Spreading sequences for underwater multiple-access communications

Multiple-access communication networks are being more and more considered for use in acoustic underwater (AUW) channels. Spread-spectrum communications are generally used if the signal-to-noise ratio (SNR) is low. They fight against intersymbol interference (ISI) caused by the multipath propagation. This wide-band transmission is more robust against jamming (traffic noise) or fading problems than PSK communications. And they are particularly interesting in a multi-user environment. The long propagation delays in AUW channels preclude the use of time-division multiple-access (TDMA). Therefore, code-division multiple-access (CDMA) seems to be a very robust and efficient solution. In CDMA systems, a different spreading sequence is dedicated to each user. Due to hardware limitations and the number of users required, the length and size of the sequence families are often fixed. The classical families exist only for a few lengths and sizes. To design efficient systems, spreading sequences have to be chosen by minimizing a criterion on both even and odd correlation functions. The high complexity of the problem led us to use methods, derived from combinatorial optimization, which are described. These methods allow the design of variable length and size sequences. Their performance also exhibits better odd correlation properties than the classical families. Some sequences thus obtained are given. The inherent time-diversity of the CDMA may allow the overall performance to be increased. RAKE reception is discussed in an AUW medium. Spatial diversity was also tested and showed improvements using a small number of sensors.