Wireless channel estimation for high-speed rail communications:Challenges,solutions and future directions

With the development of High-Speed Rail(HSR),countries and individual passengers alike have enjoyed far ranging benefits as a result-economic,social,environment and in added convenience.One of the important parts of HSR construction is the signaling system,where wireless communications play a key role in the transmission of train control data.Channel estimation has a significant impact on the quality of the wireless communication,however,whose performance is degraded due to the fast mobility of HSR.This paper focuses on the channel estimation technology in HSR.We first summarize the key challenges for HSR channel estimation,especially the Inter-Carrier Interference(ICI)faced by Orthogonal Frequency Division Multiplexing(OFDM)systems.Then we provide a comprehensive review of existing pilot-aided channel estimation schemes from three points:channel model,estimation algorithm,joint channel estimation and ICI mitigation schemes.Lastly,we present the challenges of channel estimation for the Orthogonal Time Frequency Space(OTFS)system and Reconfigurable Intelligent Surface(RIS),which are promising techniques for HSR systems in the future sixth Generation(6G)wireless communication.

MmWave extra-large-scale MIMO based active user detection and channel estimation for high-speed railway communications

The current High-Speed Railway(HSR)communications increasingly fail to satisfy the massive access services of numerous user equipment brought by the increasing number of people traveling by HSRs.To this end,this paper investigates millimeter-Wave(mmWave)extra-large scale(XL)-MIMO-based massive Internet-of-Things(loT)access in near-field HSR communications,and proposes a block simultaneous orthogonal matching pursuit(B-SOMP)-based Active User Detection(AUD)and Channel Estimation(CE)scheme by exploiting the spatial block sparsity of the XLMIMO-based massive access channels.Specifically,we first model the uplink mmWave XL-MIMO channels,which exhibit the near-field propagation characteristics of electromagnetic signals and the spatial non-stationarity of mmWave XL-MIMO arrays.By exploiting the spatial block sparsity and common frequency-domain sparsity pattern of massive access channels,the joint AUD and CE problem can be then formulated as a Multiple Measurement Vectors Compressive Sensing(MIMV-CS)problem.Based on the designed sensing matrix,a B-SOMP algorithm is proposed to achieve joint AUD and CE.Finally,simulation results show that the proposed solution can obtain a better AUD and CE performance than the conventional CS-based scheme for massive IoT access in near-field HSR communications.

High-speed railway channel measurements and characterizations: a review

Wireless communication for high-speed railways （HSRs） that provides reliable and high data rate communi- cation between the train and trackside networks is a challenging task. It is estimated that the wireless communication traffic could be as high as 65 Mbps per high-speed train. The development of such HSR communications systems and standards requires, in turn, accurate models for the HSR propagation channel. This article provides an overview of ex- isting HSR channel measurement campaigns in recent years. Particularly, some important measurement and modeling results in various HSR scenarios, such as viaduct and U-shaped groove （USG）, are briefly described and analyzed. In addition, we review a novel channel sounding method, which can highly improve the measurement efficiency in HSR environment.

Morphological evolution and flow conduction characteristics of fracture channels in fractured sandstone under cyclic loading and unloading

In coal mining,rock strata are fractured under cyclic loading and unloading to form fracture channels.Fracture channels are the main flow narrows for gas.Therefore,expounding the flow conductivity of fracture channels in rocks on fluids is significant for gas flow in rock strata.In this regard,graded incremental cyclic loading and unloading experiments were conducted on sandstones with different initial stress levels.Then,the three-dimensional models for fracture channels in sandstones were established.Finally,the fracture channel percentages were used to reflect the flow conductivity of fracture channels.The study revealed how the particle size distribution of fractured sandstone affects the formation and expansion of fracture channels.It was found that a smaller proportion of large blocks and a higher proportion of small blocks after sandstone fails contribute more to the formation of fracture channels.The proportion of fracture channels in fractured rock can indicate the flow conductivity of those channels.When the proportion of fracture channels varies gently,fluids flow evenly through those channels.However,if the proportion of fracture channels varies significantly,it can greatly affect the flow rate of fluids.The research results contribute to revealing the morphological evolution and flow conductivity of fracture channels in sandstone and then provide a theoretical basis for clarifying the gas flow pattern in the rock strata of coal mines.

