Note: Descriptions are shown in the official language in which they were submitted.
CA 02321304 2000-08-24
WO 99/44386 PCT/SE99/00184
Continuous Verification of Mobile Stations For Halidoff Operations
FIELD OF THE INVENTION
The present invention relates generally to cellular radio telecommunication
systems. More specifically, the invention relates to an improved technique for
performing verification of mobile stations for handoff operations.
BACKGROUND OF THE INVENTION
In a basic cellular telecommunication system, a mobile switching center
(MSC) is linked to a plurality of base stations by a series of digital
transmission links.
The base stations are geographically dispersed to form an area of coverage for
the
system. Each base station (BS) is designated to cover a specified area, known
as a
cell, in which two way radio communication can take place between a mobile
station
(MS) and the BS in the associated cell. As the MS moves from one cell to the
next, a
handoff procedure is initiated to transfer the communication link from the
present BS
to the next BS in order to continue the connection. Since the handoff involves
oversight from the MSC, valuable system resources are consumed for each of the
many MSs requiring handoffs.
Consider the situation illustrated in Fig. I. A simplified cellular radio
telecommunication system is depicted comprising an MSC and base stations BS1
and
BS2 serving their respective cells. The border between the respective cells
served by
BS1 and BS2 is representcd by line I10. In an example of a typical handoff
procedure,
BS 1 has an initial established connection with the MS within its cell. As the
MS
CA 02321304 2000-08-24
WO 99/44386 PCT/SE99/00184
-2
moves toward the cell served by BS2 to position MS', the signal strength of
the
connection, which is constantly being monitored, starts to decrease. A handoff
to BS2
is therefore initiated when the relative signal strength associated with BS2
indicates
that BS2 is deemed more suitable for continuing the connection i.e. signal
strength
associated with BS2 is typically stronger than that of the current connection.
This
condition typically occurs in the proximity of cell border 110. The MSC then
sends
the appropriate commands over transmission links 115 to the BSs in order to
execute
the handoff.
The evolution of cellular systems is placing an increasing demand on system
resources. For example, increases in subscriber capacity and alternative
network
configurations such as microcell environments often result in an increase in
mobile
station handoffs. Furthermore, the addition of new resource taxing services
may also
increase the loading on the system. Given the aforementioned trend, it is
desirable to
reduce system loading whenever possible. One way of doing this is to reduce
the
1 S involvement of the MSC in recurring tasks associated with handoff
operations. For
example, in prior art digital systems such as those operating in accordance
with
Digital Advanced Mobile Phone System (D-AMPS), the handoff procedure requires
several signals to be sent between the MSC and the participating BSs in order
to
complete the operation. Some of these signals are related to a verification
process
which innately requires oversight by the MSC. By way of example, when the MSC
receives a handoff request for an MS, the MSC signals the BS targeted for
handoff
directing it to perform a verification in order to determine if the base
station can
sufficiently communicate with the MS. The MSC then waits for a confirmation
signal
back from the BS targeted for handoff indicating that the MS has been verified
prior to
proceeding with the handoff. The verification is usually performed for a very
short
period of time and typically requires much signaling activity from the MSC.
Conscquently, a reduction in the involvement of the MSC during verification
can
significantly reduce the loading on the system.
Fig. 2 depicts a typical signaling sequence involved in a D-AMPS handoff
operation from an originating BSi to a neighboring BS2. A determination for
handoff
CA 02321304 2000-08-24
WO 99/44386 PGT/SE99/OOt84
-3
is typically based on any of a number of criteria which includes the received
signal
strength, bit-error rate (BER), interference levels on co-channels from other
base
stations, and propagation path loss measurements. When conditions in BSl for
handoff are reached, BS1 sends a handoff request DHOFFREQ to the MSC, as shown
in step 200. In step 205, the MSC follows with a command VERIFYMS to BS2 to
initiate a verification procedure to determine if the radio link between the
MS and BS2
is satisfactory. BS2 performs the verification procedure by tuning a
receiver~to receive
the uplink signals the MS is currently transmitting to BS1, in which the
signals are
only evaluated for a very short time.
