WO1996009696A2

WO1996009696A2 - A method for controlling transmitting power, and a cellular radio system

Info

Publication number: WO1996009696A2
Application number: PCT/FI1995/000516
Authority: WO
Inventors: Harri Honkasalo; Petri Jolma; Jukka Liimatainen
Original assignee: Nokia Telecommunications Oy
Priority date: 1994-09-22
Filing date: 1995-09-21
Publication date: 1996-03-28
Also published as: FI97926B; AU3474695A; FI944395A; FI97926C; WO1996009696A3; FI944395A0

Abstract

The invention relates to a method for controlling transmitting power in a cellular radio system comprising in each cell at least one base station (20) communicating with subscriber terminal equipments (21-22) located within its area, in which method the base station measures the power level of a signal received by it from a terminal equipment, and the terminal equipment measures the power level of a signal received by it from the base station, and reports the result to the base station. For enabling fast and accurate power control in the method of the invention, the base station (20) compares the measurement results transmitted by the terminal equipment with its own measurement results, and controls the transmitting power of the terminal equipment and its own transmitting power on the basis of the comparison of the measurement results.

Description

A method for controlling transmitting power, and a cellular radio system

The invention relates to a method for control- ling transmitting power in a cellular radio system com¬ prising in each cell at least one base station communi¬ cating with subscriber terminal equipments located with¬ in its area, in which method the base station measures the power level of a signal received by it from terminal equipments, and a terminal equipment measures the power level of a signal received by it from the base station, and reports the result to the base station.

It is typical of the cellular radio environment that the propagating conditions of radio waves vary constantly. Both in the signal received by the sub¬ scriber terminal equipment and in the signal received by the base station, constant variation i.e. fading occurs as a function of time and location. In fading of the signal, two different types of phenomena may be distinguished. Fading may be either fast or slow, and both phenomena usually occur simultaneously.

Fast fading of the signal is caused by multi- path propagation typical of a cellular radio environ¬ ment, whereby a signal propagates along several differ- ent routes between the transmitter and the receiver. Signal components arriving to the receiver different ways are summed in the receiver, and, depending on the mutual phase differences between the signals components, they either amplify or attenuate each other. The level of the signal may vary considerably, up to dozens of decibels, already within a distance of less than a half a wavelength.

Slow fading of a signal, in turn, is caused by the fact that on the radio path, there is a varying number of factors causing additional attenuation, such as natural obstacles, or buildings. The effect of slow fading on the signal is, as its name suggests, variation in the signal strength, which is remarkably slower than that caused by fast fading which causes strong power variations around the envelope caused by slow fading.

Due to the above-mentioned constant variation of the signal strength, the transmitting power, used by a subscriber terminal equipment in particular, must con¬ tinuously be controlled, and it must be endeavoured to adjust it to a suitable value at each moment of time.The aim of power control is to keep the transmitting power of a device as low as possible, still maintaining a sufficient quality of a connection, so that the signal will not interfere with other connections, and that the power consumption of a portable terminal equipment, in particular will be small.

Controlling the transmitting power is based on controlling the power of the received signal generally carried out in the receiver. The aim is usually to control the transmitting power so that the average vari¬ ation of the signal level, caused by slow fading may be compensated, since the variations caused by fast fading take place too fast so that they could be reacted to. The combined effect of fast and slow fading, however, causes problems in power control, as it is difficult to obtain a reliable measurement result that would only indicate the attenuation caused by slow fad¬ ing, since fast fading causes constant variation to the measurement results. Heretofore, rather a long measure- ent time has been required, during which the measure¬ ment results obtained are averaged, whereby the vari¬ ations caused by fast fading may be eliminated from the measurement results.

It is previously known to carry out power con- trol of a terminal equipment e.g. so that the base sta- tion measures the power level it receives from the ter¬ minal equipment by averaging it for a given time, and on the basis of this processed measurement result, and specific quality parameters it sends a power control command to the terminal equipment. Measurements carried out by the terminal equipment are used for controlling the transmitting power of the base station. The above- described method is used e.g. in the European digital mobile communication network GSM. Power control of the GSM system is described in greater detail e.g. in M. Mouly, M. Pautet : The GSM System for Mobile Communica¬ tions, published by the authors 1992, pp.342 - 346, which are incorporated herein by reference.

