Elimination And Active Utilization Of Nonlinearity In Superheterodyne Digital Receiver

As the commonly used wideband superheterodyne digital receiver does not possess the performance of a high linear dynamic range under multiple signal excitation, a new type of adaptive wideband digital receiver architecture is proposed and designed based on blind nonlinear system identification. The traditional narrowband linear receiving and channelizing technologies should not be applied to deal with the complicated multiple-signal excitation with unknown or time-varying characteristics of time domain or frequency domain. Here, the harmonic and the intermodulation components brought about by the wideband digital receiver are firstly identified and extracted in the frequency domain, then a blind identification criterion for minimizing the short-time energy of the nonlinear components is designed, and the steepest descent method or the recursive least square algorithm is applied to extract and update iteratively the parameters of the nonlinear behavior model of the wideband digital receiver. The updated model is utilized to compensate the nonlinear distortion of the receiver in real time. Secondly, a new type of transmission technique based on nonlinear modulation and demodulation by actively utilizing the nonlinear characteristic in transmission link of communication system is proposed and verified. The nonlinear distortion is used to create obstacles at the information receiving and judging process. Therefore, the nonauthorized user can not attack the communication system effectively. The specific investigations can be concluded as follows:1、 The nonlinear mechanism of the superheterodyne digital receiver is described by means of theoretical analysis and quantitive calculation. It is proved that the linear term and second-order term should not be included in the nonlinear compensation model when compensating the nonlinearity of the superheterodyne digital receiver. It can be concluded that the low-order nonlinear polynomials are the key factors to deteriorate the spur-free dynamic range (SFDR) performance of the entire wideband digital receiver including the RF front end. The construction method of the nonlinear compensation model for the superheterodyne digital receiver is proposed. The method to reduct the computation complexity of the nonlinear compensation model and the method to increase the fitting precision of the nonlinear system for the model are also proposed. The computation complexity and the fitting precision of several memory nonlinear models are compared by means of theoretical analysis and simulation experiment.2、 A new type of identification and compensation algorithm for the nonlinear distortion of the superheterodyne digital receiver is proposed. A blind identification criterion for minimizing the short-time energy of the nonlinear components is designed. The applicability of the criterion for the superheterodyne type and direct-sampling type receiver is explained. Four different types of adaptive iteration algorithms are applied to extract and update the parameters of the nonlinear behavior model. The computational complexity of the nonlinear blind identification algorithm using these four types of adaptive iteration algorithms is compared. The expected improvement for the SFDR of the superheterodyne digital receiver is analyzed by simulation. It can be concluded that the low-order nonlinear distortion components can be suppressed to the noise floor. The performance of the nonlinear blind identification and compensation algorithm is verified by different types of input signal and different types of digital receiver systems. It can be concluded that the algorithm in this dissertation is correct and effective. The experimental results show that the SFDR of the blind identification digital receiver achieves about15-20-dB higher than that of the traditional receiver under multitone or bandpass signal excitation. The blind identification receiver architecture is helpful for detecting weak signal in concomitance with in-band or out-of-band strong jammers. The convergence precision, convergence speed, stability, and applicability for arbitrary signal of the algorithm are acceptable.3、 The optimization design to improve the robustness of the nonlinear blind identification algorithm for the superheterodyne digital receiver is carried out. In order to avoid the numerical unstability caused by the recursive least square (RLS) algorithm, the method to improve the convergence speed and convergence precision of the adaptive calculation for the nonlinear compensation model is proposed. The optimization design of the blind identification criterion is also proposed for the numerous strong signals situation when using wideband superheterodyne digital receiver. The hardware implementation architecture of the nonlinear blind identification and compensation algorithm is optimized for the real-time processing requirement of the digital receiver system. The implementation architecture of pipelined real-time compensation module based on Volterra series and the implementation architecture of adaptive iteration module for Volterra kernel vector are both designed. The method of detection and avoidance for the numerical overflow of the fixed-point implemented algorithm is proposed.4> A new type of transmission technique based on nonlinear modulation and demodulation is proposed. The difference between the active nonlinear transform technique and the traditional secure communication technique is explained theoretically. The existence of nonlinear transform pair is discussed. The procedures of active memory nonlinear transform and nonlinear inverse transform are specified. The improved nonlinear blind identification criterion designed for the cooperative receiver is proposed against the possible spectrum spread phenomenon and nonlinear components lost phenomenon. The computational complexity of nonlinear inverse transform for the cooperative receiver is analyzed with the type, order, and memoey depth of nonlinear model for nonlinear transform known. With such information unknown, the computational complexity of nonlinear inverse transform for the noncooperative receiver is also analyzed. The optimization design to reduct the computation complexity of the algorithm and to improve the anti-interception ability is carried on. It can be concluded from the analysis and simulation results that the nonlinear modulation and demodulation based transmission technique has impressive effect on data covering for the signal to be transmitted. The computational complexity of the technique can be under control for the cooperative receiver. It is very hard for the noncooperative receiver to attack the communication system.