Note: Descriptions are shown in the official language in which they were submitted.
CA 02256967 2001-09-04
METHOD FOR OPTIMIZING FRAME ERROR RATE SETTINGS
DURING SOFT HANDOFFS IN A WIRELESS
TELECOMMUNICATIONS NETWORK
Technical Field:
This invention relates to wireless telecommunications networks, and more
particularly, to enhancing the compatibility and performance of such networks
during
soft handoff procedures.
Background of the Invention:
The world-wide proliferation of wireless telecommunications technology
presents an opportunity for service providers to profit from an ever-growing
demand
for convenient, reliable wireless service. As these service providers are well
aware,
controlling expenses while providing such service poses a significant
challenge.
Existing wireless service providers meet this challenge by implementing
wireless
telecommunications networks comprised of mobile switching centers (MSCs)
interconnected to base stations. The MSC completes calls between mobile
terminals
(that is, any mobile station which uses radio transmission) and other parties.
These
"other parties" may be other mobile terminals or subscribers served by the
public-
switched telephone network (PSTN). Each base station is associated with a
specific
geographic region and is an interface between mobile terminals within its
region and
the MSC.
One consequence of the explosive growth of wireless telecommunications and
the emergence of numerous service providers is the deployment of diverse,
wireless
telecommunications networks. The variety of systems and protocols used by
these
networks is the cause of complex incompatibility issue which affect
performance.
More particularly, the incompatibility of existing wireless systems results in
the
inability to reconcile frame error rates (FER) of multiple base stations
serving the
same call during a soft handoff situation. This is because wireless service
providers
set the FER at each base station in accordance with subscriber requirements
based on
the characteristics of the geographic area in which it is located. For
example, base
stations in premium service areas (e.g., urban regions and airports) usually
have more
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M. O. A. Baum 1-3-1-3 2
stringent FER requirements than those in less populated areas. These stricter
FER
requirements ensure a level of network performance to which the subscriber in
the
geographic region are accustomed. Alternatively, service providers may choose
to
provide a lower level of service to achieve greater capacity in areas of high
use.
All base stations serving a call should maintain the same FER settings to
ensure high quality and power efficient transmissions. It is well known that
the FER
setting may be enhanced by increasing signal power from the base station to
the
mobile terminal. Accordingly, if multiple base stations serving the same call
operate
at different FER settings, it is common for base stations which do not have
the
strongest radio link with the mobile terminal (i.e., the nondominant base
stations) to
increase power to the signal (i.e., the forward link gain) transmitted to the
mobile
terminal for maintaining predetermined FER requirements. The increase in
forward
link gain is an attempt, on the part of these nondominant base stations, to
achieve the
FER of base stations with strong radio links to the mobile terminal
participating in the
call. These increases in forward link gain by nondominant base stations in a
soft
handoff scenario do not appreciably enhance the transmission quality of the
ongoing
call. Indeed, such amplification of forward link gain adversely affect the
quality of
other calls served by nondominant base stations. Unfortunately, current
implementations of wireless technology do not address the problem associated
with
differing FER settings of multiple base stations participating in a soft
handoff
scenario.
Therefore, there is a need in the art for reconciling FERs among base stations
in a wireless telecommunications network.
Summary of the Invention:
This need is addressed and a technological advance is achieved in the wireless
telecommunications art by enabling multiple base stations participating in a
call to
communicate and agree upon an FER setting for controlling the performance and
quality of a call in soft handoff.
In the preferred embodiment, a wireless telecommunications system includes
an MSC with a selection/distribution unit (SDU). The SDU includes an FER
controller with a control algorithm for maintaining a uniform FER setting for
all base
stations participating in a call. More particularly, messaging between base
stations via
the SDU allows the FER controller to negotiate and control the FER setting for
all
base stations participating in a soft handoff scenario. Advantageously,
increases in
forward link gain by nondominant base stations are eliminated. Further,
performance
is enhanced by adjusting FER settings to an appropriate level for soft handoff
performing base stations as a call traverses various geographic regions.
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M. O. A. Baum 1-3-1-3 3
Brief Description of the Drawings:
FIG. 1A is an illustrative embodiment of a wireless telecommunications
system in which the present invention may be practiced;
FIG. 1 B is a simplified block diagram of the mobile switching center and two
base stations shown in FIG. 1 A; and
FIG. 2 is a message flow diagram depicting the exchange of messages
occurring in the wireless telecommunications system of FIGS. 1 A and 1 B in
accordance with the preferred embodiment of the present invention.
Detailed Description:
FIG. 1 A shows wireless telecommunications system 100 including mobile
switching center (MSC) 102 and a plurality of base stations interconnected to
the
mobile switching center. More particularly, base stations 104, 106, 108, 110,
112, 114
and 116 are interconnected to MSC 102 via interconnection trunks 105, 107,
109, 111,
113, 115 and 117, respectively. Each interconnection trunk 105, 107...117
includes
1 S three separate communication links between each base station and MSC 102.
These
links are described in detail in FIG. 1 B. Wireless telecommunications system
100 is a
CDMA system in which multiple base stations may serve a single mobile
terminal. In
this regard, MSC 102 allocates and monitors base station resources and
facilitates
relevant information exchange during a mobile call for maintaining system
performance.
