Multiple Antennas Transmission Technology For Railway Communication Systems

High speed railway (HSR) communication systems are defined to satisfy the requirements of train-ground data transmission. On the one hand, safety-critical railway signaling (e.g. train control information, security monitoring, operational services, etc.)needs to be transmitted to the ground. On the other hand, passenger's travelling experience,such as high-rate Internet access, must be supported by more reliable communication quality.Thus, there is practical motivation and interest to investigate how to improve the data rate and transmission reliability of HSR communication systems efficiently to address the different requirements of data transmission. One of the typical terrains for HSR is viaduct.The measured data and theoretical analysis have shown that the viaduct terrain may lead to a relatively clear line-of-sight (LOS) and few scatters, thereby resulting in strong spatial correlation. In addition, the train is moved with high speed under HSR scenario. Therefore,HSR wireless communication systems are quite different from public wireless communication systems. Considering the wireless channel fading characteristics and system requirements of high date rate transmission, LTE/LTE-A (Long Term Evolution-Advanced)and 5G based multiple antennas transmission technologies are investigated under HSR scenario. Fundamental theory and key technology of multiple antennas and beamforming are further researched to guarantee the requirements of train-ground data transmission and passengers travelling experience.Firstly, under HSR scenario, one of the promising solutions to improve the throughput is multi-stream beamforming. In order to maximize the throughput, a massive MIMO based adaptive multi-stream beamforming scheme is proposed, which utilizes an adaptive beam selection proposal by exploiting location information of the train. By adaptively selecting the optimal subset of beams, including active subset size and active receive antenna indices, the proposed scheme significantly outperforms single/dual -stream beamforming and conventional massive MIMO. Numerical results also show that, the throughput is not proportional to, but a nonlinear function of, the number of active receive antennas in this scenario.Secondly, for HSR, an essential characteristic of the propagation channels is the specific spatial-temporal correlation caused by dominant LOS component. Multiple antenna gain is therefore far from being achieved due to this strong channel correlation. However, it is interesting to note that, if the resulted inter-carrier interference (ICI) is elaborately controlled,high mobility may decrease the channel correlation. Spatial modulation (SM) is one of the promising multiple antenna technologies for wireless communication systems, which needs only the activation of one transmit antenna to convey information bits implicitly through the index of the active transmit antenna in addition to conveying information bits through modulation symbols during transmission. In SM, ICI is avoided which therefore helps to the implementation of the idea of decreasing channel correlation by increasing the velocity in HSR scenario. We theoretically find out that, higher velocity makes the impact of temporal correlation be more dominant, which weakens the influence of spatial correlation. In other words, spatial correlation can be weakened by high mobility. Our theoretical analysis and simulations further demonstrate that, with an appropriate scheme, massive SM MIMO is a promising solution within an appropriate range of velocity for HSR wireless communication systems.Thirdly, using higher frequency bands (e.g. mmWave) is the most effective and straightforward way to alleviate the bands scarcity at lower frequency bands (e.g. microwave)while simultaneously to support ever-growing data rate demands for future cellular networks.Cellular network based HSR wireless communication systems also need to be evolved to satisfy the requirements of train-ground data transmission by using higher frequency bands.A hybrid SM beamforming scheme operating at mmWave frequency bands is proposed for future 5G HSR wireless communication systems. The proposed scheme is designed in both analog and digital domains. In digital domain, SM is used to activate antenna array (AA)indices to convey information bits. RF beams are predefined and the optimal beams are selected to transmit modulation symbols in analog domain. Theoretical analysis and numerical results indicate that, the proposed scheme achieves a good compromise between performance and complexity. Numerical results further show that, the performance of the proposed scheme is not sensitive to the number of predefined RF beams, but it is to some extent sensitive to both AAs at base station (BS) and antenna elements in A A on the train.Then, as one of the key performance indexes (KPIs) in future wireless networks,network reliability needs to be guaranteed to provide the steady data transmission along the railway lines, especially when safety-critical railway signaling information is delivered.Therefore, based on the fact of fruitful and unlicensed frequency resource in mmWave bands,a novel beamforming scheme, namely, non-uniform steady mmWave beamforming solution is proposed to guarantee the network reliability for future HSR wireless communication systems. Then, a bisection based beam boundary determination (BBBD) method is suggested to determine the network coverage of each predefined RF beam. Numerical results demonstrate that, the proposed non-uniform steady mmWave beamforming can provide the steady data transmission along the railway lines. Consequently, our non-uniform steady mmWave beamforming is a promising solution for future HSR wireless communication systems.Lastly, HSR wireless communication system is quite different from the public wireless communication system, because it bears the train control messages, which are related to the safety of train transportation closely. Thus the system has much higher requirements for the safety and reliability. In order to guarantee the safety of train control messages, a physical layer secure scheme with artificial noise (AN) is proposed for LTE/LTE-A based wireless communication system in HSR. In the proposed scheme, beamforming and AN transmit directions are adjusted according to the onboard legitimate receiver's channel state information (CSI) and eavesdropper's partial CSI only to degrade the eavesdropper's channel SINR (Signal to Interference and Noise Ratio) as well as to maximize the system secrecy capacity. Numerical results demonstrate that the proposed scheme will decrease the outage probability under the secrecy capacity constraint, and satisfy the safety communication requirements.