RF and Microwave Power
Amplifier and TransmitterTechnologies. Feedforward and Predistortion
Amplifier and TransmitterTechnologies. Feedforward and Predistortion
FEEDFORWARD
The very wide bandwidths (10 to 100 MHz) required in multicarrier applications can render feedback and DSP impractical. In such cases, the feedforward technique can be used to achieve ultralinear operation. In its basic configuration, feedforward typically gives improvements in distortion ranging from 20 to 40 dB.
Operation
In its basic form, a feedforward amplifier consists of two amplifiers (the main and error amplifiers), directional couplers, delay lines and loop control networks [110]. The directional couplers are used for power splitting/combining, and the delay lines ensure operation over a wide bandwidth. Loop-control networks, which consist of amplitude and phase-shifting networks, maintain signal and distortion cancellation within the various feed- forward loops.
The input signal is first split into two paths, with one path going to the high-power main amplifier while the other signal path goes to a delay element. The output signal from the main amplifier contains both the desired signal and distortion. This signal is sampled and scaled using attenuators before being combined with the delayed portion of the input signal, which is regarded as distortion-free. The resulting "error signal" ideally contains only the distortion components in the output of the main amplifier. The error signal is then amplified by the low- power, high-linearity error amplifier, and then combined with a delayed version of the main amplifier output. This second combination ideally cancels the distortion components in the mainamplifier output while leaving the desired signal unaltered.
In practice, there is always some residual desired signal passing through the error amplifier. This is in general not a problem unless the additional power is sufficient in magnitude to degrade the linearity of the error amplifier and hence the linearity of the feedforward transmitter.
Signal Cancellation
Successful isolation of an error signal and the removal of distortion components depend upon precise signal cancellation over a band of frequencies. In practice, cancellation is achieved by the vector addition of signal voltages. The allowable amplitude and phase mismatches for different cancellation levels are shown. For manufactured equipment, realistic values of distortioncancellation are around 25 to 30. The limiting factor is nearly always the bandwidth over which a given accura- cy can be obtained.
Efficiency
The outputs of the main and error amplifiers are typically combined in a directional coupler that both isolates the PAs from each other and provides resistive input impedances. For a typical 10 dB coupling ratio,90 percent of the power from the main PA reaches the output. For the same coupling ratio, only 10 percent of the power from the error amplifier reaches the load, thus the error amplifier must produce ten times the power of the distortion in the main amplifier. The peak-to-average ratio of the error signal is often much higher than that of the desired signal, making amplification of the error signal inherently much less efficient than that of the main signal. As a result, the power consumed by the error amplifier can be a significant fraction (e.g., one third) of that of the main amplifier. In addition, it may be necessary to operate one or both amplifiers well into back-off to improve linearity. The overall average efficiency of a feedforward transmitter may therefore be only 10 to 15 percent for typical multicarrier signals.
Automatic Loop Control
Since feedforward is inherently an open-loop process,
changes in device characteristics over time, temperature, voltage and signal level degrade the amplitude and phase matching and therefore increase distortion in the transmitter output. An automatic control scheme continuously adjusts the gain and phase to achieve the best signal cancellation and output linearity. The first step is to use FFT techniques, direct power measurement, or pilot signals to determine how well the loop is balanced. Both digital and analog techniques can be used for loop control and adjustment. Signal processing can be used to reduce the peaks in multicarrier signals and to keep distortion products out of the nearby receiving band [111].
Performance
An example of the use of feedforward to improve lin- earity is shown in Figure 48. The signal consists of a mix of TDMA and CDMA carriers. The power amplifiers are based upon LDMOS transistors and have two-tone IMD levels in the range –30 to –35 dBc at nominal output power. The addition of feedforward reduces the level of distortion by approximately 30 dB to meet the required levels of better than –60 dBc. The average efficiency is typically about 10 percent.
8c. PREDISTORTION
The basic concept of a predistortion system involves the insertion of a nonlinear element prior to the RF PA such that of both is linear.Predistortion can be accomplished at either RF or the combined transfer characteristic baseband.
RF Predistortion
The block diagram of a simple RF predistorter is shown. A compressive characteristic, created by the nonlinearity in the lower path (e.g., a diode) is subtracted from a linear characteristic(the upper path) to generate an expansive characteristic. The output of the linear path (typically just a time delay)is given by:
vl(vin) = a1vin (1)
and that of the compressive path is given by
invc(vin) = a2vin – bv3 (2)
Subtracting the above equations gives
invpd(vin) = (a2 – a2) vin – bv3 (3)
This is now an expansive characteristic with a linear gain of a1 – a2, and may be used to predistort a compres-
sive amplifier characteristic (cubic in this example) by
appropriate choice of a1, a2 and b.
An example of the results from using a simple diode-
based RF predistorter with a 120-W LDMOS PA amplifying an IS-95 CDMA signal is shown. When applied to π/4-DQPSK modulation in a satellite application, the same predistorter roughly halves the EVM, improves the efficiency from 22 to 29 percent, and doubles the available output power.
Predistortion bandwidths tend to be limited by similar factors to that of feedforward, namely gain and phase flatness of the predistorter itself and of the RF PA. In addition, memory effects in the PA and the predistorter limit the degree cancellation, and these tend to become poorer with increasing bandwidth.
Better performance can be achieved with more complex forms of RF predistortion such as Adaptive Parametric Linearization (APL®), which is capable of multi-order correction [106]. Most RF-predistortion techniques are capable of broadband operation with practical operational bandwidths similar to, or greater than, those of feedforward.
Digital Predistortion
Digital predistortion techniques exploit the considerable processing power now available from DSP devices, which allows them both to form and to update the required predistortion characteristic. They can operate with analog-baseband, digital-baseband, analog-IF, digital-IF,or analog-RF input signals.Digital-baseband and digital-IF processing are most common.
The two most common types of digital predistorter are termed mapping predistorters [107] and constant-gain predistorters [108]. A mapping predistorter utilizes two look-up tables, each of which is a function of two variables (IIN and QIN), as shown . This type of predis- torter is capable of excellent performance. However, it requires a significant storage and/or processing overhead for the look-up tables and their updating mechanism, and has a low speed of convergence. The low convergence speed results from the need to address all points in the I/Q complex plane before convergence can be completed.
A constant-gain predistorter requires only a single-dimensional look-up table, indexed by the signal envelope. It is therefore a much simpler implementation and requires significantly less memory for a given level of performance and adaptation time.It uses the look-up table to force the predistorter and associated PA to exhib-it a constant gain and phase at all envelope levels. The overall transfer characteristic is then linear:
GPD(IIN(t),QIN(t))×GPA(IPD(t),QPD(t)) = k (4)
An example of the improvement in the amplitude-transfer characteristic by an RF- input/output digital predistorter [109]. The plot is based upon real-time using samples from a GSM-EDGE signal. Both the gain expansion and compression are improved by the linearizer. EVM is reduced from around 4.5 to 0.7 percent. The ACPR for IS-136 DAMPS modulation
(π/4- DQPSK) is reduced by nearly 20 dB. When generating mask-compliant EDGE modulation at full out- put power (850-900 MHz), the linearized PA has an efficiency of over 30 percent. An example of linearization of a PA with two 3G W-CDMA signals by a digital baseband-input predistorter. The linearized amplifier meets the required spectral mask with a comfortable margin at all frequency offsets. The noise floor is set by the degree of clipping employed on the waveform, which limits the ACPR improvement obtained. It clearly demonstrates, however, that digital predistortion can be used in broadband as well as narrowband applications.
Neyker Stewart Zambrano
CRF
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