Cordless telephone

A multichannel access cordless telephone has a base unit and a handset unit which, in a standby mode, each independently scan a plurality of communication channels to detect interference thereon and to record those channels on which interference is detected. Thereafter, in a talk mode of the cordless telephone, one of the base unit and the handset unit scans the communication channels other than the detected ones previously recorded to detect a vacant one of the communication channels and to establish communication between the handset unit and the base unit thereover.

FIELD OF THE INVENTION 
The present invention relates generally to cordless telephones, and in 
particular relates to a cordless telephone operable to avoid interference 
from other cordless telephones. 
BACKGROUND OF THE INVENTION 
The standard telephone consists of a base unit and a handset unit connected 
to each other by an electrical cord. The base unit itself is connected by 
another cord to a receptacle on a wall, telephone pole or a similar 
immovable structure to which the telephone network line extends. 
Therefore, the range of movement of the operator of the telephone is quite 
limited. Even when the cords connecting the handset unit to the base unit 
and the base unit to the wall are long, it can be cumbersome either to 
move the entire telephone around to make calls from different locations or 
to walk around with the handset unit once a call has been placed. The 
simple fact that there is always a continuous physical connection between 
the person making the phone call and the immovable wall or other fixed 
structure can be a great inconvenience. 
The cordless telephone represents a significant improvement over the 
standard telephone. In the conventional cordless telephone, the base unit 
is still connected to the receptacle on the immovable wall or the like by 
a cord so that message signals from the telephone network line may be 
received and transmitted. However, the handset unit of the cordless 
telephone is an independently operative unit from which calls may be made 
and by which calls may be received with no physical connection to the base 
unit. The handset unit has a transmitting/receiving system or transceiver, 
a loudspeaker in the earpiece and a microphone in the mouthpiece. The base 
unit and the handset unit of the cordless telephone communicate with each 
other over a communication channel established by the transmission and 
reception of electromagnetic waves, conventionally radio waves. The 
handset unit may then be taken considerable distances from the base unit 
while still making and receiving telephone calls. Since there is no 
telephone cord extending between the handset unit and the base unit, the 
operator is free to move about without hindrance. 
Typically there are ten duplex channels permitted for each system occupying 
a bandwidth set by the Federal Communications Commission. Cordless 
telephones using all ten duplex channels therefore may not have a separate 
control channel over which control information may be transmitted for 
determining which duplex channel will be selected for use during a 
particular conversation. This control information must be transmitted over 
one of the ten duplex channels themselves. 
In order to transmit such control information without interfering with 
other telephone calls already in progress, a multichannel access (MCA) 
system has been proposed in which the cordless telephone searches for a 
vacant channel whenever an outgoing call is to be made or an incoming call 
is received and then the handset unit and base unit communicate with each 
other over the vacant channel. This permits a number of cordless 
telephones to be used simultaneously within the same general area without 
creating interference for each other by multiple transmissions over the 
same duplex channel. However, it is important to ensure that the cordless 
telephone does not erroneously attempt to establish communication over a 
channel with interference or already in use. 
Although ten duplex channels are permitted for cordless telephones, many 
conventional cordless telephones are built with only one duplex channel 
available therein. As a result, if an MCA system cordless telephone 
identifies a vacant channel at the start of an incoming/outgoing call and 
attempts to establish communication thereover, another cordless telephone 
built to use only the identified channel may start to use the identified 
channel during the initial set-up procedure and may cause interference 
with the MCA system cordless telephone. 
Furthermore, the MCA system cordless telephone will scan every channel in 
sequence at the start of the talk mode to identify a vacant channel and so 
may take a relatively long time to establish communication. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved cordless telephone which avoids the above-described difficulties 
of the prior art. 
It is another object of the present invention to provide an improved 
cordless telephone which is capable of eliminating the above-identified 
interference. 
It is yet another object of the present invention to provide an improved 
cordless telephone which will not establish communication over a channel 
already in use. 
It is a further object of the present invention to provide a cordless 
telephone which detects interference separately at the base and handset 
units to permit a double check against such interference. 
It is still a further object of the present invention to provide a cordless 
telephone which rapidly establishes communication over a channel without 
interference. 
In accordance with the present invention, a cordless telephone has a base 
unit connectable to a telephone network line for receiving and 
transmitting signals therethrough and a handset unit selectively separable 
from the base unit, the base and handset units including respective means 
for the transmission and reception of signals therebetween over any one of 
a plurality of communication channels, the cordless telephone further 
comprising channel scanning means for scanning the plurality of 
communication channels, the channel scanning means being operative in a 
standby mode of the cordless telephone for detecting the ones of 
communication channels on which there is interference, memory means for 
recording the detected ones of the communication channels on which there 
is interference, the channel scanning means further being operative in a 
talk mode at one of the base and handset units to scan the communication 
channels other than the detected ones of the communication channels 
recorded in the memory means to detect a vacant one of the communication 
channels, and control means for establishing communication between the 
handset unit and the base unit in the talk mode of the cordless telephone 
over the detected vacant communication channel. 
In a further development of the present invention, the channel scanning 
means detects the communication channels on which there is interference 
separately at the handset and base units and avoids establishing 
communication over any channel detected with interference at either unit. 
