Wireless network Latest technology: multiple input multiple output

The multiple input multiple output (Multiple-input, Multiple-output;mimo) technology provides an excellent way to extend the range of the Wireless area Network (WLAN), and has recently become the focus. MIMO technology began in 1985, but only now applies to wafer-level devices to dramatically improve transmission range and capacity.

Since MIMO is not a single concept, but consists of a variety of wireless RF technology, so we must fully understand the operation and performance of MIMO. When applied to WLANs, some MIMO technologies can be compatible with current WLAN standards (such as 802.11a, 802.11b and 802.11g), thus extending their transmission range; Conversely, some MIMO technologies can only be used for MIMO devices that are incompatible with the standard WLAN standards.

The term "MIMO" generally refers to any system that has multiple inputs in the Transporter section and has multiple outputs in the receiver part (Fig. 1) that describes its basic concepts. According to a 1965 observation by the MIMO improver Jack Salz, while the MIMO system may contain wired-linked devices, the entire system is typically a wireless system, such as the Code division used in multiple antenna systems, 3G mobile phone systems (wireless systems) multiple Access (CDMA) systems, even DSL systems (wired systems) that generate crosstalk (crosstalk) using multiple phone lines.

This paper will first introduce the history background and operation of MIMO, and discuss the extent to which MIMO can extend WLAN transmission range.

The development process of MIMO 20 years

The origin of MIMO in wireless dating dates back to 1984, when Jack Winters, who served at Bell Laboratories, had a breakthrough in development. The MIMO pioneer has used multiple antennas from the transmitter and receiver to transmit data from multiple users to the same frequency/time channel. (Computer science)

In 1985, Bell Laboratories's Jack Salz published a study on the application of MIMO in Wired communications, after which a number of academics and researchers were involved in the field of MIMO and published research reports. It is noteworthy that Jack Winters expanded the Salz report in 1985 to the Wireless communications field, proving that users can wirelessly transmit multiple streams simultaneously through the transmitter and receiver's multiple antennas.

To meet strong market demand, many WLAN, Wi-max and mobile communications companies have provided (or are planning to provide) solutions based on MIMO technology. MIMO has a number of functions, such as transmission beamforming (transmit beamforming)/maximum ratio combination (maximal Ratio COMBINING;MRC), spatial multi-work (spatial multiplexing) and space-time coding ( Space-Time coding).

Leading manufacturers in the wireless market, such as Intel, Cisco, Qualcomm, Samsung, Mitsubishi, Panasonic, Philips, Toshiba, Sony, and Atheros work together to Developed a number of mutually operational and scalable WLAN MIMO technology. These leading vendors will work together to bring a widely accepted standard solution to consumers.

MIMO Technology Advantages

Why is MIMO so attractive? By increasing the number of transmitters and receivers, MIMO can improve the data transmission rate and provide greater coverage and transmission range. However, MIMO is not an all-in-one (one-size-fits-all) technology. MIMO can increase data transmission rate, but also depends on whether the design engineer will apply. How to choose the right MIMO tool depends on the operating environment and the type of system used.

Over the past 20 years, many MIMO technologies and applications have been fairly successful, including:

• Transmission beamforming/maximum ratio combination (transmit beamforming/maximal ratio combining)

• Space multi-worker (spatial multiplexing)

• Space-Time coding (space-time coding)

Double transmission efficiency

Transmission beamforming (Fig. 2) is the technology of a transmitter that transmits data (VanVeen88, Spencer04) between the transmitter and the receiver in the most powerful path. Through the precise operation method, the transmitter can drive multiple antennas, so it can transmit most of the RF power to the receiver. Therefore, the whole device can achieve the highest transmission efficiency.

In performing the transmission beamforming, two or more power amplifiers and antennas need to be used, with the phase shift method (Phase-shifting algorithm) controlled to centralize the transmission of radio frequency to the receiver. In this way, the amplitude of the effective transmission power (effective transmit) is the square of the number of transmitted antennas, for example, if there are two transmission RF, the effective transmission efficiency will be four times times.

To achieve the above efficiency growth, there are two main factors: power elevation and array elevation. Power elevation is the transmission of multiple transmission antennas in the air data, as the number of antennas increased, the overall power also increased. The array elevation is because the power being transmitted is concentrated in the same direction, thus reducing the power wasted in other directions. If two antennas are used for transmission, the total power transmitted to the receiver is increased by 1 time times. Therefore, when combined with these two factors, the effective transmission power will be able to increase the net 4 times times.

The direction concentration process of transmitting beam can be adjusted according to the frequency characteristic of the transmission channel. As shown in (Figure 3), if the transmitting beam forming system receives at least one packet from the receiving device, the system can master the Channel response (either at the transmitting or receiving end or the same channel response) to adjust the phase of the transmitted signal so that the receiver antenna receives the signal, Can be based on the frequency of each group to make meaningful integration of data. This approach can reduce the effect of multiple paths, even when transmitting data to a non-MIMO device.

