Note: Descriptions are shown in the official language in which they were submitted.
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CIRCUIT AND METHOD FOR OPERATING --
A WIRELESS COMMUNICATION SYSTEM ~
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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.
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Previously, wireless communication systems providing 1 -
communication between one or more remote devices and a
15 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 ~ ~ `
20 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 cornmunication system which could operate with a `;
25 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.
~herefore, communication between the multiple ~remote devices l ;
30 and the single base station can be accommodated. However,
cordless telephone systems presently do not include multiple
base stations. 'l~l"j'''''
As in any wireless communication system having multiple
base stations, there is a need to regulate the communication
~5 between a remote device and the base stations o~ a cordless
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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 requireme,nt 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.
Sum,m, ~r~o~ the Invention
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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 ~ -
remo~e device. A data link also couples each base station to a i
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
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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 Rl- 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 th~ active base
station foreach remotedevice. Each basewilltransmit an
information signai indicating the strength of the 2F
communication signal received from each remote device. ; ~
Accordingly, each active bas~ 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 i j
signals to determine which base station is receiving the strongest -~
RF communication signal frorn the remote device. Finally, an RF
communication link is established between the remote device and
the base station having the strongest RF communication signal l ;
strength. ~ `
The method of the present invention also contemplates a
portable communication system having rnultiple remote devices. ~ ~ `
. . . . .
The method is similar to a wireless communication system having
one remotedevice. However, each base station in
35 communication with a remote device will act as an active base
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station. Accordingly, each active base station will query the other
base s~ations 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-
5 active base station is receiving a stronger RF communicationsignal.
Brief Q~sc!iption o~ the [~ra~iaQ~
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. '
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FIG. 4 is a flow chart of the method for operating a portable ` ~
communication system according to the present invention. ~ `
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Des~rj~jg~rnent
Referring to FIG. 1, a plan view of à 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. E3ase stations 11 are
coupled by a clata 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 t~f RF communication with base
stations 11. An example of a base istation and an associated
remote device includes a cordless telephone. A cordless
telephone which could employ the circuit and method of the
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present invention is disclosed in US l'atent 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, ~he data iink 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 radiotransceiver20 `;
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 includethe frequency
25 or channel which the remote device occupies, a security code
assi~ned to the remote device to allow the remote device to `
operate in the wireless communication system 10, and any `i
message signals. The message signals may depend on the
remote device, but could include voice signals, facsimile data or ~,
computer data. ~ i
Radio transceiver 20 provides communication signals on a ; -
line 22 which is coupled to a signal transceiver circlJit 24. Signal ` -transceiver circuit 24 includes a standard telephone company ~ -
interface (Telco) circuit 26 ~or transmitting communication signals.
Standard Telco circuits are well known in the art and provide '
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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
5 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
10 security code assooiated 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 -
20 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 rernote device from the base stations to determine if
25 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 ~ -
30 coupling signals to the Up 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
35 audible range and will not therefore be heard by the user of the
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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. F:inally, a 30 KHz signal will be ;
filtered by the public system telephone network. While a 30 KHz `~
5 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 ~4 also includes a receiver circuit
32 for receiving information signals from other base stations by
1 0 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 ~f 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
20 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 isshown in detail. Transmittercircuit30 includes an
25 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
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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 transislor 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 tran$former 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 KQ. 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
KQ. 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 KQ.
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 KQ.
The detailed signal transceiver circuit 24 shown FIG. 3 is
one example of a circuit which could be employed. However, it
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will be understood that other signal transceiver circuits for ~ ;
transrnitting 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.
Generaily, in a system having multiple remote devices, each
remote device which is in use will be in active RF communication
10 with asinglebace station. Active RFcommunication is
considered a transfer of communication signals between the a
remote device and a base station. Although each base statiion will -r~
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 , -i;
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 i; -
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 ba.se station. - ~ ~-
Turning now to Fl(3. 4, the method for operating a portable
communication system i5 described in detail in reference to ~low
chart 100. Flow chart 100 is directed to the RF communication ~
Iink between one remote device and one active base station. ~;;
3S However, it will be understood that flow chart 100 applies to all ~, ~
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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
5 be able to communicate with the base stations of the system
unless the base stations have the sec:urity code on a list. A
method for assigning security codes to the base stations is
described in a U.S. Application (attorney docket number CE76~R)
invented by James Francis Goedken, Thomas Perszyk and
10 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 stsp 110 whether another base is
receiving ~ 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 ~or RSSI signals. If the RSSI
signal is not low, the home base wili continue normal operation for
a certain tim~out period established at step 114. After the tim~ ~- ~
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out period, the base station again checks whether RSSI is low at
step 112.
However If the horne base station determined that some ;`
other base station has a stronger R',SI signal at step 110, the ~ ,
5 home base s~ation 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 communica~ion link with
another rernote device, the home base station will ~ransfer the RF
10 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 ~etermine whether the RSSI signal is ~;
low at step 112 to either continue normal operation at step 114 or l -
query other base stations at step at step 108. ;
If all base stations having a grea~er RSSI signal at step 116
already have an RF communication link with a remote device, the
horne base st~tion will then determine whether the RSSI signal is ~ ;
Iow 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.
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