These and other objects, features and advantages of the present invention 
will become apparent from the following detailed description of the 
preferred embodiment taken in conjunction with the accompanying drawings, 
throughout which like reference numerals designate like elements and parts 
.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings in detail, and initially to FIG. 1 thereof, a 
cordless telephone 10 of a type to which the present invention may be 
applied is there shown to include a handset unit 1 and a base unit 2. Base 
unit 2 is connected by a telephone cord 3 to an outlet or receptacle in a 
wall, a telephone pole, or another fixed structure, so that it may both 
receive and transmit telephone message signals through a telephone network 
line 3a and also may be powered thereby. An antenna 100 on handset unit 1 
and a corresponding antenna 200 on base unit 2 are used to create a 
communication channel between the two units by the transmission and 
reception of radio waves, schematically illustrated in FIG. 1 by the 
arrows extending between the antennas. As is conventional, handset unit 1 
includes a ten-key panel 151 for making or dialing outgoing calls, and a 
mouthpiece 101 and an earpiece 102 with which a microphone and a 
loudspeaker (not shown) are, respectively, associated. A telephone number 
may be entered on ten-key panel 151, and corresponding information is 
transmitted over the communication channel to base unit 2 and thence to 
telephone network line 3a. Alternatively, when base unit 2 receives a 
message signal from the telephone network line indicating that an incoming 
call is present, a signal from base unit 2 causes a ringing sound in 
handset unit 1 to indicate the existence of the incoming call. 
The standard maximum separation of such a handset unit 1 and base unit 2, 
which is called the service area, is about 300 meters and is set by the 
Federal Communications Commission. The ten duplex channels typically 
permitted for each system are provided with the upper channel having a 
frequency in the 49 MHz band and the lower channel having a frequency in 
the 46 MHz band. Of course, such operating parameters are set by the FCC 
and do not form a part of the present invention. 
As defined in the present application, cordless telephone 10 is considered 
to be in a standby mode when it is powered but no telephone message 
signals appear at base unit 2 and handset unit 1 is disabled, for example 
by means of a talk switch 152 discussed below with reference to FIG. 3. 
Correspondingly, cordless telephone 10 is considered to be in a talk mode 
whenever either of base unit 2 and handset unit 1 is in a "talk" mode, 
that is, whenever a telephone message signal appears at base unit 2 or 
whenever handset unit 1 is put in its talk mode by talk switch 152. 
Referring now to FIGS. 2A-2C, there are illustrated therein the scanning 
operations in the standby and talk modes of an MCA system cordless 
telephone as previously proposed by the assignee of the present invention 
in the Official Gazette of Laid-Open Japanese Patents Publication No. 
59-186436 (Japanese Patent Application No. 58-61301). With regard to FIGS. 
2A-2C, the diagrams on the left under heading (1) illustrate the channel 
scanning operations of handset unit 1, while those on the right under 
heading (2) illustrate the channel scanning operations of base unit 2. 
Numerals 1-10 designate channel numbers, symbol 0 indicates a channel not 
in use, symbol 0 indicates a channel used by cordless telephone 10 itself, 
and symbol X indicates a channel used by another cordless telephone. 
As shown in FIG. 2A, when cordless telephone 10 is in the standby mode, 
both handset unit 1 and base unit 2 repeatedly and independently scan the 
channels in sequence, each waiting either to be enabled or to receive a 
call from the other. As shown in FIG. 2B, when handset unit 1 is enabled 
to place cordless telephone 10 in the talk mode to place a telephone call, 
handset unit 1 searches to detect a vacant channel and stops its scanning 
procedure when such a vacant channel is detected. In the present example, 
it is assumed that channel No. 3 is detected as vacant. Thereupon, handset 
unit 1 attempts to establish communication between itself and base unit 2 
by repeatedly transmitting to base unit 2 over channel No. 3 an 
identifying code uniquely identifying cordless telephone 10. At the same 
time, handset unit 1 looks for base unit 2 to retransmit back the same 
identifying code over channel No. 3 so as to complete the establishment of 
communication therewith. Meanwhile, base unit 2 continues to scan the 
channels in succession and will receive the identifying code from handset 
unit 1 when it next scans the channel selected by handset unit 1, that is, 
channel No. 3. Base unit 2 is thereby enabled and transmits the 
identifying code back to handset 1. When handset unit 1 receives the 
identifying code transmitted back from base unit 2, communication is fully 
established with both handset unit 1 and base unit 2 set in the talk mode. 
Thereafter, a telephone number may be transmitted from handset unit 1 to 
base unit 2 over channel No. 3 in a manner to be described below and then 
a telephone call may be put through and completed. 
