Analysis of high-frequency atmospheric windows for terahertz communication between the ground and the satellite

The atmospheric absorption effect greatly increases the transmission loss of terahertz (THz) waves, which limits the transmission distance. In this article, with the support of sufficient antenna gain, a series of high-frequency THz transmission windows in the ground-satellite link are presented. First, the more ideal ground-based site is determined. Based on the path loss model and the layered transmission theory, the total path loss is obtained. Combined with signal transmit power, antenna gain, signal-to-noise ratio, and noise power, total usable bandwidths and transmission windows in the 0.2–15 THz corresponding to antenna gain from 0 to 100 dBi are given. By taking the high-altitude platform as a relay link, the usable transmission windows are also given. The first six transmission windows are identified as [0.4776–0.48645 THz], [0.4906–0.49975 THz], [0.6618–0.6811 THz], [12.1015–12.10355 THz], [12.10425–12.11285 THz], and [12.11505–12.14355 THz] bands against 100 dBi gain near 1 and 10 THz bands, respectively. Furthermore, the influence of ice clouds on the transmission characteristics is analyzed. The additional attenuation in the partial usable atmospheric window frequency range can reach 1.84 dB typical, which is directly related to the effective radius and shape of particles.

Common spatial pattern (CSP) is a popular algorithm for spatial filtering and subsequent feature extraction in motor imagery-based (MI) brain-computer interface (BCI) systems. The performance of CSP, however, depends heavily on the subject-specific frequency band and time segment used for classifying mental tasks. Accurate selection of most informative frequency band and time segment poses a great challenge. In this study, quantum particle swarm optimization is proposed for sole selection of frequency band and joint selection of frequency band and time segment, which are realized by a wrapping approach, incorporating CSP for feature extraction and support vector machine for classification into the classification model. The classification error rate is used as the fitness function of quantum particle swarm optimization. The classification performance of quantum particle swarm optimization based CSP algorithm for joint selection of frequency band and time segment is evaluated by comparing with other three CSP algorithms using either fixed frequency band and time segment or fixed time segment and the frequency band selected by particle swarm optimization and quantum-behaved particle swarm optimization, on two MI data sets with different number of channels and trials. Experimental results suggest that the proposed algorithm outperforms the other three algorithms in terms of classification error rate. Across all subjects from the two data sets, the averaged error rate of the proposed algorithm was 7.45%, 2.97% and 2.05% lower than the CSP with fixed frequency band and time segment, that with selected frequency bands by particle swarm optimization and that with selected bands by quantum particle swarm optimization. The proposed algorithm can facilitate the real-world application of BCIs.

The increasing demand of unoccupied and unregulated bandwidth for wireless communication systems will inevitably lead to the extension of operation frequencies toward the lower THz frequency range. Since atmospheric transmission windows exist in the lower THz frequency range, it can be realized that carrier frequencies of 300 GHz and beyond will be used for communications once the technology for high bitrate data transmission is available. However, the free-space path-loss and the attenuation due to molecules in the atmosphere can significantly reduce the transmittable data rate in the lower THz frequency range.The main factor affecting the behavior of terahertz band is the absorption by water vapor, which not only attenuates the transmitted signal, but also disperses the signal. A new model of the terahertz wave atmospheric propagation of attenuation and dispersion is developed by using the radiation transmission theory and the empirical continuum absorption based on the HITRAN database. Theoretical aspects of absorption are presented, emphasizing those that deserve special attention as frequency increases. The THz wave atmospheric attenuation experimental results and self- and foreign-continuum coefficients obtained with the improved THz-time domain spectroscopy (THz-TDS) technique are analyzed by this model. The intensities and locations of the observed absorption lines are in good agreement with spectral databases. This model accounts for the group velocity dispersion and the total path loss that a wave in the THz band suffers when propagating 1 km distance. The channel capacity of the THz band is investigated by this model under different conditions including antenna gains, channel bandwidth and transmitter power. In order to keep the considerations as general as possible, the derivations are based on simple assumptions and equations. The special requirement for antenna is also discussed.Three communication channels (340 GHz, 410 GHz and 667 GHz) are obtained in terms of the spectrum. The four parameters of the three channels, i.e., available bandwidth, center frequency, dispersion and transmittable data rate, are summarized and quantized. The signals through the atmosphere for the three communication channels within the corresponding atmospheric windows are not easy to broaden due to the low group velocity dispersion; high data rates of up to 10 Gbps or beyond per 1 GHz bandwidth can be transmitted via these channels if the antennas with high gains are used.

