Multi-input multi-output (MIMO) or multi-input multi-output antenna (MTMRA) technology is a major breakthrough of smart antenna technology in the field of wireless mobile communication. This technology can double the capacity and spectrum utilization rate of the communication system without increasing the bandwidth, and it is a key technology that must be adopted in the new generation of mobile communication systems.
So what exactly is MIMO technology?
In fact, MIMO technology has a long history. As early as 1908, Marconi proposed to use it to resist fading. In 1970s, MIMO technology was proposed to be used in communication system, but the basic work to greatly promote the application of MIMO technology in wireless mobile communication system was completed by scholars in at&T Bell Laboratories. 1995 Teladar gives the MIMO capacity under fading conditions; 1996, Foshinia proposed a multi-input multi-output processing algorithm-diagonal-Bell Laboratories layered space-time (D-BLAST) algorithm; 1998 Tarokh et al. discussed the space-time codes of MIMO. 1998, Wolniansky et al. established a MIMO experimental system by using the vertical Bell Laboratories layered space-time (V-BLAST) algorithm, and achieved a spectrum utilization rate of more than 20 bit/s/Hz in indoor experiments, which is extremely difficult to achieve in ordinary systems. These works have aroused great concern of scholars all over the world, and made the research work of multi-input and multi-output develop rapidly.
In a word, MIMO systems use multiple antennas to suppress channel fading. According to the number of antennas at the transmitter and receiver, MIMO can also include SIMO (single input and multiple output) system and MISO (multiple input and single output) system compared with ordinary SISO (single input and single output) system.
The concept of MIMO
Usually, multipath will cause fading, so it is regarded as a harmful factor. However, the research results show that multipath can be used as a favorable factor for MIMO system. MIMO system adopts multi-antenna (or array antenna) and multi-channel at both transmitter and receiver, and MIMO is aimed at multi-path wireless channel. Figure 1 is the schematic diagram of MIMO system. The transmission information stream s(k) is space-time coded to form n information substreams ci(k), I= 1, ... These n substreams are transmitted by n antennas and received by m receiving antennas after passing through the spatial channel. The multi-antenna receiver can separate and decode these data substreams by using advanced space-time coding processing, thus achieving optimal processing.
Especially, these N substreams are sent to the channel at the same time, and each transmitted signal occupies the same frequency band, so the bandwidth is not increased. If the channel responses between transmitting and receiving antennas are independent, MIMO systems can create multiple parallel spatial channels. By transmitting information independently through these parallel spatial channels, the data rate can certainly be improved.
MIMO optimizes multipath wireless channel, transmission and reception as a whole, thus achieving high communication capacity and spectrum utilization. This is a near-optimal spatio-temporal joint diversity and interference cancellation processing.
System capacity is one of the most important symbols of communication system, which indicates the maximum transmission rate of communication system. For MIMO systems with n transmit antennas and m receive antennas, assuming that the channel is an independent Rayleigh fading channel and both N and M are large, the channel capacity C is about: C=[min(M, N)]Blog2(ρ/2).
Where b is the signal bandwidth, ρ is the average signal-to-noise ratio at the receiving end, and min(M, n) is the smaller of m and n. The above formula shows that when the power and bandwidth are constant, the maximum capacity or the upper limit of capacity of MIMO system increases linearly with the increase of the minimum number of antennas. Under the same conditions, the capacity of an ordinary smart antenna system with multiple antennas or antenna arrays at the receiving end or the transmitting end only increases with the logarithm of the number of antennas. Relatively speaking, MIMO has great potential for improving the capacity of wireless communication systems.
It can be seen that the channel capacity at this time increases linearly with the increase of the number of antennas. That is to say, the capacity of wireless channel can be improved exponentially by using MIMO channel, and the spectrum utilization can be improved exponentially without increasing bandwidth and antenna transmission power. Using MIMO technology can improve the capacity of the channel, improve the reliability of the channel and reduce the bit error rate. At present, another research hotspot in the field of MIMO technology is space-time coding. Common space-time codes include space-time block codes and space-time trellis codes. The main idea of space-time code is to realize certain spatial diversity and time diversity by using space-time code, so as to reduce the channel error rate.
Research status of MIMO
At present, scholars all over the world are extensively studying the theory, performance, algorithm and implementation of MIMO. There are many literatures about the theory and performance research of MIMO system, and the contents involved are also very extensive. However, because the MIMO channel of wireless mobile communication is a time-varying and non-stationary MIMO system, there are still many problems to be studied. For example, most literatures assume that the channel is a piecewise constant fading channel. This is not enough for broadband signal 4G system and outdoor fast moving system, and complex models must be used for research. There has been a lot of work in this field, that is, the channel is frequency selective fading and the mobile station moves fast. In addition, in the basic literature, it is assumed that the receiver knows the multipath channel parameters accurately, so it is necessary to send training sequences to train the receiver. However, if the mobile station moves too fast and the training time is too short, then fast channel estimation or blind processing becomes an important research content.
