Research And Application Of Front-Ends Impairments On Ieee 802.11 AC Protocol RF Conformance Test

IQ imbalance which is refered as mismatched magnitude and pahse between the in-phase and quadrature branch in transceiver degrades the performance of the communication system, so it’s vital to improve the performance of IQ imbalance estimation and compensation algorithms. This paper mainly researches on the IQ imbalance estimation and compensation scheme for the RF test system based on IEEE 802.11ac standard. The main contents of this paper are organized as follows:Firstly, the evolution of wireless LAN standards, development of RF conformance for 802.11ac as well as the state-of-art research situation of IQ imbalance are introduced and the key technology in 802.11ac protocol including the advantages and disadvantages, MIMO-OFDM spatial multiplexing and spatial diversity technique are highlighted. Special attention is given to the lauching process when explaining the physical layer specification in detail.Secondly, both the time domain and frequency domain models are established for frequency-independent IQ imbalance on the receiver side. On that basis, classical IQ estimation algorithms are studied to lay foundation for further research and an analysis of SNR loss for different compensation schemes is presented.Thirdly, the model is rebuilt for frequency-independent IQ imbalance on the transmitter side and three IQ imbalance algorithms are proposed. The first algorithm uses the LS criterion to jointly estimate and equalize subcarriers and their mirror components in a frequency-selective channel environment. The second algorithm is based on the adaptive filtering principle, the smoothing feature of frequency channel response is used to provide intial values of equalization sequences. The third algorithm mainly takes a usage of the statistical independence characteristic of the pilot by defining the autocorrelation and pseudo-autocorrelation functions of complex signals. Furthermore, an improved frequency-domain equalization algorithm which fully employs the noise correlation in order to reduce the variance of the LS estimation error by estimating and suppressing the noise in the signal subspace is proposed.Last but not least, the key indicators for RF test system are illustrated. By combining with the synchronization module, channel estimation and equalization module and phase tracking module, the proposed IQ estimation and compensation module is adopted to establish the baseband signal processing system. The results demonstrate that the proposed algorithm provides high accuracy for the estimation of amplitude and phase mismatch. It indicates that the proposed IQ imbalance estimation and compensation scheme is competent for the usage in RF test systems.