Research On Massive MIMO And Orbital Angular Momentum Wireless Transmission Technologies

With the large-scale commercial application of the 4th generation mobile communication technologies and the rapid growing of the number of mobile communication terminals,the wireless throughput in the next decade will rise to 1000 times comparing with the wireless throughput today.This is a big challenge for today's mobile communication technologies.Massive multiple-input multiple-output(MIMO)technology is able to significantly increase the spectral efficiency of mobile communication systems,which is considered as one of the key technologies to deal with the challenge.When practically deploying massive MIMO antennas,hundreds of antennas in the massive MIMO base station will probably be very close to each other due to the limited size of the antenna array.Moreover,complex deploying scenarios,such as buildings facades and signage,will lead to irregular antenna spacings.In this case,the mutual coupling effect between antennas and the channel spatial correlation cannot be neglected when investigating the performance of massive MIMO systems.In addition,a great number of radio frequency chains in massive MIMO base stations have tremendous hardware complexity and consume huge amounts of energy.Reducing the energy consumption and hardware complexity caused by these radio frequency chains is of great importance.Meanwhile,the orbital angular momentum(OAM)technology is capable of providing a new degree of freedom for wireless communications,which may bring great increment in the system capacity.But in the microwave and millimeter wave frequency band,how to efficiently exploit this degree of freedom provided by OAM still remains a controversial problem and needs further investigation.Motivated by the above gaps,this thesis concentrates on the performance study and energy efficiency optimization of massive MIMO systems considering practical issues.Furthermore,the design and performance of wireless communication systems applying OAM technologies is also studied.The main contributions of this thesis are summarized as follows:Firstly,the single cell multiuser massive MIMO system equipped with a rectangular array is studied considering mutual coupling effects.Based on the mutual coupling between antennas,the uplink signal received by the base station is modelled.When maximum ratio combining detection is assumed at the base station,the lower bound of the uplink achievable rate of massive MIMO systems is derived with analytical results.The impact of the antenna spacing and the number of antennas on the uplink achievable rate is further studied through numerical simulation.Simulation results reveal that when the size of the antenna array is fixed,the system uplink achievable rate increases with the increasing of the number of antennas.The mutual coupling effects evidently degrades the uplink achievable rate,especially when the antenna spacing is small or the number of antennas is large.Secondly,the multiuser massive MIMO systems with irregular antenna arrays are investigated considering the mutual coupling between antennas.The mutual coupling effects and array steering matrix are firstly modelled based on the distribution characteristics of the antennas in the irregular array.Then according to the mutual coupling and array steering matrix model,the channel matrix correlation coefficient and the ergodic received gain are identified and derived to evaluate the impact of mutual coupling on irregular antenna arrays.Based on the distribution of the eigenvalues of the channel correlation matrix,numerical results illustrates that when the size of the array is smaller than a threshold,the irregularity of the array helps to decrease the channel correlation.When the size of the array is fixed,the ergodic received gain firstly rises then declines with the increasing of the number of antennas.Furthermore,the lower bound of the uplink achievable rate,the symbol error rate and the outage probability of the massive MIMO systems with irregular antenna arrays are analytically derived.Asymptotic results are also given when assuming the number of antennas grows without bound.Based on the derived results,numerical simulation results shows that mutual coupling obviously degrades the system performance.When the number of the antennas is larger than a threshold,the uplink achievable rate of the massive MIMO systems with irregular antenna arrays is larger than that with regular antenna arrays.Numerical simulation results also match well with the Monte-Carlo simulation results.Thirdly,the energy efficiency optimization problem of the hybrid precoding massive MIMO systems is studied considering the hardware cost and energy consumption.Based on the massive MIMO energy consumption model and the constraints for practical systems,the energy efficiency optimization problem is firstly modelled.Then by combining the RF precoding and the baseband precoding,the original problem without tractable global solutions is transformed into the problem with solvable local optimization results.The locally optimal hybrid precoding matrices and the number of RF chains are obtained accordingly.And the energy efficient hybrid precoding(EEHP)algorithm is also proposed.To further reduce the cost and complexity of the RF chain system,the energy efficient hybrid precoding algorithm with the minimum number of RF chains(EEHP-MRFC)is then proposed.When the number of RF chains is the minimum,the numbers of base station antennas and user equipments that optimize the energy efficiency is derived.Based on the derivation,the critical number of antennas searching(CNAS)algorithm and the number of user equipments optimization(NUEO)algorithm are proposed.Simulation results illustrates that the proposed EEHP and EEHP-MRFC algorithms effectively increase the base station energy efficiency,and the maximum energy efficiency is improved by 120%and 71%comparing with conventional zero-forcing precoding algorithm,respectively.Experiment results reveal that the EEHP-MRFC algorithm maximumly saves 46.1%and averagely saves 23.4%of the transmission power comparing with the zero-forcing precoding when the experiment transmission rate is 100 Mbps.At 60 Mbps transmission rate,the EEHP-MRFC algorithm maximumly saves 48.3%and averagely saves 27.1%of the transmission power comparing with the zero-forcing precoding.Fourthly,we investigate the application of OAM technologies in wireless communications and propose the orbital angular momentum-spatial modulation wireless communication system.The transmitter-receiver structure and the modulation-demodulation method are designed based on the propagation and intensity spatial distribution characteristics of OAM signals.The OAM wireless channels in the proposed system are also modelled.According to the proposed system model,the capacity,average bit error rate and energy efficiency of the OAM-SM system is derived.Numerical results shows that the OAM-SM system evidently improves the capacity through exploiting the degree of freedom of OAM technologies in the code domain.Comparing with conventional point-to-point MIMO systems,when the transmission distance is long,OAM-SM systems achieve high capacity without increasing the average bit error rate.Additionally,when the total transmit power is the same,OAM-SM systems achieve much higher energy efficiency comparing with conventional MIMO systems.In summary,this thesis makes innovative research on massive MIMO systems' actual performance considering practical deployment,the energy efficiency optimization of massive MIMO systems and applications of the OAM technology in wireless communications.The research results are instructive for the practical deployment of massive MIMO systems.Furthermore,the complexity and energy comsumption of massive MIMO systems can be reduced by the research findings.The results also provide references for the practical design of OAM wireless communication systems.