Abstract:Terahertz(THz) technology has great potential in the next generation communication, biological imaging, quality monitoring and other fields, and the development of high-quality terahertz waveguides makes it possible to creat lightweight and miniaturized terahertz systems, which is vital to promote the development of terahertz technology. In this paper, terahertz waveguides are divided into metal waveguides, polymer waveguides and composite waveguides according to different materials. Terahertz polymer waveguides with high flexibility and low cost are emphatically introduced, and different guiding mechanisms are discussed. Finally, the technology of guiding terahertz wave is summarized, and the bottleneck problem restricting the commercial application of terahertz waveguide is preliminarily discussed.

Abstract:Phase shifters are important devices in phased array systems. As the frequency increases, the skinning depth of the metal and the roughness of the waveguide surface have a non-negligible effect on the device, which cause the increase o f device loss. To address the difficulties of poor phase shift accuracy and high loss in terahertz phase shifters, a planar digital terahertz phase shifter based on a loaded line is proposed. By connecting two branches in parallel on a microstrip line, the switching diodes are loaded on the parallel branches, the electrical lengths of the two branches are adjusted to obtain the desired phase shift, and the switches states are controlled to achieve different phase shift angles. The simulation results show that the reflection coefficients of both on and off are less than -10 dB, the insertion loss is less than 0.5 dB, and the phase shifting error is less than 5° in the 192~210 GHz band range, while the phase shifting error is less than 1°within the 5 GHz band. The proposed planar phase shifter is easy for processing and system integration with low cost, and has a wide range of applications in terahertz phased array systems.

Abstract:Terahertz metamaterial phased array antenna has significant application value in integrated detection and communication systems due to its strong beam manipulation capabilities. In order to enhance the communication and detection performance of the system, it is necessary for terahertz metamaterial phased array antenna to achieve flexible switching between wide beams and narrow beams. Therefore, a beamwidth control method of terahertz metamaterial phased array antenna based on inverse phase interval(inverse phase) coding is proposed in this paper. The array size is reversed by the antenna 3dB beamwidth expression and the target beamwidth, the phase encoding is kept unchanged while the remaining elements outside the array size are inversely encoded, aiming to achieve phase cancellation by phase inversion. The simulation results demonstrate that the proposed method accurately controls the beamwidth of terahertz metamaterial phased array antenna, allowing for flexible switching between wide beams and narrow beams.

Abstract:A Grounded Coplanar Waveguide(GCPW) to slotline power divider is firstly built and verified based on the structural properties of both the GCPW and the slotline. Then an in-phase power divider and an out-of-phase power divider based on a GCPW-slotline-GCPW structure are designed according to the electric field distribution characteristics of the cross-section of the slotline. The simulation results show that the insertion loss of the in-phase power divider is better than 4 dB and the return loss is better than 9.6 dB in the range of 175~225 GHz; the insertion loss of the out-of-phase power divider is better than 4 dB and the return loss is better than 10.5 dB in the range of 185~215 GHz, and the amplitude imbalance is less than 0.24 dB and the phase imbalance is less than 1.3°. Compared to other terahertz power dividers, the power dividers presented in this study are simpler, more compact, and easier to integrate, with comparable insertion and return losses.

Abstract:The E-band broadband high-Intermediate Frequency(IF) single balanced mixer is proposed to meet the demand of the communication, radar and test instruments in the millimeter-wave broadband field. The Radio Frequency(RF) and Local Oscillator(LO) signals are input through the multi-branch broadband broadened waveguide quadrature coupler. The signal is transitioned from the waveguide transmission mode to the microstrip mode through the grounding fin transition structure, providing a broadband IF signal loop and DC grounding. The IF output low-pass filter can effectively suppress the LO and RF signals and provide an equivalent grounding circuit for them. The mixer employs the nonlinearity of the Schottky barrier diode to achieve mixing, and ultimately achieves a wideband low-loss mixing effect through the microstrip matching circuits. The mixer utilizes three LO frequencies of 57.6 GHz, 62.4 GHz, and 67.2 GHz to segmentally down-convert the RF signal within the range of 67~ 85 GHz to the IF range of 9.4~17.8 GHz. The test results indicate that within the 67~85 GHz RF range, when the RF input power is -15 dBm and the LO input power is 12 dBm, the frequency conversion loss of the mixer is 7.1~10.1 dB, and the undesired mixing product rejection is above 36 dBc.

