A New IDMA System Based on CSK Modulation for Multiuser Underwater Acoustic Communications

Abstract

This work proposes a novel interleave-division multiple access (IDMA) system based on cyclic shift keying (CSK) modulation, named CSK-IDMA, for multiuser underwater acoustic (UWA) communications. The CSK modulation uses a circular cyclic shift of the spreading sequence to represent the information, which can be viewed as a special $M$ -ary spread spectrum modulation. Compared with conventional direct-sequence spread spectrum (DSSS) technique, CSK overcomes the spreading gain versus data rate limitations. The proposed scheme adopts the IDMA technique to reduce the cochannel interference. At the receiver, we employ passive time reversal (PTR) technique to compress the channels and improve the signal-to-interference-noise ratio (SINR) at first. In order to perfectly incorporate with the IDMA receive structure, we propose a soft CSK demodulation which can iteratively exchange extrinsic information with a soft channel decoder to form a turbo equalization. Simulation results show the proposed CSK-IDMA scheme can offer significantly improved performance. The data processing results from a lake experiment show that the proposed scheme can support 8 users and 10 users with zero error bits for 2.3 km transmission and 2.7 km transmission, respectively. The corresponding bit error rate (BER) for 16 users are $\text{8.4} \times \text{10}^{-4}$ and $\text{9.7} \times \text{10}^{-4}$ , respectively. The data rate is 62.5 bit/s per user within 4 kHz bandwidth and spreading sequences of 256 chips. To the best of our knowledge, this is the first work which can support more than 8 users for multiuser UWA communications with a high bandwidth efficiency.