A simple matched filter approach to demodulating AIS signals in co-channel interference

Automatic Identification System (AIS) signals contain ship tracking information used for collision avoidance, in rescue missions, and for port security. AIS systems employ GMSK modulation and 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. We apply Laurent's Amplitude Modulated Pulse (AMP) decomposition of constant amplitude binary phase modulated signals to demodulate a target AIS signal in collision with an interfering AIS signal. AMP decomposition is technically complex, which obscures any intuitive relationship between components and the GMSK signal. We introduce a conceptually simpler demodulation method employing a set of filters matched to GMSK tribit states and phases. We compare bit error rates (BER) for the two methods, (1) as a function of carrier frequency difference (CFD) between equal power target and interfering AIS signals, and (2) as a function of power ratio at a small, fixed CFD. Our experiments show that our matched filter (MF) approach matches AMP accuracy for (1), and is better for (2). Moreover, MF is easier to understand, motivate and implement.