Millimeter wave (mmWave) communication and intelligent reflecting surface (IRS) are both promising solutions for the next generation of wireless communication technology. In this article, the near-field channel models based on the spherical wave assumption and parabolic wave assumption are proposed for double-IRS assisted mmWave communication systems, where the parabolic wave model serves as an approximation of the spherical wave model. Under the parabolic wave assumption, the directional-dependant rayleigh distances and near-field reflection phases are investigated, and explicit expressions for the normalized array gains and path power gains are obtained using the sine integral. Based on the obtained path power gains, the suboptimal IRS rotation angles are also explored. We first consider the range limits of the activation conditions for the rotation angles, and then take the derivative of the explicit expression for the amplitude of IRS-assisted link to obtain the suboptimal rotation angles for different links. Using the designed near-field reflection phases, the approximate achievable rate is obtained and verified by numerical results. From these results, an interesting finding emerges: under reasonable IRS dimensions, the performance gains of two IRSs working noncooperatively are significantly greater than those of two IRSs working together. In conclusion, this work highlights the importance of IRS rotation angles and the intrinsic nature of double-IRS assisted communications.

Reconfigurable intelligent surfaces (RIS) have attracted significant attention due to their capability of establishing virtual line-of-sight (VLoS) links. This paper proposes a channel model for RIS-assisted millimeter wave (mmWave) communication systems that incorporate the effective aperture (EA) of RIS elements, the horizontal and vertical rotation angles of the RIS, the servomechanism limitations associated with these rotation angles and the activation criteria to constrain the feasible range of these rotation angles. To enhance the system performance, we jointly optimize the horizontal and vertical rotation angles of the RIS with the objective of maximizing the signal-to-noise ratio (SNR) based on the proposed model. An alternating optimization (AO) algorithm is developed to solve this problem efficiently. Specifically, the original optimization problem is decomposed into two subproblems corresponding to independent optimization of the horizontal and vertical angles, and closed-form optimal solutions are derived for each subproblem. Updating iteratively these closed-form solutions yields suboptimal horizontal and vertical rotation angles for the RIS. Moreover, a global optimal solution of closed-form to the original optimization problem is derived for the special case where the base station (BS) is positioned directly in front of the RIS. Numerical results demonstrate that the suboptimal rotation angles obtained by the AO algorithm closely approximate the optimal solutions. Furthermore, the proposed AO algorithm, which jointly optimizes both rotation angles, significantly outperforms the methods that individually optimize either the horizontal or vertical angle.

Intelligent reflecting surface technology (IRS) is emerging as a major innovation in wireless communications due to its unique advantages. It takes advantage of a large number of low-cost passive elements with adjustable phase-shift capabilities, which can reflect incident signals independently. When these elements work together, IRSs can achieve three-dimensional passive beamforming without the use of any transmit RF link. This mechanism not only enhances spectrum efficiency but also reduces the energy consumption of communication systems. Based on this advantage of IRSs, the paper explores IRS-assisted multiuser wireless systems, where IRSs are cleverly deployed between a multi-antenna access point (AP) and multiple single-antenna users. By jointly optimizing the transmission beamforming of active antenna array of the AP and the passive phase-shift beamforming of the IRSs, the objective is to minimize the total transmit power of APs, while ensuring that each user’s signal-to-interference-plus-noise ratio (SINR) requirement is met. The optimization problem is challenging to solve, as it is a nonconvex quadratically constrained quadratic programming problem, and the optimization variables are highly coupled with each other. To address this challenge, a low-complexity and efficient optimization algorithm, known as the linearized alternating direction multiplier method (LADMM) algorithm is proposed to address the transmit power minimization problem. The simulation results indicate that the LADMM algorithm provides superior system performance and significantly lower complexity compared to other existing methods.

Reconfigurable intelligent surface (RIS) has the potential to shape the wireless channel into an intelligent programmable wireless propagation environment. RIS-assisted millimeter wave (mmWave) technology is considered as a potential technology for sixth generation (6G) wireless communications. In this article, an RIS-assisted mmWave system is considered, and the corresponding physics-based channel model under the parabolic wavefront assumption, which is a second-order approximation to the spherical wavefront assumption, is established. Based on the parabolic wavefront assumption, the approximate closed-form expression of the direction-dependent Rayleigh distance is derived, which is a supplement to the classical Rayleigh distance. Also, the RIS reflection phase, consisting of a conventional far-field reflection phase and an addition near-field reflection phase, is obtained. The far-field phase compensates the phase variations from the mismatch in the azimuth and elevation angles, and the near-field phase compensates the phase variations caused by the distance differences from the transmitter/receiver to different RIS unit cells. Based on the conventional far-field reflection phase and the designed reflection phase, the received signal power is explored, and the approximate expressions are also obtained by using the Fresnel functions, which are validated by using numerical results. In addition, the numerical results show that the mmWave channel model under parabolic wavefront assumption and the corresponding near-field reflection phases are necessary to explore the RIS-assisted mmWave communication systems.

