Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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EXTENDED RANGE CORDLESS TELEPHONE SYSTEM
Background of the Invention
1. Technical Field
This invention relates to cordless telephone systems and more
particularly to an arrangement for extending the operating range of a portable
unit
within a cordless telephone system.
2. Description of the Prior Art
Cordless telephones in the United States presently operate on ten
separate frequency pairs (channels) in the 46/49 Ml-iz band. These telephones
have
generally been used in the residential market to allow the user to make and
receive
telephone calls with a portable handset unit connected by a radio link to a
fixed base
unit. Because of radiated power limitations on the radio link, however, the
handset
unit must be used within a radius of a few hundred feet of the base unit. The
actual
range of operation in any given situation depends on the radio propagation
characteristics of a particular premises, which are affected by the building
material
composition as well as the contents of the building. Generally, the operating
range
limitation has not been considered a shortcoming for the cordless telephones
that are
used in residential premises, but now that cordless telephones are being
provided for
use in business premises, this operating range limitation is considered a much
more
2U critical problem. Moreover, business premises generally provide a harsher
electro-
magnetic environment than residential premises, with the building material
composition and building contents being such that they significantly reduce
the
operating range of the the cordless telephone from what it would otherwise be
if
operated in a residential premises. In addition, the business customer is more
critical
of communication problems than the residential customer and will view radio
dead-
spots as unacceptable.
In order to cope with the limited operating range of the cordless
telephone system, one multiple channel access system employs a dedicated
common
signaling channel to set up calls. Operation of the system is such that the
handset
unit bids for access to the plurality of radio frequency channels. Access to
channels
is provided by a centralized controller through a transceiver at a cell site.
Since
cordless telephones have been allocated a limited number of communication
channels over which they may operate, this system has the disadvantage of
reducing
the available number of channels by minimally committing one of these as the
common signaling channel. This system also has the disadvantage of requiring
manual location registration wherein the user of the portable handset unit
must
remember to register the location of his or her handset unit, else the
centralized
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controller will not know the whereabouts of this portable handset unit. One
alternative to manual location registration is the suggestion of periodic
polling of
each portable handset unit through the transceivers at the cell sites. This is
also
undesirable, however, in that it may cause needless interference to users of
other
nearby cordless telephone systems.
In addressing the problems of limited available radio frequency
spectrum and the resulting interference between transceivers operating at the
same
frequencies, one cordless telephone system employs a line-of sight or optical
transmission link for communications between portable handset units and
dispersed
base units. This system, described in United States Patent 4,456,793;
specifically
utilizes modulated infrared transmissions between the portable handset units
and
each of the dispersed base units. Although this system generally avoids the
problems associated with having limited available radio frequency spectrum, it
has
the disadvantage of having a limited operating range between base and handset
units
because of its line-of-sight operating requirement. It unfortunately thus
requires a
. large number of base units dispersed through a premises in order to obtain
the
desired continuous coverage for a user of a handset unit as he or she roams
through
the premises.
Summary of the Invention
In accordance with the invention, a cordless telephone system provides
an extended operating range for a portable or handset unit within the system.
The
cordless telephone system comprises a plurality of handset units and a
plurality of
base units for communicating over a communication channel. The base units are
dispersed so as to divide a location of the telephone system into a number of
reception areas for telephone service with the handset units. A control unit
connects
to each of the plurality of base units for switching the telephone service for
each
handset unit between the plurality of base units. In accordance with a feature
of the
invention, at least one of the plurality of base units provides the telephone
service
only to a selected ore of the plurality of handset units. This assures that a
base unit
will always be available for the user of a particular handset unit.
Operable as an adjunct to a private branch exchange (PBX) switch or a
key system, the cordless telephone system employs multiple base units and
multiple
handset units, with only one of the multiple base units being activated at a
time for
communicating with a given handset unit. Each of the multiple handset units
has an
assigned or dedicated base unit with which only it may communicate. Each of
the
multiple handset units may also communicate with other shared base units which
are
also included in the system.
