Main characteristics and key technologies of 4G mobile communication system
1 Introduction
With the increasing demand for mobile communication systems, the 2G, 2.5G and 3G systems currently in commercial use are no longer sufficient to meet the increasing high-speed multimedia data services of modern mobile communication systems. Many countries have invested in Research and development of 4G mobile communication system. This article will briefly introduce the main technical characteristics of the 4G mobile communication system and discuss the relevant key technologies that may be used in the 4G system.
2. Main features of 4G mobile communication system
Compared with 3G, the technology of 4G mobile communication system has many surpassing points, and its characteristics are as follows:
(1) High rate. For large-scale high-speed mobile users (250km / h), the data rate is 2Mb / s; for medium-speed mobile users (60km / h), the data rate is 20Mb / s; for low-speed mobile users (indoor or pedestrian), the data rate It is 100Mb / s.
(2) Mainly based on digital broadband technology. In the 4G mobile communication system, the main transmission band of the signal is millimeter wave, and the cell will be correspondingly smaller, which greatly increases the user capacity, but it will also cause a series of technical difficulties.
(3) Good compatibility. The 4G mobile communication system realizes a unified global standard, allowing users of all mobile communication operators to enjoy common 4G services, and truly realize that a mobile phone can communicate at any place in the world.
(4) Strong flexibility. The 4G mobile communication system adopts intelligent technology to enable it to adaptively allocate resources, process the changing service flow size in the communication process to meet the communication requirements, and adopt intelligent signal processing technology to deal with various complex environments with different channel conditions The normal transmission and reception of signals have strong intelligence, adaptability and flexibility.
(5) Multi-type users coexist. The 4G mobile communication system can perform adaptive processing according to the dynamic network and changing channel conditions, so that low-speed and high-speed users and various user equipment can coexist and intercommunicate, thereby meeting the needs of multiple types of users in the system.
(6) Integration of multiple services. The 4G mobile communication system supports richer mobile services, including high-definition image services, conference TV, virtual reality services, etc., so that users can obtain any required information services anywhere. Combining industries such as personal communications, information systems, broadcasting, and entertainment as a whole to provide users with a wider range of services and applications more safely and conveniently.
(7) Application of advanced technology. 4G mobile communication system is based on several breakthrough technologies, such as: OFDM multiple access, smart antenna and space-time coding technology, wireless link enhancement technology, software radio technology, efficient modulation and demodulation technology, high-performance Transceiver and multi-user detection technology, etc.
(8) Highly self-organizing and adaptive network. The 4G mobile communication system is a fully autonomous and adaptive network with self-management capabilities for the structure to meet the changing needs of users in terms of business and capacity.
3. The key technology of 4G mobile communication system
In order to meet the increasing demands of mobile communication users for high-speed multimedia data services, and specifically realize the advantages of 4G systems over 3G, 4G mobile communication systems will mainly use the following key technologies:
(1) Access method and multiple access scheme
OFDM (Orthogonal Frequency Division Multiplexing) is a high-speed transmission technology in a wireless environment. Its main idea is to divide a given channel into many orthogonal sub-channels in the frequency domain, and use one sub-carrier for modulation on each sub-channel. Each subcarrier is transmitted in parallel. Although the total channel is non-flat, that is, frequency selective, each sub-channel is relatively flat, and narrow-band transmission is performed on each sub-channel, and the signal bandwidth is smaller than the corresponding bandwidth of the channel. The advantage of OFDM technology is that it can eliminate or reduce interference between signal waveforms, is insensitive to multipath fading and Doppler frequency shift, improves spectrum utilization, and can realize low-cost single-band receivers. The main disadvantage of OFDM is its low power efficiency.
(2) Modulation and coding technology
The 4G mobile communication system uses new modulation technologies, such as multi-carrier orthogonal frequency division multiplexing modulation technology and single-carrier adaptive equalization technology, to ensure spectrum utilization and extend the battery life of user terminals. 4G mobile communication systems use more advanced channel coding schemes (such as Turbo codes, concatenated codes, LDPC, etc.), automatic repeat request (ARQ) technology, and diversity reception technology, etc., to ensure that the system is adequate under low Eb / N0 conditions. performance.
(3) High-performance receiver
The 4G mobile communication system places high demands on the receiver. Shannon's theorem gives the minimum SNR required to achieve reliable transmission of capacity C in a channel with a bandwidth of BW. According to Shannon's theorem, it can be calculated that for a 3G system if the channel bandwidth is 5 MHz and the data rate is 2 Mb / s, the required SNR is 1.2 dB; and for a 4G system, 20 Mb / s data must be transmitted on a 5 MHz bandwidth , Then the required SNR is 12dB. It can be seen that for 4G systems, due to the high rate, the performance requirements of the receiver are also much higher.
