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With the rapid development of science and technology, mobile communication technology has undergone profound changes, from 1G to 2G, to 3G, to 4G and 5G, constantly changing and continuing. On December 4, 2013, the fourth-generation mobile communication technology was officially put into operation in The Chinese market, which means that China's mobile communication industry has entered the 4G era. At this time, the research institute and the world's well-known enterprises engaged in communication technology research have entered the new generation of mobile communication, namely 5G (the fifth generation of mobile communication system), research and development. No matter which generation of communication system, the technology studied is to analyze the characteristics of wireless communication channel and overcome noise interference. There are a lot of researchers looking at Massive MIMO technology. How does it differ from the antenna technology used in 3G/4G communication systems? Will it become the core technology of the next generation of wireless communication?
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In December of 2013, China formally entering the 4 g (fourth generation mobile communication system) communications network era, in the 4 g mobile communication system, the OFDM (orthogonal frequency division multiplexing) technology, OFDM technology for its high spectrum efficiency and good anti-multipath fading performance and being hyped, 5 g in the future communication network will also be for OFDM technology related research.
The key technologies of 4G communication system include:
A. OFDM technology;
B. MIMO technology;
C. Multi-user detection technology;
D. Software radio;
E. Smart antenna technology;
F. IPv6 technology.
Figure 1 shows the layout of Massive MIMO antennas in A 5G communication network. I am studying Massive MIMO technology in my lab. Figure 1 shows the users communicating with each other centered on a large-scale antenna.
The performance of wireless communication system is mainly restricted by mobile wireless channel. Wireless channel is very complex, and its modeling has always been a difficulty in system design. Generally, statistical methods are used to make statistics according to the measured values of the communication system in a specific frequency band.
Wireless fading channel is divided into large scale fading channel model and small scale fading channel model. The so-called large scale fading model describes the variation of field strength over a long distance (hundreds or thousands of meters) between transmitter and receiver, and reflects the variation of received signal power with distance caused by path loss and shadow effect. The small scale fading model describes the fast fluctuation of the received field intensity over a short distance or time.
The large scale fading channel model is caused by the influence of the surface profile (such as mountains, forests, buildings, etc.) between the receiver and the receiver. The small scale fading channel model is caused by multipath effect and Doppler effect. If there are a large number of reflection paths but no LOS (direct signal) signal component, the small scale fading is called Rayleigh fading, and the envelope of the received signal is described statistically by The Rayleigh probability density function. If LOS is present, the envelope follows Rician distribution. Multipath effect phenomenon causes flat fading and frequency selective fading.
In 3G/4G/5G wireless communication system, how to overcome the phenomenon of multipath effect, based on my research, here some analysis and views?
F. Software radio.
1.2 Key technologies of 4G communication system
A. Large-scale MIMO technology;
From 1G to 4G, communication equipment is inseparable from antennas. When the traditional antennas cannot meet the needs of communication networks, mobile communication enters the 3G era. Researchers have developed smart antennas. Only TD-SCDMA systems use this technology. Smart antenna is a new mobile communication technology based on the principle of adaptive antenna. It has incomparable advantages in eliminating interference, expanding cell radius, reducing system cost and increasing system capacity.
In order to achieve the purpose of high-speed communication, smart antenna is indispensable, and it must be used more effectively. In 3G communication network, intelligent antenna can not solve the problems such as multipath interference whose delay exceeds the width of the chip and Doppler effect caused by high-speed movement, which will be effectively solved in 4G.
2.2.2 MIMO technology in 4G communication system
The 4G communication system uses multiple input multiple output (MIMO) technology, which is not used in 3G systems. In 4G communication network, the antennas of most base stations adopt the structure of one receiver and two receivers. MIMO technology and
The combination of OFDM technology, the core technology adopted by 4G system, namely MIMO-OFDM technology, forms a new technology that meets people's needs, greatly improves the data rate, avoids narrowband fading, improves the spectrum utilization rate and the ability to resist multipath fading.
The first reason is that OFDM technology belongs to multi-carrier modulation (MCM) technology, and OFDM technology has good anti-noise performance and anti-multipath interference ability, as well as the characteristics of high frequency spectrum utilization.
The second reason is that using MIMO technology not only increases the wireless channel capacity exponentially, but also the frequency spectrum utilization rate can be increased exponentially without increasing the bandwidth and antenna transmission power. It plays a key role in improving anti-interference ability. MIMO technology systems have significant advantages:
A. Reduced inter-symbol interference;
B. Improved spatial diversity gain;
C. Improved wireless channel capacity and spectrum utilization.
(7)
Where, is a coded modulation symbol, an orthogonal matrix, which sends each line symbol directly through the antenna; Is the modulation symbol (obtained from the m-QAM /PSK symbol). Further development, Alamouti code is extended to general STBC[2] code, and STTC[3] space time lattice code, etc. STTC has excellent diversity gain and coding gain, but its decoding complexity is very high. In order to satisfy the degree of freedom and diversity, an orthogonal matrix with 8 rows and 8 columns can be encoded at most. Beyond that, you degrade the speed performance, you don't get full speed.
B. The complexity of receiving detection is low
Its disadvantages:
A. The number of sending antennas must be smaller than or equal to that of receiving antennas
B. The time domain and spatial processing are not combined, and the error code performance is inferior to space time code
LDC code principle: in the structure diagram, the matrix is composed of certain criteria. After the source bit data flow is in series, it is modulated by M-PSK/QAM, and then the modulation symbols of each path are multiplied by the matrices of each path respectively to form the matrix symbols. Each matrix symbols are added together, and finally sent through the MIMO antenna. LDC code is a summary of THE OSTBCs scheme, BLAST scheme, or OSTBCs, BLAST is a special case of LDC code. Comparison of LDC Code, STBC, BLAST and ST Trellis Code in MIMO technology
The working principle of information bitstream mapping is as follows: The input bit number is B=3 bits. The first two bits are used to activate one of the four antennas, and the last bit is used as the modulator input. Then the activated antenna sends the modulated BPSK symbol to the receiver.
Figure 6. CSTSK structure diagram
How it works: In the system model, the input source data stream, after a series parallel transformation, is divided into two data streams, one data stream is used as one of the selection matrix. The other data stream is mapped into the modulator and the modulation symbol is output. After the first path is selected, the symbol matrix is multiplied by the modulation symbol and sent to the receiver through the space-time Mapper.
STSK scheme has the advantages of enhancing spatial diversity and time diversity. The multipath effect fading is further overcome.
STSK is combined with OFDM technology to form STSK-MIMO-OFDM system, which can obtain spatial, time and frequency diversity gain, and greatly improve the performance of wireless communication.
In the future 5G communication system, whether to combine STSK with MIMO-OFDM technology in large-scale MIMO technology has not been decided yet, and it is still under laboratory research.
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