The MIMO transmitter can greatly enlarge the transmission range, so that the farther receiver can receive high frequency signal. Therefore, MIMO can provide better coverage for a large area of the home or office environment. MIMO transmitters allow users to still receive signals within the range limit, making WLAN settings easier.

Double reception Efficiency

The highest ratio combination (MRC) is a receiver technology that integrates signals from different antennas uniformly (see Figure 4). MRC improves the signal noise ratio (signal-to-noise Ratio;snr) of the receiving signal, and the improvement is proportional to the number of antennas used. Multiple receivers not only increase the receiving power, but also reduce the effect of multiple channels by integrating the receiving signals of each frequency component individually. This process, known as the "subcarrier maximum ratio combination" (subcarrier-based maximal ratio combining), can greatly enhance the overall gain, especially in multiple path environments. As indicated in the previous (Figure 3), in a multipath environment, the signal is reflected through multiple objects, causing two antennas to receive signals of different characteristics, or one of the antennas is unclear about the frequency of the signals.

MRC integrates the frequency signals received by the antenna, thereby increasing the power of the signal. If the signal is of a similar intensity, the receiver will selectively combine its signal strength, so that even if only two antennas are used, the frequency power can be increased by one fold.

Receive Combining cannot be confused with antenna diversity (antenna diversity). The antenna diversity does not select signal elements based on the intensity of different signals in different frequencies or from the intensity of the signal enhanced by using two antennas. A receiver with an antenna diversity chooses only the antenna that provides the best performance, while the second antenna is not. Although this technique does have its advantages over the absence of any antenna diversity, it still does not reduce the interference of multiple channels or improve signal quality.

It is noteworthy that even if the transmitter does not adopt the MIMO technology, it can still enjoy the benefits of the MCR function in the receiver. In the existing devices, such as hotspot access points, home network gateway, desktop and notebook computers, and so on, its transmitters are fully compatible with this type of MIMO technology. This technique can also be applied to the 2.4 and 5GHz spectrum to improve the performance of all 802.11a and 802.11g standard devices. Unlike other technologies, the use of receive combining technology at one end of a wireless connection can improve overall performance.

Increase the efficiency of transmission and reception

When combined with the transmission beam forming with MRC and using multiple transmit and receive antennas, it becomes a MIMO system (see Figure 5). The transmission beam forming/MRC can greatly improve the system performance. More importantly, such a combination can be fully integrated with all terminal devices to ensure interoperability and reliability of systems using these technologies, such as wireless regional networks. Since the use of MIMO applications in 802.11 has not yet been standardized, it is currently able to provide reliable and stable pre-standard solutions, only those that use the/MRC technology of transmission beamforming, such as Atheros and its OEM customers currently offer VLocity MIMO Solutions.

The industry has now developed an independent technology called spatial multiplexing (see Figure 6), which transmits two or more independent streams of data between the transmitter and the receiver, thereby increasing the data transfer rate. This technology has been developed since 1987. Because this technology uses the multiple independent data stream of the stream, and the traditional system design can only accept a single data flow, so the traditional system has a problem that cannot decode the signal.

To use space for multiple workers, you must have a known boot (preamble) or training sequence (training sequence) that allows the receiver to learn how to separate overlapping data streams. In addition, most systems integrate the response of the receiver into the transmitter as a reference for choosing the appropriate mode of operation. This is why the WLAN Pre-standard solution using a space multi-worker may not be compatible with the 802.11N standard expected to be launched in early 2006. We will explain further in the next paragraph. In addition, according to the study, "MOBPIPE04" will not work with each other when they are communicated in a standard-compatible non-MIMO mode. Even without these problems, the existing WLAN system with 802.11A/B/11G Standard is designed for the non-spatial and multi-working standards, so the use of space and multiple workers can not improve performance.

Spatio-Temporal coding (space-time coding) is the technique of using different paths between transmission and receiver without the knowledge of the transmitter. When the transmitter is designed with the receiver, space-time coding is a relatively easy to implement mechanism. In addition, space-time coding is the technology used by the 3G mobile phone system. However, because this technique does not need to understand the signal path, its data transmission rate is lower than that of the/MRC and Space division of the transmission beam. Space-Time coding usually requires multiple transmission antennas, while the receiving antenna is one or more.

Over the past 20 years, these MIMO derivative technologies, whether individual or integrated, have significantly improved the effectiveness of wireless and cable systems. For example, mobile phone operators actively promote space-time coding, while radar solutions and fixed wireless systems are widely used in transmission beamforming.

802.11N will define MIMO for future WLAN

As WLAN is widely used globally, consumers and businesses are increasingly reliant on the operation of the network, carrying out important business activities or handling sensitive personal matters.