Cordless telephone system capable of quickly establishing connection during call setup phase

The present invention relates to a cordless telephone system.

Cordless telephone systems have currently been in widespread use due to their excellent mobility. The system includes a base station interfacing the public or private switched telephone network and a cordless station for exchanging radio control signals with the base station to establish a connection. During a standby mode, the base station constantly scans several speech channels to detect idle channels. In response to a call request from the cordless station, the base station specifies one of the idle speech channels as a possible candidate and informs the cordless station of the identifier of the specified channel over the control channel and waits for a response from the cordless station until a prescribed period expires. On receiving this information through the control channel, the cordless station switches to the specified channel and checks for the availability of the specified channel. Because of different field intensities between the sites of the base station and cordless station, the latter may determine that the specified channel is not actually available. Under such circumstances, the cordless station switches to the control channel and repeats the transmission of a call request to the base station. Failing to receive a response from the cordless station before the prescribed period expires, the base station recognizes that the specified channel is not available and switches to the control channel to wait for the retransmission of a call request. In response to the retransmission, the base station specifies another idle channel and informs this channel identifier to the cordless station to allow it to perform a channel check on the newly specified channel. The process will be repeated until an available speech channel is detected. Similar situations occur when an incoming call is received from the network.

Therefore, the time taken to establish a connection is substantial if busy conditions are repeatedly encountered at the site of the cordless station.

JP-A-1 208 919 discloses a radio communication system wherein a base station sends only one idle channel identifier and waits for the end-of-switching signal. If the latter is not received within a predetermined interval, the base station repeats the same process by scanning the channel spectrum to find a new idle channel and send its identifier.

EP-A-0 189 920 and EP-A-0 213 929 also disclose a radio channel control method for mobile communication.

It is therefore an object of the present invention to provide a cordless telephone system capable of establishing a connection in a short period of time following the reception of an incoming call from the network or an originating call from the cordless station. This object is solved with the features of the claim.

According to the present invention, several speech channels are periodically scanned by a base station during a standby mode to detect idle channels from the scanned channels. In response to receipt of a call request from a cordless station or an incoming call from a switched telephone network, the base station transmits a channel indicating signal to the cordless station over a control channel indicating that the detected idle channels can be used as possible candidate channels and switches to a first one of the candidate channels and waits for receipt of an end-of-switching signal from the cordless station during a prescribed period of time. On receiving the channel indicating signal, the cordless station switches to the first candidate channel and tests the first candidate channel for availability and transmits an end-of-switching signal if the first candidate channel is determined as being idle. The base station switches to a second one of the candidate channels if the end-of-switching signal is not received from the cordless station during the prescribed period of time. The cordless station switches to the second candidate channel and tests the second candidate channel for availability if the first candidate channel is determined as being busy during the prescribed period of time and transmits the end-of-switching signal over the second candidate channel if it is determined as being idle.

The present invention will be described in further detail with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none" compact="compact"><li>Fig. 1 is a block diagram of a cordless telephone system of the present invention;</li><li>Figs. 2A, 2B and 2C are flowcharts describing programmed instructions which are performed by the base station;</li><li>Figs. 3A and 3B are are flowcharts describing programmed instructions which are performed by the cordless station; and</li><li>Figs. 4A and 4B are illustrations of control signals exchanged between the base station and cordless station during a call originating mode and a call terminating mode, respectively.</li></ul>

