Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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oFF-LOAa c~vr~x sysTEM Fox oFF-LOU~n~
c~,vr.~ sERVrcE ~xoM A row c~vz~ sysT~
TO I1~CREASE CELL'QLAR SLRYICE CAPACITY
~,ACKGROtI~i~7 OF THE INVENTION
Field of the Invention
The present invention generally relates to a
cellular communication system used to off-load telephone
calls from a main cellular system, and, more
to particularly, to a cellular system providing telephone
service for a mobile telephone in a cell which is located
in an area geographically inside a larger cell of a main
cellular system requiring a high concentration of
cellular service.
Description of the Related A, -fit
Cellular mobile telephones (mobile telephones) are
typically employed in automobiles, ships or the like, and
thus, are transportable with the user iato various
different geographic zones. In each of the different
geographic zones, one or more mobile telephone switching
offices (MTSO) are provided to complete call connections.
Once connected to the mobile telephone switching office,
the mobile telephone may then be connected to another
mobile telephone within a specific zone or, through
land-based networks such as a public switching telephone
network (PSTN), to a land-based telephone or to another
cellular mobile telephone in a different zone.
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Cellular mobile telephone systems (cellular systems)
service specific geographic areas, or zones, each of
which is typically divided into a plurality of cells.
Each cell includes a stationary transmitter receiver
(transceiver) station coupled to transmit and receive
antennas, which is used to establish radio communication
connections with mobile telephones physically located
within the associated cell. The stationary transmitter
receiver stations, in turn, are connected to a mobile
telephone switching office, associated with the zone
where the stationary transmitter receiver stations are
located, which switches the call in an appropriate
manner. Each cell within a specific zone has a number of
frequencies assigned to it to establish radio
communications with a mobile telephone. The frequencies
are divided into control channels, paging channels and
voice channels. The paging and control channels are used
for the mutual identification between the mobile
telephone and the cellular system providing the cellular
mobile telephone service. The location or frequency of
the control channels contained in the frequency range
assigned to each cell identifies the type of cellular
system which is being used. Typically, one set or range
of frequencies is called an "A" cellular system and a
second set of frequencies is called a °B" cellular
system. Generally, a specific geographic area contains
one of each type of cellular system (i.e., an A and a B
cellular system) thereby to provide alternative cellular
service to that area.
A mobile telephone must identify itself to the
cellular system provider before service for the mobile
telephone is established. The identification process
begins using the control channels. When first switched
on, the mobile telephone scans through the control
channels and measures the signal strength of each
channel. It will then tune to the strongest control
channel and lock on to the overhead message stream. When
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the mobile makes an access attempt, the reverse setup
channel access message contains various data which
permits the cellular system to identify the mobile
telephone and thereby determine whether the mobile
telephone should be provided with cellular service. A
detailed description of the interface between the mobile
telephone and the land-based transceiver station may be
found in the United States Federal Register, Vol. 46, No. 98, Thursday,
May 21, 1981 - app. D, pages 27680-27706.
In a typical cellular telephone system, as a mobile
unit travel along a path that passes from one cell to
another, a handoff occurs. The handoff action is
-' controlled by the mobile telephone switching office which
monitors the signal strength received from the mobile
telephone. The handoff command is typically generated
when the signal received from the mobile telephone falls
below a preselected signal strength, thus indicating that
the mobile telephone is at the cell boundary of one cell
and requires a transfer of the cellular service to an
adjacent cell which is able to receive a signal from the
mobile above the preselected signal strength.
As a mobile telephone passes from one cell to
another cell, the handoff command instructs the new cell
which the mobile telephone is entering, to begin
transmitting at a frequency which is different from the
frequency which was transmitted by the cell from which
the mobile telephone is exiting. This procedure is
followed as the mobile telephone passes into each next
successive, adjacent cell. The assigned frequencies of
each adjacent cell are different, and such assigned
frequencies are not repeated except for cells that are
far enough away from each other so that no interference
problems will occur.
One example of a stationary transmitter receiver
station, also referred to as a cell site, is the AT&T
series I and II cell sites (model 1 and model 2
architecture) used in the family of AUTOPLEX'" cellular
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telecommunications systems which are commercially
available from the American Telephone and Telegraph
Company of New York, NY.
Because of increased demands for more radio channels
or frequencies resulting from an increased number of
cellular customers, cellular system providers often
desire to expand their networks to serve an increasing
number of customers within a geographic location.
Various solutions.have been proposed to increase the
capacity of cellular systems and thereby to meet the
foregoing need. For example, a cell may be split into
four smaller cells, each with a radius of half the radius
of the original cell permitting cellular service to be
increased four fold. Naturally, the smaller the cell,
the greater the number of handoffs are required in a
cellular telephone system for a given capacity. These
smaller cell sites are typically known as microcell sites
which operate functionally, similarly to a traditional
macrocellular or main cellular system but only provide
cellular service for a small geographic area. The
microcell sites are typically directly connected to the
main cell site using coaxial transmission lines,
microwave links or an optical fiber cable network which
interconnects the main cell site with associated
microcell sites using a nonstandard protocol. See, for
example, U.S. Patent No. 5,067,173 citing
additional references.
While cells may be subdivided into smaller cells to
provide cellular service for an increasing number of
customers, shrinking cell cites creates additional
considerations and problems. For example, the rate at
which mobile telephones move through the cell and the
non-uniformity of the electromagnetic field' generated in
the cell affect the performance of a microcell system.
