Pilot-Based Channel Estimation In Ofdm System

To implement coherent detection, receivers in OFDM system need reliable and efficient channel estimation. However, due to time variance of channel state information, channel estimation tends to be inaccuracy in real system. Therefore it is vitally important to improve the reliability of channel estimation. This thesis studies the pilot-based channel estimation in OFDM system from two aspects, one is to mitigate the channel estimation error, and the other is to decrease the effect of time-varying channel.The thesis firstly gives an overview of wireless fading channel, and then studies the time-varying linear baseband equivalent model and discrete-time baseband model of the wireless channel. Due to the direct correlation between small-scale fading and multi-path channel impulse response, the wireless multi-path channel can be built as a linear time-varying channel. Then the thesis discusses both time-domain model and frequency-domain model of OFDM system, and introduces general procedures of the pilot-based channel estimation scheme in OFDM system. This scheme consists of three core issues:1) pilot design at transmitter terminal,2) channel estimation based on the pilots at receiver terminal, and3) the reconstruction of channel impulse response.In terms of pilot design at transmitter, this paper firstly describes the basic principles of pilot design:the maximum Doppler shift, which determines the minimal coherence time, and the maximum delay spread, which determines the minimal coherence frequency, jointly affect the interval between pilot pattern in both time and frequency domain. Then this thesis further elaborates the overlapping pilot design methodology and orthogonal pilot design methodology. In the perspective of channel estimation based on the pilots at receiver terminal, this thesis discusses two typical channel estimation algorithm:LS and MMSE. These two algorithms represent the trade-off between computation complexity and performance. Combined with interpolation algorithm, LS is often used to reconstruct the channel impulse response in frequency domain, while MMSE is hard to implement, due to the large computation complexity. Therefore, it is necessary to propose a novel algorithm that can improve the performance and lower the complexity simultaneously, such as LMMSE and SVD-based algorithm.With regard to the reconstruction of channel impulse response, this thesis focuses on three major scenarios, based on which, the channel estimation is conducted. From the theoretical analysis and simulation result, the conclusion can be obtained that the proposed scheme achieves compromise between computation complexity and performance.●In OFDM system with virtual subcarriers, the channel information is partially lost in the area of virtual subcarriers. Eventually, it leads to the energy leakage in time domain. This paper mainly discusses two improved channel estimation algorithms in OFDM system with virtual subcarriers, one is based on virtual subcarrier pilot interpolation, and the other is based on iteration in time domain. Simulation results illustrate that both of these two algorithms reduce the energy leakage brought by virtual carrier, and improve the system performance.●In fast time-varying channel, due to the Doppler shift, the channel impulse response is time-variant during one OFDM symbol. It plays a negative effect on orthogonality between subcarriers in OFDM system, which eventually leads to inter-cell interference (ICI). This thesis proposes an improved channel estimation algorithm on the basis of linear-approximation and piecewise approximation. The simulation results demonstrate that the proposed scheme achieves the better compromise between channel estimation performance and computation complexity.●In multi-cell TDD-OFDM system, the pilot contamination, resulted from non-orthogonal pilot training sequence used by different users, can cause larger ICI. This paper analyzes pilot contamination, and designs a novel channel estimation algorithm to mitigate the pilot contamination. Simulation results show that after cyclic shift pilot interpolation and pilot interference cancellation, the maximum transmission rate is obtained.To sum up, through research and analysis, channel estimation is the key technology in the new generation of OFDM communication system. It is beneficial to optimize the channel estimation methods to improve the system performance, to gain higher transmission rate, and to enhance reliability.