Researches On Key Techniques Of Asynchronous Transmission Signal Design And Detection In MIMO Systems

The next generation mobile communications system, IMT-Advanced, has already begun collecting technology needs in the world. Beyond the demand for very high transmission data rate, there are also three bottleneck problems restricting the technology development of the next generation mobile communications system: where to set up a new base station, how to reduce electromagnetic radiation, and spectrum resource tension.From the signal design and detection point of view, a possible way to solve the bottleneck problems that the next generation mobile communications system faces to, is to design a high spectral efficiency space-time multiple input multiple output (MIMO) architecture that can be detected by a low computational complexity receiver over the ill-conditioned MIMO channels. This architecture is also the focus in this thesis.First, this thesis focuses on a modified Vertical-Bell Laboratories LAyered Space-Time (V-BLAST) system, where different delay offsets are intentionally applied to the spatially multiplexed data streams. The proposed system can use only one receive antenna to recover the transmitted information by zero forcing (ZF) detection, whereas the conventional synchronous V-BLAST system requires that M_r≥M_t, (where M_t and M_r are the number of transmit and receive antennas, respectively). Compared with the conventional synchronous V-BLAST system, the modified system can achieve M_r th-order diversity using ZF detection; however, the increase in the diversity order comes at the cost of the multiplexing gain. Although delay offsets in the system introduce an attenuation factor that decreases the instantaneous signal-to-noise ratio, the BER performance of the modified system with proper system parameters demonstrates a performance improvement over the conventional synchronous V-BLAST system.Second, the correlation of the spatial shaping pulses is analyzed to be one of the available characteristics that can be exploited to realize the spatial multiplexing over the ill-conditioned MIMO channels, for example, Ricean MIMO channels. From the correlation point of view, we propose an asynchronous layered space-time architecture and derive the exact bit error rate (BER) performance in Ricean fading channels. The theoretical analysis and simulation results show that the proposed layered space-time strategy offers a significant performance improvement with the increase of Ricean K-factor.Finally, we employ the asynchronous transmission idea to the multiple antennas spread spectrum systems. We note that the asynchronous scheme has its advantage with BER compared to the conventional synchronous situation. It menas if the requirement for the BER is the same, the asynchronous scheme can save the transmission power to reduce interference in the networks, therefore, the user capacity will be close to the theoretical case.The research results in this thesis provide a space-time coding strategy to meet the physical layer technology needs in the next generation mobile communications system. By employing the correlation of the spatial shaping pulses, the receiver can use the low computational complexity detector to realize the spatial multiplexing over the ill-conditioned MIMO channels. The asynchronous layered space-time architecture can be applied in the IMT-Adavancd, IEEE 802.11, and IEEE 802.16 systems.