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Automatic Identification System (AIS) Coherent Demodulation Techniques

Posted on:2013-02-17Degree:MasterType:Thesis
Country:ChinaCandidate:Z C ZhangFull Text:PDF
GTID:2212330371960261Subject:Communication and Information System
Abstract/Summary:PDF Full Text Request
The automatic identification system is a new type of ship navigation identification equipment and a communication system using self-organizing time division multiple access mode in the VHF marine mobile communication frequency band. It is capable of automatically transmitting and receiving ship dynamic and static information, realizing the communication, monitoring and recognition between ships and shores. AIS can display the navigation state and other information of the ship and peripheral ships in a standardized interface. It can enhance vessel traffic services used with the radar, effectively avoid ship collision, reduce marine accidents, protect the navigation security, maintain the marine ecological environment and promote modern shipping transportation development.AIS uses Gaussian filtered minimum shift keying (GMSK) modulation, having features of constant amplitude, continuous phase, small bandwidth, high outer band attenuation and low adjacent channel interference. The focus of this paper is the coherent demodulation algorithm of GMSK signal. Coherent demodulation, which has good error performance, is one of the important ways in GMSK signal demodulation. In the first, the paper introduces the structure, function and development of AIS and discusses the technical standards of AIS. Then, we focus on the GMSK signal modulation principle. After that, the paper makes a study of two kinds of GMSK signal demodulation algorithm, which are linear coherent demodulation and Viterbi coherent demodulation, which includes Viterbi coherent demodulation based on maximum likelihood sequence detection (MLSD) and simplified Viterbi coherent demodulation. The paper introduces the demodulation process in detail, including frame detection, carrier frequency offset and phase estimation, clock synchronization, carrier phase tracking, matched filter, whitening filter and Viterbi decoding. This paper makes a simulation of signal modulation and demodulation process using the Matlab software, tests the bit error rate performances, draws bit error rate curves and compares performances of these demodulation algorithms. In addition, the signal received by AIS VHF receiver is put into the program for demodulation to verify the correctness of the algorithms and realize monitoring on a variety of sailing ship information. Finally, this paper also makes the Matlab simulation on linear coherent demodulation and Viterbi coherent demodulation in single channel with interference, draws the bit error rate curves with signals in different carrier to interference ratios (CIR), analyses and compares the error performances of the algorithms in single channel with interference. The experimental results show that, linear coherent demodulation and Viterbi coherent demodulation are able to achieve the expected performances in the condition of interference free and single channel with interference, and the simplified Viterbi coherent demodulation is a promising coherent demodulation algorithm, which has certain advantages in error rate performance and structural complexity. The Matlab program can be applied directly to the central processor of AIS for GMSK signal software coherent demodulation, thus the shipping information monitoring and identification can be realized. At the same time, it provides the basis for the research of AIS in the software simulation and has positive significance and a promoting effect on AIS product testing and development.
Keywords/Search Tags:automatic identification system, Gaussian minimum shift keying, coherent demodulation, Viterbi decoding
PDF Full Text Request
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