Ldpc Codes And Wireless Lan Research

Wireless LAN has been developed into one of the most important parts of the computer network with the increasing demands for broadband wireless communication. It is the successful establishment and consecutive development of IEEE802.11 protocol that causes WLAN become the reliable and flexible network connection scheme in the places that cannot be accessed by wired network.Low density parity check (LDPC) codes were invented by Gallager in 1962. They are defined by sparse parity check matrices, with various construction methods. Simulation results show that such codes have near Shannon limit performance when decoded using an iterative probabilistic decoding algorithm. LDPC code has to be shown to perform as well as the turbo code but lower complexity and without the error floor problems.Orthogonal Frequency Division Multiplexing (OFDM) is a special case of multicarrier modulation, in which a single high data rate datastream is transmitted using a number of lower data rate subcarriers. In July 1998, the IEEE standardization group selected the OFDM modulation technique for their new IEEE802.11a standard, targeting data transmission from 6 up to 54 Mbps. Meanwhile, this new IEEE standard is the first one to use OFDM in packet-based communications.Furthermore, OFDM can be combined easily with some advanced technique, such as time-space coding, diversity, interference cancellation and smart antenna, to improve the transmission reliability in WLANs. The LDPC-based STC can significantly improve the system performance by exploiting both the spatial diversity and the selective-fading diversity in wireless channels.The performances of WLAN depences on not only the physical sublayer but also the MAC sublayer. This paper analyzes the CSMA/CA performance of MAC layer, and then discusses the realization of WLAN QoS: based on priority mechanism and based on fair source share mechanism. Simulation results show that priority mechanism brings in the unfair distribution of network resources, such that stations with low priority tend to "thirsty". This paper proposes an algorithm that modifies AIFS dynamically by estimates obtained resources; simulation results show this algorithm realizes fairness.Combined with the national high technology project named "Key techniques of the wireless module in fieldbus facilities", this thesis focuses on the baseband technologies of WLAN physical sublayer and the QoS of MAC sublayer. The main contents are as follow:Chapter 1 is an introduction of LDPC, OFDM and WLANs' MAC sublayer.Chapter 2: Analyzes all factors influenced on the OFDM coherent demodulation and puts forward an algorithm of carrier frequency offset synthetical estimation for OFDM system in Wireless LAN environment based on IEEE 802.11a OFDM baseband system. The simulation results and analysis are also presented in this chapter.Chapter 3: Introduces the history of Low density parity check codes and some construction methods and BP decoding algorithm. This chapter presents an OFDM decoding algorithm based on LDPC with multilevel modulation. Simulation results show that LDPC codes are strong competitor for forward error correction in WLAN. Finally, some discussions are presented.Chapter 4: OFDM may be combined with antenna arrays at the transmitter and receiver to increase the diversity gain and/or to enhance the system capacity in WLAN to meet the increasing demands for broadband wireless communication. This chapter introduces Samsung Electronics' LDPC, and proposes LDPC coding for multi-antennae WLAN based on SVD technique. Simulation results and analysis are also presented in this chapter. Finally, some problems are discussed.Chapter 5: Analyzes the CSMA/CA performance of MAC layer and proposes a new backoff algorithm.Chapter 6: Discusses the realization of WLAN QoS and proposes an algorithm that modifies AIFS dynamically by estimates obtained resources.Chapter 7: The research work is summarized, the existing problems are pointed out and possible solutions are given. At the same time, the development of LDPC and WLAN in the future is pro...