Both factors relate to the time required to determine the
relative location of the mobile telephone and to process
a handoff for the mobile telephone from the stationary
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transmitter receiver station of one cell where the mobile
telephone is currently located but is preparing to exit,
to the stationary transmitter receiver station of another
cell, where the mobile telephone is entering. If a
handoff is required, one or more candidate cells must be
queried for their idle channel status and for a
verification of the mobile telephone's signal strength in
that candidate cell. The processing of the decision,
status, and verification usually requires the
3.0 intervention of higher level system control functions in
addition to the control function in the serving and
candidate cells. In addition, the mobile telephone must
be instructed to tune to a frequency available in the
candidate cell and verification of its presence after the
handoff must be made by the candidate cell. Thus, a
significant amount of time is used for haadoff
processing.
In addition, since microcell systems are usually
directly connected to a main cellular system via a
nonstandard protocol, it is impossible to integrate or
include a generic microcell system to off-load cellular
sezvice from the main cellular system since microcell
systems are manufactured with a proprietary nonstandard
protocol. This inability of competing products to
provide cellular sezvice results in microcellular systems
being expensive and difficult to integrate with other
cellular systems.
Thus, it is desirable to provide for easily and
inexpensively off-loading cellular service from a main
cellular system using a microcellular system.
Specifically, it is desirable to enable establishing a
microcellular system in a small area, requiring a high
concentration of cellular service, of a main cellular
system to thereby off-load a significant number of
customers from the main cellular system. For example, a
large office building or shopping mall may require a
significant amount of cellular service which could be
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easily provided using a microcellular system since mobile
telephone users typically establish cellular service
while walking from office to office in a building or from
store to store in a shopping mall. Thus, mobile
telephones in these small areas are less likely to
rapidly move across large geographic areas, making use of
a microcellular system ideal.
In addition, there is currently no method of
off-loading cellular service from a main cellular system
to another or off-load cellular system without
reconfiguring the main cellular system to ignore the
cellular service in the off-load cellular system which is
providing cellular service in the same area where the
main cellular system had previously provided the cellular
service. It is, therefore, also desirable to provide
this additional or off-load cellular service for
customers in a high concentration area of the main
cellular system Without reconfiguring the main cellular
system, thereby providing off-load cellular sezvice which
is transparent to the main cellular system.
Further, it is desirable to off-load cellular
service frown a main cellular system using, for example,
an off-load or microcellular system to provide cellular
service in an area of high concentration Without
requiring the microcellular system to interface with the
mobile telephone switching office (MTSO). The reason
that it is undesirable for the off-load cellular system
to be required to interface with the mobile telephone
switching office is that no standard interface has been
developed which allows the off-load cellular system to be
easily integrated with the mobile telephone switching
office without the off-load cellular system being
required to implement the specific interface requirements
of the mobile telephone switching office. Since various
vendors provide mobile telephone switching offices for
cellular service using different interface protocols, the
off-load cellular system would be required to know the
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interface protocol used by each cellular system. Thus,
the off-load cellular system would be complex and not
easily integratable into an already existing cellular
system.
Finally, it is desirable that a mobile telephone,
which is being serviced by the off-load cellular system
be able to establish telephone service with telephone
equipment which is located outside the off-load cellular
system. Thus, even though the off-load cellular system
is transparent to the main cellular system, the off-load
cellular system must still be able to provide telephone
service with telephone equipment located outside the
" serving area of the off-load cellular system.
SUNll~IARY OF THE INVENTION
It is, therefore, an object of the invention to
provide an off-load cellular (microcellular) system
within an existing main cellular system for off-loading
calls in areas of high concentration from the main
cellular (macrocellular) system while operating
transparently to the main cellular system.
It is another object of the invention to provide an
off-load cellular system which permits the mobile
telephone within the off-load cellular system to
establish telephone connection with telephone equipment
located outside of the off-load cellular system.
It is another object of the invention to provide an
off-load cellular system which is not required to
interface with the mobile telephone switching office of
the main cellular system thereby eliminating the use of a
nonstandard protocol interface.
These and other objects of the present invention are
realized by providing a page rebroadcast system in an
off-load cellular system, including a receiver circuit
monitoring and receiving a page signal broadcast from a
main cellular system. In addition, the page rebroadcast
systems also includes a rebroadcast unit receiving the
page signal output from the receiver circuit, and
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rebroadcasting the page signal as a rebroadcasted signal,
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enabling a mobile telephone located in the off-load
cellular system to receive the page signal from the main
cellular system.
The present invention also provides a control system
controlling cellular service between a main cellular
system and an off-load cellular system. The control
system includes a receiver circuit monitoring and
receiving an origination message broadcast from a mobile
telephone located in an off-load cellular system. In
addition, the control system includes a controller
receiving the origination message output from the
receiver circuit, and instructing the off-load cellular
system whether to provide off-load cellular service to
the mobile telephone responsive to the origination
message.
Finally, the present invention provides an off-load
cellular system for controlling cellular sezvice between
a main cellular system and the off-load cellular system
and for providing off-load cellular service to a mobile
telephone. The off-load cellular system includes a
receiver circuit, monitoring and receiving an origination
message broadcast from the mobile telephone located in
the off-load cellular system, and a system controller,
for receiving the origination message output from the
receiver circuit, and for outputting a connect signal
indicating whether the off-load cellular service is to be
provided to the mobile telephone responsive to the
origination message. In addition, the off-load cellular
system includes a connection controller for receiving the
connect signal output from the system controller, for
transmitting to the mobile telephone a voice connect
message indicating a frequency to obtain the off-load
cellular sezvice responsive to the connect signal, and
for connecting the mobile telephone with a calling party,
providing the off-load cellular service to the mobile
telephone.
The present invention uses a combination of nearby
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forward setup channel monitoring, rebroadcast of
monitored mobile pages, and selected shedding of mobile
access attempts to provide an off-load cellular system
which can operate within an existing cellular system
transparently without requiring nonstandard protocol
interfacing with different mobile telephone switching
offices. The invention allows the use of a standard
mobile telephone in the off-load cellular system as well.
Thus, no new equipment is needed by the mobile telephone
user.
