Title: Method and apparatus for providing an alert with information signal between a mobile switching center and a base station
Abstract: The invention is a method and apparatus for providing an Alert With Information Message signal between a mobile switching center and a base station in a cellular communication system. The invention recognizes that current provisions of the IS-634 standard lack support for certain required functionality on the interface between a mobile switching center and a base station, or the A-Interface. An Alert With Information Message signal is introduced on the A-Interface to support the required functionality. Introduction of the Alert With Information Message provides support for reliable call processing on the A-Interface for the call waiting procedure, the hard handoff procedure, and the resolution of a glare condition.
Patent Number: 6,870,823 Issued on 03/22/2005 to Krishnamurthi, et al.
| Inventors:
|
Krishnamurthi; Rajeev (San Diego, CA);
Mohanty; Bibhu P. (San Diego, CA);
Quick, Jr.; Roy F. (San Diego, CA)
|
| Assignee:
|
Qualcomm Inc (San Diego, CA)
|
| Appl. No.:
|
863457 |
| Filed:
|
May 27, 1997 |
| Current U.S. Class: |
370/335; 370/522; 379/241 |
| Intern'l Class: |
H04B 007//21.6; H04M 007//00 |
| Field of Search: |
370/335,342,320,338,328,522,434,441
379/201,215,241,230,210-212
455/414,417,422,432,436-439,442-446,560-561
375/130,200
|
References Cited [Referenced By]
U.S. Patent Documents
| 5590177 | Dec., 1996 | Vilmur et al. | 370/335.
|
| 5633868 | May., 1997 | Baldwin et al. | 370/331.
|
| 5822420 | Oct., 1998 | Bolon | 370/434.
|
| 5873036 | Feb., 1999 | Vucetic | 455/439.
|
| 5878036 | Mar., 1999 | Spartz et al. | 370/335.
|
| 6049543 | Apr., 2000 | Sauer et al. | 370/335.
|
| 6157828 | Dec., 2000 | Krishnamurthi | 455/414.
|
Other References
"Mobile Station-Base Station Compatibility Standard for Dual Mode Wideband
Spread Spectrum Spread Spectrum Cellular System" TIA/EIA Document/IS-95-A
Feb. 27, 1996, pp. 6-160 through 6-161.
"Mobile Station-Base Station Compatibility Standard for Dual Mode Wideband
Spread Spectrum Spread Spectrum Cellular System" TIA/EIA Document/IS-95-A
Feb. 27, 1996, pp. 6-186 through 6-202.
|
Primary Examiner: Nguyen; Steven
Attorney, Agent or Firm: Wadsworth; Philip R, Nguyen; Thien
Parent Case Text
This application claims the benefit of Provisional Application No.
60/040,319, filed Feb. 14, 1997.
Claims
We claim:
1. A CDMA cellular telecommunications messaging system, comprising:
a message generator in a mobile switching center for generating an IS-634
message signal, said message signal for transmission to a base station for
triggering said base station to transmit a subsequent message signal to a
mobile station; and
a message receiver in said base station for receiving said message signal;
wherein said message signal is generated upon detection by said mobile
switching center of a condition whereby a mobile subscriber attempts to
originate a call while another party is attempting to call the mobile
subscriber;
wherein said condition is detected when said mobile switching center
receives a message indicative of a mobile station originated call and a
message indicative of said mobile station terminated call in quick
succession; and
wherein said message signal is generated in response to the receipt of a
Release Message by said mobile switching center, said Release Message
having been generated by a network servicing a party which is to terminate
its connection with said mobile station, said party being one of two
parties communicating with said mobile station in a call waiting
procedure.
2. The messaging system of claim 1 wherein said message indicative of said
mobile station originated call is a Service Request Message and said
message indicative of said mobile station terminated call is a Paging
Response Message.
3. The messaging system of claim 1 wherein said message signal is an Alert
With Information Message signal and said subsequent message signal is an
Alert With Information Message signal.
4. The messaging system of claim 2 wherein said message signal is an Alert
With Information Message signal and said subsequent message signal is an
Alert With Information Message signal.
