Research On Massive MIMO Under High-Speed Railway Scenario

Being able to improve the system capacity manyfold without increasing the frequency spectrum resource and transmission power, multiple-antenna technologies have been regarded as one of the key technologies of next-generation communications. Through further increasing the number scale of antenna elements, Massive MIMO system is able to provide more powerful potential for conventional multiple-antenna technologies. However, with the significantly increased number of antennas, many new challenges is being aroused. In contrast with the researches to Massive MIMO in cellular network, the communication network architecture and predictable mobility in high-speed railway can remarkably simplify the application of this technology. Therefore, it’s meaningful to research the key technologies of Massive MIMO under the high mobility and railway communication scenario according to the characteristics of high-speed railway.Through the analysis to the environment features of high-speed railway communication and the characteristics of multiple-antenna technologies, this thesis mainly carries out the Massive MIMO researches from the aspects of adaptive multiple-stream multiplexing solution in highly correlated high-speed channel and the adaptive beamforming technology under high-speed mobility.To tackle with the strong correlation in high-speed railway communication channel brought by dominant LOS (Line Of Sight) transmission and limited scatterrers, this thesis proposes to reconstruct the highly correlated high-speed railway channel into an equivalent aggregated channel with lower correlation by utilizing the method of principal component analysis. Then, a new optimization model for the adaptive multi-steam multiplexing technology is developed based on this aggregated channel. In this optimization process, the objective is to maximize the system capacity under the constraint of required condition number of channel matrix so as to guarantee the link reliability. Through simulation, the proposed solution is verified to be capable of significantly improving the system capacity when the train is running near the BS (Base Station) and maintaining the performance of conventional transmission solution when the train is near the cell edge.The high-speed railway channel with sparse scatterrers can only utilize less than 30% spatial degree of freedom to transmit multiple streams parallelly and conventional beamforming is also invalid in improving system performance when the energy reaches the amount that the highest modulation and coding level requires. In order to further explore effective spatial degree of freedom and enhance the engergy efficiency in high-speed railway, a new DOA (Direction Of Arrival) based adaptive multi-beam beamforming solution is proposed based on the train mobility pattern in railway environment. In this solution, the insulation in space among beams is explored to enhance parallel transmission with multiple beams. Through simulation, the proposed beamforming solution is verified to have better system capacity and energy efficiency than these in conventional beamforming solutions.