Correspondingly, as shown in FIG. 2C, if an incoming telephone message 
signal is received over telephone network line 3a, the roles of handset 
unit 1 and base unit 2 are reversed. Now it is base unit 2 which first is 
enabled and scans to detect a vacant channel, now assumed to be channel 
No. 9. Base unit 2 transmits the identifying code to handset unit 1 over 
channel No. 9. When handset unit 1 detects the identifying code on 
reaching channel No. 9 during its scanning process, it retransmits the 
identifying code back to handset 1 to place both units in the talk mode 
and establish communication. It will be seen that the MCA system cordless 
telephone is able to prevent interference when an outgoing call is made or 
when an incoming call is received even though other telephones may be 
simultaneously operating near by. However, if a conventional cordless 
telephone outside the MCA system begins communication over its one duplex 
channel, and that duplex channel corresponds to one previously detected as 
vacant by the MCA cordless telephone, there will be interference. 
Before discussing the specific features of the present invention, a brief 
description of the underlying construction and operation of cordless 
telephone 10 will be given as background. Referring now to FIGS. 3 and 4, 
important portions of the circuitry contained within handset unit 1 and 
base unit 2, respectively, are therein illustrated. It should be noted 
that the circuitry within these two units contains many corresponding 
elements, so that the two figures and their accompanying description 
should be considered together. 
Handset unit 1 (FIG. 3) has a transceiver which includes a transmission 
system 110 comprised of elements bearing reference numerals 111 to 115, 
and a receiving system 120 comprised of elements bearing reference 
numerals 121 to 128. Base unit 2 (FIG. 4) similarly has a transceiver 
which includes a transmission system 210 comprised of elements bearing 
reference numerals 212 to 215, and a receiving system 220 comprised of 
elements bearing reference numerals 221 to 227. Base unit 2 further 
includes a connecting circuit 230, including elements 231 and 232 and 
serving to connect transmission system 210 and receiving system 220 to 
telephone network line 3a. The function and operation of these elements 
will be described as signals are transmitted between the two units. In the 
illustrated embodiment, the signals are transmitted over a selected one of 
the 10 allowed duplex channels, each containing an upper channel and a 
lower channel. The present invention is particularly directed to the 
selection of the duplex channel, as will be fully described below. 
For the transmission of a signal from handset unit 1 (FIG. 3), an audio 
signal St from a microphone 111 contained within mouthpiece 101 of handset 
unit 1 is supplied through a low frequency amplifier 112 to a voltage 
controlled oscillator (not illustrated) within a phase locked loop (PLL) 
circuit 113, which outputs a corresponding high frequency FM signal Su in 
the upper channel of a selected one of the duplex channels. For this 
purpose, PLL circuit 113 is provided with a channel selecting signal CH 
used to determine the frequency of a local oscillator signal used to 
frequency convert the FM signal to radio frequencies. Signal Su is 
supplied through a high frequency amplifier 114 and a band pass filter 115 
with a pass band including all the upper channels to antenna 100, by which 
it is transmitted as a radio frequency wave to base unit 2 over the 
communication channel. 
Referring now to FIG. 4, signal Su transmitted from handset unit 1 is 
received at base unit 2 by antenna 200 and is supplied through a band pass 
filter 221, also having a pass band including all the upper channels, and 
a high frequency amplifier 222 to a mixing circuit 223. Mixing circuit 223 
receives a local oscillation signal from a local oscillator (PLL) circuit 
224 receiving channel selecting signal CH to frequency convert the signal 
Su to an FM intermediate frequency signal. This FM signal is supplied 
through an intermediate frequency amplifier 225 to an FM demodulating 
circuit 226, wherein it is demodulated to produce audio signal St. Audio 
signal St is passed to telephone network line 3a through an audio 
frequency amplifier 227 and the signal transmission circuit 230 comprising 
hybrid circuit 231, and relay circuit 232. Hybrid circuit 231 has solid 
state components and moving contacts to form a 2-line to 4-line transition 
between the electronic receiving system 220 and electromechanical relay 
circuit 232. Relay circuit 232 includes a relay of the type to make and 
break the connection between base unit 2 and telephone network line 3a, 
and a hold relay which in a first position can hold a connection with 
telephone network line 3a while disconnecting it from hybrid circuit 231, 
so that telephone communication with telephone network line 3a is 
temporarily interrupted while keeping the incoming call available on 
telephone network line 3a. The hold relay has a second position in which 
telephone network line 3a is made available for connection to hybrid 
circuit 231 to release telephone communication. 
The transmission of signals from base unit 2 to handset unit 1 will now be 
described. When incoming message signals are received from telephone 
network line 3a, an audio signal Sr is supplied through relay circuit 232, 
hybrid circuit 231 and low frequency amplifier 212 to a VCO (not 
illustrated) of a PLL circuit 213, which outputs a corresponding high 
frequency FM signal Sd in the lower channel of the same duplex channel as 
signal Su. PLL circuit 213 is supplied with channel selecting signal CH 
for this purpose. Signal Sd is supplied through a high frequency amplifier 
214 and a band pass filter 215 with a pass band including all the lower 
channels to antenna 200, by which it is transmitted to handset 1 over the 
communication channel. 
Referring back to FIG. 3, signal Sd is received by antenna 100 and is 
supplied through a band pass filter 121 with a pass band including all the 
lower channels and a high frequency amplifier 122 to a mixing circuit 123. 