5. A CDMA cellular communication system, comprising:
a mobile station;
a base station in communication with said mobile station via an
Air-Interface, said base station defining a first cellular coverage area;
a mobile switching center in communication with said base station via an
IS-634 A-Interface;
a message generator at said mobile switching center for generating a
message signal for transmission to said base station on said A-Interface;
a message receiver at said base station for receiving said message signal,
wherein upon receipt of said message signal, said base station transmits a
subsequent message signal to said mobile station on said Air-Interface;
a first party in a first network;
a second party in a second network, said first and second parties being in
communication with said mobile station in a call waiting procedure,
wherein said second network generates a Release Message indicative of the
termination of communication of said second party with said mobile
station, said Release Message being transmitted to said mobile switching
center to cause said mobile switching center to generate said message
signal;
wherein said message generator generates said message signal when said
mobile switching center detects a condition whereby a mobile subscriber
attempts to originate a call while another party is attempting to call the
mobile subscriber.
6. The cellular communication system of claim 5 wherein said message signal
is an Alert With Information Message signal and said subsequent message
signal is an Alert With Information Message signal.
7. The cellular communication system of claim 5 wherein said condition is
detected when said mobile switching center receives a message indicative
of said mobile station originated call and a message indicative of said
mobile station terminated call in quick succession.
8. The cellular communication system of claim 7 wherein said message
indicative of mobile station originated call is a Service Request Message
and said message indicative of said mobile station terminated call is a
Paging Response Message.
9. The cellular communication system of claim 8 wherein said message signal
is an Alert With Information Message signal and said subsequent message
signal is an Alert With Information Message signal.
10. In a CDMA wireless communication system, a method for messaging between
a mobile switching center and a base station, comprising:
detecting the occurrence of a condition whereby a mobile subscriber
attempts to originate a call while another party is attempting to call the
mobile subscriber;
generating a message signal in said mobile switching center based on a
positive result of said detecting;
transmitting said message signal to said base station on an IS-634
A-Interface, wherein upon receipt of said message signal, said base
station transmits a subsequent message signal to a mobile station on an
Air-Interfaces;
establishing communication between a first party and said mobile station
via said mobile switching center and said base station;
establishing communication between a second party and said mobile station
via said mobile switching center and said base station while putting said
first party on hold; and
generating a Release Message in a network servicing said second party to
indicate that the second party is terminating communication with said
mobile station, said Release Message transmitted to said mobile switching
center, wherein said message signal is generated responsive to receipt of
said Release Message by said mobile switching center.
11. The method of claim 10 wherein said detecting comprises:
determining if a message indicative of said mobile station originated call
is received at said mobile switching center; and
determining if a message indicative of said mobile station terminated call
is received at said mobile switching center.
12. The method of claim 11 wherein said message indicative of a mobile
station originated call is a Service Request Message and said message
indicative of said mobile station terminated call is a Paging Response
Message.
13. The method of claim 12 wherein said message signal is an Alert With
Information Message signal and said subsequent message signal is an Alert
With Information Message signal.
14. An apparatus of a CDMA wireless communication system, comprising:
means for detecting an occurrence of a mobile subscriber attempting to
originate a call while another party is attempting to call the mobile
subscriber;
means for generating a message signal in a mobile switching center based on
a positive result of said detecting;
means for transmitting said message signal to a base station on an IS-634
A-Interface, wherein upon receipt of said message signal, said base
station transmits a subsequent message signal to a mobile station on an
Air-Interface;
means for establishing communication between a first party and said mobile
station via said mobile switching center and said base station;
means for establishing communication between a second party and said mobile
station via said mobile switching center and said base station while
putting said first party on hold; and
means for generating a Release Message in a network servicing said second
party to indicate that the second party is terminating communication with
said mobile station, said Release Message transmitted to said mobile
switching center, wherein said message signal is generated responsive to
receipt of said Release Message by said mobile switching center.
15. The apparatus of claim 14 wherein said means for detecting comprises:
means for determining if a message indicative of said mobile station
originated call is received at said mobile switching center; and
means for determining if a message indicative of said mobile station
terminated call is received at said mobile switching center.
16. The apparatus of claim 15 wherein said message indicative of a mobile
station originated call is a Service Request Message and said message
indicative of said mobile station terminated call is a Paging Response
Message.
17. The apparatus of claim 16 wherein said message signal is an Alert With
Information Message signal and said subsequent message signal is an Alert
With Information Message signal.
