Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
6 1 3
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METHOD FOR OPERATING A COMMUNICATION
SYSTEM HAVING MULTIPLE BASE STATIONS
Field of the In~D~
The present invention is generally related to i~
cornmunication systems, and more particularly to a method for : `
operating a wireless communication system having multiple base `--
stations.
E~çl~r~nd~ he Inv~n~
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. Typically, cordless telephone systems ;-`
include one or more wireless remote devices or handsets which i`
are associated with a single base station. Each of the handsets
may communicate with the base station on a different frequency.
Therefore, communication between the multiple handsets and the ~ .
single base station can be accommodated. -
In a cordless telephone system having multiple base ,
stations, it is necessary to allow the handset to communicate with
each base station, while preventing unauthorizecl handsets from `;
communicating with the base stations. Accordingly, there is a
3i5 need to assign security codes to each handset and maintain a list
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of the security codes of all active handsets allowed to operate in
the system. Further, when a handset is waiting to communicate
with a base station, the handset should be on the same channel
as a base station within range to reduce the amount of time
5 necessary to provide a radio frequency (RF) communication link
between the handset and a base station. Accordingly, there is a
need to align the channel of the handset to a base station which is
within range even when the handset is not communicating with a
base station. ~;~
10Also, because the strength of the RF communication - `
signals between the handset and the base stations may vary with
the relative location of the hands~t to the base stations when the
handset is communicating with a base station, it is useful to
proYide an RF communication link between each handset and a
15 base station which has the strongest RF communication link.
Accordingly, there is a need to transfer the communication signals
between base stations in a wireîess communication system ``
having multiple base station.
As in any wireless communication system having multiple
20 base stations, there is a need to regulate the communication
between a handset and the base stations. In particular, there is a
need for regulating which base station will communica~e with a
particular handset. It is beneficial to regulate communication
between a handset and the base stations without employing a
25 separate controller of the base stations. Eliminating any
requirement for separate controller wil! reduce both the cost and
complexity of ths system. Accordingly, there is a need for a
method for operating a wireless communication system having
multiple base stations such as a cordless telephone system which
30 eliminates any requirement for a separate controller of the base
stations.
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~mm~lnvention
The present inven$ion encompasses a method for
establishing and maintaining communication in a portable ` ~
telephone system adapted to operate with a plurality of base -
stations and at least one handset on a plurality of channels. The
methocl generally includes steps of assigning a security code to
the handset(s) operating in the portable communication system to
enable RF communication betwe,on the handset(s) and the ` ~ ~
plurality of base stations; communicating the security code `;
assigned to the handset(s) to the plurality of base stations to ~ ~ ,
maintain within each of the plurality of base stations the security ~
code(s) of the handset(s) operating in the system; and aligning -;
the channel of the handset~s) with a channel of a base station to 1
allow for communication between each handset and one of the
plurality of base stations.
In another aspect, the present invention discloses a
method for assigning security codes for a portable communication
system adapted to operate with a plurality of handsets and at ;;
least one base station. The method comprises the steps of
providing the current security code of each of the handsets to the `
base station; assigning a new security code to each of the
handsets; and placing each new security code on the master list ;
of security codes in the base station.
The present invention further discloses a method for
maintaining communication between a handset and one of a "
plurality of base stations when the handset is not actively `
communiçating with a base station. In one aspect of the
invention, a method comprises, for each handset, the steps of -
periodically sending a handshake request from the handset to the ~:
plurality of base stations: waiting for a return handshake from a
base station; and changing the channel of the handset to
correspond with the channel of the base station which retumed
the handshake.
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Finaily, in another aspect of the invention, a method for
maintaining communication in a portable communication system
having a plurality of base stations and at least one handset when
a handset is actively communicating with a base station is
5 disclosed. In particular, the method includes steps of evaluating
the RSSI level of a signal received at a base station from a
handset; instructing remaining base stations to report an RSSI
level and an operating channel; selecting the base station
reporting the strongest RSSI signal; instructing the handset to
10 switch to the operating channel; and providing a handshake
between the handset and the base station reporting the strongest
RSSI signal.
In an alternate embodiment of a method for maintaining
communication in a portable communication system having a
15 plurality of base stations and at least one handset when a
handset is actively communicating with a base station, the
method comprises the steps of evaluating the RSSI level of the
signal received at a base station frorn a handset; placing the base
station on hold; the handset requesting to make a connection to
20 the base station reporting the strongest RSSI signal; providing a
handshake between the handset and the base station reporting
the strongest RSSI signal.
