In the upcoming B5G and 6G, orbital angular momentum (OAM) can be used as the new dimension for data transmission between the base-station (BS) and the self-backhauled small cell (SBSC) station. However, the OAM beam is not suitable for the long distance transmission due to its inverted cone shape. Moreover, the commonly used uniform circular array (UCA) communication system with fixed radius can only obtain limited channel capacity. In order to obtain higher channel capacity, this paper proposes an optimized transmission system combining UCA and non-twisted parabolic antenna (PA). The optimization of UCA deployment radius enables the combined system to obtain the maximum channel capacity for the transmission at a specific distance. Compared with the traditional OAM system and the multiple-input multiple-output (MIMO) system, the channel capacity can be increased nearly up to two and four times respectively without increasing the bit error rate (BER) and complexity significantly.

With the same transmission rate and Bit Error Rate (BER) as the traditional Direct Sequence Spread Spectrum (DSSS) system, the stronger anti-interception performance can be obtained by Orbital Angular Momentum (OAM). In other words, the contradiction between the transmission rate and the security can be solved by the double-channel OAM Spread Spectrum (SS) system. In this paper, the performance of the low probability of intercept and the low probability of detection are evaluated by Computational Complexity (CC) and Channel Exposure Time (CET), respectively. In addition, the case of trans-mitting the generator polynomial coefficients by OAM, as well as the cases with encryption algorithms, are proposed and verified. As a result, these proposed schemes greatly increase the CC and reduce the CET, which means that the great improvement of the anti-interception performance can be achieved.

Besides promotion in the spectrum efficiency, orbital angular momentum (OAM) can bring additional benefits to physical layer security. In this study, the authors will highlight the benefit of covert transmissions to deal with the contradiction between transmission rate and security in the traditional direct sequence spread spectrum (DSSS) system. Based on OAM, a secure high‐speed spread spectrum (SS) transmission system is proposed. Specifically, the system architecture and mathematical models are established. Afterwards, the detection method for the proposed OAM SS system is investigated. Meanwhile, the corresponding bit error rate (BER) and performance of anti‐interception are analysed. The performance of data anti‐interception is evaluated based on computational complexity, while the performance of signal anti‐interception is studied by Neyman–Pearson criterion. Furthermore, Monte–Carlo simulations are also implemented to show the effectiveness of the proposed system. Compared with the traditional DSSS system, the higher security can be achieved by the proposed system. Besides, the transmission rates and BERs are the same for the traditional DSSS system and proposed system. Therefore, the proposed OAM SS transmission system successfully handles the contradiction of the transmission rate and security. In other words, the secure high‐speed transmission can be achieved by the proposed OAM SS system.

In order to deal with the power leakage of side lobes in physical layer security, this paper investigates a secure transmission system aided by Orbital Angular Momentum (OAM) jamming. The receiver, located at the center of the beam, remains unaffected due to the power null of OAM radio wave. Based on its distinctive power null, a secure transmission system with imperfect Channel State Information (CSI) and random eavesdroppers is proposed in this paper. To be specific, the artificial noise and the desired information signal are transmitted by the radio wave with OAM and without OAM respectively. Meanwhile, a power allocation optimization algorithm is designed to acquire the target secrecy capacity. Simulations are implemented to evaluate the effectiveness of the proposed algorithm compared with other methods. Furthermore, we find that there is a secrecy capacity upper bound in the presence of imperfect angle measurement and random eavesdroppers. Moreover, the proposed power allocation algorithm leads to a higher secrecy capacity upper bound.

Electro-Magnetic (EM) Waves with different Orbital Angular Momentum (OAM) modes can be multiplexed to enjoy the high spectrum efficiency and transmission rate. However, the OAM waves can hardly be multiplexing transmitted to long-distance in free space especially at the Radio Frequency (RF) band due to its inverted cone beam structure. As a promising solution to overcome the difficulties, the OAM multiplexing with distributed pseudo-coplanar relay is highlighted with experimental demo and evaluation. In this paper, we realize a real-time multiplexing transmission experiment with four OAM modes at 10 GHz. Specifically, the pseudo-coplanar scheme is also demonstrated by a helicopter experiment, which emulates the ground-to-air transmission process between the ground station and the satellite formations when OAM EM waves are applied in the satellite communications.

