Research And Improvement On Physical Layer Techniques For WiMAX System

WiMAX system provides a method for inter-operational broad band wireless access effectively, which is in the condition of multi-manufacturer for one point to multipoint in the Metropolitan Area Network.In the existing WiMAX system, the techniques of random scramble, RS code and OFDM in physical layer has been used to ensure the reliability of transmission, but for the more reliable application at higher rate, it requires to improve the anti-interfere and error-correcting capability of the system.The paper is mainly on setting up the physical simulation platform to simulate the BER of WiMAX system, and then proposes a method to improve the existing RS coding/decoding scheme with CRC. Finally, the paper improves the adaptive modulation code scheme of WiMAX. An adaptive allocation algorithm was proposed to improve the performance in WiMAX system, which is based on the transmission bandwidth optimization.First of all , the paper sets up the whole physical simulation platform for WiMAX, according to the parameters in IEEE 802.16e standards. The platform is composed of random scrambler, RS coder, convolutional coder, constellation symbol mapping, and OFDM modules. The simulation results show that the demond of the lowest SNR to receivers has not arrived at the level as low as possible at the premiss of ensuring the decoding performance in the different modulation modes for WiMAX.Then, based on the impacts of various effects of wireless channel on the system performance, an RS with CRC coding scheme is proposed. Firstly, a transmission matrix is selected. Afer the padding, the data is checked with CRC. Hence the receicing end could identify the error location in RS code according to the result of CRC. With the known error location, the RS decoding can correct twice the amount of bytes as orignal. Thus the correction capability of the proposed RS scheme is significantly improved. As the paper focuses on the reliablity of high rate transmission, then the additonal network overhead caused by CRC can be neglected. So, the RS with CRC coding scheme could put into practice.Finally, according to frequency selective fading and time-varying characteristics in wireless channel, an adaptive allocation algorithm was proposed in WiMAX system, which is based on the transmission bandwidth optimization. Through building the physical layer transmission platform for WiMAX systems and channel models, the simulation was made to focus on the application of the new algorithm in WIMAX system. The simulation results show that: the adaptive allocation algorithm, which is based on transmission bandwidth optimization, not only guarantees BER performance of WiMAX system, but also makes WiMAX system response to the changes in wireless channel more quickly to realize the BER performance, comparing to the traditional algorithm.