Short PN Sequences for Direct Sequence Spread Spectrum Radios

Abstract

Traditional military spread spectrum systems use Direct Sequence Spread Spectrum (DSSS) for its jam resistance and low probability of intercept. Additionally, the transmission security afforded by long PN sequences in the waveform is important as is the ability to do code division multiple access (CDMA). DSSS systems as proposed for the unlicensed ISM band use a form of DS that has few of these attributes. The sequences used are so short that special attention must be placed on what are the best sequences to use. The Barker sequences are often chosen, but do they have the right characteristics? A Barker word has good autocorrelation sidelobes when preceded and succeeded by all zeros. Does it have good correlation sidelobes when preceded and succeeded by more Barker words with QPSK modulation? That is, the adjoining sequences can have any of four phases relative to it and this may cause some sequences that initially look good to not be. This paper examines 11, 13, 15, and 16 bit sequences for these properties and makes sequence recommendations.