A Multiple Input Multiple Output (MIMO) system refers to a communication system that uses multiple antennas simultaneously at the transmitting end and the receiving end. The research proves that MIMO technology is very suitable for wireless broadband communication systems in complex wireless signal propagation environments in cities. The spectrum efficiency in indoor propagation environment can reach 20~40 bit/s/Hz; while using traditional wireless communication technology in mobile cellular The spectral efficiency is only 1 to 5 bit/s/Hz, and it is only 10 to 12 bit/s/Hz in a point-to-point fixed microwave system. In general, the multipath of the RF signal causes fading and is therefore considered a detrimental factor. However, the results show that multipath can be used as a favorable factor for MIMO systems. As an important means to improve the data transmission rate, MIMO technology has attracted more and more attention and is considered as a revolution in the new generation of wireless communication technology.
1 3 main technologies of MIMO system
Currently, MIMO technology mainly utilizes three kinds of multi-antenna technologies such as space-time coding, spatial multiplexing and beamforming of transmit diversity to improve wireless transmission rate and quality.
1.1 Space-time coding of transmit diversity
There are two main types of space-time coding based on transmit diversity technology, namely Space Time Block Code (STBC) and Space Time Code (STTC). Although the space-time coding scheme cannot directly increase the data rate, the information is transmitted independently and uncorrelatedly through these parallel spatial channels, so that the signal obtains the diversity gain at the receiving end, which creates conditions for the data to achieve high-order modulation.
1.1.1 Space Time Block Code (STBC)
STBC jointly encodes the data stream at the transmitting end to reduce the symbol error rate caused by channel fading and noise. It increases the redundancy of the signal at the transmitting end, so that the signal obtains the diversity gain at the receiving end, and the space-time block code The same information is orthogonally encoded and transmitted from multiple antennas. The principle of the MIMO system is as shown in FIG. 1. The transmission information stream s(k) is space-time coded to form N information substreams ci(k), i=1, . . . , N. The N information substreams are transmitted by N antennas and received by M receiving antennas after the spatial channel. Multi-antenna receivers use advanced space-time coding to separate and decode these data substreams for optimal processing. In particular, the N substreams simultaneously transmit signals, each of which occupies the same frequency band and thus does not increase the bandwidth. If the channel responses between the transmitting and receiving antennas are independent and independent, the multiple input and multiple output system can create multiple parallel spatial channels.
STBC is a very simple transmit diversity technology proposed by AlamouTI in 1998. Due to its simple structure and good performance, it quickly entered the 3GPP standard. STBC essentially transmits the same information from two antennas after orthogonal coding. Because of the orthogonality, the two signals can distinguish two independent signals at the receiving end, and only need to do simple linear combination. You can get the diversity gain.
However, there are certain problems in the construction of STBC orthogonal code groups. For a real-signal constellation (PAM constellation), it can construct a space-time coding algorithm with a coding rate of 1. However, for a common complex signal constellation, such as MQAM (such as 16QAM) or MPSK (such as 8PSK), when the number of transmit antenna elements is greater than 2, whether there is a code group with a coding rate of 1 remains to be further studied. At present, for systems with a number of transmit antennas equal to 3, 4, and greater than 4, if a complex signal constellation is used, the maximum space-time coding rate can only reach 3/4 and 1/2. It can be seen that for a high-rate multi-antenna wireless communication system using high-order modulation, if the space-time block coding algorithm is directly used, it is impossible to fully utilize the effectiveness of the system. Therefore, finding a better space-time block code has become a research direction. In addition, how to make full use of the advantage of space-time block code in frequency selective channel and time selective channel is also a research topic. In summary, the current STBC is also based on a transmit diversity technique in which the number of transmit antenna elements is equal to two.
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