Telecommunications – Transmitter – With feedback of modulated output signal
Reexamination Certificate
1999-07-09
2004-09-14
Gesesse, Tilahun (Department: 2684)
Telecommunications
Transmitter
With feedback of modulated output signal
C455S127100, C455S115100, C330S149000
Reexamination Certificate
active
06792251
ABSTRACT:
BACKGROUND
The present invention relates to a multi-carrier power amplifying arrangement and to a method of controlling at least the average phase error of a multi-carrier power amplifying arrangement. The invention also relates to an array antenna with an amplifying arrangement. Particularly the invention relates to minimizing radiated power intensity for intermodulation products.
In cellular mobile communication systems the geographical coverage area of a mobile communication network is divided into cells. For each of these cells a stationary base station is arranged to communicate with a plurality of mobile stations. Each base station (BS) is connected to an antenna, or a set of antennas, covering the cell in which the BS is arranged. This means that a base station on the downlink transmits in such a manner that all mobile stations within the cell can receive the signal. Most of the transmitted information is, however, point-to-point information intended for one single mobile station only. Most of the RF power will thus be transmitted in directions in which no transceiver will receive it. If the base station could concentrate the transmitted power vertically as well as horizontally to the desired directions, using antennas with radiation patterns characterized through narrow antenna lobes, a higher efficiency could be achieved.
If the same kind of antennas are used for reception in the base station as well (on the uplink), a corresponding improvement of the reception sensitivity is obtained in the desired directions. This concentration of the output power and the reception sensitivity can be used for increasing the transmission range or for lowering the requirements on the transmitter in the base station as well as on the transmitter in the mobile station. Since the channel frequency reuse potentially can be increased with this technique, the total capacity of the mobile communication network can be enhanced. These and other advantages have in the last few years resulted in a growing interest in antenna arrays with narrow antenna beams.
It is well known in the art that all amplifiers distort an input signal. The distortion becomes greater as the power levels are increased. When the amplifier is exposed to multiple input signals, intermodulation (IM) products are introduced. These IM products must be kept low because they interfere with the system in general and with other users in particular.
Current cellular communication standards impose stringent limits on radiated power intensity for intermodulation products. The use of a central multi-carrier power amplifier (MCPA), as in current mobile communication cellular systems, suffers from a disadvantage in this respect since not only the desired signal, but also the intermodulation products will be amplified/enchanced. The consequence thereof, in combination with the power requirements on the single multi-carrier amplifier, will be that the single multi-carrier amplifier needs to employ rather sophisticated techniques to reach the desired limits on the carrier to intermodulation ratio (C/I-ratio).
Since the amount of linearisation that can be provided is technologically limited, a central amplifier has to use a significant amount of back-off to reach the desired levels of intermodulation. This leads to a decreased efficiency which in turn results in a low DC to RF conversion efficiency.
In the article “Application of Linearised Amplifiers in Adaptive Antennas” by Hongxi et al, published in the IEEE MTT-S 1995 International Topical Symposium on Technologies for Wireless applications, and in which the use of adaptive antenna arrays for future mobile communication systems is discussed, a linearisation technique called feed-forward technique is proposed as the most suitable approach for suppression of intermodulation in the base stations.
The feed-forward technique, which has been applied for linearisation of a central multi-carrier power amplifier (MCPA) basically consists of two independent steps. The first step is to extract the distortion introduced by the main amplifier by comparing the amplified signal with the input signal. This step is referred to as extracting an error signal. The second step is to amplify this error signal and inject it in anti-phase and time aligned at the output of the feed-forward amplifier to thereby cancel out the distortion.
The performance of the feed-forward technique is dependent on the ability to add rotated signal vectors correctly in anti-phase and equal amplitude. This process determines how well a distortion component can be extracted or suppressed. The ability to control these variables, i.e., gain and phase, are therefore of crucial importance in feed-forward amplifier systems. Hence, phase and amplitude adapters are employed. These adapters for a given environment can be tuned to give a minimum of phase and amplitude error. However, due to, e.g., temperature drift, an imbalance may occur, resulting in insufficient intermodulation suppression. For this reason, the general feed forward linearisation method is normally combined with an adaptive phase/amplitude control.
There are two main methods of how to implement this. The first method uses the actual distortion caused by the main amplifier to control the settings of the gain and phase controls. This is possible since it equals the first step in the feed-forward process. In this case, the error signal content is minimized at the output. The second method uses a known distortion simulating signal which is injected in the amplifier path and minimized at the output, thereby also reducing the distortion introduced by the main amplifier. The term distortion shall be understood to mean any signals present in the output of a device which were not present at the input.
U.S. Pat. No. 5,386,198 discloses an example of a method of the second type for controlling a feed-forward compensated power amplifier. A spread spectrum technique is used to cover a control signal(s) and injects a composite signal at a suitable point into the feed-forward amplifier system to reduce distortion. Control signals after remapping of the spread spectrum at the output of the system are correlated in a match filter correlator and the result is used to control, in either polar or Cartesian co-ordinates, the injection, in anti-phase, of the extracted distortion into the feed forward amplifier output.
It is a drawback that the requirements as to linearisation of each amplifying means are too high and difficult, if not impossible, to meet in order to reach an acceptable carrier to intermodulation ratio (C/I-ratio).
It is particularly disadvantageous that the average phase error may be high and particularly it is disadvantageous that, when the conditions change, for example as compared to a first situation in which for a given temperature, the used control circuitry is tuned to give a zero average phase and amplitude error, the average phase error will increase considerably as for example the temperature changes (even if temperature compensated circuits are used). The average phase error will also increase considerably for high antenna gains and the limits as to the maximum allowed intermodulation intensity may easily be exceeded. It is also substantially impossible to detect and remove a resulting average phase error using the hitherto known arrangements and methods.
SUMMARY
What is needed is therefore an amplifying arrangement, particularly a multi-carrier arrangement, through which the radiated power intensity of intermodulation products can be kept low in a simple and efficient manner. Particularly an arrangement is needed in which the requirements as to linearization are low on the respective amplifying means forming the amplifying arrangement, particularly lower than in hitherto known arrangements. An arrangement is also needed through which the power of intermodulation products can be kept low irrespectively of whether the conditions change in relation to any initial conditions, or generally under varying conditions and for a high antenna gain etc. Particularly an arra
Johannisson Björn
Luberek Jarek
Gesesse Tilahun
Telefonaktiebolaget LM Ericsson (publ)
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