Description
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to spread spectrum telecommunications
systems. More particularly, the present invention relates to a novel and
improved messaging method and apparatus in a CDMA communication system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is
one of several techniques for facilitating communications in which a large
number of system users are present. Although other techniques such as time
division multiple access (TDMA), frequency division multiple access
(FDMA), and amplitude modulation (AM) schemes such as amplitude companded
single sideband (ACSSB) are known, CDMA has significant advantages over
these other techniques. The use of CDMA techniques in a multiple access
communication system is disclosed in U.S. Pat. No. 4,901,307, entitled
"SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR
TERRESTRIAL REPEATERS," assigned to the assignee of the present invention
and incorporated by reference herein.
In the CDMA cellular system, a large number of mobile telephone system
users, each having a transceiver, communicates through satellite repeaters
or terrestrial stations which are also referred to as cells. Each cell
includes a physical plant called a base station. A cell covers a limited
geographic area and routes calls carried over cellular telephones to and
from a telecommunication network via a mobile switching center. When a
cellular telephone user moves into the geographic area of a new cell, the
routing of that user's call may be eventually made through the new cell by
a process called a "handoff".
A cellular telephone or, more specifically, a mobile station, broadcasts a
signal that is received by a base station. The signal is then relayed to a
mobile switching center which in turn routes the signal to the public
switched telephone network and to telephone lines or other mobile
stations. Similarly, a signal may be transmitted from the public switched
telephone network to a mobile station via a base station and a mobile
switching center. The communications channel allocated for communication
of information between the mobile station and the base station is called
the traffic channel.
The interface between the mobile station and the base station is referred
to as the Air-Interface. The telecommunications industry association (TIA)
has provided a standard for CDMA call processing on the Air-Interface
entitled "IS-95-A Mobile Station--Base Station Compatibility Standard for
Dual Mode Wideband Spread Spectrum Cellular System," hereinafter IS-95-A,
which is incorporated by reference. The interface between the base station
and the mobile switching center is referred to as the A-Interface. The TIA
has provided for call processing on the A-Interface through the standard
provided in "IS-634 Mobile Switching Center--Base Station Interface for
Public 800 MHz," which is also incorporated by reference. IS-95-A and
IS-634 both define the messages and signals that are sent on their
respective interfaces for the operation of a CDMA cellular telephone call.
The call flow in a CDMA environment requires processing on both the
Air-Interface and the A-Interface. The successful progression of a call
requires that the proper messages and signals are sent at the right times
on both the Air-Interface and the A-Interface. The IS-634 standard is
being developed to provide for call processing on the A-Interface. A
number of problems and deficiencies are present in IS-634 which currently
does not support some of the necessary operations on the A-Interface. Some
of these problems and deficiencies are recognized and solved by the
present invention in the manner described below.
SUMMARY OF THE INVENTION
The present invention is a novel and improved method and apparatus for
providing a new message on the interface between a mobile switching center
and a base station, also known as the A-Interface, of a cellular telephone
system. The present invention recognizes that the A-Interface, as
presently defined by the IS-634 standard, does not support some of the
operations necessary for reliable call processing. Introduction of the new
message will provide support for required functionality on the
A-Interface.
The present invention recognizes that improvements are needed for the
A-Interface to reliably handle the call waiting and the hard handoff
procedures. In addition, a better technique is needed for resolving a
glare condition. An Alert With Information Message is introduced on the
A-Interface to improve the operation of these procedures.
Using the call waiting feature, a mobile station may have a first party of
a public switched telephone network put on hold while being connected to a
second party. If the second party hangs up, then according to the current
procedure, a Release/Clear Message (or an equivalent message) is sent to
the mobile station, causing the mobile station to release its traffic
channel. A new traffic channel must then be reestablished to reconnect the
mobile with the first party. The present invention recognizes that the
current procedure is unreliable in that difficulties may arise in the
process of reconnecting the mobile with the first party. Moreover,
Air-Interface resources are used inefficiently as the current procedure
requires the release of the traffic channel and the reassignment of a new
channel. To overcome these shortcomings, an Alert With Information Message
is sent on the A-Interface to the base station, triggering the transmittal
of an Alert With Information Message on the Air-Interface. Upon receipt of
the Alert With Information Message, the mobile station is prompted to
transmit a Connect Message to the mobile switching center via a base
station. The mobile switching center then reconnects the first party to
the mobile station, without the need to reestablish a new traffic channel.
