On The Linear Dispersion Code And Its Application In Cooperative Diversity Technology

Space-time code is a kind of key technology of Multiple Input Multiple Output (MIMO) system which can provide an efficient way to improve the system capacity and (or) reliability in the wireless communication field by using the space dimensionality. In order to obtain the advantages of MIMO technology in some scenes which could not be deployed with multiple antennas, cooperative diversity technology appeared. As a new spatial diversity technology, cooperative diversity had become a research hotspot in the communication filed in recent years. This dissertation is intended to research the linear dispersion code which apperared in recent years and its application in space-time cooperative diversity technology. The main works of this paper were summarized as follows:A new architecture of space-time codes as a combination of Orthogonal Space-Time Block Codes (OSTBC) and Linear Dispersion Codes was proposed in order to improve the bit error rate (BER) performance of OSTBC. The scheme proposed was named Linear Dispersion Orthogonal Space-Time Block Codes (LDOSTBC). In LDOSTBC scheme, firstly, the data was coded into LDC codewords. Then, the coded LDC substreams were coded into OSTBC codewords again. Compared with OSTBC scheme, the performance of LDOSTBC scheme with the rate MR LDC could be improved without decreasing the data rate where M was the number of transmit antennas and R was the spectral efficiency of the modulation constellation. If some rate penalty was allowed, the performance of LDOSTBC with the rate less than LDC could be improved further.Aiming at single input OFDM system, a new transmission scheme which named virtual LDC was proposed in this paper. The virtual LDC scheme transferred the LDC substreams through the multiple uncorrelated OFDM subchannels which could achieve the coding and diversity advantage as well as LDC in MIMO. It was shown that the virtual LDC scheme could improve the BER performance compared with plain OFDM system.The BER performance of the space-time cooperative diversity scheme based on LDC overcomes the traditional OSTBC cooperative diversity scheme in high rate system. Contraposing selective relaying protocol, a LDC space-time cooperative diversity scheme was proposed. By selecting different rate liner dispersion code, the scheme could trade off between BER performance and spectral efficiency. Comparing with the system adopting lower rate liner dispersion code, when the channel condition was good enough between the source and relays, the system adopting higher rate liner dispersion code could gain higher spectral efficiency by sacrificing only a little BER performance. Contraposing amplify-and-forward protocol, two kinds LDC space-time cooperative diversity schemes were proposed. The main difference between the schemes was when the noises of the relays were added to the liner dispersion codes, before or after the coding. Firstly, we proved the two schemes have the same BER performance. Secondly, the characteristic of the two schemes was analyzed and concluded that the two schemes had different characteristic in the aspect of complexity and data rate.In the space-time cooperative diversity system, the cooperative nodes often worked in an asynchronous condition for the different location. Generally speaking, there are two methods to overcome the impact of synchronization. One method is to design special structure of space-time code, the other method is to use the OFDM technology. Aiming at the first method, an asynchronous linear dispersion code (LDC) cooperative diversity scheme was proposed. The new scheme added the zero padding (ZP) between the linear dispersion codeword to mitigate the effect of asynchronism. The length of ZP was decided by the relative timing errors between different relays. An easy decoding method of our scheme by restructuring the stacked channel matrix was also given in this paper. Analysis and simulation showed that the new scheme could work well in asynchronous condition.Aiming at the second method, a LDC cooperative diversity scheme in OFDMA system was proposed. In this scheme, the subcarriers of the first cooperative phase were allocated according to the structure of liner dispersion code and the uncorrelation of OFDM subcarriers. By this way, the substreams of the liner dispersion code in the different row experienced independent fading. The receiver restructured the receiving information of the first phase to virtual space-time code and combined it with the information of space-time cooperation received during the second phase and decoded. Comparing with traditional strategy, the BER performance was improved further.