~ When the uplink signals have been received satisfactorily or if a preset
time
period has elapsed, BS2 sends a response MSVERIFIED to the MSC at step 2I0
which includes an indication of whether the verification was successful. If
successful,
the MSC signals BS2 with a STARTVC command (step 2I5) to set up a digital
traffic
channel (DTC), which includes setting up a frequency and timeslot for the new
connection. After the DTC has been set up, BS2 signals the MSC with a
VCSTARTED acknowledgment message at step 220 to notify the MSC that a traffic
channel has been set up. At this point, the MSC issues a handoff order to BS1
with a
STARTHOFF command (step 225) which BS1 then forwards to the MS (step 230).
The MS responds back with an acknowledgment (step 235) to BS 1 which
terminates
the connection with the MS and sends a message HANDOFFSYNC (step 240) to the
MSC to confirm the release of the MS from the connection with BS1. Next, the
MS
tunes to the new frequency of the new traffic channel establishing a
connection with
BS2 in step 245. With the connection established, BS2 signals the MSC in step
250 to
confirm that the handoff was completed with a HANDOFFCONF message indicating
that the communication Link has been completely transferred to cell 2.
Although the handoff procedure of the prior art works relatively well,
modifications may be made to improve efficiency and reliability. For example,
the
large number of signals being passed, particularly during the verification
procedure,
causes the handoff operation to be resource intensive. In addition, a known
problem
with the prior art handoff procedure is that incorrect handoff decisions may
sometimes
CA 02321304 2000-08-24
WO 99/44386 PCT/SE99/00184
-4
occur e.g. due to co-channel interference. This can happen when the
verification step,
which is performed for a very short time, erroneously verifies an MS operating
on the
same frequency (co-channel) belonging to another cell. This can result in an
inappropriate handoff decision involving the wrong MS. It is therefore an
objective of
S the present invention to provide a verification technique of mobile stations
in handoff
operations that requires less participation from the MSC thus reducing system
loading.
A further objective is to provide a mare reliable verification technique to
reduce the
occurrence of incorrect handoff decisions.
SUMMARY OF THE INVENTION
Briefly described, and in accordance with an embodiment thereof, the
invention discloses a method and apparatus for reducing the loading on a
cellular
telecommunication system by reducing the involvement of the mobile switching
ce~uter
(MSC) in recurring tasks such as the verification process for mobile station
handoffs.
1 S In a cellular telecommunication system having an MSC linked to a plurality
of base
stations, a mobile station (MS) has an established wireless connection with a
first base
station (BSI). When the MS comes into receiving range of the second base
station
(BS2) e.g. near the cell border, and while the MS is still in radio
communication with
BS1, BS2 eavesdrops on the uplink transmissions sent by the MS to BSI. BS2
immediately initiates a verification procedure on the MS, prior to a handoff
being
initiated, and analyzes the uplink signals to determine if a satisfactory
connection can
be established with BS2. At the time when a handoff is determined to be
necessary by
BS1, a handoff message is relayed to BS2 via the MS in the slow associated
control
channel (SACCH) on the digital traffic channel (DTC). When the verification is
completed satisfactorily, the MSC is notified by BS2. The MSC then transmits
the
command for handoff to BS 1 which then orders the MS to establish a connection
with
BS2. The connection with BS 1 is released completing the transfer of the
connection to
BS2.
CA 02321304 2000-08-24
WO 99/44386 PCT/SE99/00184
-5
The present invention provides a method in which handoffs are performed with
improved efficiency since the verification step will have been completed or be
underway before the time the handoff occurs. The verification of the mobile
station is
preemptive and carried out in the "background" by neighboring base stations
thereby
substantially reducing the participation of 'the MSC by eliminating several
communication steps with the associated base stations. Furthermore, the
verification
procedure has improved reliability since it is performed over a relatively
longer period
of time. This is because the verification starts well before a handoff
procedure is
initiated by the currently connected BS i.e. at the time when BS2 is first
able to
receive uplink signals from the MS. These and other advantages of the present
invention will become apparent upon reading the following detailed
descriptions and
studying the various figures of the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention, together with further objects and advantages thereof, may best
be understood by reference to the following description taken in conjunction
with the
accompanying drawings in which:
Fig. 1 illustrates a mobile station handoff from one base station to a
neighboring base station in a cellular telecommunication system;
Fig. 2 depicts a signaling sequence involved in a typical prior art handoff
procedure;
Fig. 3 illustrates the verification process operating in accordance with an
embodiment of the present invention.
