Technical Overview of MIMO
For more information on the implementation of MIMO in IEEE802.11n WLAN and WiMAX explore the links below.
MIMO (Multiple-Input, Multiple-Output) is a smart antenna technology. MIMO uses multiple antennas at both the transmitter end and the receiver end to make more efficient use of the RF spectrum.
|Figure 1. Simplified representation of 2x2 MIMO radio system.|
Mathematical algorithms are used to spread the user data across the multiple transmitters. The transmitted signals are three dimensional and described in terms of time, frequency and space. This spatial multiplexing is a common transmission technique in MIMO to transmit independent and separately encoded data signals from each of the multiple transmit antennas. Therefore, the space dimension is reused, or multiplexed, more than one time. At the receiver, a special channel calibration signal at the beginning of the packet allows the different signals to be identified during the recombination process. The technique of separating out different paths in the radio link is what allows the MIMO radio to transmit multiple signals at the same time on the same frequency, and thereby improve the use of the spectrum.
Currently wireless signals transmitted via single antennas are distorted by hills, buildings, valleys and other landscape features. These alternative signal paths separated in time, multipaths, result in distortions such as fading, picketing or cliff effects. This loss of signal integrity prevents the wider adoption of wireless technology. MIMO radio works by taking advantage of the multiple paths a radio signal takes between the transmitter and receiver. The signals are now spatially diverse. Additionally the multiple paths or channels provide a greater signal capacity. This additional capacity may be used for higher data rates and data redundancy thereby improving the chances of signal recovery at the receiver.
Ultimately, the goal of MIMO is to measurably improve the spectral efficiency (bit/sec/Hz), the coverage area (cell radius) and the signal quality (bit-error rate or packet-error rate). As these goals are realized there are more applications for emerging wireless technologies; such as WLAN, Broadband Wireless Access (BWA) and cellular. These advances do come at some cost. Multiple antennas increase RF costs and complexities and mathematically complex DSP algorithms challenge the designers and manufacturers.
There are many bodies interested in the development of MIMO; the European Marquis project, the Wireless Gigabit with Advanced Multimedia Support (WIGWAM), among others. The leading MIMO adopters are WLAN IEEE 802.11n and Mobile WiMAX IEEE 802.16. Each adoption is unique but shares some common threads. WLAN IEEE 802.11n uses Orthogonal Frequency Division Multiplexing (OFDM) modulation and WiMAX uses Orthogonal Frequency Division Multiple Access (OFDMA) modulation. These modulation schemes efficiently handle the problems created by the multiple paths inherent in MIMO and provide robust signals with low signal to noise ratios.
Agilent is at the forefront of these technologies and is moving ahead producing test equipment for each wave of development. Whatever implementation you are working on, the added signal complexity requires robust testing. The successful development of a multi-channel radio will involve a combination of single- and multi-channel measurements. Agilent provides a number of tools both for two-channel signal analysis and multi-channel signal generation.
Sources: MIMO Wireless LAN PHY Layer [RF] Operation & Measurement, Agilent Technologies Application Note 1509, Agilent Webinars, Wikipedia.org, techworld.com/mobility/features
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