The off-load cellular system may be a stand alone
custom application used in office buildings, hospitals,
shopping malls, etc., or the off-load cellular system may
be a cellular system which provides selective service to
customers of the main cellular system to off-load
cellular service which has been previously provided by
the main cellular system. The present invention also
provides the intersystem functions of customer
verification, call delivery and handoff using either
existing cellular clearing house sezvices, the EIA/TIA
IS-41 interface specifications or a combination of the
two. Flexible registration criterion is used to limit
the mobile telephone s ability to register with the
off-load cellular system when the mobile telephone is
considered an unsuitable user for the off-load cellular
service.
These, together with other objects and advantages
which will subsequently become apparent reside in the
details of construction and operation as more fully
hereinafter described and claimed, reference being had to
the accompanying drawings forming a part hereof, wherein
like numerals refer to like parts throughout.
BRIEF DESCRIPTION OF TFiE DRAWINGS
Fig. 1 is a basic illustration of the conceptual
construction of the present invention;
Fig. 2 is an illustration of the interaction between
the cellular system of the present invention of a main
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cellular system according to the first embodiment of the
invention;
Figs. 3A-3B together comprise a block diagram of the
circuit construction of the present invention according
5 to the first embodiment;
Fig. 4 is a flowchart describing the mobile access
attempt process of the present invention;
Fig. 5 is a flowchart describing the mobile
registration message process of the present invention;
10 Fig. 6 is a flowchart describing the mobile
origination message process of the present invention;
Fig. 7 is a flowchart of the mobile page response
message process of the present invention;
Fig. 8 is a flowchart of a route attempt process of
the present invention;
Fig. 9 is a flowchart of the mobile de-registration
process of the present invention;
Fig. 10 is a block diagram of the circuit
construction of the monitoring circuit of the first
embodiment of the present invention;
Fig. 11 is a black diagram of the circuit
construction of the transceiver control and interface
circuit of the present invention;
Fig. 12 is an illustration of the interaction
between the cellular system of the present invention with
a mobile telephone switching office of a main cellular
system according to the second embodiment of the
invention;
Figs. 13A-13C together comprise a block diagram of
the circuit construction of the present invention
according to the second embodiment; and
Fig. 14 is a block diagram of the circuit
construction of the transceiver control and interface
circuit of the second embodiment of th present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Fig. 1 illustrates the conceptual construction of
the off-load cellular system of the present invention.
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In Fig. 1, cellular system equipment 2 is constructed of
conventional cellular equipment which can be purchased
from AT&T, for example, and is used to establish cellular
service with a mobile telephone by connecting a mobile
telephone to, for example, a public switching telephone
network. A main cell page monitor 4 monitors the
overhead control channels to deterxaine whether the main
cellular system has transmitted a page message to a
mobile telephone which may be located in the off-load
cellular system. If the main cell page monitor 4
receives a page message from the main cellular system,
the main cell page monitor 4 notifies the controller 6
that a page message has been received and transmits the
page message to the controller 6. The controller 6
formats the page message to be accepted by cellular
system equipment 2 for rebroadcasting within the off-load
cellular system. This rebroadcasting of the page message
permits a mobile telephone to receive a page message from
the main cellular system even while located within the
off-load cellular system. Rebroadcasting the page
message is necessary since standard mobile telephones
will naturally tune to the frequency or channel having
the highest signal strength within a cellular service
system or provider. Thus, with this configuration, a
mobile telephone located within an off-load cellular
system will be able to receive page messages from the
main cellular system to permit the main cellular system
to access the mobile telephone while the off-load
cellular system appears transparent to the main cellular
system.
In addition to the rebroadcasting of the page
message to permit the mobile telephone to receive pages
from the main cellular system, controller 6 may also be
used to determine whether a mobile telephone which is
located in the off-load cellular system should be
serviced by the off-load cellular system or whether the
mobile telephone should be selectively shed from the
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off-load cellular system to receive cellular service from
the main cellular system. The mobile telephone's access
attempt is shed by broadcasting a message instructing the
mobile telephone to tune to the main cellular system. As
will be discussed, the message preferably conforms to
EIA-553 interface specifications and includes a directed
retry message as discussed in these specifications.
Fig. 2 illustrates the basic interconnections
between the off-load cellular system with a main cellular
system including an IS-41 signalling network. In Fig. 2,
the off-load cellular system includes the off-load
cellular system switch 8 and the off-load cellular system
" transceiver 10. The off-load cellular system switch 8
may be connected to multiple off-load cellular system
transceivers 10 for transmitting a call to and receiving
a call from mobile telephones located within the off-load
cellular system. In Fig. 2, the off-load cellular system
switch 8 is connected to a public switching telephone
network (PSTN) 16 for connecting a mobile telephone with
telephone equipment connected to the public switching
telephone network 16 via, for example, a central
switching box or a private branch exchange. The mobile
telephone switching office 12 of the main cellular system
may also connect to the public switching telephone
network 16 or another public switching telephone network
in order also to provide telephone service between a
mobile telephone in the main cellular system with
telephone equipment connected to a public switching
telephone network. The mobile telephone switching office
12 is also connected to multiple land-based transceiver
stations 14 which are used to transmit control and voice
data to a mobile telephone located in the main cellular
system. Further, the off-load cellular system may be
connected to the main cellular system for purposes of
delivering a cellular call from the main cellular system
' to the off-load cellular system. As shown in Fig. 2,
off-load cellular system switch 8 is connected to main
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cell mobile telephone switching office 12 via an IS-41
signalling
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network 17.
Figs. 3A and 38 together comprise a block diagram of
the present invention according to the first embodiment.
In Figs. 3A and 3H, receive antenna system 20 is used to
receive signal transmissions from the mobile telephone.
