Research On Key Techniques Of MIMO Systems

As the growing of the number of wireless users and business needs, the traditional SISO communication system has been unable to meet the next-generation wireless communication systems for data transmission rate and transmission quality. Limited wireless communication resources impel the high spectral efficiency communication technologies continue to emerge. MIMO technology partitions the space while multi-carrier technology partitions the frequency, and two degrees of freedom can be obtained. Making use of multi-path components in the space, MIMO technology can increase system capacity or enhance the robustness of communication systems without extra bandwidth. However, it is very sensitive to the frequency selective fading. OFDM technology, a multi-carrier modulation, can transform the frequency selective fading channel into a flat fading channel, which effectively compensates for the defect of MIMO technology. Besides, the sub-carries in OFDM systems are mutually orthogonal and overlapping, which can improve the system spectral efficiency. But OFDM system performance deteriorates seriously because of the multi-path effects. MIMO technology combined with OFDM technology can give full play to the advantages of both meanwhile make up for their deficiencies. Therefore, MIMO-OFDM technology is considered as the key technology of next generation wireless communication systems, and MIMO multi-carrier systems are also a hot topic.In this dissertation, MIMO multi-carrier system is studied in-depth, including the analysis of wireless channel, the derivation of the optimal channel capacity, the exploration of optimal utilization of space degrees of freedom and the cancellation of MIMO-OFDM system phase noise, as follows:1) In-depth analysis of SCM proposed by the 3GPP. First, SCM is pretreated to be suitable for the digital simulation. Then SCM sub-channel cross correlation is analyzed under multi-antennas condition in both theory and simulation. Finally, SCM channel capacity is derived without and with mutual coupling effect and the simulations is provided to verify the theoretical derivation.2) The optimal channel capacity is derived based on the three-dimensional channel partition theory. MIMO multi-carrier systems have time, frequency, and space degrees of freedom, and this three aspects can be taken into account to optimize and design the communication systems. Based on previous research, this dissertation first defines the conception of three-dimensional joint resources and gives the system model with a group of symbols as a studying unit. Then the corresponding constraints are established, and the objective function of the optimal capacity is derived. Finally, the upper bound of the system capacity under the three-dimensional channel partition is obtained.3) A generalized spatial modulation is put forward. In traditional MIMO systems, the antennas are employed to form physical links for information transmission. In generalized spatial modulation systems, the combinations of transmit antennas are used to convey information bits in addition to forming physical links. The selected transmit antennas are considered as virtual spatial constellation which is adopted to modulate information bits. Compared to SISO systems, the generalized spatial modulation systems improve the system spectral efficiency. In addition, since all the selected transmit antennas transmit the same digital modulation symbols, the channel interference can be avoided while diversity gain can be obtained. Therefore, the robustness of the communication systems is enhanced. In this dissertation, the proposed algorithm is analyzed in theory and verified by the simulations.4) The code words of SSK modulation are optimized. For the property of reconfigurable antenna, it can be adopted to optimize the SSK code words. Theoretical analysis shows that the signal detection accuracy of SSK modulation systems with such code words is improved. The bit error rate simulation validates the results of theoretical analysis.5) The methods that employing space redundancy to correct the signal error is studied. In traditional communication systems, time redundancy is adopted to do signal error detection and error correction. MIMO technology, which makes the space can be divided, increases the degree of freedom of the communications system. Two methods to implement channel coding in space domain are studied in this dissertation. The system block diagrams are presented and the principles are analyzed. The simulations are done to illustrate the system performance.6) The phase noise cancellation algorithm of MIMO-OFDM systems is explored. The improved phase noise cancellation algorithm and MIMO-OFDM system demodulation based on all phase fast Fourier transform are discussed respectively. For the improved phase noise cancellation algorithm, the phase noise property is first studied. Base on it, the signal structure is designed. The phase noise of MIMO-OFDM systems with this signal structure can be reduced. For all phase FFT demodulation, because all phase FFT does not affected by the phase shift, therefore it can effectively avoid the phase noise. The superiorities of the algorithms are verified both by theoretical and simulation analysis in this dissertation.