The terahertz frequency band has the advantages of large bandwidth, narrow beam, strong penetration, and high security, and is an important direction for the frequency expansion of the next generation of satellite communications. In this paper, according to the satellite-ground and inter-satellite terahertz communication frequency bands divided by the International Telecommunication Union, the atmospheric transmission loss model of the satellite-ground terahertz communication link is constructed and simulated, and the key components of terahertz communication in different frequency bands are analyzed based on the research status at home and abroad. At the same time, the transmission performance comparison of satellite-ground and inter-satellite terahertz communication links in different frequency bands was carried out. Finally, through a comprehensive analysis of the transmission loss, device availability and link transmission performance of different terahertz communication frequency bands, the recommendations for the selection of frequencies for satellite-ground and inter-satellite terahertz communication.

Terahertz (THz) communication has been envisioned as a key enabling technology for sixth-generation (6G). In this paper, we present an extensive THz channel measurement campaign for 6G wireless communications from 220 GHz to 330 GHz. Furthermore, the path loss is analyzed and modeled by using two single-frequency path loss models and a multiple-frequencies path loss model. It is found that at most frequency points, the measured path loss is larger than that in the free space. But at around 310 GHz, the propagation attenuation is relatively weaker compared to that in the free space. Also, the frequency dependence of path loss is observed and the frequency exponent of the multiple-frequencies path loss model is 2.1. Moreover, the cellular performance of THz communication systems is investigated by using the obtained path loss model. Simulation results indicate that the current inter-site distance (ISD) for the indoor scenario is too small for THz communications. Furthermore, the tremendous capacity gain can be obtained by using THz bands compared to using microwave bands and millimeter wave bands. Generally, this work can give an insight into the design and optimization of THz communication systems for 6G.

When an aircraft flies at a hypersonic speed within the atmosphere, the temperature of the infrared window (IRW) on the aircraft will rise rapidly due to the high-speed incoming flow will produce a severe aerodynamic heating to its optical detection window. The infrared (IR) radiation of the high-temperature gas and optical window will generate severe pneumatic thermal radiation effect upon the detection system, with the performance of the IR detector possibly being reduced or even destroyed. To evaluate the influence on the target imaging made by the IRW radiation, the experiment on the basis of building a simulating model is conducted by the means of ray tracing so that the accurate transmittance of the IRW can be observed under the different temperature. And then the radiation distribution of the thermal radiation on the detector generated by the IRW radiation noise and target signal can finally be obtained. This paper also records the different parameters in the detection system being set in the experiment, and analyzes the different influences brought by various factors to the Signal to Noise Ratio (SNR). It is also expected that it will provide a data reference to the following research of radiation noise suppression and design of IR detection system.

This paper presents a performance evaluation of IEEE 802.16e protocol. The 802.16e PHY/MAC protocol for wireless MAN has been applied to a high altitude platform (HAP) architecture with another protocol family member: 802.16SC (single carrier). In this work an HAP serves a rural scenario with low multipath effects. Two different time-variant wireless channels have been considered: the first one for a HAP- mobile user link that is characterized by path loss and doppler effects and the second one for a mobile user - mobile user link that presents also multipath fading. Many simulation scenarios with different mobility user speeds and different modulations are reported and interesting results concerning the PER of both channel, the first under single carrier (SC), and the second under an OFDM modulation, a specific multicarrier modulation, are presented. Thus PER evaluation under different modulation and for different payload sizes are presented. Simulation results showed good channel performance if the mobility of the user is low or the mobile user is near the vertical axis of the HAP segment, but also if QPSK modulation is used. Finally Matlab elaboration to obtain PER polynomial function are presented.