A hard handoff occurs in a cellular system when a mobile station moves from
the coverage area of one base station to the coverage area of another base
station. During a mobile station terminated call, in order for the mobile
to undertake handoff, it must first receive an Alert With Information
Message transmitted by the base station. Under current procedure, the
Alert With Information Message is generated and transmitted solely by the
base station in whose coverage area the mobile is currently located. The
present invention recognizes that the current procedure may cause the hard
handoff procedure to be unreliable. If the mobile moves from the coverage
area of a first base station to the coverage area of a second base
station, the mobile station may never receive the Alert With Information
Message from the first base station, resulting in a dropped call. Thus,
the present invention provides for an Alert With Information Message to be
generated and transmitted by the mobile switching center. While the mobile
station is moving from the coverage area of the first base station to that
of the second base station, the mobile switching center transmits the
Alert With Information Message to the second base station. The second base
station in turn transmits an Alert With Information Message to the mobile
station. Having received the Alert With Information Message, handoff can
now proceed without the call being dropped.
A mobile subscriber may attempt to originate a call while another party is
attempting to call the same mobile subscriber, resulting in a glare
condition. The Air-Interface standard, IS-95-A, provides the mobile the
option of aborting the call that it is originating and responding to the
call that it is receiving. However, the present invention recognizes that
the mobile may not successfully receive the call because the base station
is not configured to transmit the proper messages to the mobile when a
glare condition occurs. In order for the mobile subscriber to respond to a
call, it is necessary for the mobile to receive an Alert With Information
Message. However, the standard currently does not address this situation.
As a result, the call may be dropped. The present invention recognizes
that the resolution of the glare condition should be undertaken at the
mobile switching center. The mobile switching center, upon receiving a
message indicative of a mobile originating a call and subsequently
receiving a message indicative of the same mobile receiving a call, should
recognize that the mobile is proceeding with receiving a call. The mobile
switching center should then transmit an Alert With Information Message to
the base station, triggering the base station to transmit an Alert With
Information Message to the mobile station. As the Alert With Information
Message is received by the mobile station, the glare condition is resolved
without the call being dropped.
Benefits and advantages of the present invention will become apparent from
the following description of the preferred embodiment when it is
considered with reference to the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The features, objects, and advantages of the present invention will become
more apparent from the detailed description set forth below when taken in
conjunction with the drawings in which like reference characters identify
correspondingly throughout and wherein:
FIG. 1 is a block diagram presenting a schematic overview of an exemplary
CDMA cellular telephone system in accordance with the present invention;
FIG. 2 is a block diagram illustrating an exemplary call-waiting scenario
according to the current IS-634 standard;
FIG. 3 is a block diagram illustrating a call-waiting scenario according to
the present invention;
FIG. 4 is a flow chart illustrating an exemplary embodiment of the
processing steps involved in call-waiting as implemented by the processing
elements of FIG. 3;
FIG. 5 is a block diagram illustrating an exemplary hard handoff scenario
in a cellular telephone system;
FIG. 6 is a state diagram illustrating the states of a mobile station in a
mobile station terminated call;
FIG. 7 is a block diagram illustrating an exemplary embodiment of the hard
handoff scenario according to the present invention;
FIG. 8 is a flow chart illustrating an exemplary embodiment of the
processing steps involved in hard handoff as implemented by the processing
elements of FIG. 3;
FIG. 9 is a block diagram illustrating an exemplary embodiment of the
processing elements involved in the resolution of a glare condition; and
FIG. 10 is a flow diagram illustrating the occurrence and the resolution of
a glare condition.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
An exemplary CDMA cellular mobile telephone system in which the present
invention is embodied is illustrated in FIG. 1. The CDMA cellular
telephone system is indicated generally by 9 and includes a mobile
switching center (MSC) 10, also referred to as a mobile telephone
switching office (MTSO), that includes interface and processing circuitry
for providing system control to the base stations. The MSC 10 routes
telephone calls from a public switched telephone network (PSTN) 11 to the
appropriate base station for transmission to the appropriate mobile
station. The MSC 10 also controls the routing of calls from the mobile
stations via one or more base stations to the PSTN 11. In addition, the
MSC 10 may direct calls between mobile stations via the appropriate base
stations. The MSC 10 may be coupled to the base stations by various
conventional means, such as dedicated telephone lines, optical fiber
links, or by radio frequency communications.