In another known technique for carrying out power control of a terminal equipment, the terminal equipment controls its own transmitting power on the basis of the signal strength received from the base station, in addition to which the base station measures the power received from the terminal equipment, and transmits power control commands to the terminal equip¬ ment. This method is disclosed e.g. in the reference EIA/TIA Interim Standard: Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, TIA/EIA/IS-95, July 1993. Both of the above-mentioned methods, however, are attended by the drawback, as already mentioned before, that when reliable power control parameters are needed, the measurement results must be averaged for a relatively long time. The object of the present invention is to pro¬ vide a method for power control, by means of which method the power control parameters may be calculated quickly, and the effect of fast fading on the measure¬ ment results may be reduced without a long averaging time. This is achieved with a method of the type set forth in the introduction, which is characterized in that the base station compares the measurement results sent by the terminal equipment with its own measurement results, and that the base station controls the trans¬ mitting power of the terminal equipment, and its own transmitting power on the basis of the comparison of the measurement results.

Another object of the invention is a cellular radio system, comprising in each cell at least one base station communicating with the subscriber terminal equi¬ pments located within its area, the base station com¬ prising means for measuring the power level of the sig¬ nal received from the terminal equipments, the terminal equipment comprising means for measuring the power level of the signal received from the base station, and means for reporting the measurement results to the base sta¬ tion. The cellular radio system of the invention is characterized in that the base station comprises means for comparing the measurement results sent by the ter¬ minal equipment with its own measurement results, and means for controlling the transmitting power of the ter¬ minal equipment and its own transmitting power on the basis of the comparison of the measurement results. By means of the method of the invention, the drawbacks of prior art methods may be avoided upon elim¬ inating the effect of fast fading on the measurement results. Power control parameters are obtained faster than has been possible heretofore, since the averaging time may be shortened in the method of the invention. The method of the invention may be used for controlling both the transmitting power of the terminal equipment and the transmitting power used by the base station. In the following, the invention will be dis¬ closed in greater detail with reference to the examples in the attached drawings, in which

Figure 1 illustrates an example of fast and slow fading,

Figure 2 illustrates a cellular radio system in which the method of the invention may be applied,

Figure 3 is a block diagram illustration of the base station receiver of the cellular radio system of the invention, and

Figure 4 is a block diagram illustration of the structure of a terminal equipment of the cellular radio system of the invention.

Figure 1 illustrates an example of fast and slow fading. The figure describes the received signal so that the horizontal axis represents time and the vertical axis represent the power level of the received signal. The power level in the receiver varies constant¬ ly, and deep fadings 10 of the power level represent fast fading, which is due to multipath propagation of signal components. The average power level 11 of a sig¬ nal also varies slowly, and this is termed as slow fad¬ ing, which is due to factors causing attenuation and occurring on the radio path. The aim of power control is to compensate variation of the power level caused by slow fading, in particular.

In the following, the method of the invention will be disclosed by way of an example of power control of the terminal equipmen . Accordingly, by means of the method of the invention, the transmitting power used by the base station may also be controlled.

Figure 2 illustrates a part of the cellular radio system in which the method of the invention may be applied. The figure shows a base station 20 serving its own coverage area. The base station is connected via a transmission link 26 to a base station controller 23. The transmission link 26 is implemented with prior art methods, such as transmission lines or a radio link. A mobile services switching centre may also function as the base station controller 23. The base station con¬ troller 23 controls the operation of the base station. In the situation shown by the figure, there are two sub¬ scriber terminal equipments 21, 22 within the coverage area of the base station, with which terminal equipments the base station has a bidirectional connection 24, 25.

The subscriber terminal equipments 21, 22 measure continuously, at given intervals the strength of the signal 24, 25 received from the base station, and report the measurement results by using prior art methods, such as control channels, to the base station. In the digital GSM system, for instance, the terminal equipments report to the base station a measurement result at intervals of 480 ms. Accordingly, the base station 20 continuously measures the signal received from each terminal equipment.

In the method of the invention, the base sta¬ tion compares the obtained measurement results with the measurement results reported by the subscriber terminal equipment from the same time period, and makes a deci- sion on power control on the basis of this comparison. Other quality parameters, such as the bit-error ratio may also have an effect on power control.

The aim of the comparison of the measurement results carried out by the base station is to eliminate the effect of fast fading from the measurement results, so that it would be possible to compensate the effect of slow fading with power control. The comparison may advantageously be carried out so that in the base sta¬ tion, the mean value of the measurement results of the base station and the terminal equipment is calculated. Since fast fading is independent of different directions of transmission (from the base station to the terminal equipment, and from the terminal equipment to the base station) , and thus different in strength in different directions of transmission, whereas slow fading, is, in turn, similar in strength in both directions of trans¬ mission, the effect of fast fading may be reduced, and correspondingly, the effect of slow fading may be weighted as a result of averaging. Thus, the desired power control is achieved without long averaging times.

