Study On Enhancement Technologies For Pilot-based Channel Estimation In Wireless Communication Systems

With the rapid development of wireless communication technologies,wireless communication systems try to achieve reliable data transmission in high-speed mobile channel environment,which undoubtedly put forward higher requirements on the channel estimation technologies.In the high-speed mobile scenes,the large Doppler frequency offset brings the fast time-varying fading channel,resulting in the inter carrier interference.At the same time,the serious multipath effects causes the channel to exhibit frequency selective fading characteristics,resulting in the inter symbol interference.The two factors above-mentioned make the system performance seriously degraded.In order to ensure the transmission performance of wireless communication systems,this paper investigates the channel estimation technologies from the aspects of improving the channel estimation accuracy and reducing the computational complexity to solve the problem of multipath time-varying in the high-speed mobile channel environment.The contributions are summarized as follows:1.This paper studies the iterative channel estimation technologies based on block-type pilots,and proposes a low complexity weighted LS soft iterative mobile channel estimation algorithm.The proposed algorithm directly uses the bits soft information from the soft demodulator to iteratively estimate the channel without the need of re-encoding,which simplifies the computational complexity of the reconstructed symbol module.Meanwhile,considering the influence of noise in each iteration of channel estimation,we modify the weights of the weighted LS channel estimation to improve the estimation accuracy.Furthermore,the fixed polynomial model is used to fit the estimation,which further reduces the computational complexity and improves the estimation accuracy.Based on the 3GPP LTE uplink system,our simulation results show that the block error rate can be 10^-4 when the Doppler frequency offset is 800Hz?the RF frequency of 2.6GHz and the moving speed is 330Km/h?.Compared with the linear interpolation channel estimation algorithm based on the least squares LS and the weighted LS iterative channel estimation algorithm,the proposed algorithm improves the signal-to-noise ratio by about 6dB and 0.7dB respectively.2.This paper studies the time-frequency domain cascaded interpolation channel estimation techniques based on scattered pilots,and a joint time-frequency-domain interpolation channel estimation algorithm is proposed based on the weighted virtual pilots for OFDM systems.The proposed algorithm,in time-frequency domain,uses different interpolation algorithms to reduce the computational complexity.We design a kind of virtual pilots based on the scattered pilot pattern in the proposed algorithm.Furthermore,a weighting function is also designed for the various channels with different Doppler frequency shifts and maximum multipath delay.In order to improve the estimation accuracy,the proposed algorithm adaptively adjusts the weights on virtual pilots in time-frequency direction.The simulation results show that when the BER can be 10^-5 when the Doppler frequency offset is 350Hz?the RF frequency of 800MHz and the moving speed is 473Km/h?.Compared with the time-frequency domain cascaded linear interpolation channel estimation algorithm based on LS and the simplified Wiener cascaded interpolation channel estimation algorithm,the proposed algorithm improves the SNR by about 2.5dB and 1.5dB respectively.Compared with the Wiener channel estimation algorithm based on MMSE criterion,the proposed algorithm only has a slight performance degradation?about 0.5dB?.