Abstract:In response to the issue of precise altitude measurement in complex terrains where existing proximity sensors struggle to adapt, a novel imaging-based adaptive ground altitude estimation method in the image domain is proposed. This method applies the principle of Doppler sharpening imaging to the forward-looking view scene, taking advantage of the high resolution in the terahertz frequency band and the short synthetic aperture length. It utilizes terahertz forward-looking imaging for terrain perception, transforming the one-dimensional distance measurement of traditional proximity sensors into two-dimensional imaging measurement, thus achieving precise perception of the terrain and topography directly below the motion trajectory. After obtaining the ground target image, the altitude estimation is realized by fitting image domain features using the characteristics of the ground image. Theoretical analysis and simulation verification were conducted on the parameter design and imaging algorithm of the forward-looking Doppler sharpening imaging. A terahertz proximity detection system operating at 220 GHz frequency band was developed and unmanned aerial vehicle-mounted tests were carried out. The tests show that this method can effectively perceive the ground environment, eliminate interference from corner reflectors, trees, and other targets on ground altitude measurement, and achieve an altitude measurement accuracy of 0.5 m. This demonstrates the feasibility of the terahertz imaging-based ground altitude estimation method and lays a foundation for improving the adaptability and perception capability of proximity sensors to complex terrains based on image domain features.

Abstract:In order to improve the accuracy of motion parameter estimation and imaging quality of moving targets in terahertz Circular Synthetic Aperture Radar(CSAR) mode, a terahertz CSAR ground moving target parameter estimation and refocusing method is proposed. Firstly, the traditional spectral peak measurement method is improved to estimate the azimuth velocity by combining the shadow position of the moving target. Compared with the existing methods, the proposed method takes into account the influence of the real position of the target on the Doppler frequency shift, and improves the accuracy of the moving target's azimuth velocity estimation. Then the Fractional Fourier Transform(FrFt) is employed to estimate the Doppler modulation frequency of the azimuth phase, and the range velocity of the target can be estimated together with the azimuth velocity. Finally, the first-order and second-order phase compensation functions are constructed for azimuth compensation based on the estimation results, and the Phase Gradient Autofocus(PGA) algorithm is utilized to compensate for the residual quadratic and higher phase errors. The moving target motion parameters in the simulated and real CSAR data are accurately estimated and good focusing is achieved, which proves the effectiveness of the proposed method and greatly improves the imaging quality of the moving target.

Abstract:In Circular Synthetic Aperture Radar(CSAR), the range equation between radar and ground moving target is complicated because the radar platform flies around the circumference of the observation scene, and there is a complex coupling relationship between range direction parameters and azimuth direction parameters of target, which makes it difficult to estimate the range velocity of moving target independently. A method for estimating the range velocity of moving target in terahertz CSAR based on sub-aperture division is proposed. By selecting a suitable angle of sub-apertures, sub-aperture division is performed on CSAR. Under the condition of sub-aperture, the decoupling between the range velocity of moving target and other parameters can be realized by small angle approximation, and the range velocity of moving target can be accurately estimated, which can achieve accurate relocation of moving target. The effectiveness of the proposed algorithm is verified by simulation and measured data.

Abstract:The evolution of future communication technologies towards higher carrier frequencies is inevitable to meet the escalating demands for data transmission rates. The terahertz(THz) spectrum emerges as a critical enabler for communication systems characterized by high stability and rapid data transfer rates. This paper reviews research progress on terahertz near-field channel characteristics and channel modeling. Firstly, the necessity of near-field channel analysis in the terahertz frequency band is introduced, the difference between near and far field channels is compared, and the spherical wavefront modeling and the model based on electromagnetic wave theory are introduced. Secondly, combining the channel modeling technology and the characteristics of the near-field channel, the methodology that can be used in near-field channel modeling is presented. Finally, the paper encapsulates the findings on terahertz near-field channel characteristics and offers insights into potential future research.