The receive antenna 20 may, for example, simply be a
leaky coaxial cable connected to a 3-DB mobile antenna
mounted in various floors of an office building. Once
the receive antenna system 20 has received a signal from
the mobile telephone, the signal is transmitted to
conventional filter 22 via, for example, a coaxial cable
which may be connected to filter 22 using an N-type
connecter. Filter 22 is preferably a band pass filter
which limits the band width of the receive signal of
receive antenna system 20. Filter 22, therefore, is used
to limit the frequencies of channels which are to be
considered by the off-load cellular system, i.e., filter
22 excludes signals which are not of interest to the
off-load cellular system but which may be of interest to
other systems, such as other cellular systems or marine
based systems, etc. Filter 22 then transmits the
filtered signal to the low noise amplifier (LNA)/splitter
24 via, for example, a coaxial cable which may be
connected to the low noise amplifier/splitter 24 via an
N-type connector.
Low noise amplifier/splitter 24 is conventional and
amplifies the signal received from filter 22 and splits
the signal into various identical signals which are then
output to each transceiver 26. The signal is amplified
in the low noise amplifier/splitter 24 since there is a
great deal of loss in the signal when the signal is
split. The low noise amplifier/splitter 24 is connected
to the transceiver 26 via, for example, a coaxial cable
using, for example, an N-type coaxial connection.
Conventional transceiver 26 receives the signal from
the low noise amplifier 24 which is in the standard
interface format used between mobile telephones and
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cellular systems i.e., Electronic Industries Association
(EIA)-553 publication. The transceiver 26 boosts the
received signal using a preamplifier and then demodulates
the signal into 10 kHz Manchester encoded data. The
transceivers 26 may be preprogrammed by the transceiver
control and interface system 28 to receive specific
channels of interest which are broadcast by the mobile
telephone for call registration, call origination or page
response messages. The transceiver is connected to the
transceiver control and interface system 28 using, for
example, a 25 conductor cable assembly with a D-sub
connector.
The transceiver control and interface system 28
receives the Manchester encoded data frown the transceiver
26, decodes the Manchester encoded data received from
transceiver 26 and extracts information received from the
mobile telephone such as mobile identifier, electronic
serial number, telephone number, etc. The transceiver
control and interface system 28 then sends the decoded
data to the system control computer 30 using, for
example, a conventional RS-232 interface cable
connection. The protocol used between the transceiver
control and interface system 28 and the system control
computer 30 can be any standard protocol such as an
asynchronous 8 bit transmission protocol. The data Which
is received by the system control computer 30 initially
transmitted from the mobile telephone is typically either
a mobile telephone registration,-origination or page
response message.
The operations which are performed by the system
control computer 30 for the various messages are shown in
Figs. 4-11. When the mobile access attempt from the
mobile telephone is a registration message, the system
control computer 30 in step S2 of the Fig. 4 determines
that the mobile registration process should be performed
in step S4 which is shown in Fig. 5. The mobile
registration process is then started in step S6 of Fig. 5
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by the system control computer 30. The off-load cellular
system will permit mobile registration, when the off-load
cellular system is configured for autonomous
registration, in the system control computer 30.
5 Autonomous registration is typically used in cellular
systems to permit the cellular system to verify that a
mobile telephone user may be provided with cellular
service before the mobile telephone user has dialed a
calling number and pressed a send key on the mobile
10 telephone. Thus, autonomous registration permits the
mobile telephone to be immediately connected with the
calling party when dialing a calling number since the
mobile telephone has been previously validated. When
autonomous registration is not used, the mobile telephone
15 placing the call must be validated, which requires
additional time before the mobile telephone is connected
with the calling party.
The registration process of the present invention
utilize the conventional registration process which
detects the presence of mobile telephones prior to a call
attempt and which is described in the cellular radio
telecommunications system operations interface
specification EIA-553. In order for the mobile telephone
to perform autonomous registration, the system control
computer 3o pre-programs the transceiver control and
interface system 28 to transmit to the mobile telephone
the standard interface message including registration
bits which are set to indicate to the mobile telephone
upon examination of the registration bits to perform
3o autonomous registration according to the interface
specifications EIA-553.
In addition, a mobile telephone may be validated,
for example, for credit worthiness, using a conventional
visitor location register (VLR) 32 which would be
connected to the system control computer 30 via an RS-232
data interface; the visitor location register 32 may then
interface with a conventional home location register
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(HLR) service using an IS-41 cellular signalling network
or alternatively, the visitor location register 32 could
access directly via, for example, a dial-up modem, a
clearing house, such as GTE Telecommunication Services,
which can validate the mobile telephone for the off-load
cellular system.
As shown in Fig. 5, the system control computer 30
determines whether the mobile telephone (or "mobile") is
already registered in step S8, and if the mobile
telephone is already registered, the system control
computer 30 resets the de-registration counter of the
de-registration process in step S10. The de-registration
process constantly monitors whether the mobile telephone
is attempting to register with the off-load cellular
system. As shown in Fig. 9, the system control computer
30 decrements the de-registration counter in step P2
based upon a predetermined time interval. In step P4,
the system control computer 30 determines whether the de-
registration counter is equal to 0, indicating that the
mobile telephone has failed to register again Within the
prescribed amount of time. If the de-registration
counter in step P4 is not 0, then control is directed
back to step P2 for decrementing the de-registration
counter at the next specified time interval. If,
however, the de-registration counter is 0, then the
system control computer 30 deletes the mobile telephone
from a registration customer list maintained by the
system control computer 30 thus indicating that the
mobile telephone is now no longer registered with the
off-load cellular system. From step S10 in Fig. 5, the
mobile registration process and the mobile access attempt
process are then exited until the system control computer
30 receives another access attempt from the transceiver
control and interface system 28.