In Fig. 1, two cordless telephone systems are illustrated as adjacent systems which share the same frequency spectrum. Each cordless telephone system comprises a base station 2 connected to a public or private switched telephone network 1 through a subscriber loop 3, and a cordless station 4. Base station 2₁ includes a hybrid 10 having a two-wire circuit coupled through subscriber loop 3₁ to the network 1, the transmit portion of the four-wire circuit being coupled to the input of a radio transmitter 11 and the receive portion of the four-wire circuit being coupled to the output of a radio receiver 13. Transmitter 11 and receiver 13 are provided with a respective frequency synthesizer which is controlled by a control circuit 12. The output of transmitter 11 is connected to a transmit antenna 16 and the input of receiver 13 is connected to a receive antenna 17. A channel status memory 14 is connected to control circuit 12 to store busy/idle status of all speech channels. As will be described, this is done by sweeping the frequency synthesizer of receiver 13 across the allocated spectrum so that all speech channels are scanned at periodic intervals and detecting their field intensities. For this purpose, a carrier detector 15 is connected to receiver 13 to supply the control circuit 12 with a busy or idle status signal indicating that the detected carrier intensity of a switched channel is higher or lower than a prescribed threshold, respectively. The busy/idle conditions of all speech channels are stored into channel status memory 14 and periodically updated during a standby mode. During a call processing mode, the memory 14 is accessed by control circuit 12 to select idle speech channels to generate a channel indicating signal indicating that the detected idle channels can be used as possible candidate channels.

In cordless station 4₁, radio signals from base station 21 are received by a receive antenna 28 and supplied to a radio receiver 20. Radio signals from base station 2₂ of adjacent system are also received by antenna 28 and fed into receiver 20. The audio-frequency output of receiver 20 is applied to a speech circuit 24 whose output is, in turn, coupled to the input of a radio transmitter 21. Speech circuit 24, which is normally turned off, is activated by control circuit 22 in response to a turn-on signal sent from base station 2₁. The output of transmitter 21 is coupled to a transmit antenna 29 for transmission of signals to receive antenna 17 of the base station. Receiver 20 and transmitter 21 are provided with a respective frequency synthesizer which is controlled by a control circuit 22. A carrier detector 23 is coupled to receiver 20 to detect the intensity of transmitted carrier to supply control circuit 22 with information on the busy/idle status of a channel to which the station is being switched. A keypad 25 supplies dialing information as well as off-hook signal to control circuit 22. An idle channel memory 27 is connected to control circuit 22 to store idle channel data supplied from base station 2₁ to enable control circuit 22 to sequentially switch the station from one idle speech channel to the next until it detects an actually available speech channel. Receipt of a ringing signal sent from base station 2₁ causes control circuit 22 to activate a tone ringer 26 and the off-hook condition of the station or receipt of a turn-on signal for speech circuit causes control circuit 22 to deactivate it.

The system has a single control channel and several speech channels. These control and speech channels are commonly used by adjacent systems, so radio signals from antenna 16 of base station 2₁ are also received by cordless station 4₂ as shown at A₁ and signals from base station 2₂ are also received by antenna 28 of station 4₁ as shown at A₂. Likewise, signals from antenna 29 of station 4₁ are also received by base station 2₂ as shown at B₁ and signals from station 4₂ are also received by antenna 17 of base station 2₁ as shown at B₂.

The control circuits 12 and 22 are a microprocessor-based controller which is programmed to perform a sequence of instructions as will be described hereinbelow.

In Fig. 2A, during standby modes, base-station control circuit 12 is shown to perform a standby routine in which it constantly scan all speech channels and examines their busy/idle status and updates the channel status memory 14. The program execution starts with step 31 in which variable i is set equal to 1. Exit then is to step 32 to cause base-station transmitter 11 and receiver 13 to switch to speech channel S_i and proceeds to step 33 to check the carrier intensity of channel S_i detected by carrier detector 15 against the prescribed threshold to determine if channel S_i is idle or currently used by any cordless telephone system and store the busy/idle status of the channel into channel status memory 14. Exit then is to step 34 to increment variable i by one and moves ahead to decision step 35 to check to see if all speech channels are examined. If the answer is negative, control returns to step 32 to repeat the process on the next speech channel S_i+1 until all speech channels are tested.

If the answer is affirmative in step 35, control proceeds to step 36 to control the transmitter 11 and receiver 13 so that the base station is switched to the control channel to receive control signals (decision steps 37 and 38). If a call request is made from cordless station 4₁, the decision in step 37 is affirmative and control exits to step 41 (Fig. 2B), and if there is an incoming call from the network, the decision in step 38 is affirmative and control exits to step 51 (Fig. 2C). If negative decision is made in both steps 37 and 38, control returns to step 31 to repeat the standby routine.

