Abstract: With flexibility, convenience and mobility, unmanned aerial vehicles (UAVS) can provide wireless communication networks with lower costs, easier deployment, higher network scalability and larger coverage. This paper proposes the deep deterministic policy gradient algorithm to jointly optimize the power allocation and flight trajectory of UAV with constrained effective energy to maximize the downlink throughput to ground users. To validate the proposed algorithm, we compare with the random algorithm, Q-learning algorithm and deep Q network algorithm. The simulation results show that the proposed algorithm can effectively improve the communication quality and significantly extend the service time of UAV. In addition, the downlink throughput increases with the number of ground users.
Abstract: Unmanned aerial vehicle (UAV) as a powerful tool has found its applicability in assisting mobile users to deal with computation-intensive and delay-sensitive applications (e.g., edge computing, high-speed Internet access, and local caching). However, deployment of UAV-aided mobile services (UMS) faces challenges due to the UAV limitation in wireless coverage and energy storage. Aware of such physical limitations, a future UMS system should be intelligent enough to self-plan trajectories and best offer computational capabilities to mobile users. There are important issues regarding the UAV-user interaction, UAV-UAV cooperation for sustainable service provision, and dynamic UMS pricing. These networking and resource management issues are largely overlooked in the literature and this article presents intelligent solutions for cooperative UMS deployment and operation. Mobile users’ locations are generally private information and changing over time. How to learn on-demand users’ truthful location reporting is important for determining optimal UAV deployment in serving all the users fairly. After addressing the truthful UAV-user interaction issue via game theory, we further study the UAV network sustainability for UMS provision by minimizing the energy consumption cost during deployment and seeking UAV-UAV cooperation. Finally, for profit-maximizing purpose, we analyze the cooperative UAVs’ deployment, capacity allocation, and dynamic service pricing.
Abstract: Unmanned aerial vehicles (UAVs) have recently been regarded as a promising technology in Internet of things (IoT). UAVs functioned as intermediate relay nodes are capable of establishing uninterrupted and high-quality communication links between remotely deployed IoT devices and the destination. Multiple UAVs are required to be deployed due to their limited onboard energy. We study a UAV pair-supported relaying in unknown IoT systems, which consists of transmitter and receiver. Our goal is that transmitter gathers the data from each device then transfers the information to receiver, and receiver finally transmits the information to the destination, while meeting the constraint that the amount of information received from each device reaches a certain threshold. This is an optimization problem with highly coupled variables, such as trajectories of transmitter and receiver. On account of no prior knowledge of the environment, a dueling double deep Q network (dueling DDQN) algorithm is proposed to solve the problem. Whether it is in the phase of transmitter’s receiving information or the phase of transmitter’s forwarding information to receiver, the effectiveness and superiority of the proposed algorithm is demonstrated by extensive simulationsin in comparison to some base schemes under different scenarios.
Abstract: To improve the time-varying channel estimation accuracy of orthogonal frequency division multiplexing air-ground datalink in complex environment, this paper proposes a time-varying air-ground channel estimation algorithm based on the modulated learning networks, termed as MB-ChanEst-TV. The algorithm integrates the modulated convolutional neural networks (MCNN) with the bidirectional long short term memory (Bi-LSTM), where the MCNN subnetworks accomplish channel interpolation in frequency domain and compress the network model while the Bi-LSTM subnetworks achieve channel prediction in time domain. Considering the unique characteristics of airframe shadowing for unmanned aircraft systems, we propose to combine the classical 2-ray channel model with the tapped delay line model and present a more realistic channel impulse response samples generation approach, whose code and dataset have been made publicly available. We demonstrate the effectiveness of our proposed approach on the generated dataset, where experimental results indicate that the MB-ChanEst-TV model outperforms existing state-of-the-art methods with a lower estimation error and better bit error ratio performance under different signal to noise ratio conditions. We also analyze the effect of roll angle of the aircraft and the duration percentage of the airframe shadow on the channel estimation.