Probing primordial gravitational waves is one of the core scientific objectives of the next generation CMB polarization experiment. Integrating more detector modules on the focal plane and performing high accurate observations are the main directions of the next generation CMB polarization telescope, like CMB S4. Also, multi-band observation is required by foreground analysis and reduction, as it is understood that foregrounds have become the main obstacles of CMB polarization measurements. However, ground observation is limited by the atmospheric window and can be usually carried out in one or two bands, like what BICEP or Keck array have done in the south pole. In this paper, we forecast the sensitivity of tensor-to-scalar ratio r that may be achieved by a multi-frequency CMB polarization experiment, basing on which to provide guidance for further expanding frequency bands and optimize the focal plane of a telescope. At the same time, the realization of having two frequency bands in one atmospheric window is discussed. With fixed number of detectors, the simulation results show that, in order to get a good limit, more frequency bands are needed. Better constraints can be obtained when it includes at least three bands, i.e., one CMB channel (95 GHz) + one dust channel (high frequency) and one synchrotron channel (low frequency). For example, 41 + 95 + 220 GHz, which is better than only focusing around the CMB band, like 85 + 105 + 150 GHz, and 95 + 135 + 155 GHz, and this frequency combination is even better than the combination of 41 + 95 + 150 + 220 GHz. As CMB S4 plans to consider two frequency bands in each atmospheric window, and along this way, we find that one CMB band and more bands in synchrotron and dust channels are helpful, for example, 2 bands in lower frequency, 30 + 41 GHz, 2 bands in higher frequency, 220 + 270 GHz, i.e. 30 + 41 + 95 + 220 + 270 GHz, can get better constraints, and in this case, more detectors are asked to be assigned in the CMB channel.

Terahertz (THz) communications are envisioned to be a promising technology for 6G thanks to its broad bandwidth. However, the large path loss, antenna misalignment, and atmospheric influence of THz communications severely deteriorate its reliability. To address this, hybrid automatic repeat request (HARQ) is recognized as an effective technique to ensure reliable THz communications. This paper delves into the performance analysis of HARQ with incremental redundancy (HARQ-IR)-aided THz communications in the presence/absence of blockage. More specifically, the analytical expression of the outage probability of HARQ-IR-aided THz communications is derived, with which the asymptotic outage analysis is enabled to gain meaningful insights, including diversity order, power allocation gain, modulation and coding gain, etc. Then the long term average throughput (LTAT) is expressed in terms of the outage probability based on renewal theory. Moreover, to combat the blockage effects, a multi-hop HARQ-IR-aided THz communication scheme is proposed and its performance is examined. To demonstrate the superiority of the proposed scheme, the other two HARQ-aided schemes, i.e., Type-I HARQ and HARQ with chase combining (HARQ-CC), are used for benchmarking in the simulations. In addition, a deep neural network (DNN) based outage evaluation framework with low computational complexity is devised to reap the benefits of using both asymptotic and simulation results in low and high outage regimes, respectively. This novel outage evaluation framework is finally employed for the optimal rate selection, which outperforms the asymptotic based optimization.

When a hypersonic vehicle is flying in the atmosphere,serious aero-thermo-radiation effects can reduce or even destroy the performance of infrared( IR) detection,and high-temperature IR window becomes the main factor of the effects due to the aerodynamic heating. Based on thermal radiation transfer model for IR window,a method was proposed to measure thermal radiation characteristics of IR window materials.And thermal radiation characteristics of a sapphire IR window material applied to mid-wave infrared( MWIR)detection system were measured. The results indicate that thermal radiation characteristics of the sapphire IR window material with thickness of 0. 1 mm,in the wavelength region 3. 7- 4. 8 μm,have an approximate cubic relationship with temperature,changing from 100℃ to 350℃. With the rise of temperature,the transmittance decreases,while the self-radiation increases. The intensive self-radiation can make detector into saturation state easily,of which the influence on the MWIR detection system is bigger than that of transmittance which decreases detecting signal to noise ratio( SNR).

In future wireless networks, integrating Terahertz (THz) communication with cell-free massive multiple-input multiple-output (CFMM) systems presents a promising approach to achieving high data rates and low latency. This paper investigates the use of recurrent neural network (RNN)-based hybrid precoding in CFMM systems operating in the THz band. The proposed method jointly designs analog and digital precoders to adapt to dynamic channel conditions and user mobility. However, THz communication is challenged by high path loss and sparse scattering, which complicate accurate channel estimation. To address this, the RNN is trained to predict optimal precoding weights by learning spatial and temporal channel patterns, thereby improving channel estimation and mitigating pilot contamination. Simulation results show that the proposed method achieves higher spectral efficiency and lower bit error rate (BER) than conventional techniques. Specifically, the RNN-based approach attains a spectral efficiency of 10 bps/Hz at a signal-to-noise ratio (SNR) of 30 dB, compared to 8.2 bps/Hz for minimum mean square error (MMSE) precoding. For 16-QAM, the RNN-based method achieves a BER of 10⁻⁶ at an SNR of 11 dB, while MMSE requires 12.5 dB to reach the same BER. Overall, the RNN-based hybrid precoding consistently outperforms traditional methods across various SNR levels, antenna configurations, and user densities, underscoring its potential in next-generation THz wireless systems.

