WO1995003663A1

WO1995003663A1 - Circuit and method for operating a wireless communication system

Info

Publication number: WO1995003663A1
Application number: PCT/US1994/006560
Authority: WO
Inventors: James Mielke
Original assignee: Motorola Inc.
Priority date: 1993-07-20
Filing date: 1994-06-13
Publication date: 1995-02-02
Also published as: CA2142461A1; BR9405521A; DE4495338T1; GB9507326D0; KR950703829A; GB2286316A; JPH08501917A

Abstract

A circuit and method for operating a wireless communication system (10) having multiple base stations (11) allows for the transfer of RF communication between a base station (11) and a remote device (12) to another base station (11) without a separate controller regulating the base stations. The base station (11) communicates with each other by way of the RING and TIP line of the public system telephone network (18) at a frequency of approximately 30 KHz. Accordingly, the communication signals transferred between the base stations (11) are outside the audible range of the user and will be filtered by the public system telephone network (18). The base station (11) in active communication with a remote device (12) will determine whether another base station (11) is receiving a stronger RF communication signal from the remote device (12) and will transfer the communication link to the other base station (11).

Description

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CIRCUIT AND METHOD FOR OPERATING A WIRELESS COMMUNICATION SYSTEM

Field of the Invention

The present invention is generally related to a wireless communication system, and more particularly to a circuit and method for coupling a wireless remote device to a base station in a wireless communication system.

Background of the Invention

Previously, wireless communication systems providing communication between one or more remote devices and a plurality of remote base stations included a controller to regulate the communication between a remote device and the base stations. The controller would generally provide the hand-off between the base stations when necessary. One example of a wireless communication system having a number of base stations is a cellular telephone system. Generally, the base stations of the cellular telephone system are controlled by a cellular control station.

A cordless telephone system is another example of wireless communication system which could operate with a plurality of base stations. Presently, cordless telephone systems include one or more remote devices or handsets which are associated with a single base station. Each of the remote devices may communicate with the base station on a different frequency. Therefore, communication between the multiple remote devices and the single base station can be accommodated. However, cordless telephone systems presently do not include multiple base stations.

As in any wireless communication system having multiple base stations, there is a need to regulate the communication between a remote device and the base stations of a cordless telephone system having multiple bases. In particular, there is a need for regulating which base station will communicate with a particular remote device. Further, because the strength of the RF communication signals between the remote device and the base stations may vary with time and the relative location of the remote device to the base stations, it is useful to couple each remote device to a base station which has the strongest RF communication link.

In a cordless telephone system, it is beneficial to regulate communication between a remote device and the base stations without employing a separate controller of the base stations. Eliminating any requirement for separate controller will reduce both the cost and complexity of the system. Accordingly, there is a need for a circuit and method for operating a wireless communication system having multiple base stations such as a cordless telephone system which eliminates any requirement for a separate controller of the base stations.

Summary of the Invention

The present invention encompasses a wireless communication system for use with a public system telephone network comprising at least one remote device adapted to transmit RF communication signals to and receive RF communication signals from a plurality of base stations. An example of a remote device could be a cordless telephone remote device. A data link also couples each base station to a telephone line for transmitting communication signals to and receiving communication signals from the telephone line. One base station is the "active" base station and provides communication signals to the telephone line by way of the data link and RF communication signals to the remote device.

The data link also couples the base stations to one another to enable communication between the base stations. Each base station provides information signals which are transmitted to the other base stations by way of the data link. The information signals include a signal indicating the strength of the RF communication signal received from the remote device. The active base station processes the information signals from each base indicating the strength of the RF communication signal received by each base to determine which base station should be active.