Abstract: Interference source localization with high accuracy and time efficiency is of crucial importance for protecting spectrum resources. Due to the flexibility of unmanned aerial vehicles (UAVs), exploiting UAVs to locate the interference source has attracted intensive research interests. The off-the-shelf UAV-based interference source localization schemes locate the interference sources by employing the UAV to keep searching until it arrives at the target. This obviously degrades time efficiency of localization. To balance the accuracy and the efficiency of searching and localization, this paper proposes a multi-UAV-based cooperative framework alone with its detailed scheme, where search and remote localization are iteratively performed with a swarm of UAVs. For searching, a low-complexity Q-learning algorithm is proposed to decide the direction of flight in every time interval for each UAV. In the following remote localization phase, a fast Fourier transformation based location prediction algorithm is proposed to estimate the location of the interference source by fusing the searching result of different UAVs in different time intervals. Numerical results reveal that in the proposed scheme outperforms the state-of-the-art schemes, in terms of the accuracy, the robustness and time efficiency of localization.
Abstract: Space-air-ground integrated network is capable of providing seamless and ubiquitous services to cater for the increasing wireless communication demands of emerging applications. However, how to efficiently manage the heterogeneous resources and protect the privacy of connected devices is a very challenging issue, especially under the highly dynamic network topology and multiple trustless network operators. In this paper, we investigate blockchain-empowered dynamic spectrum management by reaping the advantages of blockchain and software defined network (SDN), where operators are incentive to share their resources in a common resourced pool. We first propose a blockchain enabled spectrum management framework for space-air-ground integrated network, with inter-slice spectrum sharing and intra-slice spectrum allocation. Specifically, the inter-slice spectrum sharing is realized through a consortium blockchain formed by the upper-tier SDN controllers, and then a graph coloring based channel assignment algorithm is proposed to manage the intra-slice spectrum assignment. A bilateral confirmation protocol and a consensus mechanism are also proposed for the consortium blockchain. The simulation results prove that our proposed consensus algorithm takes less time than practical Byzantine fault tolerance algorithm to reach a consensus, and the proposed channel assignment algorithm significantly improves the spectrum utilization and outperforms the baseline algorithm in both simulation and real-world scenarios.
Abstract: The special position of terahertz wave in the electromagnetic spectrum makes it possess the characteristics of orientation, broadband, penetration and low energy, which promotes the extensive research of terahertz wave in the fields of communication, radar, imaging, sensing, security inspection and so on. The solid-state terahertz sources based on semiconductor devices have attracted extensive attention in the field of terahertz information technology due to their characteristics such as being able to work at room temperature, being small in size, being easy to integrate and having good frequency stability. Terahertz planar Schottky diode is a kind of low parasitic semiconductor device. Its high cutoff frequency makes it work well in the terahertz range. The frequency multiplier based on planar Schottky diode is an important part of terahertz solid state source. In this review, the development of Schottky diodes technology in recent years have been introduced, including the structures and preparation of Schottky diodes. In addition, the current situation and performance of different types of terahertz sources based on Schottky diodes are further introduced, and the future development trend is discussed.
Abstract: Terahertz (THz) communications are considered as very promising for the sixth-generation (6G) ultra-dense wireless networks. However, THz signals suffer from well-known severe path loss, which consequently shortens the coverage of THz communication systems. To deal with this issue, precoding technique is expected to be beneficial to extend the limited coverage by providing directional beams with ultra large number of antenna arrays. In this paper, we overview the state-of-the-art developments of THz precoding techniques such as reconfigurable intelligent surface based precoding, hybrid digital-analog precoding and delay-phase precoding. Based on the survey, we summarize several open issues remaining to be addressed, and discuss the prospects of a few potential research directions on THz precoding, such as one-bit precoding, precoding for hardware impairments and THz security precoding. This overview will be helpful for researchers to study innovative solutions of THz precoding in the future 6G wireless communications.
