1.1 The Concept of Communication
- communication【通信】：It is the process of using signals to transmit messages containing information in space. To put it simply, communication is the spatial transmission of information【信息的空间传递】
- Information【信息】： It is an uncertain description of the state of motion or the mode of existence of all things, and the law of movement of things that people want to know or want to express, usually in the form of messages (such as voice, text, music, data, pictures or moving images, etc.).信息是抽象的，是消息的内涵
- Message【消息】： It is the external manifestation of information or the logical carrier of information. Information is the connotation of a message.【信息的形式载体&表达形式】
消息类似于容器，信息就是容器里的物品。一条消息可以包含多个信息，也可以不包含信息。一条信息可以有多种表达形式。
Since "transportation" and "communication" have a strong analogy, we use some examples of transportation (including road and rail transportation, etc.) as the object of comparison, such as: transportation/communication, transportation/transmission, vehicle/signal, cargo/information, road/channel, etc., in order to help us understand more thoroughly many concepts and problems in the principles of communication.
signal【信号】是信息或消息的物理载体，是通信任务实施的具体对象。
消息可以分为两大类：连续（模拟）消息和离散（数字）消息。连续消息是指消息的状态是连续变化或者不可数。离散消息则是消息的状态是可数的或者离散的。
1.2 Communication Systems
1.2.1 Definition & Component
Communication system: A collection of device hardware, software, and transmission media used to communicate.用于进行通信的设备硬件、软件和传输介质的集合
通信系统的三个要点：source destination channel
发送设备 transmitting equipment: modulation调制
接收设备 receiving device: demodulation解调
carrier machine: 载波机
lead: 电线
调制的定义是：把输入信号变换为适合于通过信道传输的波形，这一变换过程称为调制。通常把原始信号称为调制信号，也称基带信号；被调制的高频用于运载原始信号，因此称载波。
1.2.2 Classification
1. According to the channel transmission signal
【这里的信号看的是信道而不是信源】分为模拟通信系统和数字通信系统
An analog communication system模拟通信 can be said as a system that transmits analog messages in the form of analog signals (see Figure 1-1 for the model), such as a telephone communication system.
A digital communication system can be said as a system that transmits analog messages in the form of digital signals (see Figure 1-3(a) for the model).
For example, digital telephone communication systems and mobile communication systems (mobile phones).
信源信宿处理的都是模拟消息，模拟通信信道传输的是模拟信号，数字通信传输的是数字信号
数字通信比模拟通信多了信源译码（编码）和信道译码（编码）【特点】
信源编码：将模拟信号转变为数字信号；信源译码功能相反
信道译码：数字信号形成的脉冲序列变成检错功能的脉冲序列；信道译码功能相反
A data communication system【数据通信】 can be said as a system that transmits data services with digital or analog signals .
For example, the communication system between a computer and a printer. In general, data communication mainly refers to communication between computers (or digital terminals).
模拟信号的特点：波动性 连续性 无穷多个点
电/光信号传输特点：衰减--传输距离越长，频率越快，衰减越大。
信号的波形发生畸变（由噪声和衰减造成）
Because the direct cause of digital communication is to improve the quality of analog communication,为了提高模拟通信的质量
Therefore, digital communication can be understood as an upgrade of analog communication.
模拟与数字的主要差别在：数字通信在信宿上接收到的信号质量更好
Digital communication has the following characteristics:
1.Strong anti-interference ability.抗干扰能力
2.It is convenient for signal processing and processing.
3.Improved transmission quality.--控制误码
4.Digital information is easy to encrypt and highly confidential.
5.Increased flexibility and versatility通用性 of communication systems.
Disadvantages: Increased channel bandwidth【用带宽换性能】--数字通信的频带利用率较低
【从信息传输的角度看】The analog communication system can be regarded as a signal waveform transmission system【信号波形传输系统】, while the digital communication system and data communication system (most) are signal state transmission systems【信号状态传输系统】. Schematic diagram of signal transmission of two communication systems:
模拟通信在传输上采用“逐级”放大，数字通信更多的用“再生”方式
2. According to the channel transmission medium【介质】
按照传输介质分类，分为无线通信技术和有线通信技术
Wireless communication systems，A system that uses radio waves, infrared, ultrasonic, and lasers to communicate.【无线电波，红外线，超等波，激光】
Wired communication system，A communication system that uses wires (including cables, fiber optic cables, waveguides, etc.) as a medium.【导线包括电缆、光纤等】，有线通信质量更高
3. According to the channel transmission modulation【按照调制与否分类】
可分为基带通信系统和调制通信系统
Baseband communication system：A system that transmits a signal that has not undergone any modulation.【基带信号】电信号固有形式
Modulation communication systems：A system that transmits a signal that has been modulated.
已调（带通信号）在远距离上能更好的传输
4. according to the type of service【业务】
For example: telephone communication system, telegraph communication system电报通信, broadcast communication system广播通信, television communication system电视通信, data communication system数据通信
5. according to the operating band【工作波长】
For example: long-wave communication system, medium-wave communication system, short-wave communication system, microwave communication system微波 and optical communication system光波
1.3 Communication method
Communication method：The work form and signal transmission【工作方式和信号传输方式】 mode between two (or more) parties of a communication. It is an issue that each part must determine firstly before the communica
