On The Pre-rake And Related Techniques Based On New Coding Approaches In TDD-CDMA System

Compared with the frequency division duplex (FDD) CDMA system, time division duplex (TDD) CDMA has the advantages of better bandwidth utilization, flexible spectrum allocation, channel reciprocity between uplink and downlink and so on, thus is a potential candidate for the next generation mobile communication technology. For TDD-CDMA system, spreading codes and error correcting codes play very important role, where the correlation properties of spreading codes is essential for reducing various interference, and error correcting codes offer extra protection for digital signal transmission. In this thesis, the Pre-rake and related techniques based on new coding approaches in TDD-CDMA System are investigated.First of all, in TDD-CDMA system, more multipath interference is introduced by Pre-rake combining. The generalized orthogonal codes have been given to reduce greatly the transmission delay caused multipath interference. By theoretical analysis, the power of received signal to interference and noise ratio(SINR) has been derived. Compared with the conventional orthogonal codes, by the proposed generalized orthogonal codes based Pre-rake combining in TDD-CDMA, higher SINR can be achieved, resulting better bit error rate (BER) and bigger capacity.In TDD-CDMA system, because of the time variation characteristic of wireless channel, the channel state of uplink time slot and the next downlink time slot cannot be exactly the same, and the Pre-rake combining base on imperfect channel estimation will lead to the reduction of received SINR. In the thesis, the Pre-rake combining with generalized orthogonal codes under imperfect channel estimation has been discussed, the simulation results show that the proposed scheme has good performance even under imperfect channel estimation, which can be explained that the most significant interference arrived in the generalized orthogonal zone can be eliminated.The combining method of Pre-rake has been investigated, and OT-MRC based Pre-rake combining method is proposed. With the pre-selected threshold, stable performance of Pre-rake combining can be achieved, and at the same time, the Pre-rake combining complexity can also be reduced. Furthermore, a Pre-rake combining with minimum Interference employing generalized orthogonal codes is proposed. Due to the correlation properties of generalized orthogonal codes and partial combining method, diversity combining can be achieved, and interference can be reduced to very low level.