Performance of terahertz channel with multiple input multiple output techniques

Terahertz (THz) communication is being considered as a potential solution to mitigate the demand for high bandwidth. The characteristic of THz band is relatively different from present wireless channel and imposes technical challenges in the design and development of communication systems. Due to the high path loss in THz band, wireless THz communication can be used for relatively short distances. Even, for a distance of few meters (>5m), the absorption coefficient is very high and hence the performance of the system is poor. The use of multiple antennas for wireless communication systems has gained overwhelming interest during the last two decades. Multiple Input Multiple Output (MIMO) Spatial diversity technique has been exploited in this paper to improve the performance in terahertz band. The results show that the Bit Error Rate (BER) is considerably improved for short distance (<5m) with MIMO. However, as the distance increases, the improvement in the error performance is not significant even with increase in the order of diversity. This is because, as distance increases, in some frequency bands the signal gets absorbed by water vapor and results in poor transmission. Adaptive modulation scheme is implemented to avoid these error prone frequencies. Adaptive modulation with receiver diversity is proposed in this work and has improved the BER performance of the channel for distance greater than 5m.     

Estimation of time-varying channels for pilot-assisted OFDM systems

Orthogonal frequency division multiplexing (OFDM) systems encounter performance degradations because of the time-varying (TV) channels common in wireless environments. The channel variations within one OFDM symbol introduce intercarrier interference. In this case, the frequency domain channel matrix is no longer diagonal, hence the corresponding channel estimation is challenging. In this article, two novel TV channel estimation approaches are proposed for the pilot-assisted OFDM systems, where the channel is approximated by the high-order linear model or the piece-wise linear model in time domain. The least square estimation is derived for the two kinds of channel approximations. The simulation is performed based on realistic TV channels with a fairly high Doppler spread. The results show the significant decreasing of the estimation mean square error using the proposed approaches.     

Potentiality of semiconducting diamond as the base material of millimeter-wave and terahertz IMPATT devices

An attempt is made in this paper to explore the potentiality of semiconducting type-IIb diamond as the base material of double-drift region（DDR） impact avalanche transit time（IMPATT） devices operating at both millimetre-wave（mm-wave） and terahertz（THz） frequencies. A rigorous large-signal（L-S） simulation based on the non-sinusoidal voltage excitation（NSVE） model developed earlier by the authors is used in this study. At first,a simulation study based on avalanche response time reveals that the upper cut-off frequency for DDR diamond IMPATTs is 1.5 THz, while the same for conventional DDR Si IMPATTs is much smaller, i.e. 0.5 THz. The L-S simulationresultsshowthattheDDRdiamondIMPATTdevicedeliversapeakRFpowerof7.79Wwithan18.17%conversion efficiency at 94 GHz; while at 1.5 THz, the peak power output and conversion efficiency decrease to6.19mWand8.17%respectively,taking50%voltagemodulation.AcomparativestudyofDDRIMPATTsbasedon diamond and Si shows that the former excels over the later as regards high frequency and high power performance at both mm-wave and THz frequency bands. The effect of band to band tunneling on the L-S properties of DDR diamond and Si IMPATTs has also been studied at different mm-wave and THz frequencies.     

The Research on 220GHz Multicarrier High-Speed Communication System

This paper presents our investigation into a 220 GHz multicarrier highspeed communication system based on solid state transceivers.The proposed system has eased the demand of high sampling rate analog-to-digital converter(ADC)by providing several signal carriers in microwave band and converting them to 220 GHz channel.The system consists of a set of 220 GHz solid-state transceiver with 2 signal carriers,two basebands for 4 GSPS ADCs.It has achieved 12.8 Gbps rate real-time signal transmission using 16QAM modulation over a distance of 20 m without any other auxiliary equipment or test instruments.The baseband algorithm overcomes the problem of frequency difference generates by non-coherent structure,which guarantees the feasibility of long-distance transmission application.Most importantly,the proposed system has already carried out multi-channel 8K video parallel transmission through switch equipment,which shows the multicarrier high-speed communication system in submillimeter wave has great application prospects.To the best of the authors’knowledge,this is the first all-solid-state electronics multicarrier communication system in submillimeter and terahertz band.     