Due to the existence of wind and pressure variation, the unmanned aerial vehicle (UAV) will undergo pitch and roll wobbles in 3-D space, which will further induce the position offset of the aerial intelligent reflector surface (AIRS) deployed on the UAV platform. In this article, we consider the impact of the UAV pitch and roll wobbles and the effective AIRS structure, which is the projected structure of AIRS reflection unit in the signal propagation direction, on the channel statistical characteristics, and propose a novel AIRS-assisted beam channel model for mobile-to-mobile (M-to-M) multiple-input–multiple-output (MIMO) communication systems. In the proposed model, the relationship between the scattering gain of the AIRS unit and its size is also considered. Based the proposed model, the power scaling law of the AIRS-assisted link is derived, and it is demonstrated that the average received signal power is proportional to the square of the total geometric area of the AIRS in the far-field region. The simulation results show that the UAV wobbles with small angles significantly affects the spatial correlation function and the average received signal power, and the Doppler spread can be significantly reduced by increasing the number of AIRS reflection units. The simulation results also show that the effective AIRS structure in the signal propagation direction is essential in AIRS-assisted channel modeling and the proposed AIRS-assisted M-to-M MIMO communication system can obtain higher performance gain than traditional M-to-M MIMO communication system.

Intelligent reflecting surface (IRS) via using massive low-cost passive elements that can reflect the signals by adjusting phase shifts provides a cost-effective and energy-efficient solution to enhance the wireless communication system’s performance. In the article, we consider an IRS-aided multiuser multi-input–single-output (MISO) downlink system. We tackle the weighted sum-rate (WSR) maximization by jointly optimizing the active beamforming at the base station (BS) and the passive beamforming at the IRS. We first decouple the nonconvex optimization problem by the Lagrangian dual transform, then resort to fractional programming to address the active and passive beamforming optimizations. We develop the mirror descent (MD) method and the accelerated projected gradient (APG) method to solve subproblems. The simulation results show that the MD and APG algorithm get the comparable WSR gain and convergence speeds as existing methods, but with a significantly lower computational complexity.

Due to the large path loss in millimeter wave (mmWave) band, the transmission path between transmitter (Tx) and intelligent reflecting surface (IRS) is considered as a Rayleigh fading channel, and a physics-based channel model is proposed for IRS-assisted mmWave communication system in urban scenario. Also, the horizontal and vertical rotation angles of IRS and the relationship between the scattering gain of IRS reflecting unit and its effective aperture in the incident direction and the desired reflection direction are considered in the proposed model. For the considered communication scenario, the existing reflection phases, which are designed to align the virtual line-of-sight (VLoS) components among Tx, IRS, and receiver (Rx) with the LoS components between Tx and Rx, are not the appropriate reflection phases. Based on the proposed model, we first obtain the statistical phases of the virtual scattering components within a cluster by minimizing phase differences between different IRS reflection units, and then obtain the reflection phases by minimizing the phase differences of the derived statistical phases for all clusters. By comparing with the existing reflection phases, the designed reflection phases can significantly enhance the system performance gains of mmWave communications. Using the designed reflection phases, the expressions of received signal power and upper bound of ergodic sum capacity are derived in this paper, which are validated by using Monte-Carlo simulation results. Numerical results show that the proposed mmWave channel model could accurately simulate the propagation characteristics of IRS. Also, numerical results show that the performance gains of IRS-assisted systems are equivalent to that of large-scale communication systems without using IRS.

In this paper, we use directional antenna arrays instead of large-scale antenna arrays to combat the severe propagation path loss, and propose a novel beam channel model for millimeter-wave (mmWave) unmanned aerial vehicle (UAV) multiple-input multiple-output (MIMO) communication systems. The sparsity of mmWave channel and the directivity gain of mmWave antenna arrays are considered in the proposed model, and the antenna directivity gain is affected by the beamwidth, i.e., the narrower the beam, the higher the antenna directivity gain. Also, the beamwidth affects the number of active clusters, i.e., the wider the beam, the more the active clusters. Based on the proposed beam channel model, the influence of antenna directivity gain on the channel statistical characteristics including space-time correlation, frequency correlation function, and achievable rate is investigated. It is found that the directional antenna arrays can reduce Doppler spread caused by the movement of UAV and receiver and the mmWave communication systems using directional antenna arrays with high directivity gains can achieve an equivalent performance gain to that of the mmWave communication systems using omni-directional large-scale antenna arrays. Furthermore, considering the correlation between transmit and receive antenna elements, we obtain a closed-form expression of the tight upper bound of achievable rate, which is validated by using previously reported analytical upper bound and the simulation results.

