Telecommunications – Transmitter and receiver at separate stations – With control signal
Reexamination Certificate
1999-03-17
2002-06-11
Hunter, Daniel (Department: 2694)
Telecommunications
Transmitter and receiver at separate stations
With control signal
C455S522000, C370S318000, C370S342000
Reexamination Certificate
active
06405021
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of controlling the transmission power in a cellular system wherein a plurality of mobile stations communicate bidirectionally with each other via a base station situated in a service area, and a base station apparatus in such a base station.
2. Description of the Related Art
In a cellular system which employs a code division multiple access (CDMA) process for radio communications, a number of base stations and mobile stations communicate bidirectionally with each other in respective established links at the same frequency. The received power (desired power) of a signal in a certain link acts as interference power that disturbs other links. In an uplink transmission where a mobile station transmits a signal and a base station receives the transmitted signal, when the desired power exceeds a predetermined power level, the interference power increases, thus reducing the link capacity.
To prevent the above phenomenon from occurring, it is necessary to strictly control the transmission power of mobile stations. According to one uplink transmission power control process, the desired power from a mobile station is measured by a base station and the measured level of the desired power is compared with a control target level. If the measured level of the desired power is greater than the control target level, then the base station transmits a control command for reducing the transmission power to the mobile station. Conversely, if the measured level of the desired power is smaller than the control target level, then the base station transmits a control command for increasing the transmission power to the mobile station. This transmission power control process is discussed in detail in U.S. Pat. No. 5,056,109 entitled “Method and apparatus for controlling the transmission power in a CDMA cellular system”, issued to Gilhousen et al.
According to the above transmission power control process, however, if a control command transmitted from a base station to a mobile station is judged erroneously so as to increase or reduce the transmission power in a manner opposite to the control command, then the desired power may become excessive or insufficient in level. If the desired power becomes excessive, then the interference power which affects other links increases. If the desired power becomes insufficient, then the quality in the link decreases.
Japanese unexamined patent publication No. 1997-312609 discloses a process of ignoring a received control command if the quality of the received control command is poor, so that the transmission power from a mobile station will be prevented from increasing or decreasing in a wrong direction for thereby alleviating the above drawback.
Code division multiple access (CDMA) cellular systems use a technique known as “soft handoff” which allows a mobile station, as it moves from one cell to another, to communicate with a plurality of base stations in the vicinity of the boundary of the cells while changing links. The soft handoff is disclosed in detail in U.S. Pat. No. 5,101,501 entitled “Method and system for providing a soft handoff in communications in a CDMA cellular telephone system”, issued to Gilhousen et al.
According to an uplink transmission power control process with a soft handoff function, a plurality of base stations measure the desired power from a mobile station, and independently transmit respective control commands based on the measured levels of the desired power to the mobile station via downlinks. When the mobile station receives the control commands from the respective base stations, the mobile station controls the transmission power based on the received control commands. If the mobile station receives different control commands, then the mobile station controls the transmission power based on the control command for reducing the transmission power. This process is described in TIA/EIA Interim Standard, Mobile Station—Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, TIA/EIA/IS-95-A, Telecommunication Industry Association, May 1995, 6.6.6.2.7.2 Reverse Traffic Channel Power Control During Soft Handoff.
As described above, if the mobile station receives different command signals transmitted from the respective base stations, then the mobile station controls the transmission power on the basis of the control command for reducing the transmission power, from among the control commands transmitted from the respective base stations and received by the mobile station. Therefore, the desired power does not exceed a control target level in any of the base stations, for thereby achieving a high uplink capacity. In this process, consequently, it is important to reduce the probability that the mobile station will fail to receive a control command for reducing the transmission power.
In a downlink transmission where a base station transmits a signal and a mobile station receives the transmitted signal, the transmission power from the base station is controlled to achieve a predetermined ratio between the desired power and the interference power for thereby accomplishing a high link capacity.
According to the uplink transmission power control, as described above, the base stations independently transmit respective control commands for controlling the transmission power to the mobile station via downlinks. It is important that, while the soft handoff function is being performed, the downlink transmission power be controlled in order for the mobile station to be able to receive control commands transmitted from the base stations.
One solution is to control at the mobile station the downlink transmission power levels for equalizing respective desired power levels from the base stations. According to this solution, in those base stations which suffer a large downlink propagation loss, the transmission power is set to a correspondingly large level. Therefore, the interference power increases, resulting in a reduction in the downlink capacity. Such a reduction in the downlink capacity can be suppressed by controlling the transmission power levels so as to equalize transmission power levels from the base stations, as disclosed in Anderson, “Tuning the macro diversity performance in a DS-CDMA system,” Proc. IEEE 44th Vehicular Technology Conference, pp. 41-45, June 1994.
According to the above transmission power control scheme, however, the desired power levels from the base stations which suffer a large downlink propagation loss are so reduced that the mobile station is more likely to fail to receive control commands from those base stations and hence it is more probable that the mobile station will fail to receive control commands to reduce the transmission power.
Instantaneous level variations of the propagation loss differ from frequency to frequency. In a system where different frequencies are employed for uplink and downlink transmissions, uplink and downlink propagation losses differ from each other. Therefore, on the assumption that an uplink propagation loss from a mobile station to a base station is small and the desired power received by the base station is greater than a control target level, when a control command for reducing the transmission power is transmitted from the base station to the mobile station, if a downlink propagation loss from the base station to the mobile station is larger, then the desired power received by the mobile station is smaller, with the result that the mobile station may possibly fail to receive the transmitted control command for reducing the transmission power.
If all control commands transmitted from other base stations to the mobile station at this time are for reducing the transmission power, then the transmission power is increased by the mobile station, and the desired power in the base station which has transmitted the control command that the mobile station has failed to receive becomes excessive. Consequently, the interference power will be greater than if the mobile
Hunter Daniel
NEC Corporation
Scully Scott Murphy & Presser
Tran Pablo N
LandOfFree
Method of controlling transmission power in cellular system... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of controlling transmission power in cellular system..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of controlling transmission power in cellular system... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2955267