Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-10-04
2004-05-25
Corsaro, Nick (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S442000, C455S453000, C370S332000
Reexamination Certificate
active
06741859
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to cell designs of a mobile communication system that is based on a CDMA (code division multiple access) scheme typically used in an IS-95-A scheme.
2. Description of the Related Art
As a number of customers increases in a mobile-communication system, there is an increasing need for a system that can accommodate a large number of customers.
FIG. 10
is an illustrative drawing showing a configuration of a typical related-art mobile-communication system.
In the system of
FIG. 10
, a public telephone network is connected to a mobile network via mobile switch center MSC. The mobile switch center MSC has base station controllers BSC connected thereto, and the base station controllers BSC in turn have base stations BTS connected thereto. Each of the base stations BTS communicates with mobile stations MS residing in its cell (i.e., area of control) so as to render services such as a telephone service. In such a mobile-communication system, a CDMA (code division multiple access) scheme, a TDMA (time division multiple access) scheme, or a FDMA (frequency division multiple access) scheme is typically employed for the purpose of providing multiple accesses.
[CDMA Scheme]
The CDMA scheme is used in the IS-95-A scheme. In the CDMA scheme, a base station uses the same frequency for communicating with different mobile stations residing in its own cell. Channels for communicating with respective mobile stations are established by using predetermined codes, which are called dispersion codes, and serve to discriminate respective signals of mobile stations. Data exchanged between the base station and a mobile station is encrypted (frequency dispersed) by convolving the data with a dispersion code. On the receiver side, the received data is further convolved with the same dispersion code in order to identify a channel.
In the CDMA scheme, a transmitter side of a base station uses two types of dispersion codes. One is a short code, which is used for discriminating the base station from other base stations. The other is a long code, which is used for discriminating a mobile station as a destination. These two codes are convolved with transmission data.
Further, a transmitter side of a mobile station uses two types of dispersion codes. One is a short code again, which is used by a base station for obtaining a data timing of data received from the mobile station. The other is a long code, which serves to discriminate the mobile station from other mobile stations. These two codes are convolved with transmission data.
Such dispersion codes as described above are used for channel-discrimination purposes in the CDMA scheme. Because of this, each mobile station can selectively pick up a channel directed to itself from a relevant base station even when each mobile station simultaneously receives signals of the same radio frequency from a plurality of base stations.
In this manner, the CDMA scheme allows base stations to transmit the same frequency to all the mobile stations, and allows all the mobile stations to transmit the same frequency to the base stations. Please note, however, that the transmission frequency of the base stations is different from the transmission frequency of the mobile stations.
[Hand-off of CDMA Scheme]
“Hand-off” refers to an operation performed when a mobile station moves from a cell of a given base station to a cell of an adjacent base station while engaging in a call. The CDMA scheme performs a soft hand-off operation to insure a continuous call without a break.
During a period of a soft hand-off state, two base stations having bordering cells transmit the same data received from the base-station controller to a mobile station currently positioned around the border of the cells. The mobile station combines the received signals sent from the two base stations, thereby improving a reception gain. Each of the two base stations receives a signal sent from the mobile station, and forwards the signal to the base-station controller. The base-station controller compares the two signals sent from the two respective base stations, and select one having a better signal quality. Selected data is then sent to the mobile-switch center. In this manner, a call never breaks during a soft hand-off period as long as either one of the two base stations securely receives signals from the mobile station.
A mobile-communication system based on the TDMA scheme typically employs a different type of a hand-off operation called a hard hand-off. In a hard hand-off operation, a radio frequency is switched after a mobile station comes sufficiently close to a first base station when moving from a second base station to the first base station with an aim of achieving a secure shift. This means, however, that the mobile station becomes distanced from the second base station before the hand-off operation is actually performed. A hard hand-off thus requires a greater transmission power than a soft hand-off. Further, a communication suffers a brief moment of disconnection at the time of switching.
Even the CDMA scheme may use a hard hand-off operation when two base stations cannot use the same frequency to provide respective services to a mobile station, for example. In such a case, a brief moment of disconnection is observed before a switched channel is reconnected.
[Number of Subscribers in CDMA]
The CDMA scheme achieves division of channels by use of codes, and uses the same radio frequency shared by a large number of mobile stations. When a base station attempts to receive a signal from a given mobile station, other signals transmitted from other mobile stations using the same radio frequency appear to be nothing but sources of interferences for the base station. Namely, an increase in the number of mobile stations adding to the number of transmission signals is tantamount to an increase in noise. The acceptable number of mobile stations that can communicate using the same radio frequency is obviously limited by the degree of interference. It is important, therefore, to reduce interferences by using as small transmission power as possible for each mobile station. This is the most important issue to be addressed in deciding the number of mobile stations than can be accommodated in the same cell, i.e., the number of customers of a single system.
In order for a mobile station to reduce its transmission power around a border of cells, a soft hand-off is suitable because it requires only a minimum transmission power that achieves communication with the closest base station.
As a mobile station shifts its position, a building may come into a line between the mobile station and the base station, or may go out of the line. When the mobile station is obscured by a building, the base station in the CDMA system increases transmission power in response to weakening signals if the CDMA system is not using a soft hand-off. Such an increase in transmission power is an increase of noises as far as other mobile stations are concerned. When the mobile station comes out from behind the building, the transmission power is decreased. Such an adjustment of transmission power is repeated as the mobile station moves.
In a system which employs a soft hand-off, even when a base station is obscured by a building, a mobile station may maintain a connection with another mobile station. In such a case, necessary transmission power is smaller compared to the case of no soft hand-off operation. Namely, a noise effect on other mobile stations is smaller.
Accordingly, a system employing the soft hand-off operation can accommodate a larger number of mobile stations than a system using no soft hand-off, thereby achieving a smaller system cost per user.
[System Configuration of CDMA Scheme]
FIG. 11
is an illustrative drawing showing a configuration of areas (cells) of related-art base stations employing the CDMA scheme.
As previously described, the number of channels that a single base station can use with a common radio
Otsuka Yasunori
Oyama Jun
Corsaro Nick
Fujitsu Limited
Muchin Katten
Rosenman Zavis
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