Multi-channel access (MCA) radio telephone system

A multi-channel access (MCA) radio telephone system completes calls from conventional telephone systems over a plurality of shared radio channels to a base station serving a plurality of substations connected to a second telephone set. The MCA radio telephone system provides a method for clearing calls after they are completed. The method comprising the steps of: (a) detecting the hang-up at the first and conventional set involved in the clearing call; (b) maintaining the D.C. loop of the subscriber line to the first and conventional telephone set in response to the detection of an on-hook signal and returning the radio channel to an idle and waiting state; (c) detecting the hang-up of the second telephone set which is served by the base station re-establishing a radio channel between the base station and the substation involved in the call being cleared; (d) sending a release signal from the clearing substation through the re-established radio channel to the base station; and (e) opening the D.C. loop to the first and conventional telephone set in response to the release signal and placing the re-established radio channel into an idle and waiting state.

BACKGROUND OF THE INVENTION 
The present invention relates to a multi-channel access (MCA) radio 
telephone system and, more particularly, to a method for clearing such an 
MCA radio telephone system. 
In a conventional MCA radio telephone system, such as a marine telephone 
system, a plurality of shore subscriber telephone sets are connected 
through a shore telephone exchange and a concentrator to a base station, 
which is coupled to a plurality of substations, such as ships, over a 
plurality of radio channels. In such a system, when a shore subscriber 
hangs up first, the clearing process can be accomplished by either one of 
the following two methods. 
In the first method, in response to the hanging-up of the shore subscriber, 
the shore exchange sends a busy tone through the concentrator to the base 
station. The busy tone is further carried with the present radio channel 
from the base station to the substation the subscriber of which can hear 
the busy tone as an audible signal indicating that the shore subscriber 
had hung up. The substation subscriber, therefore, hangs up his own 
telephone set. Only then is the radio channel released, and thereby made 
available for use by another substation. 
A disadvantage of this method is that the radio channel remains busy until 
the substation subscriber hangs up even though the call is finished. This 
is detrimental to the efficient utilization of radio channels in an MCA 
system, wherein these radio channels are shared by a plurality of 
substations. 
In the second method, in response to the hanging-up of the shore 
subscriber, the shore exchange sends a busy tone to the concentrator which 
detects this busy tone and finds that the shore telephone is already 
on-hook. Also, the concentrator sends to the substation a signal 
(disconnection signal) to let it know that the shore subscriber has hung 
up. This disconnect signal releases the radio channel shared by a 
combination of the base station and the substation, and further releases 
the direct current (D.C.) loop of the line connecting the subscriber to 
the shore exchange. Upon detection of the disconnect signal, the 
substation releases its radio channel and causes an internal circuit 
therein to generate a busy tone (a local busy tone), which is heard in the 
telephone set. This local busy tone keeps on sounding until the substation 
subscriber hangs up. 
This method is favorable because it is a more efficient utilization of 
radio channels. Nevertheless, it has a disadvantage since the D.C. loop of 
the subscriber line is open as viewed from the shore exchange. Although 
the substation is deemed to have already hung up and to be ready to accept 
a call from the exchange, at any time, the telephone set connected to the 
substation is actually still off-hook, so that no call can be received as 
yet. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to provide an MCA radio 
telephone system having a clearing method which permits an efficient 
utilization of radio channels. 
Another object of the invention is to provide an MCA radio telephone system 
having a clearing method which is capable of avoiding a situation wherein 
another exchange subscriber originates a call to the substation subscriber 
during the clearing operation of a substation coupled to an exchange 
subscriber. 
According to the present invention, a multi-channel access (MCA) radio 
telephone system completes calls over a plurality of radio channels shared 
by a combination of a base station coupled to a first telephone set, and a 
plurality of substations connected to a second telephone set. The MCA 
radio telephone system provides a method for clearing calls comprising the 
steps of: 
(a) detecting the hanging-up at the first telephone set; 
(b) maintaining the D.C. loop of the subscriber line of the first telephone 
set in response to the detection of the hung-up state and returning the 
radio channel to a waiting state; 
(c) detecting the hanging-up of the second telephone set and again 
establishing a radio channel between the base station and the substation; 
(d) sending through the re-established radio channel a release signal from 
the substation to the base station; and 
(e) opening the D.C. loop in response to the release signal and placing the 
re-established radio channel into an idle and waiting state.