Performance analysis of broadband satellite communication system based on OFDM/TDM

Orthogonal frequency division multiplexing （OFDM） is attracting more attention for its capability of high speed transmission. However, the OFDM possesses an obvious shortage in its high ratio of the peak power to the average power （PAPR）, which has become the main issue holding it back to be applied to the broadband satellite communication system. OFDM combined with time division multiplexing （TDM）, dividing the subcarriers of OFDM into some blocks in time tune, can decrease the high PAPR of OFDM. Meanwhile, the advantages of OFDM can be preserved. In this paper, OFDM/TDM is applied to the broadband satellite communication system. This paper theoretically analyses OFDM/TDM system model as well as its restraining effect on PAPR, and proposes frequency domain multiplexing-pilot （FDM-Pilot） channel estimation algorithm. Simulation results show OFDM/TDM in broadband satellite communication system has approving performance and decreased the PAPR.     

AoA-Based Channel Estimation for Massive MIMO OFDM Communication Systems on High Speed Rails

Channel estimation is a well-known challenge for wireless orthogonal frequency division multiplexing(OFDM)communication systems with massive antennas on high speed rails(HSRs).This paper investigates this problem and design two practicable uplink and downlink channel estimators for orthogonal frequency division multiplexing(OFDM)communication systems with massive antenna arrays at base station on HSRs.Specifically,we first use pilots to estimate the initial angle of arrival(AoA)and channel gain information of each uplink path through discrete Fourier transform(DFT),and then refine the estimates via the angle rotation technique and suggested pilot design.Based on the uplink angel estimation,we design a new downlink channel estimator for frequency division duplexing(FDD)systems.Additionally,we derive the Cramér-Rao lower bounds(CRLBs)of the AoA and channel gain estimates.Finally,numerical results are provided to corroborate our proposed studies.     

Performance of closed loop transmit diversity in a Rayleigh fading channel

In this paper, the closed loop transmit diversity technology for the Wideband Code Division Multiple Access（WCDMA） systems is investigated in a multipath Rayleigh fading channel. The RAKE receiver model and the weighing vector algorithm are presented. The performance is theoretically analyzed in terms of the average maximal Signal-to-Noise Ratio（SNR） gain available over the Space-Time block coding based Transmit Diversity（STTD） technology. Theoretic analysis and simulation results show that the closed loop transmit diversity can provide a 3dB performance gain over the open loop scheme in a single path fading channel, while the performance gain decreases dramatically with the increasing inherent multipath diversity of the wireless channel.     

A Survey on Channel Measurements and Models for Underground MIMO Communication Systems

The high reliability of the communication system is critical in metro and mining applications for personal safety, channel optimization, and improving operational performance. This paper surveys the progress of wireless communication systems in underground environments such as tunnels and mines from 1920 to 2022, including the evolution of primitive technology, advancements in channel modelling, and realization of various wireless propagation channels.In addition, the existing and advanced chann...     

Principle and Key Technologies of OFDM-RoF

The merging of optical communication and wireless communication is a tendency in the development of future communication.Orthogonal Frequency Division Multiplexing (OFDM) technology is becoming a core technology in the physical layer of next generation wireless communication system.Taking advantages of both wireless communication and optical communication,OFDM Radio over Fiber (OFDM-RoF) system is characterized by high speed,large capacity and high spectral efficiency.However,it still has some problems to be addressed,including dispersion and nonlinearity effects.The nonlinearity effect of fiber and modulator can be mitigated by PAPR reduction algorithms,the dispersion effect can be eliminated using Cyclic Prefix (CP) and channel estimation,and the fiber nonlinearity can be reduced by techniques such as digital phase conjugation,Partial Carrier Filling (PCF),nonlinearity precompensation and serial correlation reduction.     