Propagation characteristics of wideband high-speed railway channel in viaduct scenario at 2.35 GHz

In order to obtain accurate characteristics of wireless channels in the viaduct area of China, a channel meas- urement was taken in a railway viaduct scenario of the Zhengzhou-Xi＇an passenger dedicated line with a bandwidth of 50 MHz at 2.35 GHz. The single-slope log-distance model is used to analyze the path-loss （PL）, and the distribution of shadow fading （SF） is obtained by statistical methods, which shows that the normal distribution fits the samples well. Ricean K-factor is analyzed by the method of moments, and the variation of K-factor is given along the measured route. Small scale such as delay spread and Doppler behavior are parameterized. Based on empirical channel measurement, this paper provides parameters for the evaluation and simulation work on viaduct scenarios of high-speed railway.

Fracture Analysis of Composites in High-speed Tension

In this paper the results of a high-speed tension experiment of the SiC_w/Al composite have been reported and a simplified theoretical model has been developed to study the fracture mechanism of composites in high-speed tension. This theoretical model provides a new explanation for the increase of dynamic fracture strength of composites in high-speed tension.

Doppler frequency offset estimation and diversity reception scheme of high-speed railway with multiple antennas on separated carriage

The challenges of severe Doppler effects in high-speed railway are considered. By building a cooperative antenna system; an algorithm of joint channel estimation and Doppler frequency offset （DFO） estimation is proposed based on Ricean channel model. First, a maximum likelihood estimation （MLE） algorithm for DFO is designed, show- ing that the Doppler estimation can be obtained by estimating moving velocity of the train and the path loss with the exploitation of pilots that are placed inside the frame. Then a joint detection algorithm for the receiver is proposed to exploit multi-antenna diversity gains. Last, the theoretical Crammer Rao bound （CRB） for joint channel estimation and DFO estimation is derived. The steady performance of the system is confirmed by numerical simulations. In particular, when the Ricean fading channel parameter equals 5 and the velocities of train are 100 m/s and 150 m/s, the estimation variances of DFO are very close to the theoretical results obtained by using CRB. Meanwhile, the corresponding sig- nal to noise ratio loss is less than 1.5 dB when the bit error rate is 10-5 for 16QAM signals.

A Method for Identifying Channeling Paths in Low-Permeability Fractured Reservoirs

Often oilfield fractured horizontal wells produce water flowing in multiple directions.In this study,a method to identify such channeling paths is developed.The dual-medium model is based on the principle of inter-well connectivity and considers the flow characteristics and related channeling terms.The Lorentz curve is drawn to qualitatively discern the geological type of the low-permeability fractured reservoir and determine the channeling direction and size.The practical application of such an approach to a sample oilfield shows that it can accurately identify the channeling paths of the considered low-permeability fractured reservoir and predict production performances according to the inter-well connectivity model.As a result,early detection of water channeling becomes possible,paving the way to real-time production system optimization in low-permeability fractured reservoirs.

胜利油田GF84区块CCUS气窜封堵技术及其应用

胜利油田GF84区块为低渗透油藏,采出程度较低,CO_(2)驱是提高该区块采收率的有效措施之一。在前期开发中注采井之间已形成明显的气窜通道,现阶段亟需进行气窜封堵,提高CO_(2)驱波及系数,实现均衡驱替。通过分析GF84区块气窜特征与开发矛盾,将气窜类型划分为“裂缝型气窜”和“基质型气窜”,并制定了“裂缝封堵”和“基质调剖”的治理策略。在明确气窜特征的基础上,通过室内实验研发了硅盐树脂堵剂、CO_(2)气溶性发泡剂和高温冻胶堵剂,并形成了“硅盐树脂裂缝封堵+CO_(2)气溶性发泡剂、高温冻胶基质调剖”化学封堵分级调控技术。结果表明,该技术在GF84区块成功应用4口井,其中硅盐树脂裂缝封堵2口井,CO_(2)气溶性发泡剂基质调剖2口井,措施有效率100%,有效期在0.5 a以上。研究成果可为低渗透油田CO_(2)驱开发提供技术支撑。