Mixing circuit 123 is supplied with a local oscillation signal from a 
local oscillator (PLL) circuit 124 receiving channel selecting signal CH, 
so that signal Sd is frequency converted to an FM intermediate frequency 
signal. This FM signal is supplied through an intermediate frequency 
amplifier 125 to an FM demodulating circuit 126, wherein it is demodulated 
to the audio signal Sr and supplied through an audio frequency amplifier 
127 to an electro-audio transducer or loudspeaker 128 contained within an 
earpiece of handset unit 1. 
In accordance with an important aspect of the present invention, handset 
unit 1 further contains a control circuit, generally designated by 
reference numeral 140 in FIG. 3, and base unit 2 similarly contains a 
control circuit, generally indicated by reference numeral 240 in FIG. 4. 
These control circuits may each advantageously be comprised in whole or in 
great part by a microprocessor or microcomputer contained on a single 
integrated chip. An advantageous example is the MSM-6404 microcomputer IC 
manufactured by Oki Electric Industry Co., Ltd. Control circuit 140 
includes a central processing unit (CPU) 141, for 4-bit parallel 
processing, a read only memory (ROM) 142, which stores control programs 
for controlling the operation of CPU 141, a random access memory (RAM) 143 
serving as a work and storage area, and an input/output (I/0) port 144. 
Similarly, control circuit 240 includes a CPU 241, a ROM 242, a RAM 243 
and an I/0 port 244, each performing a function corresponding to that of 
the element of control circuit 140 having the same two final digits in the 
reference numeral. 
Importantly RAM 143 includes three storage areas A.sub.1, A.sub.2 and 
A.sub.3, collectively referred to herein as area A.sub.i. RAM 243 
similarly includes three areas A.sub.1 ', A.sub.2 ' and A.sub.3 ', 
collectively referred to herein as areas A.sub.i '. Areas A.sub.i and 
A.sub.i ' are each adapted to store one of ten identifiers uniquely 
respectively associated with the ten duplex channels and, in particular, 
to store an identifier of a channel on which interference is detected. A 
default identifier having a value of 0 is stored in areas A.sub.i, A.sub.i 
' of RAMs 143, 243 when no channel identifier is stored therein. In 
addition, ROM 142 stores a control program 300 and ROM 242 stores a 
control program 400. These programs 300, 400 are used to control the 
operations of control circuits 140, 240, respectively, in scanning the 
communication channels and establishing communication between handset unit 
1 and base unit 2, as discussed below. 
Connected to control circuit 140 for controlling the latter are ten-key 
panel 151, talk switch 152 and an ID ROM 153. Ten-key panel 151 has 
conventional non-lock push button switches for inputting the telephone 
number to be called. Talk switch 152 is a three position change-over 
switch used for selecting an operational mode of handset unit 1. When a 
movable contact 152a of talk switch 152 contacts fixed contact O (the OFF 
position), the power is turned off to all the elements within handset unit 
1 except control circuit 140, to place handset unit 1 in an off mode. When 
movable contact 152a contacts fixed contact S (the STANDBY position), 
handset unit 1 is placed in a standby mode to await the signal Sd from 
base unit 2 and to perform a standby scanning operation as discussed 
below. When movable contact 152a contacts fixed contact T (the TALK 
position), handset unit 1 is placed in its talk mode and the communication 
channel between handset unit 1 and base unit 2 may be established. 
ID ROM 153 stores an identifying code ID uniquely identifying cordless 
telephone 10. A corresponding ID ROM 253 is connected to control circuit 
240 and stores the same identifying code ID. Handset unit 1 and base unit 
2 are designed to establish communication only with each other by means of 
the exchange of identifying code ID so as to avoid interference with other 
cordless telephones and to remove the possibility of wiretapping and 
unauthorized use. The general operation of cordless telephone 10 in 
exchanging identifying code ID is as follows. 
An MSK (minimum shift key) modulating circuit 161 is connected to control 
circuit 140 for converting binary signals supplied thereto to an MSK 
signal Sm in the audio frequency band. These binary signals include 
identifying code ID, a dial signal DS corresponding to the telephone 
number entered through ten-key panel 151 and possibly other control 
signals. MSK signal Sm is supplied to audio frequency amplifier 112 for 
transmission to base unit 2. In particular, before communication has been 
established between handset unit 1 and base unit 2, MSK signal Sm is 
designed to occupy an audio frequency band as illustrated in FIG. 7A and 
at this initial stage is transmitted at high speed, for example, 1.2 kbps, 
so as to rapidly establish such communication. On the other hand, after 
communication has been established, if it is desired to continue 
transmitting identifying code ID or other control signals, MSK signal Sm 
is transmitted in a frequency band lower than that of signals St and Sr at 
a relatively lower rate, for example, 110 bps, as illustrated in FIG. 7B. 
In base unit 2 (FIG. 4) the output of FM demodulator 226 is supplied to an 
MSK demodulator 263 wherein it is demodulated to provide the identifying 
code ID and other control signals encoded in MSK signal Sm. The 
demodulated signals are supplied to control circuit 240 for comparison 
with the identifying code ID stored in ID ROM 253. Base unit 2 similarly 
includes an MSK modulating circuit 261 for encoding the identifying code 
ID and other control signals in a signal Sm transmitted from base unit 2 
to handset unit 1 in the same manner as described above. In handset unit 
1, an MSK demodulating circuit 163 is supplied with the output of FM 
demodulating circuit 126 to demodulate the identifying code ID, control 
signals and other signals encoded in MSK signal Sm. The demodulated 
signals are supplied to control circuit 140. 