It should be understood that although the present invention is described
herein within the context of a CDMA cellular communications system, it is
equally applicable to other types of communication systems, such as the
personal communication system (PCS). Furthermore, systems utilizing other
well known transmission modulation schemes such as TDMA and FDMA as well
as other spread spectrum systems may employ the present invention.
In FIG. 1, two exemplary base stations (BSs), 12 and 14, along with two
exemplary mobile stations (MSs) 16 and 18, each including a cellular
telephone, are illustrated. Arrows 20a and 20b represent forward and
reverse code channels that define a possible communication link between BS
12 and MS 16. Arrows 22a and 22b define a possible communication link
between BS 12 and MS 18. Similarly, BS 14 can establish a two-way
communication link with MS 18 as represented by arrows 24a and 24b and
with MS 16 as represented by the arrows 26a and 26b.
As previously mentioned, the interface between BSs 12 and 14 and MSs 16 and
18 is called the Air-Interface, and in the exemplary embodiment, call
processing on the Air-Interface is governed by IS-95-A. Also as previously
mentioned, the interface between MSC 10 and BSs 12 and 14 is called the
A-Interface, and call processing on the A-Interface is governed by IS-634.
The present invention recognizes that the A-Interface as currently defined
by IS-634 lacks support for some of the necessary functionality for
reliable call processing. In addition, the present invention recognizes
that introduction of a new message on the A-Interface will cure some of
the deficiencies of the A-Interface.
An exemplary procedure in which deficiencies are recognized on the
A-Interface is the call waiting procedure. Call waiting is one feature
available to the CDMA cellular subscriber. The call waiting feature allows
a cellular subscriber to toggle between two parties on the telephone
network. Call waiting may be provided in a CDMA telephone system according
to the techniques disclosed in copending U.S. patent application Ser. No.
08/535,998, entitled "Early Detection of Mobile to Mobile," filed Sep. 29,
1995, which is assigned to the assignee of the present invention and
incorporated by reference herein.
An exemplary call waiting scenario is illustrated in FIG. 2. In FIG. 2,
party 128 and party 230 are two parties of a PSTN. A call has been
established between MS 32 and party 128. A traffic channel has been set up
between MS 32 and BS 34 on the Air-Interface. Furthermore, BS 34 is
connected to MSC 36 on the A-Interface by any of various conventional
means as described above.
If party 230 calls MS 32 while MS 32 is communicating with party 128, MSC
36 will send signaling data to BS 34 indicating that another party is
attempting to contact MS 32. The means for combining traffic data and
signaling data is described in detail in U.S. Pat. No. 5,511,073, entitled
"METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION,"
assigned to the assignee of the present invention and incorporated by
reference herein. To inform MS 32 that another call is coming in, an
alerting signal is relayed by BS 34 to MS 32 either by a Flash With
Information message or an inband tone. Then, MS 32 can invoke the call
waiting feature by pushing a specific key on the MS keypad. This causes
the generation of a Flash With Information Message, which is sent on the
Air-Interface to BS 34, which in turn relays the Flash With Information
Message on the A-Interface to MSC 36. MSC 36 then connects party 230 to MS
32 while putting party 1 on hold. Thereafter, MS 32 may toggle between
party 128 and party 230 by pushing the key on the keypad to cause the
generation of the Flash With Information Message. Each time MSC 36
receives the Flash With Information Message, it connects MS 32 to the
party that was previously put on hold and puts on hold the party that was
previously on the line.
Referring still to FIG. 2, assume that party 128 is on hold and MS 32 is
connected to party 230. If party 230 hangs up, then according to the
current procedure, the network which is servicing party 2 generates a
Release Message. This Release Message propagates to MSC 36, which then
sends the Release Message to BS 34 on the A-Interface. BS 34 in turn sends
a Release Message to MS 32 on the Air-Interface to cause MS 32 to release
its traffic channel. MSC 36 then signals to BS 34 that party 128 is
attempting to contact MS 32. This causes BS 34 to send a Paging message to
MS 32 to indicate that a call is incoming. In response, MS 32 generates a
Paging Response Message which is transmitted to BS 34. BS 34 then
transmits the Paging Response Message to MSC 36. On receipt of a Paging
Response Message, MSC 36 assigns a new traffic channel to MS 32 and MS 32
is reconnected to party 128.
