In pilots-aided OFDM systems, the channel estimation relies on the pilots inserted in the systems. Since the number of pilots is usually limited, the performance of the channel estimate may not be satisfactory when the delay spreard of the channel is large. Recently, a joint time and frequency domain estimator, which can grealty outperform the conventional frequency domain channel estimator, has been proposed for DVB-T systems. However, this method need to collect four OFDM symbols in order to conduct the estimation. This will significantly increase the memory size and cause problems in time-variant channels. In this thesis, we propose new joint time and frequency domain methods to solve the problem. In the first part of this thesis, we propose new methods for the time-invariant channel estimation. In the second part of this thesis, we extend the proposed methods to accommodate time-variant channels. The distinct feature of the proposed methods is that only one OFDM symbol is required even when the pilot density is very low. Simulations show that the performance of the proposed methods can approach the optimum.