Returning to the exchange of signals, when an incoming call is detected in 
base unit 2 and signal Sd is transmitted to handset unit 1, in handset 
unit 1, the output of FM demodulating circuit 126 is supplied to a 
reception detecting circuit 162 for detecting the presence or absence of 
signal Sd in the output. As is conventional, the frequency components of 
the output are different in the presence or absence of signal Sd. A 
detection signal NSQL generated by reception detecting circuit 162 when 
signal Sd is present is supplied to control circuit 140 and an appropriate 
control signal is supplied to a ring tone generator 164 for generating a 
bell sound signal. The bell sound signal is supplied to a ringer 165 
causing it to generate a bell sound, i.e. handset unit 1 rings. 
Control circuit 140 of handset unit 1 generates a transmission enable 
control signal TXEN and supplies it to PLL circuit 113 to control whether 
or not FM signal Su is output therefrom. As discussed in more detail 
below, the communication channel is established only when identity between 
identifying codes stored in base unit 2 and handset 1 and exchanged over a 
vacant channel is detected. When identity is detected, signal TXEN enables 
PLL circuit 113 to output signal Su, whereas if identity is not detected, 
signal TXEN disables PLL circuit 113. 
Control circuit 140 also generates a muting signal MUTE supplied to audio 
frequency amplifier 127 when no telephone conversation is in progress, to 
prevent extraneous noise from being generated and output through 
transducer 128. 
In base unit 2, a tone generating circuit 264 generates a tone encode 
signal TE corresponding to the telephone number for an outgoing call, 
which signal TE is supplied to audio frequency amplifier 227 and thence to 
hybrid circuit 231 for transmission to telephone network line 3a. A bell 
signal detecting circuit 265 is connected to telephone network line 3a to 
detect an incoming bell signal indicating the presence of an incoming 
call. The output detected signal BL of bell signal detecting circuit 265 
is supplied to control circuit 240, which is responsive thereto to send a 
control signal in MSK signal Sm to control circuit 140, which in turn 
causes call tone generator 164 to generate its bell sound signal. Thus, 
handset unit 1 will ring in response to the detection of an incoming 
telephone call. 
In the preferred embodiment illustrated in FIG. 4, base unit 2 is without a 
switch corresponding to talk switch 152 and instead it is assumed that 
base unit 2 is normally in a standby mode until and unless a telephone 
message signal is received on telephone network line 3a or it is enabled 
through communication established by handset unit 1 in making an outgoing 
telephone call. 
Turning now to the specifics of the present invention, cordless telephone 
10 while in the standby mode detects which of the communication channels 
have interference thereon and records or memorizes these detected channels 
so as to avoid or skip them during a subsequent attempt to establish 
communication, that is, when an incoming telephone call is received or an 
outgoing telephone call is made while in the talk mode. Such interference 
may most likely be caused by another cordless telephone, but may also be 
caused by other apparatus or natural phenomena. It is recognized in the 
present invention that handset unit 1 and base unit 2 may be separated to 
such an extent that one unit may pick up interference on a particular 
channel while the other unit may fail to detect such interference, by 
reason of being considerably more removed from the source of the 
interference. Thus, in the preferred embodiment, each of handset unit 1 
and base unit 2 independently scans the duplex channels to detect 
interference thereon and stores in the respective ROMs 143, 243 the 
identifiers of the detected channels. This provides a double check to 
avoid the use of a channel carrying interfering signals. 
The operations of handset unit 1 and base unit 2 in the standby and talk 
modes are controlled in accordance with the respective control programs 
300 and 400 stored in ROMs 142 and 242 and loaded into CPUs 141 and 241, 
respectively. These operations will now be described in connection with 
FIGS. 5A and 5B, together illustrating the flowchart for control program 
300 for handset unit 1, and FIGS. 6A and 6B, together illustrating the 
flowchart for control program 400 for base unit 2. 
Referring first to FIG. 5A, when handset unit is in the standby mode, PLL 
circuits 113 and 124 are both controlled at step 301 by control circuit 
140 producing a value (CH) of channel selecting signal CH to the up and 
down channels, respectively, of a present one of the duplex channels 
having an identifier associated therewith equal to a value (CH) of signal 
CH. In other words, each value (CH) of signal CH, which may for example 
range from 1 to 10 corresponding to the ten duplex channels, constitutes a 
unique identifier associated with a particular one of the duplex channels. 
At step 302, the position of talk switch 152 is checked. Initially, it is 
assumed that talk switch 152 of handset unit 1 is set in the standby 
position at contact S placing handset unit 1 in its standby mode. 
