Wireless communication technology is essentially the use of wireless transmission media to achieve interconnection between terminals. This wireless medium can be electromagnetic waves or light waves. In fact, according to the theory of quantum mechanics, light waves are also electromagnetic waves. The difference is that The wavelength is different.
With the transmission medium, the next step is coding, which means how to modulate the electromagnetic signal to represent the information we want to transmit. According to the characteristics of electromagnetic waves, information can be expressed by modulating the amplitude, frequency, phase, etc. of electromagnetic waves, that is, our common amplitude modulation, frequency modulation, and phase modulation.
With coding, the next step is to communicate. If there are only two communication terminals, then things are simple. For example, by amplitude modulation coding, the amplitude is 1 for 1, and the amplitude is 0 for 0. If there are many communication terminals? Will the communication between them interfere with each other? I am very sure of that. How to do? This shows the encoding method for identifying the identity of the communication terminal. The common ones are as follows:
FDMA, translated into Chinese, is a frequency division multiple access technique, that is, by modulating radio waves into a number of frequencies that do not interfere with each other, each frequency represents a communication line. To be professional, the total bandwidth is divided into multiple orthogonal channels, each occupying one channel. For example, the frequency band allocated for wireless cellular telephone communication is divided into 30 channels, each of which is capable of transmitting voice calls, digital services, and digital data.
TDMA, translated into Chinese, is a time division multiple access technology, which divides time into multiple periods, each period is divided into multiple time slots, and the cycle is repeated. Each time slot corresponds to one user's transmission information, and is differentiated according to time slots. The user signal of the address, thus completing the multiple access connection.
CDMA, translated into Chinese, is a code division multiple access technology. The technical principle is based on spread spectrum technology, which is about to transmit certain information bandwidth data, and is modulated with a high-speed pseudo-random code whose bandwidth is much larger than the signal bandwidth, so that the original data signal The bandwidth is extended, then modulated by the carrier and sent out. The receiving end uses the identical pseudo-random code to perform correlation processing with the received bandwidth signal, and the narrowband signal of the original information data is despreaded to realize information communication.
Wireless LAN technology. The three technologies of FDMA, TDMA, and CDMA can provide a proprietary connection for each communication channel. It can be understood that the medium is not shared, so that the remote communication can be ensured without being interfered, and therefore it is mostly used for remote communication. The same is the use of electromagnetic waves, the technology used in wireless LAN is CSMA / CA technology, its working principle is:
(1) First, it is detected whether the channel is used. If it is detected that the channel is busy, it waits for a random time before sending the data.
(2) If the receiving end correctly receives the frame, it will send an acknowledgement frame ACK to the transmitting end after a period of time.
(3) The sender receives the ACK frame and determines that the data is correctly transmitted. After a period of time, an idle time occurs.
It can be seen that the CSMA/CA technology used by the WLAN is a shared transmission medium, and the two communication terminals are implemented by competing to the transmission medium. If there are too many terminals, the transmission channel will be crowded, so it is not suitable for a wide range of remotes. Communication is only suitable for small-scale local communication. Because of this, we call the wireless communication network using CSMA/CA technology as the wireless local area network.
Well, let's briefly introduce it here. The purpose of this article is to give readers a little insight into wireless communication, and it will not be discussed in depth.
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