Research On MC-CDMA Technology In Underwater Acoustic Networks

With the advances in acoustic communication technology and acoustic modem technology that enabled high-rate, real-time reliable point-to-point communications between underwater nodes such as ocean bottom sensors and autonomous underwater vehicles (AUVs), current research focuses on communication between various remote underwater nodes within a network structure to meet the emerging demand for worldwide applications, such as environmental data collection, offshore exploration, pollution monitoring, navigation support, and military surveillance. The design of an underwater acoustic (UWA) network is commonly carried out in the form of a layered architecture, and recent research focuses on the first three layers, i.e. the physical layer, the data link layer, and the network layer. However, the quality of such networks is mainly limited by characteristics of the underwater acoustic channel, such as low available bandwidth, highly varying multipath, large propagation delays and limited energy supply. Within such an environment, designing an UWA network that maximizes throughput and reliability becomes a worldwide and very difficult task. In particular, managing channel access is difficult, as many nodes have to share the same limited source.This paper focuses on the multiple access methods of the UWA network's physical layer. We choose the multi-carrier CDMA (MC-CDMA) technology as our multiple access strategy, and, as some difficulties and key technologies we may face when we apply MC-CDMA to our network, the paper is mainly to do some research about the following four aspects:1. Spreading codes selection for the MC-CDMA system. The characteristics of a good spreading code in spectrum spreading systems is to have low cross correlation and high autocorrelation while being capable of supporting as many users as they can. In this paper, we research the correlation performance of Walsh code, pseudo noise sequence, and the carrier interferomety code (CI code), the double orthogonal code (DO code), which are new family spreading sequeces with many advances.2. We address channel estimation based on pilot signal to MC-CDMA system in our UWA network. Using the time domain and frequency domain structure of MC-CDMA, we present the LS estimators in the frequency domain of the system. The simulation and pool-testing indicates that this channel estimation can ensure our MC-CDMA working under an acceptable BER.3. In this paper, we address different detection strategies while MC-CDMA users work on downlink and uplink, separately. As for the downlink communications between multiple users, we survey different combining methods, including EGC, MRC, ORC, and so on. and the paper shows a very well performance the combining methods have in our UWA networks finally.4. In the uplink, with the increase of multiple access interference, and the loss of synchronicity between users, we propose several multi-user detection (MUD) technologies to MC-CDMA systems. The researched MUDs in this paper covers DEC, PIC, and DEC-PIC-EGC, DEC-PIC-ORC, DEC-PIC-MRC.Furthermore, we analyze the performance of our research not only in simulation, but also in pool-testing. The conclusion of this paper will give some helps to the design of our UWA network, which is the paper's future work.