Therefore, program 300 proceeds to step 303 wherein it is determined from 
signal NSQL whether a signal in the down channel set at step 301, which 
may be FM signal Sd or an interference signal, is being received. If no 
signal is being received, program 300 proceeds to step 304 in which the 
first identifier (A.sub.1) stored in area A.sub.1 of RAM 143 is compared 
with identifier (CH) of the channel set at step 301. If (CH) does not 
equal (A.sub.1), program 300 proceeds to step 305 to compare identifier 
(CH) with identifier (A.sub.2) stored in area A.sub.2. If again (CH) does 
not equal (A.sub.2), program 300 proceeds to step 306 wherein the value 
(CH) of channel selecting signal CH is incremented by 1 to designate the 
next channel in sequence, and program 300 returns to step 301. For so long 
as no signal or interference is detected on any of the channels, program 
300 loops through steps 301-306, repeatedly scanning all the channels in 
turn, as illustrated in FIG. 2A. 
The operation of handset unit 1 in its standby mode to detect interference 
on any of the communication channels and to store the respective 
identifiers in areas A.sub.i will now be described. In step 303, if it is 
detected that an FM signal Sd at the frequency of the down channel of the 
duplex channel set at step 301 is present, the program proceeds to step 
311 to determine whether signal Sd includes therein identifying code ID 
identical to the identifying code ID stored in ROM 153. If identity is 
detected, then base unit 2 is attempting to establish communication upon 
receiving an incoming telephone call and program 300 proceeds to step 331 
for a procedure to be discussed below. For the present, assuming no such 
identity is detected, that is, a different identifying code from another 
cordless telephone is included or there is no identifying code at all, 
program 300 proceeds to step 312 in which identifier (CH) is compared with 
identifiers (A.sub.i) stored in area A.sub.i to determine whether the 
present channel has already been detected as containing interference. If 
identifier (CH) is equal to any of the stored identifiers, program 300 
proceeds to step 306 to increment (CH). However, if identifier (CH) is 
unequal to all of the identifiers (A.sub.i), program 300 proceeds to step 
313 wherein it is checked whether the default identifier is stored in area 
A.sub.1. If at step 313 the default identifier is stored in area A.sub.1, 
so that (A.sub.1) equals 0, program 300 proceeds to step 317 to store 
identifier (CH) in area A.sub.1, and then returns to step 306. However, if 
a channel identifier is already stored therein, that is if (A.sub.1) does 
not equal 0, program 300 proceeds to step 314 to check whether the default 
identifier is stored in area A.sub.2. If the default identifier is stored 
in area A.sub.2, so that (A.sub.2) equals 0, program 300 proceeds to step 
316 to store identifier (CH) in area A.sub.2, and then returns to step 
306. If, however, identifier is stored in area A.sub.2, so that (A.sub.2) 
does not equal 0, program 300 proceeds to step 315 to store identifier 
(CH) in area A.sub.3, and the program returns to step 306. 
Thus, as described above, when a channel is newly detected as containing 
interference during the standby mode, it is initially stored in area 
A.sub.3 and thereafter is successively moved to area A.sub.2 and then to 
A.sub.1 as additional channels are detected with interference. If a fourth 
and higher order channels are detected with interference, their 
identifiers will successively displace each other in area A.sub.3 during 
each cyclical scan of the ten duplex channels. 
Returning now to the fundamental scanning operation of steps 301-306, if 
interference has been previously detected on a channel, its corresponding 
channel identifier, here assumed to be identifier (A.sub.1), will be 
stored in area A.sub.1 of RAM 143, and therefore, if the interference then 
stops, at some point during the successive scanning of the channels step 
304 will detect that (CH) equals (A.sub.1). At such point program 300 
proceeds to step 307 in which identifier (A.sub.1) stored in area A.sub.1 
and the identifier (A.sub.3) stored in area A.sub.3 are switched. If no 
channel identifier has yet been stored in area A.sub.3, the execution of 
steps 304 and 307 will cause the identifier (A.sub.1) to be stored in area 
A.sub.3 and area A.sub.1 will become vacant. In such a manner the channel 
identifier for this channel now without interference will be shifted to 
area A.sub.3 and will be dropped when an interference is detected on a new 
channel. Program 300 then proceeds to step 306 to increment value (CH) to 
designate the next channel. 
Correspondingly, if a channel identifier (A.sub.2) is stored in area 
A.sub.2 of RAM 143, then at step 305 at some point during the scanning, 
(CH) will equal (A.sub.2). At such point, program 300 proceeds to step 308 
in which the identifier (A.sub.2) stored in area A.sub.2 and the 
identifier (A.sub.3) stored in area A.sub.3 are switched, and program 300 
then proceeds to step 306. By this means, the channel identifiers are 
stored in the ordered areas A.sub.i in a weighted order in which A.sub.1 
stores the most recently detected channel. 
The operation of base unit 2 in the standby mode in detecting channels with 
interference and in storing the respective identifiers in ordered memory 
locations A.sub.1 '-A.sub.3 ' of RAM 243 is illustrated in FIG. 6A under 
the control of program 400. It will be seen that the operations effected 
by base unit 2 at steps 401 and 404-417 correspond precisely to the 
operations effected by handset unit 1 at steps 301 and 304-317, 
respectively. It is important to remember, however, that the channels set 
at steps 301 and 401 are not necessarily the same and indeed this feature 
helps to prevent the erroneous establishment of communication when base 
and handset units 1, 2 detect interference on different channels, as 
discussed below. Furthermore, the operations at steps 402 and 403 by base 
unit 2 are different from those of handset unit 1 at step 302 and 303. 