For a wireless communication system having multiple remote devices, a separate base station will be the active base station for each remote device. Each base will transmit an information signal indicating the strength of the RF communication signal received from each remote device. Accordingly, each active base station will determine which base station is receiving the strongest RF communication signal. The present invention also encompasses a method for operating a wireless communication system having a plurality of base stations and at least one remote device. The method initially establishes an RF communication link between the one remote device and one of the base stations, called the "home" base station or the initial active base station. The remote device also provides RF communication signals to the remaining base stations. Each of the base stations generates an information signal indicating the signal strength of the RF communication signals received from the remote device. The home base station provides a query to the other base stations for the information signals to determine which base station is receiving the strongest RF communication signal from the remote device. Finally, an RF communication link is established between the remote device and the base station having the strongest RF communication signal strength.

The method of the present invention also contemplates a portable communication system having multiple remote devices. The method is similar to a wireless communication system having one remote device. However, each base station in communication with a remote device will act as an active base station. Accordingly, each active base station will query the other base stations to determine the base station having the strongest RF communication signal. Each active base station will transfer the RF communication link to a non-active base station if a non- active base station is receiving a stronger RF communication signal.

Brief Description of the Drawings

FIG. 1 is a plan view of a wireless communication system

10 having multiple base stations 11 and multiple remote devices 12 for employing the circuit and method of the present invention.

FIG. 2 is a block diagram of the circuit for operating a wireless communication system according to the present invention.

FIG. 3 is a circuit diagram of the preferred signal transceiver circuit 24 shown in the block diagram of FIG. 2.

FIG. 4 is a flow chart of the method for operating a portable communication system according to the present invention.

Description of the Preferred Embodiment

Referring to FIG. 1 , a plan view of a wireless communication system 10 having a plurality of base stations 11 and a plurality of remote devices 12 employing the circuit and method of the present invention is shown. Base stations 11 are coupled by a data link 14 to a single line 16 of a public system telephone network (PSTN) 18. While any number of remote devices 12 could be incorporated in the wireless communication system, only one remote device is required. Remote devices 12 could include any device capable of RF communication with base stations 11. An example of a base station and an associated remote device includes a cordless telephone. A cordless telephone which could employ the circuit and method of the present invention is disclosed in US Patent No. 5,140,635, assigned to Motorola, Inc., the entire patent of which is incorporated by reference.

As shown in FIG. 1 , each base station 11 is in communication with the other base stations by way of data link 14 (indicated by the solid lines between the base stations). Preferably, the data link could incorporate the standard ring and tip lines of the public system telephone network by coupling each base station to the same telephone line 16 of the public system telephone network. Alternatively, the communication between the base stations could be accomplished by a separate RF communication circuit. Finally, each remote device 12 communicates with each base station 11 by way of RF communication signals (shown by the broken lines in FIG. 1). Turning now to FIG. 2, a block diagram for base station 10 shows the relevant portions of the base station for providing information signals between the base stations by way of the data link. Each base station generally includes a radio transceiver 20 for transmitting RF communication signals to and receiving RF communication signals from each remote device 12. Radio transceivers are well known in the art and could be employed in the present invention. Preferably, the radio transceiver circuit disclosed in the aforementioned US Patent 5,140,635 is employed. The RF communication signals include the frequency or channel which the remote device occupies, a security code assigned to the remote device to allow the remote device to operate in the wireless communication system 10, and any message signals. The message signals may depend on the remote device, but could include voice signals, facsimile data or computer data.

Radio transceiver 20 provides communication signals on a line 22 which is coupled to a signal transceiver circuit 24. Signal transceiver circuit 24 includes a standard telephone company interface (Telco) circuit 26 for transmitting communication signals. Standard Telco circuits are well known in the art and provide communication signals to the standard tip and ring lines of the public system telephone network.

Radio transceiver 20 also generates an information signal on line 22. The information signal could include a receiver signal strength indicator (RSSI) signal and an associated security code for the remote device. The RSSI signal indicates the strength of RF communication signals received from remote device 12. In a system having multiple remote devices, an RSSI signal is generated for each remote device 12 and is identified by the security code associated with the remote device. The RSSI signal could be an on/off indicator or could represent a discrete level within a predetermined range of levels representing the signal strength. Information signals including an RSSI signal are provided to a microprocessor 28. Microprocessor 28 in each base station 10 maintains an

RSSI signal indicating the signal strength of the RF communication signal received from each remote device 12. Microprocessor 28 also communicates with signal transceiver circuit 24 to transmit the RSSI signals to the other base stations and receive RSSI signals from the other base stations by way of data link 14. As will be described in detail in reference to the operation of the circuit of the present invention, Microprocessor 28 of each active base station compares the RSSI signals associated with a given remote device from the base stations to determine if another base station is receiving a stronger RF communication signal.