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Brief Desçription of ~l~a~i~9 ~
In describing the present invention, reference is made to ,
5 the following drawings wherein:
FIG. 1 is a plan view of a conventional wireless ~ :~
communication system having multiple base stations and multiple : . ~
wireless handsets employing the method of the present invention; :: :
FIG. 2 is a flow chart of the method for operating a
communication system having multiple base stations according ~o
the present invention; i;
FIG. 3 is a flow chart of a method for transferring security ~ -
codes between base sta~ions according to the present invention; ~-
FIG. 4 is a flow chart of a method for maintaining a channel ~:
alignment between base stations and handsets according to the
present invention;
FIG. 5 is a flow chart of a method for establishing an offhook
condition of a handset according to the present invention;
FIG. 6 is a flow chart of a method for executing a handoff ~- ;
between base stations according to the present invention; and
FIG. 7 is a flow chart of an alternate method for executing a :
handoff between base stations according to the present invention.
FIG~ 8 is a block diagram of the preferred circuit for
operating a wireless communication system according to the
present invention. :.
FIG. 9 is a circuit diagram of the preferred signal transceiver
circuit shown in the block diagram of FIG. 8.
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Description ~f the Pr~red Em~Q~liment
Referring to FIG. 1, a plan view of a wireless
5 communication system 10 having a plurality of base stations 1 1
and a plurality of handsets 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, aithough the method of the present invention
10 could be employed on a system of base stations coupled to ~ -
multiple phone lines. While any number of handsets 12 could be
incorporated in the wireless communication system, only one
handset is required. Handsets 12 could include any device ~ ~
capable of RF communication with base stations 11. An exampl~ I ;
15 of a base station and an associated handset includes a cordless ` ~
telephone. A cordless telephone which could employ the 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 ~
25 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 RF communication or
over an AC power line. Finally, each handset 12 communicates , ;
with each base station 11 by way of RF communication signals
30 (shown by the brol<en lines in FIG. 1).
Turning now to FIG. 2, a flow chart shows the preferred
method for operating a communication system having multiple
base stations according to the present invention. To avoid ~ ;~
interference between base stations in a multibase system, each `,35 base station is preferably restricted to a certain subset of the total ~
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available channels. For example, in a frequency division multiple ~ -
access (FDMA) 49 Megahertz cordless system, there are
presently a total of ten channels available for use. In a system
having two base stations, a first base station may be assigned
5 channels 1-5 and a second base station may be assigned
channels 6-10. Preferably, the channels assigned to a single
base station are not adjacent frequencies in the spectrum. Base ~ -
stations always communicate within their channel domain, except -
when asked to report their RSSI level on a specific channel for
handoff purposes, discussed infra. -
In assigning the channel domains for the base stations, it is
preferred that no two base stations that could ever be in the RF
range of the same handset may have intersecting channel
domains. That is, while the RF range of the base stations
intersect to provide seamless handoff, it is preferred that the base -;
stations which have intersecting RF ranges do not have
intersecting RF channel domains. Accordingly, if there are less
base stations than total available channels, it is possible to
ensure that no two domains intersect by exclusively assigning ~ `
20 channels to base stations. Therefore, the base stations may be
placed wherever they are desired. It should be noted that if there
are less base stations than available channels, the base stations
could have overlapping channel domains if the base stations are
carefully placed to avoid channels having overlapping channel
25 domains being in the same RF range. However, if a system has
more base stations than available channels, care must be taken
as to the positioning of base stations having intersecting channel
domains.
There are a number of ways to implement domain
3Q assignments for the base stations according to the present
invention. The channel assignments could be manually assigned -
by using the keypad of the base station to select channels.
Alternatively, the base stations could be preprogrammed at the
time of manufacture. Finally, the base stations may be allowed to
35 arbitrate with each other to ensure their non-simultaneous use of
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RF channels when the base stations have overlapping radii of RF
coverage, and when the bases are within range of a specific
remote device. There are a number of ways to arbitrate. For
example, the base stations could arbitrate by RF communication
5 when idle or add a separate RF transceiver for arbitration.
