Bai Zhao

Bio: Bai Zhao is an academic researcher from Nanjing University of Posts and Telecommunications. The author has contributed to research in topics: Computer science & Beamforming. The author has an hindex of 1, co-authored 3 publications receiving 1 citations.

Outage Constrained Robust Secure Beamforming in Cognitive Satellite-Aerial Networks

Abstract: This letter proposes a robust beamforming scheme to enhance the physical layer security (PLS) of multicast transmission in a cognitive satellite and aerial network (CSAN) operating in the millimeter wave frequency band. Based on imperfect channel state information (CSI) of both eavesdroppers (Eves) and primary users (PUs), we maximize the minimum achievable secrecy rate (ASR) of the secondary users (SUs) in the aerial network under the constraints of the interference to the PUs in the satellite network, the quality of service (QoS) requirements of the SUs and per-antenna power budget of the aerial platform. To tackle this mathematically intractable problem, we first introduce an auxiliary variable and outage constraints to simplify the complex objective function. We then convert the non-convex outage constraints into deterministic forms and adopt penalty function approach to obtain a semi-definite problem such that it can be solved in an iterative fashion. Finally, simulation results show that with the transmit power increase, the minimal ASR of SUs obtained from the proposed BF scheme well approximate the optimal value.

IRS-Aided Uplink Transmission Scheme in Integrated Satellite-Terrestrial Networks

Abstract: This paper proposes an intelligent reflecting surface (IRS)-aided uplink transmission scheme to expand the wireless coverage and improve spectral efficiency for an integrated satellite-terrestrial network (ISTN). Here, multiple earth stations communicate with the satellite via non-orthogonal multiple access (NOMA) technology, while multiple direct users and blockage users access the cellular network through space division multiple access and IRS-aided NOMA technology, respectively. In particular, we first aim at maximizing the ergodic sum rate of the ISTN subject to the quality-of-service requirements and transmit power budgets of all users, yielding a constrained optimization problem. Then, based on the assumption that only the statistical channel state information (CSI) is available, we propose a two-layer alternating algorithm to solve the non-convex original problem. The algorithm combines successive convex approximation with $S$-procedure approach to calculate the phase shifts of IRS and transmit powers of all users in the first layer, and uses Charnes-Cooper method with semi-definite programming to calculate beamforming vectors at the BS in the second layer. Furthermore, with the help of the zero-forcing beamforming principle, we propose a low-complexity algorithm to solve the problem that can reduce the computational load effectively. Finally, simulation results demonstrate that our proposed algorithms can obtain a higher ergodic sum rate than the benchmarks.

Robust Downlink Transmission Design in IRS-Assisted Cognitive Satellite and Terrestrial Networks

Abstract: Cognitive satellite and terrestrial network (CSTN) is considered as a promising technology to provide ubiquitous connectivity for various users within wide-coverage. This paper proposes a robust downlink transmission scheme for multiple intelligent reflecting surfaces (IRSs) assisted CSTN. Here, the satellite network adopts multigroup multicast transmission scheme to serve many earth stations, while the terrestrial network exploits space division multiple access and multi-IRS-enhanced non-orthogonal multiple access technology to communicate with many terrestrial users. By assuming that these two networks share the same frequency band having only the angular information based imperfect channel state information of each user, we formulate an optimization problem to minimize the total transmit power subject to the constraints of quality-of-service requirement for each user, per-antenna transmit power budgets of satellite and BS, and unit-modulus requirement for each reflecting element. To tackle this mathematically intractable problem, we then employ angular discretization together with the successive convex approximation method to obtain the active beamforming (BF) vectors of satellite and BS, the passive BF vector of IRS, and the power allocation coefficients. Moreover, we propose a generalized zero forcing BF and alternative optimization to obtain the suboptimal solutions of the optimization problem with low computational complexity. Finally, simulation results are given to demonstrate the effectiveness and superiority of the proposed two schemes over the benchmarks.

Robust Beamforming for RIS Enhanced Transmissions in Cognitive Radio Networks

Abstract: We propose a robust beamforming (BF) scheme for reconfigurable intelligent surface (RIS) enhanced transmission to support heterogeneous services with diverse signal-to-interference-plus-noise ratio requirements in cognitive radio networks (CRNs). Here, the CRN coexisting with a primary network offers connection-centric services and content-aware services through space division multiple access and RIS-aided multicast technology, respectively. Using imperfect statistical channel state information, the RIS enhanced transmission scheme is formulated as a non-convex optimization problem with outage constraints. To address this intractable problem, we first use the cumulative distribution function of a standard normal distribution and Schur complement approaches to transform the non-convex outage constraints into solvable ones. Then, a robust BF algorithm integrating alternate optimization with semidefinite relaxation methods is proposed to obtain the active BF weight vectors at the cognitive base station and the phase shift matrix at the RIS. Our simulation results demonstrate the robustness of the proposed BF algorithm and the superiority of the RIS enhanced wireless transmission.

