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Mobile Phone Patent Abstract
Disclosed is a hard handoff system for a mobile phone in a CDMA
radio communication system having at least two adjacent first and
second cellular service areas, which comprises a first base station
for serving the mobile phone in the first cellular service area
through one of preset multiple traffic channels, a second base station
for serving the mobile phone in the second cellular service area
through one of preset multiple traffic channels, and a control station
for allocating another one of the multiple traffic channels to the
first and second base stations to serve the mobile phone moving
from the first to the second cellular service area if the traffic
channel serving the first base station is not available for the
second base station.
Mobile Phone Patent Claims
What is claimed is:
1. A handoff system for a mobile phone in a code division multiple
access (CDMA) radio communication system having at least two adjacent
cellular service areas, comprising:
a first base station for serving said mobile phone in a first cellular
service area over a traffic channel having a first frequency;
a second base station for serving said mobile phone in a second
cellular service area; and
a control station for respectively controlling said first base
station and said second base station to allocate a traffic channel
having a second frequency for servicing said mobile phone over said
traffic channel having said second frequency, when said mobile phone
moves from said first cellular service area to said second cellular
service area through an overlapping service area, by determining
that a traffic channel having said first frequency is not available
in said second base station.
2. The handoff system as set forth in claim 1, wherein said control
station services said mobile phone through both said traffic channel
having said second frequency allocated by said first base station
and said traffic channel having said second frequency allocated
by said second base station until a power strength of said second
base station detected through said traffic channel having said second
frequency allocated by said second base station reaches a preset
value.
3. The handoff system as set forth in claim 2, wherein said control
station cuts off said first base station from servicing said mobile
phone an services said mobile phone through said second base station
only, when said power strength of said second base station detected
through said traffic channel having said second frequency allocated
by said second base station reaches said preset value.
4. The handoff system as set forth in claim 1, wherein said control
station cuts off said first base station from servicing said mobile
phone an services said mobile phone through said second base station
only, when a power strength of said second base station detected
through said traffic channel having said second frequency allocated
by said second base station reaches a preset value.
5. The handoff system as set forth in claim 1, wherein said control
station services said mobile phone only through said second base
station when a power strength of said second base station detected
through said traffic channel having said second frequency allocated
by said second base station exceeds a power strength of said first
base station by a preset value.
6. A handoff system for a mobile phone in a code division multiple
access (CDMA) radio communication system having at least two adjacent
cellular service areas, comprising:
a first base station for serving said mobile phone in a first cellular
service area over a traffic channel having a first frequency;
a second base station for serving said mobile phone in a second
cellular service area; and
a control station for respectively controlling said first base
station and said second base station to allocate a traffic channel
having a second frequency for servicing said mobile phone over said
traffic channel having said second frequency, when a power strength
of said second base station exceeds a power strength of said first
base station by a predetermined value as said mobile phone moves
from said first cellular service area to said second cellular service
area through an overlapping service area, by determining that a
traffic channel having said first frequency is not available in
said second base station.
7. The handoff system as set forth in claim 6, wherein said control
station services said mobile phone through both said traffic channel
having said second frequency allocated by said first base station
and said traffic channel having said second frequency allocated
by said second base station until a power strength of said second
base station detected through said traffic channel having said second
frequency allocated by said second base station reaches a preset
value.
8. The handoff system as set forth in claim 7, wherein said control
station cuts off said first base station from servicing said mobile
phone an services said mobile phone through said second base station
only, when said power strength of said second base station detected
through said traffic channel having said second frequency allocated
by said second base station reaches said preset value.
9. The handoff system as set forth in claim 6, wherein said control
station cuts off said first base station from servicing said mobile
phone an services said mobile phone through said second base station
only, when a power strength of said second base station detected
through said traffic channel having said second frequency allocated
by said second base station reaches a preset value.
10. The handoff system as set forth in claim 6, wherein said control
station services said mobile phone only through said second base
station when a power strength of said second base station detected
through said
traffic channel having said second frequency allocated by said
second base station exceeds a power strength of said first base
station by a preset value.