Abstract:There exists severe deterioration of phase noise in the ultra wideband application scenario of terahertz communication, since terahertz carriers are achieved through multiple frequency doubling. Based on the multipole/zero phase noise model at terahertz frequency, a fully parallelized phase estimation algorithm is proposed on the basis of traditional blind phase estimation algorithms. This algorithm inserts pilots into each parallel data and uses pilot phase noise information as the initial phase to perform phase extension and rotation phase detection on the parallel data. Referring to the traditional blind phase estimation algorithm, the decision selection approach for the optimal phase estimation value is adopted. At the same time, the previous and current time pilots are employed to estimate the phase of the current parallel data from the front and back directions. The two estimated phases are weighted and summed based on the distance between the front and back pilots to obtain the optimal phase estimation information. After simulation verification, the residual phase noise is reduced by 10 dBc/Hz and 25 dBc/Hz at 1 MHz and 10 MHz, respectively, through this algorithm.

Abstract:As a new type of high-frequency band resource, terahertz wave has a higher carrier frequency, it can withstand larger bandwidth and transmission rate, therefore, it has become a research hotspot for new generation broadband communications. A 300 GHz terahertz wireless communication system is constructed based on pure electronic devices, in which the transmitter has a simple structure and the whole system is less complex than the photon-assisted system. Signal transmission experiments with wireless distances of 50 m and 100 m are conducted. The transmission performance of the system is explored and the relationships among the transmission performance, transmission rate and transmitting voltage are studied. As far as we know, 100 m is the longest distance that an all-electronic terahertz wireless transmission system can achieve without using a Power Amplifier(PA).

Abstract:Focusing on current Orthogonal Frequency Division Multiplexing(OFDM) communication systems, the integrated waveforms study is conducted based on two-Dimensional Fast Fourier Transform(2D-FFT). The sensing characteristics of THz OFDM waveforms are investigated by combining theoretical analysis with simulations to analyze the impacts of waveform parameter design under various frequencies and bandwidths, aiming to guide the design of THz OFDM Integrated Sensing And Communication(ISAC) systems. Theoretical and simulation analysis demonstrates that the THz OFDM integrated communication and sensing system possesses multi-object sensing capabilities, with the large bandwidth of THz enabling a high range resolution of the OFDM waveform sensing up to the centimeter level and a velocity resolution up to decimeter per second level. Moreover, even under low sensing Signal-to-Noise Ratio(SNR) conditions, the system can still resolve the position and velocity information of targets, confirming the capability of OFDM integrated communication and sensing waveforms to support THz narrow beam mobile communication.

Abstract:Traditional Orthogonal Frequency Division Multiplexing(OFDM) signals in terahertz communication systems usually bring about high Peak to Average Power Ratio(PAPR) problems that would seriously degrade the power amplifier efficiency and deteriorate the nonlinear effect of terahertz link. The design of constant-envelope OFDM based Integrated Sensing And Communication(ISAC) waveforms can suppress the harmful effects of high PAPR. The Ambiguity Function is used as the criteria for the range and velocity performance evaluation of three OFDM-based ISAC waveforms in the terahertz bands, namely, OFDM-16QAM(16-Quadrature Amplitude Modulation)-LFM(Linear Frequency Modulation), OFDM-BPSK(Binary Phase Shift Keying)-LFM, OFDM-MSK(Minimum Shift Keying)-LFM. The numerical simulation results demonstrate that, OFDM-16QAM-LFM waveform has excellent range-velocity performance but serious PAPR problems. OFDM-BPSK-LFM, OFDM-MSK-LFM waveforms replace the amplitude-phase modulation with phase modulation, thus maintaining constant-envelope characteristics. The range detection capability of OFDM-BPSK-LFM waveform does not degrade with the increase of subcarriers, while the velocity detection capability is seriously affected. Therefore OFDM-BPSK-LFM waveform is not suitable for the velocity detection of fast-moving objects. OFDM-MSK-LFM waveform can tolerate more subcarriers, making it suitable for multi-carrier radar detection under complex circumstances. This work provides references for the choice of ISAC waveforms in different application scenarios.