POWER CONTROL IN A TRANSMITTING STATION OF A COMMUNICATION SYSTEM

Abstract: The invention relates to a transmission system in which a receiving station transmits information to the transmitting stations so that they control their transmission power. This control is based on the receiving quality at the receiving station. The transmission power of the transmitting stations is limited to a maximum value (MAX) beyond which the performance of the transmission system no longer improves. This maximum is determined by each transmitting station as a function of its operating conditions in an initialization phase. The value obtained may be updated during the communication phase. Application: Notably video communication signal distribution systems.

Description: Power control in a transmitting station of a communication system Field of the invention The invention relates to a communication system comprising at least a transmitting station including at least a variable output power amplifier and control means for controlling the power of said amplifier based on information transmitted by a receiving station, said output power being limited to a maximum value. The invention also relates to a transmitting station intended to be used in such a system, a power control method for a variable power amplifier and a computer program including coding means for executing said method. the invention is applied to radio communication systems in general, notably to the interactive satellite broadcasting systems, to microwave transmission systems, to mobile telephony systems Background of the invention United States patent number US 4,777,653 filed by Philips Communication d'Entreprise describes an apparatus for controlling microwave transmission power based on information transmitted by the receiving station, notably based on information relating to the quality of the received signals. This apparatus enables to permanently adapt the transmission power of the transmitting stations to obtain desired performance. A maximum transmission power that is not to be surpassed is defined beyond which the domain of considerable non- linearities will be reached. In a device of this type the maximum transmission power is fixed a priori. Summary of the invention It is an object of the invention to effect an improvement of this type of device so that its efficiency is enhanced. For this purpose, a communication system according to the invention and as defined in the opening paragraph is characterized in that said transmitting station comprises evaluation means for evaluating said maximum value at least at a given instant as a function of at least one criterion linked with the behavior of the amplifier. Thus, the transmitting station compnses means for determining the maximum value of the output power as a function of the behavior of the amplifier at the moment of the evaluation. The invention is particularly advantageous when the behavior of the amplifier is likely to change as a function of the operating conditions, for example, as a function of the temperature Vanous cntenons are suitably used for determining said maximum output power In an advantageous manner this maximum value depends on the value of the output power beyond which the performance of the system no longer improves. Generally, the output power of the transmitter is increased when the receiving station indicates that the quality of the received signal is insufficient. Beyond a certain value the amplifier starts to work in a non-lmear domain which leads to an increase of the error rate for the received signal and a disturbance of neighbonng channels Thus the opposite effect to the one anticipated is obtained This cntenon permits to determine the limit on the basis of which it is no longer useful to augment the output power because this will no longer lead to an improvement of the quality of the received signal. In practice, said evaluation means compnse, for example, measunng means for measunng a ratio between output power and input power of said amplifier when the input power augments, said maximum value corresponding to the value of the output power obtained for a predefined value of said ratio. This calculation mode offers the advantage of being both simple and accurate. This determination is advantageously effected at least when the transmitting station is connected to the communication system. Other evaluations may regularly be made after the connection Bnef descnption of the drawings These and other aspects of the invention are apparent from and will be elucidated, by way of non-limitative example, with reference to the embodιment(s) descnbed hereinafter In the drawings Fig. 1 represents a first example of a communication system according to the invention, Fig 2 represents a second example of a communication system according to the invention, and Fig. 3 represents a flow chart descnbmg a method according to the invention of controlling the power of a vanable power amplifier Descnption of preferred embodiments In Fig 1 is represented a first example of a communication system according to the invention. It compnses a receiving station 1 and a transmitting station 2 The transmitting station 2 transmits signals to the receiving station 1 via an upstream channel 3. The receiving station 1 transmits signals to the transmitting station 2 by a downstream channel 4 The receiving station 1 compnses signal receiving means 11 and analyzing means 12 for analyzing received signals. The