Telecommunications – Transmitter and receiver at separate stations – Distortion – noise – or other interference prevention,...
Reexamination Certificate
1999-03-26
2003-06-03
Trost, William (Department: 2683)
Telecommunications
Transmitter and receiver at separate stations
Distortion, noise, or other interference prevention,...
C455S062000, C455S448000, C455S437000, C455S441000, C455S443000, C455S444000, C455S452200, C370S335000, C370S331000
Reexamination Certificate
active
06574456
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of preventing interference of adjacent frequencies in a cellular system, and more particularly to a method of preventing interference of adjacent frequencies in a cellular system that may be caused by adjacent carrier frequencies for uplink channels used by the independent cellular systems.
2. Description of the Related Art
In general, a specified frequency band is allocated to a cellular system, and a plurality of carrier frequencies which different frequencies are predetermined within this frequency band. A mobile station establishes a channel between itself and a base station in a service area using the carrier frequencies to perform two-way communication therebetween. The base station and the mobile station are designed so as to control leakage power into frequency bands adjacent to both sides of the used carrier frequency as little as possible. However, the leakage power can not be fully suppressed, so that an interference signal power is supplied to a signal of an adjacent frequency at predetermined rate. In one cellular system, each of the mobile stations establishes a channel between itself and the base station closest to it to perform communication. In an uplink channel which is a channel established from a mobile station to a base station, the transmission power of the uplink channel for transmitting a signal from the mobile station to the base station is controlled such that the difference between received powers of the signals from the mobile station, received by the base station may not be large. For this reason, an interference signal power supplied from a signal of an adjacent frequency becomes smaller than a desired signal power. On the other hand, in a downlink channel which is a channel established from a base station to a mobile station, if a transmission power of the base station is constant, an interference signal power from signals transmitted on a frequency adjacent to that of a desired wave signal reaches the mobile station through the same transmission path to be attenuated similarly. Accordingly, the interference signal power from the signal having a frequency adjacent to that of the desired wave signal is smaller than the desired signal power. Thus, in one cellular system, such interference of adjacent frequencies may not pose a serious problem.
However, the interference of adjacent frequencies may pose a serious problem when another cellular system having an allocated frequency band adjacent to the frequency band of the particular cellular system establishes a base station in the same service area and communicates with a mobile station.
As shown in
FIG. 1
, for example, when mobile station
221
located near base station
261
belonging to the other cellular system establishes a channel and communicates with base station
211
, it sends a signal with a power sufficiently large for reception to base station
211
remote from mobile station
221
, so that the signal is also received with a large power by base station
261
. When mobile station
271
establishing a channel with base station
261
controls the transmission power so that the received power is not too large at base station
261
, only the received signal from mobile station
221
may be exceedingly large. At this time, when a signal of a carrier frequency adjacent to a carrier frequency used for the transmission of mobile station
221
, is used for the transmission by mobile station
271
the interference of adjacent frequencies of the transmitted signal of mobile station
221
will be large at base station
261
, thereby deteriorating a receiving quality of the signal from mobile station
271
.
One of solutions to such a problem is to make an interval between carrier frequencies used in different cellular systems wider than that between the carrier frequencies used in the same cellular system. Because the spacing between the carrier frequencies among the cellular systems becomes wider, this solution is disadvantageous in reducing utilization efficiency of frequency bands. However, in a cellular system using many narrow band signals, the spacing between the carrier frequencies is small as compared with the total of the frequency band, so that the reduction in the utilization efficiency is not so serious. On the other hand, in a cellular system in which a small number of wide band signals in the same frequency band are used, the spacing between the frequency bands is large as compared with the total of the frequency bands, thus significantly reducing the utilization efficiency.
An approach to prevent interference between the cellular systems, has been proposed wherein prior to the start of transmission of an uplink channel by the use of a certain carrier frequency at a mobile station, interference signal power of a carrier frequency of a downlink channel used in conjunction with the carrier frequency of an uplink channel is measured, and when an interference signal power from a carrier frequency of an adjacent frequency is large, a carrier frequency other than that carrier frequency is used. However, in this approach, a certain combination of a carrier frequency of an uplink channel and a carrier frequency of a downlink channel must be used.
Furthermore, in a cellular system which adopts a code division multiple access method (CDMA) as a radio access method, despite a certain combination of carrier frequencies for an uplink channel and carrier frequencies for a downlink channel, signals to be transmitted to many mobile stations are multiplexed onto a signal of one carrier frequency of this reason, a received power of signals transmitted to other mobile stations using the same carrier frequency is received with an interference signal power from a signal of an adjacent carrier frequency mixed. Thus, despite the measurement of the received power, the interference signal power from the adjacent carrier frequency can not be obtained. Accordingly, with this method, it is impossible to prevent the interference from the adjacent carrier frequency.
Furthermore, since a transmitter in the mobile station has typically a simpler construction as compared with the base station, it has sometimes a low suppression of a leakage power into an adjacent carrier frequency. Therefore, when a certain mobile station is located near a base station belonging to another cellular system and the mobile station performs a transmission using adjacent carrier frequencies for an uplink channel as well as for a downlink channel, the mobile station is able to continue the communication with a good quality without being applied with an interference of adjacent frequencies because of a low leakage power from transmitting signals of the base station to the adjacent carrier frequency. However, because of a large leakage power from transmitting signals from the mobile station to an adjacent carrier frequency. A mobile station may cause an one-sided interference obstacle to the base station.
Although such interference of adjacent frequencies presents no problem when starting a communication, it may be a problem during the communication because as a mobile station may approach a base station belonging to another cellular system, as it moves during the communication.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of preventing interference of adjacent frequencies which is capable of preventing a degradation of communication quality due to an interference between carrier frequencies adjacent to each other, without widening the frequency interval of signals used in different cellular systems.
When two cellular systems utilizing frequency bands adjacent to each other for an uplink channel, are provided in a same service area, and a signal for the uplink channel used by a mobile station of one cellular system may cause interference of adjacent frequencies to a mobile station of the other cellular system, the method of preventing an interference of adjacent frequencies in a cellular system according to
NEC Corporation
Perez-Gutierrez Rafael
Scully Scott Murphy & Presser
Trost William
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