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In further accordance with the invention, a radio link control unit within the
cordless telephone system advantageously controls in preferred embodiments the
activation and deactivation of the base units and the connection of the voice
and PBX
switch control paths to each active base unit, i.e., the base unit actively
communicating
with a handset unit. As a user of a handset unit with a call in progress roams
around a
premises, the active base unit monitors the quality of the signal received
from the handset
unit. If this signal quality deteriorates to below an acceptable level, the
active base unit
notifies the radio link control unit which, in turn, initiates a polling
procedure to identify
another base unit that is then receiving the signal from the handset unit with
an acceptable
signal quality. The control unit then simultaneously turns off the transmitter
of the base
unit with the unacceptable received signal quality and turns on the
transmitter of the base
unit with the acceptable received signal quality and moves the voice path to
the new base
unit. With no calls in progress, both base unit and handset unit transmitters
are off, but all
receivers are active in order to respond to incoming calls and service
requests. Channel
cycling and base unit polling procedures are advantageously employed to insure
that
incoming or outgoing calls are completed quickly, even though the location of
a roaming
handset unit is unknown.
In accordance with one aspect of the present invention there is provided a
cordless telephone system comprising: a plurality of handset units each having
a first
transmitter and a first receiver; a plurality of base units each having a
second transmitter
and a second receiver for respectively transmitting to the first receiver and
receiving from
the first transmitter in the handset units over a communication channel, the
base units
being dispersible so as to divide a location of the telephone system into a
number of
reception areas for telephone service with the handset units; a control unit
for switching
the telephone service for each handset unit between the plurality of base
units; at least one
of the plurality of base units being assigned to and dedicated to a selected
one of the
plurality of handset units for providing the telephone service only to the
selected one of
the plurality of handset units, the plurality of base units further comprising
shared base
units, each shared base unit including means for providing telephone service
to each one
of the plurality of handset units; and monitoring means in each one of the
plurality of base
units, the monitoring means in the dedicated base unit monitoring the
communication
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channel selected by the selected one of the plurality of handset units and the
monitoring
means in each shared base unit monitoring the communication channel selected
by each
one of the plurality of handset units, in response to the monitoring means,
the dedicated
base unit and the shared base units providing telephone service to the
selected one of the
plurality of handset units during movement of the handset unit from a first
reception area
for a first one of the plurality of base units to a second reception area for
a second one of
the plurality of base units essentially without interruption over the selected
communication
channel, including the provision of uninterrupted service during the selected
handset unit's
movement from the reception area of its dedicated base unit to a reception
area of a shared
base unit.
In accordance with another aspect of the present invention there is provided a
method of extending the operating range of a cordless telephone having a
plurality of
handset units with a first transmitter and a first receiver, and a plurality
of base units with
a second transmitter and a second receiver for respectively transmitting to
the first receiver
and receiving from the first transmitter in the handset units over a
communication channel,
the method comprising the steps of: dispersing the base units so as to divide
a location of
the cordless telephone into a number of reception areas for telephone service
with the
handset units; connecting each of the plurality of base units for switching
telephone
service for each handset unit between the plurality of base units; dedicating
at least one of
the plurality of base units for providing the telephone service in a selected
reception area
only to an assigned selected one of the plurality of handset units; arranging
base units in
the plurality of base units as shared base units, each shared base unit
providing telephone
service to each one of the plurality of handset units; monitoring in the
dedicated base unit
the communication channel selected by the assigned selected one of the
plurality of
handset units; monitoring in each shared base unit each communication channel
selected
by each one of the plurality of handset units; and providing in the dedicated
base unit and
the shared base units telephone service to a one of the plurality of handset
units during
movement of the handset unit from a first reception area for a first one of
the plurality of
base units to a second reception area for a second one of the plurality of
base units
essentially without interruption over the selected communication channel,
including the
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provision of uninterrupted service during the selected handset unit's movement
from the
reception area of its dedicated base unit to a reception area of a shared base
unit.
Brief Description of the Drawings
This invention and its mode of operation will be more clearly understood from
the following detailed description when read with the appended drawing in
which:
FIG. 1 is a block diagram of a cordless telephone system including a radio
link
control unit and both multiple base units and handset units in the system
operating as an
adjunct to a PBX switch in accordance with the invention;
FIG. 2 is a block representation of the major functional components of one of
the handset units and one of the base units depicted in FIG. 1, operative in
accordance with
the invention;
FIG. 3 is a flow diagram of a process suitable for incorporating into the
radio
link control unit of the cordless telephone system of FIG. 1 in accordance
with the
invention;
FIG. 4 shows the telephone system of FIG. 1 with the handset units and the
base
units of this system operating according to the process described in FIG. 3
and in
accordance with the invention; and
FIG. 5 shows a system status table of information for the cordless telephone
system shown in FIG. 1 in accordance with the invention.
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Throughout the drawings, an element when shown in more than one
figure is designated by the same reference numeral in each such figure.