If the system control computer 30 determines in step
S8 of Fig. 5 that the mobile telephone is not already
registered, then the system control computer 30
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increments the registration message attempt counter in
step S12 and determines whether the registration message
attempt counter is equal to a preset data base value in
step S14. If the registration message attempt counter is
equal to the database value, then the mobile telephone
has attempted to register several times indicating that
the mobile telephone is a suitable off-load cellular
system user and, therefore, the system control computer
30 in step S16 registers the mobile telephone, and resets
the de-registration counter. The mobile registration
process is then exited until the next access attempt is
received by the system control computer 30.
If the system control computer 30 detezmiaes in step
S18 of Fig. 4 that the mobile access attempt is an
origination message, the origination message process is
started in step S22 in Fig. 6 by the system control
computer 30. The system control computer 30 first
determines in step S24 whether the mobile identifier
received from the traasceiver control and interface
system 28 is included in a conventional system control
computer database (not shown). If the mobile identifier
of the mobile telephone which has initiated the
origination message is in the system control computer
database, the system control computer 30 resets the de-
registration counter in step S26 and then attempts to
route the call using the off-load cellular trunks in step
S28 which is shown in Fig. 8.
The attempt to route out the mobile telephone call
to the off-load cellular trunks starts in step S30 of
Fig. 8. The system control computer 30 first determines
whether the number dialed by the mobile telephone will
route to the off-load cellular trunks in step S32. If
the dialed number is not valid for the off-load cellular
system, then the system control computer 30 will retrieve
a list of frequencies to be used to remove or shed the
mobile telephone from the off-load cellular system back
to the main cellular system in step S34. These
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frequencies are previously determined to be compatible
with the main cellular system, and therefore, the mobile
telephone should be able to obtain cellular service using
the main cellular system once the system control computer
30 indicates, to the transceiver control and interface
system 28, to broadcast a control message preferably
including a directed retry message in step S36. The
directed retry message is formulated according to
existing EIA-553 interface specifications and indicates,
30 to the mobile telephone, to tune to the specific
frequencies of the main cellular system Which are
included in the directed retry message. After step S36
is perfonaed, the attempt to route call process is exited
until another origination message is received which
includes a mobile identifier which matches the mobile
identifier stored in the system control computer
database.
If the dialed number will route to the off-load
cellular trunks in step S32, the system control computer
30 determines whether the mobile telephone's toll class
will allow the dialed number. If the mobile telephone's
toll class will not allow the dialed number, then the
mobile telephone is shed from the off-load cellular
system in steps S34 and S36 by the system control
computer 30 as described previously. If the mobile
telephone's toll class will allow the dialed number, then
the off-load cellular system will route the dialed number
to off-load trunks in step S40 in order to connect the
mobile telephone call to the telephone equipment
associated with the dialed number. The attempt to route
call process then exits.
If the system control computer 30 detezinines that
the mobile identifier is not in the system control
computer data base in step S24 of Fig. 6, an origination
message attempt counter is incremented in step S42 and
the system control computer 30 then determines whether
the origination message attempt counter is equal to a
WO 94n3Q7 PCTIUS94/03288
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19
predetermined database value in step S44. If the
origination message attempt counter is equal to the
database value, then the mobile is registered and the
de-registration counter is reset by the system control
computer 30 in step S46. The mobile is then shed from
the off-load cellular system in steps S48 and S50 as
previously described with respect to steps S34 and S36
shown in Fig. 8. If the origination message attempt
counter is not equal to the database value, then the
mobile is not registered and the mobile is then shed from
the off-load cellular system in steps S48 and S50. After
step S50 is performed, the origination message process is
then exited until the next origination message is
received, as discussed above.
If the system control computer 30 determines that a
page response message is received from the mobile
telephone in step S52, of Fig. 4 which is in response to
a page message from either the main or off-load cellular
systems, then the mobile page response process is
performed in step S54. The page response message process
starts in step S56 of Fig. 7, and the system control
computer 30 determines whether the page response which
has been received from the transceiver control and
interface system 28 is a result of a page message
broadcast from the main cell system. If the page
response is a result of a page message broadcast from the
main cell system, the system control computer 30
retrieves a list of frequencies in step S60 to include in
the directed retry message which is then sent to the
mobile telephone via the transceiver control and
interface system 28 in step S62. As indicated earlier,
the list of frequencies which are used for broadcasting
the directed retry message may be obtained beforehand and
stored in the system control computer based upon the
off-load cellular system's location or ability to receive
the control messages from the neighboring main cellular
systems. For example, the system control computer 30 may
WO 94113517 PCTIUS94/03Z88
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store only two or three frequencies to use for
broadcasting the directed retry message which represent
the two or three cell sites of the main cellular system
which are located near the off-load cellular system.
5 Thus, these two or three main cellular systems will
typically broadcast the strongest signal strength for the
off-load cellular system region and, therefore, the
mobile telephone needs only to be informed to retune to
these two or three frequencies when the off-load cellular
10 system sheds the mobile telephone. Alternatively, the
signal strength of the various main cell systems can be
monitored by the main cell page monitor 34, discussed
below, and the main cell page monitor 34 can inform the
system control computer 30 of the frequencies which are
15 the strongest to insure that the mobile telephone tunes
to a frequency of the main cellular system Which would be
of the strongest signal strength thereby obtaining better
reception for voice communication.
If the system control computer 30 determines that
20 the page response is not a result of a page message
broadcast from the main cellular system in step 558, then
the system control computer sends a message via the
transceiver control and interface circuit 28 to indicate
to the mobile telephone to use a voice circuit of the
off-load cellular system.
If the system control computer 30 determines that
the page response is a result of a page message broadcast
from the main cellular system in step 558, the main cell
page monitor 34 receives the main cell page and
rebroadcasts the page so that the mobile telephone
located in the off-load cellular system will receive the
main cell page.
