Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
1998-04-28
2001-08-07
Maung, Nay (Department: 2681)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S069000, C455S115200
Reexamination Certificate
active
06272355
ABSTRACT:
FIELD OF TECHNIQUE
The invention relates to a power control method used for changing step size of power control in a cellular radio system, comprising at least one subscriber terminal and one base station, receiving information on the speed of the subscriber terminal and using a fading channel.
The invention relates further to a cellular radio system used for power control and comprising at least one subscriber terminal, one base station and means for changing step size of power control, which means receive information on the speed of the subscriber terminal and which cellular radio system uses a fading channel.
PRIOR ART
In a cellular radio system, the quality of a connection between a base station and a subscriber terminal varies continuously. This variation is due to disturbing factors on the radio path and, for instance, to an attenuation of radio waves as a function of distance. The reception of the subscriber terminal is disturbed also by signals coming from base stations located within the coverage area and serving other subscriber terminals.
In a cellular network environment, users are located randomly with respect to the base station and each other. Attenuation of a signal between a base station and a subscriber terminal is described by part loss, increasing at least quadratically with increasing distance. Therefore, subscriber terminals located close to the base station may cover a transmission of more distant base stations entirely, especially when the power control of the subscriber terminals is not accurate, because even a small correlation of a strong signal may cause a great interference to the indication of a weak signal. This phenomenon is called near/far problem. In a cellular radio system, power control aims at that the power received by the base station of all subscriber terminals shall be equal irrespective of the distance between the subscriber terminal and the base station. The power control also aims at that the subscriber terminal receives a signal of accepted level all the time when it is in contact with the base station. It is, however, difficult to carry out an accurate power control, e.g. because of the quickly varying nature of a radio channel.
Attenuation of the radio path between a base station and a subscriber terminal at a single point is not only a function of distance and frequency, for instance. In a received signal, fast and slow variation occur as a function of time and place. This signal variation is called signal fading.
Fast signal fading is due to the fact that signal components coming to a receiver different ways are summed together. Depending on mutual phase difference between the signal components, they either amplify or attenuate each other. The variation of the signal components may be significant, up to tens of decibels. Great variation occurs along a distance of less than half a wavelength already. On account of fast fading, the amplitude of a received signal follows so-called Rayleigh distribution. Fast fading may be e.g. flat or frequency-selective fading depending on the bandwidth of the signal.
Slow fading of a signal is due to the fact that, on the radio path between the base station and the subscriber terminal, there is a varying amount of obstacles causing extra attenuation. Obstacles can be house walls or high terrain forms, for instance. Slow fading follows typically lognormal distribution. The average signal level in decibels received in the lognormal distribution is normally distributed. In cellular radio systems, signal variations caused by fast and slow fading are taken into consideration by increasing the transmission power or, respectively, by shortening the radio path in such a way that the received signal power is in certain probability above the threshold power of the receiver.
It is previously known to use power control in subscriber terminals to decrease signal variation caused by fading. In general, power has been controlled by a standard step size. The base station or the subscriber terminal has measured the signal and transmitted power control commands on the basis of this measurement. The power control commands have been commands to decrease or increase the transmission power, for instance. The step size of power control has been changeable, too. The variation in the step size has been dependent on the load of the base station or on previous power control commands, for instance.
However, it has not been possible to achieve an accurate power control by the known power control methods so that fading occurring in the channel could have been decreased sufficiently. These solutions have not considered the change in fading profile caused by a movement of the subscriber terminal. As the step size used for power control in the known methods has been selected some compromise value, which has not been especially good at any speed of the subscriber terminal. Consequently, the step size has worked at all speeds of the subscriber terminal, but it has not been anywhere near ideal at any speed. Due to the above, the known power control methods have not been capable of following separate fading notches in the fading profile efficiently, which notches have occurred in the radio channel.
When the subscriber terminal has moved very slowly or stayed in place, the step size of power control used in the known solutions has usually been too big. By too big a step size of power control, it has not been possible to adapt the ideal power control efficiently and accurately enough. With increasing speed of the subscriber terminal, the step size to be used for power control has usually become too small for being capable of following efficiently fast fading, in particular. If the speed of the subscriber terminal has grown even higher, the known power control methods have not been capable of following separate fading notches any more. Moreover, it is not possible to follow said fading notches, because the frequency of occurrence of fading notches has been much greater than the possibility of transmitting power control commands, for instance. The power level received in this situation has averaged the fading channel at the fading notches and the power control used has followed slow fading, primarily.
FEATURES OF THE INVENTION
An object of the present invention is to implement a solution according to which the step size to be used for power control is provided in such a way that the power control compensates for fadings occurring in the channel.
This is achieved by a method of the type described in the introduction, characterized in that the step size is changed on the basis of the information on the speed of the subscriber terminal for diminishing fadings in the channel.
The cellular radio system according to the invention is characterized in that the means change the step size to be used for power control on the basis of the information on the speed of the subscriber terminal for diminishing fadings in the channel.
Considerable advantages are achieved by the method of the invention. The step size to be used for power control in a cellular radio system according to the invention is determined in such a way that the speed of the subscriber terminal is taken into consideration. In this way, the power control will be accurate. Simultaneously, the quality and the safety of a connection between the base station and the subscriber terminal of the cellular radio system will be better. Accordingly, the solution of the invention is capable of following the ideal power control better and diminishing the channel fading on the radio path, because the step size to be used for power control is changed on the basis of the speed information received from the cellular radio system.
Preferred embodiments of the method according to the invention appear also from the attached dependent claims and preferred embodiments of the receiver according to the invention appear from the attached dependent claims.
REFERENCES:
patent: 5574984 (1996-11-01), Reed et al.
patent: 5771461 (1998-06-01), Love et al.
patent: 5862453 (1999-01-01), Love et al.
patent: 592404
Maung Nay
Nokia Telecommunications Oy
Persino Raymond
Pillsbury & Winthrop LLP
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