The Design And Simulation Of High Spectrum Efficiency Band-Limited Communication System

With the rapid development of modern society,spectrum resources in the communication field have become increasingly scarce.How to improve the utilization of frequency band resources has gradually become the focus of people's research.At the same time,in communication systems such as satellite communications and deep space communications,transmitter power amplifiers are generally required to operate in a fully saturated mode in order to generate sufficient transmit power to ensure that signals can be accurately transmitted to the receiving end.However,the power amplifier working in this state has non-linear characteristics,so the modulated signal is required to have the characteristics of constant envelope or quasi-constant envelope to reduce the occurrence of spectrum spreading.In communication systems,the spectral characteristics of the modulated signal,such as its bandwidth and out-of-band energy radiation,depend mainly on the form of the baseband symbol signal.However,there are few research methods for optimizing the form of baseband symbol signals based on mathematical models.In this thesis,a mathematical model is established from the perspective of improving the frequency band utilization rate and the modulated signal has constant envelope or quasi-constant envelope characteristics to optimize the baseband symbol signals in full response and partial response quadrature modulation systems.The purpose is to make the spectrum corresponding to the optimized baseband symbol signal have good spectral characteristics and the modulated signal has the characteristics of constant envelope or quasi-constant envelope.According to the optimization theory,by introducing the minimum out-of-band energy radiation criterion,an optimal mathematical universal function model is established.The energy-limited condition of a single symbol signal,the boundary condition of a symbol,and the condition that the modulated signal has a constant envelope characteristic are added to the universal function model.The Chebyshev polynomial is introduced to represent the optimized baseband symbol signal,and MATLAB is used to realize the numerical solution of the optimized baseband symbol signal.According to the obtained optimized baseband signal,the time-frequency characteristic curve is drawn and verified that the modulated signal has the characteristics of constant envelope or quasi-constant envelope.At the same time,in the frequency domain,specific numerical comparisons are also performed with MSK and QPSK signals.In addition,the factors affecting the frequency-domain characteristics of the optimized baseband symbol are also discussed quantitatively based on specific values.For the full response system and the partial response system,corresponding orthogonal modulation and coherent demodulation schemes are proposed.The simulations of the orthogonal modulation and coherent demodulation of the respective systems were completed by MATLAB,and it was verified that the modulated signal has the characteristics of constant envelope or quasi-constant envelope.And it can be known that as long as the appropriate sampling time and decision threshold are selected,useful information can be demodulated from the received signal.At the same time,the error rate performance of each system is analyzed.By comparing with the MSK signal error rate,it can be seen that the system's error rate performance can meet the requirements,thereby verifying the feasibility of the proposed scheme.