Research On Technology Of Ranging In MAC Layer For WiMAX System

Based on IEEE 802.16 protocol, WiMAX system provides broadband wireless access in higher speeds and greater coverage than ever. As an extension to IEEE 802.16 standard, IEEE 802.16d specifies media access control layer and physical layer of unlimited access system. This thesis focuses on Ranging mechanism and key technologies in IEEE 802.16d. IEEE 802.16d standard only specifies Ranging process and parameters that can be adjust, and leave engineering problems to designers.Ranging mechanism is also known as calibration process, it functions mainly to keep the quality of RF link between substation (SS) and basestation (BS). This thesis includes the following parts: 1) Studying the function of Ranging mechanism; 2) Key algorithm studying of the Ranging mechanism: adaptive modulation and coding algorithm (AMC), uplink frame synchronization algorithm; 3) Proposing the FEC-combined new AMC algorithm, and performance analyzing; 4) Implementation of the Ranging process on Fujitsu WiMAX SoC platform MB87M3550.AMC functions as follows: if the channel is in a better condition, AMC will choose a higher-order modulation and higher data rate channel coding so as to maximum system throughput; if the channel is in a worse condition, AMC will choose a lower-order modulation and lower data rate channel coding so as to ensure the quality of transmission. After some careful study on the system throughput performance with different modulation and coding mode in different channel conditions, this paper propose an improved AMC algorithm, which switches the modulation and coding mode with a set of definite CINR threshold to maximize system throughput, and combines forward error correction coding mechanism and SR-ARQ mechanism together. A set of optional switching threshold values is obtained by OPNET simulation of WiMAX networks.Frame synchronization is very important in Ranging. Only if the MAC frame between the substation and basestation is synchronized, can the user data be demodulated correctively. According to features of IEEE 802.16d OFDM TDD frame, this thesis proposes a modified uplink frame synchronization method. First, the long preamble is scrambled; second, the timing synchronization process is divided into two steps: rough synchronization and fine synchronization. As classic synchronization methods do, the new method runs with low complexity. In addition, by the two steps of timing capture procedure, timing capture becomes easier than usual in actual WiMAX system. Since there are long preambles in downlink frames, this method can also be applied in downlink. MATLAB Simulation shows that, the proposed method can still obtain accurate time synchronization even in multipath distorted channel.