Research And Application Of Channel Estimation In MIMO-OFDM System On IEEE 802.11ac Protocol RF Conformance Test

In mobile radio communication, the received signal is distorted by the doubly selective fading channels, which generally exhibit both time-selectivity and frequency-selectivity. Accurate channel estimation can effectively improve the performance of the whole system. This thesis focuses on and spreads out from the research and application of integrated channel estimation test in multiple-input multiple-output orthogonal frequency division multiplexing (MIMO OFDM) system applicable for IEEE 802.11ac protocol. The main contribution of the thesis is organized as follows.Firstly, the paper introduces the background of channel estimation techniques and points out the necessity and urgency to study MIMO OFDM channel estimation algorithms. Then, the paper studies the standard of IEEE 802.11ac physical layer as well as the key technologies in PHY implementation, and the importance of the research on basic principles and characteristics of MIMO OFDM systems is given.Secondly, the paper summaries several classical MIMO OFDM channel estimation algorithms, and makes comprehensive performance analysis and comparison between them. Besides, it describes various channel interpolation algorithms which may be used in MIMO OFDM channel estimation and compares their performance with each other respectively.Then we propose four MIMO OFDM channel estimation algorithms based on the classical channel estimation algorithms and the specific characteristics of the IEEE 802.11ac MIMO OFDM system. The algorithm, named as joint noise estimation and LS channel estimation algorithm, can effectively suppress the influence of noise by estimating and subtracting the noise component in the signal subspace. Compared to the traditional LS and LMMSE algorithms, the proposed method performs best in low SNR regions.. By considering the fact that IQ imbalance exists in transceiver inevitably, which can cause imaging interference on channel thus resulting in worse channel estimation performance. We propose a joint estimation algorithm of IQ imbalance and channel response. Firstly the algorithm uses a few pilots to estimate and compensate the IQ imbalance. Then it estimates the channel coefficients by virtue of training sequences. Simulation results show that the proposed algorithm can decrease the system’s BER obviously. Frequency-domain windowed MMSE channel estimation is also proposed to suppress the CIR leakage of the non-sample-spaced channel. In the first stage, a window function is applied to the CFR in the frequency domain. Next, the MMSE weighting operation is conducted on CIR in the time domain, and finally, the channel frequency response by DFT is computed. Compared to MMSE estimation algorithm, this method has a lower computation complexity while maintains high estimation performance. Considering the feature of IEEE 802.11ac frame structure, which contains both training sequences and pilot signals, we propose a new algorithm called LS channel estimation based on pilot tracking. The algorithm works as follows:firstly it estimates the channel frequency response by training sequence, then it equalizes the data with the estimated channel coefficients, and extracts pilot signals to track and compensate the residual phase. This method can track and compensate the residual phase effectively, thus the performance of the system is improved.Finally, the paper establishes a software platform of IEEE 802.11ac receiver, and explains the design and implementation of the receiver’s respective module according to the receiving process. We apply the proposed channel estimation algorithms into the actual receiver, and use the data transmitted by DUT as inputs to validate the RF conformance of the IEEE 802.11ac transmitters. By comparing the testing results of Aeroflex and ours, we are able to draw the conclusion that the channel estimation schemes proposed in this dissertation can work well in the RF conformance testing instruments.