine the start time of the superframe 
signal transmission cycle, so that collision will not occur. 
Next, a process performed by the personal stations PS1, PS2, PS3 for 
determining a cell station for connection is explained. 
FIG. 18 is a flowchart illustrating an example of the above-mentioned 
process. 
When the power to the personal station PS1, for example, is turned on in 
step S81, the communication controller 24 of the personal station PS1 
starts monitoring the control channel signal and receives, in step S82, 
the superframe signal of each of the cell stations CS1, CS2, CS3. Step S83 
frame synchronizes each of the received TDMA frames using the unique word 
(UW) 42 contained in each of the control channel signals. 
Step S84 analyzes each of the received control channel signals and step S85 
extracts the cell station identification information (CS-ID) from the 
control data (CAC) 44 contained in each of the control channel signals. 
Step S86 then measures the electric field intensity of the control channel 
signal received from each cell station. Step S87 tabulates the cell 
station identification information (CS-ID) and the corresponding electric 
field intensity information and stores the tabulated information in the 
EEPROM 27. 
Step S88 then checks if there is any superframe signal being transmitted 
from any other cell station, and if the answer to step S88 is YES, the 
processes from S83 through S87 are repeated. If there is no other 
superframe signal being transmitted, the process proceeds to step S89 to 
display in the display 29 the cell station identification information 
(CS-ID) and the corresponding electric field intensity information stored 
in the EEPROM 27. 
Step S90 then determines if any key input has been made through the input 
key panel 30 for selecting one of the displayed cell stations as the cell 
station for connection. 
If a key input is made in step S90 via the input key panel 30, step S91 
connects the personal station PS1 with the selected cell station and step 
S92 receives the superframe signal only from the selected cell station. 
If a key input is not made within a predetermined period of time from the 
time the display 29 has displayed the information in step S89, the process 
proceeds to step S94 to connect the personal station PS1 with the cell 
station which has the strongest electric field intensity among the cell 
stations whose cell station identification information are stored in the 
EEPROM 27, and step S92 receives the control channel signal only from the 
connected cell station. 
Then, step S93 monitors the electric field intensity of the control signal 
being received from the cell station that it is connected with and checks 
if the electric intensity of the control signal from the connected cell 
station reaches a level below a predetermined level. 
If the answer to the step S93 is YES, the process returns to step S82 to 
start monitoring the control channel signal and receiving the superframe 
signals again. The controller 24 updates the information stored in the 
EEPROM 27 and selects the cell station for connection again based upon the 
updated information or may display in the display 29 a message asking if 
the user desires to change the cell station for connection. 
If the electric field intensity of the control signal being received from 
the connected cell station does not reach a level below the predetermined 
level, the process ends. 
Thus, when there are a plurality of the cell stations that have the same 
electric field intensity of the control signal, the user can select the 
cell station for connection from among the plurality of cell stations. 
Therefore, for example, when it is known that the traffic of a certain cell 
station is relatively concentrated, the user can avoid selecting such a 
cell station for connection and can select a cell station whose traffic is 
relatively light. 
Further, when the user moves with the personal station and the electric 
field intensity of the control signal from the connected cell station is 
weakened, the user can select the cell station having the strongest 
electric field intensity and switch the cell station for connection to a 
cell station having the strongest electric field intensity for maintaining 
the communication quality. 
Now, the second embodiment of the present invention is explained. 
When the radio communication system is used in a certain private closed 
area, such as for example, an area inside a building or a complex of 
buildings, two control channel signals with different frequencies are 
generally provided for avoiding frequent occurrence of busy traffic. In 
the PHS system, for example, frequencies of 1895.450 MHz and 1900.250 MHz 
are allocated for the control channel signals when the system is a private 
system. 
In the radio communication system in the second embodiment, having two 
control channel signals with different frequencies, each cell station 
monitors each of the two control channel signals from the other cell 
stations and transmits the superframe signal with the control channel 
signal having lighter traffic 
FIGS. 19 and 20 are flowcharts illustrating an example of a process 
performed by the cell stations CS1, CS2, CS3 for monitoring, when two 
control channel signals with different frequencies are provided, both of 
the control channel signals and transmitting the superframe signal with 
the control channel signal having lighter traffic. In this example, the 
two control channel signals are hereinafter called a 1st control channel 
and a 2nd control channel respectively. As in the first embodiment, the 
communication controller 4 of each cell station performs this process. 
When the power to the cell station CS1, for example, is turned on, step 
S101 performs, first, for the 1st control channel, the process of 
monitoring the control channel signal and extracting the cell station 
identification information (CS-ID) from each of the received superframe 
signals for storing the information in the EEPROM 7, which corresponds to 
the process of steps S2-S8 shown in FIG. 8. 
Step S102 then performs the process of determining the vacant time cycle in 
each of the superframe signal transmission cycles of the other cell 
stations CS2, CS3, which corresponds to the processes in steps S22-S23 of 
FIG. 12. 