The system control computer 30 which implements the
above processes may be a conventional IBM compatible
personal computer using, for example, a conventional 386
type microprocessor chip.
The call delivery process is a standard function of
2159092
21
the IS-41 interface specifications which allows a
cellular system which receives a call attempt to one of
its mobile telephones to deliver the call to another
cellular system which is providing cellular service to
the mobile telephone using the standard IS-41 interface
protocol. Once the voice circuit is connected to the
incoming trunk circuit in step S64, the page response
message process ends.
The main cell page monitor 34 in Fig. 3A is used for
receiving page messages of the main cellular system via
receive antennas 36 and for indicating to the system
control computer 30 via a standard RS-232 data interface
" the content of the page messages received from the main
cellular system. The system control computer 30 will
then transmit the page message to be rebroadcast to
transceiver control and interface system 28. Transceiver
control and interface system 28 then formats the message
as described above to enable the mobile telephone to
receive the page message in the proper format according
to EIA-553 specifications. The formatted message is then
transmitted to transceivers 26 for transmitting the
message to the mobile telephone. The signals which are
to be transmitted may be amplified in conventional power
amplifiers 38 and combined in conventional combiner 40.
Alternatively, for low power implementation of the above
off-load cellular system, power amplifiers 38 are not
necessary. The combined signal is then filtered using
conventional band pass filter 42 and then transmitted via
conventional transmit antennas 44. Thus, a mobile which
is currently locked on to the frequencies in the off-load
cellular system will still be able to receive its pages
from the main cellular system. Once the mobile is
registered (steps S16 or S46) via IS-41 procedures, the
main cellular system may provide busy features such as
indicating to a telephone equipment user trying to reach
the mobile telephone that the mobile telephone is
currently in the busy status as well as providing call
WO 94123527 PCTIUS94/03188
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22
forwarding and call waiting features when the mobile
telephone is provided with cellular service from the
off-load cellular system. In addition, the mobile
telephone which is locked on to the off-load cellular
system will be able to receive similar call treatment as
the main cellular system such as specific dialing
patterns, restrictions, activated features, etc.
The off-load cellular system provides the above
features in accordance with IS-41 interface
specifications when an incoming call is routed to the
off-load cellular system via the main cellular system.
The system control computer 30 maintains a list of all
mobile telephones currently locked on to the off-load
cellular system. The main cellular system coaanunicates
to the off-load cellular system via the IS-41 network of
the incoming call which may be originated, for example by
a PSTN. System control computer 30 determines whether
the called mobile telephone is busy, etc. by referring to
the list of mobile telephones described above and informs
the main cellular system of the called mobile telephone's
status in accordance With IS-41 interface specifications.
If the mobile telephone is not busy, transceiver control
and interface system 28 receives the incoming call from
the PSTN via, for example, conventional transmit/receive
cable pairs and broadcasts the call to the mobile
telephone to establish off-load cellular service. If the
mobile telephone is busy, then the main cellular system
may attempt, for example, call forwarding if this feature
is selected by the mobile telephone.
Fig. 10 is a detailed description of the main cell
page monitor of the present invention. In Fig. 10,
receive antenna 36 receives the pages which are
transmitted from the main cellular system. The signal is
then transmitted to a conventional filter 46 which is
preferably a bandpass filter permitting the cellular
frequencies of interest to be input to the main cellular
page monitor 34. The filtered signal is then transmitted
WO 94!?35Z7 PCT/US94I03Z88
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23
to conventional receiver subsystem 47 such as NE605 and
UMA1014 manufactured by Signetics via, for example, a
coaxial cable having an N-type connector. The signal is
first applied to conventional preamplifier 48, which
boosts the input signal to prepare the signal for
demodulation by a conventional demodulator 49. The
demodulator 49 demodulates the boosted signal based upon
synthesizer 50 which is controlled by the microprocessor
circuit 56 to indicate to demodulator 49 which channel
frequency is to be demodulated. Conventional oscillator
circuit 51 is used as a reference clock for synthesizer
50 for determining the proper 30 kHz signal which is to
be demodulated. The demodulator 49 then outputs 10 kFiz
Manchester encoded data to the Manchester data decoder 53
located in decoder 52 via a standard electrical
connection.
Conventional Manchester data decoder 53 decodes the
received data and temporarily stores the decoded data in
conventional RAM buffer 54. The Manchester data decoder
53 may be, for example, UMA 1000T manufactured by
Signetics. The data is then transmitted from the RAM
buffer 54 to conventional microprocessor circuit 56 via a
standard data bus, such as I2C. The microprocessor
circuit 56 then accepts the decoded data from decoder 52,
analyzes the forward control channel information, and
transmits the appropriate decoded data to the system
control computer 30 via a standard R.S-232 interface. In
addition, synthesizer 50 may be used to transmit the
signal strength data for each of the 30 kHz channels to
the microprocessor circuit 56 via analog to digital
converter 55 to determine which channels or frequencies
are to be used for page monitoring and the directed retry '
as discussed above.
Fig. 11 describes the transceiver control and
interface system 28 of the first embodiment of the
present invention. In Fig. 1I, the transceiver control
and interface system 28 controls the radio transceiver 26
WO 94!Z3327 PCTlUS94/03288
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24
and performs all the audio and data processing necessary
for voice channel or control channel functions.
Transceiver control and interface system 28 controls
transceiver 26 through conventional interface 100, which
has two balanced 600 ohm pairs for analog signal
processing, a ten bit transistor-transistor-logic (TTL)
level output that is used to set the transmit and receive
frequencies of transceiver 26 and a TTL output to enable
the transmitter within transceiver 26. In addition, an
analog input for signal strength measurement may also be
provided by interface 100 used for handoffs which will be
described later in connection with the second embodiment
of the present invention.
