Communications Systems | 通信系统

📚 Communications Systems | 通信系统

Communication systems transfer information from a transmitter to a receiver through a channel, usually using electromagnetic waves. This topic brings together ideas about waves, fields, circuits and digital electronics, and it underpins technologies such as radio, television, mobile phones, optical fibres and satellite links.

通信系统通过信道把信息从发射端传送到接收端，通常借助电磁波。本专题综合了波、场、电路和数字电子学等多个领域的概念，是广播、电视、移动电话、光纤和卫星链路等技术的基础。

1. The Basic Communication System | 通信系统基本模型

A general communication system consists of a source, a transmitter, a channel, a receiver and a destination. The transmitter encodes or modulates the information onto a carrier wave, and the receiver demodulates the signal to recover the original message.

一个通用通信系统由信源、发射机、信道、接收机和信宿组成。发射机把信息编码或调制到载波上，接收机对信号进行解调，恢复原始信息。

Noise can enter the system at any stage, especially in the channel. The design goal is usually to maximise the received signal power relative to the noise power while using an acceptable bandwidth and cost.

噪声可能从任何环节进入系统，尤其是在信道中。设计目标通常是在可接受的带宽和成本下，尽可能提高接收信号功率与噪声功率之比。

2. Why Modulation Is Needed | 为什么需要调制

Audio-frequency signals have low frequencies and cannot be radiated efficiently by an antenna of reasonable size. For efficient radiation the antenna length should be comparable with the wavelength, so baseband audio waves would require impractically long antennas.

音频信号频率较低，用合理尺寸的天线无法有效辐射。为了高效辐射，天线长度应与波长相当，因此基带音频波会要求长得不切实际的天线。

Modulation also allows many different programmes to be transmitted simultaneously without mutual interference, because each can be shifted to a different carrier frequency. This is the basis of frequency-division multiplexing.

调制还能让许多不同节目同时传输而互不干扰，因为每个节目可以被搬移到不同的载波频率上。这正是频分复用的基础。

3. Amplitude Modulation (AM) | 振幅调制

In amplitude modulation the amplitude of a high-frequency carrier is varied in proportion to the instantaneous amplitude of the information signal, while the carrier frequency remains constant.

在振幅调制中，高频载波的振幅按照信息信号的瞬时振幅成比例变化，而载波频率保持不变。

For a single-tone message, the AM waveform contains the carrier frequency f₀ and two sidebands at f₀ − f

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