The present invention recognizes that this procedure has several
shortcomings. First, it does not efficiently use the Air-Interface
resource, since it requires the release of the traffic channel, re-paging
of MS 32 and the assignment of a new traffic channel. Further, the need to
obtain a new traffic channel makes it difficult to reestablish the
connection between MS 32 and party 128. If MS 32 moves away from BS 34
from which it is released, the page from MSC 36 may not be received by MS
32. At the very least, this increases the amount of time that party 128
has to wait for the reconnection with MS 32. There is also the possibility
that the connection between MS 32 and party 128 cannot be reestablished,
resulting in a dropped call. A better procedure is needed to handle the
procedure for releasing party 230 from MS 32 wherein MS 32 is using the
call waiting feature.
To overcome the shortcomings, the present invention introduces a new
message for the A-Interface. A preferred embodiment of the present
invention is illustrated in FIG. 3. The preferred embodiment provides for
a new message, an Alert With Information Message, on the A-Interface. The
Alert With Information Message is currently available on the Air-Interface
under IS-95-A. According to IS-95-A, the Alert With Information Message is
sent by a base station to a mobile station to cause the mobile to ring and
to transition from a Conversation State to a Waiting For Answer State,
expecting the mobile user to intervene. By providing for an Alert With
Information Message on the A-Interface, the mobile switching center can
better control the progress of the call waiting procedure.
Referring to FIG. 3, assume again that party 128 is on hold and MS 32 is in
communication with party 230. If party 230 then hangs up, the network
servicing party 230 generates a Release Message as described above. In the
preferred embodiment, when the Release Message comes from the network, MSC
36 intercepts it, and message generator 38 in MSC 36 generates an Alert
With Information Message signal, which is sent to BS 34 on the
A-Interface. Message receiver 40 in BS 34 receives the Alert With
Information Message, and BS 34 in turn sends an Alert With Information
Message to MS 32 on the Air-Interface. When MS 32 receives the Alert With
Information Message, it generates an alerting signal in the mobile and
waits for the subscriber to answer. When the subscriber answers by
pressing a key on keypad of MS 32, a Connect Message generated by MS 32 is
sent to BS 34 and forwarded to MSC 36. On receiving the Connect Message,
MSC 36 connects party 128 to MS 32.
In the preferred embodiment, by not permitting the Release Message (or a
Clear Message) from the network to be sent to MS 32, MS 32 is not caused
to release the traffic channel when party 230 hangs up. As a result, there
is no need to expend additional Air-Interface resources for the assignment
of a new traffic channel. Furthermore, the preferred embodiment does not
require MS 32 to be paged following the release of the traffic channel to
indicate that party 128 is still waiting to be reconnected with MS 32.
Consequently, the risk that MS 32 will move away from the coverage area of
BS 34 and not receive the page is eliminated.
A brief illustration of the steps involved in call waiting as described in
the embodiment above is shown in FIG. 4. FIG. 4 shows a flow chart
illustrating some of the steps involved in the processing as discussed
with reference to FIG. 3.
Another procedure in which deficiencies are recognized on the A-Interface
is the handoff procedure. In a cellular communication system, a handoff
must take place when a mobile station moves from the coverage area of one
base station to another base station. A geographic area in a cellular
system is divided up into cells. Each cell is serviced by a corresponding
base station. In a hard handoff environment, as a mobile station moves
from one cell to another cell, communication with the original base
station is terminated before communication with the subsequent base
station is established. In a soft handoff environment, communication with
the subsequent base station is established before terminating
communication with the original base station. U.S. Pat. No. 5,267,261,
which is incorporated by reference and assigned to the assignee of the
present invention, discloses a method and system for providing soft
handoff.
The hard handoff procedure in a mobile terminated call may be understood
with reference to FIGS. 5 and 6. As illustrated in FIG. 5, as MS 42 moves
from the coverage area of BS144 to BS246, a connection needs to be
established between MS 42 and BS246 while MS 42 needs to be disconnected
from BS144. Both BS144 and BS246 are shown to be connected to MSC 48 in
FIG. 5. However, it should be understood that a hard handoff can also
occur when a mobile station moves from the control of one mobile switching
center to another. In fact, the different mobile station controllers may
support different modulation techniques such as TDMA or FDMA.
