Research On OFDM And AIS Demodulation Technique

Software radio uses a common hardware platform and communications capabilities such as setting frequency, modulation and demodulation, and synchronism are implemented by software as much as possible, thus facilitating the upgrade of the system. It overcomes the limitation of a conventional radio which is single purpose oriented and lacks flexibility by building. Based on the related theory of software radio and the background of the defense pre-research project, this paper studies the demodulation method of OFDM and automatic identification system (AIS) signals.Firstly, characteristic of software radio and digital demodulation technique based on software radio are introduced. And then the hardware based on software radio was designded and implemented to process digital signal.Secondly, blind demodulation method of OFDM signal is studied. Using properties of the auto-correlation function of OFDM signal, the methods of parameters blind estimation are presented, which are two methods to estimate the effective length, the carrier frequency estimation method, frequency offset and timing synchronization estimation method. In order to more exact demodulate signal, because the effective length of OFDM symbol has integer time relation with cycle prefix length, the methods of exactly estimating the effective length and total length of OFDM are given, also the exact estimate method of subcarrier period is given. Using normalized cumulate the modulation type is identified. According to different modulation type, the phase lock loop is set correctly and correct constellation is obtained. The demodulation results of two segments received OFDM signal validate the effectiveness of the method.Lastly, demodulation and information recovery of AIS signal are studied. After model and the demodulation principle of AIS signal are introduced, the demodulation method of phase difference and parameter estimation method are presented. The information recovery method based on AIS protocol is presented. The demodulation result of the received AIS signal validates the effectiveness of presented methods.