Specifically, at step 402, handset unit 2 detects the presence or absence 
of an incoming telephone message signal over telephone network line 3a by 
checking for the presence or absence of signal BL. If no incoming 
telephone message signal is present, program 400 proceeds to step 403 in 
which the presence or absence of a signal in the upper channel of the 
duplex channel set at step 401, such as FM signal Su is detected, as 
opposed to the detection of a signal in the lower channel in step 303 by 
handset unit 1. With these exceptions, the operations of handset unit 1 
and base unit 2 in the standby mode are identical. 
The operation of cordless telephone 10 in placing an outgoing telephone 
call from handset unit 1 over a channel without interference will now be 
explained. As described above, to initiate an outgoing telephone call, 
talk switch 152 is changed over to contact T to establish the talk mode at 
handset unit 1. This switching operation is detected at step 302 (FIG. 5A) 
and program 300 proceeds to step 321 (FIG. 5B) in which identifier (CH) is 
successively compared with the identifiers (A.sub.i). If identifier (CH) 
does not equal any of identifiers (A.sub.i), that is, if the present 
channel set at step 301 has not previously been detected as containing 
interference, program 300 proceeds to step 322 to check whether the 
present channel now is being used by another cordless telephone. 
Specifically, signal NSQL will indicate whether the channel set at step 
301 is being used by another cordless telephone, for example by a 
conventional cordless telephone which will use only one particular channel 
regardless of the presence or absence of other telephone communications on 
that channel. If in fact the channel set at step 301 is not being used, 
program 300 proceeds to step 323 and control circuit 140 supplies enabling 
control signal TXEN to PLL circuit 113 to enable the transmission of 
signal Su. At step 324, identifying code ID is retrieved from ROM 153 and 
modulated to produce MSK signal Sm by modulator 161 and thereafter 
supplied to amplifier 112. Therefore, as illustrated in FIG. 2B, 
identifying code ID is transmitted to base unit 2 in FM signal Sd over the 
up channel of the duplex channel set at step 301. Identifying code ID is 
repeatedly transmitted at step 324 for a time period at least long enough 
to permit all the channels to be scanned by base unit 2 and an identifying 
code ID transmitted back therefrom and redetected in handset unit 1. 
Meanwhile, base unit 2 has remained in the standby mode and has been 
sequentially scanning the channels by repeating the operations of steps 
401-417 as described above. Now, however, at step 403 (FIG. 6A) when the 
channel set at step 401 is the same as the channel set at step 301 and 
over which signal Su is being transmitted from handset unit 1, signal Su 
will be detected and program 400 proceeds to step 411 to determine whether 
the correct identifying code ID is contained in the received signal Su. 
Since signal Su is in fact coming from the correct handset unit 1, program 
400 proceeds to step 431 (FIG. 6B) in which identifier (CH) is compared 
with the identifiers (A.sub.i ') previously stored in areas A.sub.i '. If 
identifier (CH) does not equal any of identifiers (A.sub.i '), thus 
indicating that at base unit 2 also the channel associated with identifier 
(CH) has not been detected as including interference, program 400 proceeds 
to step 432 in which transmission of FM signal Sd is enabled by the 
generation of signal TXEN. Then, at step 433 identifying code ID stored in 
ROM 253 is modulated to produce MSK signal Sm by modulator 261 and is 
supplied to amplifier 212. Therefore, as illustrated in FIG. 2B, 
identifying code ID is transmitted back to handset 1 in FM signal Sd 
through the down channel of the duplex channel set at step 401. 
Thereafter, the program proceeds to step 490 wherein base unit 2 is 
connected to telephone network line 3a, that is, amplifier 227 is released 
from muting by the cessation of signal MUTE and telephone network line 3a 
is connected through relay circuit 232 to converting circuit 231. 
Meanwhile, identifying code ID transmitted by base unit 2 in step 424 is 
detected at handset unit 1 in step 325 from the output signal of 
demodulating circuit 163. In practice, steps 324 and 325 are alternated 
for a time period equal to a shorter one of the period in which all the 
channels are scanned in base unit 2 and the period until transmission of 
identifying code ID from base unit 2 back to handset unit 1 is confirmed. 
As in the present case it as assumed that base unit 2 transmits the 
correct identifying code ID back to handset unit 1, program 300 proceeds 
to step 390 wherein amplifier 127 is released from muting by the cessation 
of signal MUTE and communication is established between handset unit 1 and 
base unit 2 over the present channel. In this manner, handset unit 1 is 
connected to telephone network line 3a through base unit 2. 
Thereafter, the operator may input the desired telephone number by 
operating the buttons on ten-key panel 151 of handset unit 1 and a 
corresponding dial signal DS (FIG. 3) is generated by control circuit 140, 
converted to MSK signal Sm by modulating circuit 161 and then transmitted 
to base unit 2 as described above. In base unit 2 (FIG. 4), dial signal DS 
is derived from demodulating circuit 263 and tone encoder 264 is 
controlled by control circuit 240 in response to dial signal DS to 
generate a tone encode signal TE corresponding to the input telephone 
number. Signal TE is delivered through amplifier 227, converting circuit 
231 and relay circuit 232 to telephone network line 3a. Thereafter, as is 
conventional, the telephone corresponding to the input number is called 
and thus the outgoing telephone call is completed. 