In FIG. 6, the states of a mobile station in a mobile terminated call are
briefly illustrated. The states are summarized herein and explained more
fully in the aforementioned document IS-95-A, pages 6-160 to 6-161.
The progression of the hard handoff procedure is dependent on the state of
the mobile station. Referring to FIG. 6, in a mobile station terminated
call, the mobile is in the Traffic Channel Initialization State, shown by
block 50, when a traffic channel has been established between a mobile
station and a base station. Upon receipt of a Base Station Acknowledgment
Order from the base station, the mobile moves into the Waiting For Order
State, shown by block 52. The mobile station then transitions into the
Waiting For Answer State, illustrated by block 54, when it receives an
Alert With Information Message from the base station. When the mobile
station subscriber answers the call by pushing a specific key on the
mobile station keypad, the mobile transitions into the Conversation State,
represented by block 56. The mobile transitions into the Release State,
shown by block 58, when conversation terminates.
According to the current procedure, a mobile station should be in the
Waiting For Answer State 54 or the Conversation State 56 during hard
handoff. If the mobile is in any other state, the call is likely to be
dropped during hard handoff. Thus, the mobile station must receive an
Alert With Information Message transmitted by the base station on the
Air-Interface before the mobile station undertakes handoff. Otherwise, the
handoff procedure would be unreliable. Referring again to FIG. 5, assume
that MS 42 is in the coverage area of BS144. If MS 42 subsequently moves
from the coverage area of BS144 to the coverage area of BS246 while MS 42
is in the Waiting For Order State 52, MS 42 may not receive the Alert With
Information Message sent by BS144. Without receiving the Alert With
Information Message, MS 42 cannot transition into the Waiting For Answer
State 54 in the coverage area of BS246. As a result, the call will be
dropped while handoff is in progress. Thus, the present invention
recognizes that a better method for handling hard handoff in a mobile
station terminated call is necessary.
As previously described, providing an Alert With Information Message on the
A-Interface overcomes the shortcomings in the call waiting procedure. The
present invention recognizes that providing an Alert With Information
Message on the A-Interface will also overcome the shortcomings in a hard
handoff procedure in a mobile station terminated call. Further, it should
be understood that the present invention applies to hard handoff
procedures in a variety of communication environments, including CDMA,
TDMA, or FDMA.
A preferred embodiment of the use of an Alert With Information Message on
the A-Interface in a hard handoff procedure is illustrated in FIG. 7 with
reference to FIG. 6. An Alert With Information Message is generated by
message generator 50 in MSC 48 and sent to message receiver 52 in BS246.
The Alert With Information Message is sent on the A-Interface when MS 42
is moving from the coverage area of BS144 to the coverage area of BS246
while in the Waiting For Order State. After BS246 receives the Alert With
Information Message from MSC 48, BS246 then sends an Alert With
Information Message to MS 42 on the Air-Interface. Having received the
Alert With Information Message, MS 42 can transition into the Waiting For
Answer State 54 while in the coverage area of BS246, and handoff can occur
without the call being dropped. Thus, by providing for an Alert With
Information Message on the A-Interface, MSC 48 has better control of the
hard handoff procedure. Since MSC 48 can send the Alert With Information
Message to BS246, causing BS246 to send the Alert With Information Message
to MS 42, MS 42 does not need to rely on receiving the Alert With
Information Message from BS144 in order to transition into the Waiting For
Answer State 54. Consequently, handoff can occur without the call being
dropped. A flow chart illustrating some of the processing steps involved
in hard handoff as described in the embodiment above is shown in FIG. 8.
Still another procedure in which deficiencies are recognized on the
A-Interface is the handling of a glare condition. A glare condition is
when a mobile subscriber attempts to initiate a call while another party
is attempting to call the mobile subscriber. A call initiated by a mobile
subscriber is referred to as a mobile station originated call, while a
call made to a mobile subscriber is referred to as a mobile station
terminated call.
The glare condition may be better understood by referring back to FIG. 1.