通道压裂支撑剂缝内分布规律研究

通道压裂是低渗透致密油气藏高效、低成本开发的关键技术,通过采用非连续簇团铺砂的方式,使裂缝中形成具有高导流能力的通道,但目前关于通道压裂的理论研究仍然处于起步阶段.因此,基于计算流体力学建立欧拉-欧拉固液两相流模型,对通道压裂过程中支撑剂的分布状态进行模拟,通过与实验结果进行对比,证明了所建立模型的有效性,并系统研究了不同中顶压裂液(纯压裂液脉冲段)黏度、支撑剂密度、泵注排量对通道压裂通道率的影响.结果表明,中顶压裂液的黏度从1 mPa·s增加到20 mPa·s,通道内平均通道率增加5%;支撑剂密度从1400 kg/m^(3)增加到2000 kg/m^(3)时,缝内通道率减小了7%;排量由3 m^(3)/min增加到7 m^(3)/min时,通道率由28%增加到33.5%后减小到31%.现场应用过程中应使用较高黏度中顶压裂液、较低密度支撑剂和合理的泵注排量以保证裂缝中形成有效支撑通道.

Starr架辅助复位结合通道螺钉技术治疗Tile C1.3型骨盆骨折的临床疗效观察

目的探讨Starr架辅助复位结合微创通道螺钉技术治疗Tile C1.3型骨盆骨折的疗效和安全性。方法回顾性分析川北医学院附属医院骨科2019年1月—2023年1月手术治疗的34例Tile C1.3型骨盆骨折患者资料,男性23例,女性11例;年龄21~68岁,平均42.6岁;道路交通伤19例,高处坠落伤7例,摔伤8例;合并四肢骨折11例,脊柱骨折12例,腹部损伤6例,腓总神经损伤2例。18例采用Starr架辅助复位结合通道螺钉技术为研究组,16例采用传统切开复位内固定患者为对照组。分析两组患者手术时间、术中出血量、术中透视次数及住院时间。通过CT测量比较患者术前术后骶骨垂直移位复位情况。采用Matta影像评分评价骨折复位质量。观察切口感染、血管神经损伤、下肢静脉血栓形成及异位骨化等并发症。比较患者骨折愈合时间,采用Majeed功能评分评价患者功能恢复情况。结果研究组患者手术时间(73.0±13.6)min,术中出血量(25.0±12.5)mL,平均住院时间12.1d,显著少于对照组[手术时间(138.3±47.6)min,术中出血量(226.5±102.4)mL,平均住院时间23.2d],差异有统计学意义(P均<0.001),但研究组术中透视平均31.2次,多于对照组(平均25.0次),差异有统计学意义(P=0.009)。通过CT测量数据分析两组患者术前、术后骨盆垂直移位距离[研究组,术前:(10.2±6.2)mm,术后:(4.7±3.1)mm;对照组,术前:(9.7±5.8)mm,术后:(5.1±3.4)mm],差异无统计学意义(P>0.05)。研究组和对照组患者复位质量优良率分别为88.9%、81.3%,差异无统计学意义(z=-1.116,P=0.265)。两组患者总并发症率分别为11.1%、25.0%,差异无统计学意义(χ^(2)=1.500,P=0.682)。两组患者均获得6~17个月随访,平均11.6个月。研究组患者骨折平均愈合时间16.3周,显著短于对照组22.3周,差异有统计学意义(P=0.005)。末次随访Majeed评分(85.3±6.4)分显著高于对照组(80.6±5.7)分,差异有统计学意义(P=0.032)。结论Starr架辅助复位结合经皮通道螺钉技术通过闭合复位微创固定方式治疗Tile C1.3型骨盆骨折,具有术中出血少、手术及住院时间短、骨折愈合时间明显缩短等优点,符合快速康复理念,临床疗效好。

致密油气窜波及控制体系适用性评价

我国致密油藏分布广泛,储量占比大,开发潜力巨大。此类油藏目前注气开发较多,基质渗透率、孔隙度均较低,油气流动通道主要为裂缝。但油藏裂缝跨尺度发育,受储层非均质性和黏性指进的影响,注气提高采收率技术受限,在裂缝及高渗条带存在时易形成气窜,对地层能量补充带来了诸多问题。选择泡沫和CO_(2)响应性凝胶配制气窜波及控制体系,并通过物理实验模拟评价其性能和在致密油藏的适用性;利用商业数值模拟软件表征CO_(2)注气吞吐及地下成胶反应。经过对国内某致密油藏的数值模拟研究,证明CO_(2)响应性凝胶对注气吞吐有明显的增产效果,采收率增加3.1%,验证了CO_(2)响应性凝胶体系在致密油藏开发中具有良好的应用前景。

