| Frequency-hopping communication has the characteristics of high concealment and good anti-interference because the signal is at different frequency points at each time.It can ensure that the normal communication between the two sides of the communication is not interfered and tracked.Therefore,it is widely used in the military field,and then gradually extended to civil communication.In recent years,with the rapid development of wireless communication technology,high-order modulation is used to improve the efficiency of frequency-hopping communication,which makes the peak-to-average power ratio to the signal increase day by day,resulting in serious nonlinear distortion after the frequency hopping signal passes through the power amplifier in the transmitter,and finally the communication cannot be carried out normally.This problem needs to be solved urgently.At present,digital predistortion is often used to solve the nonlinear distortion problem of power amplifier,but the relevant research is aimed at the traditional communication with fixed frequency point,while the research on frequency hopping communication with time-varying frequency point is less.Based on this background,this thesis studies the frequency-hopping digital predistortion scheme.Firstly,the basic principle and important parameters of frequency-hopping communication are introduced,and the nonlinear characteristics,common predistortion models and parameter identification method of power amplifier are also explained.Then it is analyzed that the particularity of frequency-hopping communication compared with traditional fixed frequency communication: because the carrier frequency is changing at all times,the power amplifier will show different nonlinear distortion at different frequencies,so predistortion technology cannot be directly applied to frequency hopping communication system.Secondly,the baseband frequency hopping scheme is adopted to replace the traditional local oscillator hopping scheme,and the baseband frequency-hopping signal is directly transmitted,which reduces the cost and complexity of the system.On this basis,by controlling the three key parameters of frequency-hopping points,frequency-hopping interval and frequency-hopping rate,two experimental schemes are used to observe the distortion of power amplifier,and then explore the influence of frequency hopping communication mode on the nonlinear distortion of power amplifier.According to the ACPR and NMSE indexes obtained from the experimental results,for the fixed frequency point at a certain time,there is little difference between the frequency hopping communication mode and the nonlinear distortion of the power amplifier under the traditional communication mode,which provides a basis for the subsequent frequency hopping predistortion compensation technology.Finally,two frequency-hopping digital predistortion schemes are proposed: integral predistortion and ergodic predistortion.The essence of both is to use a predistorter to compensate the frequency-hopping signal nonlinearly.The difference lies in which signal is used for training.The former uses a broadband signal containing all frequency hopping points,while the latter uses a new signal spliced by all frequency hopping points.In this thesis,a test platform with thousands of hops per second is built.The 64 QAM signal with a bandwidth of 120 k Hz,four frequency hopping points and an interval of 500 k Hz is used to simulate the short-wave frequency-hopping communication at the carrier frequency of 5 MHz,and the two digital predistortion schemes are verified.The results show that under the two schemes,the nonlinearity of four frequency hopping points is compensated,and the ACPR is reduced from-35 dBc to-48 dBc and-50 dBc respectively,which meets the communication standard.Different frequency hopping predistortion schemes are compared. |
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