📚 Comparison of Different Channels | 不同信道的比较
In modern communication systems, the term 'channel' refers to the physical medium or frequency band used to transmit information from a transmitter to a receiver. Different channels exhibit distinct characteristics in terms of bandwidth, attenuation, interference, and suitability for various applications. Understanding these differences is essential for selecting the most appropriate channel for a given communication requirement.
在现代通信系统中，“信道”一词指用于将信息从发射机传输到接收机的物理介质或频带。不同信道在带宽、衰减、干扰以及适用性方面展现出不同的特性。理解这些差异对于为特定的通信需求选择最合适的信道至关重要。
1. Introduction to Communication Channels | 通信信道简介
A communication channel can be a guided medium such as copper wires and optical fibres, or an unguided medium such as free space for radio waves. Guided channels confine the signal physically, whereas unguided channels radiate electromagnetic energy into the environment. The performance of a channel is often evaluated by its available bandwidth, signal-to-noise ratio, attenuation per unit length, and susceptibility to external interference.
通信信道可以是有导介质，如铜线和光纤，也可以是无导介质，如用于无线电波传播的自由空间。有导信道会对信号进行物理约束，而无导信道则将电磁能量辐射到环境中。评估信道性能的指标通常包括可用带宽、信噪比、单位长度衰减以及对外部干扰的敏感程度。
2. Twisted-Pair Cable Channels | 双绞线信道
Twisted-pair cables consist of two insulated copper conductors twisted together. The twisting reduces electromagnetic interference by ensuring that induced voltages in adjacent loops cancel each other out. However, the bandwidth is relatively low, typically up to a few megahertz, which limits data rates. Attenuation is significant over long distances, and crosstalk between pairs can degrade the signal.
双绞线由两根互相缠绕的绝缘铜导体组成。这种缠绕方式通过使相邻环路中的感应电压相互抵消来减少电磁干扰。然而，其带宽相对较低，通常仅达几兆赫兹，这限制了数据传输速率。长距离传输时衰减显著，而且线对之间的串扰也会使信号质量下降。
Twisted pairs are widely used in telephone subscriber lines and Ethernet LAN cables (e.g., Cat5e, Cat6). They are inexpensive and easy to install, making them suitable for short-range, low-cost communication. Compared with coaxial cable or optical fibre, twisted-pair channels suffer from higher attenuation and greater vulnerability to noise, but they remain popular due to their low material cost and flexibility.
双绞线广泛用于电话用户线和以太网局域网电缆（如 Cat5e、Cat6）。它们价格低廉且易于安装，因此适合短距离、低成本的通信。与同轴电缆或光纤相比，双绞线信道的衰减更高且更易受噪声影响，但因其材料成本低和灵活性好，依然得到普遍应用。
3. Coaxial Cable Channels | 同轴电缆信道
A coaxial cable features a central copper conductor surrounded by an insulating layer, a metallic braided shield, and an outer jacket. The shield confines the electromagnetic field to the dielectric, effectively reducing radiation loss and external interference. Coaxial cables offer higher bandwidth than twisted pairs, typically up to hundreds of megahertz, and lower attenuation coefficients, making them suitable for cable television and broadband internet services.
同轴电缆具有一个中心铜导体，周围依次包裹绝缘层、金属编织屏蔽层和外护套。屏蔽层将电磁场限制在电介质内，有效地降低了辐射损耗和外部干扰。同轴电缆提供的带宽高于双绞线，通常可达数百兆赫兹，衰减系数也更低，因此适用于有线电视和宽带互联网业务。
Despite these advantages, coaxial cables are bulkier and more expensive than twisted pairs. In comparison with optical fibre, their bandwidth is still limited, and they are prone to signal leakage if the shield is damaged. Coaxial channels are often used in baseband and broadband networks where moderate bandwidth and good noise immunity are required.
尽管有这些优势，同轴电缆比双绞线更粗重，价格也更高。与光纤相比，其带宽仍然有限，且若屏蔽层损坏，容易发生信号泄露。同轴信道常用于需要中等带宽和良好抗噪能力的基带与宽带网络。
4. Optical Fibre Channels | 光纤信道
Optical fibre channels use glass or plastic fibres to transmit signals in the form of light. Total internal reflection keeps the light within the core, resulting in extremely low attenuation — as little as 0.2 dB/km for single-mode fibres. The bandwidth of optical fibres is immense, extending into the terahertz range, which enables data rates of terabits per second over long distances. Moreover, optical fibres are immune to electromagnetic interference and crosstalk, providing secure and reliable communication.
