MIMO: New wireless broadband communication technology with unlimited business opportunities
With the rapid development of wireless communication technology, people have higher and higher requirements on the performance and data rate of wireless local area networks. In the future wireless broadband communication system, there are two most severe challenges: channel multipath fading and spectrum efficiency. Orthogonal frequency division multiplexing (OFDM) as the basic technology of wireless broadband communication can solve the problem of multipath fading, and can achieve the high rate that people expect without the limitation of bandwidth and power, but the bandwidth and power are limited. The wireless LAN system is not very suitable. The multiple input multiple output (MIMO) technology can generate independent parallel channels in space and simultaneously transmit multiple data streams, which effectively improves the transmission rate of the system, that is, increases the spectrum efficiency without increasing the system bandwidth. . Therefore, the combination of MIMO and OFDM will become a key technology to meet the development requirements of the next generation wireless LAN. In the next few years, the related research work of this technology will be further emphasized, especially the application research in the field of wireless LAN will become a hot spot. The two complement each other and give birth to a broad market for wireless broadband communications technology.
So what exactly is MIMO technology?
MIMO is the abbreviation of MulTIple Input MulTIple Output. As the name implies, it means multiple input and multiple output. After multiple cuts, wireless network communication data is transmitted synchronously through multiple antennas; because wireless signals are susceptible to various factors and interference during the transmission process, they will take different reflections or cross paths, so the time to reach the receiving end is inconsistent . In order to prevent data from being out of sync and unable to be recombined, the receiving end is received by multiple antennas, and the received signal is recalculated by DSP. According to the time difference, the separated data is recombined and the correct and fast data stream is output. Since the wirelessly transmitted data is transmitted by division, not only the single data traffic is reduced, the transmission distance can be extended, but also the antenna reception range is expanded. Therefore, MIMO technology can not only speed up the data transmission rate of the existing wireless network spectrum, but also do not need to occupy additional spectrum, more importantly, it can also extend the signal reception distance. From this perspective, MIMO is undoubtedly equivalent to opening up multiple lanes for wireless transmission, while ensuring high speed and high efficiency, and truly allowing wireless communication to ride on the information highway.
How does MIMO technology work?
Figure 1 briefly illustrates the MIMO operation. 
Figure 1 MIMO system operation
The MIMO system divides a data stream into several data streams; 2. Each data stream is modularly encoded; 3. It is transmitted on the same frequency channel at the same time through different RF antenna chains; 4. After multipath reflection, each A receiving antenna RF chain has a linear integration of multiple transmitted data streams; 5. The MIMO algorithm is used in the receiver to separate these data streams. The algorithm is based on all channels between each transmitter and receiver to make estimates .
Each multipath route can be viewed as a single channel that creates multiple "virtual routes", all of which can be used to carry signals. Multiple spatially separated antennas of MIMO can make full use of virtual lines created by multipath and transmit more data. In addition to doubling the transmission rate, since each receiving antenna calculates a single transmitted data stream, the transmission range can also be increased, which is also an advantage of antenna diversity.
Figure 2 shows a basic MIMO-OFDM transmitter flow. The figure shows that the main processing modules include digital (green), analog (yellow) and mixed signal (blue) functions. There are two transmitter antennas and two sets of OFDM modulator, digital-to-analog converter (DAC), analog modulator (RF front end), power amplifier (PA) and omni-patterned antenna. The dual-antenna MIMO transmitter is a digital modulator composed of two sets of analog chains (DAC and RF circuits) from the same source and omnidirectional antennas. Therefore, MIMO-OFDM transmission and two OFDM transmissions synchronized on the same channel It is exactly the same, except that different digital data is sent.
Figure 2 Basic architecture of MIMO-OFDM transmitter
As shown in Figure 2, after the information source is spurious, it is encoded in a redundant forward error correction (FEC, Forward Error CorrecTIon redundancy) manner. In order to randomize the transmission order of these coded bits, these coded bits will be placed in different antenna transmission chains, that is, consecutive coded bits are randomly sent to different OFDM modulators, each modulator Send the coded bits back to the transmission processing chain and antenna.
