AIS GMSK Modulator – Scicos Simulation


AIS Automatic Identification System for Ships is a traffic system that uses VHF transponders on vessels to periodically send out location information. It was developed by Swedish inventor Hakan Lans and is similar in concept to the ADS-B transponder system used on aircraft for position reporting.

AIS uses two VHF marine channels 87B (161.975 MHz) and 88B (162.025 MHz). Transmission is by 9.6Kbps GMSK modulation using HDLC packet protocol. VHF channel access is by SOTDMA Self Organized Time Division Multiplexing. AIS uses navigational information from external on board GPS, Inertial Navigation and Ship Control systems. AIS has several different classes of equipment such as Class A, Class B, Base Station and Aids to Navigation. Depending on the Class, different types of information are transmitted when the ship is in port, or underway (SOTDMA/CSTDMA).

In this post I will show how we can simulate an AIS modulator using the Open Software ScicosLab/Scicos.

Fig.1 – AIS Frame/Slot

Figure 1 shows the construction of an AIS frame. The frame consists of 2250 slots within a 1 minute time frame, which means 26.667msec per slot. Each slot consists of 256 bits with a payload of 168bits. HDLC NRZI coding is used with an inversion on a “0”. Figure 2 shows ais_slot_data.sce which is used to create an AIS slot packet and then convert the data to NRZI. Figure 3 shows ais_data_read.cos which is used to visualize the data and Figure 4 shows the original data on top and the NRZI data on the bottom trace. The NRZI data created is bipolar in order to drive the phase continuous FM modulator.

Fig.2 ais_slot_data.sce
Fig.3 ais_data_read.cos
Fig.4 AIS Data & AIS Data_NRZI

Figure 5 is an AIS GMSK modulator driven by a continuous PRBS source in order to get excellent spectrum visibility. The parameters chosen are:
-Data Rate R = 9600bps
-Frequency Shift = R/2 = 4800Hz = +/- 240Hz from Fc
-Fc = 10KHz (For visibility not an AIS frequency)
-Gaussian Filter BT = 0.4

Figure 6 shows the single lobe output spectrum centered around Fc with very little out of channel content. Figure 7 shows what happens if we remove the Gaussian Filter. This shows a huge out of band spectral content. Figure 8 shows what happens if we don’t chose Frequency Shift = R/2 but R instead. Here we have spectral lines at Fc +/- R/2.

The beauty of GMSK modulation is that it produces a constant envelope signal with zero line spectrum and low out of channel spectral content. Thus a perfect signal for an RF amplifier with non ideal characteristics.

Fig.5 AIS GMSK Modulator with PRBS Source
Fig.6 AIS GMSK Spectrum
Fig.7 AIS MSK Spectrum (No Gaussian Filter)
Fig.8 AIS FSK Spectrum Shift = R = 9600Hz
Fig.9 AIS GMSK Modulator with Slot Data
Fig. 10 AIS GMSK Slot Data and FM Modulated Output

Figure 9 shows the same AIS GMSK Modulator fed by the AIS Slot Data of Figure 4, instead of the PRBS source. Figure 10 shows the NRZI slot data, the Gaussian smoothed data and the Carrier FSK output. Note there are no phase discontinuities at the zero crossings, because we use a phase continuous VCO.

Fig.11 AIS GMSK Modulator – Scicos Simulation Video

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#1 – “Automatic Identification System”, Wikipedia

#2 – “All About AIS”, Created by IMO/IEC Experts

#3 – “Recommendation ITU-R M.1371-5 Technical characteristics for an automatic identification using time division multiple access in the VHF maritime mobile frequency band”

#4. ScicosLab with Modnum Toolbox

#5. “Learn Telecommunications by Simulation” EBook