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IRS-Aided Uplink Transmission Scheme in Integrated Satellite-Terrestrial Networks

Abstract: This paper proposes an intelligent reflecting surface (IRS)-aided uplink transmission scheme to expand the wireless coverage and improve spectral efficiency for an integrated satellite-terrestrial network (ISTN). Here, multiple earth stations communicate with the satellite via non-orthogonal multiple access (NOMA) technology, while multiple direct users and blockage users access the cellular network through space division multiple access and IRS-aided NOMA technology, respectively. In particular, we first aim at maximizing the ergodic sum rate of the ISTN subject to the quality-of-service requirements and transmit power budgets of all users, yielding a constrained optimization problem. Then, based on the assumption that only the statistical channel state information (CSI) is available, we propose a two-layer alternating algorithm to solve the non-convex original problem. The algorithm combines successive convex approximation with $S$-procedure approach to calculate the phase shifts of IRS and transmit powers of all users in the first layer, and uses Charnes-Cooper method with semi-definite programming to calculate beamforming vectors at the BS in the second layer. Furthermore, with the help of the zero-forcing beamforming principle, we propose a low-complexity algorithm to solve the problem that can reduce the computational load effectively. Finally, simulation results demonstrate that our proposed algorithms can obtain a higher ergodic sum rate than the benchmarks.

Outage Analysis of Alamouti-NOMA Scheme for Hybrid Satellite–Terrestrial Relay Networks

Abstract: In this article, we investigate the performance of hybrid satellite–terrestrial relay networks with multiuser downlink nonorthogonal multiple access. The satellite applies Alamouti space–time block coding, and users exploit a receive antenna selection technique. Communication between the satellite and users is assumed to be established with the aid of a half-duplex terrestrial relay equipped with multiple receive antennas and operating in amplify-and-forward mode, because of the heavy masking effects attributed to environmental obstacles. Subsequently, the satellite–relay link is exposed to shadowed-Rician fading, whereas the relay–users links undergo Nakagami- $m$ fading, which is a generic statistical channel model for nonterrestrial networks. To be more practical, we consider imperfect successive interference cancellation. The exact outage probability of each user and the corresponding asymptotic expression at the high signal-to-noise ratio are derived to demonstrate the system performance. The analysis indicates that the shadowing conditions do not affect the array gain when the diversity order is dominated by the multiantenna configuration of the second hop. The theoretical derivations are validated by using Monte Carlo simulations. The numerical results indicate that the proposed scheme significantly improves the performance of each user under various shadowing conditions. It also outperforms orthogonal multiple access when proper power levels are allocated to users.

SelfieStick: Towards Earth Imaging from a Low-Cost Ground Module Using LEO Satellites

Abstract: Real-time access to overhead Low-Earth Orbit (LEO) satellite imagery from a handheld device can have transformative applications: tracking wild-fire, natural disasters and weather events. Today, real-time access to images from LEO satellites overhead is challenging to obtain. LEO satellite ground receivers are bulky, expensive and sparsely deployed in the world. Despite the exponential increase in LEO small satellites orbiting the planet today-there is a significant time gap between an image capture on such a satellite and users who need it the most in remote and ecologically-sensitive regions. This paper presents SelfieStick, a novel satellite receiver system that explores reducing this barrier of access to real-time satellite imagery data using a single low cost (< $ 30) tiny receiver. SelfieStick's core approach takes advantage of the multiplicity of overhead Low-Earth Orbit satellites due to their exponential rise in recent years. While signals from such satellites may be individually weak, especially at a low-cost receiver, SelfieStick stitches together noisy RF captures containing underlying images of the same part of the Earth across many such satellites to generate clean Earth images. This is made possible by combining weak signals in the RF domain (rather than the traditional image domain) after appropriately transforming and aligning the RF signals accounting for different satellite perspectives, their orbits and wireless channels. A detailed experimental evaluation on the RTL-SDR platform on satellite captures from the NOAA constellation demonstrates a PSNR improvement of 5 dB through combining of images across 10 satellites.

The attack-reconnaissance squadron as a new formula and a new quality of the attack helicopter squadron of the Polish Armed Forces

Abstract: This paper takes a comprehensive look at the current state of army aviation attack helicopter squadrons of the Polish Armed Forces. The aim of the article is to present a concept for the functioning of the attack reconnaissance squadrons of the Polish Armed Forces, which takes into account the identified requirements of the Land Forces of the Polish Armed Forces in relation to army aviation on the battlefield and the current limitations in the functioning of the attack helicopter squadrons of the Polish Armed Forces. To meet this aim, qualitative research included interviewing, observation and the collection and qualitative analysis of texts and documents. The study revealed that it is necessary to organise attack-reconnaissance squadrons with a modular organisational structure including: a headquarters, a command company, three attack-reconnaissance companies, an aviation maintenance company and a supply company, capable of conducting autonomous operations in independent directions (areas). The squadrons should be equipped with new attack-reconnaissance helicopters and unmanned aerial vehicles as well as equipment for their technical and logistical support that will be part of their individual subunits. The results of the research are the basis for further, in-depth research on the issue of improving the functioning of attack helicopter squadrons, so that they are fully capable of supporting the land forces of the Polish Armed Forces in large scale combat operations.