Comparison of MIMO and other wireless communication transmission technologies
MIMO technology is a technology that can send and receive two or more data signal streams in a single radio frequency channel. It uses a multi-dimensional transmission scheme to send and receive two or more signal streams through a single radio frequency channel. The system can achieve twice or more data transmission in a channel. In the system, more than one highly integrated RF upconverter and antenna are used to transmit these multiple signals, and there are also more than one highly integrated RF downconverter and antenna to receive these multiple signals. In general, a MIMO system is a system that uses multiple antennas to suppress channel fading. According to the number of antennas at both ends of the transceiver, MIMO can also include a SIMO (single-in, multiple-out) system and a MISO (multiple-in, single-out) system compared to the ordinary SISO (single-in, single-out) system. Using MIMO technology, the maximum data transmission rate of each channel will increase linearly with the number of different data streams transmitted in the channel.
With the ability to transmit multiple data streams simultaneously, MIMO can double the transmission capacity of wireless signals without adding additional spectrum. The peak transmission rate of the MIMO system increases with the number of data streams transmitted in the radio frequency microwave channel. Because multiple signals are transmitted in different antennas and channels, MIMO signals are sometimes referred to as "multi-dimensional signals" (mulTI-dimensional signals)
signal).
In addition to doubling the signal rate in the same channel, a properly designed MIMO system can also increase coverage and stability through high spectral efficiency and higher remote transmission rate (throughput-at-range). The MIMO system improves the "effective throughput" (transmission rate measured at a specific distance from the transmitter) compared to the "peak throughput" (peak throughput, which is very close to the transmitter The measured transmission rate is better. The independent test shows that a well-designed WLAN MIMO system can increase the effective coverage by eight times, and also increase the effective transmission rate by six times.
Some other multi-antenna transmission and reception technologies are often confused with MIMO technologies. These technologies include transmit beamforming (transmitter
Beam-forming is sometimes referred to as "smart antennas" and receiver diversity. Although this type of technology can improve the coverage of general traditional one-dimensional signals and is also very suitable for specific applications such as outdoor point-to-point connections (wireless backhaul), they still cannot achieve the effect of doubling the transmission capacity of a true MIMO system.
Figure 3 shows the dual antenna beamforming (smart antenna) system of the digital signal processing part of the transmitter. The information data is encoded and inserted into the OFDM carrier. In this case, the interleaver (Interleaver) is not the encoded bit. It is sent to different antennas, but only to different frequencies. This transmitter uses only one OFDM modulator. The waveform is adjusted according to the amplitude and phase of a single antenna, and then divided into w (1) and w (2) and sent to the antenna. Each OFDM may have different phase and amplitude values.
Figure 3 Basic architecture of OFDM transmit beamforming
Although this approach has its advantages for specific applications such as outdoor point-to-point wireless connections, it does not increase the overall transmission rate of the wireless network, nor does it provide a reliable environment for multi-user wireless networks in homes and offices.
Although beamforming can also provide a wider transmission range for certain applications, some of its serious disadvantages cannot be ignored, such as some hidden nodes; the number of terminal devices that can be supported will be reduced; and its high power consumption restrictions Limit the number of RF transmitters. Let's take a look at the antenna diversity scheme again. In order to receive the strongest signal and improve reliability, the receiving antenna switches between several diversity antennas, but because there is no additional signal processing, the signal quality has not improved . The situation of receiving integration technology is also very similar. Although it can receive signals through multiple antennas, this technology cannot increase the data transmission rate or transmission capacity due to the effects of signal attenuation and multipath reflection.
