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 letter, we study an energy-efficient algorithm for beamspace high altitude platform with non-orthogonal multiple access (HAP-NOMA) systems over Rician fading channels. We first derive the non-line-of-sight (NLOS) theorem of beamspace HAP channel in large scale array regime according to the asymptotic property of the Toeplitz matrix. Then, we propose the beamspace HAP-NOMA scheme based on the derived NLOS theorem, which can be partitioned into two stages, i.e. user grouping scheme and power optimization scheme. The statistical channel state information (CSI) is only required in the proposed algorithm, which avoids acquiring instantaneous CSI. NOMA can serve users in each beam simultaneously to compensate for the loss of spectral efficiency due to the beamspace HAP. The digital precoder can be obtained by zero forcing (ZF) based on the equivalent channel. Numerical results show the performance enhancement of the proposed beamspace HAP-NOMA algorithm.

A three dimensional (3-D) geometry-based channel model is proposed for intelligent reflecting surface (IRS)-assisted unmanned aerial vehicle (UAV) multiple-input multiple-output (MIMO) communication scenarios where the users are located on the edge of IRS service area. In order to reduce the interference between different IRS reflection units, we first construct an optimization problem about the statistical propagation phases of different IRS reflection units, and then derive the optimal statistical propagation phase by solving the designed optimization problem. Finally, we align the proposed statistical propagation phases of IRS with the stronger components to derive suboptimal IRS reflection phases. Using derived IRS reflection phases, the statistical properties are investigated. The numerical results show that the proposed IRS reflection phases can enhance the received signal strength and reduce the multipath fading effect. Also, it can be observed that when the number of IRS reflection units increases, the spatial correlation significantly decreases.

This letter investigates an energy-efficient scheme for the downlink active reconfigurable intelligent surface (ARIS) aided non-orthogonal multiple access (NOMA) on high altitude platform multiple-input single-output (HAP-MISO) with lens antenna array, i.e. ARIS aided beamspace HAP-NOMA. Specifically, by jointly considering the line-of-sight and non-line-of-sight components on HAP, we deduce the sparsity of the sparse matrix for the dimension-reduced beamspace HAP-NOMA channel and the sparse matrix is defined as the matrix with at most four nonzero elements, where the power is concentrated in the corresponding direction. The traditional user grouping strategy is employed by the sparsity. Then, we derive the closed-form equivalent (CE) of the spectral efficiency (SE) based on the sparsity, which is the upper bound on SE in practice due to the existing power leakage for beamspace HAP-NOMA. Moreover, the optimization problem is formulated to maximize the CE and the joint alternating precoding algorithm is proposed to attain the active reflecting at the ARIS and the power allocation at the HAP, which is only dependent on statistical channel state information (SCSI). Notably, users in the same beam can be served simultaneously by NOMA and the severe path loss due to the HAP-RIS-user link can be alleviated by ARIS. The integration of NOMA and ARIS would compensate the performance loss caused by beamspace HAP-MISO. Numerical results show that the proposed algorithm can converge faster with a lower complexity and gain better performance.

In this letter, a radio frequency (RF) precoding method is put forward for massive multiple-input multiple-output (MIMO) systems with limited RF chains on high altitude platform (HAP). In the proposed method, we formulate the RF precoder design as an average signal-to-leakage-plus-noise ratio (SLNR) maximization problem, where the RF precoder is composed of some columns of a specific discrete fourier transform (DFT) matrix. First, we obtain the lower bound of the average SLNR and transform it into a trace problem. Then, for ease of handling, signal-to-leakage ratio (SLR) is adopted to achieve the initial RF precoder. Finally, a greedy algorithm for RF precoding is proposed to update the initial RF precoder for better performance. Numerical simulations demonstrate that the design proposed in this letter has better performance than other RF precoding schemes.