Alternatively, the bases could arbitratle over the AC power line or
the PSTN. Arbitration would allow bases that are servicing a
handset to scan all available (i.e. all 10) RF channels, either
automatically or by user request. If a base required the use of a ` -
channel outside of its normal channel domain, and that channel ~ ~
,~
was not currently in use, it could simply trade with the base station
assigned that channel. Th~s could be done in real time, giving the
user access to all available channels. ` ~
To avoid interference from bases outside the multi ~ :
base system (e.g. a neighbor~s phone), bases that are not ~ ~ `
servicing a handset periodically check their RSSI levels to see if a
potential interferer exists. If so, the base scans for a clean or
unoccupied channel within its domain. Alternatively, the base
station could trade if necessary to moves to the unoccupied ~ ~
channel. Then when a handset scanning for a base moves into ;
that bases domain, it is likely that the connection will be
established on an unoccupied channel, without requiring any
communication on a occupied channel. ~ ;~
I laving established the base station channel domains,
security codes can be assigned to the handsets at a step 24.
Generally, security codes are assigned to ensure that only
handsets which are a part of the system are allowed to
communicate with the base stations of the system. Accordingly, a ~`~
! ~ ' 1 handset associated with foreign base station in the proximity of
30 one of the user's base stations will not be allowed to
communicate with the user's base station. Preferably, the security
code assigrled to each handset can be assigned when the
handset is cradled in the base station. The assignment of security
codes can be accomplished by way of a physical connection
between the handset and the base station. However, it is
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possible to assign security codes by way of RF communication
signals. For additional security, an access code may be required
by the user before a security code can be assigned to a handset
The security code assigned to any given handset by a
5 base station must be communicated to the remaining base
stations in the system at a step 26 to allow the handset to
communicate with any of the base stations in the system.
Preferably, the security codes assigned to the handsets can be
transferred between the base stations by way of a data link, such ~-
10 as that disclosed in a U.S. Application invented by Jarnes Mielke
and entitled "CIRCUIT AND METHOD FOR OPERATING A
WIRELESS COMMUNICATION SYSTEM" (attorney docket
number CE00811 R), such application being assigned to the
assignee of the present invention and filed on even date -
15 herewith. The aforementioned application generally discloses a
method and circuit for transferring security codes as 30 KHz
signal on a datalink coupled to the telco line. The circuit is
described in detail in reference to FIGS. 8 and 9. Alternatively,
the security codes could be preprogrammed at the time of
20 manufacture, manually programmed using the base and/or
handset keypads, or randomly generated once (or multiple times
via user request) by the handset and then transferred to each
base by a one time physical connection (i.e. through the charge
contacts). This would eliminate the need for base to base
25 communication, thereby reducing the cost and complexity of the
system.
At a step 28, the base stations maintain a channel
alignment with the handsets to ensure that an RF communication ~ :
link can be established between each handset and a base
30 station. Generally, each handset will periodically request for a
handshake with a base station via the RF communication link on
a designated channel. If the handshake request is unsuccessful,
the handset will continue to request a handshake on different
channels until it receives an acknowledge from a base station.
35 Accordingly, in response to a ringing signal or when the handset
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goes offhook to initiate a call, the handset will be on an unused
channel associated with a base station which is within range of
the handset. Although channel alignment is beneficial in a
system having a single base station, maintaining channel
5 alignment is particularly important in system having multiple base
stations. When a handset which is moved within the area
covered by the base stations, the handset will be able to
communicate on an unoccupied channel with one of the base
stations within range.
An offhook request to make communication with a base --
station is then made at a step 30. The affhook request could be ;;
rnade in response to a rin~ing signal, or when placing an
outgoing call. After a connèction is made with a base station, the
RF communication link is maintained with the base station until a
15 handoff is required at a step 32. A handoff may be necessary
when the user of the handset travels within the region of the base
stations. The user may travel outside the range of one base ; '
station and within the range of another base st~tion, requiring a ~ ~
handoff to the other base station. At the end of a conversation, `~ ~
20 the handset will go onhook at a step 34. After going onhook, the -
handset will maintain a channel alignment with the base stations
as described in reference to step 28. ;~
Turning now to FIG. 3, a detailed flow chart showing the ;~
preferred operation of assigning security codes to the handsets
25 (as shown in block 24 of FIG. 2) is described. Initially, the handset
detects that it has been placed in the base charging cradle and -`
send a request for a security code to the base station via RF or .
through the charge contact at a step 40. Preferably, the request
includss a copy of the handset's current (i.e. original) security
30 code. The base station then checks a list of all known handset -
security codes to determine if it has previously communicated
with the handset at a step 42. The base station generates a new
random security code at a step 44 and sends it to the handset.
The base station then waits to receive an acknowledge signal
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from ~he handset at a step 46 indicating that the new security ~:
code has been received.