However, if prior to the initial transmission of identifying code ID by 
handset unit 1 it is detected at step 321 (FIG. 5B) that the present 
channel set in step 301 is the same as a detected channel having its 
identifier stored in any of the areas A.sub.i, or if it is detected at 
step 322 that the present channel is already in use by another cordless 
telephone, program 300 proceeds to step 306 to increment identifier (CH) 
by 1 to designate the next channel, and then the program returns to step 
301. In other words, if in the talk mode the present channel in step 321 
is one of the detected channels, handset unit 1 does not reach step 322 to 
detect whether the present channel is currently in use. Functionally this 
means that handset unit 1 scans the communication channels other than 
those detected ones whose identifiers (A.sub.i) are stored in area 
A.sub.i. Thereafter, steps 323-325 (FIG. 5B) and steps 432 and 433 (FIG. 
6B) are repeated to detect a vacant channel over which communication 
thereafter may be carried out. 
Furthermore, if at step 325 the correct identifying code ID is not received 
back from base unit 2 at handset unit 1, program 300 proceeds to step 325 
in which control circuit 140 stops signal TXEN to disable the transmission 
of signal Su, and then returns to step 306. This may occur, for example if 
the channel set at step 301, while vacant at handset unit 1, is detected 
with interference at base unit 2 at step 431. Specifically, in base unit 
2, if it detected at step 431 that identifier (CH) set at step 401 is 
equal to any of the stored identifiers (A.sub.i ') stored in areas A.sub.i 
', program 400 proceeds to step 434 in which the FM signal Sd without the 
identifying code ID is transmitted for a short period of time, for example 
one second, and then program 400 returns to step 406. This is detected 
back in handset unit in step 325 as the absence of the identifying code 
ID. In response thereto, handset unit 1 increments its present channel and 
transmits identifying code ID over the next vacant channel and again waits 
for the same to be transmitted back through that next channel. Of course, 
FM signal Sd (or FM signal Su) without identifying code ID will be 
detected as interference by any unit in the standby mode. 
Thus, when an outgoing telephone call is to be made from handset unit 1 and 
talk switch 152 is changed over to establish the talk mode, handset unit 1 
searches for a vacant channel while skipping those channels previously 
detected with interference and calls base unit 2 over the detected vacant 
channel. When base unit 2 responds to the call, communication is 
established between handset unit 1 and base unit 2 over the detected 
vacant channel to enable the outgoing telephone call. 
Correspondingly, when an incoming telephone message signal is detected on 
network telephone line 3a at base unit 2, base unit 2 can initiate and 
establish communication with handset unit 1 in a manner corresponding to 
the operation described above with regard to an outgoing telephone call. 
Specifically, at step 402 (FIG. 6A) bell signal BL is detected and program 
400 proceeds to carry out steps 421-426/490, while program 300 at handset 
unit 1 executes steps 331-334. Once again, if the channel initially 
detected as vacant in base unit 2 is recorded as carrying interference at 
handset unit 1, cordless telephone 10 will avoid establishment of 
communication over this initially detected channel and will search further 
to find a channel vacant at both handset and base units 1, 2. 
Thus, when an incoming call is received, it is base unit 2 which searches 
for a vacant channel while skipping the previously detected channels with 
interference and calls handset unit 1 over the detected vacant channel. 
When handset unit 1 responds to the call, communication is established 
between handset unit 1 and base unit 2 to enable the incoming call to be 
received. 
In accordance with known techniques, one skilled in the art will realize 
that the operations affected by circuits 161-164 and 216, 264 may be 
realized by software. Furthermore, the telephone number may be transmitted 
by dial pulses. 
In addition, programs 300 and 400 may be modified so that at steps 324 and 
424, respectively, the presence or absence of FM signals Sd and Su may be 
respectively checked as well as identifying code ID. Only if signals Sd 
and Su are then detected will the programs respectively proceed to steps 
325 and 425 to check whether or not the identifying code ID is transmitted 
back from the other unit. 
In accordance with the present invention, an MCA system cordless telephone 
without a separate control channel may record channels detected with 
interference in its memory and thereafter, in searching for a vacant 
channel, may skip the detected channels. If a vacant channel is thereafter 
detected, communication between the handset unit and the base unit is 
established over the detected vacant channel. Thus, communication may be 
established over a vacant channel without interference from 
electromagnetic waves transmitted from another cordless telephone or other 
apparatus. A further advantage is that the communication between the 
handset unit and the base unit can be promptly established without 
attempted transmission over channels with interference. Furthermore, since 
the channels detected with interference are recorded, taken into account 
the order of detection, these channels will be skipped without error. 
Having specifically described a preferred embodiment of the invention, it 
will be apparent that the invention is not limited to such embodiment, and 
that many modifications and variations may be effected therein by one 
skilled in the art without departing form the spirit or scope of the 
present invention as defined in the appended claims.