Signal transceiver circuit 24 generally includes a transmitter circuit 30 and a receiver circuit 32. Microprocessor 28 provides an RSSI signal to signal transceiver circuit 24 for coupling signals to the tip and ring lines of the PSTN by way of data link 14. Preferably, transmitter circuit 30 transmits information signals at a frequency of approximately 30 KHz. A frequency of 30 KHz is selected to optimize the transmission of the information signals. In particular, a 30 KHz signal is outside the audible range and will not therefore be heard by the user of the remote device. A 30 KHz signal frequency is also the lowest frequency outside the audible range and, therefore, allows largest signal amplitude on the Telco line. Finally, a 30 KHz signal will be filtered by the public system telephone network. While a 30 KHz signal is preferred, any other frequency which is outside the audible range and which will be filtered by the public system telephone network could be used.

Signal transceiver circuit 24 also includes a receiver circuit 32 for receiving information signals from other base stations by way of data link 14. Preferably, receiver circuit 32 includes a bandpass filter for passing 30 KHZ signals transmitted on the data link by transmitter circuits of other base stations. The information signals are coupled to the microprocessor where they are stored. Because each base station will receive the information signals from the other base stations, any one of the base stations will be able to function as an active base station and determine which base station is receiving the strongest RF communication signal from a particular remote device.

Finally, an answering machine 33 may be incorporated in one of the base stations. Preferably, the answering machine will be a digital answering machine.

Turning now to FIG. 3, the preferred signal transceiver circuit 24 (shown in block form in FIG. 2) found in each base station 11 is shown in detail. Transmitter circuit 30 includes an AND gate 34 having a first input 36 coupled to receive a 30 KHz square wave and a second input 38 coupled to receive data. The data should include information signals describing the remote device, the channel or frequency of operation, and the RSSI signal generated by the radio transceiver as described above. The data will be transmitted as a 30 KHz square wave by ANDING the data with the 30 KHz square wave signal. Alternatively, the data could be provided by the microprocessor as a 30 KHz signal. The output 40 of AND gate 34 is coupled by a capacitor 42 to the TIP line. Preferably, capacitor 42 is approximately 200 picofarads. Output 40 of AND gate 34 is also coupled to an inverter 44. The output 46 of inverter 44 is coupled by a capacitor 48 to the RING line. Capacitor 48 is also preferably 200 picofarads.

Telco circuit 26 is also shown in detail in FIG. 3. Telco circuit 26 includes a transformer 52 for transmitting and receiving audio signals from the radio transceiver. The transformer is coupled to a relay 54. Relay 54 is controlled by a transistor 56 which is turned on or off by a voltage at a control electrode 58 coupled to an input resistor 60. Relay 54 is coupled to a bridge circuit 62 at a node 64. Nodes 66 and 68 of bridge circuit 62 are coupled to the TIP and RING lines. Bridge circuit 62 also receives signals from the TIP and RING lines at nodes 66 and 68. Finally, node 70 is coupled to transformer 52 for transmitting signals from the TIP and RING lines to the radio transceiver.

