2.1 Nonlinear property of power amplifier
Power amplifier due to its circuit characteristics, the input power increases continuously, but the corresponding output does not always show a linear amplification trend. The amplifier will gradually move from the linear region to the saturated region. It will cause harmonic distortion and intermodulation distortion and affect the transmission quality of the signal.
2.1.1 harmonic distortion
As a nonlinear device, radio frequency (RF) power amplifier can generate parasitic signals of integral frequency of the main signal frequency when amplifying the signal. Assume that the input signal is a monophonic signal:
The transfer function of RF power amplifier is defined as:
For the convenience of derivation, only the first three steps are taken and the input signal phase is assumed to be zero. At this time, the input signal is substituted in formula (2), then we can get an output expression as:
From the results, the ideal monophonic signal after nonlinear transformation will get integral multiples of harmonic components 2w,3w for the input signal frequency w. If these harmonics are not processed, the communication quality of other frequencies will be affected. But because the harmonic frequency is often far away from the main frequency, the harmonic can be filtered through a filter.
2.1.2 intermodulation distortion
In addition to the harmonic components, it is the intermodulation components that cause the distortion in and out of the band. The intermodulation component is different from harmonics in that its frequency component is often close to the main frequency and is difficult to be filtered by a filter.
Assume that the input dual-tone signal of the RF power amplifier is:
For the convenience of analysis, formula (4) can be simplified at (o1 = 0, o2 = 0) and A1=A2=1. After simplification, the input dual-tone signal can be expressed as:
Since the input and output characteristics of the power amplifier are nonlinear, the transfer function of the RF power amplifier can be defined as:
Only the third order is used for expansion, and the specific expansion form is as follows:
It can be seen that after the nonlinear amplification of dual-tone signals w1 and w2, we can obtain the first-order components, second-order components, third-order components and intermodulation components. After the signal passing through the power amplifier, in addition to the fundamental wave and frequency doubling, the mixing component is generated by the interaction between w1 and w2. So the ideal two-tone signal, after passing through the RF power amplifier, will derive the far-end harmonic component and the near-end intermodulation component.
2.1.3 memory effect
As RF power amplifier, besides nonlinear amplification of signal amplitude, it also has memory effect. In the application of narrowband signal power amplifier system, the memory effect is not obvious, but with the change of modulation mode and the continuous broadening of signal broadband, the memory performance of RF power amplifier is obvious. With the increasing requirement of communication transmission rate, the signal modulation is more and more complex, and the signal bandwidth is wider and wider. Especially for 5G amplifier, the wider the bandwidth, the higher the signal frequency, the more serious the memory characteristics will become. The AM/AM characteristic curves of signals with bandwidth of 25MHz, 50MHz, 75 MHz and 100 MHz working on RF power amplifier are respectively shown in figure 1.
It can be clearly seen that as the bandwidth gets wider and wider, the divergence degree of AM/AM characteristic graph becomes more serious and the dynamic nonlinearity becomes more obvious. Due to the strong dynamic nonlinear characteristic of high bandwidth signal, the dynamic modeling ability of the traditional model to the amplifier will be greatly reduced, so it is necessary to seek for a better model for it. Because of the better linear fitting ability of the long short-term memory neural network, we suggest using LSTM to build and simulate the dynamic nonlinear characteristics of 5G power amplifier.
2.2 digital pre-distortion
The basic principle of digital pre-distortion is to cascade a module before the input of the RF power amplifier, which is opposite to the characteristics of the RF power amplifier, so as to reverse the original input signal into the amplifier, and then input the reverse-distortion signal to the RF power amplifier as an input signal. Through the forward distortion of the RF power amplifier, the distortion effect of the RF power amplifier and the distortion effect of the pre-distorter can exactly cancel each other, so as to output the linearly amplified signal.
The input and output signals of the RF power amplifier are obtained from the experimental apparatus. The pre-distorter will embody the reverse characteristic of the RF power amplifier by compensating the input signal. At this time, the computer is regarded as a predistorter, and the compensated signals are converted into .wv format, which can be loaded into the signal generator. Then, the RF power amplifier is used to observe the improvement degree of the spectrum from the spectrum analyzer. After adjusting parameters, collecting data, analyze and sort out. We can get the better pre-distortion effect to correct the nonlinear characteristics of PA.
However, the current important digital pre-distortion test platform by the hardware infrastructure is expensive on the market. The data acquisition of RF power amplifier is limited which seriously restricts the digital pre-distortion technology related research.
Based on modern internet technology, relying on automatic control concept, computer cloud technology, we propose a method to design a power amplifier pre-distortion data acquisition and verification cloud platform. The platform realizes remote control of intelligent devices and remote monitoring of device status and remote acquisition of power amplifier data. LSTM network is trained through cloud big data for power amplifier modeling. Thus, it can obtain the efficient and accurate digital pre-distortion effect and the nonlinear characteristics of broadband power amplifier can be corrected, so as to improve the transmission efficiency of 5G signal.