11. A handoff method for a mobile phone in a CDMA radio communication
system having at least a first base station for serving said mobile
phone in a first cellular service area over a traffic channel having
a first frequency, a second base station for serving said mobile
phone in a second cellular service area and a control station for
controlling said second base station to service said mobile phone
over a traffic channel having said first frequency or for respectively
controlling said first base station and said second base station
to service said mobile phone over a traffic channel having a second
frequency, when said mobile phone moves from said first cellular
service area to said second cellular service area, said method comprising
the steps of:
detecting when a power strength of said second base station reaches
a preset power strength value;
detecting when said power strength of said second base station
exceeds a power strength of said first base station by a predetermined
value;
determining whether said second base station has a traffic channel
having said first frequency available to serve said mobile phone
when said power strength of said second base station exceeds said
power strength of said first base station by a predetermined value;
controlling, when it is determined that said second base station
does have a traffic channel having said first frequency available
to serve said mobile phone, said second base station to service
said mobile phone over said traffic channel having said first frequency;
and
respectively controlling said first base station and said second
base station to allocate a traffic channel having a second frequency
for servicing said mobile phone over said traffic channel having
said second frequency when it is determined that said second base
station does not have a traffic channel having said first frequency
available to serve said mobile phone.
12. The method as set forth in claim 11, wherein said step of respectively
controlling said first base station and said second base station
to allocate a traffic channel having a second frequency comprises
a step of controlling said mobile phone to tune to said second frequency.
13. The method as set forth in claim 12, further comprising the
steps of:
informing, by said first and second base stations, said control
station of their connection to said mobile phone;
determining when said power strength of said second base station
reaches a second preset power strength value; and
disconnecting said first traffic channel of said first base station
from said mobile phone when said power strength of said second base
station reaches said second preset power strength value.
14. The method as set forth in claim 12, further comprising the
steps of:
informing, by said first and second base stations, said control
station of their connection to said mobile phone;
determining when said power strength of said second base station
exceeds said power strength of said first base station by a second
predetermined value; and
disconnecting said first traffic channel of said first base station
from said mobile phone when said power strength of said second base
station exceeds said power strength of said first base station by
said second predetermined value.
Mobile Phone Patent Description
CLAIM OF PRIORITY
This application makes reference to, incorporates the same herein,
and claims all benefits accruing under 35 U.S.C .sctn.119 from an
application entitled Hard Handoff System For A Mobile Phone And
Method Therefore earlier filed in the Korean Industrial Property
Office on Aug. 9, 1997, and there duly assigned Ser. No. 96-33233
by that Office.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a code division multiple access
(CDMA) communication system, and more particularly a hard handoff
system for handing off the service of a mobile phone from one base
station to another base station and method therefor.
2. Description of the Related Art
Generally, in a CDMA communication system, when a mobile phone
is moving from the service area served by one base station to the
adjacent service area served by another base station, there is required
a hard or soft handoff operation to continuously serve the mobile
phone without interruption of the communication. The soft handoff
is achieved without changing frequency channel, frame offset, or
pseudo noise (PN) offset, etc. while the hard handoff requires changing
of at least one of them.
Typically, when a mobile phone is moving from a first cellular
service area served by corresponding base station to the second
cellular service area served by a different base station, the handoff
operation is started by a control station in the overlapped zone
common to the first and second cellular service areas. The control
station carries out a soft handoff or a hard handoff according to
predetermined situations. Namely, the control station carries out
the soft handoff if the traffic channel employed by one base station
is available for another base station, or else the hard handoff
is carried out. A soft handoff technique for use in a CDMA cellular
telephone system is described in U.S. Pat. No. 5,101,501 to Klein
S. Gilhousen, et al., entitled Method And System For Providing A
Soft Handoff In Communications In A CDMA Cellular Telephone System.
U.S Pat. No. 5,594,718 to Lindsay A. Weaver, Jr., et al. entitled
Method And Apparatus For Providing Mobile unit Assisted Hard Handoff
From A CDMA Communication System describes both a soft handoff technique
and a hard handoff technique for use where neighboring cell sites
may utilize either a CDMA system or an FM system. U.S. Pat. No.