In Fig. 2B, if there is an originating call from cordless station 4₁, base station 2 returns an acknowledgment (ACK) signal over the control channel by containing in it a list of idle channels S_i stored in memory 14 (step 41). Exit then is to step 42 to set variable i to 1 and switch the base station to an idle speech channel S_i (step 43). A timeout period of T₁ seconds is then set in a timer (step 44), and the receipt of an end-of-switching signal from the cordless station is checked (step 45). If the end-of-switching signal is not received, control proceeds to step 46 to check to see if timeout period T₁ has expired. If the answer is negative in step 46, control repeats steps 45 and 46 until timeout period T₁ expires. If an end-of-switching signal is received within the timeout period T₁, control advances to step 49 to send a turn-on signal to the cordless station and enters a talking mode. Otherwise, it exits to step 47 to check to see if all channels contained in the idle channel list are tested. If the answer is negative, variable i is incremented by 1 (step 48) and control returns to step 43 to repeat the process so that the next idle speech channel S_i+1 is examined. If no end-of-switching signal is received on each of the successively switched idle speech channels, the decision is affirmative in step 47 and control abandons the attempt to establish a connection and returns to the standby routine.

In Fig. 2C, if there is an incoming call from the network, base station 2 sends a channel assignment signal indicating a list of idle channels S_i stored in memory 14 and the timer is set to period T₁ (step 51). Exit is to step 52 to check for receipt of an ACK signal from the cordless station. If the answer is negative in step 52, step 53 is executed to check for the expiration of period T₁. If no ACK signal is returned within the period T₁, control returns to step 51 to retransmit the channel assignment signal. If an ACK signal is received, control moves ahead to step 54 to set variable i to 1 and goes to a looped sequence of steps 55 through 60 which are respectively identical to steps 43 through 48 of Fig. 2B.

If an end-of-switching signal is received so the answer is affirmative in step 57, control proceeds to step 61 to transmit a ringing signal over the switched speech channel to receive an off-hook signal (step 62) and transmit a turn-on signal (step 63) to enter a talking mode.

In Fig. 3A, during standby modes, the cordless station is switched to the control channel. A call originating routine begins with step 71 which checks to see if an off-hook condition has occurred. If the answer is affirmative, control exits to step 72 to switch the station to the control channel and checks the output of carrier detector 23 against the prescribed threshold to determine if the control channel is in use by another cordless telephone. If the answer is negative, control proceeds to step 73 to send a call request signal to base station 2. Exit then is to step 74 to check for the presence of a channel assignment signal. If there is none, control returns to step 73 to retransmit the call request signal, and if there is one, control proceeds to step 75 to store into idle channel memory 27 the list of idle speech channels contained in the transmitted channel assignment signal.

Variable i is then set equal to 1 (step 76). The cordless station is then switched to an idle speech channel S_i (step 77) and a timer is set to a timeout period T₁ (step 78). Exit is to step 79 to check the output of carrier detector 23 against the threshold to determine if that channel is busy. If it is, control exits to step 80 to check for the expiration of the timeout period and goes to step 81 to check to see if all channels of the idle list are tested. If the answer is negative, variable i is incremented by 1 (step 82) and control returns to step 77 to repeat the process on the next idle speech channel S_i+1. If the answer is affirmative in step 81, control returns to the starting point of the call originating routine.

If the switched idle channel is not busy, exit then is to step 83 to send an end-of-switching signal, followed by the setting of the timer to a timeout period T₂ seconds in step 84. Exit then is to step 85 to check for the presence of a turn-on signal. If there is one, control proceeds to step 87 to turn on the speech circuit 24 to enter a talking mode. If the turn-on signal is not received, a timeout check is made (step 86) to repeat step 85 until period T₂ expires, whereupon control returns to step 71.

