Figure 1. Simplified microwave application system block diagram using the SC1894.
Figure 2. ATPC Power Ramp Sequence when SC1894 is active in Track Mode.
Figure 3. Power Ramp Sequence when SC1894 correction is frozen at max power.
This document is an introduction to linearizing a microwave transmission-system PA using Maxim's SC1894 RF PA Linearizer (RFPAL) from 4GHz to 80GHz. It includes basic system design considerations to implement a linearized microwave amplifier and the effects of automatic transmit power control (ATPC) on the SC1894 performance.
Digital microwave transmission systems require low bit-error rates (BER) and must not transmit excessive adjacent-channel power (ACP), which would interfere with other wireless services. With respect to transmitter signal fidelity, BER is directly affected by the modulated-waveform error-vector magnitude (EVM). Reducing the modulated-waveform EVM is a primary goal in all transmitter design and the final stage power amplifier (PA) can degrade the EVM when operating near maximum power creating intermodulation products (IMD). These IMD products are the dominant components of ACP. A traditional solution to avoid the IMD problem is to back off (reduce) the power until the IMD distortion is at an acceptable level. Using backoff results in an inefficient transmitter design and, depending on the waveform, one cannot achieve the desired linearity specification regardless of how far the PA is backed off.
Maxim's SC1894 can improve the PA linearity and minimize EVM, IMD, and ACP levels without sacrificing the PA output power. Furthermore, predistortion linearization allows a PA to operate at significantly higher efficiency compared to another PA operating in backoff with similar output power levels. In the case of GaN devices that can be inherently nonlinear, predistortion is often the only available method to achieve the required EVM and ACP levels. By enabling operation at maximum PA output power while minimizing EVM and ACP, the SC1894 devices provide the system with lower BER, better efficiency, and lower system and operating costs.
IMPORTANT: In order to achieve optimal performance, refer to  in the References table.1.2 References
|1||ETSI 302 217-1 V1.3.1 (2010-01) regarding Automatic Transmit Power Control (ATPC)|
|2||ETSI 302 217-2-2 V1.4.1 (2010-07)|
|3||SC1894 data sheet|
Microwave point-to-point manufacturers have products that cover many frequency ranges from 4GHz to 80GHz. These radios transmit a fully occupied single carrier with an instantaneous signal bandwidth ranging from 3.5MHz to 112MHz. Modulation modes vary from BPSK to 1024QAM and peak-to-average ratios (PAR) of 4dB to 8dB. As the signal bandwidth and modulation complexity increase, so does the need for predistortion in order to meet system linearity requirements. The system output power is typically controlled through lookup tables calibrated at the OEM factory. The power-control lookup table is used to control the various VGA gains in a desired sequence. These lookup tables are then used to set the antenna output power at each operating frequency, temperature, and power level. Transmit output power is typically operated over a 25dB power range.2.1 System Block Diagram
To use the SC1894 in microwave systems operating at frequencies greater than 3.8GHz, the SC1894 must be placed in the IF section of the transmitter as shown in Figure 1. (Typical microwave systems operate using an IF at frequencies between 350MHz to 3.5GHz.)
The application represented in Figure 1 can be adapted to many different transmitter configurations having different output powers, gains and frequencies. Due to the many variations possible, the exact implementation of ALC1, ALC2, and ALC4 is left to the discretion of the system designer. These functions can be implemented as either fixed or variable gain amplifiers/attenuators. The main requirement is that the elements in the up-conversion and feedback down-conversion paths do not significantly contribute to the overall distortion of the system so that the SC1894 only compensates PA-related nonlinearity.
System-level Linearity Guidelines:
Component-level description and guidelines:
All system power changes are sensed at the RFFB input.
All filters should have flat gain and group delay over the total IMD frequency range. Gain or group delay variation increases memory2 effects; thus, reducing correction. The SC1894 injects an “inverse IMD” correction signal into the output PA and this signal should be passed through the up-conversion chain with little or no distortion. Likewise, the SC1894 measures the PA IMD using the down conversion path and any distortion on this path reduces the accuracy of this IMD measurement.
