Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
2003-01-16
2004-08-10
JeanPierre, Peguy (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C341S120000, C341S118000
Reexamination Certificate
active
06774834
ABSTRACT:
The field of the invention is that of signal processing, more specifically in the context of correcting distortion produced by an amplifier system. Correction involves comparing the characteristics of the output signal with the original signal at the input of the amplifier system in order to determine the distortion that the system produces. This makes it possible to respond by calculating inverse predistortion to be applied to the input signal to cancel the distortion and thereby improve the linearity and the electrical efficiency of the amplifier system.
The invention is based on the observation that, because the amplifier system introduces a time-delay, the comparison is not applied to the signal and to its exact replica (ignoring the intrinsic distortion of the system), and that this leads to non-negligible error and puts a theoretical limit on how well predistortion can be calculated.
BACKGROUND OF THE INVENTION
Existing predistortion techniques do not address this problem of error associated with the time-delay between the signals compared.
Because the signals are generally compared using digital techniques, it is necessary to digitize at least the analog output signal of the amplifier system. The digitized output signal at a given time t0 is the replica, distorted to a greater or lesser degree by the amplifier, of the signal on the input side of the system at time t0-&dgr;, where &dgr; is the time-delay caused by the system.
Note further that the time-delay is not fixed, and can vary with aging of the system, its temperature, the amplified signal, the power, the frequency band used, etc., which makes correction by applying a single fixed time shift to the comparison signal (reference signal) from the input side of the amplifier system somewhat ineffective.
FIG. 1
is a simplified diagram of the main components of a conventional amplifier system for a radio transmitter in which nonlinearity is corrected by predistortion. The system
2
includes a digital predistortion unit
4
which receives a digital signal X to be amplified at a first input
4
a
, in this instance from a digital modulation source
6
. The unit
4
adds predistortion to the digital signal X, thereby modifying it to produce a predistorted signal X′ at the unit's output
4
b
. The signal X′ is then converted into analog form by a digital-to-analog converter (DAC)
8
whose output feeds a transmit frequency converter stage (TX-IF and TX-RF)
10
which drives the power amplifier
12
, which is connected to a transmit antenna (not shown).
To make the comparison, the output signal Y of the power amplifier
12
is fed back via a loop
14
to a second input
4
c
of the predistortion unit
4
. The loop includes a radio frequency (RF) head
16
whose input receives the output signal of the power amplifier
12
and supplies it in a suitable form to a receiver frequency converter stage (XR-IF and XR-RF)
18
whose function is the inverse of that of the converter stage
10
(even if the intermediate frequencies are not the same). The output of the converter stage
18
is digitized by an analog-to-digital converter (ADC)
20
which supplies the digitized signal Y to the second input
4
c
of the predistortion unit.
The DAC
8
and the ADC
20
are clocked by a signal &PHgr; spl at respective clock inputs E&PHgr;. The signal &PHgr; spl fixes the period of each conversion of the instantaneous digital value of the signal X for the DAC and the period of each digitization of the instantaneous analog value of the signal detected at the output of the power amplifier
12
, i.e. the sampling rate. In this example, the signal &PHgr; spl is supplied in common to the DAC
8
and the ADC
20
by a phase-locked loop (PLL)
21
fed by a reference clock &PHgr; ref.
Note that the signals X and Y compared by the predistortion unit are subject to varying time shifts which can exceed the period of the clock &PHgr; spl and whose exact values do not correspond to an integer number of periods of that clock.
In the prior art, the base band output Y from the ADC
20
is compared directly to the signal X, with no further precautions.
This results in limitations on performance caused both by the intrinsic calibration in the factory of phase alignment and most importantly of sampling time alignment between sending (sampling in the DAC
8
) and receiving (sampling in the ADC
20
), and also by inherent propagation times in the system, which are non-negligible and liable to vary, in particular because of the presence of analog filters. Although the time shifts for the digital signals can be controlled, the problem is much more difficult in the analog domain.
This defective time alignment is particularly prejudicial to obtaining good predistortion performance at reasonable cost in modern transmitter stations, such as those used in third generation cellular systems. These systems use code division multiple access (CDMA) coding at the radio interface. For reasons of cost and size, the base transceiver stations (BTS) use only one transmit radio power amplifier
12
to transmit the signals of all users on one or more carriers. In this context, the term “transmission” refers to the complete transmission system in the base transceiver station, which includes the digital information and signal processing part and the purely analog part including in particular the power amplifier; the term “radio” refers to the part where radio emission proper takes place which comprises the last link of the chain.
The error in the predistortion calculated by this time shift could be corrected using algorithms, but this would require expensive calculation resources and would be based on approximations and extrapolations that would impose a fundamental limitation on accuracy.
Optimum use of new predistortion techniques allied with peak limiting techniques can achieve power amplifier efficiencies up to 15% to 17%.
This represents a significant advance over conventional amplification techniques known as “feedforward” techniques, the efficiency of which is limited to around 8%. Also, it reduces costs and substitutes digital processing for the complex techniques used in the high power radio frequency and analog stages.
However, to accommodate the many different types of power amplifier and transistor available, and to minimize, or even eliminate, calibration in the factory, the predistortion system must be adaptive so that it can act dynamically to track and correct changes in the transfer function of a power amplifier, especially variations in nonlinearity over the frequency band, in the number of carriers, in temperature, in aging, in memory effects (remanence), etc.
The best and fastest methods are based on conversion tables which construct the inverse of the nonlinearity of a power amplifier and apply it to the modulation signal to reproduce at the power amplifier output the required original signal with the best spectrum masking on adjacent channels.
However, and whichever variant is employed, the algorithms used to update the predistortion table are based on very linear wideband receivers producing a good copy of the radio signal as transmitted. Updating the predistortion table is based on comparing the incoming signal and the outgoing signal, that is to say on the correlation between the incoming signal and the outgoing signal. The greater the radio transmit band and the greater the band to be linearized (at least three to five times the instantaneous transmission band), the more directly is linearization gain linked to the accuracy of the comparison. Most of these methods, based on least mean squares (LMS) algorithms, are relatively insensitive to the quadrature and the gain of the signals to be compared. However, the major problem is the accuracy of the time-delays between the signals compared. In fact, the problem is in all regards identical to that of wideband time-delay locking for a “feedforward” amplifier, except that it occurs in the digital domain in the case of predistortion. In this case, the difficulty of the problem increases as the width
Evolium S.A.S.
Jean-Pierre Peguy
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