Equalization Technique And Modulation Recognition In HF Communication

Shortwave communication has many advantages.It provides remote communication without the need of any relay.The network is strongly invulnerable,maneuverable and rapidly restored.Shortwave transceivers are widely used in military,maritime and long distance communications.Shortwave communication usually adopts a wide variety of modulation formats to avoid noise interference and extend transmission range.Therefore,in non-cooperative communications,the identification of modulation formats is crucial.At the same time,communications suffer from the various interference,like multipath interference.It will heavily affects the reception of transmitted signal.Equalization is one of the most important components in a complex receiver,and also an effective way to mitigate the effect of inter-symbol interference.Therefore this research is of great significance.To further our study,the first step is to establish the shortwave channel.Based on the well-known watterson model,the equivalent baseband transmission model for digital-modulated signals is established.The factors affecting signal transmission,such as Doppler spread,Doppler frequency shift,multipath and delay,are considered in the model.Through the simulation the basic characteristics of shortwave channel and its influence on transmission are verified.In non-cooperative communications,receiver has no information on the modulation formats.Therefore,it needs to collect each feature for classification,and then extract the parameters from the features.Threshold is set according to the performance that needs to be achieved.After those procedures,signals can be feed to the classifiers.In this dissertation,we pick a well-designed decision tree to simulate the signal recognitions with different modulation formats in the watterson channel.Due to the effect of multipath interference,fading and Doppler shift,signals suffer from severe inter-symbol interference and distortion,which results in a great reduction in transmission performance.Among them,multipath interference is particularly serious.We review the traditional equalization techniques,e.g.,LMS,RLS.After that,based on the state-of-art expectation propagation algorithm,we propose an iterative decision feedback equalization algorithm.Simulations show that EP based algorithm significantly improves the performance.In this dissertation,we realize the recognition for up to eight modulation formats through the watterson channel.Based on the single carrier transmission system,we detail the derivation of EP receiver with DVB-S2 LDPC and Proakis-C channel.Simulation results also confirm the performance of proposed algorithm.