Detailed Description
Referring now to FIG. l, there is shown a private branch exchange
(PBX) control unit 10, a radio link control unit 11, multiple fixed telephone
base
units 12 through 17 and multiple portable handset units 18 and 19. The PBX
control
unit 10 is employed in a PBX switch (not shown) and controls the routing of
inconung calls to the switch received over multiple central office leads 101
as well
as those intercommunication calls. placed between two telephone units within
the
PBX switch. The radio link control unit 11, the base units 12 through 17 and
the
handset units 18 and 19 comprise a cordless telephone system which operates as
an
adjunct to the PBX switch in accordance with the invention:
The radio link control unit 11 coordinates the overall operation of the
cordless telephone system. This control unit 11 serves as an adjunct switch
for
interconnecting calls within the PBX switch directed to and from handset units
18
and 19 through respectively associated extension leads 102 and 103. Telephone
calls
originated and received at handset units 18 and 19 are also routed through one
of the
plurality of multiple base units 12 through 17 via the radio link control unit
11. The
radio link control unit 11 also coordinates the handing off of handset units
between
base units and the search for a roaming handset unit during an incoming call.
The analog terminal line (ATL) protocol is employed in the PBX
switch. It is to be understood that the cordless telephone system also is
operable in
PBX switches and key telephone systems employing other protocols. The ATL
protocol defines an information transport path between the PBX control unit 10
and
the radio link control unit 11 as well as between the radio link control unit
11 and
each of the multiple base units 12 through 17. The protocol employs a three-
level
hierarchical structure. The low, intermediate and high levels are designated
as Line,
Link and Message respectively. Interconnection between units is provided via a
standardized four-pail interface with the four pairs being allocated into (1)
void-1,
(2) voice-2, (3) control and (4) power.
Information between the radio link control unit 11 and the base units 12
through 17 is communicated in three ways in accordance with the ATL protocol.
First, the radio link control unit 11 transmits command messages to a base
unit. No
specific response message is required by the base unit from the radio link
control
unit. Second, the radio link control unit 11 queries a base unit for
information and a
response message is expected. Lastly, a base unit transmits an unprompted
message
to the radio link control unit 11 and a response message may or may not be
expected.
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2046254
These messages are exchanged during the time, typically once every 25
milliseconds, when the radio link control unit 11 polls each base unit. All
messages
received either by a base unit or the radio link control unit 11 cause an
acknowledgment to be returned to the sender. If the sender's message requires
information to be returned to him or her, the returned information serves as
the
acknowledgment. If the sender's message does not require information to be
returned, an explicit acknowledge message is returned.
The radio link control unit 11 contains a switching logic module 110
that connects ATL leads from the PBX control unit 10 to the multiple base
units 12
through 17 through ATL ports designated 1 through 6. Additionally, the radio
link
control unit 11 contains a controller 111 for executing a process or program
for
performing the functions provided by the cordless telephone system and those
required by the ATL protocol. This controller 111 may be impl8mented through
the
use of a microcomputer containing read-only-memory (ROM), random-access-
memory (RAM) and through use of the proper coding. Such a microcomputer is
known in the art and is readily available from semiconductor manufacturers
such as
Signetics, Intel and AMD.
In accordance with the invention; two modes of base unit operation are
employed in providing telephone service to the handset units: dedicated base
units
and shared base units. A dedicated base unit is usable only by a designated
handset
unit whereas a shared base unit is usable by any handset unit operating in the
cordless telephone system. Assigning each handset unit to a dedicated base
unit
assures that a base unit, typically located on a user's desk or within that
user's
immediate surroundings, will always be available for that user to access. By
way of
example, base unit 12 is dedicated to handset unit 18 and base unit 15 is
dedicated to
handset unit 19. And handset unit 18 is assigned within the PBX control unit
10 to
extension line 102 and handset unit 19 is assigned to extension line 103. Base
units
13, 14, 16, and 17 are shared units which are strategically distributed
through a
building to mv:imize the operating range for a handset unit operating in the
cordless
telephone system.
In operation by way of a brief example, if handset unit 18 is
communicating with base unit 12 and wanders out of the operating range of this
base
unit into the operating range of a shared base unit, handset unit 18 will be
handed off
from base unit 12 to the shared base unit. In this hand-off, the radio link
between
base unit 12 and handset unit 18 will be terminated and a new radio link
between the
shared base unit and handset unit 18 will be established. Thus, handset unit
18 may
be handed off to all base units except base unit 15 since this base unit is
similarly
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zo~sz54
dedicated to handset unit 19. Also, since each handset unit is assigned or
responsive
to a unique extension, upon receipt of an incoming call for this handset unit,
the
handset unit is located by this shared base, as described later herein, and
the
incoming call is routed to this handset unit through this nearby shared base
unit.
