Intra-cell inter-frequency hard handoff method in a CDMA...

Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...

Reexamination Certificate

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Details

Other Related Categories

C370S335000, C370S342000

Type

Reexamination Certificate

Status

active

Patent number

06208631

Description

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to CDMA cellular systems, and more particularly to a method for processing a hard handoff between a moving mobile station and base station.
DESCRIPTION OF THE RELATED ART
Generally, hard handoff occurs when a mobile station crosses a boundary of the service area of a base station which currently maintains a connection with the mobile station and enters the service area of another base station. Such handoff enables a mobile station to be given continuous service while moving from the service area of one base station to the service area of another neighboring base station and is typically divided to two types: soft handoff and hard handoff.
A soft handoff, which is possible to perform due to CDMA system's features, establishes a new channel without terminating the original channel. Therefore, the mobile station is in communication with at least one base station during the soft handoff process. On the other hand, hard handoff terminates the original channel before establishing a new channel, and occurs when the frequency channel, frame offset or system are changed.
The handoff is explained with reference to FIG.
1
. In
FIG. 1
, a base station
1
(BS
1
) operates two frequencies F
1
and F
2
, while a base station
2
(BS
2
) operates only on one frequency F
1
. When a mobile station using the F
1
and another mobile station using the F
2
move to a service area of the base station BS
2
, handoff is performed in the two mobile stations, respectively as follows.
Firstly, when the mobile station using the frequency F
1
moves from a cell
1
to another cell
2
. If the output signal strength of base station BS
2
is greater than a predetermined threshold value (T_ADD), the mobile station reports the output signal strength of base stations BS
1
and BS
2
to base station BS
1
which currently has a channel with the mobile station. The base station BS
1
reports the output signal strength of both base stations BS
1
and BS
2
received from the mobile station to a base station controller (BSC) (not shown) and the BSC orders the base station BS
2
to assign a channel. In response to this order, the base station BS
2
assigns the channel and reports the result to the BSC. When the base station BS
2
has completed assigning the channel, the BSC orders the mobile station to connect to the channel with the base station BS
2
via the base station BS
1
that is currently in communication with the mobile station. The mobile station maintains the chancel with the base station BS
1
and simultaneously connects a new channel with the base station BS
2
, and therefore maintains connection with the two base stations BS
1
and BS
2
. If the mobile station moves closer to the base station BS
2
, the output signal strength of the base station BS
1
, becomes weaker. The mobile station MS
1
reports the output signal strength of the two base stations BS
1
and BS
2
to the BSC via the base stations BS
1
and BS
2
, when the output signal strength of the base station BS
1
is less than a predetermined threshold value (T_DROP).
The BSC orders the base station BS
1
to break the channel with the mobile station MS
1
and orders, via the base station BS
2
, the mobile station to break the connection with the base station BS
1
and to maintain connection with the base station BS
2
. In accordance with the order, the mobile station maintains connection with the base station BS
2
other than the base station BS
1
and handoff from the base station BS
1
to the base station BS
2
is completed.
Next, there is another case when the mobile station using the frequency F
2
moves from the cell
1
to another cell
2
. Unlike the mobile station using the Frequency F
1
, the mobile station is not able to recognize a neighboring cell and accordingly is not able to report the output signal strength of the base station BS
2
, because the base station BS
2
has no output signal on the frequency F
2
. Handoff does not occur and as the mobile station moves toward base station BS
2
, the mobile station escapes the service area of the base station BS
1
and accordingly the established channel between the base station BS
1
and mobile station is discontinued.
Cell configuration of using a different frequency in a certain base station instead of the same frequency of the neighboring base stations is decided by the amount of subscriber demand and when the subscriber demand is greater in the certain area than the neighboring areas, it is very effective and useful.
In a code division multiple access (CDMA) system, handoff is necessary if a mobile station enter a base station BS
2
operating with only a frequency F
1
, while the mobile station maintains a communication channel with base station BS
1
operating on a frequency F
2
. The prior art disclosed in U.S. Pat. Nos. 5,594,718 and 5,680,395, describe a method and apparatus for processing a hard handoff between base stations. The prior art is characterized in that a pilot channel indicating the existence and identity of a base station, frequency F
2
is installed in base station BS
2
and a mobile station is capable of reporting the base station output signal strength by referring to the pilot channel while moving from the base station BS
1
to BS
2
. A BSC receives a report message concerning the output signal strength of the base stations BS
1
and BS
2
from the mobile station via the base station BS
1
and commands the mobile station to perform an inter-frequency hard handoff from the base station BS
1
to BS
2
, because the BSC already knows that the frequency F
2
of the base station BS
2
has only the pilot chancel. As a result, the mobile station in the frequency F
2
of the base station BS
1
is successfully handed off to the frequency F
1
of the base station BS
2
without discontinuing communication and additionally, this method disclosed in the '718 and '395 patents has an advantage to enable handoff with only the pilot channel without installing a number of channels corresponding to the frequency F
2
to the base station BS
2
. However, this method has problems in that the RF devices should be installed in the frequency F
2
pursuant to addition of a pilot channel, and as such, a frequency converter, filters, frequency combiners, amplifiers, etc. should be additionally installed.
U.S. Pat. No. 5,737,704 discloses a channel allocation method for removing inter-cell hard handoffs in a CDMA system. According to the disclosure, each cell is divided into a plurality of concentric circle regions and a first channel (not allocated to a neighboring cell) is allocated to the innermost concentric circle region. A second channel that is allocated to a neighboring cell is allocated to the outermost concentric circle region, and the first and second channels are allocated to the middle concentric region. The first and second regions are selectively varied in order to reduce handoff.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for performing inter-frequency hard handoff based on the output signal strength of a base station and the distance between the base station and mobile station, while avoiding the addition of another device.
According to the preferred embodiment of the present invention, the inter-frequency hard handoff method comprises the steps of: setting up a new call of a base station; confirming whether a current base station satisfies a condition of inter-frequency hard handoff occurrence; confirming whether a currently used frequency satisfies a condition of inter-frequency hard handoff occurrence; if in the step of confirming whether a current base station satisfies a condition of inter-frequency hard handoff occurrence, said condition is satisfied, performing a soft handoff and call processing; if both the conditions of the two confirming steps are not satisfied, setting a threshold value on an output signal strength and on distance; if both the conditions of the two confirming steps are satisfied, instructing the mobile station to periodically report t

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