Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
1999-01-05
2002-06-04
Pham, Chi (Department: 2631)
Pulse or digital communications
Transmitters
Antinoise or distortion
C375S224000, C330S002000, C330S149000
Reexamination Certificate
active
06400775
ABSTRACT:
The invention relates to the field of amplifiers.
More precisely, the invention relates to a method and a system for linearizing an amplifier.
BACKGROUND OF THE INVENTION
The invention is particularly, but not exclusively, applicable to linearizing amplifiers used in cellular radiocommunications systems (such as, in particular, the Global System for Mobile communications 900 MHz (GSM 900) (a public radiccommunications system operating in the 900 MHz band), the Digital Cellular System 1800 MHz (DCS 1800), the Personal Communication System (PCS), IS95, the Universal Mobile Telephone System (UMTS), the International Mobile Telecommunications 2000 system (IMT-2000), CDMA-One, etc.
A mobile terminal generally includes an amplifier in its transmission system.
Frequently, it is desirable to obtain linear amplification. Since an amplifier is never exactly linear, it is necessary to linearize the amplifier.
In radiocommunications systems, the non-linearity of the amplifier in the radio transmission system is detrimental because it distorts the transmitted signal, thereby degrading the spectrum of the signal, and thus causing interference signals to be generated in channels adjacent to the working channel. In other words, the non-linearity of the amplifier causes an increase in the level of transmission interference in the adjacent channels. This gives rise to failure to comply with specifications aiming to limit interference. Such failure to comply with the specifications is further accentuated when modulation having a non-constant envelope (which is more sensitive to non-linearity defects) is used upstream from the amplifier. It can thus be understood that it is necessary to linearize the amplifier.
Several solutions for performing such linearization are known from the state of the art.
A first known linearization solution consists in using the amplifier well within its capacities, so as to operate in a linear zone. The lower the power level, the smaller the non-linearity manifested by the amplifier. That first known solution suffers from the major drawback of degrading amplifier efficiency. Thus, if such an amplifier is included in a mobile terminal, it reduces battery life between charges.
A second known linearization solution consists in using an analog feedback correction loop (cf FIG.
1
). Unfortunately, the correction loop requires additional analog components which firstly suffer from defects affecting the performance of the resulting overall system, and secondly are subjected to drift over time, causing the effectiveness of the correction to vary.
A third known linearization solution consists in using an analog feedforward correction loop (cf FIG.
2
). The drawback with such a correction loop is that it requires a second amplifier which must be very linear.
A fourth known linearization solution is described in Patent Document EP 522 706 A2. That document describes a linearization method comprising a prior testing stage and an amplification stage. During the prior testing stage, the following are performed successively: a test signal is amplified; the corresponding output signal is sampled at a low rate over a long period; the Fourier transform is applied to the samples of the amplified test signal; the amplitudes of the harmonics are computed so as to reconstruct the output signal; the input signal is compared with the reconstructed output signal of the amplifier, so as to determine the distortion parameters specific to the amplifier, and a table is constructed of predistortion parameters which are such that they cancel the distortion parameters specific to the amplifier. During the amplification stage, the predistortion parameters are applied to the signal to be amplified, so that the amplified signal at the output of the amplifier is not distorted.
In other words, in the fourth solution, the principle of predistortion is used which consists in compensating for the non-linearity of the amplifier by adding a unit of inverse non-linearity at the input of the amplifier, so that the juxtaposition of the two non-linear units has a linear transfer function. The test stage which, in the fourth solution, consists in comparing the input test signal with the corresponding output signal, makes it possible to determine the distortion (or non-linearity) due to the amplifier.
The fourth known solution is not advantageous because it requires many samples to be taken at the output of the amplifier, and therefore requires a large amount of computation, considerable memory space, and a long time for acquiring (i.e. recovering) and processing the samples at the output of the amplifier. In addition, the results obtained are not accurate. It should be noted that a long acquisition time can be incompatible with certain operational constraints allowing only a small amount of time for acquisition: signals that are not steady, or time limits for transmission of test signals).
To sum up, it appears that all of the above-mentioned known solutions which aim to enable an amplifier to be linearized suffer from drawbacks, namely, in particular, their high cost considering the performance obtained.
OBJECTS AND SUMMARY OF THE INVENTION
A particular object of the present invention is to mitigate the various drawbacks of the state of the art.
More precisely, one of the objects of the present invention is to provide a method and a system of digitally linearizing an amplifier, the method and system requiring lower computation power than the above-mentioned known solutions.
An additional object if to provide such a method and such a system that require only a small amount of memory space.
Another object of the invention is to provide such a method and such a system that make it possible to reduce the time required to acquire the samples at the output of the amplifier, as compared with the solutions known from the state of the art.
Another object of the invention is to provide such a method and such a system that make it possible to reduce the time required to process the output samples, as compared with the solutions known from the state of the art.
A further object of the invention is to provide such a method and such a system that make it possible to obtain results that are very accurate.
Another object of the invention is to provide such a method and such a system that are low in cost.
These various objects and others that appear below are achieved by the invention by means of a method of digitally linearizing an amplifier having non-linearity, the method being of the type comprising:
a prior determination stage in which correction parameters for correcting said non-linearity are determined, as a function of an amplified test signal present at the output of the amplifier when a test signal is applied to the input thereof; and
a linear amplification stage comprising the following steps:
causing an input signal which is to be amplified by said amplifier to be predistorted as a function of said correction parameters so as to obtain a predistorted signal; and
causing said predistorted signal to be amplified by said amplifier so that the amplified predistorted signal present at the output of said amplifier corresponds to said input signal as amplified linearly;
wherein said determination stage in which the correction parameters are determined comprises the following steps:
acquiring said amplified test signal in the form of samples in baseband;
defining a modelled amplifier, by computing parameters of a predetermined amplifier model on the basis of said samples of the amplified test signal; and
computing said correction parameters on the basis of said modelled amplifier so that they make it possible to linearize said modelled amplifier.
The present invention thus provides a novel way of determining the predistortion relationship (i.e. the correction parameters) to be applied upstream from the amplifier in order, ultimately, to obtain amplification that is linear.
More precisely, the general principle of the invention consists in modelling the real amplifier, and then in computing the predistortion relation
Gourgue Frederic
Soula Veronique
Alcatel
Nguyen Dung X.
Pham Chi
Sughrue & Mion, PLLC
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