DETAILED DESCRIPTION OF THE EMBODIMENT 
Referring to FIG. 1, a telephone exchange 1 is a central office, local, or 
an end office (EO), which is either directly or through a toll office 
connected to subscriber lines 51, to which n (n is a positive integer) 
subscriber stations or telephone sets 5 are connected, respectively. A 
concentrator 2 is connected to the end office 1 by n pairs of subscriber 
lines 6. The concentrator 2 concentrates (or conversely deconcentrates) n 
pairs of subscriber lines 6 into m pairs of subscriber lines 7, which are 
connected to a radio base station 3. The letter m is a positive integer 
and generally is no greater than n, because that gives an efficient 
utilization of frequencies. More specifically, the number of 
transmitter/receivers at the base station 3 is reduced if the radio 
telephone system is a frequency division multiple (FDM) system; or, the 
frame length is shortened if it is time division multiple (TDM) system. 
The radio base station 3 has transmitter/receivers corresponding to the 
FDM or TDM system and constitutes m radio channels. Around this radio base 
station 3 are geographically scattered a plurality of substations 4-1 to 
4-m, which can be connected by radio channels to the base station 3. To 
each of the substations 4-1 to 4-m are connected one or more telephone 
sets 8. The number of the telephone sets 8 in the whole system is n, 
corresponding to the n pairs of subscriber lines 6. 
Next will be briefly described the channel composition of the MCA radio 
telephone system. The system is classified into two types: one type 
comprising one or more control channels and a plurality of speech channels 
and the other type comprising only a plurality of speech channels. In the 
other type system, an idle signal is transmitted from the concentrator 2 
on every idle speech channel. A substation originating a call scans all 
the speech channels and transmits an origination signal and an 
identification number (ID No.) signal via one of the channels on which the 
idle signal is detected. When a substation is to receive a call, the 
concentrator 2 transmits a paging signal on any or specific one of the 
idle speech channels. Upon receiving the signal, the called substation 
performs the remainder of the connecting process and then begins 
conversation. 
In the system of the former type, the concentrator 2 transmits idle speech 
channel information on one of the control channels. A substation 
originating a call selects an idle speech channel according to the idle 
speech channel information on the control channel. The calling substation 
tunes to the idle channel to transmit an origination signal and its ID No. 
signal thereon. When a substation is to receive a call, the concentrator 2 
transmits to the called substation via one of the control channels. The 
transmission includes a channel designation signal to which the selective 
call signal for the substation has been added. Since each substation is 
waiting on one of the control channels, the called substation tunes in to 
the designated speech channel upon detection of its channel designation 
signal. Then, the called substation performs the remainder of the 
connection process and begins conversation. The inventive clearing method 
used in the MCA radio telephone system is applicable to both of the 
aforementioned two types of systems. 
Referring now to FIG. 2, the concentrator 2 comprises subscriber circuits 
21, each corresponding to one of the n pairs of subscriber lines 6, a 
switching network 22, trunk circuits 23, each corresponding to one of the 
m pairs of subscriber lines, and a controller unit 24 for controlling the 
trunk circuits 21 and the switching network 22. The radio base station 3 
has transmitter/receivers 31, each corresponding to one of the m pairs of 
subscriber lines multiplexer 32 and an antenna. While the base station 3 
in this instance uses the FDM system, a TDM base station would need only 
one transmitter/receiver and memory circuits, each corresponding to one of 
the subscriber lines 7. The controller unit 24 is responsible for the 
selection of the subscriber circuit 21 and trunk circuit 23 to be used and 
for the control of the connection completed via the switching network 22. 
The unit 24 also transmits pertinent control signals, such as the idle 
signal, to substations by way of the trunk circuits 23. 
The flow of voice signals after the establishment of a speech channel is as 
follows. A voice signal from the telephone set 5 enters the end office 1 
by way of a subscriber line 51, and passes through one of the subscriber 
lines 6 to the corresponding subscriber circuit 21. Then the voice signal 
is supplied, from subscriber circuit 21 through the trunk circuit 23 and 
subscriber line 7 to the corresponding transmitter/receiver 31. The 
signals are multiplexed at 32 and radiated from the antenna to the desired 
substation 4. The radiated voice signal is received by the 
transmitter/receiver 41 of the substation and sent to the telephone set 8, 
through an interface circuit 42. A voice signal from the substation 
telephone set 8 reaches the telephone set 5 via the same route, with 
transmission in the reverse direction. 