Other technologies that may be confused with MIMO include data compression and radio channel integration
combining). In most network applications, data compression technology has not actually increased the data transmission rate; channel integration technology has encountered the problem that the frequency band is not open and cannot be used legally in many countries (such as Japan). Another problem with channel integration technology is that it may interfere with other network devices in the same network, causing a negative impact on neighboring wireless networks.
Unlike the channel integration technology, MIMO does not need to increase the number of radio frequency channels to achieve a higher data transmission rate; not only that, MIMO has downward compatibility and interoperability with other devices, and will not Of your network is causing interference.
MIMO market application prospects
For the wireless communication field, people have been looking for higher transmission rates, wider coverage and more reliable solutions. MIMO provides an innovative solution to such needs without using more spectrum. . Based on such technological advantages, many manufacturers have devoted themselves to the research and development of wireless network products based on MIMO technology and the research of a new generation of mobile communication systems, which has enabled MIMO technology to be strongly promoted.
As early as August 2003, AirgoNetworks launched the AGN100 Wi-Fi chipset, and called it the world's first mass-market product with integrated MIMO technology. AGN100 uses the company's multi-antenna transmission and reception technology to increase the Wi-Fi rate to 108Mbps per channel, while maintaining compatibility with all commonly used Wi-Fi standards, using three 5GHz and three 2.4GHz antennas. The chip supports all 802.11a, b and g modes, including the latest standards introduced by the IEEE802.11 working group (including TGi security and TGe quality service functions). Wireless devices that use the Airgo chipset can communicate with previous 802.11 devices. They can even communicate with Airgo devices at 108 Mbps while communicating with 802.11a devices at 54 Mbps. With its excellent performance in improving system spectrum utilization, MIMO technology has attracted much attention in the development of mobile communication technology. At present, MIMO chips are developing very quickly. Samsung Electronics also announced that it will launch two notebooks with built-in MIMO. This is the first time MIMO is applied to notebook computers, which once again proves the high acceptance and growth of MIMO in the market.
MIMO can improve the throughput, transmission distance and reliability of WLAN, making it a key technology in the field of wireless communication. With the advantages of MIMO technology, WLAN can be used to transmit multimedia information that does not tolerate delay and requires a lot of bandwidth, such as HDTV wireless real-time propagation. It also provides more reliable and higher-speed transmission in a larger coverage area for enterprises or homes, and allows the ever-increasing network connection speed to fully exert its benefits. Users no longer have to complain about slow network connections, insufficient coverage, or unreliable connections. In 3GPP's high-speed downlink packet access scheme, an antenna system based on MIMO technology is proposed. This system has multiple antennas on the sending and receiving sides, which can be considered as a further expansion of dual antenna diversity. However, MIMO also introduces a code-reuse (Codere-use) method, which uses the same channel coding and scrambling code to modulate multiple different data streams, making full use of mobile communication space resources. Currently, companies such as Lucent, Panasonic and NTTDoCoMo are actively advocating the application of MIMO antenna system technology. In recent years, many organizations are studying space-time coding technology based on MIMO antenna systems, in order to reduce the complexity of the equipment, improve processing capacity, and move toward the development direction of maximizing the use of space resources.
MIMO has been used to define IEEE
The basis of the 802.11n standard. In the future, MIMO will be widely used in Wi-Fi devices, from entertainment systems in the home to multimedia servers, handheld computers or VoIP phones and other devices can be found MIMO. Not only that, MIMO's advantages in spectrum efficiency and performance make it also very suitable for being used in the wide-area wireless mobile phone application market.
With the completion of the IEEE 802.11n standard and formal approval, the Wi-Fi Alliance has begun to conduct interconnectivity certification for this standard. The MIMO system will eventually become a powerful and fully compatible technical extension of today's 802.11a / b / g standard, and can retain the full functionality of the existing Wi-Fi system, because it can provide users with the best wireless transmission solutions on the market The solution is widely used, which also provides unlimited business opportunities for equipment manufacturers!
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