Intelligent reflecting surface (IRS) composed of a large number of low-cost passive reflecting elements has attracted significant attention from communication communities because of its ability to substantially improve the communication performance. In this paper, the aperture area and radiation pattern of the IRS reflecting elements are considered, and a novel geometry-based three-dimensional (3-D) wideband channel model is proposed for IRS-assisted unmanned aerial vehicle (UAV) communication systems. In the proposed model, the reflection phase is designed by jointly considering the aperture area of the IRS reflecting element and the propagation phases among UAV, IRS, and receiver (Rx), each IRS reflecting element is modeled as an anomalous reflector instead of a specular reflector, and large-scale IRS reflecting elements can jointly beamform the signal in a desired direction. Based on the proposed model, the effects of arbitrary trajectory of UAV and the number and the size of passive reflecting elements on channel statistical characteristics are considered, and the average received signal power and the ergodic sum capacity, which consider the impacts of the number and the size of passive reflecting elements, are also investigated. Furthermore, the path loss of the IRS-assisted link, which is in inverse proportion to the square of aperture area of IRS reflecting elements, is derived, and it coincides with the measured results in real outdoor scenarios. Analysis shows that the communication performance can be enhanced by increasing the number and the size of IRS reflecting elements, and it is validated by numerical results and Monte-Carlo simulation results.

In this article, a nonstationary 3-D wideband geometry-based stochastic model (GBSM) is proposed for low-altitude unmanned aerial vehicle (UAV) multiple-input–multiple-output (MIMO) communication systems. The proposed GBSM is a combination of Line-of-Sight (LoS) components, local multipath components (MPCs) scattering from the scatterers around the receiver (Rx), named as local scatterers, and far MPCs scattering from far scatterers, defined as not local scatterers, and uses 2-D one-ring and 3-D cylinder to mimic local scatterers as well as 3-D multiple confocal elliptic cylinders to mimic far scatterers. In this article, two-state continuous-time Markov chains (CTMCs) are introduced to model appearances or disappearances of the LoS components, local MPCs at the transmitter (Tx) installed on UAV, and far MPCs at the Tx and Rx, and the evolution process of the far MPCs is also investigated. The concept of the visibility region (VR) is introduced to model the birth and death processes of the local MPCs at the Rx, and the effect of the size of the VR on channel statistics is also considered. In the proposed GBSM, the inherited nature of the LoS components, the local MPCs and far MPCs, is considered, and the corresponding statistical properties are derived. The proposed nonstationary 3-D GBSM is validated by the measured results in terms of temporal correlation, and the numerical results show that the proposed 3-D GBSM is suitable for describing nonstationarity of the 3-D UAV-MIMO channel.

In this study, the authors consider a decode‐and‐forward (DF) relaying protocol with full‐duplex wireless information and power transfer, and investigate its outage performance. In this protocol, a cooperative relay can transmit the information to the destination and harvest the energy from the source simultaneously. Thus, part of the energy used by the relay can be recycled through the self‐loop channel that exists in the relay's full‐duplex operation. An approximated expression for the outage probability is derived under the scenario where the self‐loop channel is fading, and then the authors derive an exact closed‐form expression for the outage probability under a scenario where the self‐loop channel is non‐fading. Moreover, to optimise the outage performance, a power allocation scheme is also provided. The authors analyse the effect of self‐loop channel on the outage performance, and show that, benefiting from the full‐duplex operation and self‐energy recycling, the considered protocol is superior to the existing power splitting‐based and time switching‐based DF relaying protocols, in terms of outage performance. Simulations are provided to validate the authors' results.

In this paper, considering the scenario that the available channel state information (CSI) used for relay selection (RS) is outdated, we propose a RS scheme for full-duplex (FD) cooperative networks in an interference-limited environment, where the residual loop interference (RLI) at the relay and the interference comes from the direct link both affect the cooperative transmission. Specifically, based on the statistical CSI, power allocation under the sum power constraint is performed firstly according to the signal-to-interference-ratio (SIR) balancing condition, and then with the known outdated CSI and the correlation coefficient between the outdated CSI and the real one, the conditional outage probability (COP) associated with each relay is derived in a closed form. The relay with the minimum COP is selected to participate in transmission. It is shown that the proposed scheme can provide a significant performance improvement over the conventional RS schemes for both symmetric and asymmetric network configurations. Simulations are provided to validate our results.

In this paper, we investigate a decode-and-forward (DF) relaying protocol for a three-node half-duplex single-antenna network, consisting of a single source-destination pair and a relay. The proposed relaying protocol, called DF with time allocation (DF-TA), can switch its transmission mode according to the partial channel state information at the transmitter (CSIT), which is obtained through a one bit feedback from the destination. We derive the diversity multiplexing tradeoff (DMT) of DF-TA in a closed-form, and develop an adaptive time allocation strategy to achieve its optimal DMT. We show that the DF-TA significantly improves the DMT of existing time allocation schemes with or without CSIT in DF relaying channel. Moreover, in contrast to the power control schemes based on CSIT, which are performed with the assumption of long-term power constraint, the DF-TA can be generalized to the practical scenarios where a strict short-term power constraint is imposed on, due to the environmental safety and interference prevention.