What is the core technology of the fourth generation mobile communication system?

The core technology of the fourth generation mobile communication system is "OFDM". OFDM refers to Orthogonal Frequency Division Multiplexing technology, which realizes parallel transmission of high-speed serial data through frequency division multiplexing. It has good resistance to multipath fading and can support multi-user access. OFDM technology is characterized by a highly scalable network structure, good anti-noise performance and anti-multi-channel interference capabilities, and can provide higher quality (high speed, small delay) services and better performance and price than current wireless data technology. Compared with this, it can provide better solutions for 4G wireless networks.

The operating environment of this tutorial: Windows 7 system, Dell G3 computer.

The fourth generation mobile communication system is referred to as "4G". Based on the third generation mobile communication system, it is a mobile communication system with faster data transmission speed and higher quality.

The network architecture of the 4G mobile communication system can be divided into three layers from bottom to top: physical network layer, intermediate environment layer, and application environment layer.

The physical network layer provides access and routing functions: the intermediate environment layer serves as the bridging layer to provide QoS mapping, address translation, security management, etc. The interface between the physical network layer, the intermediate environment layer and its application environment layer is open, which can bring the following advantages:

• ① Make it easier to develop and provide new services Easy;

• ② Can provide seamless high data rate wireless services;

• ③ Can operate in multiple frequency bands;

• ④ Enable services to adapt to multiple wireless standards and multi-mode terminals, across multiple operators and service providers, and provide a wider range of services.

# The fourth generation mobile communication system is mainly based on orthogonal frequency division multiplexing (OFDM) as the core technology.

OFDM (Orthogonal Frequency Division Multiplexing) is orthogonal frequency division multiplexing technology. In fact, OFDM is MCM (Multi Carrier Modulation), a type of multi-carrier modulation. Parallel transmission of high-speed serial data is achieved through frequency division multiplexing. It has good resistance to multipath fading and can support multi-user access.

OFDM technology is developed from MCM (Multi-Carrier Modulation, multi-carrier modulation). OFDM technology is one of the implementation methods of multi-carrier transmission schemes. Its modulation and demodulation are implemented based on IFFT and FFT respectively. It is a multi-carrier transmission scheme with the lowest implementation complexity and the most widely used.

OFDM technology is characterized by a highly scalable network structure, good anti-noise performance and anti-multi-channel interference capabilities, and can provide higher quality (high rate, small delay) services than current wireless data technology And with better performance-price ratio, it can provide better solutions for 4G wireless networks.

For example, Wireless Area Loop (WLL), Digital Audio Broadcasting (DAB), etc. will all use OFDM technology. 4G mobile communications provide a technological response to the accelerating growth of broadband wireless connectivity requirements, ensuring seamless services across multiple wireless systems and networks, both public and private, indoor and outdoor.

By providing the best services required by users on the most suitable available network, we can cope with the expected growth in Internet-based communications, add new frequency bands, greatly expand spectrum resources, and provide different types of communication interfaces. Using routing technology as the main network architecture, Fourier transform is used to develop the hardware architecture to achieve the fourth generation network architecture. Mobile communications will develop towards data-based, high-speed, broadband, and higher frequency bands. Mobile data and mobile IP will become the mainstream services of mobile networks in the future.

The advantages and disadvantages of OFDM technology

Advantages

OFDM has many technical advantages as follows, It is used in 3G and 4G and has many advantages in communication:

(1) It can send a large amount of data even under narrowband bandwidth. OFDM technology can separate at least 1,000 digital signals at the same time, and its ability to operate safely around interfering signals will directly threaten the further development and growth of CDMA technology;

(2) OFDM technology can continuously Monitor sudden changes in communication characteristics on the transmission medium. Since the ability of the communication path to transmit data changes over time, OFDM can dynamically adapt to it and turn on and off the corresponding carriers to ensure continued successful communication;

(3) This technology can automatically detect which specific carrier under the transmission medium has high signal attenuation or interference pulses, and then take appropriate modulation measures to enable successful communication of the carrier at the specified frequency;

(4) OFDM technology is particularly suitable for use in high-rise buildings, densely populated and geographically prominent places, and areas where signals are spread. High-speed data transmission and digital voice broadcasting both hope to reduce the impact of multipath effects on signals.

(5) The biggest advantage of OFDM technology is its resistance to frequency selective fading or narrowband interference. In a single-carrier system, a single fading or interference can cause the entire communication link to fail, but in a multi-carrier system, only a small portion of the carriers will be interfered with. Error correction codes can also be used for error correction on these sub-channels.

(6) It can effectively combat interference between signal waveforms and is suitable for high-speed data transmission in multipath environments and fading channels. When frequency selective fading occurs in the channel due to multipath transmission, only the subcarriers that fall in the depression of the frequency band and the information they carry are affected, and other subcarriers are not damaged. Therefore, the overall bit error rate performance of the system is much better. many.

(7) Through joint coding of each subcarrier, it has strong anti-fading ability. OFDM technology itself already takes advantage of the frequency diversity of the channel. If the fading is not particularly severe, there is no need to add a time domain equalizer. By jointly coding each channel, system performance can be improved.

(8) OFDM technology is very resistant to narrowband interference because these interferences only affect a small part of the sub-channels.

(9) You can choose the OFDM implementation method based on IFFT/FFT;

(10) The channel utilization rate is very high, which is particularly important in a wireless environment with limited spectrum resources; when When the number of carriers is large, the spectrum utilization rate of the system tends to 2Baud/Hz. (baud is baud; 1 Baud = log2M (bit/s), where M is the encoding level of the signal).

Disadvantages

Although OFDM has the above advantages, its signal modulation mechanism also causes some disadvantages in the transmission process of OFDM signals:

(1) Very sensitive to phase noise and carrier frequency offset

This is a fatal shortcoming of OFDM technology. The entire OFDM system has extremely strict requirements on the orthogonality between each sub-carrier. Any small carrier frequency Any deviation will destroy the orthogonality between subcarriers and cause ISI. Similarly, phase noise will also cause the rotation and diffusion of symbol constellation points, forming ISI. Single-carrier systems do not have this problem. Phase noise and carrier frequency offset only reduce the received signal-to-noise ratio SNR without causing mutual interference.

(2) The peak-to-average ratio is too large

OFDM signals are composed of multiple subcarrier signals, and these subcarrier signals are independently modulated by different modulation symbols. Compared with the traditional constant envelope modulation method, OFDM modulation has a very high crest factor. Because the OFDM signal is the sum of many small signals, the phases of these small signals are determined by the data sequence to be transmitted. For some data, these small signals may be in phase and add together in amplitude to produce a large instantaneous peak amplitude. If the peak-to-average ratio is too large, it will increase the complexity of A/D and D/A, and will reduce the efficiency of the RF power amplifier. At the same time, at the transmitter end, the maximum output power of the amplifier limits the peak value of the signal, which will cause interference within the OFDM frequency band and between adjacent frequency bands.

(3) The required linear range is wide

Since the OFDM system has a large peak-to-average power ratio (PAPR) and is more sensitive to nonlinear amplification, the OFDM modulation system is more sensitive to the amplifier than the single-carrier system. The linear range requirements are higher.

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