Amplifiers – With amplifier bypass means
Reexamination Certificate
2000-04-27
2002-05-21
Pascal, Robert (Department: 2817)
Amplifiers
With amplifier bypass means
C330S149000, C375S296000, C455S063300
Reexamination Certificate
active
06392482
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for linearizing, over a wide frequency band, a transmission chain comprising a power amplifier.
The constant increase of traffic in the field of telecommunications raises new problems which are not easy to solve for the equipments. More particularly the transmission equipments must operate over an increasing frequency bandwidth in order to be able to transmit simultaneously a great number of channels which operate at different frequencies and/or with different codes. In fact in telecommunication systems each transmitter may operate either on a frequency division multiple access (FDMA) mode or on a coded division multiple access (CDMA) mode or on a combination of both. In the case of CDMA the modulation is spread over a frequency band of several MHz. It is recalled here that in CDMA to each symbol, such as a bit, is superimposed a code which is a sequence at a higher frequency.
The consequence of the increase of the number of channels to be transmitted is also an increase of the transmission power of the transmission equipments.
The most difficult problem to solve with such increase of power together with the increase of the frequency bandwidth is the linearization of the transmission chains, more particularly the linearization of power amplifiers.
Generally power amplifiers are used near saturation in order to obtain the most efficient use of such amplifiers. However near saturation each amplifier has a non linear behavior, i.e. the gain decreases sharply for high input signals compared to small input signals; moreover the output signal is phase distorted due to the well known AM-PM conversion, i.e. the conversion of amplitude modulation into phase modulation.
When several channels are transmitted simultaneously the difficulty of linearization is increased by the fact that the sum of signals corresponding to these different channels, i.e. different frequencies, presents generally a non-constant envelope, i.e. the sum of such signals varies with time. The non-constant envelope must be transmitted correctly, i.e. with an amplifier working linearly in order, on one hand, to transmit signals with a good quality and, on the other hand, to prevent the transmission of interferences that would pollute the spectrum of same or other operators.
The difficulty of the problem is also increased by the fact that the higher the frequency, the lower is the yield of the amplifiers.
Several technologies are used for linearizing power amplifiers. However none of the conventional technologies is able to provide a transmission chain which may be manufactured at reasonable costs and/or with a reasonable volume, and which is able to minimize the transmission of interferences.
For a wide band amplifier the ability to minimize the transmission of interferences is characterized by the SFDR parameter, i.e. the “spurious free dynamic range”. This parameter is expressed in dB. It is the ratio between the highest of the useful transmitted powers and the maximum stray signal in the useful frequency band. In the GSM standard this SFDR parameter must be at least equal to 75 dB and in the UMTS future standard this SFDR gain is set to be at least 70 dB.
The main solution which is known, up to now, for the linearization over a wide frequency band of a non-constant envelope signal is the feedforward technology wherein no feedback loop is provided. This technology corrects, in real time, the non-linearities, even if they are frequency dependent. The non-linearities are measured by a comparison with a linear signal before amplification and they are substracted in phase opposition from the output signal of the amplifier to linearize.
A conventional feedforward linearization circuit is represented schematically on FIG.
1
:
The digital signals to transmit are first processed by a digital input processor
10
and the processed digital signals are transmitted, through an analog chain
12
, to the input of a power amplifier
14
. For the linearization of such amplifier the output signal is transmitted to the first input
16
1
of a first subtractor
16
through an attenuator
18
. The second input
16
2
of subtractor
16
receives the input signal of amplifier
14
. The output of the first subtractor
16
is amplified by a residual error amplifier
20
, the output of which is connected to the input
22
1
of a second subtractor
22
, the other input
22
2
of which receives the output of amplifier
14
through a delay device
24
.
The purpose of the feedforward circuit, with subtractors
16
and
22
and amplifier
20
, is to linearize the amplifier
14
itself.
However in the transmission equipment non-linearities are also introduced by the analog chain
12
and error amplifier
20
, limiting the overall linearizing effect.
In order to correct the non-linearities introduced by analog chain
12
it is known to provide the circuit
10
with digital correction means which realize a digital predistortion. These correction means comprise, for instance, look-up tables which, in view of the output of amplifier
14
provide predistortion values. As shown on
FIG. 1
the output of amplifier
14
is connected to a corresponding input
10
1
of circuit
10
.
As the goal of the predistortion means in circuit
10
is to correct the non-linearities of the combination of analog chain
12
with amplifier
14
but not to correct independently the distortions introduced by the analog chain
12
, the amplifier
20
receives non linearized signals because the signals at the input of the amplifier
14
are distorted by analog chain
12
. Therefore the input signal of residual error amplifier is at a relatively high level, about 10% of the level of signals at the input of amplifier
14
. Moreover, as already mentioned, the amplifier
20
itself, which is non-linearized, may introduce non-linearities.
In brief the circuit represented on
FIG. 1
is power consuming and is unable to provide a quality which is sufficient for wide band transmissions.
The invention overcomes these drawbacks.
The invention provides also a transmission equipment for a base station of a cellular telecommunication system which can transmit, and receive, signals according to different standards, for instance signals with a multiplicity of carriers and with a multiplicity of codes.
SUMMARY OF THE INVENTION
The transmission circuit according to the invention comprises:
input digital processing means with predistortion means receiving input signals from the output of a power amplifier,
these digital processing means being connected to the input of the power amplifier through an analog chain, and
the power amplifier is associated with feedforward linearization means including a first subtractor which generates the difference between a signal representing the input signal of the amplifier and a signal representing the output signal of this amplifier, this difference being amplified by a residual error amplifier in order to be subtracted from the output signal of the power amplifier.
This transmission circuit is characterized by the fact that it comprises a reference branch for generating a signal representing the input signal of the amplifier which is provided to the first subtractor of the feedforward means, this reference branch providing a signal which is independent from the output signal of the power amplifier.
In other words the reference which is used for the feedforward circuit is independent from the predistortion, or precorrection, of the signal, or power, branch comprising the digital processing means and the analog chain. Therefore the reference signal provided to the first subtractor is not distorted by the predistortion in the signal branch and the output of the first subtractor may be at a lower level (for instance about 20 dB) than the output of the corresponding subtractor
16
in the conventional circuit represented on FIG.
1
. Moreover the error represents the residual difference between the reference signal and the predistorted amplified signal; therefore the better is predistortion, the lo
Alcatel
Choe Henry
Pascal Robert
Sughrue & Mion, PLLC
LandOfFree
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