| Shortwave communication has the advantages of long communication distance,convenient networking,and strong anti-destruction ability.It has an irreplaceable position in the field of communications.However,the shortwave channel is a typical time-varying channel.The multi-path effect in the channel is serious,the Doppler shift is large,and the fading is severe.When the signal is transported in the shortwave channel,it will be severely interfered,and the reliability of the transmitted information will be greatly reduced.In order to improve the reliability of the information transmitted in the shortwave communication system,and to resist the interference from the shortwave channel for the transmitted signal,the adaptive technology,channel estimation technology,channel coding technology and many other anti-interference technology on the platform of the shortwave adaptive data transmission system will be researched in this thesis.First,the adaptation techniques of bandwidth and transmission rate are researched.By realtime detecting and evaluating the channel quality in the shortwave adaptive data transmission system,the bandwidth and transmission rate of data can be selected adaptively,so that the shortwave adaptive data transmission system always work at the optimal bandwidth and transmission rate.Therefore,a low PAPR detection waveform for multi-channel is simulated and designed in this thesis,and normalized SNR is used to measure the channel quality.According to the channel quality,an adaptive selection scheme of transmission bandwidth and rate is given.The simulation result shows that the multi-channel detection waveform can detect the channel quality well,and the shortwave adaptive data transmission system can automatically select the transmission bandwidth and rate for different communication services,thus taking into account the reliability and effectiveness of the communication service.The technique has been applied to a practical shortwave communication system,and system tests have shown a good effect.Secondly,the channel estimation technology is studied.By estimating the channel impulse response accurately in the shortwave communication system,the receiver can compensate the channel well,thus improving the reliability of the received information.Therefore,the autocorrelation algorithm,adaptive algorithm such as LMS and RLS,compressed sensing algorithm such as Co Sa MP are used to channel estimation in this thesis,then the advantages and the disadvantages of each algorithm are analyzed.The simulation results show that the autocorrelation algorithm is simple and practical,but it is limited by the autocorrelation property of the training sequence.The LMS algorithm has slow convergence speed and large steady-state error,both of them cannot performed well at the same time.RLS algorithm has faster convergence speed and smaller steady-state error,but the complexity is higher than LMS algorithm.Both adaptive algorithms need long enough training sequences to ensure their convergence.Co Sa MP algorithm has the best channel estimation accuracy,but it is limited to estimate the sparse channel,and the algorithm complexity is higher than that of the RLS algorithm.Finally,the Raptor code is researched.When data transmitted in the shortwave communication system is encoded by Raptor code,they can supervise each other so that the decoder of receiver can detect the data error continuously and correct it,thus improving the reliability of the transmitted data.Since the Raptor code has no fixed code rate,the receiver has no need to request sender to transmit wrong data packets again.Therefore,the Raptor code and ARQ technology to data transmission is simulated and analyzed in this thesis.The simulation result shows that Raptor code can greatly reduce the data transmission time and improve the system throughput when the channel is poor quality. |
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