浅埋深中厚煤层预掘回撤通道支护技术研究与应用

为进一步提高综采工作面末采回撤速度、确保设备回撤空间,采用理论计算、现场试验相结合的方法对浅埋深中厚煤层预掘回撤通道支护技术进行初步研究,建立了工作面残余煤柱承载力学模型,理论分析得知残余煤柱宽度对煤柱承载性能的时空演化规律具有重要影响,末采期间残余窄煤柱出现区域性脆性破坏失稳状态是导致超前采动应力、顶板岩层断裂线前移的主要原因。现场试验末采段残余煤柱宽度约5 m时,整条窄煤柱在多重载荷作用下发生屈服,进而呈现大面积失稳破碎状态,巷内支护体承载受力急剧增加,锚索最大受力约为225 kN,最大顶底板移近量出现在通道中部靠工作面侧,约为100 mm,结果表明,采用高预应力强力锚杆(索)联合支护结构配合垛式支架外部支撑,可有效控制末采期间预掘回撤通道顶底板之间的闭合位移,为类似条件下预掘回撤通道支护提供工程借鉴。

通道压裂中支撑剂团排列方式对裂缝导流能力的影响

高速通道压裂是以支撑剂团支撑裂缝并获得更高裂缝导流能力的新型压裂技术。为了探究支撑剂团排列方式对裂缝整体导流能力的影响,文中在考虑闭合压力作用下支撑剂团和裂缝壁面变形的基础上,建立了一种流体在高速通道和支撑剂团中耦合的流动模型,并对交错型和正对型支撑剂团排列方式的裂缝整体导流能力进行模拟,分析了支撑剂团半径和间距对裂缝整体导流能力的影响。结果显示:支撑剂团半径的减小和支撑剂团间距的增大均会导致裂缝支撑效果变差,正对型和交错型支撑剂团的半径均不宜小于0.100 m,支撑剂团间距不宜大于0.25 m;相同条件下,交错型支撑剂团填充的裂缝导流能力要比正对型支撑剂团填充的裂缝导流能力平均高53.53%。

滨里海盆地东缘北特鲁瓦油田石炭系碳酸盐岩储层裂缝网络连通性评价

基于岩心、成像测井及裂缝网络建模结果,应用拓扑学理论,对滨里海盆地东缘北特鲁瓦油田石炭系碳酸盐岩储层裂缝网络的连通性进行了评价,并分析了其对油田开发的影响。研究结果表明:(1)滨里海盆地东缘北特鲁瓦油田石炭系碳酸盐岩储层裂缝切割类型可划分为高高切割、高低切割和低低切割,研究区A2,A3,G1,G2,G3及G4小层裂缝切割较为发育,且主要分布在构造高部位。(2)裂缝网络连通性可通过连通体面积占比、裂缝体密度、裂缝的平均长度和平均节点数4个参数进行定量评价,根据连通体面积大小、裂缝体密度大小及裂缝类型可对裂缝连通体进行定性分析和命名,研究区A2小层主要发育低低大连通体、高中大连通体和高高大连通体。(3)裂缝网络连通性和储层物性的耦合作用是油田水窜的原因,裂缝网络连通性主要与裂缝长度和裂缝体密度有关。

单侧通道经皮椎体后凸成形术联合固本益髓灸法治疗老年中上段胸椎体破裂骨折患者的疗效

目的:探讨老年中上段胸椎体破裂骨折患者接受单侧通道经皮椎体后凸成形术联合固本益髓灸法的疗效及术后血栓风险的影响。方法:选取2020年1月—2021年12月我院收治的老年中上段胸椎骨折伴锥体后壁破裂患者100例,随机分为对照组和研究组,各50例。对照组接受单侧通道经皮椎体后凸成形术治疗,研究组在此基础上联用固本益髓灸法干预。两组术后均接受骨折护理。比较两组患者的疼痛评分、功能障碍指数,并观察两组患者并发症的发生率,观察并比较患者治疗前后凝血四项指标、D-二聚体、血液流变学指标以及深静脉血栓(DVT)发生情况。结果:治疗18个月后,两组患者的VAS评分和椎体高度均明显低于治疗前(P<0.05)。研究组总并发症发生率明显低于对照组(2%vs 14%,χ^(2)=4.891,P<0.05)。研究组治疗后凝血活酶时间(APTT)、凝血酶原时间(PT)、凝血酶时间(TT)、纤维蛋白原(FIB)水平明显优于对照组患者(P<0.05)。研究组治疗后D-二聚体、全血低切黏度、全血高切黏度及血浆黏度水平明显优于对照组患者(P<0.05)。研究组患者治疗7天、治疗14天血栓DVT发生率均明显低于对照组患者(P<0.05)。治疗后,研究组患者的ODI评分显著低于对照组,改良日本骨科学会评分(M-JOA)评分高于对照组(P<0.05)。结论:行单侧通道经皮椎体后凸成形术联合固本益髓灸法治疗中上段胸椎骨折伴锥体后壁破裂患者能达到较好的临床疗效,且术后血栓的发生率也较低。