Finally, signal transceiver circuit 24 includes a receiver circuit 70. The receiver circuit acts as a bandpass filter to pass the information signals from the other base stations, and block signals from the public system telephone network. Preferably, receiver circuit 70 will pass 30 KHz information signals which are transmitted by the transmitter circuits 30 of the other base stations. Receiver circuit 70 includes a differential amplifier circuit 72 having a positive input 74 and a negative input 76. Positive input 74 is coupled to the RING line by a capacitor 78 and resistor 80 which form a low pass filter. Preferably capacitor 78 is 620 pf and resistor 80 is 10 KΩ. Input 74 is also coupled to a parallel RC network comprising a capacitor 82 and a resistor 84 to form a high pass filter. Preferably, capacitor 82 is 20 pf and resistor 84 is 220 KΩ. Negative input 76 to the differential amplifier is also coupled to a capacitor 86 in series with a resistor 88 to form a low pass filter. Preferably, capacitor 86 is 620 pf and resistor 88 is 220 KΩ. Also, a parallel configuration of a capacitor 90 and a resistor 92 is coupled between negative input 76 and the output 94 of the differential amplifier. Preferably, capacitor 90 is 20 pf and resistor 92 is 220 KΩ.

The detailed signal transceiver circuit 24 shown FIG. 3 is one example of a circuit which could be employed. However, it will be understood that other signal transceiver circuits for transmitting and receiving approximately 30 KHz information signals on the data link could be employed within the scope of the present invention.

Operation

The operation of the circuit will now be described in detail. Generally, in a system having multiple remote devices, each remote device which is in use will be in active RF communication with a single base station. Active RF communication is considered a transfer of communication signals between the a remote device and a base station. Although each base station will receive RF communication signals from each remote device for purposes of determining an RSSI signal for each remote device, only one base station (i.e. the active base station) will return RF communication signals. Similarly, each base station will be in active RF communication with only one remote device.

Each active base station will be able to transfer the RF communication link with the associated remote device to one of the remaining (i.e. non-active) base stations if one of the base stations is receiving a stronger RF communication signal. The microprocessor of the active base station will determine which base station is receiving the strongest RF communication signal from the associated remote device. If another base station which is not active is receiving a stronger RF communication signal from the remote device, the microprocessor of the active base will provide command signals to transmitter circuit 30 shown in detail in Figure 3. The command signals will be transmitted to the non- active base station to transfer the RF communication link to the non-active base station.

Turning now to FIG. 4, the method for operating a portable communication system is described in detail in reference to flow chart 100. Flow chart 100 is directed to the RF communication link between one remote device and one active base station. However, it will be understood that flow chart 100 applies to all active communication links in the system. At the initial step 102, the remote device is turned on. The remote device sends an RF communication signal indicating the identification code or security code assigned to the remote device. The remote device will not be able to communicate with the base stations of the system unless the base stations have the security code on a list. A method for assigning security codes to the base stations is described in a U.S. Application (attorney docket number CE765R) invented by James Francis Goedken, Thomas Perszyk and Charles John Malek and entitled METHOD FOR OPERATING A COMMUNICATION SYSTEM HAVING MULTIPLE BASE STATIONS, which application is also assigned to Motorola, Inc. and is filed on even date herewith, the entire application of which is incorporated by reference. The remote device will make an RF communication link to the last base station with which the remote device communicated, called the home base station, at a step 104. It will be understood that the first active base station having an RF communication link with remote device will be called the home base station for convenience. The home base station has no other significance after an RF communication link is established.

The home base station then generates an RSSI signal at a step 106. The home base station also queries other base stations at a step 108 for an RSSI signal indicating the strength of the RF communication link. The microprocessor of the home base station will then determine at a step 110 whether another base is receiving a stronger RF communication signal by evaluating the RSSI signals of received from the base stations.

If no other base has a stronger RSSI signal, the home base station will check whether the RSSI signal is low at a step 112. If the RSSI signal is low, the home base station returns to step 108 to query the other base stations for RSSI signals. If the RSSI signal is not low, the home base will continue normal operation for a certain time-out period established at step 114. After the time- out period, the base station again checks whether RSSI is low at step 112.

