Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-09-22
2002-03-19
Kincaid, Lester G. (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S437000, C455S450000, C370S332000
Reexamination Certificate
active
06360100
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates to the field of telecommunications, and more particularly to an improved method and system for handing off communications between cells or sectors of a wireless communication system.
2. Description of the Related Art
Wireless cellular communications is becoming a convention method for people to communicate with one another. A conventional wireless cellular communication system in accordance with an industry standard commonly known as IS-95B, which is issued by the Telecommunications Industry Association and Electronics Industry Association (TIA/EIA) defines the way in which one type of wireless cellular communications is performed. In accordance with IS-95B, a mobile station (such as a wireless cellular mobile telephone) communicates with other mobile stations, a conventional telephone, or other such communication devices over a communications link that includes at least one cellular base station. The mobile station transmits a radio signal to the cellular base station. The cellular base station establishes a connection to a wired network that might include conventional telephone circuits (commonly known as the public switched telephone network (PSIN)).
A mobile station need only establish communication through one base station at a time in order to communicate with a device at the other end of the communications link (i.e., make a “call”). However, as a mobile station moves, the mobile station and the base station may lose the ability to communicate over the radio link. For example, if the mobile station moves outside the range of the base station or if an obstruction comes between the mobile station and the base station, the communications between the mobile and base stations will be interrupted. Therefore, the placement of base stations is planned such that there is an overlap between the coverage areas of each base station. This overlap ensures that a mobile station can contact at least one base station in every geographic point intended to be covered by the system. This is important because if the mobile loses contact with all bases stations for any substantial amount of time, the call is “dropped”. Once a call is dropped, the call must be reestablished by the mobile station redialing the call.
Due to the substantial overlap between base station coverage areas, a procedure known as “soft handoff” can be performed. Soft handoff is a process in which the mobile station receives identical signals from both a first and a second base station. A mobile station will preferably enter soft handoff (i.e., signals from a second base station will be received by the mobile station) whenever a second station becomes available. Soft handoff ensures that a call is not dropped as the mobile station moves out of the coverage area of a first base station and into the coverage area of a second base station.
One conventional method for performing a soft handoff is illustrated in FIG.
1
.
FIG. 1
shows a mobile station
101
, a first base station
103
, a second base station
105
, and a mobile switching center (MSC)
107
. In addition, the time sequence of communications between each is illustrated as follows. Arrowheads that terminate on the vertical line
109
that descends from the mobile station
101
, for example, represent signals received by the mobile station
101
. Arrows that terminate without an arrow head (i.e., at originate) at the vertical line
109
represent signals that have been transmitted from the mobile station
101
. Arrows that are closer to the top of the figure represent signals that are transmitted before signals represented by arrows closer to the bottom of the figure. In some instances, an arrow that is above another arrow may represent a signal that is transmitted continuously and thus may be transmitted concurrent with the signal represented by the lower arrow. For example, the traffic signal represented by arrow
111
may continue to be transmitted concurrent with the pilot strength measurement message (PSMM) that is represented by arrow
113
.
As shown in
FIG. 1
, the traffic signal
111
is initially transmitted between the mobile station
101
and the base station
103
. The traffic that is transmitted from the mobile station
101
to the base station
103
is then sent on to the MSC
107
by the base station
103
. Likewise, traffic that originates at the MSC
107
is sent to the base station
103
. This traffic is then transmitted from the base station
103
to the mobile station
101
. When the mobile station
101
detects a pilot from the second base station
105
with sufficient power, the mobile station
101
transmits a PSMM to the first base station
103
indicating the pilot strength of all the pilots that are currently being received at a signal level that is above a predetermined threshold. In the case shown in
FIG. 1
, the PSMM indicates that the mobile station
101
is receiving pilot signals that are above the predetermined threshold from both the first base station
103
and the second base station
105
. This PSMM is then transmitted from the first base station
103
to the MSC
107
, as represented by arrow
115
. The MSC
107
responds to the receipt of this PSMM by requesting the second base station
105
to allocate resources to establishing a communication link between the second base station
105
and the mobile station
101
, as represented by the block
116
. In addition, the MSC
107
generates a handoff direction message (HDM). The HDM is transmitted from MSC
107
to the first base station
103
, represented by the arrow
117
, after a time delay, represented by the arrow
119
. The HDM message is then transmitted from the first base station
103
to the mobile station
101
, represented by the arrow
121
. The HDM indicates to the mobile station
101
that a request has been made for the second base station
105
to allocate resources to establishing a communications path between the second base station
105
and the mobile station
101
.
The mobile station
101
responds to the HDM by adding the second base station
105
to the “Active Set” in the mobile station
101
and transmitting a handoff completion message (HCM) to both the first base station
103
and the second base station
105
, represented by the arrows
123
,
125
. Both the first and second base stations
103
,
105
transmit the HCM to the MSC
107
, represented by the arrows
127
,
129
. The active set in the mobile station
101
indicates which base stations are actively in communication with the mobile station
101
. Traffic will then be transmitted from the MSC
107
to the mobile station
101
through both the first and second base stations
103
,
105
.
This procedure works well in most cases. However, in some cases, the pilot transmitted from the second base station
105
is received by the mobile station shortly before the signals received from the first base station
103
can no longer be received by the mobile station
101
. If the time delay between receipt of the PSMM
115
and the transmission of the HDM from the first base station
103
is such that the communication link between the mobile station
101
and the first base station
103
deteriorates before the HDM can be received from the first base station
103
by the mobile station
101
, then the call will drop.
SUMMARY OF THE INVENTION
The disclosed method and apparatus allows a soft handoff to be completed, even when the communications link between the active base station and the mobile station deteriorates before the mobile station has received the handoff direction message. The mobile station maintains a list of base stations that the mobile station is in communication with, referred to as an “Active Set”. In addition, the mobile station maintains another list of base stations that are proximate to the base stations in the active set. This list is referred to as the “Neighbor Set”. A memory within the mobile station includes information that would allow the mobile station to demodulate information transmitted from those ba
Bender Paul E.
Grob Matthew S.
Hoagland Greg M.
Karmi Gadi
Kimball Robert H.
Baker Kent D.
Kalousek Pavel
Kincaid Lester G.
Qualcomm Incorporated
Wadsworth Philip
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