Hybrid Spatial Modulation Beamforming for mmWave Railway Communication Systems

Using higher frequency bands [e.g., millimeter wave (mmWave)] is the most effective and straightforward way to alleviate the bands scarcity at lower frequency bands (e.g., microwave) while simultaneously supporting the ever-growing data rate demands for future cellular networks. Cellular-networks-based high-speed railway (HSR) wireless communication systems also need to be evolved to satisfy the requirements of train-ground data transmission by using higher frequency bands. An essential characteristic of HSR propagation channels is the strong line of sight, which results in high channel correlation. Therefore, the conventional multiple input multiple output (MIMO, e.g., VBLAST, STBC) is less effective under the HSR scenario. Moreover, the high complexity of the conventional MIMO makes it hard to achieve multiple antenna gain. Spatial modulation (SM) is a low complexity and high spectral efficiency multiple antenna technology that has emerged in recent years. The properties of SM imply that it may be a potential technique for improving the performance of HSR wireless communication systems. In this paper, 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. Multiple antenna gain, which is difficult to achieve by the conventional MIMO, can be almost achieved by the proposed scheme due to analog beamforming. 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 and antenna elements in an AA on the train.