5,649,000 to Dong-Wook Lee, et al., entitled Method And System For
Providing A different Frequency Handoff In A CDMA Cellular Telephone
System proposes a system for use in an area where a soft handoff
is not possible, e.g., a CDMA system which includes a FDMA (frequency
division multiple access) scheme, and thus utilizes a hard handoff
technique.
The soft handoff uses a diversity combining technique employing
a rake receiver, and the adjacent base station is added to the presently
serving base station whose transmission power strength is large
enough to be effective, so that there occurs hardly a problem in
the communication between the mobile phone and the control station
during handoff. Further, the soft handoff is carried out between
two channels of the same frequency, thus securing reliability of
the handoff. An example of a diversity receiver is provided in U.S.
Pat. No. 5,109,390 to Klein S. Gilhousen, et al., entitled Diversity
Receiver In A CDMA Cellular Telephone System.
Compared to the soft handoff, the hard handoff suffers from an
inherent drawback because the handoff is over a different frequency
on an adjacent traffic channel. In this case, the diversity is removed
as soon as the service channel of the mobile phone is changed to
the different traffic channel having the different frequency, and
therefore the receiving quality is degraded compared to the previous
communication environment. In addition, the power strength received
by the mobile phone differs before and after changing the frequency
channel even at the same position. Therefore, the power strength
of the presently serving base station becomes weak making it undesirable
for use in the communication between the mobile phone and the control
station.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a hard handoff
system which may keep substantially the same diversity after changing
the frequency channel, and method therefor.
Another object of the present invention is to provide a hard handoff
system which is not affected by the service ranges of the base stations
varied according to the frequency channels, and method therefor.
According to the present invention, a hard handoff system for a
mobile phone in a CDMA radio communication system having at least
two adjacent first and second cellular service areas comprises a
first base station for serving the mobile phone in the first cellular
service area through one of preset multiple traffic channels, a
second base station for serving the mobile phone in the second cellular
service area through one of preset multiple traffic channels, and
a control station for allocating another one of the multiple traffic
channels to the first and second base stations to serve the mobile
phone moving from the first to the second cellular service area
if the traffic channel serving the first base station is not available
for the second base station.
According to another aspect of the present invention, a hard handoff
method for a mobile phone in the CDMA radio communication system
comprises the steps of continuously informing the control station
of the power strengths of the first and second base stations when
the power strength of the second base station through the traffic
channel serving the first base station reaches a preset value, determining
whether the traffic channel serving the first base station is available
for the second base station or no when the power strength of the
second base station exceeds that of the first base station over
another preset value, allocating another one of the multiple traffic
channels to the first and second base stations to serve the mobile
phone moving from the first to the second cellular service area
if the traffic channel is not available for the second base station,
and handing off the service of the mobile phone from the first to
the second base station when the power strength of the second base
station through the newly allocated traffic channel reaches further
another preset value.
According to yet another aspect of the present invention, a hard
handoff method for a mobile phone in the CDMA radio communication
system comprises the steps of continuously informing the control
station of the power strengths of the first and second base stations
when the power strength of the second base station through the traffic
channel serving the first base station reaches a preset value, determining
whether the traffic channel serving the first base station is available
for the second base station or no when the power strength of the
second base station exceeds that of the first base station over
another preset value, allocating another one of the multiple traffic
channels to the first and second base stations to serve the mobile
phone moving from the first to the second cellular service area
if the traffic channel is not available for the second base
station, and handing off the service of the mobile phone from the
first to the second base station when the power strength of the
second base station through the newly allocated traffic channel
exceeds that of the first base station over still another preset
value.
