Research On Timing And Frequency Synchronization Of IEEE 802.11ac RF Conformance Test

IEEE 802.11ac is the very high throughput WLAN standard released in 2013. The technology of radio frequency (RF) test is of great significance for 802.1 lac products. So in this paper we focus on the synchronization technology for the RF test system based on IEEE 802.11ac standard. Main contributions of this paper are as follows:Firstly, This paper gives a brief description of the knowledge about IEEE 802.11ac standard including the physical layer techniques, the principle and features of OFDM technology, the frame structure, the major parameters, the models of transmitter and receiver. Then we talk about the meaning of the RF conformance test for 802.11 ac system and introduced the RF test system used in this paper. A detailed introduction of different types of measurements includes transmit power, frequency, spectral, modulation and IQ imbalance is given afterwards.Secondly, the influence of timing and carrier frequency synchronization as well as clock error to receiver is analyzed and we make a deeply research on synchronization methods for OFDM systems, both data-aided and blind method are discussed in this paper. In consideration of the characters of 802.11 ac system we mainly analyze the S&C method, Moose method and ESCA method based on training symbols and the ML method based on cyclic prefix. Then the improved methods raised by Minn and Park and an independent symbol timing method are respectively elaborated.Thirdly, taking the disadvantages of previous method into consideration, we proposed a robust timing and frequency synchronization scheme for RF test systems. In the research of timing synchronization, an improved algorithm on the basis of delay correction method is proposed to resolve the correlation platform effect. The improved method uses a sum of more than one correlation values as the judgment variable and the simulation results show that improved method is significantly enhance the accuracy of frame synchronization. In addition, a new symbol synchronization method is proposed to avoid redundant correlation peaks and make sure to find the right start position of OFDM symbols. In the research of carrier frequency offset estimation, on the basis of previous methods an improved frequency estimation method is proposed, this method averages the estimation results from different training sequence to make the result more precise.Finally, The simulation platform of the 802.11ac receivers is implemented. Then we give a detailed explanation for every modules based on the design procedure and test the proposed synchronization method on the platform. After a series of processing of the input data which come from the vector signal analyzer, we can finally get the measurements results such as frequency offset and error vector magnitude. The comparison results show that proposed methods are suitable for RF test systems and make the receiver work stable and precise.