If the base station does not receive an acknowledge from
the handset, the base station then determines if the handset is still
5 in the base station at a step 48. If the handset is in the base
station, the base station generates a new random security code at
step 44 and sends it to the handset. However, if the handset is
not still in the base station, the base station exits at step 50.
Alternatively, if the security code is sent by way of RF, the base
10 station generates a new random security code at step 44 as
shown by the dotted line if no acknowledge is returned form the
handset at step 46.
If the base station receives an acknowledge back from the
handset indicating that the new security code has been received
15 from the base station at step 46, the base station checks a list of
security codes assigned to handsets in the system to determine if
the original security code of the handset is in the list at a step 52.
If the original security code is in the list, the base station replaces
the original security code with the newly assigned security code
20 at a step 54 and exits at a step 50. However, if the original
security code is not in the list, the base station determines if their
are any empty slots in the list at a step 56. If there is an empty
slot, the base station replaces the empty slot with the new security ~ ~`
code at a step 58. However, if there are no remaining empty slots
in the base station, the base station replaces the least recently ; ~ ;~
used security code with the new security code on the list at a step - -
60. Accordingly, the base station will maintain a list of all active
handsets within the system. As stated earlier, an identical list of ;
security code is maintained in each of the base stations to allow
for communication between each handset and any of the base i ;~
stations.
Turning now to FIG. 4, the preferred method of maintaining
the channel alignment (shown in block 28 of FIG. 2) is described.
After having assigned the security codes to the handsets, the
method of the present invention maintains a channel alignment
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between each handset and a given base station. In particular, a
periodic timer expires initiating a channel alignment procedure at ~
a step 70. The handset then randomly selects a channel search ~ -
pattern with an even distribution of the channels at a step 72. The ~;
5 random distribution of channels is beneficial in a communication
system having multiple handsets. in particular, it is a simple
method of collision avoidance, shouW more than one handset be
searching simultaneously for a base station for alignment or off
hook purposes.
1 û The handset sets the operating channel onto the first ;
channel N in the pattern. Preferably, the first channel in the; `
pattern is set to the channel on which the handset was previously -
aiigned to a base station. This may decrease the time the
channel is occupied for this procedure in the event that the I;
handset is still aligned. The handset then sends a request for a
handshake to the base via the RF link on channel N at a step 74.
The method then determines if there is a base on channel N `
which is in range at a step 76. If there is a base on channel N
which is within range, the handset is aligned with the base. i `
Accordingly, the base sends an acknowledge at step 78. If the i
handset sees the acknowledge at a step 78, the handset returns
to step 70 to wait for the periodic timer to expire to complete -
another channel alignment. -~ -
If there is no base on channel N which is in range at step -
76 or the handset does not see the acknowledge from the base at ;~
step 80 within a predetermined amount of time, the handset ; `^
advances the channel number to the next channel in the pattern ` ~;
at a step 82. The handset then determines if all channels in the -
pattern have been used at a step 84. If all the channels have not~
been used, the handset sends a request for a handshake to the
base via the RF link on a new channel N at a step 74. However, if
all the channels in the pattern have been used, the handset ` ~
returns to step 72 to randomly select a new channel search " ~;
pattern and begin the channel alignment procedure again. The
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handset could optionally align to the base with the highest RSSI
level.
Accordingly, the channel alignment procedure described in
reference to FIG. 4 maintains a channel alignment of the handset
5 with a base by periodically sending a handshake request to a
base on a certain channel, and incrementing the channel if no
return handshake is received from the base. The handset will
align with a base station even when the handset is moved from
one location to another within the range of base stations while
1 0 onhook.
Turning now to FIG. 5, a flow chart describes the preferred
operation of an offhook re~luest (as shown in block 30 of FIG. 2)
according to the method of the present invention. In particular,
the user initiates a connection at the handset at a step 90. The
15handset then randomly selects a channel search pattern with an ~;
even distribution of channels at a step 92. The handset sets the
channel of the handset equal to the first channel N in the pattern.
Preferably, the first channel in the pattern is set to the channel
which was previously determined during the channel alignment ~ `
20 procedure (described in detail in reference to FIG. 4) to reduce
the time required to make an offhook connection.
The handset then sends a request to go offhook to the `
base via the RF link on channel N at a step 94. The handset then
determines if there is a base on channel N which is in range at a
25 step 96. If there is a base on channel N which is in range, the
handset determines whether the base is already servicing a
handset at a step 98. If the base is not already servicing the `~ -
handset, the base sends an acknowledge signal and goes
offhook on channel N at a step 100. If the handset sees an
30 acknowledge at a step 102, the connection is made on channel N
at a step 104.
