Performance Analysis of WLAN Medium Access Control Protocols in Simulcast Radio-Over-Fiber-Based Distributed Antenna Systems

The performance of three wireless local-area network(WLAN) media access control(MAC) protocols is investigated and compared in the context of simulcast radioover-fiber-based distributed antenna systems(RoF-DASs) where multiple remote antenna units(RAUs) are connected to one access point(AP) with different-length fiber links.The three WLAN MAC protocols under investigation are distributed coordination function(DCF) in basic access mode,DCF in request/clear to send(RTS/CTS) exchange mode,and point coordination function(PCF).In the analysis,the inter-RAU hidden nodes problems and fiber-length difference effect are both taken into account.Results show that adaptive PCF mechanism has better throughput performances than the other two DCF modes,especially when the inserted fiber length is short.

Distributed multichannel medium access control protocol without common control channel for single-hop cognitive radio networks

A novel distributed cognitive radio multichannel medium access protocol without common control channel was proposed.The protocol divided a transmission interval into two parts for exchanging control information and data,respectively.In addition to evaluating system saturation throughput of the proposed protocol,a three-dimensional multi channel Markov chain model to describe the sate of the cognitive users (CUs) in dynamic spectrum access was presented.The proposed analysis was applied to the packet transmission schemes employed by the basic,RTS/CTS access mechanism adopted in the normal IEEE 802.11.Analyzing the advantage of the two methods,a hybrid access mechanism was proposed to improve the system throughput.The simulation results show that the experiment results are close to the value computed by the model (less than 5%),and the proposed protocol significantly improves the performance of the system throughput by borrowing the licensed spectrum.By analyzing the dependence of throughput on system parameters,hybrid mechanism dynamically selecting access mechanism can maintain high throughput.

碘工质射频离子推力器放电特性仿真研究

碘作为空间推进器中极具发展潜力的工质,近些年备受关注。与氙气不同,碘工质放电的产物较为复杂,主要是I^(+),同时还包含了少量的I^(-)、I_(2)^(+)、I_(2)^(+)和I_(3)^(+)。虽然多价离子的含量较少,但对推力器的工作性能存在着不小的影响。文章基于等离子体放电建立二维轴对称模型,分别研究了质量流量、射频功率以及背景压力对两种主要电离产物I^(+)、I_(2)^(+)的影响。质量流量为20~70 mL/min,随着质量流量的增加,I^(+)略微降低,I_(2)^(+)略微增加。射频功率为100~650 W,随着射频功率的增加,I^(+)迅速增加,I_(2)^(+)略微减小。背景压力在0.001~0.1 Torr时,随着背景压力的增加,I^(+)先是快速增加,在p0=0.015 Torr左右,开始减小;I_(2)^(+)一直维持缓慢增加。在忽略电离损耗的前提下,对三个影响因素进行全因子实验设计分析(DOE),得出质量流量在60 mL/min,射频功率在150 W,背压在0.015 Torr时,得出最优产物中I_(2)^(+):I^(+)=0.204094,与仿真结果 I_(2)^(+):I^(+)=0.205069,误差为0.48%。

中波发射台射频干扰成因及对策分析

中波发射台作为广播信号传输的重要设施,其射频干扰问题一直备受关注。射频干扰不仅影响信号传输质量,降低接收效果,严重时甚至可能导致信号中断。因此,探究中波发射台射频干扰的成因并提出有效的对策,对于保障信号的稳定传输具有重要意义。基于此,以中波发射台射频干扰为重点,深入探讨中波发射台射频干扰的原因和改善中波发射台射频干扰的对策。

北广ZF-10A中波发射机射频部分的工作原理与故障检修要点

北广ZF-10A中波发射机是典型的数字振幅调制(Digital Amplitude Modulation,DAM)中波发射机,采用了全固态装配技术和成熟的数字化控制电路,具有工作性能稳定、工作效率高及保护功能完善等优点。国内大中功率中波发射台基本都在使用此类型的发射机,虽然厂家不同、功率等级不同,但整机电路组成基本相同。基于此,分析此机型射频部分的工作原理和故障检修要点。

DRM技术在中波广播中的应用研究

以数字音频广播(Digital Radio Mondiale,DRM)技术在中波广播中的应用为研究对象,首先介绍DRM技术的发展、关键组成部分、产业现状及其在国内外的应用情况,其次重点分析DRM技术在中波广播中的应用优势和应用实例,最后阐述DRM技术在偏远地区的应用,以期为DRM技术与中波广播的融合提供参考。

