Asynchronous Detection and Identification of Multiple Users by Multi-Carrier Modulated Complementary Set of Sequences

The correct detection of arrivals is a crucial step in any communication system, in order to achieve frame synchronization. This step is even more relevant in underwater acoustic communications, as the medium is characterized by hazardous phenomena such as long multipath delays and non-uniform Doppler shifts, which hinder the detection process. All these phenomena make the detection of arrivals and identification of multiple users in underwater communication systems a challenging task. In this paper, a multi-user detector is presented. The detection is based on the direct-sequence code-division multiple access technique, by using a combination of two sequences as a preamble: a Zadoff-Chu sequence that provides candidate peaks and frame synchronization, and a complementary set of sequences (CSS) that validates the true Zadoff-Chu arrival while at the same time identifies the user. Both sequences are modulated using multi-carrier techniques, and the users can transmit asynchronously to the channel. In the case of CSS, multi-carrier modulation techniques allow performing channel equalization and demodulation at the receiver in a simple manner, enabling the correlation of the CSS at bit level, thus keeping its ideal correlation properties. Sea trials were conducted in February 2017 at Suruga Bay, Japan, in which the reception of two different users was analyzed under different noise and multipath conditions, as well as uniform and non-uniform Doppler shifts. The results show that, even under strong multipath and non-uniform Doppler shifts caused by roll and pitch motions, the receiver is able to detect and identify the user in 98% of the transmitted packets.