Aplication Of Fountain Codes In Ad Hoc Network

Wireless Mobile Ad hoc Network, also known as the MANET, is determined by a number of familiar nodes which can move freely. All nodes are equipped with wireless transmitter and receiver, they can not only be treated as hosts to send data, but also as routers to forward data. When two nodes are not within communication range, they can forward data through the neighboring node as a router, eventually arriving at the receiver. Since the nodes are free to move, the network topology changes at any moment and the channel environment is also time-varying. So choose a better network coding scheme to improve the performance of Ad hoc network becomes more and more important.LT codes are rateless, i.e., the encoding packets that can be generated from the source packets on the fly are limitless. All the encoding packets are transmitted through the network to reach the destination. The destination can recover the source packets with high probability as long as it receives a sufficient number of encoding packets. So there is no need to use automatic retransmission mechanism to ensure the reliability. It is suitable for both the broadcast and multicast communication network, greatly saving in network overhead. Fountain codes have many characteristics such as universality, reliability and robustness so that it can adapt to the Ad hoc network environment. LT codes, Raptor codes and SRLDPC codes have been widely used in practical network currently.In this thesis, we introduce the encoding and decoding characteristics of LT codes and SRLDPC codes in detail, highlighting their advantages in Ad hoc network with its topology time-varying. Also we give a brief introduction about Kite codes and RA codes. The difference is that LT codes and SRLDPC codes are designed for erasure channel, while Kite codes and RA codes are designed for AWGN channel. Network transmission is based on packets. Each packet is correctly transmitted to the receiver or lost, it is similar to erasure channel. Therefore we focus on the performance of LT codes and SRLDPC codes in Ad hoc network.In this thesis, LT codes and SRLDPC codes are applied to Ad hoc network environment, compared with Kite codes and RA codes. We use two schemes to achieve multicast communication, which are based on broadcast mechanism and sharing mechanism. We use OPNET network simulation software to build Ad hoc network and compare the two schemes in their advantages. We analysis the impact on network performance of different encoding methods through the statistics collected by OPNET. The result shows that each of the schemes has its own advantages. The broadcast scheme can improve the probability of correct decoding, while the sharing scheme can reduce the amount of encoding packets to send. Both schemes do have increased the network throughput and improved the performance.