Intensity distribution function and statistical properties of fast radio bursts

Fast Radio Bursts （FRBs） are intense radio flashes from the sky that are characterized by mil- lisecond durations and Jansky-level flux densities. We carried out a statistical analysis on FRBs that have been discovered. Their mean dispersion measure, after subtracting the contribution from the interstellar medium of our Galaxy, is found to be ~ 660 pc cm-3, supporting their being from a cosmological origin. Their energy released in the radio band spans about two orders of magnitude, with a mean value of ~ 10-39 erg. More interestingly, although the study of FRBs is still in a very early phase, the published collection of FRBs enables us to derive a useful intensity distribution function. For the 16 non-repeating FRBs detected by the Parkes telescope and the Green Bank Telescope, the intensity distribution can be described as dN/dFobs = （4.1± 1.3） × 103 F-obs1.1±0.2 sky-1 d-l, where Fobs is the observed radio obs fluence in units of Jy ms. Here the power-law index is significantly flatter than the expected value of 2.5 for standard candles distributed homogeneously in a flat Euclidean space. Based on this intensity distribution function, the Five-hundred-meter Aperture Spherical radio Telescope （FAST） is predicted to be able to detect about five FRBs for every 1000 h of observation time.

Combined limit on the photon mass with nine localized fast radio bursts

A nonzero-mass hypothesis for the photon can produce a frequency-dependent dispersion of light, which results in arrival-time differences of photons with different frequencies originating from a given transient source. Extragalactic fast radio bursts(FRBs), with their low frequency emissions, short time durations, and long propagation distances, are excellent astrophysical probes to constrain the rest mass of the photon mγ. However, the derivation of a limit on mγis complicated by the similar frequency dependences of dispersion expected from the plasma and nonzero photon mass effects. If a handful measurements of redshift for FRBs are available, then the different redshift dependences of the plasma and photon mass contributions to the dispersion measure(DM) might be able to break dispersion degeneracy in testing the photon mass. For now, nine FRBs with redshift measurements have been reported, which can turn this idea into reality. Taking into account the DM contributions from both the plasma and a possible photon mass,we use the data on the nine FRBs to derive a combined limit of mγ≤ 7.1 × 10^-51 kg, or equivalently mγ≤ 4.0 × 10^-15 e V/c2 at 68% confidence level, which is essentially as good as or represents a factor of 7 improvement over previous limits obtained by the single FRBs. Additionally, a reasonable estimation for the DM contribution from the host galaxy, DMhost, can be simultaneously achieved in our analysis. The rapid progress in localizing FRBs will further tighten the constraints on both mγ and DMhost.

Alternative kind of hydrogen atoms as a possible explanation for the latest puzzling observation of the 21 cm radio line from the early Universe

There is a puzzling astrophysical result concerning the latest observation of the absorption profile of the redshifted radio line 21 cm from the early Universe(as described in Bowman et al.). The amplitude of the profile was more than a factor of two greater than the largest predictions. This could mean that the primordial hydrogen gas was much cooler than expected. Some explanations in the literature suggested a possible cooling of baryons either by unspecified dark matter particles or by some exotic dark matter particles with a charge a million times smaller than the electron charge. Other explanations required an additional radio background. In the present paper, we entertain a possible different explanation for the above puzzling observational result: the explanation is based on the alternative kind of hydrogen atoms(AKHA),whose existence was previously demonstrated theoretically, as well as by the analysis of atomic experiments. Namely, the AKHA are expected to decouple from the cosmic microwave background(CMB) much earlier(in the course of the Universe expansion) than usual hydrogen atoms, so that the AKHA temperature is significantly lower than that of usual hydrogen atoms. This seems to lower the excitation(spin) temperature of the hyperfine doublet(responsible for the 21 cm line) sufficiently enough for explaining the above puzzling observational result. This possible explanation appears to be more specific and natural than the previous possible explanations. Further observational studies of the redshifted 21 cm radio line from the early Universe could help to verify which explanation is the most relevant.

中低速磁浮系统电磁环境分析

随着我国中低速磁浮系统的线路逐渐增多,其对电磁环境的影响也备受大众关注。该文系统地通过对中低速磁浮系统的电磁环境实测,系统分析中低速磁浮列车内、外电磁环境的时域、频域特性,牵引变电所的电磁环境特性,并将实测数据与相关国内外电磁环境标准对比,在中低速磁浮车内、站台、区间和变电所等处,各电磁环境参量(包括直流磁场、低频磁场、无线电干扰场强)均符合相关标准限值要求。中低速磁浮系统的电磁环境较好,对其内部及周围的设备和人员产生电磁影响概率较低,是一种值得大力推广的绿色城市轨道交通运输方式。

无线电中波广播发射技术与维护

广电建设事业的快速发展,使得无线广播技术应用水平得到切实提升,为实现理想化无线电中波广播应用模式,业界加大对中波广播发射技术以及维护方式的研究力度。该文将通过对广播发射台基本情况的介绍,对无线电中波广播发射技术及其原理进行深度性探讨,并会就其维护方式展开研究,旨在提升无线电中波广播发射质量,实现理想化中波广播维护模式。

