Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Reexamination Certificate
1998-09-18
2002-08-06
Kizou, Hassan (Department: 2662)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C370S331000, C370S335000, C370S342000
Reexamination Certificate
active
06430200
ABSTRACT:
BACKGROUND
1. Technical Field
The present application relates generally to a communication system and, more particularly, to an apparatus and method for generating a pilot signal for performing a hard hand-off in a code division multiple access (CDMA) cellular system using an External Pilot Transmitter (EPT).
2. Description of the Related Art
In a cellular mobile telephone system, the cellular service area is divided into a plurality of sub-areas (i.e., cells), and each cell has a base station associated therewith. In a cellular communication system, a single mobile switching center (MSC) controls all base stations, and allows a mobile station to continue communication when the mobile station travels between several service cells. In a code division multiple access (CDMA) cellular communication system or a personal communication service (PCS) system, various types of hand-off operations are utilized to ensure continuous communication when the mobile station travels between cells (i.e., when a mobile station travels from a “source base station” to a “target base station”).
One type of hand-off operation is referred to as a “hard hand-off.” During a hard hand-off operation, when a mobile station enters a target cell (or target base station), communication with the source base station is terminated, and then communication with the target base station is established. The hard hand-off process is completed within a very short time and the user of the mobile station does not even recognize the temporary termination of communication.
In a CDMA cellular system, frequency assignment to the various cells is typically offset due to the unbalanced distribution of subscribers. For instance, downtown city areas generally require more traffic capacity to service a large number of subscribers, whereas suburban areas require relatively less traffic capacity due to a smaller number of subscribers. Consequently, when a target base station (in which a mobile station enters) does not have a frequency assignment through which communication is currently established, or when, if any, the frequency assignment does not have enough traffic channels, a hard hand-off cannot occur.
A hard hand-off is generally performed as follows. The source base station continuously measures the signal strength of a mobile station within its cell region to determine if the signal strength drops below a predetermined threshold value. When the received signal strength falls below the threshold, the source base station determines that the mobile station is located at the boundary of its cell region, and then signals a base station controller (BSC). The BSC then decides which base station (i.e. target base station) receives a relatively strong signal from the mobile station.
When it is determined that a particular target base station (e.g., a neighboring base station) receives a strong signal, the BSC transmits a hand-off request message to the target base station, as well as a command to the mobile station to communicate with the target base station (neighboring base station). The mobile station then performs a hand-off and, accordingly, communication between the mobile station and the target base station is established.
The BSC determines whether to perform a hand-off based on a pilot signal strength of the source base station which is in current communication with the mobile station. In the conventional system, a call will be disconnected if the hand-off to the target base station is unsuccessful. There are various reasons for a hand-off operation to fail. For instance, a hand-off operation can fail if there are no available channels in the target cell for communicating the call or if the mobile station fails to receive a hand-off message. A hand-off can also be requested when a mobile station enters a shadow area of the cell area in which the pilot signal strength becomes weak. Furthermore, it is very difficult to determine a hand-off determination parameter and time in the varying cellular communication environment. And there is trade-off between coverage areas to reduce the possibility of success to perform a hand-off.
A conventional method for performing a hard hand-off using a pilot signal is disclosed in U.S. Pat. No. 5,594,718 entitled “A Method and Apparatus For Providing Mobile Unit Assisted Hard Handoff From A CDMA Communication System To An Alternative Access Communication System.” This method uses a pilot beacon for generating an identifying pilot signal corresponding to a target base station in order to overcome the above difficulties.
Referring now to
FIG. 1
, a diagram illustrates a conventional pilot signal generator for generating a pilot signal for performing a hard hand-off in accordance with the prior art. Each base station has a RF path unit, which includes a plurality of digital MODEMs
200
,
210
and
220
for converting audio frequencies into intermediate frequencies, a plurality of transceivers
300
,
310
and
320
for converting the intermediate frequencies into radio frequencies and a plurality of power amplifiers
400
,
410
and
420
for amplifying the RF signals which are transmitted from an antenna. The digital MODEMs
200
and
210
, transceiver
300
and
310
, and power amplifiers
400
and
410
are utilized for communication, whereas a pilot signal generator (which generates an identifying pilot signal corresponding to a target base station) includes the digital MODEM
220
for generating a pilot signal, the transceiver
320
and the power amplifier
420
. As a mobile station moves to a target base station, the mobile station simultaneously receives a weak pilot signal from the source base station and a relatively strong pilot signal from the target base station. Accordingly, the mobile station will request a hand-off and the digital MODEM
220
of the pilot signal generator will only transmit the overhead channel such as, for example, a pilot, synchronization, and paging.
The transmission of the overhead channel (as described above) causes an unbalance of coverage areas of multiple frequencies, and this unbalance increases the load of the primary frequency. Specifically, when the coverage area of the pilot signal generator is greater than that of the forward link of the primary frequency, a mobile station will more frequently request a hand-off in a base station having a pilot signal generator than in the balanced base station where the coverage area of the primary frequency is almost balanced with that of the pilot signal generator. Accordingly, a call will more frequently be handed down to the primary frequency, which results in an increase of the load of the primary frequency and reduces the performance of the entire system. A pilot digital gain modulation can control the unbalance of the coverage areas, but it is very difficult to modulate a pilot digital gain taking into consideration the actual radio environment and system operation.
In addition, when the pilot signal generator is added to the base station, it requires space for installation; a digital hardware MODEM, and a channel card. The digital hardware MODEM is used for a hand-off (not for actual communication), and for generating some channels such as a pilot channel, synchronization channel and paging channel. This pilot signal generator is not portable because it is installed in an outdoor device.
There are various disadvantages associated with the conventional pilot signal generator. For example, the pilot signal generator is physically large and significantly costly due to the large space required for installation. Moreover, the conventional pilot signal generator is not portable and requires a significant amount of labor to install it, or extract it, from the system. In addition, as stated above, it causes unbalance in coverage areas of multiple frequencies.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method and apparatus for generating a pilot signal for a hard hand-off using an IF amplifier/divider without having to add the pilot signal device to frequency allocation generati
Han Deog-Su
Kim Dae-Sik
Dilworth & Barrese LLP
Kizou Hassan
Samsung Electronics Co,. Ltd.
Spafford Tim
LandOfFree
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