Abstract: Advances in 6G communication changes how machines and humans interact. The blossom of new applications demands significantly higher data bandwidth while preserving the mobility and sustainability of electronic wireless communication systems. It also demands an integrable system that allows convenient interactions between communication units and signal processing units. A review of CMOS-based THz communication system solutions is presented, with a focus on novel systematic EM-circuit co-design philosophy. This review starts with a review of THz power generation, followed by the discussion of THz localization and THz beamforming for efficient high-throughput communication.
Abstract: With the increasing number of users and emerging new applications, the demand for mobile data traffic is growing rapidly. The limited spectrum resources of the traditional microwave and millimeter-wave frequency bands can no longer support the future wireless communication systems with higher system capacity and data throughput. The terahertz (THz) frequency bands have abundant spectrum resources, which can provide sufficient bandwidth to expand channel capacity and increase transmission data rate. In addition, with the rapid development of silicon-based semiconductor technology, its characteristic size keeps decreasing, and the radio frequency performance of active devices is gradually approaching the performance of III-V semiconductor technology. The realization of THz communication systems based on low-cost, high-stability, and easy-to-integrate silicon-based process has become a feasible solution. This review summarizes the reported silicon-based THz communication systems, as well as the key sub-circuit chips in these systems, including the local oscillator, power amplifier, low noise amplifier, on-chip antenna and transceiver chip, etc.
Abstract: The use of advanced digital signal processing (DSP) technology in the high-speed terahertz (THz) communication can effectively compensate the linear and nonlinear effects of the system and further improve the transmission performance of the system. This paper introduces the principle and application of advanced DSP algorithms such as probability shaping (PS) technology, Volterra series nonlinear compensation algorithm, Kramers-Kronig receiver, look-up table (LUT) pre-distortion compensation technology, pre-equalization and decision-directed least-mean-square equalization algorithm. Combined with DSP algorithms such as PS and LUT pre-distortion, using photon-assisted technology to successfully realize the wireless transmission of vector THz signals higher than 1 Tbit/s on the sub-THz band (D-band) 4 × 4 multiple input multiple output system.
Abstract: The research on high power 170 GHz frequency doubler based on the GaAs Schottky diodes is proposed in this paper. This basic doubler cell is developed with a 50- μ m-thick, 600- μ m-wide, and 2-mm-long AlN substrate with high thermal conductivity to reduce the thermal effect. Besides, power combined frequency doubler has been fabricated to improve the power capacity by a factor of two. Great agreement has been achieved between the simulated results based on electro-thermal model and measured performances. At room temperature, the 3 dB bandwidth of the single doubler based on GaAs Schottky diodes is 11.8 % over the frequency range from 160 to 180 GHz with pumping power of 150 to 330 mW. And the peak efficiency of the doubler is measured to be 33.1 % , while the maximum output power is 101.7 mW at 174.08 GHz. As for power combined circuit, the best efficiency is 30.1 % with a related output power of 204.6 mW. The proposed methods of developing high power multipliers can be applied in higher frequency band in the future.
Abstract: This paper presents an E-band reflector antenna fed by dual circularly polarized feed system. Axial displaced ellipse reflector is adopted for high gain and low blockage mechanisms. The feed system mainly composed of orthomode transducer, iris polarizer and horn antenna, and possesses dual circular polarization. The orthomode transducer offers high isolation performance in broadband by double symmetry and the iris polarizer achieves phase shift by introducing discontinuities. In addition, the ridge and iris in the feed are optimized to be wide enough to fabricate and maintain good mechanical properties in higher frequency band. The entire antenna is simulated and fabricated. The measured gain of 45 ± 1.2 dBi corresponds to about 52 % efficiency. The measured results of reflection coefficients less than −18 dB, isolation over 27 dB, axial-ratio value less than 2.3 dB are achieved from 71 to 86 GHz.
