Using direct sequence spread spectrum in marine radio communication

This paper proposes application of the DSSS CDMA method (Direct Sequence Spread Spectrum Code Division Multiplexing Access) in marine voice and data transmission. The spread spectrum method is based on conversion of signal into the form of wider band, while signal power remains on the same level. This conversion causes decrease of power spectral density, which is proportional to the spreading factor. Spectrum spreading is achieved by multiplying the input signal by a code sequence (spreading code). Simultaneous transmission can be achieved by spreading multiple signals by uncorrelated and orthogonal sequences. Power spectral density of CDMA is usually lower than noise power, therefore the transmission remains transparent for other receivers and does not affect other systems. Another advantage of the CDMA is its robustness against the narrowband noise. For a correct reception of the CDMA signal, a precise synchronization is necessary. This results from a method of despreading, which is based on another multiplication of signal and the spreading code. This operation should be performed in-phase. If not, on the input of detector appears additional noise.