Step S103 then determines if there is a sufficient vacant time cycle in the 
1st control channel. As described previously, because one slot is 625 
.mu.s, it is determined that there is not sufficient vacant time if the 
vacant time is less than 1 ms in this embodiment. If there is not 
sufficient vacant time, step S104 switches the control channel signal from 
the 1st control channel to the 2nd control channel. Then, step S105 
performs for the 2nd control channel the process of monitoring the control 
channel signal and extracting the cell station identification information 
(CS-ID) for storing the information in the EEPROM 7 as in the steps S101 
for the 1st control channel, and step S106 performs the process of 
determining the vacant time cycle in each of the other cell stations as in 
the step S102 for the 1st control channel. 
Then, step S107 checks if there is a sufficient vacant time cycle. If there 
is a sufficient vacant time cycle in step S107, the process proceeds to 
step S111 of FIG. 20 to determine whether or not the superframe signal 
will collide with any of the superframe signals being transmitted from the 
other cell stations if the superframe signal is transmitted during the 
vacant time cycle of the 2nd control channel as determined. If the answer 
to step S111 is NO, step S112 determines the superframe signal 
transmission cycle of the cell station CS1 which is determined as not 
causing collision in step S111. 
Step S113 then transmits the superframe signal with the cycle as 
determined. Step S114 displays in the display 9 a message informing the 
user that the superframe signal is being transmitted and step S115 
extinguishes the LED 12 informing the user that no failure has occurred 
during the superframe signal transmission. 
If it is determined in step S111 that a collision will occur, the process 
proceeds to step S116 to stop the transmission of the superframe signal. 
Step S117 displays in display 9 a message informing the user that the 
transmission of the superframe signal has been stopped and step S118 
lights the LED to indicate the stoppage of the transmission. 
If it is determined in the step S107 of FIG. 19 that there is no sufficient 
vacant time cycle in the 2nd control channel also, the process proceeds to 
step S116 of FIG. 20 to stop the transmission of the superframe signal. 
Thus, each of the cell stations CS1, CS2, CS3, when there are two control 
channel signals provided, can transmit the superframe signal with the 
control channel signal having lighter traffic. 
In the above-mentioned second embodiment, alternatively, each of the cell 
stations may automatically switch the control channel signal for 
transmitting the superframe signal between the two control channel signals 
alternately after a predetermined period of time. 
FIG. 21 is a flowchart illustrating an example of a process of switching 
the control channel signal between the two channels. The process is 
performed by the communication controller 4 of the cell stations CS1, CS2, 
CS3. 
As illustrated in FIG. 21, if there is a sufficient vacant time cycle in 
each the 1st control channel and the 2nd control channel in the processes 
in steps S121-S128, step S129 starts transmission of the superframe signal 
with the 1st control channel and at the same time starts a timer. Step 
S130 then judges if a predetermined time "t" has passed, and if the answer 
to step S130 is NO, the transmission of the superframe signal with the 1st 
control channel is continued. If the answer to the step S130 is YES, the 
process proceeds to step S140 to stop the transmission of the superframe 
signal with the 1st control channel. 
Step S141 then transmits the superframe signal with the 2nd control channel 
and starts the timer. Step S142 judges if the predetermined time "t" has 
passed, and if the answer to the step S142 is NO, the transmission of the 
superframe signal with the 2nd control channel is continued. If the answer 
to the step S142 is YES, the process proceeds to step S143 to stop the 
transmission with the 2nd control channel and return to the step S129. 
Thus, each of the cell stations CS1, CS2, CS3 switches the control channel 
signal to transmit the superframe signal between the two channels 
alternately after the predetermined time "t". 
Therefore, the radio communication system of this invention can avoid, when 
two control channel signals with different frequencies are provided, a 
situation in which the traffic of the control channel signal gets crowded 
onto one of the channels, even when the number of the cell stations in the 
system increases and the amount of traffic becomes busy. 
Further in the above-mentioned embodiment, each of the cell stations CS1, 
CS2, CS3 may switch the control channel signal for transmitting the 
superframe signal between the two channels each time a request for 
connection is received from each of the personal stations PS1, PS2, PS3. 
FIG. 22 is a flowchart illustrating an example of a process of switching 
the control channel signal for transmitting the superframe signal between 
the two channels each time a request for establishing a radio connection 
is received from a personal station. 
When the power to the cell station CS1, for example, is turned on in step 
S131, the cell station CS1 performs a process of checking if there is a 
sufficient vacant time cycle in each of the 1st control channel and the 
2nd control channel, and if there is a sufficient vacant time cycle in 
each of the two control channels, step S132 starts the transmission of the 
superframe signal with the 1st control channel. 