Abstract: In this paper, A 220 GHz orthogonal modulator based on two symmetrical subharmonic mixers is designed, analyzed and measured, where the mixers are implemented basde on antiparallel Schottky diodes. The orthogonal modulator consists of a 90° phase shifted 220 GHz 3 dB coupler, two sub-harmonically pumped mixers and an in-phase 110 GHz 3 dB power divider. The IF output signals are in-phase (I) branch and quadrature (Q) branch operating from DC±3.5GHz. In the back to back experimentaltests, it is shown that the conversion loss of the two 220 GHz I/Q mixers is less than 30 dB when the IF operating frequency is DC−3.5 GHz, IF input power is −10 dBm and the LO power is around 6 mW. Based on floating-point simulation, the amplitude and phase imbalance of the I/Q mixer are less than 0.2 dB and 2 degree respectively. When the 220 GHz modulator and demodulator are set in a back to back configuration, a signal-to-noise ratio of 21 dB can be obtained using the 16QAM modulation type.
Abstract: This paper presents the research on a 220 GHz multi circuit inte-grated front end based on solid-state circuits. This integrated front end integrates a 220 GHz subharmonic mixer, a 110 GHz tripler, a 110 GHz 8 dB hybrid coupler and a 220 GHz waveguide bandpass filter (BPF) in one single block. Compared to the traditional transceivers which usually use cascade connection of the independent mixers and multipliers, the size of the proposed multi circuit integrated front-end block is 25 mm × 20 mm × 20 mm, ten times smaller than the cascading transceiver. In order to check the tripler’s output power, a modified compact 110 GHz 8 dB hybrid coupler is set between mixer and tripler. Due to the characteristics of the hybrid coupler, the deterioration of cascading transceiver’s performance caused by mismatch has also been improved. In addition, to achieve single sideband (SSB) communication, a 220 GHz BPF with high selectivity is integrated in the circuit. The measured conversion loss of the fabricated multi circuit integrated front end is less than 11 dB, where the LO and RF frequency are 37 and 210−220 GHz. Based on this front-end, a 220 GHz high speed communication system has been setup and it can achieve 10 Gbps data transmission using 16QAM modulation.
Abstract: A reflective phase shifter is proposed to realize the two-dimensional beam-scanning reflectarray. The reflectarray is composed of double-dipole resonant elements in which the phase shift is implemented by applying a driving voltage to the liquid crystal (LC). A 30 × 30 phase shifter sample is fabricated on a quartz substrate with a 480 μ m thickness and a 4 cm × 4 cm area. According to the experimental results, 0 to 360° phase shift can be achieved within the bandwidth of 5 GHz. In order to accomplish two-dimensional beam-scanning and reduce the negative impact of driving lines on the reflectarray, a new LC driving method is developed. To verify the accuracy of the proposed approach, three samples of different driving line numbers are designed using the above-mentioned phase shifter, which all have achieved a 0−360° phase shift in the bandwidth of 4 GHz. Considering the influence of LC anisotropy and inhomogeneity, an improved calculation result is obtained and compared with experimental data.
Abstract: As the carrier frequency goes into the terahertz band, the phase noise of the signal source has increasing impacts on the performance of the communication system. Considering a 16QAM high-order digital modulation terahertz communication system (DMTCS), by comparing the influence of two kinds of local oscillator signal sources with different phase noise characteristics on the bit error rate (BER) performance of the system based on theoretical analysis and experimental research, it is found that the near-end phase noise of local oscillator signal source has a great influence on the BER performance of the DMTCS. The suppression of phase noise of the local oscillator signal source based on choosing loop bandwidth of the digital phase-locked loop (DPLL) at the receiving end is also discussed. It is found that the negative impacts of phase noise on BER performance of the system can be efficiently decreased by reasonably selecting the loop bandwidth of the DPLL.