光纤信道利用玻璃或塑料纤维以光的形式传输信号。全内反射将光限制在纤芯内，从而实现了极低的衰减——单模光纤低至 0.2 dB/km。光纤的带宽非常巨大，可达太赫兹范围，这使得它能以每秒太比特的速率进行长距离传输。此外，光纤不受电磁干扰和串扰的影响，可提供安全可靠的通信。
However, optical fibre installation and splicing require precision equipment and skilled technicians, leading to higher initial costs than copper-based systems. Fibres are also more fragile and require protective cabling. In the comparison of channels, optical fibre excels in long-haul, high-capacity links, such as intercontinental submarine cables and modern fibre-to-the-home (FTTH) networks, whereas metallic channels still dominate short-distance, cost-sensitive applications.
不过，光纤的安装和接续需要精密设备和熟练技术人员，导致前期成本高于铜缆系统。光纤本身也更脆弱，需要保护套管。在信道的比较中，光纤在长途、大容量链路中优势突出，如洲际海底光缆和现代光纤到户（FTTH）网络；而金属信道在短距离、成本敏感的应用中仍占主导地位。
5. Radio Wave Channels: Ground Wave Propagation | 无线电波信道：地波传播
Ground wave propagation occurs at frequencies below about 2 MHz, where vertically polarised waves travel along the Earth's surface. The Earth acts as a guiding medium, and the wave follows the curvature of the planet due to diffraction. This channel is relatively stable and can provide coverage over hundreds of kilometres. However, the attenuation increases with frequency, so it is mainly utilised in AM radio broadcasting and maritime communication.
地波传播发生在约 2 MHz 以下的频率，垂直极化波沿地球表面传播。大地起到了导向作用，波因衍射而沿着地球的曲面前进。该信道相对稳定，能够覆盖数百公里。然而，衰减随频率升高而增加，因此它主要用于调幅（AM）广播和海上通信。
In terms of channel comparison, ground wave propagation does not require any artificial infrastructure between transmitter and receiver. Its main drawbacks are low bandwidth and sensitivity to ground conductivity variations. Compared with sky wave or satellite channels, ground waves are less affected by atmospheric conditions but cannot achieve global coverage on their own.
在信道比较中，地波传播不需要在发射机和接收机之间架设任何人造设施。其主要缺点是带宽低且对地面电导率变化敏感。与天波或卫星信道相比，地波受大气条件的影响较小，但无法单独实现全球覆盖。
6. Radio Wave Channels: Sky Wave Propagation | 无线电波信道：天波传播
Sky wave propagation uses the refraction and reflection of radio waves by ionised layers in the upper atmosphere (the ionosphere). Frequencies typically range from 3 MHz to 30 MHz (HF band). This channel enables long-distance communication well beyond the horizon, often spanning continents, because the waves can bounce between the ionosphere and the ground. It is widely used in international broadcasting, amateur radio, and military communication.
天波传播利用高层大气（电离层）的折射和反射。频率范围通常为 3 MHz 至 30 MHz（高频波段）。由于电波可以在电离层和地面之间多次反弹，这种信道能够实现远超视距的长距离通信，经常跨越大陆。它被广泛应用于国际广播、业余无线电和军事通信。
The performance of sky wave channels varies with time of day, solar activity, and season because the electron density in the ionosphere changes. Critical frequency and maximum usable frequency (MUF) must be considered. Compared with ground wave channels, sky waves offer much longer range but suffer from fading, interference, and unpredictable propagation conditions. In modern communication, satellite channels have largely supplemented sky wave links for reliable long-range coverage.
天波信道的性能随一天中的时间、太阳活动以及季节而变化，因为电离层中的电子密度会改变。必须考虑临界频率和最高可用频率（MUF）。与地波信道相比，天波能提供更远的传输距离，但存在衰落、干扰和不可预测的传播条件。在现代通信中，卫星信道已在很大程度上补充了天波链路，以提供可靠的远距离覆盖。
7. Radio Wave Channels: Space Wave Propagation | 无线电波信道：空间波传播
Space wave propagation occurs when radio waves travel in a straight line between the transmitting and receiving antennas, including a direct wave and a ground-reflected wave. It is dominant at VHF (30–300 MHz), UHF (300 MHz–3 GHz), and higher frequencies. Because the wave does not follow the Earth's curvature, the communication range is limited to the line-of-sight, which depends on antenna heights.