However If the home base station determined that some other base station has a stronger RSSI signal at step 110, the home base station will determine whether the other base station has an RF communication link with another remote device at a step 116. If one or more of the base stations which have a stronger RSSI signal do not have a communication link with another remote device, the home base station will transfer the RF connection to the base station having the strongest RSSI at a step 118. This base station will become the new active base station. The active base will then determine whether the RSSI signal is low at step 112 to either continue normal operation at step 114 or query other base stations at step at step 108. If all base stations having a greater RSSI signal at step 116 already have an RF communication link with a remote device, the home base station will then determine whether the RSSI signal is low at step 112 to continue normal operation at step 114 or query other bases at step 108. In summary, the circuit and method for operating a wireless communication system enables base stations to transfer information signals between base stations to determine which base station is receiving the strongest RF communication signal from a remote device. The circuit and method reduces cost and complexity by eliminating the requirement for a separate controller of the base station.

Claims

1. A wireless communication system for use with a public system telephone network comprising: at least one remote device adapted to transmit and receive

RF communication signals; a plurality of base stations adapted to transmit RF communication signals to and receive RF communication signals from said at least one remote device; a communication link coupling each said base station to a telephone line of said public system telephone network for transmitting communication signals between said telephone line and said plurality of base stations and coupling each said base station of the plurality of base stations to each other base station for transmitting information signals between said plurality of base stations.

2. The wireless communication system according to Claim 1 wherein each said base station includes a receiver signal strength indicator circuit to generate an information signal indicating the strength of an RF communication signal received from said at least one remote device.

3. The wireless communication system according to Claim 1 wherein said information signals are transmitted between said plurality of base stations at a frequency of approximately 30 KHz.

4. The wireless communication system according to Claim 1 wherein said information signals comprise a security code assigned to said at least one remote device to enable communication with said plurality of base stations.

5. The wireless communication system according to Claim 1 wherein each said base station includes a transmitter circuit for transmitting said information signals to said plurality of base stations and a receiver circuit for receiving said information signals from said plurality of base stations.

6. A wireless telephone system for use with a public system telephone network comprising: at least one remote device adapted to transmit and receive RF communication signals; a plurality of base stations adapted to transmit RF communication signals to and receive RF communication signals from said at least one remote device; a communication link coupling each said base station to a telephone line of said public system telephone network for transmitting communication signals between each base station of said plurality of base stations and said telephone line and coupling each said base station of the plurality of base stations to the other base stations for transmitting signals between said plurality of base stations, wherein one of said plurality of base stations provides communication signals to said telephone line by way of said communication link and RF communication signals to said at least one remote device; each said base station having a receiver signal strength indicator circuit for generating an information signal indicating the signal strength of RF communication signals received from said at least one remote device; and each said base station having a transmitter for transmitting said information signal to said plurality of base stations and a receiver for receiving said information signals from said plurality of base stations by way of said data link.

7. A method for operating a wireless communication system having a plurality of base stations adapted to communicate with a public system telephone network by way of a telephone line and adapted to communicate with at least one remote device by way of RF communication signals comprising the steps of: providing a communication link between said plurality of base stations and a telephone line of a public system telephone network, said communication link enabling communication between said base stations and enabling communication between said each base station and said telephone line; transmitting information signals between said plurality of base stations; and providing an RF communication link between said at least one remote device and at least one of said plurality of base stations.

8. A method for operating a wireless communication system having a plurality of base stations and at least one remote device comprising the steps of: establishing an RF communication link between said at least one remote device and one of said plurality of base stations; generating a signal for each said base station indicating the signal strength of RF communication signals received from said at least one remote device; providing a query from said one of said plurality of base stations to the other base stations for said signal generated at each said base station indicating the strength of said RF communication signals; determining which said base station is receiving the strongest RF communication signal from said at least one remote device; and establishing an RF communication link to said base station having the strongest signal strength of said RF communication signal.

9. The method for operating a wireless communication system having a plurality of base stations and at least one remote device according to Claim 8 wherein the step of generating a signal for each said base station indicating the strength of said RF communication signal includes providing a communication link between said plurality of base stations.

10. The method for operating a wireless communication system having a plurality of base stations and at least one remote device according to Claim 8 wherein signals transmitted on the communication link are transmitted at a frequency of approximately 30 KHz.