[1] Introduction to wireless communication
1. Basic concepts of wireless communication
Wireless communication is a communication method that uses the radiation and propagation of electromagnetic waves to transmit signals through space. The use of wireless communication can transmit telegraph, telephone, fax, data, images, and radio and television programs and other communication services. Wireless communication systems can be divided into mobile cellular network systems (2G, 3G, 4G, 5G), wireless local area network systems (WLAN), broadcast television systems, satellite communication systems, etc.
2. Wireless communication multiple access technology
The multiple access technologies discussed in this article include:
Frequency division multiple access technology, time division multiple access technology, code division multiple access technology and space division multiple access technology correspond to FDMA, TDMA, CDMA and SDMA respectively.
What is multiple access technology? Why does multiple access technology appear? Understanding of each multiple access technology?
it's actually really easy
Early wireless telegraphy did not require multiple access technology, because its communication method was able to send and receive, and the user load was not large, and only point-to-point communication was required. In order to achieve higher communication efficiency, mobile communication now uses a basic network architecture. In this basic network architecture, many base stations are included, and the base stations are connected to each other. When the mobile phone communicates, it does not directly communicate with another mobile phone via radio, but first sends a signal to the base station closest to itself, and the base station sends the signal to the base station closest to the other mobile phone, and then this base station passes wirelessly. To the destination mobile phone.
Then, there will be multiple mobile phones communicating with one base station at the same time. How does the base station distinguish the signals of different mobile phones? This requires multiple access technology.
- FDMA (adopted in the first generation of communications):
As shown in the figure: Frequency division multiple access technology uses different users to occupy different frequency channels, thus avoiding mutual interference and realizing differentiation.
Which frequency the mobile phone chooses can be achieved through a filter. Because the blocking of the filter has a transition zone. Therefore, a certain bandwidth must be reserved between two adjacent frequencies as protection. (Later we will talk about OFDMA technology, which belongs to Orthogonal Frequency Division Multiple Access technology, does not reserve bandwidth protection and improves frequency band utilization)
- TDMA (adopted in second-generation communications)
As shown in the figure: In principle, TDMA is similar to FDMA, except that the frequency is replaced by time. Time resources are divided into frames, and each frame is divided into several time slots. Timers are required and synchronization with the base station is required. .
- CDMA (adopted in third-generation communications)
CDMA and OFMDA are the multiple access technologies of the third and fourth generation mobile communications respectively. In fact, mathematically, the difference between these two technologies is very small.
What is used now is CMDA is direct sequence spread spectrum, CDMA uses code to distinguish users. The so-called code is a sequence composed of 1 and -1, such as 1 1 -1 -1 is a spreading code of length 4.
The spread spectrum operation is to multiply the user data symbol, assumed to be x0, by the above code to obtain x0, x0, -x0, -x0. In other words, after spreading, one symbol becomes 4 symbols. If you want to keep the user's rate unchanged, you need 4 times the bandwidth. Therefore, this technique is called direct sequence spread spectrum.
The explanation of CDMA technology is really complicated, and there will be a chapter later to deduce the whole process in detail.
- OFDMA (adopted in the fourth and fifth generations)
In order to avoid mutual interference between various channels of traditional FDMA, a certain guard band needs to be reserved, and there is a certain spectrum loss. However, there is no guard band between the various sub-carriers of OFDM, and they are orthogonal, and there is no interference between each other, so it has a larger spectrum efficiency advantage than FDMA. However, such a standard is too low, and orthogonality between sub-carriers is a natural thing.
The explanation of OFDMA technology will also have a later chapter, carefully deriving the principles of OFDM technology, and the principles of QPSK, 16QAM, 64QAM, 256QAM, etc.
Summary:
Multiple access technology is the basic technology of wireless communication. We introduced FDMA TDMA CDMA OFDMA according to the process of technological development. They are the multiple access technologies of the first to fifth generation mobile communications. The basis for implementing FDMA is filter technology, which is the first to be applied because it is the simplest. TDMA is the first step towards broadbandization, and it needs to rely on the development of timing technology. The problem with TDMA is that there is only one dimension of resource division, that is, time, and resource allocation is difficult when moving toward higher bandwidth. But the problem with CDMA is that it is a self-interference system. Although it uses fast power control, UFR, soft handover and other technical means, it still cannot overcome its shortcomings. The multi-user detection of the CDMA system is the fundamental method to solve this problem, but due to the limitation of the actual system design, the multi-user detection technology cannot be applied. OFDM is also a special kind of CDMA, which utilizes the characteristics of linear systems and overcomes the multipath effect and multi-user interference of the channel at a relatively small cost. It is called the multiple access technology of the fourth generation of mobile communications.
3. Wireless communication duplex mode
Duplex is divided into half duplex and full duplex
- Half-duplex means that only one device is allowed to transmit information at the same time. If another device wants to transmit information, it needs to wait for the original transmitting device to complete the transmission before processing. Walkie-talkies are a typical application of half-duplex technology.
- Full duplex refers to allowing two devices to transmit information in both directions at the same time. General telephones and mobile phones are full-duplex systems, because the voice of the other party can be heard while talking. The duplex technology we usually talk about usually refers to the full-duplex technology.
The duplex mode is divided into frequency division duplex (FDD) and time division duplex (TDD)
- Frequency Division Duplex (FDD): Use frequency to distinguish between sending and receiving signals. Advantages: Simple to implement; Disadvantages: Low spectrum utilization efficiency when uplink and downlink services are asymmetric (mainly data services); Typical applications: GSM, CDMA2000, WCDMA system.
- Time Division Duplex (TDD): Use time slots to distinguish between transmitted and received signals. Advantages: When the uplink and downlink services are asymmetric, different numbers of time slots can be flexibly allocated to the uplink and downlink, and the spectrum utilization efficiency is high. Disadvantages: the implementation is more complicated and requires GPS When it is synchronized and used with CDMA technology, it is difficult to control interference between uplink and downlink; typical applications: TD-SCDMA, TD-LTE systems.