In transceiver interface and control system 28, when
an analog/audio signal is received by transceiver 26 from
the mobile telephone through interface 100 on a balanced
600 ohm pair, the audio signal is transmitted to analog
signal processing circuit 59 via conventional
transformers 58 for impedance matching. Transformers 58
may be, for example, N6385 manufactured by PICO
Electronics, Inc. Analog signal processing ,circuit 59
filters and conditions the received signal according to
EIA-553 specifications and outputs the conditioned signal
to audio interface 60. Analog signal processing circuit
59 may be constructed of, for example, an NE5751/NE5750
chip set manufactured by Philips Semiconductors.
The processed audio signal which has been filtered,
deemphasized, and expanded by analog processing circuit
59 is then transmitted to conventional audio interface
circuit 60. Audio interface circuit 60 may be, for
example, a CH1834 telephone line interface module
manufactured by Cermetek Microelectronics. In addition,
manchester encoder/decoder circuit 62 monitors the
supervisory audio tone (SAT) and signalling tone to
determine connection status. Audio interface circuit 60
receives the analog signal prepares the conditioned
analog signal for outputting to a Public Switching
WO 942517 PCTIUS94103288
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Telephone Network (PSTN) via conventional analog
telephone interface 101. Microprocessor 64 also controls
audio interface 60 for sending dialing tones, seizing and
releasing the telephone line connected to interface 101.
5 Likewise, the analog/audio signal received from the
PSTN is conditioned by audio interface circuit 60 and is
then transmitted to analog processing circuit 59. The
analog signal is then filtered, compressed,
pre-emphasized and limited to a maximum frequency
l0 deviation preferably of +/- 12 FOiZ. Audio interface 60 is
controlled by microprocessor circuit 64 through a
conventional serial address/data bus structure. The
processed analog signal is then transmitted from analog
signal processing circuit 59 via transformers 58 to
15 transceiver 26 through interface 100 using the 600 ohm
balanced transmission pair.
When the signal received from transceiver 26 is used
as a setup channel, the signal is transmitted to
transformers 58 and then to analog signal processing
20 circuit 59 as discussed previously. The analog signal
processing circuit 59 then transmits the conditioned
setup signal to manchester encoder/decoder 62 which
decodes the received setup signal according to EIA-553
specifications for microprocessor circuit 64.
25 Manchester encoder/decoder circuit 62 may be logic
programmed using conventional programmable logic devices
such as EPM 5128-J2 manufactured by Altera.' To buffer the
data into and out of the manchester coder/decoder circuit
62, conventional field programmable gate array is
programmed in two serializing First-In First-Out (FIFO)
64x8 memories are preferably~provided in the manchester
coder/decoder circuit 62 which may be, for example, an
iFX780 Field Programmable Gate Array manufactured by
Intel Corporation. One of the FIFOs is used to buffer
incoming decoded manchester data, and the second FIFO is
used to buffer outgoing data to be encoded by manchester
coder/decoder circuit 62.
WO 942'i527 PCTlUS941Q3288
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26
Microprocessor circuit 64 reads the data from
manchester coder/encoder circuit 62 to determine the
function necessary to process a call attempt or customer
vertical feature as well as connection status. The data
is then transmitted from microprocessor circuit 64 to
system control computer 30 through interface 102 which is
a conventional RS-232 port via conventional S-232 circuit
66. RS-232 circuit 66 may be constructed by, for example,
an 82510 circuit manufactured by Intel and a buffer
MC145406 manufactured by Motorola. Data received from
system control computer 30 through interface 102 is
processed by microprocessor circuit 64 and data to be
transmitted to transceiver 26 is written to the output
buffer FIFO of manchester coder/decoder circuit 62 as
previously discussed.
When microprocessor circuit 64 writes data to
manchester encoder/decoder circuit 62, manchester encoded
data is transmitted to analog processing circuit 59 where
it is modulated and summed with the analog output and
transmitted to transceiver 26. Likewise, decoded
manchester data received from transceiver 26 is output to
the FIFO where it is read by microprocessor circuit 64.
Microprocessor 64 is conventional and may be, for
example, an 80C186EB circuit manufactured by Intel.
Once the system control computer 30 receives the
encoded data from the transceiver control and interface
system 28, the system control computer 30 will process
the received data which will typically relate to, for
example, providing a call waiting feature, or indicating
to a calling party that the mobile telephone is currently
busy on another trunk circuit. Whenever a message needs
to be sent or broadcast by transceiver 26 to a mobile
telephone in the off-load cellular system (e.g., for
indicating to mobile telephone to retune to main
33 cellular-system), system control computer 30 sends a
message to microprocessor 64 through interface 102.
Microprocessor circuit 64 then formats the received data
WO 94!13527 ' PGTlUS94J03288
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27
and writes the data to the FIFO used to transmit data in
manchester coder/decoder circuit 62 which encodes the
data to be broadcast by transceiver 26 as discussed
above.
A conceptual description of a second embodiment of
the present invention is described with reference to Fig.
12. In Fig. 12, the off load cellular system 8 is
connected to the main cellular system 12 via the IS-41
signalling network 17 to provide the call delivery
feature as described in the first embodiment as well as a
direct connection with the main cellular system 12 via an
IS-41 trunk circuit which is used for intersystem handoff
between the main cellular system and the off-load
cellular system. Thus, the present invention, according
to the second embodiment, also includes the feature of
being able to handoff a mobile telephone call between a
main cellular system and an off-load cellular system
which may be located in a high concentration area of the
main cellular system. Figs. 13A-13C show the basic
structural configuration of the second embodiment of the
present invention. Since the second embodiment includes
additional features not provided by the first embodiment
of the invention, only the differences between the first
and second embodiments will be discussed. In Figs.