In wireless telecommunications system 100, each base station is associated
with a particular geographic region. In a vast area of this region, it is
assumed that the
base station acts as the"dominant base station" (that is, the base station
with the
strongest radio link to an active mobile terminal). Although a dominant base
station is
identified, multiple base stations may serve the mobile terminal along
geographic
region border. In this example, base station 104 is the dominant base station
associated with geographic region 120, base station 106 is the dominant base
station
for geographic region 122, base station 108 dominates geographic region 124,
base
station 110 is associated with geographic region 126, base station 112 is the
dominant
base station associated with geographic region 128, base station 114 is
associated with
geographic region 130, and base station 116 is the dominant base station
associated
with geographic region 132. Also shown in FIG. 1A are locations in which the
geographic regions of each base station overlap. One such location is
identified as
intersecting region 142. In intersecting region 142, a mobile terminal may be
served
by base station 106, base station 108, base station 110 or some combination
thereof.
Although multiple base stations may serve a mobile terminal, there is
generally one
base station which maintains a dominant forward link with the mobile station
during
CA 02256967 2001-09-04
4
an active call. There are, however, times when multiple base stations may have
essentially equal forward links to the mobile terminal.
MSC 102 determines the allocation of base station resources to any particular
mobile station and addresses all system requests. For example, a mobile
station
traveling in an overlap geographic region, such as intersecting region 142,
may
request a soft handoff with another base station. The handoff request is
triggered by a
base station when the power level between the mobile terminal and another base
station increases above a predetermined threshold level. In the preferred
embodiment,
during soft handoff procedure, the MSC 102 reconciles different operating
parameters
of base stations involved in a soft handoff procedure. One such operating
parameter
is the FER setting of each base station participating in the call.
FIG. 1B is a simplified block diagram of MSC 102 and its interconnections
with base stations 106 and 108. The system architecture shown in FIG. 1B is
described in the prior art. In this embodiment, MSC 102 comprises control
processor
204, switch fabric 209 and SDU 208 including FER control system 206. Switch
fabric
209 is interconnected to SDU 208 via link 235.
Base station I 06 includes call controller 212, interconnection processor 214
and channel element 216. Channel element 216 is interconnected to the call
controller
and interconnection processor via links 237. Also shown is base station 108
including
interconnection processor 224 and call controller 222 which are interconnected
to
channel element 226 via links 229. In this embodiment, mobile station 260 is
being
simultaneously served by base stations 106 and 108.
Base stations 106 and 108 maintain established interconnection links to MSC
102. More particularly SDU 208 is interconnected to call controller 212 and
interconnection processor 214 of base station 106 via interconnection links
243 and
239, respectively. MSC control processor 204 is interconnected to call
controller 212
of base station 106 via interconnection link 231. Similarly, SDU 208 is
interconnected
to call controller 222 and interconnection processor 224 of base station 108
via
interconnection links 245 and 241, respectively. Control processor 204 is
interconnected to call controller 222 of base station 108 via interconnection
link 233.
In other words, each base station maintains three separate interconnection
links to
MSC 102. Further, call controller 212 of base station 106 and call controller
222 of
base station 108 are interconnected via link 205.
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M. O. A. Baum 1-3-1-3
FER control system 206 is responsible for monitoring all FER settings of all
base stations in wireless telecommunications system 100. More particularly,
FER
control system 206 receives FER setting data from base station interconnection
processors 214 and 224. During a soft handoff procedure, a base station sends
a
handoff required request to MSC 102 so that the MSC can allocate resources at
another base station. FER control system 206 determines a least restrictive
FER
setting for the call. If, for some reason one or both stations do not transmit
FER data,
MSC 102 applies a default FER setting which may be comfortably met by all base
stations in the system. In this manner, FER control system 206 reconciles
potentially
incompatible FER requirements of base stations in handoff for a given call.
FIG. 2 is a message flow diagram illustrating the exchange of messages in
wireless telecommunications system 100 during a soft handoff procedure in
accordance with the preferred embodiment of the present invention.
For purposes of example, assume that mobile terminal 260 is located in
intersecting region 142 where it is being simultaneously served by base
stations 106
and 108. In alternative embodiments, more than two base stations may serve a
mobile
terminal in an intersecting region and the base stations may be associated
with
different MSCs. The soft handoff procedure begins when base station 106
extends a
handoff required message to MSC 102. MSC 102 receives the handoff required
message and subsequently extends a soft handoff request message to base
station 108.
In response to the handoff request message, base station 108 extends a
resource availability message to SDU 208 of MSC 102. The resource availability
message identifies the radio link base station 108 will allocate to the call
and includes
the expected FER setting of the call. In this embodiment, base station 106
extends a
requested FER setting which may or may not reflect the current FER applied to
the
call by the base station. All received FER data is extended to FER control
system 206
which determines an FER that can be applied to the call. In most instances,
the FER
setting determined by FER control system 206 corresponds to the least
restrictive FER
setting for the call based on data received from all base stations involved in
the call.
The FER setting is then reported by SDU 208 to all participating base stations
as the
FER setting to be applied to the call. The application of the FER setting may
take
place prior to or in parallel to establishing a soft handoff call.
In this manner, attempts by one base station to match the FER setting of other
base stations by increasing forward link gains are eliminated. More
particularly, the
SDU in an MSC coordinating a soft handoff procedure determines a FER setting
which is acceptable to all base stations involved in the soft handoff process.
Advantageously, unnecessary increases in power levels to the forward link are
avoided
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M. O. A. Baum 1-3-1-3 6
so that base stations in a wireless telecommunications network may efficiently
use
resources.
Although the present invention has been described with respect to an
illustrative embodiment, those skilled in the art will recognize that numerous
other
arrangements may be devised without departing from the scope of the invention.
