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Wireless communication refers to the communication method that uses electromagnetic waves such as radio waves or light waves to transmit information. Compared with wired communication, it has the advantages of wide coverage and freedom of movement, and has become one of the important technologies in the field of modern communication. The following are the basic principles of wireless communication:

For example, if you want to send a voice message to your friend, but you are far away and cannot communicate face to face, then you need to:

Analog signal sampling: analog quantity → digital quantity, convert voice information into electrical signal (electrical signal = original baseband signal, low frequency, generally speaking, we can regard the original baseband signal as a continuous sound wave, electrical signal or digital signal , such as the voice of our speech, the music file in the mobile phone, etc., these signals themselves have not been modulated into a carrier signal, that is, they have not been encoded into a signal that can be transmitted through radio waves ).
The low-frequency original baseband signal is converted into a high-frequency signal suitable for wireless transmission through modulation technology . (Amplitude modulation can be used to change the amplitude of the original baseband signal to generate a modulated signal. The sound will generate a sound wave, and the sound wave is a signal whose amplitude changes with time. By performing amplitude modulation on the sound wave signal and the carrier signal, you can generate a modulated signal)
The modulated signal is transmitted to your friend through the wireless network. He can restore the modulated signal (high frequency) to the original baseband signal (low frequency) through demodulation technology , and then play it through the speaker, and you can communicate smoothly.

1. Signal transmission

Wireless communication first converts information signals into electromagnetic waves such as radio waves or light waves for transmission. During signal transmission, factors such as bandwidth, frequency, power, and modulation method of the signal need to be considered.

2. Modulation

Signal modulation is a technology in wireless communication, which converts the original baseband signal (or called baseband signal, low frequency) into a high frequency signal suitable for wireless transmission (also called carrier signal, high frequency) through modulation technology. Signal modulation in wireless networks can be divided into two types: analog modulation and digital modulation.

analog modulation

Analog modulation is the process of converting an analog baseband signal into a modulated signal through an analog circuit. Common analog modulation methods are: amplitude modulation (AM), frequency modulation (FM) and phase modulation (PM).

Amplitude modulation (AM): A modulated signal is generated by changing the amplitude of the original baseband signal so that it changes over time.
Frequency Modulation (FM): A modulated signal is produced by changing the frequency of the original baseband signal so that it varies over time.
Phase Modulation (PM): A modulated signal is generated by changing the phase of the original baseband signal so that it varies over time. digital modulation

digital modulation

It is the process of converting digital baseband signals into modulated signals through digital circuits. There are two main types of digital modulation: Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK) and Phase Shift Keying (PSK).

Amplitude Shift Keying (ASK): Digital modulation is realized by corresponding "0" and "1" of the digital baseband signal to different amplitudes.
Frequency Shift Keying (FSK): Digital modulation is realized by corresponding "0" and "1" of the digital baseband signal to different frequencies.
Phase Shift Keying (PSK): Digital modulation is realized by corresponding "0" and "1" of the digital baseband signal to different phase angles.

3. Demodulation

At the receiving end, it is necessary to demodulate the received modulated signal and convert the signal into the original information signal. Demodulation needs to take into account factors such as signal noise and interference.

Direct detection and demodulation (Envelope Detection): After the modulated signal passes through a nonlinear element (such as a diode), the envelope signal is extracted through a low-pass filter, thereby realizing the demodulation of the modulated signal.
Coherent Demodulation (Coherent Detection): At the receiving end, the phase information of the modulated signal is recovered from the reference signal, and then the modulated signal is demodulated into the original baseband signal through the amplitude demodulator.
Frequency Discrimination (Frequency Discrimination): The received signal is demodulated through the Frequency Discriminator, which is mainly used for the demodulation of frequency modulation (FM) signals.
Phase Demodulation (Phase Demodulation): The received signal is compared with the signal generated by the local oscillator through the Phase Comparator to obtain the phase information of the modulated signal, and then the modulated signal is demodulated to the original baseband through the dephaser Signal.

4. Channel coding

In order to improve the reliability and security of wireless communication , channel coding technology is often used to encode information signals to enhance the error correction capability and confidentiality of the channel.

Channel coding is a technique that improves the reliability of data transmission by adding redundant information . Commonly used channel coding methods include the following:

Parity Check Code (Parity Check Code): It is the simplest encoding method, which only needs to add a parity bit to the data to detect errors in the data.
Cyclic Redundancy Check (CRC): It is a common check code that can detect multi-bit errors and is used to detect errors in data transmission.
Hamming Code: It is a coding method that can correct unit bit errors and has a high error correction capability.
Convolutional Code: It is a method of coding through a sliding window, which has high error correction capability and good code rate and delay characteristics, and is widely used in mobile communication and satellite communication and other fields. Turbo code (Turbo Code): It is a multi-layer iterative coding method with high error correction capability and good performance, and is widely used in wireless communication, digital TV and other fields.

5. Multiplexing

In wireless communication, each user uses a wireless channel for communication. If each user occupies an independent channel, wireless channel resources will be used up quickly, resulting in low communication efficiency.

Multiplexing technology allows multiple users to share the same channel to achieve more efficient use of channel resources. This technology divides the channel into several sub-channels, and allocates each sub-channel to different users, so as to transmit data of multiple users on the same channel at the same time. Specifically, the multiplexing technology can divide a channel into multiple sub-channels according to time, frequency, or chips, and different users can communicate on different sub-channels to avoid mutual interference.

Common multiplexing techniques are:

Time Division Multiplexing (TDM): The channel is divided into several time slots according to time, and different users transmit data in different time slots, and each user occupies a fixed time slot.
Frequency Division Multiplexing (FDM): The channel is divided into several sub-channels according to frequency, and different users transmit data in different sub-channels, and each user occupies a fixed sub-channel.
Code Division Multiplexing (CDMA): By using different spreading codes, the channel is divided into several chip sequences, and different users use different spreading codes for encoding and decoding, so as to realize the transmission of data on the same channel.

In practical applications, different multiplexing technologies are often used in combination to achieve better channel utilization efficiency and data transmission effects.

6. Power control

In order to ensure the reliability of wireless communication and save energy, it is necessary to control the transmission power of radio waves. Power control needs to consider factors such as channel loss and interference to determine the optimal transmit power.

【Wireless Network Technology】Basic Principles of Wireless Communication