In Fig. 3B, the cordless station starts a call terminating routine with step 91 which checks for the presence of a channel assignment signal. If there is one, control proceeds to step 92 to store the list of idle speech channels contained in the received signal into memory 27 and goes to step 93 to return an ACK signal. Steps 94 through 100 are executed in a manner identical to steps 76 through 82 of Fig. 3A, so that idle channels will be successively tested if each test encounters a busy channel.

If an idle speech channel is available, control exits from step 97 to step 101 to send an end-of-switching signal, causing the base station to send a ringing signal (step 61, Fig. 2C). If the ringing signal is received by the cordless station (step 102), the tone ringer 26 is activated (step 103) to urge the user to go off hook. If an off-hook condition is detected (step 104), control proceeds to step 105 to deactivate the tone ringer and goes to step 106 to send an off-hook signal, causing the base station to send a turn-on signal (step 63). The timer is set to period T₂ (step 107) to wait for the reception of the turn-on signal (step 108) until period T₂ expires (step 109). If the answer is affirmative in step 108, control exits to step 110 to turn on the speech circuit 24.

The operation of the system will be described with reference to Figs. 4A and 4B together with the flowcharts described above. Assume that, in a simplified example, speech channels S₁ and S₂ are detected as being idle by the base station during a standby mode.

In Fig. 4A, if a call is originated from cordless station 4₁, a call request signal is sent (step 73, Fig. 3A) to base station 2₁. On receiving it (step 37, Fig. 2A), base station 2₁ returns an ACK signal containing a list of idle speech channels S₁ and S₂ (step 41, Fig. 2B). Data indicating the idle speech channels S₁ and S₂ are stored into idle channel memory 27 (step 75) and the cordless station is switched from the control channel to speech channel S₁ (steps 76, 77) and the busy/idle status of channel S₁ is checked during timeout period T₁ (steps 78, 79, 80). If channel S₁ is used by cordless station 4₂, the signal from base unit 2₂ will be received by antenna 29 and carrier detector 23 produces an output. Therefore, the answer in step 79 is negative. After period T₁ expires, control exits from step 80 to step 81, and repeats the process so that cordless station 2₁ is then switched to speech channel S₂.

Following transmission of the ACK signal, base station 2₁ is switched to idle channel S₁ (steps 42, 43, Fig. 2B) expecting the receipt of an end-of-switching on that channel from cordless station 4₁. Since this signal is not received within the timeout period T₁ (step 46), base station 2₁ switches to channel S₂. If this channel is detected by cordless station 4₁ as being idle, an end-of-switching signal is sent on channel S₂ (step 83, Fig. 3A). In response to this signal (step 45), base station 2₁ transmits a turn-on signal (step 49), which is received by the cordless station (step 85) to tum on the speech circuit 24.

In Fig. 4B, if an incoming call is received from the network (step 38, Fig. 2A), base-station controller 12 sends a channel assignment signal (step 51, Fig. 2C) containing in it the list of idle channels S₁ and S₂. Cordless station 4₁ responds to this signal by returning an ACK signal (step 92, Fig. 3B) and switches to channel S₁ to check for its busy/idle status. If this channel is detected by cordless station 4₁ as being busy, period T₁ lapses and the cordless station switches to channel S₂ and transmits an end-of-switching signal on channel S₂ (step 101).

Base station 2₁ responds to the ACK signal from cordless station 4₁ by switching to channel S₁ (steps 54, 55). Since this channel is found busy on the cordless station side of the system, period T₁ expires also in the base station (steps 58, 59) and it switches to channel S₂ (steps 60, 55) to receive the end-of-switching signal (step 57). A ringing signal is then sent to the cordless station (step 61).

When cordless station 4₁ activates its tone ringer (steps 102, 103) in response to the ringing signal, goes off hook (step 104) and deactivates the tone ringer (step 105), an off-hook signal is then sent (step 106), which is received by the base station (step 62) and a turn-on signal is sent to the cordless station (step 63) to allow it turn on its speech circuit.

It is seen that while the cordless station is switched successively to an idle channel if each channel test reveals a busy condition, the base station is switched successively to the same channel in response to the end-of-switching signal from the cordless station signaling that the switched speech channel is also idle on the cordless station side of the system.