FPGA Implementation of FFT Algorithm for OFDM Based IEEE 802.16d （Fixed WiMAX） Communications

The IEEE 802.16d communication standard uses orthogonal frequency division multiplexing （OFDM）. In the widely used OFDM systems, the fast Fourier transform （FFT） and inverse fast Fourier transform pairs are used to modulate and demodulate the data constellation on the sub-carriers. In this paper, a high level implementation of a high performance FFT for OFDM modulator and demodulator is presented. The design has been coded in Verilog and targeted into Xilinx Spartan3 field programmable gate arrays. Radix-22 algorithm is proposed and used for the OFDM communication system. The design of the FFT is implemented and applied to fixed WiMAX--IEEE 802.16d communi- cation standard. The results are tabulated and the hardware parameters are compared. The proposed architecture is least in number of multipliers used and the memory size, and second to the least in number of adders used.     

Study of atmospheric attenuation characteristics of terahertz wave based on line‐by‐line integration

Terahertz band communication promises new solution for satisfying the increasing demand for ultrahigh‐speed wireless communication. Channel models capturing the unique peculiarities of the terahertz (THz) band are required for communication systems designing. Extreme high molecular absorption is a distinctive phenomenon that has to be involved in terahertz communication models. Research in this field has mainly focused on the characteristics along the horizontal propagation path. In this paper, we developed a unified molecular absorption model along the slant propagation path of the THz wave, based on the line‐by‐line integration method developed by Van‐Vleck and Weisskopf and combining the molecular spectral line in the HITRAN database. Then, an in‐depth analysis on the THz channel characteristics is carried out by the developed propagation models. The attenuation characteristics of terahertz waves with frequencies in the range of 0.1 to 1 THz are analyzed by theoretical and mathematical modeling. The results show that the terahertz communication channel has a strong dependence on both the molecular composition of the medium and the transmission distance. The experimental results also indicate the strong absorption frequency points, weak absorption frequency points, and spectral windows.     

THz wireless communication system based on wavelet transform

With a very wide frequency band not allocated at present, THz waves have many optimal characteristics such as high transmission rate, large capacity, and high security. The research of THz communication technology has become a hotspot in wireless communication. For THz wireless communications, it is crucial to study advanced electrical signal processing techniques. In this paper, in view of the shortcomings of traditional orthogonal frequency division multiplexing (OFDM) technology, we propose wavelet transform orthogonal multicarrier modulation method in THz system. In addition, we study THz channel coding technology to ensure that the THz wireless communication baseband system has better bit error rate (BER) performance and low computational complexity. Based on above, a THz wireless communication baseband system is conceived.     

Performance Analysis of UWB Channels for Wireless Personal Area Networks

Ultra-wide band (UWB) communication is one of the most promising technology for high data rate networks over short-range communication. The ultra-wide bandwidth offers pulses with very short duration that provides frequency diversity and multipath resolution. Ultra-wide band (UWB) channels raise new effects in the receiver, the amplitude fading statistics being different compared to the conventional narrow band wireless channels. This review paper focuses on modeling of ultra-wide band channels, especially for simulation of personal area networks and also discusses the benefits, application potential and technical challenges in wideband communication. The concept of Orthogonal Frequency Division Multiplexing (OFDM) has recently been applied in wireless communication systems due to its high data rate transmission capability with high bandwidth efficiency and its robustness to multi-path delay. UWB OFDM communication was proposed for physical layer in the IEEE 802.15.3a standard which covers wideband communication in wireless personal area networks. Since the channel model for multicarrier UWB communication is different from that of plain ultra-wide band channel, a novel modification method in UWB channel model is proposed with specific center frequency and multipath resolution. Moreover, dynamic channel estimation is necessary before demodulation of UWB OFDM signals since the radio channel is time varying and frequency selective for wideband systems. The performance of the proposed method is statistically analyzed using LS and MMSE based channel estimation methods.     

基于Lambertian模型的星间太赫兹通信信道特性分析

随着数据传输速率的不断增长,6G无线通信系统需要采用新的频谱资源.太赫兹频段因其具有高带宽、高速率等特点,在未来的无线通信领域具有极大的优势.另一方面,为了应对未来无线通信网络全球覆盖的要求,6G通信网络需要集成地面通信网络和空间通信网络.本文利用射线追踪理论,对太赫兹频段的星间通信链路的确定性信道模型进行了研究.由于...     