Figure 3 illustrates the base station receiver of a cellular radio system, in which the method of the invention may be applied. The receiver comprises an antenna 30 by means of which the received signals are applied to radio frequency means 31, in which the received signal is converted onto an intermediate freq¬ uency. The signal is further applied to converter means 32, in which an analog signal is converted into a digi¬ tal form. The receiver further comprises detector means 33, in which the received signal is detected and from which means 33 the signal is further applied to other parts of the receiver. The receiver further comprises control means 34, in which the above-mentioned blocks are controlled and calculation is carried out. The receiver further comprises means 31, 34, in which the power of the signal received from the terminal equipment is measured, and means 34, in which the measurement result of the base station is compared with the measure¬ ment result sent by the terminal equipment along with the other signal, and means 34, for calculating a power control command to be transmitted to the terminal equip¬ ment, said power control command depending on said com¬ parison. Accordingly, in means 34, transmission power used by the individual transmitter of the base station may also be calculated. The base station receiver also comprises other components, such as filters and ampli¬ fiers, but they are unessential to the invention. The receiver described above is a receiver of a digital cellular radio system, but the invention may be also be applied in an analog system.

Figure 4 illustrates a terminal equipment of a cellular radio system, in which equipment the method of the invention may be applied. The terminal equipment comprises means 40 for coding the signal to be trans- mitted, means 41 for interleaving the coded signal, means 42 for forming a burst to be transmitted, modulat¬ ing means 43, the output signal of which is applied via a transmitter unit 44 and a duplex filter 45 to an antenna 46. The terminal equipment further comprises a receiver unit 47, in which the received signal is con¬ verted onto an intermediate frequency, and converter means 48, in which the signal is converted into a digi¬ tal form, means 49 for detecting the converted signal, deinterleaving means 50, and means 51 for decoding the received signal, in which means both channel and speech decoding are carried out. The terminal equipment further comprises control and calculation means 52 for control¬ ling the operation of the other blocks mentioned above. The terminal equipment of the cellular radio system of the invention further comprises means 47, 52 for measur¬ ing the strength of the signal received from the base station, and means 40-44, 52 for transmitting the infor¬ mation on the measurement result to the base station. The terminal equipment also comprises other components, such as filters, but they are unessential to the inven¬ tion. The above-disclosed terminal equipment is a ter¬ minal equipment of a digital cellular radio system, but the invention may correspondingly also be applied in an analog system. Although the invention has been disclosed above with reference to the example in accordance with the attached drawings, it is obvious that the invention is not limited thereto, but it may be modified in various ways within the scope of the inventive idea set forth in the attached claims.

Claims

Claims :

1. A method for controlling transmitting power in a cellular radio system comprising in each cell at least one base station (20) communicating with sub¬ scriber terminal equipments (21-22) located within its area, in which method the base station measures the power level of a signal received by it from a terminal equipmen , and the terminal equipment measures the power level of a signal received by it from the base station, and reports the result to the base station, c h a r a ¬ c t e r i z e d in that the base station compares the measurement results sent by the terminal equipment with its own measurement results, and that the base station controls the transmitting power of the terminal equip¬ ment, and its own transmitting power on the basis of the comparison of the measurement results.

2. A method as claimed in claim 1, c h a r a - c t e r i z e d in that comparison between the measure- ment results of the base station (20) and the terminal equipment (21-22) is carried out by means of averaging the results.

3. A method as claimed in claim 1, c h a r a c¬ t e r i z e d in that the base station (20) transmits the terminal equipment (21-22) a power control command by using a fast control channel .

4. A cellular radio system, comprising in each cell at least one base station (20) communicating with the subscriber terminal equipments (21-22) located with- in its area, the base station (20) comprising means (31, 34) for measuring the power level of the signal received from the terminal equipments, the terminal equipment comprising means (47, 52) for measuring the power level of the signal received from the base station, and means (40-44, 52) for reporting the measurement results to the base station, c h a r a c t e r i z e d in that the base station comprises means (34) for comparing the measurement results sent by the terminal equipment with its own measurement results, and means (34) for control- ling the transmitting power of the terminal equipment and its own transmitting power on the basis of the com¬ parison of the measurement results.

5. A cellular radio system as claimed in claim

1, c h a r a c t e r i z e d in that the base station comprises means (34) for carrying out a comparison between the measurement results of the base station and the terminal equipment by averaging the results.