空间波传播是指无线电波在发射天线和接收天线之间沿直线传播，包括直射波和地面反射波。它以甚高频（30–300 MHz）、特高频（300 MHz–3 GHz）及更高频率为主。由于电波不沿地球曲面前进，通信距离被限制在视距范围内，该距离取决于天线高度。
Space wave channels provide much wider bandwidth than ground or sky wave propagation, making them ideal for FM radio, television broadcasting, and mobile cellular systems. The main challenge is obstruction by buildings or terrain, which causes shadowing and multipath fading. In channel comparisons, space wave links are typically short-range but offer high data rates, and they can be extended by installing repeaters or elevated antennas.
空间波信道比地波或天波传播提供了更宽的带宽，这使其成为调频广播、电视广播和移动蜂窝系统的理想选择。主要挑战在于建筑物或地形造成的遮挡，这会引起阴影效应和多径衰落。在信道对比中，空间波链路通常距离较短，但能提供高数据速率，并且可以通过增设中继器或升高天线来延长距离。
8. Microwave Line-of-Sight Channels | 微波视距信道
Microwave channels operate at frequencies from about 1 GHz to 30 GHz, using highly directional antennas to establish point-to-point links. Parabolic dishes focus the beam, allowing long-distance line-of-sight transmission if towers are spaced every 50 km or so to overcome the Earth's curvature. These channels carry large volumes of telephone and data traffic, and they serve as backhaul links in cellular networks.
微波信道工作频率约为 1 GHz 至 30 GHz，利用高定向天线建立点对点链路。抛物面天线会聚波束，只要每隔大约 50 km 设置一个塔以克服地球曲率，就可以实现长距离视距传输。这些信道能承载大量电话和数据业务，并充当蜂窝网络中的回传链路。
Compared with satellite channels, microwave links have lower latency since the signal travels a shorter path. However, they are susceptible to rain attenuation, especially at frequencies above 10 GHz, where raindrops absorb and scatter the waves. In terms of installation cost, microwave towers are expensive to build and maintain, but they do not require digging trenches like fibre optic cables. Microwave channels complement optical fibre networks in areas where laying cables is impractical.
与卫星信道相比，微波链路延迟更低，因为信号传播路径更短。然而，它们容易受到降雨衰减的影响，特别是在 10 GHz 以上的频率，雨滴会吸收和散射电波。在安装成本方面，微波塔的建造和维护费用昂贵，但它们不需要像光纤那样开挖沟槽。在不便铺设电缆的区域，微波信道可作为光纤网络的补充。
9. Satellite Communication Channels | 卫星通信信道
Satellite channels rely on a spacecraft in orbit that receives signals from an Earth station on the uplink frequency, amplifies them, and retransmits on a different downlink frequency. Geostationary satellites at an altitude of approximately 35,800 km appear stationary relative to the Earth and can cover roughly one-third of the planet. This channel enables broadcasting, global telephony, and broadband internet to remote areas.
卫星信道依赖轨道上的航天器，它以上行频率接收来自地球站的信号，将其放大，并以下行频率转发。位于约 35,800 km 高度的地球静止卫星相对于地球看似静止不动，能够覆盖大约地球表面的三分之一。这种信道可将广播、全球电话和宽带互联网送达偏远地区。
The main disadvantage of a geostationary satellite channel is the large propagation delay of about 240 ms for a round trip, which can degrade real-time interactive applications such as voice calls and gaming. Signal power is spread over a vast area, requiring high-gain antennas and careful link budget calculations. Compared with optical fibre, satellite channels offer universal coverage but limited bandwidth per beam. Low Earth orbit (LEO) satellite constellations are emerging to reduce latency, yet they add complexity in tracking and handover.
地球静止卫星信道的主要缺点是较大的传播延迟，往返约 240 ms，这会使语音通话和游戏等实时交互应用的质量变差。信号功率散布在广袤的区域，需要使用高增益天线并仔细进行链路预算计算。与光纤相比，卫星信道能提供无所不在的覆盖，但每束带宽有限。正在兴起的低地球轨道（LEO）卫星星座旨在降低延迟，但它们增加了跟踪和切换的复杂性。
10. Comparison of Key Characteristics | 关键特性比较
The following table summarises the essential differences among the most common communication channels. The values are typical and may vary with specific implementations.
