We study inter-channel interferences in VHF transmissions for the maritime automatic identification system and propose solutions to avoid them. The inter-channel interference is caused by the brevity of SOTDMA frames used by ships to transmit their position.In order to avoid the spreading of the spectrum, two solutions are considered. A Tukey window is first used to avoid a too hard transition between consecutive temporal slots. The second solution consists in using a Chebyshev lowpass filter. We compare these two solutions in term of interference and show that their association can be envisaged with reasonable complexity.