干湿循环作用下渠道膨胀土裂隙演化规律及强度特性

为探究干湿循环作用下膨胀土渠道边坡裂隙演化特征与强度衰减规律,对北疆某渠道膨胀土开展了不同压实度的干湿循环试验,利用图像处理技术提取膨胀土表面裂隙参数,利用直剪试验测得膨胀土抗剪强度,并分析黏聚力随裂隙率的变化规律。结果表明:在干湿循环作用下,黏聚力的衰减速率随着压实度的增大而增大,内摩擦角总体呈减小趋势,裂隙率的增加速率随着压实度的增加而减小;裂隙相对面积呈先增后减的变化规律。最后,拟合获得不同压实度膨胀土黏聚力与表面裂隙率之间的关系式,通过确定膨胀土表层裂隙率可计算出其黏聚力。研究成果为进一步揭示剧烈干湿变化诱发的膨胀土渠道边坡失稳破坏提供了重要依据。

煤矿充水因素评价及涌水量预测

为真实掌握煤矿的涌水量和充水条件,在对煤矿水文地质分析的基础上,首先对充水条件和充水因素进行评价,得出煤矿的充水通道主要为导水裂隙带,基于层次分析得出通道条件的权重因子为0.2667最大,为该煤矿的充水的首要条件;在对煤矿历年涌水量观测结果分析的基础上,采用大井法对涌水量进行预测,综合分析最终得出该煤矿正常涌水量为462 m^(3)/h,最大涌水量为693 m^(3)/h。

浅埋近煤层群采动“三区”漏风裂隙场时空演化规律数值模拟

浅埋近煤层群采动下,地面采空区工作面“三区”之间易形成连通的漏气通道引发工作面低氧和采空区煤自燃,如何厘清采动效应下浅埋煤层群“三区”漏气通道的时空分布演化规律是有效防控煤自燃和低氧的关键。以陕煤柠条塔煤矿S1232工作面为研究背景,采用理论与数值模拟相结合的方法,研究了单一煤层、复合煤层重复开采下覆岩破坏特征,分析了煤层间距与采厚对裂隙二次发育的影响规律。结果表明:浅埋近煤层群开采下上覆岩层裂隙仅存在垮落带与裂缝带,主关键层破断后裂缝带快速发育至地表,关键层周期破断控制地表裂隙周期生成与下沉;上煤层裂隙密度发育由原始阶段先后经历快速增长、稳定和二次增长3个阶段;随采厚增大,各区间裂隙密度增大,大裂隙发育更充分,裂隙宽度整体增大;随煤层间距增大,小于0.2 m的裂隙宽度随采厚增大,且增加的速度逐渐加快,大于0.2 m的裂隙宽度随采厚增大,但增加速度逐渐变缓;采厚相同时,裂隙宽度大于0.2 m的裂隙密度随着煤层间距的增大逐渐减小。

聚合物基透明导电薄膜断裂韧性的测量方法

在航空透明件领域,厚聚合物基底上透明导电薄膜的断裂韧性是量化抗开裂性的关键力学特性,因此基于纳米压痕和通道开裂试验评估了薄膜的断裂韧性。采用磁控溅射法将氧化铟锡(ITO)薄膜沉积在聚碳酸酯(PC)基底上。纳米压痕试验中,纳米压痕在ITO薄膜上引起脆性断裂,通过对所得的载荷-深度曲线进行积分来计算断裂韧性;通道开裂试验中,使用原位拉伸法获得开裂应变,并结合力学模型计算断裂韧性。结果表明,不考虑残余应力时,2种测试方法获得了相对一致的断裂韧性,500 nm厚ITO薄膜的断裂韧性为1.62~1.81 MPa·m^(1/2);对于200 nm以下厚度的薄膜或存在大残余应力的薄膜,宜选择通道开裂试验以确定断裂韧性。