The present invention will now be described more specifically with
reference to the drawings attached only by way of example.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the present invention, and many
of the attendant advantages thereof, will become readily apparent
as the same becomes better understood by reference to the following
detailed description when considered in conjunction with the accompanying
drawings in which like reference symbols indicate the same or similar
components, wherein:
FIG. 1 is a diagram for illustrating a mobile phone moving from
one cellular service area to an adjacent cellular service area in
a CDMA communication system;
FIGS. 2A, 2B and 2C illustrate a conventional soft handoff operation;
FIGS. 3A, 3B, 3C and 3D illustrate a conventional hard handoff
operation;
FIGS. 4A, 4B, 4C and 4D illustrate a hard handoff operation according
to the principles of the present invention; and
FIG. 5 is a diagram for illustrating the operational sequence between
the base stations and mobile phone and control station for handoff
according to the principles of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, when a handset (mobile phone) 30 is moving
from the first cellular service area CELL1 served by a first base
station (BS1) 10 to the second cellular service area CELL2, a handoff
operation is started by a control station 40 while mobile phone
30 is in the overlapped zone OL commonly occupied by the first and
second service areas CELL1 and CELL2. Control station 40 carries
out a soft handoff as shown in FIG. 2 or hard handoff as shown in
FIG. 3 according to the situation. Namely, control station 40 carries
out a soft handoff if the traffic channel employed by first base
station 10 is available to the second base station (BS2) 20. If
the traffic channel is not available to second base station 20,
control station 40 carries out a hard handoff.
Describing the soft handoff, with reference to FIGS. 2A-2C, when
mobile phone 30, served through a traffic channel TCH1 of a frequency
f1, travels into overlapped zone OL and detects that the transmission
power strength of second base station 20 reaches a preset value
T.sub.-- add at time t1 at position P1 of the overlapped zone OL
as shown in FIGS. 2B and 2C, it informs control station 40 of the
transmission power strength of second base station 20. Then, control
station 40 determines whether the traffic channel THC1' of the frequency
f1 of second base station 20 is available or not. If traffic channel
TCH1' is available, control station 40 controls mobile phone 30
to be continuously served through both the traffic channel TCH1
of first base station 10 and the traffic channel TCH1' of second
base station 20.
As the mobile phone 30 moves continuously towards second base station
20, the transmission power strength of the first base station 10
is reduced below the preset value T.sub.-- add at time t2 at position
P2 of the overlapped zone OL as shown in FIGS. 2B and 2C, and then
control station 40 assigns only second base station 20 to serve
mobile phone 30 by dropping first base station 10. Namely, control
station 40 hands off mobile phone 30 from service area CELL1 to
service area CELL2 without changing the channel of the frequency
f1. Such a soft handoff uses the diversity combining technique such
that, for example, the signal-to-noise ratio of both the first and
second base stations are combined to increase the quality and reliability
of the communication.
Compared to a soft handoff, the hard handoff suffers some inherent
drawback. Referring to FIGS. 3A-3D, when mobile phone 30, served
through a traffic channel TCH1 of a frequency f1, moves into the
overlapped zone OL and detects that the transmission power strength
of second base station 20 reaches a preset value, it informs control
station 40 of the transmission power strength of second base station
20. Then, control station 40 determines whether traffic channel
THC1' of the frequency f1 of second base station 20 is available
or not. If traffic channel TCH1' is not available, control station
40 hands off the service of mobile phone 30 to traffic channel TCH0'
of frequency f0 of second base station 20 when the power strength
of second base station 20 exceeds the power strength of first base
station 10 over a preset value of, for example, 2.5 dB.
In this case, the diversity is removed as soon as the service channel
of mobile phone 30 is changed to traffic channel TCH0' of frequency
f0, and therefore the receiving quality is degraded compared to
the previous communication environment of FIG. 2A. In addition,
because the cell planning is carried out per frequency channel,
the service ranges of the base stations are varied according to
the frequency channels, as shown in FIGS. 3B and 3C. Thus, the power
strength received by the mobile phone differs before and after changing
the frequency channel even at the same position. For example, although
the power strength of the channel of the frequency f1 is large enough
to effectively serve the mobile phone, it becomes considerably degraded
when changing the channel to the frequency f0. Moreover, the time
of changing the frequency channel should be when the power strength
of the adjacent base station exceeds that of the presently serving
base station over a certain value. Therefore, the power strength
of the presently serving base station becomes weak making it undesirable
to the communication between the mobile phone and the control station.