Point-to-point microwave link propagation losses are nominally low by design, with the radio operating at an output level well below maximum power. This is by regulatory requirement to reduce interference while still maintaining the needed SNR or BER level for transmission integrity. However, operating the link with just enough power to maintain good BER makes the system vulnerable to fading conditions such as heavy rain and multipath. When a fading condition is detected, ATPC compensates for the fading condition by increasing the PA power up to 100dB/s in order to maintain the SNR/BER rate required by the system.
IMD is a function of PA power. As the power increases, the predistortion system must track or adjust to the new operating point as fast as possible. The rate of power change and the location of gain change are the key parameters driving special processing by the SC1894. See Figure 2 and Figure 3 for an overview of the ATPC power-ramp sequences.
To accommodate ATPC power-level changes when the PA is actively linearized, Maxim developed specialized firmware (FW) that allows the SC1894 to better correct ACP during and after PA power changes. In optimized (default) mode, any change of the power level into RFFB initiates a full recalibration of the internal SC1894 gain settings. Maxim has developed a fixed gain mode called "smooth adaptation" mode. In this mode, there is no recalibration of SC1894 internal gain when the power is changed.
For this discussion, the two locations in a microwave transmission system where RF power changes can occur are before and after the SC1894. When the RF power changes before the SC1894 RFIN port, we call this "RFIN ATPC". When power changes after the RFOUT port, we term this; "RFOUT ATPC."
Both RFOUT ATPC and RFIN ATPC require the use of smooth adaptation mode.4.1 Option 1: RFOUT ATPC
With RFOUT ATPC, the PA power is changed using ALC2 and ALC4 (see Figure 1). Changing PA power with ALC2 and ALC4 also changes the predistortion signal from the SC1894, requiring new correction settings to be calculated and incurring a delay in adjusting to the new power level. To avoid recalculating correction with every power-level change, Maxim has developed FW that performs a look up table (LUT) function using factory-trained correction coefficients. The FW also continuously saves the best correction settings at each power level as it operates in the field, updating the LUT with these settings.
Best solution for system signal to noise ratio, but not the best for dynamic behavior.4.2 Option 2: RFIN ATPC (Ideal For the SC1894)
Smooth adaptation mode allows the SC1894 to continue uninterrupted ACP correction, but only when power is changed before RFIN using ALC1. Using RFIN ATPC and Smooth Adaptation mode, the RF input power to the SC1894 tracks the PA input power, dB-for-dB. Likewise, the SC1894 correction signals (generated from RFIN using Volterra series coefficient multiplication) tracks the PA IMD levels almost exactly. This feature greatly improves the dynamic power performance of the SC1894 predistortion. An LUT is also implemented with RFIN ATPC, but the variations of correction coefficients (for the Volterra series) are very small and the performance of RFIN ATPC with either the LUT or frozen coefficients at maximum PA power is almost the same.
(Performance of RFIN ATPC with standard wireless FW when coefficients are frozen at maximum power is very similar. However, reading and writing an LUT is not available with this FW so the ATPC FW is a better choice when an LUT function is required.)
With the SC1894 smooth adaptation mode, RFIN ATPC power changes using ALC1 before RFIN providing the best dynamic behavior, but not ideal for system signal-to-noise ratio.4.3 Option 3: RFIN/RFOUT ATPC (Recommended)
Since a power amplifier is able to meet most linearity requirements when operated in backoff, in general, only about the top 3dB to 6dB of the PA power range requires predistortion. Below this point, the SC1894 correction can be frozen until the power returns to this range. ALC1 is used for at least the first 3dB to 6dB of PA backoff from maximum power. Thereafter, ALC2 and ALC4 can be used if adaptation is frozen.
Option 3 allows the microwave system architect to allocate the ATPC gain reduction to different parts of the system to maintain the minimum required system signal-to-noise ratio and achieve best dynamic range performance.4.4 Smooth Adaptation Mode
Smooth mode is enabled by factory calibrating the SC1894 when the system is operating at maximum RMS power and maximum PEP.
Often, a standby system is deployed in parallel to the main system. The standby system PA is therefore inactive for long periods of time, but can be driven to PMAX at any time. The same FW used to perform the LUT function for ATPC can be used for this case to accelerate convergence time of the backup PA by properly seeding the coefficients before enabling adaptation. Any LUT must have frequency, modulation, power, and temperature as indices.Footnotes