Study On Mixed Pilot-Aided Channel Estimation Of OFDM Systems

The target of broad band wireless communication is to transmit data fast and reliably, while there are two tough problems in front of us, they are multipath fading channel and bandwidth efficiency. In Orthogonal Frequency Division Multiplexing (OFDM) system, a high-rate serial data stream is split into many low-rate parallel streams, thus changed the frequency selective multipath fading channel into flat fading channel in frequency domain which effectively mitigates the effects of multipath propagation. At the same time, parallel subcarriers of the system are orthogonal and overlap each other in frequency domain, this can directly and effectively increase the transmission efficiency. Due to the above desirable advantages, OFDM provide a reasonable resolvent for our new generation mobile communication. In case of getting the benefit form OFDM systems, a series technology need to be carried out. Channel estimation is one among those. Channel estimation is a kind of technology that estimates the channel impulse response between transmit antenna and receive antenna. For the sake of resuming the data from the transmitter, the estimation of wireless channel is necessary at the receiver. Besides, the frequency domination equalization of OFDM system also requests the operation of Channel estimation. So channel estimation is the basis of system's coherent detection, demodulation and equalization, plays an important role in OFDM systems.OFDM channel estimation methods can be divided into three kinds. The first one is based on pilot or training sequence, named Non-Blind Channel Estimation (NBCE). The second class is blind channel estimation method, The third class is semi-blind channel estimation method. NBCE has good capability and is easy to realize, so it's the emphasis of this thesis. The basic principle of NBCE is make use of some known pilots in transmitter and estimate channel parameter at the receiver take use of these known pilots. The estimation then can be divided into methods based on frequency domination pilot and methods based on time domination training sequence.Most OFDM systems uses channel estimation algorithm based on pilot. The general form of the pilot is block-type, comb-type, star-type, the paper investigates principles and performances of the existing channel estimation methods in block-type, comb-type, star-type pilot arrangement. To block-type pilot arrangement, the performance of channel estimation is discussed with least square criterion, minimum mean squared error estimation criterion and mend linear minimum mean squared error to comb-type pilot arrangement, the paper introduces three interpolation methods including linear interpolation. In all three pilot, when sub-channel information is estimated, the estimated accuracy is low, and the distribution and selection of the pilot are not flexible enough. To solve the above shortcomings, this paper will combine block-type and comb-type to a new joint pilot. Then using the new pilot to estimate the sub-channels is good. In conclude: In the time-varying channel, as the comb-type owes its characteristics, it can track the channel variation in time, so in terms of bit error rate it is higher than comb-type. As the MMSE criterion improves the estimation performance of the algorithm, so is better than LS criterion. The joint pilot can track the time-varying channel, and the selection and distribution of the pilot has a lot of flexibility.