(4) Smart antenna technology
Smart antennas have intelligent functions such as suppression of signal interference, automatic tracking, and digital beam adjustment, and are considered to be key technologies for future mobile communications. The smart antenna uses digital signal processing technology to generate a spatial directional beam, align the main beam of the antenna with the direction of arrival of the user signal, and the side lobe or null to the direction of arrival of the interference signal, so as to fully utilize the mobile user signal and eliminate or suppress the interference signal . This technique can both improve signal quality and increase transmission capacity.
(5) MIMO technology
MIMO (Multiple Input Multiple Output) technology refers to the technology of using multiple transmit and multiple receive antennas for spatial diversity. It uses discrete multiple antennas, which can effectively decompose the communication link into many parallel sub-channels, thus greatly Increase capacity. Information theory has proved that when different receiving antennas and different transmitting antennas are not related to each other, the MIMO system can well improve the system's anti-fading and noise performance, thereby obtaining huge capacity. For example, when the number of receiving antennas and transmitting antennas is 8 and the average signal-to-noise ratio is 20dB, the link capacity can be as high as 42bps / Hz, which is more than 40 times the capacity of a single antenna system. Therefore, in wireless channels with limited power bandwidth, MIMO technology is a spatial diversity technology that achieves high data rates, increases system capacity, and improves transmission quality. Today, when wireless spectrum resources are relatively scarce, MIMO systems have already demonstrated their superiority and will continue to be used in 4G mobile communication systems.
(6) Software radio technology
Software radio is a new technology with an open structure that takes standardized and modularized hardware functional units through a common hardware platform and uses software loading to implement various types of radio communication systems. The core idea of ​​software radio is to use broadband A / D and D / A converters as close as possible to the antenna, and define wireless functions as much as possible with software. Various functions and signal processing are implemented in software as much as possible. Its software system includes various types of wireless signaling rules and processing software, signal flow conversion software, source coding software, channel error correction coding software, modulation and demodulation algorithm software, etc. Software radio makes the system flexible and adaptable, and can adapt to different networks and air interfaces. Software radio technology can support multiple
Mode mobile phones and base stations can achieve variable QoS for various applications.
(7) IP-based core network
The core network of the 4G mobile communication system is an all-IP-based network. Compared with existing mobile networks, it has fundamental advantages, that is, it can realize seamless interconnection between different networks. The core network is independent of various specific wireless access solutions, can provide end-to-end IP services, and is compatible with existing core networks and PSTN. The core network has an open structure, which can allow various air interfaces to access the core network; at the same time, the core network can separate services, control, and transmission. After adopting IP, the adopted wireless access methods and protocols are separated from the core network (CN) protocol and link layer. IP is compatible with a variety of wireless access protocols, so it has great flexibility when designing the core network, and does not need to consider what method and protocol is used for wireless access.
(8) Multi-user detection technology
Multi-user detection is the key technology of anti-interference in broadband CDMA communication system. In the actual CDMA communication system, there is a certain correlation between the signals of various users, which is the source of the existence of multiple access interference. The multiple access interference generated by individual users is very small, but as the number of users increases or the signal power increases, multiple access interference has become a major interference in broadband CDMA communication systems. The traditional detection technology is based on the classic direct sequence spread spectrum theory, and the spreading code matching process is performed on each user's signal separately, so the ability to resist multiple access interference is poor; the multi-user detection technology is based on the traditional detection technology. All user signal information of multiple access interference detects the signal of a single user, so it has excellent anti-interference performance, solves the problem of near-far effect, and reduces the system's requirements for power control accuracy, so it can use link spectrum resources more effectively , Significantly increasing system capacity. With the continuous development of multi-user detection technology, a variety of high-performance but not particularly complex multi-user detector algorithms are constantly being proposed. It will be feasible to use multi-user detection technology in 4G actual systems.
4. Summary
The 4G mobile communication system is currently only a basic concept and is in the laboratory research and development stage. Many people in the industry believe that although 4G mobile communication technology has stronger advantages than 3G, if 4G is put into practical application, it will need to update the existing mobile communication infrastructure, which will trigger a series of Funds, concepts and other issues, to a certain extent, slow down the speed of 4G officially entering the market. But what is certain is that with the rapid development of the Internet, 4G will continue to develop at a high speed. 4G will be a multi-functional integrated broadband mobile communication system and a new generation of mobile communication system that meets future market demands.
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