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

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.