Correlation analysis of nonlinear distortion

The amplitude of the output signal voltage of the amplifier circuit is limited by the saturation region and the cutoff region. Under the condition of given circuit parameters, the amplitude of output voltage without obvious distortion is called the maximum output amplitude, which is usually expressed by peak value or peak-to-peak value.

Limited by the saturation region, the maximum output voltage can only reach (UCEQ -UCES), and limited by the cutoff region, the maximum output voltage can only reach IC. Therefore, the maximum amplitude of the actual output voltage can only be twice the smaller value in (UCEQ-UCES) and IC (peak-to-peak).

The setting of static working point has great influence on the maximum output amplitude. In order to obtain a larger output amplitude, the Q point should be set near the midpoint of the AC load line. The distortion caused by transistor working in nonlinear region is called nonlinear distortion. The causes of nonlinear distortion come from two aspects: one is the nonlinearity of transistor characteristics; Second, the Q-point setting is inappropriate or the input signal is too large.

It is explained that the distortion caused by the high or low selection of Q point makes the transistor enter the saturation region or the cutoff region in part of the time of the input signal, which is called saturation distortion and cutoff distortion respectively.

In order to avoid the cutoff distortion caused by the instantaneous operating point entering the cutoff zone, we should:

IC≥ICm +ICEO GS02 18

In order to avoid saturation distortion caused by instantaneous operating point entering saturation region, we should make:

The multimedia of UCE≥Uom+ UCES GS02 1 wireless communication service is one of its future development directions, and the multimedia service needs high-speed data transmission to support it, so broadband transmission is the inevitable trend of wireless communication development. Orthogonal frequency division multiplexing (OFDM) technology can effectively resist the interference between signal waveforms, has excellent anti-noise performance and anti-multipath fading ability, and has high spectrum utilization rate, which is suitable for high-speed data transmission in wireless mobile channels with multipath propagation and Doppler frequency shift. With its inherent ability of resisting delay spread and high spectrum utilization, OFDM technology has quickly become a research hotspot and the core technology of the next generation wireless communication.

As we all know, OFDM signals have high peak-to-average power ratio, which requires high linearity of high power amplifier (HPA). Otherwise, nonlinear distortion will occur, which will lead to spectrum expansion and in-band signal distortion and worsen system performance. Therefore, it is necessary to suppress the nonlinear distortion of the system.

A distortion compensation technique combining partial transmission sequence (PTS) and recursive least squares (RLS) is proposed, which can effectively reduce the nonlinear distortion of high power amplifier. 2. 1 partial transmission sequence

Partial transmission sequence (PTS) divides each OFDM symbol into V sub-blocks, multiplies each sub-block by a phase factor, and then performs IFFT operation on X ′ (k) to obtain X ′ (n). The phase factor bi should be selected to make the peak-to-average power ratio of x ′ (n) the lowest.

2.2 Adaptive compensation

Therefore, the amplitude predistortion is realized by inverting the AM/AM characteristic curve of HPA, and the phase predistortion is realized by subtracting the AM/PM response of HPA from the phase of the original signal. Considering the OFDM system with the number of subcarriers N=256, the subcarriers are modulated by 16QAM, the number of PTS blocks is V=4, and the OFDM time domain signal is generated by adjacency method, δ = 0.004, λ=l, ωA(O)=0, ωP(0)=0. In communication systems, the predistortion performance is usually independent of multipath fading, so it is assumed that the channel is an ideal additive white Gaussian noise channel, there is no intersymbol interference, and the clocks of the transmitter and receiver are accurately synchronized.

Where Pmax represents the maximum output power of the amplifier and po represents the average power of the output signal of the amplifier. Fig. 2 shows signal constellations of receivers with and without predistortion under different output power compensation conditions. It can be seen that the predistortion can effectively compensate the nonlinear distortion caused by the power amplifier (Figure 2(a) and (b)). At the same time, it can be seen that with the decrease of output power back-off, the high power amplifier enters the limit region. At this time, even pre-distortion can not completely eliminate the nonlinear distortion introduced by the power amplifier (Figure 2(c) and (d)).

When obo = 4.5 db, the error rate curve of the system with or without predistortion is shown in Figure 3. In order to make the high power amplifier work most effectively, it is necessary to compromise between the maximum output power of the amplifier and the minimum nonlinear distortion of OFDM signal. Therefore, the total degradation TD(TotalDegTadation) of the system is defined as:

Where Eb/No(HPA) represents the low EB/NO required when using a nonlinear power amplifier under a specific bit error rate condition; Eb/No(AWGN) represents the minimum EB/no required by a pair of Philippine linear power amplifiers without using the same bit error rate. The total degradation of the system changes with the change of output power regression, and there is a minimum value. The corresponding OBO value is called optimal power backoff, and its value is usually used to evaluate the performance of distortion compensation algorithm.