Fig. 4 depicts a signaling sequence for a handoff procedure operating in
accordance with the embodiment; and
CA 02321304 2000-08-24
WO 99/44386
PGT/SE99/00184
' -6-
Fig. 5 illustrates an exemplary application of the continuous verification
function.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A discussion directed toward the signaling associated with prior art handoff
operations was provided in the preceding sections. A primary motivation of the
present invention is to reduce system loading by reducing the involvement of
the MSC
in recurring tasks such as overseeing the verification process during handoff
operations. In the prior art, the MSC is burdened with directing the
verification
procedure after a handoff decision has been made involving the currently
connected
BS. In contrast and in accordance with the present invention, an improved
method of
performing verification of mobile stations (MSs) in handoff operations without
substantially involving the MSC is described.
In an exemplary embodiment, verification on the MS is performed by all
neighboring base stations (BSs) capable of receiving signals from the MS while
it is
still in communication with the originating BS. When the MS moves within the
receiving range of neighboring BSs, those that can adequately receive signals
from the
MS will immediately initiate verification of the MS in anticipation of a
handoff. Since
a neighboring BS will likely be the target for handoff, the verification will
be
completed by the time the actual handoff is performed.
Fig. 3 illustrates the verification process utilized in a simplified two-cell
radio
telecommunication system operating in accordance with the exemplary
embodiment.
An MS initially located in cell 1 is maintaining a connection with BS1 i.e.
transmissions of uplink 315 and downlink 320 signals are made between the MS
and
BS 1. As the MS moves near cell border 310, it comes 'into the receiving range
of BS2.
In BS2, and in addition to the primary transceivers 325, is an array of
receivers 330
configured to receive signals from MSs operating in neighboring cells.
Receivers in
the receiver array 330 are capable of listening to active frequencies of the
neighboring
CA 02321304 2000-08-24
WO 99/44386 PCT/SE99/00184
_~_
cells e.g. the operating frequencies of BS1. When the MS moves toward cell
border
310, and while still maintaining its connection with BS 1, uplink signals 3
I5'
transmitted by the MS to BS1 are received by a receiver in array 330 in BS2
tuned to
the uplink frequency. Upon reception of the uplink signals, BS2 immediately
begins a
verification procedure on the MS. As known by those skilled in the art, the
verification procedure typically includes signal strength measurements and the
evaluation of the signal based on the measurements by, for example,
determining the
color code and evaluating various statistics such as the number of
satisfactory
measurement samples.
By the time a handoff to,BS2 is performed, the verification of the MS by BS2
will be completed satisfactorily without requiring the involvement of the MSC.
The
verification is effectively performed in the "background" by the neighboring
base
stations which are constantly monitoring MSs that are within receiving range
in
anticipation of a handoff. Another advantage of the method is that the
verification that
takes place is typically performed over a relatively longer time than that
performed in
the prior art and thus is more reliable. It should be noted that the
application of the
invention to a teiecommunicadon system comprising a large number of base
stations,
having equivalent equipment and capability, is readily attained and that the
simplified
embodiment was given for purposes of illustration.
Fig. 4 illustrates a typical signaling sequence for a handoff procedure
operating
in accordance with the exemplary embodiment. The embodiment represents a
signaling sequence used in a cellular system operating in accordance with the
Digital
Advanced Mobile Phone System (D-AMPS) standard. In the handoff example, a
mobile station (MS) traveling within a cell served by an originating BS 1,
with which it
has an established connection, towards a neighboring cell served by BS2. BSI
detects
that the signal strength of the connection is getting weaker and determines,
based on
recent signal strength measurements, that a handoff to a more suitable cell
served by
BS2 should be undertaken. At this point (step 400) BS1 sends a message in a
presently
existing Slow Associated Control Channel (SACCH) on the digital traffic
channel
(DTC) to the MS to indicate that BS1 wishes to initiate a handoff to BS2. The
SACCH
CA 02321304 2000-08-24
WO 99/44386 PCT/SE99/00184
_8_
is utilized on both the uplink and downlink of the traffic channel for sending
system
information, thus "piggybacking" the handoff message on the SACCH requires no
additional bandwidth. The MS receives this message and retransmits it on the
current
uplink frequency, as shown in step 410. The message is also received by BS2
since it
S is tuned to and listening to MS transmissions for verification.
