The orbital angular momentum (OAM) technique provides a new degree of freedom for information transmissions in millimeter wave communications. Considering the spatial distribution characteristics of OAM beams, a new OAM spatial modulation (OAM-SM) millimeter wave communication system is first proposed for future mobile networks. Furthermore, the capacity, average bit error probability and energy efficiency of OAM-SM millimeter wave communication systems are analytically derived for performance analysis. Compared with the conventional multi-input multi-output (MIMO) millimeter wave communication systems, the maximum capacity and energy efficiency of OAM-SM millimeter wave communication systems are improved by 36% and 472.3%, respectively. Moreover, numerical results indicate that the proposed OAM-SM millimeter wave communication systems are more robust to path-loss attenuations than the conventional MIMO millimeter wave communication systems, which makes it suitable for long-range transmissions. Therefore, OAM-SM millimeter wave communication systems provide a great growth space for future mobile networks. Index TermsMillimeter wave communications, orbital angular momentum, spatial modulation, capacity, average bit error probability, energy efficiency.with respect to the transmission SNR is illustrated in Fig. 11. When the transmission SNR is less than 10 dB, the energy efficiency of OAM-SM millimeter wave communication systems increase with the increase of the transmission SNR. When the transmission SNR is larger than or equal to 10 dB, the energy efficiency of OAM-SM millimeter wave communication systems decrease with the increase of the transmission SNR. There exist a maximum energy efficiency of OAM-SM millimeter wave communication systems considering different transmission SNR values. When the transmission SNR is fixed, the energy efficiency of OAM-SM millimeter wave communication systems is always larger than the energy efficiency of MIMO millimeter wave communication systems. To be specific, compared with the energy efficiency of MIMO millimeter wave communication systems, the maximum energy efficiency of OAM-SM millimeter wave communication system is improved by 472.3%. VI. CONCLUSIONS Exploiting spatial distribution characteristics of OAM beams, a new OAM-SM millimeter wave communication system is first proposed for future mobile networks. Important performance metrics like capacity, ABEP and energy efficiency of OAM-SM millimeter wave communication systems are analytically obtained in closed forms. It is shown analytically that significant performance gains can be achieved by the OAM-SM millimeter wave communication systems over the conventional MIMO millimeter wave communication systems, e.g., the maximum capacity

The Orbital Angular Momentum (OAM) wireless communication technology has received much attention in recent years for its natural orthogonality between different OAM states. Combining the OAM technology with the Orthogonal Frequency Division Multiplexing (OFDM) technology, we proposed a new OAM-OFDM wireless communication system to explore a new approach for enhancing the transmission capacity of the wireless communication system. Atmospheric turbulence is an important factor for influencing the capacity of the OAM-OFDM wireless communication system. In consideration of the effect of the atmospheric turbulence, we derived a crosstalk model of the proposed OAM-OFDM wireless communication system. Furthermore, a capacity model of the OAM-OFDM wireless communication system is proposed in consideration of the effect of atmospheric turbulence on both the OAM and OFDM signals. Compared with the conventional OAM wireless communication system, the capacity performance of the proposed OAM-OFDM wireless communication system is significantly improved and the average improvement is 751%. INDEX TERMS Orbital angular momentum, capacity, bit error ratio, orthogonal frequency division multiplexing.

The orbital angular momentum (OAM) wireless communication technique is emerging as one of potential techniques for the Sixth generation (6G) wireless communication system. The most advantage of OAM wireless communication technique is the natural orthogonality among different OAM states.However, one of the most disadvantages is the crosstalk among different OAM states which is widely caused by the atmospheric turbulence and the misalignment between the transmitting and receiving antennas. Considering the OAM-based multiple-input multiple-output (OAM-MIMO) transmission system with unaligned antennas, a new channel model is proposed for performance analysis. Moreover, a purity model of the OAM-MIMO transmission system with unaligned antennas is derived for the non-Kolmogorov turbulence. Furthermore, the error probability and capacity models are derived for OAM-MIMO transmission systems with unaligned antennas. To overcome the disadvantage caused by the unaligned antennas and non-Kolmogorov turbulence, a new optimization algorithm of OAM state interval is proposed to improve the capacity of the OAM-MIMO transmission system. Numerical results indicate that the capacity of OAM-MIMO transmission system is improved by the proposed optimization algorithm. Specifically, the capacity increment of the OAM-MIMO transmission system adopting the proposed optimization algorithm is up to 28.7% and 320.3% when the angle of deflection between the transmitting and receiving antennas is -24 dB and -5 dB, respectively.

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