The radio link control unit 11 provides for coordinating the search for a
handset unit during an incoming call. Because of the potential for causing
needless
interference, it is undesirable to provide communications in a cordless
telephone
system via a radio link when a call is neither active nor being originated.
Thus the
cordless telephone system is advantageously arranged to quickly locate a
roaming
handset unit upon receipt of an incoming call. When an incoming call is
received,
the dedicated base unit and the idle shared base units are polled such that
each base
unit attempts to locate the handset unit. This polling process is fully
described later
herein and with reference to FIG. 3.
Referring next to FIG. 2, there is shown a block representation of the
major functional components of the base unit 12 and the handset unit 18, both
depicted in FIG. 1 and operative in an extended range cordless telephone
system in
accordance with the principles of the invention. With regard to the functional
components illustratively described in base unit 12, the dedicated base units
and the
shared base units of FIG. 1 incorporate these same components and are
operationally
identical. Similarly, the handset unit 19 incorporates the same components as
handset unit 18 and is also operationally identical.
Included in the base unit 12 is a control unit 120 which advantageously
provides a number of control functions. This control unit 120 interfaces with
a
control unit 180 in the handset unit 18 and, while the handset 18 is located
in a
mating cradle in the base unit 12, generates and transfers security code data
to the
control unit 180 in accordance with the teachings of United States Patent
4,736,404
issued to R. E. Anglikowski, et al. on April S, 1988. This control unit 120
also
compares the received security code data with its stored security code data
during the
establishing cf a two-way radio f:equency ~R~ communications link between the
handset unit 18 and the base unit 12. A favorable comparison of the data from
the
two security codes must be achieved in order for the base unit 12 to respond
to a
request-for-service signal from a handset unit. This control unit 120 also
receives
and processes opcode data provided by the handset unit 18 for dialing and
providing
signaling information out to the radio link control unit 11 via a telephone
circuit 121
and over ATL leads 104. Control unit 120 may be implemented through the use of
a
microcomputer containing ROM, RAM and through use of the proper coding. Such
a microcomputer is known in the art and is readily available from
semiconductor
_6_
manufacturers such as Signetics, Intel and AMD.
Both the base unit 12 and the handset unit 18 are operable on a plurality
of communication channels. The control unit 120 configures a radio transmitter
122
and a radio receiver 123 in the base unit 12 for proper operation on the
selected one
of the plurality of channels when actively communicating with the handset unit
18.
The control unit 120 also provides the appropriate channel control information
to the
handset unit 18.
The transmit and receive signals of the base unit 12 are coupled to a
duplexer 125 which permits the transmitter 122 and the receiver 123 to both
simultaneously operate over antenna 124 while preventing the output of
transmitter 122 from being coupled directly to the input of the receiver 123.
When
the base unit is in an idle state awaiting an incoming telephone call or a
request-for-
service signal from'a handset unit, transmitter 122 is turned off while
receiver 123
remains on to detect the request-for-service signal. The telephone circuit 121
serves
as an interface for control signals provided over the ATL leads 104 between
the
radio link control unit 11 shown in FIG. 1 and the control unit 120. The
telephone
circuit 121 also serves as the interface for voice signals over the ATL leads
104
which are sent to transmitter 122 and received from receiver 123.
Referring next to the handset unit 18, there is shown the control unit 180
which stores the security code data that is generated by the base unit 12 and
provided
to the handset unit 18. This security code data stored in control unit 180 is
transmitted from the handset unit 18 to the base unit 12 while establishing
initial
communications through a request-for-service signal as well as during the
transfer of
subsequent opcode data to the base unit. These signals are transmitted in a
frequency shift keying (FSK) format and includes a synchronizing signal
immediately followed by a data field which includes the security code
generated by
the control unit 120. Like the control unit 120, this control unit 180 may be
implemented through the use of a microcomputer containing ROM, RAM and
through use o~ the proper coding. Such a microcomputer is known in the art and
is
readily available from semiconductor manufacturers such as Signetics, Intel
and
AMD.