After receiving the relayed SACCH message from BS1 via the NiS,~BS2 sends
a handoff request HOFFREQ to the MSC in step 415. The handoff request includes
the cell identification of BS1 and associated channel information for use by
the MSC.
In addition, the HOFFREQ indicates to the MSC that the verification has been
completed satisfactorily. In step 4I7, the MSC issues a command STARTVC to BS2
to start transmitting and receiving on a specified frequency and timeslot. The
MSC
then issues a handoff order to BS 1 by sending a STARTHOFF command in step
420.
The STARTHOFF command includes information on the new frequency and timeslot
to be used. BS1 then sends a handoff message to direct the MS (step 425) who
then
responds back with a mobile acknowledgment in step 430. At this point the
connection
with BS 1 can be released and a message HANDOFFSYNC indicating same is sent to
the MSC in step 435. The MS retunes to the specified frequency to establish a
connection with BS2 at step 440. And finally, BS2 signals the MSC confirming
the
handoff is completed by sending a HANDOFFCONF message in step 445. The
communication link has been completely transferred to cell 2 at this point.
In the present invention, the MS 'is capable of repeating the message received
..
on the SACCH from BS 1 to BS2 as illustrated in step 410. The concept of
utilizing the
MS as a relay mechanism for transmitting information between base stations
further
reduces the necessity for the information to be sent from BS1 to the MSC to be
passed
on to BS2. As a consequence, the handoff procedure of the present invention
eliminates several communication steps between the MSC and base stations as~
compared to the prior art handoff procedure, thereby significantly reducing
the
loading on the system. Furthermore, the technique of the present invention is
especially suitable for applications involving many fast occurring handoffs in
a short
amount of time such as fast moving traffic along highways, for example.
CA 02321304 2000-08-24
WO 99/44386 PC1'/SE99100184
. _9_
Fig. 5 illustrates an application for continuous verification function as
described. An array of receivers 520 is placed in BS 5I5 which continuously
listen to
MS uplink transmissions of active frequencies of neighboring cells served by
BS 505
and BS 510. In an embodiment of the application, a receiver in the array
searches the
spectrum by scanning for frequencies that are being used by MSs communicating
with
neighbor base stations such as BS 505 and BS 510. The scanning pattern may
follow a
succession algorithm such as, for example; round robin or any other suitable
model.
When an MS transmitting on a new frequency is detected but not yet monitored,
a
receiver in the array is assigned to continuously listen to that particular
MS. In
another embodiment, the searching for active frequencies may follow a
predetermined
Iist of neighbor frequencies.
The listening procedure of the receivers is relatively straightforward since
the
direction of oncoming traffic and the operating frequencies of the adjacent
cells are
known. Thus, an MS traveling in either direction along a highway 500 will
initially
have an established connection with either BS 505 or BS 5I0. BS 515, who is
the
target of the handoff, will perform the verification on the oncoming traff c
as
described above. The handoff will be more efficiently handled since the
verification
step will have been completed by the time the handoff is performed to BS 515.
Furthermore, there is increased reliability of the verification because it is
performed
over a relatively longer period of time i.e. the verification starts at the
time the MS
enters into the receiving range of receivers 520 of BS 515. Additionally, the
verification procedure can be further corroborated by detecting whether the MS
is
moving toward or away from BS 515 by analyzing whether the signal strength is
increasing or decreasing.
The embodiment of the present invention not only reduces system signaling for
improved efficiency but also produces much more reliable handoffs. The result
is a
more accurate ~ verification process that is continuous in nature and is
better able to
confirm the location of the correct MS. Those skilled is the art will
appreciate that the
prior art verification procedure only measures very brief bursts of data in
which the
CA 02321304 2000-08-24
WO 99/44386
PCT/SE99/00184
-10-
color code is analyzed in order to determine where the MS is located relative
to the
BS.
It should be understood that the embodiment shown herein is merely exemplary
and that one skilled in the art can make variations and modifications to the
signaling
steps without departing from the spirit and scope of the invention.
Furthermore, the
inventive concept of continuous verification of mobile stations for. handcfPf'
operations
may be applied to other telecommunication systems operating in accordance with
standards such as Global System for Mobile Communication (GSM), Personal
Digital
Cellular (PDC), and Advanced Mobile Phone System (AMPS). All such variations
and modifications are intended to be included within the scope of the
invention as
defined by the appended claims.
