Key Techniques For Ranging System In Wireless OfDMA Networks

Synchronization of the multiple access users in the uplink can be achieved by aranging process specified in the orthogonal frequency division multiple access(OFDMA) based IEEE802.16standard, and which involves many aspects such as theranging code design of access user, the resources allocation of access channel, themultiuser detection and synchronization parameter estimation of multiuser access at thebase station, etc. Duo to the problems of the selective fading of ranging channel, thetiming offset of access users, the correlation property of ranging codes and the signalpower differences of ranging signals led to the near-far effect made the received rangingsignal distortion, and which result in the multiple access interference (MAI) on accesschannel. The magnitude of the MAI seriously affects the ranging performance andlimits the access capacity for OFDMA systems, and which can not meet the multiuseraccess requirement for the next-generation OFDMA wireless communication systems.To solve these problems, we analyze and research the key ranging techniques from theranging signal design method, the multiuser detection method with interferencecancellation, and the ranging method with muitlple antennas for OFMDA systems,respectively. The main research works and innovation points are listed as follows:1. For the multiuser wireless access in OFDMA uplink, we introduced the rangingprocedure and the ranging resource allocation in the frequency domain and the timedomain, described the ranging structure and signal model for OFDMA systems,investigated the frequency correlation based ranging method and derived the correctdetection probability and the false detection probability in theory. For rangingperformance greatly depending on the magnitude of the MAI on the access channel, themain factors including the selective fading of access channel, frequency correlationproperty of ranging codes and the number of ranging users which affect the rangingperformance are analyzed in detail for OFDMA systems.2. In the ranging code design and precoding ranging method, accrding to thecharacteristics of OFDMA ranging systems, a principle for ranging codes design withgood related property and lower peak-to-average power ratio is proposed on the premiseof the quantity of ranging codes in OFMDA uplink. In precoding based ranging method,according to the cell radius and the multiple symbols structure of ranging signal, byprecoding the ranging code of access user, a precoding based ranging signal is designedfor macro-cell and micro-cell, respctively. By using the orthogonality of precodingvector, the received ranging signal is divided into several decoded groups, and a new ranging method is presented for OFDMA systems. Simulation results show that theproposed method reduces the active access users on the same decoded group, andsuppresses the accumulating MAI in multiuser detection process, so it can significantlyimprove the ranging performance and the access capacity in the OFDMA uplink.3. In the interference cancellation method of multiuser detection, the successivemultiuser detection (SMUD) and interference cancellation ranging method is analyzedfor OFDMA systems, by selecting the valid path with the maximum signal tointerference plus noise ratio (SINR) and further mitigating the MAI of the coefficientestimate of valid path, a novel SINR based successive interference cancellation rangingmethod is proposed for OFDMA systems. Compared with the SMUD, the proposedmethod can improve the accuracy of the coefficient estimate of valid path, reduce theresidual MAI in the successive interference cancellation procedure, and improve theranging performance of multiuser detection. According to the high computationalcomplexity in the SMUD, by using parallel detection and parameter estimate of validpaths, a low complexity ranging method with iterative parallel interference cancellationis presented, and the performance impact of multiuser detection is analyzed based on theaccuracy of coefficient estimate of valid paths and the number of iterative detection.Simulation results show that the proposed method improves the accuracy of coefficientestimate of valid paths and increases the ranging performance with low-complexity, soit improves the access efficiency for OFMDA ranging systems.4. In OFDMA ranging method with multiple receiving antennas, the rangingmethods are presented for receiving antenna array with multiple-input multiple-output(MIMO) and smart antenna at the base station, respectively. For MIMO antennas at thebase station, a novel coherent combination based ranging method is proposed by usingthe limited channel state information for MIMO-OFDMA systems, and the correctdetection probability and the SINR of the ranging signal after coherent combination areanalyzed. Simulation shows that the proposed method significantly improves theranging performance and increases the capacity of multiuser access. For smart antennasat the base station, ranging methods based on switched beamforming and adaptivebeamforming are proposed for MIMO-OFDMA systems, respectively. In switchedbeamforming based ranging method, the cell is divided into several sectors in the spatialdomain by a group of beamforming vectors, and the multiuser detection and timingoffset are performed by the successive interference cancellation method and adaptivethreshold at each sector. Simulation results show that the proposed method can reduce the active users in a given sector and which mitigate the MAI from other sectors, theproposed method has a much better ranging performance and accommodates moreacitve users simultaneously accessing into a cell. In adaptive beamforming basedOFDMA ranging method, according to the angle of arrival (AOA) difference of validpath of ranging signals, adaptive beamforming weight vectors are introduced to point tothe directions of the desired ranging signal of valid path, a new adaptive beamformingbased OFDMA ranging method is proposed by using the multiuser parameter estimationand interference cancellation of the valid path. In the proposed method, the residualMAI and the computational complexity are analyzed. Simulation results show that theresidual MAI is efficiently reduced by using the spatial filter based on the adaptivebeamforming, and the proposed method yields dramatic ranging performance and theaccess capacity improvements with a little increase in computational complexity.Compared with MIMO based ranging method, smart antennas based ranging methodhas a much better ranging performance and improves the capacity of ranging users.