In addition, the experimental system is an important step in the research of MIMO technology. An important problem in practical system research is to realize multi-antenna and multi-channel reception in mobile terminals, and scholars are making great efforts in this regard. Because mobile terminal equipment requires small size, light weight and low power consumption, there is still a lot of work to be done. At present, major companies are developing experimental systems.
The BLAST system of Bell Laboratories [4] is the earliest MIMO experimental system. The working frequency of the system is 1.9 GHz, with 8 antennas for transmitting and 12 antennas for receiving, and D-BLAST algorithm is adopted. The spectrum utilization rate reaches 25.9 bit/(Hz? S). However, the system only studies narrowband signals and indoor environment, and there is still a long way to go in 3G and 4G applications. Setting multiple antennas at transmitter and receiver can provide spatial diversity effect and overcome the adverse effects of radio wave fading. This is because properly arranged multiple antennas provide multiple spatial channels, and all these antennas will not fade at the same time. In the above specific experimental system, each base station is equipped with two transmitting antennas and three receiving antennas, and each user terminal is equipped with 1 transmitting antennas and three receiving antennas, that is, the downlink path is equipped with 2×3 antennas, and the uplink path is equipped with 1×3 antennas. Compared with SISO, the transmission benefit is 10 ~ 20 dB, and the system capacity increases accordingly. Moreover, the two transmitting antennas of the base station can be used to transmit different data signals when necessary, and the data rate transmitted by users can be doubled.
The layered space-time (BLAST) technology of Lucent Bell Laboratories is the leading MIMO application technology in the field of mobile communication and the further development of its smart antenna. BLAST technology, as far as its principle is concerned, is to use the unique "spatial identification" of signals on each pair of transmitting and receiving antennas to "recover" signals at the receiving end. BLAST technology is like establishing multiple non-interfering and parallel subchannels in the original frequency band, and using advanced multi-user detection technology to transmit user data accurately and efficiently, which will greatly improve the forward and reverse link capacity. BLAST technology proves that using multi-antenna arrays at the transmitter and receiver of antennas can make full use of multipath propagation, achieve the effect of "turning waste into treasure" and improve the system capacity. Theoretical research proves that the spectrum efficiency of the system can increase linearly with the number of antennas by using BLAST technology, that is to say, as long as the number of antennas is allowed to increase, the system capacity can be continuously improved. This also fully proves the great potential of BLAST technology. In view of its outstanding contribution to wireless communication theory, BLAST technology won the Thomas Edison Invention Award in 2002. From June, 5438 to October, 2002 10, the world's first BLAST chip appeared in Bell Laboratories of Lucent, and the design team of Bell Laboratories announced the launch of the industry's first chip combining Bell Laboratories' BLAST) MIMO technology. The chip supports the maximum antenna layout of 4×4, and the highest data rate it can handle reaches 19.2Mbps. This technology is used in mobile communication, and the BLAST chip enables the terminal to receive the data of 19.2 Mbps in the 3G mobile network. Now, Lucent Technology has begun to apply this BLAST chip to its Flexent OneBTS series base stations, and plans to authorize terminal manufacturers to use this BLAST chip to improve the ability of wireless 3G data terminals to support high-speed data access.
In August, 2003, Airgo Networks launched the AGN 100 Wi-Fi chipset, and claimed that it was the first mass market product integrating MIMO technology in the world. AGN 100 uses the company's multi-antenna transmitting and receiving technology to increase the current Wi-Fi rate to 108Mbps per channel, while maintaining compatibility with all commonly used Wi-Fi standards. The product integrates two chips, including a baseband /MAC chip (AGN 100BB) and a radio frequency chip (AGN 100RF), and adopts an extensible structure, so manufacturers can use only one radio frequency chip to realize a single antenna system, or add other radio frequency chips to improve performance. The chip supports all 802. 1 1 a, B and G modes, including the latest standards (including TGi security and TGe quality service function) introduced by the IEEE 802. 1 1 working group. Airgo's chipset is compatible with the current Wi-Fi standard, supports 802. 1 1a, "b" and "g" modes, and uses three 5-GHz and three 2.4 GHz antennas. Wireless devices using the Airgo chipset can communicate with previous 802. 1 1 devices, even in.