Design Of Cdma-based MAC Protocol For Underwater Sensor Networks

The21st century opened a new era of comprehensive exploitation and utilizationof marine resources. As an emerging information network, Underwater SensorNetwork (UWSN) has become a hotspot research field, and has potentially broadapplication in the fields of oceanic environment monitoring, offshore exploration,assisted navigation, disaster prevention and military surveillance, etc.Due to water absorption, the energy of radio suffers from serious attenuation, andthe attenuation increases with the increasing of frequency. Experiments showed thatthe transmission distance of radio is very short in the water which is testified to be50-100cm for nodes of IEEE802.11b/g or IEEE802.15.4protocol, thus radio waveis unsuitable for UWSN communication. So, acoustic communication is usuallyemployed in UWSN. The speed of acoustic in the water is about1500m/s，and fiveorders of magnitude less than radio. Acoustic channel is characterized by longpropagation delay, high bit error, low bandwidth, and dynamic topology etc., whichresult in conventional terrestrial-based MAC protocols inefficient for UWSN.However, current available underwater MAC protocols exist a wide range of issuesand need to be further improved and developed.This thesis described the characteristics of UWSN, analyzed the research statusof MAC protocols and the limitations of traditional MAC protocols，and proposedCDMA-based MAC protocol. CDMA-based MAC protocol highlighted thepersonalization of each node, and used divisive probability function to assign thespread code for each node. The codes assignment algorithms based on divisiveprobability function makes up the shortcoming of traditional code assignmentalgorithm, further reduces the risk of interference. Our MAC protocol based onCDMA used the state field in neighbor table to avoid the hidden terminal and exposedterminal problem caused by RTS/CTS handshaking. The design of power controlminimizes “the near-far” effect inherent in CDMA system. The simulations based on network topologies of bus, star, ring and random deployment indicate thatCDMA-based MAC protocol outperforms U-Aloha and Slotted-FAMA in terms ofthroughput, resource-consumption and end-to-end delay, etc.