A comparison of two methods for demodulating a target AIS signal through a collision with an interfering AIS signal

D. C. Smith, D. J. Nelson

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review

11 Scopus citations

Abstract

Automatic Identification Systems (AIS) are commonly used in navigation for collision avoidance, and AIS signals (GMSK modulation) contain a vessel's identity, position, course and speed - information which is also vital in safeguarding U.S. ports. AIS systems employ Self Organizing Time Division Multiple Access (SOTDMA) regions in which users broadcast in dedicated time slots to prevent AIS collisions. However, AIS signals broadcast from outside a SOTMDA region may collide with those originating inside, and demodulation in co-channel interference is desirable. In this article we compare two methods for performing such demodulation. The first method involves Laurent's Amplitude Modulated Pulse (AMP) decomposition of constant amplitude binary phase modulated signals. Kaleh has demonstrated that this method is highly accurate for demodulating a single GMSK signal in additive Gaussian white noise (AWGN). Here we evaluate the performance of this Laurent-Kaleh method for demodulating a target AIS signal through a collision with an interfering AIS signal. We also introduce a second, far simpler demodulation method which employs a set of filters matched to tribit states and phases of GMSK signals. We compute the bit error rate (BER) for these two methods in demodulating a target AIS signal through a collision with another AIS signal, both as a function of the signal-to-interference ratio (SIR), and as a function the carrier frequency difference (CFD) between the two signals. Our experiments show that there is no outstanding advantage for either of these methods over a wide range of SIR and CFD values. However, the matched filter approach is conceptually much simpler, easier to motivate and implement, while the Laurent-Kaleh method involves a highly complex and non-intuitive signal decomposition.

Original languageEnglish
Title of host publicationMathematics for Signal and Information Processing
EditorsMark S. Schmalz
DOIs
StatePublished - 2009
Externally publishedYes
EventMathematics for Signal and Information Processing - San Diego, CA, United States
Duration: Aug 2 2009Aug 5 2009

Publication series

NameProceedings of SPIE - The International Society for Optical Engineering
Volume7444
ISSN (Print)0277-786X

Conference

ConferenceMathematics for Signal and Information Processing
Country/TerritoryUnited States
CitySan Diego, CA
Period08/2/0908/5/09

Keywords

  • AIS co-channel interference
  • AIS systems
  • GMSK demodulation
  • Laurent AMP decomposition

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