Communications with the base unit 12 are provided via a radio
transmitter 181 and a radio receiver 182 in the handset unit 18. The output of
the
transmitter 181 and input for the receiver 182 are commonly coupled through a
duplexer 183 to an antenna 184. The receiver 182 demodulates voice signals
transmitted by the base unit 12 and couples these signals to an acoustical
device such
as, for example, loudspeaker 185. The transmitter 181 has as its input speech
signals
_. 2~~~~~4
from a microphone 186, security code data from control unit 180 and opcode
data
representative of entries on a keypad 188, all of which it transmits to the
base
unit 12. The key pad 188 is used for entering dial digits and control
functions
executable by the control unit 180 or transnutted to the base unit 12. A
battery 187
is also included in the handset unit 18 for providing operating power for all
circuitry
in this unit.
In order to conserve battery power, a controlled power up/power down
mode of operation for the handset. unit 18 is implemented in accordance with
the
teachings of United States Patent 4,731,814 issued to W. R. Becker et al. on
March 15, 1988. The battery 187 in the handset unit is normally charged while
the
handset unit is placed in a cradle located in, for example, its dedicated base
unit.
When the handset unit is removed from this base unit and is in an idle or
standby.
state awaiting a telephone call, power to the control unit 180, receiver 182
and
certain other selected circuitry in the handset unit 18 is controlled to
minimize power
consumption. Power to other non-essential circuitry in the handset unit 18 is
turned
completely off during this state. The handset unit automatically turns on to a
full
operating mode from the controlled power up/power down mode in response to
events such as a user depressing a key on the keypad 188 or the receipt of a
ring
indication from a base unit, the ring indication being indicative of an
incoming call
directed to the handset unit.
With reference to FIG. 3, there is shown a flow chart illustrating the
operation of the radio link control unit 11 in routing an incoming (or
intercommunication) call and providing telephone service to the correct
handset unit
via the appropriate base unit. Initially, the PBX control unit 10, shown in
FIG. 1,
determines whether the assigned extension for the handset unit to which the
incoming call is directed is busy or idle. If the extension is busy, either a
busy signal
is provided to the calling patty, a call waiting signal provided to the called
party, or
other alternate routing determined by the configuration of the PBX control
unit 10 is
executed. If the ex_te.nsien is idle, then the incoming call is routed to the
radio link
control unit. The functions performed by controller 111 of FIG. 1 in routing
the
incoming call to the intended handset unit are advantageously determined by a
process or program contained in the memory (not shown) located in this
controller.
The process is entered at step 301 where the incoming call is received.
Also at this step, the process determines the correct dedicated base unit for
the
handset unit having the extension over which the call is being received and,
in
step 302, causes a ring indication to be transmitted from this base unit. The
process
advances to decision 303 to check for the receipt of a ring acknowledge from
the
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~0~~2~~
handset unit associated with the dedicated base unit. If a ring acknowledgment
is
received by the dedicated base unit, then the process advances to step 304
where the
signaling base unit continues to send a ring indication and the called handset
unit, in
return, continues to send a ring acknowledgment until the called handset unit
goes
off-hook or the incoming call is terminated.
If at decision 303, a ring acknowledgment is not received by the
dedicated base unit, the process advances to step 305 where the process causes
a
first one of the shared base units to transmit a ring indication to the
handset unit
having the assigned extension over which the call is being received. The
process
next advances to decision 306 where it checks to see if a ring acknowledge
from the
called handset unit is received by the signaling shared base unit. If a ring
acknowledgment is received by the first one of the shared base units, then the
process proceeds to step 304 where this base unit continues to send a ring
indication
and the called handset unit, in return, continues to send a ring
acknowledgment until
the called handset unit goes off hook or the incoming call is terminated.
If a ring acknowledge from the called handset unit is not received by the
first one of. the shared base units in decision 306, the process advances to
decision 307 where a determination is made as to whether all shared base units
have
been selected for sending a ring indication to the handset unit having the
assigned
extension over which the call is being received. If all shared base units have
not
been selected, the process advances to step 308 where a next one of the shared
base
units is selected by the radio link control unit. The process then returns to
step 305
where this selected shared base unit transmits a ring indication to the
handset unit
having the assigned extension over which the call is being received. The
process
determined by steps 305 and 308 and decisions 306 and 307 is repeated until
all
shared base units are selected for signaling the called handset unit or a ring
acknowledge is received by the signaling one of the shared base units. If all
shared
base units have been selected and no ring acknowledge received from the called
handset unit, the process returrvs to the step 302 where the dedicated base
unit once
again transmits a ring indication in its attempt to find the called handset
unit. This
process is repeated until the called handset unit is located or the incoming
call is
terminated. Although the period is adjustable, the radio link control unit 11
is
arranged to typically cycle through all the base units approximately every six
seconds.
