2.1.Digital communication

Digital communication refers to the transmitter sending signals from a finite set. For example, when transmitting 1000 bits, we choose one from 1000 possible binary sequences for communication. To convey this selection, we choose a signal suitable for the current channel. Regardless of the signal chosen, the communication method is digital. A simple way to achieve this is to let each bit determine the amplitude of the carrier wave within a specific time interval, such as the first bit determining the amplitude from time 0 to T, the second bit determining the amplitude from T to 2T, and so on. This is the basic form of Pulse Amplitude Modulation (PAM). Many reasonable methods exist for mapping bits to waveforms suitable for a specific channel; regardless of the mapping method chosen, it falls under the category of digital communication.

2.2.Analog Communication

When a transmitter sends one of a series of consecutive possible signals, we call it analog communication. For example, the transmitted signal could be the output of a microphone, where even a small change in the signal can represent a valid signal. In analog communication, the source signal is used to modify a parameter of the carrier signal; two common methods are amplitude modulation (AM) and frequency modulation (FM). In AM, the carrier amplitude varies with the source signal; in FM, the carrier frequency varies with the source signal.

2.3.Analog Communication System Model

For much of the 20th century, analog communication systems dominated with their continuously varying amplitude, frequency, or phase signals. These systems reflected the continuity of nature, such as the human voice, whose sound waves vary continuously in amplitude and frequency. AM and FM radio broadcasting and traditional wired telephone systems are typical applications of analog communication systems that readers encounter daily.

In analog systems, message transmission begins with an input sensor (transducer), which converts the raw signal (such as sound) into an electrical signal, often called the message signal or baseband signal. Voice signals range from 300Hz to 3000Hz, while television signals range from 0Hz to 6000kHz.

This signal is then modulated and combined with a carrier signal. The transmitter modulates the electrical signal into a format suitable for transmission over a specific channel. This process involves loading the message signal onto a high-frequency carrier signal. Different channels may require different types of transmitters to adapt to their characteristics. The transmitter needs to be able to adjust flexibly when channel conditions change to ensure the signal remains within the effective communication range. Common modulation methods include amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM).

The modulated signal is amplified and transmitted through a channel such as air or cable. At the receiving end, the signal is detected, amplified, and demodulated to reconstruct the original message. Finally, the output sensor converts the electrical signal back to its original form, such as sound emitted through a speaker.

Although analog communication systems are simple in structure and can naturally handle continuous signals, they are susceptible to noise interference and signal attenuation, especially in long-distance or multi-level transmission.

2.4.Digital Communication System Model

Digital Communication System Model Nature typically presents information in a continuous manner, such as beautiful mountain scenery or the melodious chirping of birds. However, modern communication systems tend to use digital signals, whose amplitude and time are discrete values. Digital signals are favored in part because they are easier to transmit reliably than analog signals. When damage to the transmission system begins to affect signal quality, it can be restored to its standard form before reaching its final destination through detection, shaping, and amplification. The diagram below illustrates an ideal binary digital pulse propagating along a transmission line; the pulse shape degrades as the line length increases. Within the propagation distance where the signal can still be reliably identified, a digital amplifier amplifies the pulse, restoring its original ideal shape, thus regenerating the signal. Analog signals, due to their infinite diversity of shapes, cannot undergo such shaping. Therefore, the farther the signal travels and the more processing it undergoes, the more severe the degradation it suffers from even small errors.

Classic Digital Communication System

In the classic digital communication system shown left, the process begins with source coding, which converts analog input into digital format and often includes data compression. The digital data then undergoes channel coding, adding redundancy for error detection and correction. Digital modulation techniques map the data into symbols for transmission, such as Phase Shift Keying (PSK) or Frequency Shift Keying (FSK). The receiving process performs these steps in reverse order, adding error correction and signal processing. The ability to detect and correct errors is a significant advantage of digital systems, allowing for more reliable communication over noisy channels.