In a mobile station originated call, an Origination Message is transmitted
from a mobile station, such as MS 16, to a base station, such as BS 12,
upon initiation of a call by MS 16. After transmission of the Origination
Message but before a connection is established between MS 16 and the
called party (party A of PSTN 11), another party (party B of PSTN 11) may
attempt to contact the mobile subscriber and MSC 10 may page MS 16,
causing in a glare condition. The Air-Interface standard, IS-95-A, permits
MS 16 the option of aborting the call to party A and responding to the
call from party B. MS 16 then follows the state transition sequence for a
mobile station terminated call as illustrated in FIG. 6.
Referring now to FIG. 6, in a mobile station termination scenario, the
mobile expects to receive an Alert With Information Message to transition
from the Waiting For Order State, represented by block 52, to the Waiting
For Answer State, represented by block 54. If the Alert With Information
Message is not received, the mobile would time out and release the call.
It is recognized by the present invention that current procedures under
IS-95-A and IS-634 do not properly resolve the glare condition. Currently,
the base station is not configured to transmit an Alert With Information
Message to the mobile station upon occurrence of a glare condition. The
problem is explained in greater detail with reference to FIG. 9. Referring
to FIG. 9, when MS 60 originates a call, an Origination Message is sent
from MS 60 to BS 62. In response to the Origination Message, BS 62
transmits a BS Acknowledgment Order to MS 60. Meanwhile, MSC 64 receives
an incoming call to the same MS 60, unaware of the origination effort in
progress, and sends a Paging Request Message to BS 62 to initiate a mobile
terminated call setup. BS 62 in turn sends a Page Message containing the
MS address, and the Page Message is received by MS 60.
Due to the mobile origination attempt, BS 62 constructs a Service Request
Message and sends the message to MSC 64. However, after receiving the Page
Message, MS 60 aborts the origination call setup, and responds by
transmitting a Paging Response Message, which is received by BS 62. But
because MSC 64 had received a Service Request Message from BS 62, MSC 64
in response sends an Assignment Request Message to BS 62, requesting
assignment of Air-Interface resources for the mobile station originated
call. Upon receipt of the Assignment Request Message, BS 62 assumes that a
mobile station originated call is in progress, and does not transmit an
Alert With Information Message to MS 60. As a result, MS 60 times out
waiting for the Alert With Information Message, and the mobile station
terminated call is dropped.
In order to better resolve the glare condition, the present invention
provides for still another use of the Alert With Information Message on
the A-Interface. First, note that after BS 62 receives a Paging Response
Message from MS 60, BS 62 in turn sends a Paging Response Message to MSC
64. Then, in a preferred embodiment, the present invention provides for an
Alert With Information Message to be transmitted on the A-Interface if MSC
64 receives a Service Request Message and then a Paging Response Message.
Still referring to FIG. 9, it can be seen that the Alert With Information
Message may be generated in MSC 64 by message generator 66, while the
Alert With Information Message may be received in BS 62 by message
receiver 68. After BS 62 receives the Alert With Information Message from
MSC 64, BS 62 then sends an Alert With Information Message to MS 60 on the
Air-Interface. Having received the Alert With Information Message, MS 60
can transition into the Waiting For Answer State, and the glare condition
is resolved without the call being dropped.
Referring now to FIG. 10, a flow chart illustrating some of the processing
steps involved in the resolution of the glare condition as discussed with
reference to FIG. 9 is shown. An Alert With Information Message is
provided on the A-Interface to resolve the glare condition.
The present invention recognizes that for the call waiting, hard handoff,
and resolution of a glare condition procedures, there are certain
deficiencies in call processing on the A-Interface as currently defined by
IS634. By providing an Alert With Information Message on the A-Interface,
the present invention overcomes these deficiencies. The Alert With
Information Message is already available on the Air-Interface under
IS-95-A to cause a mobile station to transition from the Waiting For Order
State to the Waiting For Answer State. An Alert With Information Message
is provided on the A-Interface to trigger the transmittal of the Alert
With Information Message on the Air-Interface, solving the problems of the
call-waiting and handoff procedures as well as resolving the glare
condition.
The previous description of the preferred embodiments is provided to enable
any person skilled in the art to make or use the present invention. The
various modifications to these embodiments will be readily apparent to
those skilled in the art, and the generic principles defined herein may be
applied to other embodiments without the use of the inventive faculty.
Thus, the present invention is not intended to be limited to the
embodiments shown herein but is to be accorded the widest scope consistent
with the principles and novel features disclosed herein.
*