13A-13C a switch matrix 70 is used for connecting the
mobile to, for example, a main cell system or to a public
switching telephone network via tellco interface module
set 72. The switch matrix 70 is conventional and may be
MT8980D manufactured by Mite! Semiconductor, and the
tellco interface module 72 is conventional as well and
may be D4 Channel Bank DCB24 manufactured by Telco
Systems. The tellco interface module 72 prepares the
signal for proper connection to, for example, a PSTN via
a tip/ring pair. In addition, switch matrix 70 is
connected directly to the main cellular system via a
trunk cable such as a T-1 cable for implementing handoffs
in a conventional manner. Switch matrix 70 is used to
WO 94l?3527 PGT/US94103288
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switch or cross-connect voice data received from the
transceiver and interface system 28 via a conventional
digital bus interface, such as a PCM bus interface, and
is controllable via conventional microprocessor 76 which
controls the switching performed by switch matrix 70.
The microprocessor 76 is also connected to the system
control computer 30 which indicates to microprocessor 76
the desired switching to be performed in switch matrix
70. System control computer 30 also preferably receives
the signal strength level of the message received from
the mobile telephone. The signal strength is preferably
determined by transceiver control and interface module 28
in a conventional maaner. Oace system control computer
30 receives the signal strength, system control computer
30 preferably determines whether the mobile telephone is
required to be handed off to the main cellular system.
Additionally, conventional tone generator 74 is also
provided which is connected to switch matrix 70 for
providing tones for call processing and call treatment.
Fig. 14 is a block diagram of the circuit
construction of the transceiver control and interface
circuit 28 of the second embodiment of the present
invention. The operation of the transceiver control and
interface circuit 28 has been previously discussed with
reference to the first embodiment (Fig. 11), and
theref ore, only the differences between the embodiments
will be discussed.
The additional features of the transceiver control
and interface circuit 28 involve handoff capability
between the off-load and main cellular systems. For
example, in the second embodiment, since switch matrix 70
is provided, audio interface circuit 60 must be able to
interface with switch matrix 70. Thus, audio interface
circuit 60 may be a coder/decoder circuit such as MT8962
manufactured by Mitel. Coder/decoder circuit (audio
interface 60) converts the processed analog signal into a
digitized Pulse Code Modulation (PCM) signal. The PCM
WO 94I?3527 PCTIUS94103288
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29
signal is then transmitted by a conventional serial bus
interface 101 to switch matrix 70 where the transmitted
PCM signal can be switched via interface 101. Interface
101 includes two conventional serial PCM buses that
provide a conventional 64 Rilobit/sec PCM path to the
switch matrix 70. Likewise, the PCM signal received from
switch matrix 70 is converted to an analog signal by the
coder/decoder circuit and is then transmitted to analog
processing circuit 59. The analog signal is then
filtered, compressed, pre-emphasized and limited to a
maximum frequency deviation preferably of +/- 12 RHZ.
The processed analog signal is then transmitted from
analog signal processing circuit 59 via transformers 58
to transceiver 26 through interface 100 using the 600 ohm
1.5 balanced transmission pair.
Microprocessor circuit 64 sets the transceiver
channel of transceiver 26 by writing a 10 bit code to
conventional latch circuit 63. Latch circuit 63 may be
constructed of, for example, two 74LS374 eight bit
latches manufactured by Texas Instruments. The TTL level
outputs of latch circuit 63 are transmitted to
transceiver 26 through interface 100. Additionally, one
of the output bits from latch circuit 63 is also
transmitted to transceiver 26. These TTL level outputs
are used to enable the transmitter in transceiver 26. In
response to the 10 bit channel code output by latch 63,
transceiver scans certain frequencies and takes a receive
signal strength indicator (RSSI) measurement to measure
the strength of the mobile telephone's signal. The RSSI
signal is then transmitted back to microprocessor 64
through interface 100 and conventional analog-to-digital
(A/D) converter circuit 61 to be used to determine
whether a handoff between the main and off-load cellular
systems is appropriate according to standard EIA-553
specifications. A/D converter circuit 61 may be, for
example, an ADC0820 manufactured by Analog Devices. This
collected data is then sent from microprocessor 64 to
SUBSTITUTE SHEET (RULE 2~
WO 94123527 PCTlUS94103288
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system control computer 30 via interface 102 to deterznine
whether a handoff operation is to be performed.
Microprocessor 64 is also connected to the coder/decoder
(audio processing circuit 60) for power up/down and to
5 set the channel or time slot to transmit the data to
switch matrix 70.
Thus, the second embodiment of the present invention
permits the off-load cellular system to hand off a mobile
telephone user who is leaving the off-load cellular
10 system to a main cellular system without requiring the
mobile telephone user to place a new call. The handoff
procedure implemented according to the second embodiment
is a standard handoff feature which is performed
according to standard EIA-553/IS-41 interface
15 specifications. Visitor location register 32 is used to
inform the main cellular system via IS-41 cellular
signalling network of the specific trunk channel where
the mobile telephone Which is to be handed off is located
as determined by system control computer 30. Thus, when
20 switch matrix 70 switches the mobile telephone to the
main cellular system, the main cellular system knows
where the mobile telephone is for connecting the mobile
telephone call thus c~leting the handoff feature from
the off-load cellular system. This procedure performed
25 by the system control computer 30 and visitor location
register 32 as described above, is conventional and is in
accordance EIA-553/IS-41 interface specifications.
The many features and advantages of the invention
are apparent from the detailed specification and thus is
:30 intended by the appended claims to cover all such
features and advantages of the invention which fall
within the true spirit and scope of the invention.
Further, since numerous modifications and changes will
readily occur to those skilled in the art, it is not
desired to limit the invention to the exact construction
illustrated and described, and, accordingly, all suitable
modifications and equivalents may be resorted to, falling
WO 94n3S17 PCT/US94/03288
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31
within the scope of the invention.