Referring now to FIGS. 4A-4D and 5, the inventive hard handoff
operation of a mobile phone 30 from one base station 10 to the adjacent
base station 20 is characterized by including the steps of changing
the present traffic channel TCH1 of base station 10 with another
traffic channel TCH0 and handing off the service of the mobile phone
to traffic channel TCH0' of adjacent base station 20. This is different
from the hard handoff operation described above which contemplated
a hard handoff of the service of mobile phone from traffic channel
TCH1 of first base station 10 directly to traffic channel TCH0'
of second base station 20.
First, it is supposed that mobile phone 30 is moving from cellular
service area CELL1 of first base station 10 to second cellular service
area CELL2 of second base station 20. In steps S1A-S1C of FIG. 5,
mobile phone 30 continuously informs control station 40 of the pilot
strength measurement message (PSMM) representing the power strengths
of the presently serving base station 10 and the adjacent base station
20 from the time when the power strength of second base station
20 detected through the channel TCH1' has a preset value T.sub.--
add. Then, control station 40 makes a handoff determination in step
S2 when the power strength of base station 20 exceeds the power
strength of base station 10 by a preset value of, for example, 2.5
dB. Namely, control station 40 determines that a soft handoff is
to be performed when traffic channel TCH1' of second base station
20 employing the same frequency f1 as the presently serving traffic
channel TCH1 of the first base station 10 is available. When traffic
channel TCH1' of second base station 20 employing the same frequency
f1 as the presently serving traffic channel TCH1 of the first base
station 10 is not available, control station 40 determines that
a hard handoff is to be performed.
Upon determining the hard handoff, control station 40 demands first
base station 10 and the adjacent second base station 20 to respectively
allocate new traffic channels TCH0 and TCH0' commonly employing
the same frequency f0, in steps S3 and S4. Then, base stations 10
and 20 inform control station 40 of the result, in steps S5 and
S6, after newly allocating traffic channels TCH0 and TCH0', respectively.
Being informed of the new allocation of the frequency channel, control
station 40 transmits in step S7, a handoff direction message (HDM)
to mobile phone 30 to tune to base stations 10 and 20 at the allocated
frequency f0. Subsequently, mobile phone 30 receives the services
from both the first and second base stations 10 and 20 through their
respective channels TCH0 and TCH0', as shown in FIG. 4C. Base stations
10 and 20 then inform control station 40 of their connections with
mobile phone 30 through respective channels TCH0 and TCH0', in steps
S9 and S10.
Thereafter, detecting, through the channel TCH0', when the power
strength of second base station 20 reaches preset value T.sub.--
add, mobile phone 30 informs the control station 40 of the respective
power strengths of the base stations 10 and 20. Further, as the
power strength of second base station 20 reaches a given value preset
by control station 40, mobile phone 30 transfers a handoff completion
message (HCM) to control station 40 in step S11. In the present
embodiment, the control station 40 makes a determination to hand
off the service of mobile phone 30 from first base station 10 to
second base station 20 when the power strength of second base station
20 exceeds the power strength of first base station 10 by another
preset value T.sub.-- COMP (2.5 dB). Alternatively, control station
40 may determine to hand off the service of mobile phone 30 from
first base station 10 to second base station 20 when the power strength
of second base station 20 exceeds the preset value T.sub.-- add.
Finally, upon receiving the HCM from mobile phone 30, control station
40 releases channel TCH0 of first base station 10 in step S12, so
that the mobile phone 30 is served only by second base station 20.
Namely, the service of the mobile phone 30 is completely handed
off from first base station 10 to second base station 20.
Thus, the inventive hard handoff system provides means to make
the presently serving station continue to service a mobile phone
while the mobile phone is being handed off to the adjacent station
so that diversity is maintained after changing the frequency channel.
In addition, no problem occurs if the service range is altered after
changing the frequency channel. Thus, cell planning is made easy
because the optimization between the frequency channels hardly has
to be considered. Consequently, the reliability of the hard handoff
is improved to the level of the soft handoff.
Although the present invention has been described with reference
to specific embodiments together with accompanying drawings, it
will be readily appreciated by those skilled in this art that various
modifications may be made without departing the gist of the present
invention.
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