太赫兹频段星地通信信道建模与仿真

太赫兹频段作为至今尚未被完全开发的超高通信频段,具有超大带宽等优点,将其应用于第五代（the 5th Generation,5G）、后五代（Beyond 5G,B5G）移动通信系统,除实现更高速率传输外,还可实现地面移动网络与卫星网络频谱资源的共享,有利于推动新一代空天地一体化通信网络建设.文章提出了一种适用于星地通信系统的太赫兹信道建模与仿真方法,分析了自由空间损耗、分子吸收损耗、云雾衰减、雨衰减及多普勒频移等太赫兹信道的影响因素,构建了星地太赫兹通信信道建模流程,并给出了分步骤信道参数的生成方法.通过数值仿真,对不同天气状况下传输距离和频率对传输信号的影响进行了分析,并基于所生成的信道响应对误码率进行评估,从而验证了所提出模型和方法的可用性.所提建模方法能够提供不同传输条件下的动态太赫兹信道响应数据,从而为今后太赫兹频段无线通信系统的设计与开发提供评估与测试依据.     

基于THz QCL 和THz QWP 的数字通信演示系统

随着无线通信速率需求的增加和材料生长、器件工艺制作水平的提高,太赫兹（THz）通信已成为未来高速无线通信系统发展的一个重要方向。介绍了太赫兹通信的特点以及国际上太赫兹通信系统的发展现状,并报导了一种利用太赫兹量子级联激光器（THz QCL）作为发射源,太赫兹量子阱探测器（THz QWP）作为接收器的太赫兹数字通信演示系统。该系统采用On-Off-Key（OOK）调制和直接强度检测方式,通信频点为3.9 THz,通信距离为2.2 m,传输速率可达1 Mbps 以上。最后探讨了该系统的带宽限制因素及其在通信速率方面的潜力。     

0.14 THz 10 Gbps无线通信系统

太赫兹通信由于其固有的宽带特性,在Gbps以上的高速无线通信领域受到广泛关注。本文描述了一种工作在0.14 THz频段的无线通信系统,传输速率达10 Gbps。该系统基于超外差结构,中频采用数字信号处理技术进行16QAM高阶数字信号调制解调,依靠肖特基二极管次谐波混频技术实现从中频到太赫兹信号的频谱搬移。目前该系统已经通过了500 m 10 Gbps距离无线传输实验验证,通信频段为133.8 GHz～137.4 GHz,带宽3.6 GHz,发射功率0 dBm,传输误码率低于10-6。     

二氧化钛光子晶体波导的太赫兹波传输特性

本文应用时域有限差分法研究了二维二氧化钛光 子晶体波导的禁带范围和太赫兹波的 传输特性,分别设计了带隙宽度为0.226 THz 、0.2728 THz和0.316 THz的直线型、直角型和T 型二氧化钛光子晶体波导结构。研究发现,相比传统波导,本文设计的二维二氧化钛光子晶 体波导不仅在直线路径中有较高的太赫兹波传输效率,而且在转角的路径中也有很高的太赫 兹波传输效率。此研究结果为太赫兹器件的设计和制作提供重要理论依据,为高速宽带无线 通信系统的发展提供重要理论借鉴。     

地空路径太赫兹波雨天衰减特性分析

雨滴的散射和吸收作用会严重增加地空链路上太赫兹波的传输损耗,降低无线通信的性能。为实现太赫兹波在地空链路上的传输应用,必须对太赫兹波在降雨环境中的传输特性进行深入研究。本文对原有的雨衰模型进行了修正,基于Mie理论,分析了降雨率的变化对地空链路上太赫兹波传输的影响,并与原有模型的计算结果进行了对比。结果表明：在整个太赫兹频段,雨衰减损耗会随降雨率的增加而增大,随频率的增加先增大后减小,且高频太赫兹波段相对0.1~1 THz频段范围的雨衰损耗更小;同时,当频率超过1 THz时,大气窗口越靠近10 THz,损耗越小,在降雨天气环境进行无线通信传输时将更具有通信优势,且频率越低,天顶角越大,模型修正前后的差异性更加明显。