If the base is already servicing a handset, the base ignores
the handset request at step 106. Similarly, if there is no base on
channel N which is in range at step 96, or the handset does not
3~ see an acknowledge at step 102, the handset advances the
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channel number to the next one in ~he pattern at a step 108. If all
the channels in the pattern have not been used at a step 110, the
handset sends a request to go offhook to the base via the RF link ~-
on the next channel in the list at a step 94. However, if all the
channels in the pattern have been used, the handset randomly `
selects a new channel search pattern with an even distribution at
a step 92 to begin an offhook request again. ~ - `
Turning now to FIG. 6, one method for implementing the ;~
function of block 32 of FIG. 2 is shown. Having established a
connection with a base station, the original base station (i.e. the ; `~ ~
base station having a connection before any handoff to a new ~ ~ `
base station) will periodica!ly determine whether a handoff is `
required by establishing a time-out period at step 120. The
original base station will determine if the original base station or
1~ handset RSSI has fallen below a certain threshold at a step 122. ~ ~-
lf the original base station and/or the handset RSSI has fallen
below the threshold, the original base station instructs all base
stations which are not currently serving a handset to go to its ~ ~ ~
channel at a step 124. The original base station also instructs - ~ ~`
each base station not currently serving a handset to report (i) the
RSSI level of the handset and (ii) the channel which it will be
operating on a step 126. All base stations are then instructed to
go back to their original channels, and the original base station
selects the base station having the highest RSSI signal at a step ,~
128. The original base station also send a "go to channel X" to ~ ~ ~
the handset. "
The handset then initiates a handshake with the new base
and waits for a return handshake at a step 130. If there is no
return handshake, the handset returns to the original base at a `~
step 132. If upon returning, the handset can not re-establish - ~`
communication with the original base station, the handset will
search for a new base station using the same sear~h pattern as
described in detail in reference to FIG. 5 for going offhook. ' `
However, if there is a return handshake, the new base station
~6 sends a hangup command to the original base station at a step
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134. Accordingly, if there is base to base communication
provided by the TELCO line, an AC line or separate RF
communication, it is possible to complete the handoff procedure
with little interruption of the communication.
Turning now to FIG. 7, an alternate method for :
implementing block 32 of FIG. 2 is shown. In particular, a method
for handing off in a system having no separate base to base
communication is described in reference to FIG. 7. The original ~ -
base station establishes a certain timeout period at a step 140. If ;
10 the original base station determines that the original base station ;
and/or handset RSSI level falls below a certain threshold at a
step 142, the original base station goes on hold and the handset
begins an offhook request àt a step 144. The offhook request is
identical to the offhook request described in reference to FIG. 5
15 and will not be repeated again here in detail. If the handset is
unable to find another base station at a step 146, the handset ;
returns to the original base station and takes itself off hold with a
data message at a step 148. However, if the original base station
sees the new base station go off hook by sensing a change in the
20 telco line impedance at a step 150, the original base station
hangs itself up. ~
Turning now to FIG. 8, a preferred circuit for communicating ;
between base stations is shown. A block diagram for base station
10 shows the relevant portions of the base station for providing
25 information signals between the base stations by way of the data
link. Each base station generally includes a radio transceiver
220 for transmitting RF communication signals to and receiving RF
communication signals from each remote device 12. Radio
transceivers well known in the art could be employed in the
30 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
35 operate in the wireless communication system 10, and any
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message signats. The message signals may depend on the ~ ~ -
remote device, but could include voice signals, facsimile data or
computer data.
Radio transceiver 220 provides communication signals on
5 a line 222 which is coupled to a signal transceiver circuit 224.
Signal transceiver circuit 224 includes a standard telephone
company interlace (Telco) circuit 226 for transmitting
communication signals. Standard Telco circuits are well known in
the art and provide communication signais to the standard tip and
10 ring lines of the public system telephone network (PSTN).
Radio transceiver 220 also generates an information signal
on a line 222. 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 ;
15 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 microprocessor228.
Microprocessor 228 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 228 also communicates with signal transceiver :
circuit 224 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 ; -~228 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. ~
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.
