Study Of Skywave Interference Eliminating Method Based On The Multiuser Detection Technology

The oceanic wireless navigation system adopts spread spectrum frame and works at a medium and short wave band, using the ground wave signals to positioning accurately. There are mainly two kinds of interferences at this frequency band. One interference is caused by the sky wave and the other one is the near-far effect. The interference of the sky wave is more seriously, because after a certain distance the signal strength of the sky wave will exceed the strength of ground wave apparently, which makes the receiver lock on the sky wave by error. This causes the positioning solution deviate from the actual position of carrier. The result of practical measurement shows that the sky wave signal whose amplitude changes greatly and fleetly is a typical kind of fast time-varying signal. In order to improve the capturing and tracking performance of the ground signals, the interference of the sky wave must be eliminated from the received signals.Aiming at this problem, on the basis of researching the characteristics of medium and short wave band channel, this thesis brings forward an adaptive parallel interference cancellation multi-user detection method.The main research contents are as follow:Firstly, the character of the medium wave channel is researched in detail, and the method of calculating the strength of ground wave is given. In allusion to the complicated calculation of field intensity of ground wave in condition of single path, this thesis introduces the tentative approach, the accurate calculation and the improvement of accurate calculation, and analyses the calculating method of field intensity of ground wave. In allusion to the calculation of strength of ground wave in condition of mixed paths, this thesis brings forward that the mixed paths should be disposed, and an effect counting factor would be added at boundary of different spread mediums, and also introduces the detailed calculating method and simulating results. The calculating method of ground wave time delay and sky wave field intensity time delay are also introduced in this thesis, and on the basis of this, the simulating model of sky wave interference channel is established.Secondly, the thesis analyses the theories of some typical multi-user detection methods such as traditional multi-user detection method, decorrelation multi-user detection method, MMSE multi-user detection method, serial interference cancellation multi-user detection method, parallel interference cancellation multi-user detection method ,and carries out the simulations of all the multi-user detection methods, which lays the foundation for the follow-up research of adaptive parallel interference cancellation multi-user detection method.Finally, in order to eliminate the sky wave interference, this thesis brings forwards an adaptive parallel interference cancellation multi-user detection method, designs and simulates the practical project of the system. The simulating results show that the multi-user detectors based on this method can adapt to the fast time-varying channel, reduce BER of system effectively, and improves the anti-multiple access ability. The method also owns the characteristics of low complexity and small time delay. On this basis, the main functions of adaptive parallel interference cancellation multi-user detector are achieved and the schematic and simulating and debugging results based on FPGA are introduced.