Orbital Angular Momentum (OAM) has been considered as a new dimension against the traditional domains based on the electric field strength of Electro-Magnetic (EM) waves. For the long distance transmission more than 1 km, especially for the EM waves under 300 GHz, only a partial wavefront can be received. In this case, the orthogonality of OAM cannot be easily maintained as it does in the coaxial all phase plane receiving. In this paper, we introduce the concept of OAM nondegenerate index mapping in the partial phase plane receiving, in which the user information bits are mapped to the indices of the available OAM mode combinations (by adjusting the on-off status of available OAM modes). Then, besides the classical two-dimensional signal constellations in the traditional main channel, the OAM-based index mapping transmission scheme can be established and provide the additional index channel to convey information by the variation of OAM modes. Moveover, in order to confirm the validity and efficiency, a 30.6 km long distance transmission experiment with the proposed OAM index mapping scheme is also carried out.

Establishing the passive magnetic multi‐input multi‐output (MIMO) transmission is seldom reported and still confronts some challenging problems, such as (i) since the sensor energy is induced from the transmitted signals by the sensor coils, only one‐round ‘ping pang access (PPA)’ to the access point (AP) is permitted; (ii) Channel state information (CSI) should also be achieved within aforementioned PPA; and (iii) for more applications, both one‐dimensional (1D) coil and three‐dimensional (3D) coils should be considered. In this study, the magnetic positioning information is employed to solve the above problems and successfully establish the open‐loop passive magnetic MIMO transmission, i.e. (i) AP sends the enquiring signal with 3D coils; (ii) sensors with 3D or 1D coils capture the signal and measure the magnetic field from AP. Then, the position and CSI from AP to the sensor are calculated. The access time‐slot is determined according to the position; and (iii) sensors send out the response signals within the corresponding time‐slots, i.e. according to the space‐time mapping procedure. The passive massive MIMO communications can work within the successive PPAs. The proposed scheme gives a promising solution to the magnetic communication and positioning with large magnetic passive sensors in 3D space.

Due to the typical features of Air-to-Ground (A/G) communications, such as the dominant requirement for safety, as well as the high-speed mobile and three-dimensional macrocells, etc., traditional terrestrial cognitive radio communication schemes cannot be directly transplanted. In particular, the scheme of spectrum decisions only depending on spectrum sensing in the mobile terminals, i.e., the aircrafts in A/G communications, should be abandoned because of safety requirements. A novel central cognitive structure for broadband A/G communications is established in this paper. Meanwhile, Interference Cognition Table (ICT) is maintained in the database of the Central Management Station (CMS) with electromagnetic computation and modified strong tracking Kalman filtering of the spectrum sensing result. With the prior computation based on ICT, the potential harmful interference to the primary user is avoided, and the safety is guaranteed. In addition, a fast channel assignment scheme with low complexity and the corresponding Non-Continuous Interferometry Orthogonal Frequency Division Multiplex (NCI-OFDM) transmission are highlighted for improving the system capacity and transmission efficiency.

In order to provide autonomous broadband communications among mobile vehicles, especially in an environment without fixed terrestrial vehicular base stations, a Cognitive Non-Continuous Carrier Interferometry Orthogonal Signal Division Multiplexing (Cognitive NCI-OSDM) transmission system is proposed in this paper. Specifically, Orthogonal Signal Division Multiplexing (OSDM) provides the system with high transmission capacity and good adaptability to variable mobile speeds. Carrier Interferometry (CI) codes make a system with limited Peak to Average Power Ratio (PAPR) of the transmitted signal. The incorporation of adaptive OSDM scheme provides an efficient approach to control Inter-Carrier Interference (ICI). Moreover, the non-continuous scheme is quite suitable in cognitive radio applications. Compared with traditional broadband OFDM-based transmission, such as Non-Continuous OFDM (NC-OFDM) in IEEE802.22, Cognitive NCI-OSDM has advantages of higher transmission capacity and spectrum efficiency, lower ICI, limited PAPR, adaptability to its cognitive environment and variable mobile speed. In this paper, the application system and the structure of Cognitive NCI-OSDM transmission is proposed for autonomous vehicular communications. Furthermore, the performance is evaluated, including the Bit Error Rate (BER), throughput, PAPR and Power of Inter-Carrier Interference (PICI). Finally, the system complexity, which is considered as the potential drawback when compared with an OFDM-based system, is investigated and confirmed to be controllable. The results reported in this paper show the promise of the proposed cognitive NCI-OSDM system and identify some possibilities for its future application in autonomous vehicular communications.

A novel convolutional code spread scheme for multi- carrier system with high diversity gain is proposed in this paper. This scheme, named by convolutional code spread multicarrier (CCSM), convolutionally spreads the data symbols onto several consecutive subcarriers with complementary sequences. Compared with the traditional OFDM and OFDM-CDM systems, CCSM enjoys high BER performance with equivalent spectrum efficiency in multipath Rayleigh fading channels. Moreover, the special structure of convolutional spreader in CCSM makes the system suitable for high order modulation, which undoubtedly endows the CCSM with the attractable capability of high transmission capacity.