Packet Transmission And Multiple Access In Wireless Networks Based On Rateless Codes

With the rapid increase in the number of wireless network users and the rapid popularity of smart mobile devices,wireless networks which can support high data rate transmission and multi-users’ concurrent transmission is an urgent objective. However, in the existing wireless networks, such as 802.11, when the channel quality become poor or the number of users increases, it is hard for network to meet users’ demands. In 802.11, the physical layer’s and MAC layer’s efficiency is not high. On one hand,a fixed-rate channel coding PHY is unable to adapt to the dynamic change of channel. On the other hand, coordination overhead and MAC layer’s retransmission wastes a lot of channel resources. Rateless codes are the latest research results of information theory, rateless codes can reach to the channel capacity, and can automatically adapt to changes in channel state. Rateless codes initially are mainly applied in the application layer of network. In the nearly two years, there is a trend that rateless codes been implemented in physical layer. Research on rateless codes mostly focus on how to design the coding and decoding algorithm, we argue that, in order to fully tap the advantages of rateless codes, message transmission mechanism and multi-users’ channel access mechanism must be designed to exploit rateless codes’ advantages. One of the most important defects of traditional wireless packet transmission is that,MAC layer‘s long frame is an indivisible atomic unit when transmitted. Once decodeing is unsuccessful, the entire packet need to be retransmitted. What’s more,in conventional multiple access scheme, like 802.11 MAC protocols, multiple access mechanism is inefficient. In the time domain,it need a lot of competition overhead, retransmission overhead. In the frequency domain, it does not consider the frequency diversities of multi-users’ link, so the overall efficiency is not good.First,this paper presents a fine-grained packet transmission mechanism based on rateless codes. After rateless coding, transmission of long-length message is no longer an indivisible process. Rateless encoder converts a long source message to encoded output symbols, producing a continuous stream of symbols. Through fine-grained packets, the encoded symbol is transmitted to a receiving node. The receiving node receives the coded symbols, as long as enough coded symbols is received, the receiving node can decode the original long message information, an individual fine-grained packet’s error does not cause the original message to be retransmitted. In this paper, we will address the finegrained packet’s package design problem, fine grained packet’s deployment problem in OFDM, packet’s detection problem, packet length optimization problem and link protocol and other issues, then we give the experimental results and analysis results. Results show that the fine-grained packet can get very good PER performance, and can improve the user’s data transmission throughput.Secondly,for network with many users, this paper proposes an efficient multi-user access mechanism based on rateless coded transmission, called RCF-MAC, based on the design of subchannel, RCF-MAC employ a fast mutil-user conflict resolution strategy and resource allocation algorithm based on channel fine-grained packets. The main advantage of RCF-MAC is that(1)ACK confirmation is asynchronous, the link which has less information to delivered or whoes channel quality is good can firstly complete the transmissions, and vacate this sub channel for other nodes;(2)The transmitting node can adjust transmission strategy when transmitting symbols to adapt to the dynamic change of channel in real time.Finally, we have implement rateless encoding and decoding algorithm, fine-grained packet transmission and multi-user channel access mechanism RCF-MAC in the software define radio platform, USRP N210, and have done a large number of experimental tests. Results show that, compared to the existing network protocol, the proposed schemes result in the improvement of network throughput performance.