| Aircraft flying at a high speed will create plasma sheath, which will lead to the communication blackout. For the reason that the magnetic field strength is much stronger than the electric field strength in the near-field energy and small loop antenna is a kind of magnetic antenna which is widely used in short-wave communications with small size to carry, A small LF loop antenna can be used as the transmitting antenna to reach the goal of communicating across Blackout for the reason that the plasma shields the electric field while does not shield the magnetic field.Our purpose is to choose a proper narrowband modulation method to communicate using a small LF loop antenna with the center frequency of 10.099 MHz and the bandwidth of 5k Hz.Besides, the choice of communication method is restricted to the frequency characteristics of the antenna. To ensure the highest bandwidth efficiency, The narrowband modulation method chosen should have the bandwidth as narrow as possible and the bit-rate as high as possible.The UNB communication system takes a small LF loop antenna as the transmitting antenna. After the circuit model is developed and the transfer function is solved to the small LF loop antenna, The non-linear phase frequency characteristics and the non-constant group delay of the small LF loop antenna are found. After detailed analysis to applications of various kinds of narrowband modulation method is done, A narrowband modulation method called Very-minimum Waveform Difference Keying(VWDK) is chosen. VWDK, combining amplitude, frequency and phase modulation, has continuous phase and has no direct component. Research on structure and features of VWDK waveform has been done. Theoretical expression of the power spectral density as while as theoretical bit error rate of correlation demodulation have been derived, which have been verified through the computer simulation. Reduction to the bandwidth through removing the discrete spectrum and design on the sending filter are also accomplished Analysis to signal spectrum, bit error rate of correlation demodulation and bandwidth utilization after filtered are also involved. To solve the waveform distortion brought by the nonlinear phase frequency characteristics of antenna, a FIR digital filter is designed. The influence of characteristics of antenna on signal transmission can be compensated by putting the FIR pre-distortion filter before the small LF loop antenna. At the end of the paper, compensation results, including the correlation of signal before and after transmitting and the correlative detection performance before and after pre-distortion, are compared.The simulation of the entire small LF loop antenna communication system with Matlab has been completed in the condition of 100 kbps bit-rate and 10.099 MHz carrier frequency and the modulation parameter ? is equal to 0.92 and bandwidth of the antenna is 5k Hz.The bandwidth is 5k Hz after a IIR filtering with the band utilization rate as high as 20bps/Hz. It can be demodulated successfully when adding pre-distortion filter to the system while failed to demodulate when there is no pre-distortion filter. To reach the bit error rate at-510, signal-to-noise ratio should be 32 d B, 6d B higher than bit error rate of VWDK demodulation in ideal circumstances of no small LF loop antenna. |
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