analyzing means 12 of the received signals produce at least one information signal relating to the received signals, for example, a quality information signal Q. The receiving station 1 also includes transmitter means 13 for transmitting signals which convey service information (notably quality information Q) and/or useful information to one or vanous transmitting stations by the downstream channel 4 The transmitting station 2 compnses signal receiving means 21 for receiving the signals transmitted by the receiving station 1, and processing means 22 for processing received signals, notably to extract therefrom the quality information Q. The transmitting station 2 also includes transmitter means 23 for transmitting signals to the receiving station 1 These transmitter means 23 notably compnse a vanable power amplifier 24 and means 25 for controlling the amplifier 24. These control means 25 receive, on the one hand, the quality information Q produced by the processing means 22 and, on the other hand, a maximum power value MAX produced by evaluation means 26 Thus, the transmission power of the amplifier 24 may be augmented or diminished as a function of the receiving quality of the signals which were previously sent b\ the transmitting station 1, while remaining below a maximum power whose value is determined by the transmitting station as a function of the context at a given instant, for example, at the moment where the transmitting station connects to the communication system. To simplify the descnption, only one receiving station 1 and one transmitting station 2 are shown in Fig. 1. It will be evident that a system according to the invention may compnse a plurality of receiving stations 1 and a plurality of transmitting stations 2 In Fig. 2 is represented a second example of a communication system according to the invention. This system is used for broadcasting video communication signals in conformity with the project of the DVB standard defined by ETSI. It comprises a head-end station 50, which is a receiving station in the sense of the present invention, and a plurality of interactive user stations 61 to 65, which are transmitting stations in the sense of the present invention. The head-end station 50 transmits in a first frequency band Ku (12-14 GHz) signals, which convey useful information and service information (notably the quality information Q). These signals are relayed to the interactive user station 61 to 65 by a satellite 70. The interactive user stations 61 to 65 transmit signals in a second frequency band Ka (20 to 30 GHz). These signals are relayed to the head-end station 50 by the satellite 70. Each interactive user station comprises at least: - a user terminal, for example, a personal computer or a digital television decoder for generating or receiving signals, - an intermediate module which comprises a modulator for modulating the signals generated by the terminal, a converter for converting the signals modulated at an intermediate frequency, a variable output power amplifier for amplifying the intermediate frequency signals, and a controller of said amplifier, - an antenna system formed by a parabola in front of which are located a transmitting device and a receiving device which notably perform frequency conversion operations. In addition to the transmit signals SIGN, the user terminals transmit to the intermediate modules a power control command CTRL. This power control command CTRL is used by the controllers of the intermediate modules to control the power of their amplifier. The command is produced in a conventional manner by each user terminal based on the quality information Q extracted from received signals coming from the head-end station 50. In practice, the intermediate module is generally physically separated from the terminal so as to make the terminal not too cumbersome and for reasons of shielding. But it may also be considered that the intermediate module forms an integral part of the user terminal. The same intermediate module and/or the same antenna system may be shared by various interactive user stations. In Fig. 2 are represented three different types of interactive user stations: - The interactive user station 61 comprises a user terminal 101 connected to an antenna system 201 via an intermediate module 301. - The user stations 62 and 63 form part of a collective distnbution network SMATVl They compnse each a user terminal (102 and 103 respectively) and an intermediate module (302 and 303 respectively) which belong to them These intermediate modules ensure, in addition to the functions already mentioned, an interface function with the collective distnbution network SMATVl For that matter, the distnbution network SMATVl is connected directly to an antenna system 202, which is thus shared by the two user stations 62 and 63 - The user stations 64 and 65 form part of another collective distnbution network SMATV2 They compnse each a user terminal (104 and 105 respectively) These user terminals 104 and 105 are directly connected to the collective distnbution network SMATV2 and ensure, in addition to the functions already mentioned, an interface function with this network SMATV2. In addition, the collective distnbution network SMATV2 is connected to a single antenna system 204 via a single intermediate module 304 The antenna system 204 and the intermediate module 304 are thus shared by the two user stations 64 and 65 In Fig. 3 is represented an example of a