The Parameter Estimation Methods Based On The Noise Subspace Theory In OFDM System

Orthogonal frequency division multiplexing (OFDM) has recently been applied in wireless communication systems due to its high data transmission capability with high bandwidth efficiency and its robustness to multipath delay.In this letter we focus on the study of accurate and efficient time-frequency synchronization methods since OFDM system is extremely sensitive to synchronization errors and carrier frequency offset contrast to single carrier modulation system. Estimation of the channel state information is a prerequisite to realize the coherent demodulation of the received signal. In order to obtain accurate channel estimation, OFDM system is to send a large number of pilot symbols, resulting in a decline in the efficiency of spectrum. How to transport the control information implicitly using pilot symbols is also the main content of this study.First, we study the symbol synchronization in OFDM system which gives a novel timing synchronization method based on noise subspace. This method is based on the cyclic structure of OFDM symbols and well-known DFT-based channel estimation techniques. First, transfer the frequency domain least mean square channel estimation which is after the phase rotation compensation into time domain, and then define the former G values in the channel impulse response as the signal subspace, the remaining Np-G values is defined as the noise subspace, and finally estimate the starting point of the symbol by calculating the variance of the noise subspace. Through the oretical analysis and computer simulation, we found that compared with the traditional timing error estimation method based on correlation,the method proposed in this paper can get unbiased estimation results under the multipath channel environment, and the traditional method based on the correlation are biased estimation in the multipath environment.Secondly, we study the carrier frequency offset estimation in OFDM system which gives a novel carrier frequency offset estimation method based on noise subspace. First, we derive the channel impulse response estimation, and then divide the noise subspace by the signal subspace of the impulse response mean square value using the subspace theory, finally find the ratio which makes the minimum value of frequency deviation to obtain the correct frequency offset estimation. Through the simulation we find that, compared with the traditional design through the training sequence for the method for frequency offset estimation, the proposed method of frequency offset estimation range can reach the entire system bandwidth, and does not affect the performance of channel estimation, which can be used for communication systems with any form of training sequence.Finally, this paper studies the channel parameters and control signaling blind estimation method. The fourth generation mobile communication system is an environment adaptive system, which can adjust the system parameters according to the symbol actual communication environment. It can improve the information transmission efficiency, but it also brings a big increase of the control signaling. For this reason, this paper studies transmission technology of implicit signaling and the blind detection methods at the receiver. This paper puts forward the method of implicit signaling blind detection method based on the noise subspace theory. Through the simulation we find that the proposed method overcomes the dependence problem of traditional method on adjacent subcarrier channel correlation, but also reduce the error detection probability of the pilot sequences. Another advantage of this method is it can estimate the carrier frequency offset without doing channel estimation.lt can also achieve the integer frequency offset and achieve control information and channel parameters joint blind estimation.