中波广播发射台供配电系统智能化升级改造方案设计

针对通用型中波广播发射台的组织架构合理性、供配电系统实用性进行分析研究,探讨智能化升级改造的必要性,针对自动化程度低、人机结合效能差以及有人值守效率低等问题,从关键设备、处理算法方面设计一套中波广播发射台供配电系统智能化升级改造方案,通过硬件设备升级、智能化监测系统运用等方式有效解决难点问题,满足中波广播发射台电视节目安全播出的实际需求,提高系统整体工作效率,进一步促进广播台站的智能化发展。

食品射频灭菌过程加热均匀性调控研究进展

作为一种新型物理灭菌技术,射频(radio frequency,RF)加热具有穿透深度大、传热效率高、无化学残留、适合批量处理等特点,可有效杀灭食品中的致病菌。然而,受物料本身特性参数、射频系统结构、物料周围介质等因素影响,非均匀加热一直是射频灭菌规模化应用面临的主要挑战之一。为进一步明确食品射频灭菌过程均匀性调控的最新研究进展,本文基于射频杀菌的机制及特点,对调控射频加热均匀性的具体措施进行了梳理,包括调控物料介电参数、改变环境介质、优化射频加热系统等,为加热均匀性调控的进一步研究及射频灭菌技术的进一步应用奠定基础。

中波广播发射台防雷与接地措施研究

中波发射台多建设在较为宽广的空旷地带,是电视广播发送信号的关键环节。中波发射台在运行过程中,受外部环境因素的影响存在一定的安全隐患,其中雷电问题是影响中波发射台正常运行的因素之一,目前多数中波发射台都采取了防雷技术,但是在实际应用的过程中仍然存在不足,因此需要进一步加强防雷技术的应用,做好防雷保护措施。

中波广播发射天线的原理及其维护研究

随着我国无线广播事业的发展,中波广播发射技术的应用也日趋广泛。中波广播发射天线是用来发射无线电波的一种装置,天线信号容易受到各类因素干扰,因此对中波广播发射天线的维护具有重要价值。在明确中波广播发射天线原理的基础上,阐述其架设高度与地网设计,并提出了中波广播发射天线的维护措施,以期为中波广播发射天线技术的运用提供帮助,减少由于故障干扰引起的信号传输不良等问题。

广播中波发射台防雷设施改造实例

针对贵州省广播电视局五〇一台防雷防护措施存在的不足,提出整改方案并进行改造。改造完成后,各项防雷指标均顺利通过专业机构检测。防雷设施能够更好地保障广播中波发射台站所有广播发射机、卫星接收机、天馈系统和供电系统等设备的稳定运行。

中波广播发射塔台双塔辐射场强测算与影响分析

为了科学测算中波广播电视发射塔台双塔辐射场强,并进一步解析塔台样式、天线类型、系统参数以及发射模式等对电磁辐射的影响,提出一种基于场强理论计算模型的测算方法。该方法利用覆盖区实测数据,结合理论计算模型估计场强衰减因子,从而精准测算覆盖区的场强分布,并分析辐射强度对环境的影响,能够为中波广播电视发射塔台的运行管理提供重要参考。

监测系统中波广播接收效果改善创新实践

由于传输和接收过程中的无用信号及外部噪声等因素干扰,中波广播接收一直存在障碍,影响接收效果。为有效改善中波广播监测接收效果,实现中波广播的安全播出及对特定频率的有效监测,山东省广播电视监测中心在对全省中波广播监测站点按照缩短接收距离原则重新选择的基础上,采用“初级线圈+次级线圈”的无源调谐技术模式自主研制天线,明显提升了输出信噪比,大幅改善了接收效果。

中波广播发射机故障分析与处理方法

中波广播发射机的运行情况,直接影响中波广播无线覆盖网的覆盖。尽管当前的中波广播发射机组件及整体的运行性能得到了提升,但在实际运行中,发射机仍然会出现一些故障,影响广播节目传输的效果。基于此,在介绍中波广播发射机的基础上,就当前主要使用的故障分析方法以及典型故障的处理方法进行分析研究,为中波广播发射机故障的科学分析与处理提供参考。

中波传输技术在广播电视业务中的应用与探讨

中波传输技术由于其稳定性和高传输效率,已成为我国广播电视技术发展的一个重要组成部分.中波技术能够通过地波进行通信,具有很好的抗干扰性,能够为广大地区的用户提供高品质的广播电视节目,而中波技术则是目前国内无线网络覆盖的主要技术力量.根据多年的工作经验,对中波传输技术进行了较为详尽的分析,并对其在广播、电视服务方面的应用进行了讨论.