Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
2001-10-19
2004-05-04
Gelin, Jean (Department: 2681)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S069000
Reexamination Certificate
active
06731949
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a cellular type mobile communication system where a mobile station carries out communication with one or more base stations installed in a service area and, in particular, to a method of controlling transmission power in the cellular type mobile communication system where each base station carries out control of the transmission power in accordance with a control command for the transmission power that the mobile station transmits.
As is well known in the art, various multiple access types have been adapted in a mobile communication system. One of the multiple access type is a CDMA (code division multiple access) cellular type. The CDMA cellular type mobile communication system assigns to each channel with a particular code, transmits to the same repeater a modulated wave to which a carrier having the same carrier frequency is spectrum-spread with the code, establishes code synchronization in each receiving side, and identifies a desired channel. The CDMA cellular type mobile communication system may be called a SSMA (spread spectrum multiple access) cellular type mobile radio communication system.
Inasmuch as a plurality of channels use the same frequency in the CDMA cellular type mobile communication system, a received wave power (a desired wave power) of a signal in a channel becomes an interference wave power for other channels. Accordingly, in a reverse link or an upward channel through which the mobile station transmits an upward channel signal and each base station receives the upward channel signal, when the desired wave power is equal to a predetermined reference power or more, a channel capacity decreases because the interference wave power increases. To prevent this, it is necessary to severely control transmission power of the mobile station. The transmission power control for the reverse link is carried out as follows. That is, the base station measures the desired wave power of the upward channel signal and compares the desired wave power with a control target power. When the desired wave power is larger than the control target power, the base station transmits a reverse control command indicating decrement of a reverse transmission power to the mobile station. When the desired wave power is smaller than the control target power, the base station transmits the reverse control command indicating increment of the reverse transmission power to the mobile station. The mobile station decreases or increases the reverse transmission power for the upward channel signal in accordance with the reverse control command. This method of controlling the reverse transmission power is described in U.S. Pat. No. 5,056,109 which is issued to Gilhousen et al. on Oct. 8, 1991 in detail and which has a title of “Method and apparatus for controlling transmission power in a CDMA cellular mobile telephone system.” In order to transmit the reverse control command in this transmission power control, a forward link or a downward channel through which the base station transmits a downward channel signal to the mobile station is used.
On the other hand, a large channel capacity is realized in the forward link by carrying out a forward link transmission power control so that a ratio of the desired wave power to the interference wave power or a signal-to-interference ratio (SIR) has a predetermined value. The transmission power control for the forward link is carried out as follows. That is, the mobile station measures a reception quality for the forward link and compares the reception quality with a control target quality. When the reception quality is higher than the control target quality, the mobile station transmits a forward control command indicating decrement of the forward transmission power to the base station. When the reception quality is lower than the control target quality, the mobile station transmits the forward control command for indicating increment of the forward transmission power to the base station. In addition, the base station increases or decreases the forward transmission power for the downward channel signal in accordance with the forward control command.
However, in the above-mentioned methods, when propagation loss from the mobile station to the base station suddenly increases with moving of the mobile station, the base station may cannot receive the forward control command from the mobile station and the mobile station may cannot receive the reverse control command. Under the circumstances, in a conventional method where the base station controls the forward transmission power in accordance with only the forward control command from the mobile station, the base station does not increase the forward transmission power for the downward channel signal while the base station cannot receive the forward control command from the mobile station when a state where the propagation loss increases continues. In this event, a problem arises that an interruption condition continues in communication between the base station and the mobile station. This is because the mobile station cannot receive the reverse control command from the base station and the mobile station does not increase the reverse transmission power for the upward channel signal of the reverse link.
In addition, generally, a part of user information such as a speech signal or data in a received signal in the base station is encoded with a relatively longer information amount collected so as to correctly decode by carrying out error correction or the like although a received error occurs momentarily. In addition, on decoding, the base station collectively decodes the received signal having the long information amount with a relatively longer time interval. However, in a case where the mobile station moves at a high speed, an error of decision in the forward control command is comparatively much when a high-speed transmission power control is carried out so that the reception quality is maintained at a constant by following variation of a high-speed fading in a propagation path. This is because it is impossible for the decision of the forward control command to obtain the effects of the error correction or the like although the user information may be correctly decoded because it is necessary to momentarily carry out the decision of the forward control command.
Such an error of the decision of the forward control command continuously occurs at a relatively high probability. This is because the error of the decision of the forward control command occurs related to increasing and decreasing of the propagation loss. In addition, when the error of the decision of the forward control command continuously occurs, the base station cannot control the forward transmission power for the downward channel signal in accordance with the forward control command and the mobile station may be put into a state where the mobile station does not correctly receive the downward channel signal. On the other hand, inasmuch as the mobile station cannot receive the reverse control command included in the downward channel signal from the base station, it may be impossible to control the reverse transmission power for the upward channel signal. Under the circumstances, the error of the decision of the forward control command in the upward channel signal occurs frequently and it may be impossible for the base station to correctly receive the user information. In such a case, a problem occurs that the interruption condition continues in the communication between the base station and the mobile station.
In addition, in the cellular type mobile communication system, there is a technique called a soft handover or a soft handoff where a mobile station simultaneously establishes channels with a plurality of base stations at the vicinity of a boundary in cells when the mobile station moves between the cells and changes the channels between the cells. This technique is especially an important technique in the cellular type mobile communication system which adapts the CDMA as the multiple access.
Hamabe Kojiro
Yoshida Shousei
Gelin Jean
Scully Scott Murphy & Presser
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