Step S133 judges if there is a request for establishing a radio connection 
from any of the personal stations, and if there is no request the process 
returns to the step S132 to continue the transmission of the superframe 
signal with the 1st control channel. If there is a request for connection, 
the process proceeds to step S134 to perform a process of establishing the 
connection with the personal station from which the request for connection 
has been received, and step S135 stops the transmission of the superframe 
signal with the 1st control channel. 
FIG. 23 is a chart illustrating an example of a sequence for establishing a 
radio connection between a cell station and a personal station. 
In FIG. 23, the communication controller 4 of the cell station CS1 stops 
the transmission of the superframe signal when the process between the 
cell station and the personal station moves from the sequences for the 
control channel, which are performed during a time denoted as ti in the 
drawing, and to the following sequences for the communication channel, 
which are performed during a time denoted as t2 in the drawing. Such 
timing of stopping the transmission of the superframe signal is indicated 
as point A in the drawing. 
Then, step S136 (FIG. 22) switches the control channel signal for 
transmitting the superframe signal to the 2nd control channel and 
transmits the superframe signal with the 2nd control channel. Then, step 
S137 judges if there is any request for establishing a radio connection 
from any of the personal stations, and if there is no request the process 
returns to step S136 to continue the transmission of the superframe signal 
with the 2nd control channel. If there is a request for connection, the 
process proceeds to step S138 to perform the process of establishing a 
radio connection with the personal station from which the request for 
connection has been received and step S139 stops the transmission of the 
superframe signal with the 2nd control channel and returns to step S132. 
Thus, each of the cell stations CS1, CS2, CS3 switches the control channel 
signal for transmitting the superframe signal between the two channels 
each time a request for establishing a radio connection is received from 
each of the personal stations PS1, PS2, PS3. 
Therefore, the radio communication system of this invention, when two 
control channel signals with different frequencies are provided, can avoid 
a situation in which the traffic crowds onto one of two control channel 
signals, even when traffic with a particular personal station is busy. 
Now, a process of the personal stations PS1, PS2, PS3 for connecting with 
the cell station, when each of the cell stations switches the control 
channel signal for transmitting the superframe signal between two control 
channels signals, is explained. 
FIG. 24 is a flowchart illustrating an example of such process. 
When the power to the personal station PS3, for example, is turned on in 
step S151, step S152 starts monitoring the 1st control channel and at the 
same time starts a timer. Step S153 checks if a superframe signal is 
received on the 1st channel from a cell station. If the answer to the step 
143 is YES, the process proceeds to step S158 to connect to the cell 
station. 
If the answer to the step S153 is NO, step S154 checks if a predetermined 
time "t" has elapsed. If the predetermined time has not elapsed in step 
S154, the process returns to the step S152 to continue monitoring the 1st 
control channel. If the predetermined time has elapsed in step S154, the 
process proceeds to step S155 to switch the control channel to the 2nd 
control channel and start the timer. 
Then, step S156 checks if the superframe signal is received on the 2nd 
channel from a cell station. If the answer to the step S156 is YES, the 
process proceeds to step S158 to connect to the cell station. 
If the answer to the step S156 is NO, step S157 checks if the predetermined 
time "t" has elapsed. If the answer to the step S157 is NO, the process 
returns to step S155 to continue the monitoring of the 2nd control 
channel. If the predetermined time has elapsed in the step S157, the 
process returns to step S152 to switch the control channel to the 1st 
control channel to monitor the 1st control channel again. 
Thus, the personal station PS3 can connect to the cell station even when 
the cell station transmits the superframe signal with either of the two 
control channels. 
Therefore, even if the traffic of the system increases and each of the cell 
stations switches the control channel signal for transmitting the 
superframe signal between the two channels, each of the personal stations 
can easily connect to the cell station. 
Although the above-described embodiments include three sub-systems, each 
including a cell station and a personal station, the present invention can 
be applied to a radio communication system having any number of cell 
stations and personal stations. 
Further, although the present invention has been explained for the case of 
the digital cordless telephone system, the invention can be applied to any 
radio communication system including a plurality of base stations which 
communicate with a plurality of radio terminals respectively, such as a 
wireless printer system including a plurality of printers, each of which 
communicates with a plurality of personal computers over radio waves, a 
wireless LAN system including a plurality of base stations, each of which 
is connected to a LAN cable and communicating with a plurality of 
terminals over radio waves. 
Furthermore, the controller 4 of this invention may be conveniently 
implemented using a conventional microprocessor programmed according to 
the teachings of the present specification, as will be apparent to those 
skilled in the computer art. Appropriate software coding can readily be 
prepared by skilled programmers based upon the teachings of the present 
disclosure, as will be apparent to those skilled in the software art. The 
invention may also be implemented by the preparation of application 
specific integrated circuits or by interconnecting an appropriate network 
of conventional component circuits, as will be readily apparent to those 
skilled in the art. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein. 
This application is based upon the Japanese patent application no. 
08-077130 filed in the Japanese patent office on Mar. 29, 1996, the entire 
contents of which are hereby incorporated by reference.