The spectrum sharing and open nature of wireless channels enable integrated sensing and communication (ISAC) susceptible to hostile jamming attacks, particularly in the context of partial band/broadband jamming attacks. How to significantly improve the anti-jamming performance of ISAC systems with unknown jamming channel state information (CSI) and limited bandwidth is an urgent problem to be exploxied. Due to the intrinsic orthogonality and rich angular information of orbital angular momentum (OAM), vortex electromagnetic waves with helical phase fronts have shown great potential to achieve high-precision position estimation of radar and strong anti-jamming capability of wireless communication. Focusing on solving the anti-jamming problem of ISAC mentioned above, in this paper we propose a novel ISAC for anti-jamming with OAM scheme, where the ISAC transmitter can simultaneously sense the position of jammers with dynamic behavior and send data to multiple OAM legitimate users. Specifically, we develop the enhanced multiple-signal-classification (EMUSIC) based three-dimensional (3D) position estimation scheme with continuous sensing in both two-dimensional (2D) frequency and angular domains to accurately estimate the position of the jammer, thus acquiring the jamming CSI. According to the estimated jamming CSI, we design the joint transmit-receive beamforming and power allocation alternating optimization scheme, where the transmit and receive beamforming matrices are dynamically adjusted to significantly mitigate inter-mode interference, inter-user interference, and jamming, thus maximizing the achievable sum rates (ASRs) of all users. Numerical results demonstrate that our proposed scheme can significantly increase the ASR under broadband jamming attacks and achieve high-precision estimation of targets as compared with the conventional multiple-input-multiple-output (MIMO)-based ISAC.

The harsh environment and scarce resources post-disaster drive the equipment to be miniaturized and portable. Based on this, integrated sensing and communication (ISAC) systems play a significant role in providing emergency wireless networks. In order to reduce the hardware cost, a hybrid beamforming (HBF) assisted millimeter-wave (mmWave) ISAC system, which exploits the limited number of radio frequency (RF) chains, is considered in this paper. However, the HBF structure reduces the spatial degrees of freedom, thus leading to increased interference among communication users and radar sensing. To solve this problem, a rate-splitting multiple access (RSMA) strategy is adopted to enhance the emergency mmWave-ISAC system. We formulate the weighted sum rate (WSR) maximization objective by jointly designing common rate allocation and HBF. Then, we propose the penalty dual decomposition (PDD) coupled with the weighted mean squared error (WMMSE) method to solve this high-dimensional non-convex problem. Numerical results demonstrate the effectiveness of the proposed algorithm and show that the RSMA-ISAC scheme outperforms other benchmark schemes.

As the most driven force of future 6G communications, Internet of Everything (IoE) requires energy-constrained IoE devices (IDs) to realize interconnection among users, information, and things. Wireless-powered orbital angular momentum (OAM) communications can be used for IDs to achieve simultaneous wireless power transfer and multiple independent information transmissions by different OAM modes. However, the practical blockage and access capability enhancement impose crucial challenges for wireless-powered OAM communications. Therefore, for blocked line-of-sight scenario, this paper proposes non-orthogonal multiple access (NOMA)-enhanced intelligent reflecting surface (IRS) for wireless-powered OAM communications by jointly optimizing power allocation and passive beamforming. First, we formulate the information capacity and harvested energy optimization problem under the constraints of unit-modulus reflecting phase shift, minimum power transfer requirement, and minimum information transmission demand. Then, the harvested energy is converted to the achievable capacity by the energy utilization for information transmission, thus forming the sum capacity maximization problem. Finally, we decompose the sum capacity maximization problem into three subproblems, and obtain the optimal power allocation and passive beamforming by iteration. Simulation results validate the proposed NOMA-enhanced IRS of wireless-powered OAM communications, and demonstrate that the joint power allocation and passive beamforming achieves better capacity performance than the other optimization schemes.

Due to the broadcast and open characteristics of wireless channels, near-field physical layer security has attracted much attention to facilitate wireless information security against illegitimate eavesdropping. However, highly correlated channels between the legitimate transceivers and eavesdroppers for existing near-field line-of-sight multiple-input multiple-output (MIMO) systems with low degrees of freedom make it difficult to efficiently distinguish eavesdropping channels and legitimate channels in the angular domain, thus resulting in low secrecy rates. Fortunately, orbital angular momentum (OAM) with rich phase information shows great potential to enhance the physical layer security. To significantly increase the secrecy rates of near-field wireless communications, in this paper we propose the double-reconfigurable-intelligent-surface (RIS) assisted OAM secure scheme, where RISs with few reflecting elements are easily deployed to reconstruct the direct links blocked by obstacles between the legitimate transceivers, mitigate the inter-mode interference caused by the misalignment of legitimate transceivers, and adjust the OAM beams direction to interfere with eavesdroppers. Meanwhile, due to the unique orthogonality among OAM modes, the OAM-based joint index modulation and artificial noise scheme is proposed to weaken the information acquisition by eavesdroppers while increasing the achievable rate with the low cost of legitimate communications. To maximize the secrecy rate of our proposed scheme, we develop the Riemannian manifold conjugate gradient (RMCG)-based alternative optimization (AO) algorithm to jointly optimize the transmit power allocation of OAM modes and phase shifts of double RISs. Numerical results show that our proposed double-RIS-assisted OAM near-field secure scheme outperforms the existing works in terms of the secrecy rate and the eavesdropper’s bit error rate.

