Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-12-21
2003-04-15
Trost, William (Department: 2683)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S330000, C455S442000, C455S436000, C455S437000, C455S438000, C455S443000
Reexamination Certificate
active
06549524
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to an intra-frequency handoff method in a mobile communication system based on a code division multiple access (hereinafter, referred to as “CDMA”)
2. Discussion of Related Art
Generally, in a CDMA mobile communication system, all base stations share the available frequency (FA) and each base station has sectors or areas in which service is provided to mobile stations. The probability of call drops for a mobile station which moves between base stations having different FA is relatively high. Thus, the CDMA mobile communication allows a number of respective base stations to have common number of FA within overlapping zones of operation.
When a mobile station moves from one base station to another, a handoff allows a continual communication by switching radio lines to connect speech paths between the base stations. A handoff may largely be classified into two kinds as follows.
A soft handoff occurs when a mobile station moves from one base station to another base station wherein the base stations have a common FA. In the soft handoff, a communication with a new base station begins before communication with a base station currently in service ends, thereby maintaining the traffic path. The system may allow a use of two traffic paths by utilizing a rake reception function of the CDMA. In such case, a handoff between sectors of a same base station is known as a softer handoff.
A hard handoff occurs when a mobile station moves from one base station to a base station belonging to a different mobile switching center (MSC) wherein the base stations have traffic channels of different FA or traffic channel of different frame offsets. In the hard handoff, a communication with a new base station ends before communication with a base station currently in service starts.
Such handoffs as described above occurs frequently in a CDMA mobile communication system having a multiple frequency environment.
FIG. 1
is a flow chart showing messages between a mobile station and a base station during a handoff based on a mobile assist handoff (MAHO) method of the CDMA mobile communication system in the related art. The MAHO method is initiated by a request of a handoff from a mobile station. In the MAHO method, a soft, a softer or a hard handoff is executed according to factors such as a resource relation between a base station servicing a current call and an adjacent base station, and a systematic construction of the adjacent base station. The operation of the MAHO method in a CDMA mobile communication system during a handoff is as follows.
Referring to
FIG. 1
, mobile station first checks the strength of the same frequency pilot signal from an adjacent base station or cell, and transmits a pilot strength measurement message (PSMM) to a base station through a traffic channel servicing the current call (S
100
). The base station and a base station controller analyze the transmitted PSMM and the information of the adjacent base station which the base station already possesses, to decide whether an ADD handoff procedure for establishing a new traffic path or a DROP handoff procedure for withdrawing one of an established traffic paths should be executed. Thus, an extended handoff direction message (EHDM) is transmitted to the mobile station (S
101
).
A pilot PN included in the transmitted EHDM is registered in an active set of the mobile station, and according to the registration, the mobile station executes for example an establishment/withdrawal of a new traffic path or an establishment/withdrawal of a simultaneous traffic path. After the execution, the mobile station transmits a handoff complete message (HCM) to the base station (S
102
). In the above-described MAHO method, the mobile station can transmit the PSMM only when the frequency of a current traffic call and the frequency used in a destination adjacent base station are common,
FIG. 2
is a block diagram showing a CDMA mobile communication system having an environment of a multiple frequency. Referring to
FIG. 2
, the CDMA mobile communication system includes a mobile station MS
207
, base stations
204
~
206
for taking charge of a radio interface with the mobile station
207
, control stations BSC
202
and
203
for controlling the base stations
204
~
206
and for processing a vocoding of a voice packet, a mobile switching center MSC
200
for translating a subscriber's number to switch a call, and a home location register HLR
201
for storing and processing positional information of subscribers required for a paging.
A CDMA mobile communication system of the multiple frequency environment occurs in a mass subscribed area and non-mass subscribed neighboring area as the number of the subscribers increases. For example, suppose that a cell A under the charge of the first base station
204
is a massed-subscribed area and cells B and C respectively under the charge of the second and third base stations
205
,
206
are non-massed-subscribed areas. The mobile station
207
positioned at the neighboring area of the first and second base stations
204
,
205
would have the multiple frequency environment.
In the multiple frequency environment, if the number of frequencies used in the first base station
204
and the number of frequencies used in the second and third base stations
205
,
206
are different, a mobile station having a call established using a non-common frequency may move from the first base station
204
to the second base station
205
. In such case, an intra-frequency handoff should be performed.
However, the intra-frequency handoff cannot be performed in the MAHO method as described above, because the mobile station cannot transmit the PSMM when the frequency is different from the frequency of an adjacent base station. To solve this problem, a pilot generating equipment capable of generating a pilot may be additionally implemented in every base station positioned within the multiple frequency environment. Particularly, a pilot generating equipment is capable of transmitting a pilot in all frequencies used in a multiple frequency environment including the frequencies used by the first base station, i.e. the boundary base station, and the non-common frequencies Thus, the pilot generating equipment would generate the PSMM when a mobile station moves to the second base station, i.e. the destination base station.
As a result, the base stations
204
~
206
and the base station controllers
202
,
203
analyze the generated PSMM to execute the intra-frequency handoff procedure when the PSMM is generated through the pilot generating equipment. The intra-frequency handoff method based on a conventional technique is as follows.
The pilot generating equipment for transmitting a pilot of frequency during conversation is already installed in the destination base station or the second base station
205
. Thus, when the mobile station moves from the first base station
204
to the second base station
205
, the pilot generating equipment of the second base station
205
generates pilot for a corresponding frequency. Based upon the generated pilot, the mobile station can measure a pilot signal strength of the second base station
205
. Accordingly, when the pilot signal strength is larger than a comparison value from the first base station
204
and the base station controller
202
, the PSMM is transmitted to the first base station
204
as a decision message to execute a handoff process.
Subsequently, the first base station
204
analyzes the PSMM and recognizes the intra-frequency handoff. If the signal strength of the second base station
205
meets the condition to perform the intra-frequency handoff, the first base station
204
establishes a new traffic path between a corresponding mobile station and the second base station
205
by using the EHDM and the HCM. Simultaneously, the first base station
204
withdraws its own traffic path.
However, the intra-frequency handoff method as describ
Fleshner & Kim LLP
LG Electronics Inc.
Trost William
Yun Eugene
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