FIG. 3 is a circuit diagram illustrating a part of the subscriber circuit 
21 of the concentrator 2 shown in FIG. 2. Reference numerals 120 and 121 
identify terminals to which a pair of subscriber lines 6 (leading to 
subscriber stations) are connected; 122 and 123, switching circuits; and 
124, a resistor forming a terminating circuit. In a line-locked state to 
be described in detail with reference to FIGS. 4A and 4B, the switching 
circuits 122 and 123 terminate the subscriber line 6 with the terminating 
circuit 124. Reference numeral 125 identifies another switching circuit 
for forming a direct current (D.C.) loop, and 126, an inductance, which 
has a low resistance to a D.C. signal and a high impedance to the voice 
signal (0.3 kHz to 3.4 kHz). Upon formation of a D.C. loop by the 
switching circuit 125, a D.C. signal flows from terminal 120 through the 
switching circuit 122, switching circuit 125, inductance 126, switching 
circuit 123 and terminal 121, in that order. Or, the current may be in the 
completely opposite direction if the polarity of the D.C. signal is 
reversed. Reference numeral 127 identifies a 4-line/2-line converter 
circuit; 129, an outgoing speech line for feeding voice signals to the 
base station side; 130, another and incoming speech line for feeding voice 
signals to the end office side; 128, a busy tone detector; and 131, a 
terminal for detection of the busy tone. The busy tone detection signal at 
terminal 131 is supplied to the controller unit 24 (FIG. 2). The opening 
and closing of the switching circuits 122, 123 and 125 are controlled by 
the controller unit 24. 
Next will be described in detail the inventive clearing method, with 
reference to FIGS. 4A and 4B. The flow charts of FIGS. 4A and 4B represent 
an MCA telephone system which has no control channel that is separate from 
speech channels. The call described in these flow charts is originated by 
a substation 4. It has to be noted, however, that the inventive clearing 
method can also be applied even if the channel composition is different 
and if a call is received by a substation, as described earlier. 
In both FIGS. 4A and 4B, the flow on the left hand side is that of the 
controller unit 24 of the concentrator 2. The flow on the right hand side 
is that of the controller unit 43 of the substation 4. 
In FIG. 4A, the concentrator 2 transmits (at Step 201) an idle signal on an 
idle speech channel. 
When the substation 4 originates a call, it first detects (at Step 301) the 
idle signal on one of the speech channels. At Steps 302 and 303, the 
off-hook state is detected. When the substation subscriber raises the 
handset off-hook, the substation actuates (at Step 304) its 
transmitter/receiver and, at the same time, transmits an origination 
signal to the concentrator 2, via the base station 3. 
The concentrator, detecting this origination signal (at Step 202), ceases 
(at Step 203) to transmit the idle signal on the particular speech channel 
over which the origination signal has been sent. The substation detects 
the cessation of the idle signnal (at Step 305) and transmits (at Step 
306) its own identification number (ID No.) signal for loop checking. The 
concentrator detects the ID No. signal from the substation (at Step 204) 
and sends back (at Step 205) this ID No. signal together with a dial tone 
from the end office 1 (FIG. 1). The concentrator also forms (at Step 206) 
a D.C. loop across the subscriber line 6 which extend from the end office 
1. The end office 1 detects the formation of the D.C. loop, finds that the 
substation has originated a call, and supplies the dial tone to the 
concentrator. 
The substation subscriber, hearing the dial tone from the end office, dials 
(at Step 308). The concentrator passes on (at Step 207) this dial number 
signal to the end office, which calls the desired subscriber telephone 5. 
As the called subscriber raises the handset off-hook, the speech path is 
enabled (at Steps 208 and 309). The procedures of these Steps 201 to 208 
and 301 to 309 are the same as those of the conventional MCA telephone 
system. 
Next will be described the clearing method which is characteristic of the 
present invention. This clearing takes effect when the first subscriber to 
hang up is at the telephone set 5 which is connected to the end office 1. 
In response to the hanging-up by the subscriber at the telephone set 5 
(FIG. 1), the end office 1 transmits a busy tone by way of the subscriber 
line 6 to the concentrator 2. At Step 209 (FIG. 4A), the concentrator 
detects this busy tone, by the use of the busy tone detector 128 (FIG. 3), 
and transmits (at Step 210) a disconnection signal to the substation 4. 