The advent of 6G wireless networks marks a paradigm shift in wireless communication, aiming to achieve unprecedented data rates, ultra-low latency, and massive connectivity. As the demand for high-speed and energy-efficient networks grows, Terahertz (THz) communications has emerged as a viable solution to address spectrum congestion and enhance wireless capacity. However, the implementation of THz communication presents challenges, including high path loss, signal attenuation, and energy efficiency concerns. To mitigate these limitations, Intelligent Reflecting Surfaces (IRS), Massive Multiple-Input Multiple-Output (MIMO), and advanced Internet of Things (IoT) architectures are being integrated into 6G frameworks to optimize performance. Intelligent Reflecting Surfaces (IRS) utilize reconfigurable metasurfaces to dynamically control signal propagation, improving coverage and mitigating path loss in high-frequency bands. Meanwhile, Massive MIMO enhances spectral efficiency and network robustness by leveraging large antenna arrays to enable precise beamforming and spatial multiplexing. Furthermore, energy-efficient IoT architectures are crucial in 6G networks to support ultra-low power, sustainable, and scalable communication for interconnected devices. These architectures leverage machine learning, edge computing, and blockchain technology to optimize resource allocation and reduce energy consumption. By combining IRS, MIMO, and advanced IoT architectures, 6G networks can harness the potential of THz communication while overcoming its limitations. This convergence will facilitate next-generation applications such as immersive extended reality (XR), autonomous systems, and ultra-reliable low-latency communications (URLLC). Future research should focus on seamless integration, security, and sustainability in 6G-enabled THz networks to achieve a truly intelligent and energy-efficient wireless ecosystem.

TeraHertz (THz) communications can satisfy the high data rate demand with massive bandwidth. However, severe path attenuation and hardware imperfection greatly alleviate its performance. Therefore, we utilize the reconfigurable intelligent surface (RIS) technology and investigate the RIS-aided THz communications. We first prove that the small-scale amplitude fading of THz signals can be accurately modeled by the fluctuating two-ray distribution based on two THz signal measurement experiments conducted in a variety of different scenarios. To optimize the phase-shifts at the RIS elements, we propose a novel swarm intelligence-based method that does not require full channel estimation. We then derive exact statistical characterizations of end-to-end signal-to-noise plus distortion ratio (SNDR) and signal-to-noise ratio (SNR). Moreover, we present asymptotic analysis to obtain more insights when the SNDR or the number of RIS’s elements is high. Finally, we derive analytical expressions for the outage probability and ergodic capacity. The tight upper bounds of ergodic capacity for both ideal and non-ideal radio frequency chains are obtained. It is interesting to find that increasing the number of RIS’s elements can significantly improve the THz communications system performance. For example, the ergodic capacity can increase up to 25% when the number of elements increases from 40 to 80, which incurs only insignificant costs to the system.

In this paper, propagation modeling and performance analysis on the HAPs are obtained. Elevation angle is the dominant parameter on the model. All possible propagation environments are divided into four groups: suburban (SU), urban (U), dense urban (DU) and urban high rise (UHR). These groups are modeled using well-known statistical models with a dependence on elevation angle. The main parameters are the elevation angle, Rayleigh and Ricean propagation factors, and percentage of time a given fade depth is exceeded. To observe the effects of the parameters on the model, the correlation coefficients between model parameters and the fade depth are calculated. This new HAP model contains two cases-line of sight (LOS) and non-line of sight (NLOS) between a HAP and a user. In a conclusion, obtained models are combined with free space path loss and full formulations of total path loss for the four possible HAPs propagation environments at three different frequencies (2, 3.5, and 5.5 GHz) are given.

In this paper, we investigate techniques for optimizing the coexistence performance of providing WiMAX (IEEE802.16a) from High Altitude Platforms (HAPs) and terrestrial deployments in shared 3.5 GHz frequency bands are presented. The paper will show that it is effective to provide WiMAX services from HAPs with optimized performance by appropriate choice of parameters, including varying the HAP deployment spacing radius and directive antenna beamwidth based on the adopted antenna models for HAPs and receivers. Illustrations and comparisons of changing the antenna pointing offset, narrowing the transmitting and receiving antenna beamwidth as well as keeping the accepted CINR performance across the HAP coverage area, will demonstrate that that efficiently utilizing these techniques are able to enhance the HAP system performance while effectively coexisting with the terrestrial WiMAX systems.