Owing to the spatial orthogonality, orbital angular momentum (OAM), which describes the helical phase fronts of electromagnetic waves, shows an extensive application prospect for increasing the spectrum efficiency and enhancing the physical layer security of wireless communications. However, efficient anti-jamming results of existing mode hopping (MH) schemes are achieved with pre-shared hopping sequences. Such MH schemes require the strict synchronization between the legitimate transmitter and receiver, thus leading to complex system design. Also, pre-shared hopping sequences greatly limit the MH application scenarios. To solve the problem of no pre-shared hopping sequences between the legitimate transmitter and receiver for anti-jamming, in this paper we propose the precoding-based mode hopping (PoM) scheme. Specifically, we design the transmitter by dividing the input information into the index information and signal information. Based on the index information, the activated OAM-modes for hopping and the phase shift for precoding are determined. OAM-mode and phase shift randomization make jammers difficult to disrupt the communication. Then, the receiver with OAM decomposition and phase shift decoding is designed to extract legitimate signals. Also, the lower bound of achievable rate for our proposed PoM scheme is derived. Numerical results show that our proposed PoM scheme is superior to the conventional index-modulation based mode division multiplexing (IM-MDM) schemes in terms of achievable rate under hostile jamming.

Orbital angular momentum (OAM), which has attracted much attention recently, is a potential technology to achieve high capacity for future wireless communications. In existing literatures, the OAM-based transmission is assumed to be used in ideal line-of-sight (LoS) scenarios. The non-line-of-sight (NLoS) scenarios, however, is more practical for wireless communications. In this paper, we focus on OAM-based transmission in sparse multipath environments. We build the OAM-based transmission model for Rician fading channels and derive the corresponding capacity of radio vortex wireless communications. Also, we analyze the key parameters of OAM-based transmission in sparse multipath environments. Conducted numerical results verify that the capacity of OAM-based transmission is superior to that of multiple-input-multiple-output (MIMO) based transmission in sparse multipath environments.

Orbital angular momentum (OAM) based mode hopping (MH) scheme is expected to be a potential anti-jamming technology in radio vortex wireless communications. However, it only uses one OAM-mode for hopping, thus resulting in low spectrum efficiency (SE). Index modulation offers a trade-off balance between the SE and performance reliability. In this paper, we propose an MH with OAM-based index modulation scheme, where several OAM-modes are activated for hopping, to achieve high SE at a given bit error rate in radio vortex wireless communications. Based on the proposed scheme, we derive the upper bound and lower bound of achievable SEs. Furthermore, in order to take advantage of index information, we derive the optimal hopped OAM-modes to achieve the maximum SE. Numerical results show that our proposed MH with index modulation scheme can achieve high SE while satisfying a certain reliability of radio vortex wireless communications.

Orbital angular momentum (OAM) based radio vortex wireless communications have the potential to significantly increase the spectrum efficiency (SE) without increasing extra time and frequency resources. In this paper, we propose a hybrid orthogonal division multiplexing (HODM) scheme, which jointly uses the orthogonal mode division multiplexing and the conventional orthogonal frequency division multiplexing (OFD-M) to increase the SE of radio vortex wireless communications. In particular, we develop the mode and frequency dimensions based two-dimension inverse fast Fourier transform (2D-IFFT) algorithm at the transmitter to modulate transmit signal and two-dimension fast Fourier transform (2D-FFT) algorithm at the receiver to demodulate received signal, respectively. Then, we propose an optimal power allocation scheme to maximize the SE of our proposed HODM scheme. Numerical results show that the HODM scheme can significantly increase the SE in comparison with the conventional OFDM schemes.

Orthogonal frequency division multiplexing (OFDM), which has attracted much attention during the past few decades, can be used for not only anti-multipath but also high spectrum efficiency in wireless communications. However, as the spectrum becoming more and more crowded and the amount of traffic eventually increasing, it is very difficult to increase the spectrum efficiency in wireless communications. Recently, orbital angular momentum (OAM), which provides a novel mode dimension, offers the potential to achieve high spectrum efficiency for wireless communications. In this paper, we propose the orthogonal mode division multiplexing (OMDM) scheme for high capacity in wireless communications. In particular, we propose to transmit half-integer and integer OAM-modes signals within two narrowbands to avoid the inter-mode interference. Second, we propose the mode inverse fast Fourier transform (M-IFFT) algorithm at the transmitter to modulate signals and mode fast Fourier transform (M-FFT) algorithm at the receiver to demodulate signals, respectively. Finally, we develop the dynamic power allocation scheme to solve the problem of how to maximize the capacity of OMDM. Numerical results show that our developed OMDM scheme can achieve high capacity. Furthermore, we can jointly use our developed OMDM scheme with the conventional OFDM scheme to increase the capacity of wireless communications.