method according to the invention of controlling the power of the amplifiers. In an initialization phase (for example, when a transmitting station connects to the communication system), the intermediate module determines a maximum value MAX not to be exceeded by the output power of the amplifier (step 400). Dunng the communication phase the terminal transmits the power control command CTRL to the intermediate module at the same time as the signals to be transmitted SIGN. When the intermediate module receives a power control command CTRL (step 410), it compares the power PI indicated in this command with the current maximum value MAX If the power PI indicated in the command CTRL is lower than the maximum power MAX not to be exceeded, the power of the amplifier is adjusted to PI (step 420). If not, the power of the amplifier is limited to the maximum value MAX (step 430) In an embodiment which is particularly advantageous in the case of long connections, step 400 of determining the value of the maximum power MAX is repeated, for example, at regular time intervals after the initialization phase Vanous cntenons may be used for determining the maximum power MAX not to be exceeded Within the context that has just been descnbed, this maximum value is representative of the value beyond which an augmentation of the transmission power does not lead to any improvement of performance of the system. In practice, this value may be evaluated in vanous ways. - In a first example the intermediate module determines the output power for which the spectral increase (that is a difference between the maximums of the mam and second frequency spectrum envelopes of the signals to be transmitted) is equal to a predefined value, for example, 20 dB. - In a second embodiment, which is simpler to use, the intermediate module searches for the output power value that corresponds to the appearance of harmonics. - In a third embodiment the intermediate module determines a compression point of the output power. For this purpose, it measures the values of the ratio between output power and input power of the amplifier when the input power augments. The maximum value MAX is determined as being the value of the output power obtained for a predetermined value of said ratio (for example 1 dB - this is thus expressed as a compression point at 1 dB). This mode of calculation offers the advantage of being both simple and accurate.

Claims: CLAIMS 1 A communication system compnsmg at least a transmitting station including at least a vanable output power amplifier and control means for controlling the power of said amplifier based on information transmitted by a receiving station, said output power being limited to a maximum value, charactenzed in that said transmitting station compnses evaluation means for evaluating said maximum value at least at a given instant as a function of at least one cntenon linked with the behavior of the amplifier 2 A communication system as claimed m claim 1, charactenzed in that the cntenon used is representative of the value of the output power beyond which the performance of the system no longer improves 3. A communication system as claimed in claim 1, charactenzed in that said evaluation means compnse measunng means for measunng a ratio between output power and input power of said amplifier when the input power augments and in that the cntenon used is the value of the output power that corresponds to a predefined value of said ratio 4 A communication system as claimed in claim 1, charactenzed in that said evaluation means are used at least when the transmitting station connects to the transmission system 5. A transmitting station compnsmg at least a vanable output power amplifier and control means for controlling the power of said amplifier based on information transmitted by a receiving station, said output power being limited to a maximum value, charactenzed in that the receiving station compnses evaluation means for evaluating said maximum value at a given instant as a function of at least one cntenon linked with the behavior of the amplifier o 6 A transmitting station as claimed in claim 5, charactenzed m that the cntenon used is representative of the output power value beyond which the performance of the system no longer improves. 7. A transmitting station as claimed in claim 5, charactenzed in that said evaluation means compnse measunng means for measunng a ratio of output power to input power of said amplifier when the input power augments and in that the cntenon used is the value of the output power that corresponds to a predefined value of said ratio 8. A transmitting station as claimed in claim 5, charactenzed m that said evaluation means are used at least when the transmitting station connects to a transmission system. 9. A power control method for a vanable power amplifier of a transmitting station, said method using at least information transmitted by a receiving station, and said power being limited to a maximum value, charactenzed m that the method compnses an evaluation step of evaluating said maximum value at a given instant as a function of at least one cntenon linked with the behavior of the amplifier. 10. A computer program including encoding means for executing the method as claimed in claim 9.

Inventors: KIENTZ ALFRED, KIENTZ, ALFRED
Applicants: KIENTZ ALFRED, KONINKL PHILIPS ELECTRONICS NV, KIENTZ, ALFRED, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: H04B7
IPC codes: H04B1, H04B7