Research On Multirate MAC Protocol Of Underwater Acoustic Sensor Networks

As a new network form and monitoring means, Underwater Acoustic Sensor Networks (UWASNs) has become a recent research focus, which has wide application prospect in fields such as environment monitoring and protection, resources exploration, supplemental navigated water economy, disaster rescue and military defense; also has influence that is far-reaching on the safeguard of national defense and oceanic economy. However, in underwater acoustic channel, the limited frequency bandwidth, strong multi-path interference, random channel changes in time and space domain and high background noise in ocean environment together turn high speed and robust underwater acoustic communication into a recognized challenge. Medium Access Control (MAC) protocol is the basic protocol for UWASNs. It is the key network technology to ensure high efficient communication of WSN and has great impact on the performance of WSN. However, the exiting terrestrial MAC protocol is not suitable for underwater environment because of the uniqueness of the underwater environment and application. Aiming to shorten delay, highten throughput and decrease energy consumption, this paper studies the MAC protocol for UWASNs.This paper first proposes a MC-CDMA based MAC protocol suitable for UWASNs. The protocol utilizes the relationship between transmission range and transmission rate in acoustic link and chooses different transmission rates according to different lengths of the links. This protocol improves network effiency as well as maintains network connection. In this protocol, a negotiation mechanism is adopted to achieve same rate between transmitter and receiver. And a dynamic subcarrier selection algorithm is proposed to realize multirate and improve Bit Error Rate (BER) performance. The allocation algorithm of spreading codes resists the potential interference of the spatio-temporal uncertainty. Simulation results show that Multi-rate (MR-MAC) has smaller end-to-end delay and higher network throughput than traditional CSMA protocol and UW-MAC based on CDMA. Simulation results also show that the dynamic subcarrier selection algorithm can not only realize multirate but also reduce the BER.Fully considering the characteristics of the underwater link, we further propose an improved TDM A multirate MAC protocol. The protocol uses a beacon-based multirate negotiation mechanism to reduce the control overhead and DCSK mechanism which can effectively resist the multi-path interference and frequency selective. We made improvements on the traditional TDMA protocol to improve the utilization of the channel. This protocol also chooses different transmission rates according to different lengths of the links and improves network efficiency as well as maintains network connection.At last, after study the applications of TDMA and CDMA in underwater; we proposed a multirate MAC protocol for static routing. This protocol uses a mechanism combines TDMA and CDMA, allowing nodes which have different destination nodes to transmit in the same slot. The protocol not only improves the utilization efficiency of the slot, thereby improves network throughput and reduces the end-to-end delay but also improves network scalability. When new node is added or some node is dead because of energy exhaustion or destruction, we just need to assign and recycle spreading codes correspondingly, but don't need to assign slots again, solving the difficult problem of slots assignment underwater. Finally, we prove the protocol is better than traditional TDMA protocol in performance by underwater experiments. Then we implement this protocol to wireless environment and both the simulation and experiment results show it is better than traditional WSN protocol.