下表总结了最常见通信信道之间的基本差异。所列数值为典型值，可能因具体实现方式而异。
| Channel | Bandwidth | Attenuation | Interference Immunity | Typical Range | Cost / Complexity |
|---|---|---|---|---|---|
| Twisted-pair | Up to ~100 MHz (Cat6) | High (20 dB/100 m at 100 MHz) | Low | ~100 m | Low |
| Coaxial | Up to ~1 GHz | Moderate (6 dB/100 m at 100 MHz) | Medium | ~500 m (broadband) | Medium |
| Optical fibre | THz range | Very low (~0.2 dB/km) | Excellent (immune to EMI) | 100 km+ (without repeater) | High (installation) |
| Ground wave (MF) | ~10 kHz (AM radio) | Increases with frequency | Medium (atmospheric noise) | Hundreds of km | Low |
| Sky wave (HF) | ~3–30 MHz (shared) | Variable (fading) | Low (interference-prone) | Thousands of km | Low |
| Space wave (VHF/UHF) | Up to tens of MHz | Moderate (free space loss) | Medium | Tens of km (LOS) | Medium |
| Microwave (1–30 GHz) | Hundreds of MHz | Rain fade, path loss | Medium (directional) | ~50 km per hop | Medium–High |
| Satellite (C/ Ku/ Ka) | 500 MHz–several GHz | High path loss (~200 dB) | High (directional) | Global (geostationary) | Very high |
11. Analog vs. Digital Channels | 模拟信道与数字信道
Another dimension of channel comparison lies in whether the transmitted signal is analog or digital. An analog channel conveys continuously varying signals, such as voice or video in their original form. Digital channels transmit bits, usually encoded into pulses. Digital transmission offers better immunity to noise because regenerators can reconstruct the original bit stream even from a degraded signal, removing accumulated noise. Analog signals, on the other hand, accumulate noise over distance, and amplification also amplifies the noise.
信道比较的另一个维度在于传输信号是模拟信号还是数字信号。模拟信道传输连续变化的信号，比如原始的语音或视频。数字信道传输比特，通常编码为脉冲。数字传输具有更好的抗噪能力，因为再生器甚至可以从劣化的信号中重建原始比特流，从而消除累积的噪声。而模拟信号在传输距离上会累积噪声，且放大过程也会同时放大噪声。
Digital channels also allow multiplexing techniques such as time-division multiplexing (TDM) and enable encryption for security. However, they require higher bandwidth than the corresponding analog signals unless source compression is applied. In modern hybrid networks, optical fibre and satellite channels predominantly carry digital data, while analog channels remain in legacy AM/FM radio and certain CCTV systems. The trend is clearly towards all-digital communication due to its flexibility, error correction capabilities, and compatibility with data processing.
数字信道还允许采用时分复用（TDM）等多路复用技术，并能进行加密以保证安全。然而，除非采用信源压缩，否则它们需要比相应模拟信号更大的带宽。在现代混合网络中，光纤和卫星信道主要承载数字数据，而模拟信道仍存在于传统的 AM/FM 广播和某些闭路电视系统中。由于数字通信的灵活性、纠错能力以及与数据处理的兼容性，其发展趋势显然是完全数字化。
12. Conclusion | 结论
No single channel is universally superior. The choice depends heavily on the required range, bandwidth, reliability, and cost. Guided channels like twisted-pair and coaxial cables are economical for short distances and moderate bandwidths. Optical fibre provides unmatched bandwidth and low attenuation over long hauls. Unguided channels — ground wave, sky wave, space wave, microwave, and satellite — offer varying degrees of mobility and coverage, from local radio to global satellite links. In practice, modern communication networks integrate multiple channels, using optical fibre for backbone infrastructure, microwave for backhaul, and wireless radio channels for last-mile access, each optimised for its role.
没有哪一种信道是放之四海而皆优的。选择很大程度上取决于所需的通信范围、带宽、可靠性和成本。双绞线和同轴电缆等有导信道在短距离和中等带宽下具有经济性。光纤在长途传输中提供了无与伦比的带宽和低衰减。无导信道——地波、天波、空间波、微波和卫星——则提供了从本地无线电到全球卫星链路的不同移动性和覆盖能力。在实践中，现代通信网络融合了多种信道，光纤作为骨干基础设施，微波用于回传，无线电信道用于最后一英里接入，各自在其角色中实现优化。
Published by TutorHao | Physics Revision Series | aleveler.com
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