Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-12-31
2002-01-08
Chang, Vivian (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S437000, C370S332000
Reexamination Certificate
active
06337984
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a mobile communication system, more particularly, to a method for controlling a handoff in a mobile communication system so that its call quality and capacity are improved.
2. Background of the Related Art
In the mobile communication system using a code division multiple access (CDMA), some unnecessary handoffs emerge and then deteriorate the call quality. Therefore, a control method for the handoffs is required.
Generally, in order to keep the movability and the continuity of the communication in the CDMA mobile communication system, soft handoff or softer handoff has been used. The soft and softer handoffs are the mobile assisted handoff (MAHO) method based on the information that a mobile station provides. These soft and softer handoff methods consist of the process that the mobile station reports the information about the pilot signal strength of the base station and the process that the handoff control system (that is, a base station and a base station controller) performs the wireless channel resource assignment and release based on the information of the pilot signal strength of the base station.
During communicating a mobile station measures periodically the pilot signal strength of the active base set with which active communication path is established and the pilot signal strength of the candidate base station or the neighbor (or remaining) base station. If the measured pilot signal strength satisfied the handoff condition provided from the handoff control system, the mobile station attaches the corresponding measured result to a pilot strength measurement message (PSMM) and then sends the PSMM to the handoff control system. The handoff control system received the PSMM from the mobile station adds a new traffic path or drops the existing traffic path according to the strength information of the pilot signal included in the received PSMM.
Generally, the elements that is used by the mobile station to trigger the handoff are four parameters (T_ADD, T_DROP, T_COMP, T_TDROP). The parameter (T_ADD) is the threshold value of the pilot strength for the base station which is to be added to the traffic path. The parameter (T_COMP) is used for the additional generation of the PSMM. In other words, if the pilot signal strength of a base station which is measured by a mobile station is over the parameter (T_ADD), the mobile station reports the fact to the handoff control system through the PSMM. However, if the communication to the base station has not yet been established, the base station is managed as the candidate base station. The parameter (T_COMP) is used for the additional generation of the PSMM. On the other hand, the parameter (T_DROP) denotes the release condition for the currently active communication path and is the threshold value of the pilot signal strength to the base station which belongs to the communication path to be released. The parameter (T_TDROP) denotes the time threshold value which is the maximum strength of the pilot signal of the base station until the time just before the communication path presently established is released.
The conventional soft and softer handoff processes is explained by referring to FIGS.
1
and
2
.
FIG. 1
is a diagram showing the pilot signal strength change of an arbitrary base station which is monitored, during communicating, by a active mobile station and the event according to the change. Referring to
FIG. 1
, the establishment and release of a new communication path between an arbitrary base station and a mobile station is explained.
First, referring to the time a
1
in
FIG. 1
, since the presently measured strength of the pilot signal of a base station exceeds the parameter (T_ADD), the corresponding mobile station sends the PSMM to the handoff control system and moves the corresponding pilot from the neighbor set to the candidate set.
At the time a
2
of
FIG. 1
, the handoff control system sends the mobile station the handoff direction message to establish a communication path with the corresponding base station. At the time a
3
of
FIG. 1
, the mobile station receives the handoff direction message and moves the pilot of the corresponding base station from the candidate set to the active set and establishes a communication path with the corresponding base station. After then, the mobile station sends the handoff completion message to the handoff control system (Soft/Softer Add process).
However, at the time a
4
in
FIG. 1
, if the measured strength of the pilot signal of the corresponding base station is smaller than the parameter (T_DROP), the mobile station transfers the value of the parameter (T_TDROP) into the handoff drop timer and then drives the timer. At the time a
5
in
FIG. 1
, if the timer stops, the mobile station transmits the PSMM to the handoff control system.
Then, at the time a
6
in
FIG. 1
, the handoff control system sends the mobile station the handoff direction message that is to release the communication path with the corresponding base station.
The mobile station, as shown at the time a
7
in
FIG. 1
, moves the pilot of the corresponding base station from the active set to the neighbor set. In other words, the existing communication path with the corresponding base station is released and the mobile station sends the handoff control system the handoff completion message (Soft/Softer Drop process).
As described beforehand, a handoff starts by a mobile station.
FIG. 2
is a diagram showing the PSMM generation condition according to the pilot of a candidate set. Therefore,
FIG. 2
shows the condition for generating another PSMM when any response from the handoff control system or the direction to establish a communication path to the corresponding base station is not received after the mobile station sends the handoff control system the PSMM to add a new communication path. In other words,
FIG. 2
shows the PSMM generation condition for a pilot when the pilot moves from the neighbor set to a candidate set but does not further move to an active set.
In
FIG. 2
, P
0
is the strength of the pilot of the candidate base station. P
1
and P
2
are the strength of the pilot of the currently active base stations which have the communication path with a mobile station, respectively.
At the time t
0
in
FIG. 2
, since P
0
exceeds the parameter (T_ADD), the mobile station sends the handoff control system the PSMM for the pilot ol the candidate base station. At the time t
1
in
FIG. 2
, since P
0
is larger than P
1
by the value (T_COMPx 0.5 dB), the mobile station sends the handoff control system the PSMM of the pilot of the candidate base station. At the time t
2
in
FIG. 2
, since P
0
is larger than P
2
by the value (T_COMP×0.5 dB), the mobile station sends the handoff control system the PSMM of the pilot of the candidate base station.
However, the conventional handoff method described above has the following problems.
First, in the process determining the necessity of the handoff, the conventional handoff method uses the strength of the pilot of the base station as a criterion. However, if only the pilot strength of a base station is used for the criterion, in the midtown circumstances in which the propagation situation is changed rapidly and widely according to the movement of the mobile station or to the changes of the surrounding interferences, the unnecessary handoffs that deteriorates the call quality and the system performance may be caused.
For example, if the pilot strength of a base station exceeds temporarily the parameter (T_ADD), the handoff can be caused. And, the communication path between the mobile station and the corresponding base station will be established. But, the established communication path will not serve as a real communication path and be released soon. The signaling between the handoff control system and the mobile station for the handoff does not have any meaning. But, the signaling deteriorates the call quality and increases the load of the communication system. Furthe
Hong Sung Hyuck
Lee Hyun Goo
Yoon Ja Hyeok
Appiah Charles N.
Chang Vivian
Fleshner & Kim LLP
LG Information & Communications Ltd.
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