Further, (at Step 211), the concentrator releases the radio speech channel 
and connects the terminating circuit 124 across line 120, 121 at contacts 
122, 123 to terminate the subscriber line 6, and hold the D.C. loop. The 
substation receives the disconnection signal (at Step 310), releases (at 
Step 311) the radio channel and at the same time generates a local busy 
tone. 
Since the D.C. loop of the subscriber line 6 is now being maintained, as 
viewed from the end office 1, the busy tone continues to be transmitted. 
On the other hand, the local busy tone is generated from the substation. 
Thus, the busy/idle states of the base station and of the substation agree 
with each other. As a result, another subscriber connected to the end 
office 1 can hear the busy tone from the end office; therefore, he or she 
may not originate a call to the substation subscriber. The line state 
which occurs during the above described busy/idle agreement is referred to 
as a "line-locked" state. Because the radio channel is "off" in this 
line-locked state, another substation can use this radio channel, thus 
resulting in a more efficient utilization of radio channels. 
Referring to FIG. 4B, when the subscriber at substation 4 hangs up the 
telephone set 8, under the line-locked state, the on-hook condition has to 
be made known to the concentrator 2. However, the radio channel used for 
the already ended conversation has been released and is now off. Thus, it 
cannot be used for transmitting this on-hook information. Therefore, a 
radio channel is again established in accordance with the principle of the 
MCA system. Thus, the Steps 212 to 216 and 312 to 317 in flow charts FIG. 
4B are followed to re-establish the required radio channel in the same 
manner, as described for the prior art. These Steps are exactly the same 
as those of Steps 201 to 205 and 301 to 306, respectively, in FIG. 4A. 
When its ID No. signal is confirmed (at Step 318), substation 4 transmits 
(at Step 319) a line-lock release signal. The concentrator receives the 
line-lock release signal (at Step 217 and then transmits a line-lock 
disconnect signal (at Step 218). Later (at Step 219), it again releases 
the radio channel, and also releases the D.C. loop of the subscriber line 
6 by opening the path through the switching network 22 and trunk circuit 
23. The substation 4 receives the line-lock disconnect signal (at Step 
320), and again releases (at Step 321) the radio channel, to return to an 
idle and waiting state as indicated at 201, 301. 
FIG. 5A is a time chart showing the time segments of the flow charts of 
FIGS. 4A and 4B, from the detection of busy tone at concentrator 2 (at 
Step 209) to the subscriber line release (at Step 219). FIG. 5B is a 
similar time chart showing the timing from the detection of a 
disconnection signal by substation 4 (at Step 310) to a release of the 
radio channel (at Step 321). The concentrator detects the busy tone at a 
time point a, transmits the disconnection signal to the substation at a 
time point b, terminates the subscriber lines with 600.OMEGA. resistance 
at a time point c, and discontinues transmission from the radio base 
station at a time point d. This discontinuation releases the radio channel 
to make it available for use by another substation. 
The substation 4 receives the disconnection signal, discontinues its 
transmission at a time point e, and gives the local busy tone to its 
telephone set 8 at a time point f. The substation remains in this state as 
long as its telephone handset is off-hook. When the telephone handset is 
put on-hook at a time point g, the local busy tone is terminated at a time 
point h, transmission is re-established at a time point i, and the 
line-lock release signal is transmitted to the concentrator 2 at a time 
point j. 
The channel over which the line-lock release signal is transmitted at time 
point j may be different from the channel that was used earlier for 
conversation. The concentrator, receiving the line-lock release signal, 
transfers this signal to the end office, re-establishes transmission, 
sends the line-lock disconnection signal to the substation at a time point 
k, and release the D.C. loop at a time point l. At a time point m, the 
busy tone from the end office is terminated. At a time point n, the 
substation turns off its transmission to complete the clearing process. 
As heretofore described, the present invention enables radio channels to be 
more efficiently utilized because each channel is released at once when 
the subscriber on the end office side hangs up first, after the 
conversation has finished. Further, the D.C. loop is released by opening 
the terminating circuit in the concentrator 2 when the telephone set 8 on 
the substation 4 side is on-hook. The busy/idle state of the subscriber 
lines 5, as viewed from the end office side, and of the telephone set 8 
connected to the substation 4 can be made to agree with each other. 
Those who are skilled in the art will readily perceive how to modify the 
system. Therefore, the appended claims are to be construed to cover all 
equivalent structures which fall within the true scope and spirit of the 
invention.