As a further illustration of this polling operation, reference is made next
to FIG. 4 which shows the cordless telephone system of FIG. 1 and further
shows the
operation of handset units 18 and 19 when communicating with other than their
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zo~sz~~
respectively dedicated base units. Although FIG. 4 is functionally equivalent
to
FIG. l, some of the detail of FIG. 1 is not again shown in FIG. 4 for ease of
understanding and for highlighting the specific teaching illustrated in FIG.
4. As in
FIG. l, base unit 12 in FIG. 4 is the dedicated base unit for the handset unit
18 and
base unit 15 is the dedicated base unit for the handset unit 19. Initially,
handset
unit 18 is in the range of base unit 17 and is idle, and handset unit 19 is
within the
range of and communicating with base unit 14. An incoming call over extension
102
(lead 102) for handset unit 18 is received by the radio link control unit 11.
This
control unit 11 causes dedicated base unit 12 to transmit a ring indication
that is not
.received by handset unit 18 since this handset is not then within the
reception range
of the transmitted signal from base unit 12. Base unit 12 then informs the
radio link
control unit 11 that a response was not received from the handset unit 18. The
radio
link control unit 11 next causes a ring indication to be transmitted, for
example, by
shared base unit 13 to the handset unit 18. This signal is also not received
and base
I5 unit 13 informs the radio link control unit 11 that a response was not
received from
the handset unit 18. The radio link control unit 11 skips base unit 14 since
this base
unit is communicating with handset unit 19. The radio link control unit 11
also skips
base unit 15 since this base unit is dedicated to handset unit 19. The radio
link
control unit 11 next causes base unit 16 to transmit a ring indication which
also is
not received and base unit 16 informs the radio link control unit 11 that a
response
was not received from the handset unit 18. Finally, the radio link control
unit 11
causes base unit 17 to transmit a ring indication that is received and
acknowledged
by the handset unit 18 which is within the reception range of base 17. The
radio link
control unit 11 then routes the incoming call to base unit 17.
In order for an idle handset unit to request service from a base unit (e.g.,
to originate a call or change channels), the handset unit must be within the
operating
range of a base unit and the base unit must be on the same channel as the
handset
unit. An idle dedicated base unit always monitors the same channel on which
its
assigned Handset unit last operat,:d and each idle shared base unit cycles
through and
monitors each channel known to be used by a handset unit in the cordless
telephone
system. The radio link control unit 11 advantageously informs the base units
of the
channels being used by the handset units as well as when a handset unit is
changed
from one operating channel to another. If a user executes a channel change
operation
on the handset unit he or she is then using, this information is passed via
the active
base unit back to the radio link control unit 11, which informs the other base
units of
the change. Thus, if the handset unit is communicating with its dedicated base
unit
when the user changes channels, the shared base units are informed of the
change
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and which new channel to periodically go to and monitor in order to receive a
service request from that handset unit. Similarly, if the handset unit is
communicating with a shared base unit when the user changes channels, the
other
shared base units are informed of the handset unit changing channels and which
new
channel to also periodically go to and monitor (one channel for each handset
unit
operating in the system). The dedicated base unit is also informed of the new
channel which it must monitor.
Channel cycling is employed by the idle shared base units to assure that
all handset units service requests are received when a handset unit is beyond
the
operating range of its dedicated base unit. In the operation of channel
cycling, each
shared base unit is informed by the radio link control unit 11 of the
frequency
channels being used by each handset unit. Idle shared base units then listen
for
service requests on each one of the channels assigned to a handset unit
operating
within the cordless telephone system. The idle shared base units cycle through
each
of the channels, dwelling for a short period, typically 0.5 seconds, on each
channel in
order to receive any service requests from a handset unit then using that
channel.
The channel cycling operation is executed by an idle shared base unit until
either a
task of greater priority is presented by the radio link control unit 11 or it
receives a
service request from a handset unit.
When either a dedicated or idle shared base unit receives a service
request from a handset unit, the base informs the radio link control unit 11
of the
request. The radio link control unit 11 then grants permission to the base
unit to
establish a radio link with the handset unit. If more than one base unit
receives the
service request, the radio link control unit 11 grants percussion to the first
base unit
that receives the handset unit with the best radio link quality, to be
described in
greater detail later herein, and inform the other base units to ignore the
service
request. The radio link control unit 11 also switches the ATL control signals
to the
appropriate base unit.
