Handoff trial method by a mobile station

Details Handoff trial method by a mobile station Handoff trial method by a mobile station

1999-05-13

2002-12-17

Hunter, Daniel (Department: 2684)

Multiplex communications

Communication over free space

Having a plurality of contiguous regions served by...

C370S331000, C455S433000, C455S436000, C455S437000, C455S440000

Reexamination Certificate

active

06496493

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a handoff method of a cellular communication system, a personal communication system and a satellite communication system using a Code Division Multiple Access (CDMA) system, more particularly to a handoff trial method by a mobile station that compares a pilot power strength of a base station providing communication service to the mobile station with a pilot power strength of a neighboring base station and tries handoff based on the comparing results.
BACKGROUND OF THE INVENTION
In a mobile communication system, service area of a mobile station may be changed according to moving of the mobile station. The service area, a cell may be classified according to a pilot offset and a pilot power strength to be outputted through a pilot channel, in which the pilot offset and the pilot power strength differentiate the pilot channel. Therefore whether a mobile station situated at a cell border region crossing two cell belongs to any one out of the two base station service area may be recognized by detecting a received pilot power strength having a pilot offset of the mobile station transmitted to associated base station. Also, even if the mobile station moves in inter-cell, as it is necessary to provide communication service to the mobile station without corruption, handoff that automatically changes over communication line in inter-cell moving of the mobile station to sequentially provide communication service must be performed.
Referring to
FIG. 1
, there is shown a border diagram of handoff between two base stations in a general mobile communication system.
In the drawing, the reference number
10
designates a mobile station for receiving communication service while moving, the reference number
20
designates a base station for providing communication service to the mobile station
20
, reference number
25
designates other base station neighboring the base station
20
and reference number
30
designates a border that is in the cell (right oval) of the base station
20
while the cell (left oval) of the base station
25
originates. In addition, the reference number
35
designates border that is in the cell (left oval) of the base station
25
while the cell (right oval) of the base station
20
originates and the reference number
40
designates border that the base station
20
has the same pilot power strength as the base station
25
.
In
FIG. 2
there is illustrated a flowchart for general method that the mobile station
10
tries handoff in situation of FIG.
1
. Handoff procedure of the prior art of
FIG. 2
will be explained in conjugation with the
FIG. 1
First of all, when the mobile station
10
has been in the cell of the base station
20
that provides communication service to the mobile station
10
and is gradually moving to the border
30
, the mobile station
10
measures and stores a pilot power strength (pilot_A) of the base station
20
and a pilot power strength (pilot_B) of the base station
25
(ST
1
-ST
2
). Then the mobile station
10
compares the pilot_B with pilot threshold_
0
that is an absolute value set itself to try handoff (ST
3
). With the result that, if the pilot_B is larger than the pilot threshold_
0
, the mobile station
10
transmits a signal measurement message having the pilot offset and power strength of the pilot_B to the service base station
20
to try handoff to the base station
25
(ST
4
). The signal measurement message is transmitted in sequence to the base station
20
until the mobile station
10
moves in sequence to the cell region of the base station
25
and the handoff will have completed. Otherwise, if the pilot_B is smaller than the pilot threshold_
0
, flow returns to the ST
1
in FIG.
2
.
Meanwhile, when the mobile station
10
has been in the cell of the base station
25
, which provides communication service to the mobile station
10
, and is gradually moving to the border
35
, the mobile station
10
measures and stores the pilot power strength of the base station
20
. If the measured pilot power strength of the base station
20
is larger than a pilot threshold set to the mobile station
10
to try handoff, the mobile station
10
transmits a signal measurement message having the pilot power strength of the service base station
25
to try handoff to the base station
20
. As described in the above, each handoff region of the base stations
20
and
25
is between the border
30
and border
35
.
Accordingly, it is noted in the handoff method of the prior art that the pilot power strength of neighboring base station is only inspected without inspecting the pilot power strength of service base station. If the pilot power strength of neighboring base station is larger than the pilot threshold set to the mobile station to try handoff, the mobile station unconditionally sends the signal measurement message for handoff to the base station. Although handoff isn't needed as the pilot power strength of the service base station is enough to provide communication service, handoff will be tried if the above condition is satisfied.
Therefore, the handoff trial method of the mobile station in the prior art have to send the signal measurement message to the service base station at broad inter-cell region such as region between the border
30
and border
35
in
FIG. 1
, which causing to increase power consumption to handoff. Further, as handoff region is broad, the prior art has the problem in that the base station has much loads to process the signal measurement message received from the mobile station and use many channels due to frequent handoff.
U.S. Pat. No. 5,737,704 discloses channel allocation method for reducing inter-cell hard handoffs in a CDMA system which divides a service area into a plurality of cells and allocates a channel to each cell. The '704 patent comprises a first step of dividing a cell into a plurality of concentric circle regions, a second step for allocating a first channel not allocated to a neighboring cell, a third step of allocating a second channel allocated to the neighboring cell to the outermost concentric circle region of the cell and a forth step of allocating the first and second channels allocated by the second step and third step to a middle concentric circle region of the cell region. Also, the first and second region is selectively varied in order to reduce handoff.
The U.S. Patent shows a method that divides a service area into several cells by Round Trip Delay and allocates one channel to each cell, in which a cell is divided into 3 parts of an innermost concentric circle region, a middle concentric circle region and an outermost concentric circle region, channels of innermost and outermost regions being allocated to the middle concentric circle region, so as to reduce handoff by means of cell dividing method. The method, however, disadvantages in that has complex procedures such as dividing one cell into several regions and measuring RTD several times.
SUMMARY OF THE INVENTION
Again referring to
FIG. 1
, standing at the border
40
of forward handoff, the mobile station
10
receives same pilot power strengths from the base stations
20
and
25
, but receives pilot power strength larger by a predetermined value from the base station
20
at the cell border
30
. Thus, comparing the pilot power strength of service base station
20
with that of the neighboring base station
25
, we could find where the mobile station
10
stands. This inventor pays attention to the above point to solve the prior art problem.
It is therefore an object of the present invention to provide a handoff trial method by a mobile station which measures and stores a pilot power strength of a base station providing communication service to the mobile station, measures a pilot power strength of a neighboring base station and compares the stored pilot power strength of the service base station with the measured pilot power strength of the neighboring base station to try handoff according to the comparing result and to reduce occurrence frequency of

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