MIMO OFDM will become a key technology in broadband wireless field[Copy link]
In the future broadband wireless communication system, there are two most severe challenges: multipath fading channel and bandwidth efficiency. OFDM reduces the impact of multipath fading by converting frequency-selective multipath fading channel into a flat channel in the frequency domain. MIMO technology can generate independent parallel channels in space to transmit multiple data streams at the same time, which effectively improves the transmission rate of the system, that is, increases the spectrum efficiency without increasing the system bandwidth. In this way, combining OFDM and MIMO technologies can achieve two effects: one is to achieve a very high transmission rate, and the other is to achieve strong reliability through diversity. At the same time, adding appropriate digital signal processing algorithms to MIMO OFDM can better enhance the stability of the system.
Recently, various signs indicate that "wireless + broadband" has become an important selling point for future wireless communications. MIMO OFDM technology uses array antennas in OFDM transmission systems to achieve spatial diversity, and uses three diversity technologies of time, frequency and space to greatly increase the tolerance of wireless systems to noise, interference and multipath.
In order to further improve the system transmission rate, the wireless communication network using OFDM technology needs to increase the number of carriers, which will increase the complexity of the system and increase the bandwidth of the system, which is not suitable for today's bandwidth-limited and power- limited wireless communication network systems. MIMO technology can multiply the capacity and spectrum utilization of the communication system without increasing the bandwidth. Therefore, combining MIMO technology with OFDM technology is a trend that meets the development requirements of the next generation of wireless LAN. The equipment
currently under developmentconsists of two groups of IEEE802.11a transceivers, two transmitting antennas and two receiving antennas (2×2), and a MIMO OFDM system responsible for computing and processing, which can achieve a maximum transmission rate of 108Mbit/s. The transmission rate between the AP and the client is 108Mbit/s. When the client does not support this technology (in the case of IEEE802.11a clients), the transmission rate is 54Mbit/s. The next generation wireless LAN standard 802.11n uses MIMO OFDM technology, with a transmission rate of up to 320Mbps and a net transmission rate of 108Mbps. For a long time, multipath interference has always been a difficult problem to solve. The general method is to eliminate interference or turn harm into benefit. The former is to try to separate the strongest useful signal and eliminate the interference signal from other paths. This is the basic idea of using diversity technology. OFDM can effectively combat multipath propagation, so that the interfered signal can be received reliably. OFDM has a low code rate and adds a time protection interval, which has a strong ability to resist multipath interference. Its multipath delay is less than the protection interval, so the system is not troubled by inter-code interference. However, for high-speed wireless communication, a simple OFDM system is not enough to combat multipath fading in the wireless environment. It must be combined with MIMO technology to better play its role. Wireless signals propagate in complex wireless channels and produce Rayleigh fading, and their fading characteristics are different at different spatial locations. If the distance between two locations is greater than the correlation distance between antennas (usually more than ten signal wavelengths apart), it is considered that the signals at the two locations are completely unrelated, so that signal spatial diversity reception can be achieved. MIMO technology eliminates channel fading in wireless transmission through spatial diversity. The key to MIMO OFDM technology is that it can transform the multipath influencing factors existing in traditional communication systems intoenhancement factors that are beneficial to user communication performance through spatial diversity. At the same time, adding appropriate digital signal processing algorithms to the MIMO OFDM system can better enhance the stability of the system. At present, analog cellular mobile communications in various countries have made great progress, but they still cannot meet the needs. Solving the capacity problem of mobile communications has become a top priority. The use of MIMO OFDM technology is a very effective means. The combination of MIMO technology and OFDM technology is a major breakthrough in smart antenna technology in the field of wireless communications. MIMO technology can multiply the capacity and spectrum utilization of the communication system without increasing the bandwidth; and OFDM technology is generally considered to be a key technology that must be adopted by the new generation of wireless communication systems. MIMO OFDM technology can provide spatial multiplexing gain for the system, thereby greatly increasing the channel capacity. The spatial multiplexing of MIMO technology is to use multiple antennas at the receiving and transmitting ends, make full use of the multipath components in spatial propagation, and use multiple data channels (MIMO sub-channels) in the same frequency band to transmit signals, so that the capacity increases linearly with the increase in the number of antennas. This increase in channel capacity does not occupy additional bandwidth or consume additional transmission power, so it is a very effective means to increase channel and system capacity. MIMO technology can utilize the multipath components in propagation to a certain extent, that is, MIMO can resist multipath fading, but MIMO technology is still powerless against frequency selective deep fading. The current solution to frequency selective fading in MIMO technology can be combined with OFDM technology to convert frequency selective fading into flat fading on subcarriers. In addition, OFDM technology is the core technology of 4G, and OFDM has limited effect in improving spectrum utilization. Reasonable development of space resources on the basis of OFDM, that is, MIMO + OFDM, can provide reliable data transmission rate. Countries around the world and major telecommunications manufacturers have now carried out research on the next generation of mobile communication systems. The combination of MIMO and OFDM has great potential in improving the transmission rate and reliability of wireless links , and will become a key technology in the future broadband wireless field.
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变色卡
千斤顶