The radio link control unit 11 insures that no two handset units will
occupy the same frequency channel at the same time. A user of a handset unit
may
select a channel for operation that is different from the one selected by the
cordless
telephone system during initialization, for example. Because the radio link
control
unit 11 monitors and controls the channels selected for use by all handset
units, it
prevents the selection of a frequency channel previously selected by another
handset
unit. Thus, if a user initiates a channel change, the base unit with which the
handset
unit is then communicating will only change (and cause the handset to follow)
to a
channel not in use by or previously selected and presently occupied by any
other
20~~2~~
handset unit.
The radio link control unit 11 also informs the shared base units of the
security codes that they are allowed to recognize. Selection of a security
code for
each handset unit is respectively provided by its dedicated base unit. Each
one of
S these dedicated base units randomly selects the code and transfers it to the
associated
handset unit each time the handset unit is placed in a mating cradle in the
dedicated
base unit. In the same manner in which it monitors the frequency channels
selected
by the handset units, the radio link control unit 11 similarly monitors the
security
codes selected by the dedicated base units. If a second dedicated base unit
happens
to select a security code that is then being used by a first dedicated base
unit and
handset unit combination, the radio link control unit 11 instructs the second
base unit
to select another security code. The base units are thus controlled such that
no two
dedicated base units will be able to have the same security code at the same
time.
Thus, when a user returns his handset unit to the dedicated base unit, this
base unit
will be allowed to only select a security code that is not then being used by
any other
base unit and handset unit combination.
In order to achieve continuous coverage throughout a premises, each
active base unit includes signal strength monitoring circuitry for monitoring
the
strength of the received signal from the handset unit while communicating with
that
handset unit. Although the signal strength monitoring circuitry is provided in
the
base unit, it is understood that such monitoring may also performed with
similar
circuitry includable in the handset unit. When the received signal strength
deteriorates to below an acceptable level, an active base unit deterniines
that its radio
link quality is poor and informs the radio link control unit 11 which then
instructs all
idle shared base units (and the dedicated base unit, if it is idle) to listen
on the
appropriate channel and report radio link quality (GOOD/TOLERABLE/POOR) to
the radio link control unit.
In response to a request from the radio link control unit 11 to idle base
units to report radio link qualit~~, a dedicated base unit just determines
radio link
quality and reports it to the radio link control unit. Idle shared base units,
however,
suspend the channel cycling process, determine the radio link quality for the
appropriate channel, reports it to the radio link control unit, and then
resume channel
cycling. As the base units are polled, the data reflective of the radio link
quality for
the appropriate channel is received and interpreted by the radio link control
unit 11
which, in turn, selects the appropriate new base unit for handing off the
handset unit
thereto and establishing communications therewith. The hand-off is
accomplished as
follows: the radio link control unit instructs both the old and the new base
units to
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20 46254
mute their voice paths, instructs the new base unit to turn on its
transmitter, switches
the control ATL lead to the new base unit, instructs the old base unit to turn
off its
transmitter, and instructs both base units to unmute their voice paths.
In preferred embodiments, each of the base units detect radio link
quality through use of the signal strength monitor circuit 126 shown in FIG. 2
which
may be, for example, a received signal strength indicator (RSSI) circuit. The
RSSI
circuit produces an output voltage that is proportional to the strength of the
received
signal from the handset unit. The base unit then compares this voltage
relative to
both a predetermined upper and a lower threshold level. An output above the
higher
threshold level indicates a GOOD link, an output between the two threshold
levels
indicates a TOLERABLE link, and an output below the lower threshold level
indicates a POOR link. If a POOR link exists between the handset unit and the
base
unit, the radio link control unit 11 first attempts to hand-off the handset
unit to a base
unit with a GOOD link. If after polling all available idle base units, no base
unit is
found with a GOOD link, the radio link control unit 11 immediately executes a
hand-off of the handset unit to the first base unit found with a TOLERABLE
link. If
no base unit was found with a TOLERABLE link, the radio link control unit 11
does
not hand-off the handset unit. Rather, the control unit 11 causes the base
unit to
generate an audible and distinctive tone which is recognizable to the user
that he or
she is then approaching the outer limit of the operating range for that
handset unit in
the cordless telephone system.
A carrier detect circuit may alternatively be employed as the signal
strength monitor circuit 126 in detecting radio link quality. As provided in
the RSSI
circuit, the carrier detect circuit similarly provides the base unit with the
capability
of determining if a handset unit has been detected. By way of example,
operation of
the carrier detect circuit in the 46/49 MHz cordless telephone channels is
such that
when a earner is present (i.e., a transmitter operating at the base unit's
receiver
frequency), the power of the baseband signal is negligible beyond 4 KHz. When
no
carrier is present, there is significant power (white noise) out to 1S KHz.
