| As one of Multi-carrier transfer technologies, OFDM can reduce Inter-symbol Interference (ISI) in high data rate systems. And it can also mitigate multi-path and dispersive channel interference. So OFDM has already become one of the most attractive technologies for new generation wireless communications.Because of the characters of wireless channel, channel estimation is a key technique to reduce multi-path and dispersive channel interference. The receiver of OFDM system cannot demodulate the useful signal without channel estimation. In OFDM systems, Channel estimation has to get the real-time channel response as precise as possible. Now the popular Channel estimation algorithms are based on Pilot Symbol Assisted Modulation Channel estimation. This kind of algorithms need inserting pilots into data frame, so it must waste certain amount of bandwidth of the system to send pilot.Superimposed-pilot-based channel estimation algorithm is to superimpose pilot on data signal. In this way, it will not use the limited sub-carriers and time slots. At receiver, utilize the condition that data signal and noise both are zero-mean to acquire the precise channel response.Currently available superimposed-pilot-based channel estimation algorithms have two main problems. One is the large transmit power; the other is high PAPR (Peak to Average Power Ratio). The two problems are related to the format of the pilot chosen. Now two types of pilots are often used as superimposed pilot——Periodic impulse sequence and Constant magnitude sequence. Both of them have their merits and weaknesses. Periodic impulse sequence has a small transmit power, but it will bring high PAPR and thus deprave the system performance. It also can't be used in Multi-carrier OFDM system. Constant magnitude sequence has a low PAPR and precise channel estimation, but it will greatly increase the transmit power.Based on these points, this paper introduces a new channel tracking algorithms using superimposed pilots. This algorithm combine above two superimposed pilot sequence to release the conflict between transmit power and PAPR. Meanwhile the simulation result shows the algorithm has a good performance in channel estimation. But the algorithm can just be used in the channel which changes slowly. This paper raised another algorithm which combine superimposed pilot with conventional pilot to tracking the change of the channel. This algorithm can be used for channel which changes a little faster. Since we have already got the first time channel performance through superimposed pilot channel estimate, the tracking part could insert fewer pilots. It can also save the bandwidth of the system. The simulation result shows the algorithm also has a good performance in channel estimation. |