Signal transceiver circuit 224 generally includes a
transmitter circuit 230 and a receiver circuit 232. Microprocessor
228 provides an RSSI signal to signal transceiver circuit 224 for
coupling signais to the tip and ring lines of the PSTN by way of
5 data link l4. Preferably, transmitter circuit 230 transmits
information signals at a frequency of approximately 30 KHz. A
~requency 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
10 remote device. A 30 KHz signal frequency is also the lowest
frequency outside the audible range and, therefore, allows largest
signal amplitude on the Te!co line. Finally, a 30 KHz signal will be
filtered by the public systeni telephone network. While a 30 KHz
signal is preferred, any other frequency which is outside the ~'
15 audible range and which will be filtered by the public system
telephone network could be used.
Signal transceiver circuit 224 also includes a receiver :
circuit 232 for receiving information signals from other base
stations by way of data link 14 Preferably, receiver circuit 232
20 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
25 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 233 may be incorporated in
one of the base stations. Preferably, the answering machine will
30 be a digital answering rnachine.
Turning now to FIG. 9, the preferred signal transceiver -
circuit 224 (shown in block form in FIG. 8) found in each base
station 11 is shown in detail. Transmitter circuit 230 includes an
AND gate 234 having a first input 236 coupled to receive a 30
35 KHz square wave and a second input 238 coupled to receive
~1~2i~3
- 18
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 ~-
5 ANDING the data with the 30 KHz square wave signal.
Alternatively, the data could be provicled by the microprocessor as
a 30 KHz signal. The output 240 of AND gate 234 is coupled by a
capacitor 242 to the TIP line. Preferably, capacitor 242 is
approximately 200 picofarads. Output 240 of AND gate 234 is
10 also coupled to an inverter 244. The output ?46 of inverter 244 is
coupled by a capacitor 248 to the RING line. Capacitor 248 is
a!so preferably 200 picofarads. ~
Telco circuit 226 is also shown in detail in FIG. 9. Telco ~ `
circuit 226 includes a transformer 252 for transmitting and
15 receiving audio signals from the radio transceiver. The
transformer is coupled to a relay 254. Relay 254 is controlled by a `
transistor 256 which is turned on or off by a voltage at a control
electrode 258 coupled to an input resistor 260. Relay 254 is ``
coupled to a bridge circuit 262 at a node 264. Nodes 266 and ~ i
268 of bridge circuit 262 are coupled to the TIP and RING lines. ; ~-
Bridge circuit 262 also receives signals from the TIP and RING -
lines at nodes 266 and 268. Finally, node 270 is coupled to `
transformer 252 for transmitting signals from the TIP and RING
lines to the radio transceiver.
Finally, signal transceiver circuit 224 includes a receiver
circuit 270. 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 270 will pass 30 KHz information signals which are ; ~ ,`;
transmitted by tihe transmitter circuits 230 of the other base ~;
stations.
Receiver circuit 270 includes a differential amplifier circuit
272 having a positive input 274 and a negative input 276.
Positive input 274 is coupled to the RING line by a capacitor 278
and resistor 280 which form a low pass filter. Preferably capacitor
.,
~,':
` 21~6:~ 3
- 19 -
278 is 620 pf and resistor 280 is 10 KQ. Input 274 is also coupled
to a parallel RC network comprising a capacitor 282 and a resistor
284 to form a high pass filter. Preferably, capacitor 282 is 20 pf
and resistor 284 is 220 KQ. Negative input 276 to the differential
5 amplifier is also coupled to a capacitor 286 in series with a
resistor 288 to form a low pass filter. Pre~erably, capacitor 286 is
620 pf and resistor 288 is 220 K~2. Also, a parallel configuration
of a capacitor 290 and a resistor 292 is coupled between negative
input 276 and the output 294 of the differential amplifier.
1 0 Preferably, capacitor 2gO is 20 pf and resistor 292 is 220 KQ.
The detailed signal transceiver circuit 2~4 shown FIG. 9 is
one example of a circuit which could be employed. However, it
will be understood that othèr signal transceiver circuits for
transmitting and receiving approximately 30 KHz information
1 5 signals on the data link could be employed within the scope of the
present invention.
In summary, the present invention establishes and
maintains communication in a portable telephone system
adapted to operate with a plurality of base stations and at least
20 one handset on a plurality of channels. The method generally
assigns security codes to the handset(s) operating in the portable - `
communication system, and provides copies of the security codes
to each of the base stations to enable RF communication between
the handset(s) and the base stations. The method also aligns the
25 channel of the handset(s) with a channel of a base station to
allow for communication between each handset and one of the
plurality of base stations. In particular, when a particular handset
is not in use, the method mainitains a channel alignment with a ; ~
base station within range. Further, during RF communication with ~ -
30 a handset, a base station will handoff between base stations to
maintain communication. Such a handoff could be provided by
communication between the bases.