And the
carrier detect circuit checks for the presence of power at 9 KHz. Thus if
power is
detected, it is assumed that there is no carrier and that a handset unit has
not been
detected or that the radio link quality is bad. If power is not detected,
however, it is
assumed that there is a carrier, a handset unit has been detected and that the
radio
link quality is GOOD. As provided with the RSSI circuit, the hand-off of the
handset unit is then made to the first base unit reporting a good radio link
quality.
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~~4~254
In coordinating the overall operation of the cordless telephone system,
the radio link control unit 11 is required to receive and maintain certain
information
from the base units and, in return, provide some portions of this information
to the
base units for updating their operating conditions. With reference to FIG. 5,
there is
illustratively shown a system status table of information with values that are
provided at system initialization for the cordless telephone system shown in
FIG. 1.
Reference is also made to FIGS. 1 and 2 for illustrating the applicable
components of
the cordless telephone system which correspond to those entries made in the
system
status table. For example, the entries in column 1 of the table correspond to
the base
units shown in FIG. 1.
After initialization, the radio link control unit 11 first requests that the
dedicated base units 12 and 15 provide data indicative of their respectively
selected
frequency channels and security codes. Upon receipt of this information, the
radio
link control unit 11 completes entries 2, 3, 5 and 6 of columns 2 and 3 of the
table.
These entries are respectively for shared base units 13, 14, 16 and 17 shown
in
column 1. Next, the radio link control unit 11 transmits these table entries
to these
shared base units informing them of the channels they are to periodically
monitor
and the security codes they are to recognize.
The base unit class of DEDICATED or SHARED is defined in column 4
of the table and the dedicated handset unit extensions are entered in column
5. Each
dedicated handset unit has an assigned extension on the PBX switch over which
it
makes and receives telephone calls. Column 6 defines the call status of each
base
unit. All entries in this column are initially IDLE. When a call is in
progress, the
base which is communicating with the handset unit involved in the call is
assigned a
call status of ACTIVE.
The transmitter status of each base unit is indicated in column 7. The
transmitter status is BASE when the transmitter is under the control of the
base unit,
and is OFF when the transmitter in the indicated base unit is turned off by
the radio
lit:k control unit 11. With the status of BASE assigned to ~. transmitter, a
base unit is
permitted to go directly off-hook during the initiation of a call or on-hook
upon the
completion of a call. When a transmitter in a base unit has been assigned the
status
of OFF, the radio link control unit 11 directly controls the enabling of the
transmitter. The OFF status is therefore used by the radio link control unit
11 for
switching transmitters and achieving a proper hand-off for a roaming handset
unit
from one base unit to another base unit.
- 14-
2~~6~5~
Various other modifications of this invention are contemplated and may
obviously be resorted to by those skilled in the art. For example, the polling
process
depicted in FIG. 3 and executed by the base units in finding a roaming handset
could
be configured such that the base unit covering the last known location of the
roaming
S handset is the first one that attempts to locate the handset by transmitting
a ring
indication upon receipt of an incoming call. This alternative implementation
of the
process allows the radio link control unit to give preference to the last
known
location of the handset which in many situations will also be the location at
the time
of the incoming call. Those locations closest to the last known location could
also
be given a higher order of preference in the search for the handset unit.
Another alternative implementation of the invention that is
contemplated is a modification to the channel cycling process such that each
idle
shared base unit not only listen on the designated frequency channels in the
cordless
telephone system to detect a service request from the handset units in the
system, but
also to monitor and report its own radio link quality of a radio link existing
during
communications between a handset unit and another base unit. The dedicated
base
unit similarly monitors its designated frequency channel when it is not
communicating with its handset and reports its radio link quality of any
communications on its designated frequency channel to the radio link control
unit.
The radio link control unit then stores and continually updates this
information in
temporary memory in its controller. In this manner, the radio link control
unit
accumulates data reflective of radio link quality from all of the idle base
units. As a
user of a handset approaches the edge of the coverage area of a base unit, the
radio
link control unit already has available the information it needs to execute a
hand-off
to the appropriate base unit. Since the radio link control unit may be
informed of the
actual dispersement of the base units, such data could also be utilized in
predicting
the probability of the handset unit moving into a certain location covered by
a
specific base unit. These alternative implementations may be advantageously
obtained through an alteration of the process or program contained in memory
in the
controller 111 of the radio radio link control unit 11. It is therefore to be
understood
that within the scope of the appended claims, the invention may be practiced
otherwise than as specifically described
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