Multiple input/multiple output (MIMO) uses its multiple transmitters, receivers, and antennas to achieve greater link distance and reliability as well as higher data rates. So it shouldn't be much of a surprise, then, to tell you that MIMO is now an option in most of the latest wireless technologies.
Already, it's being used in 802.11g/n Wi-Fi wireless localarea networks (WLANs). It's also being picked up in some of the new WiMAX products. And there's no doubt that in the near future, we'll see it applied to Ultra-Wideband (UWB) and fourthgeneration cell phones using Long Term Evolution (LTE) and Ultra Mobile Broadband (UMB).
This hot technology isn't new, per se, but adoption has been slow because it's complex and difficult to test. Its complexity comes from the basic computationally intense MIMO reception process, as well as from the fact that MIMO usually is part of an orthogonal frequency-division multiplexing (OFDM) process.
OFDM is the modulation/multiplexing darling of wireless these days, since it's been adopted by the standards that also embrace MIMO, like LTE and UMB. While several MIMO test products are already out there, Keithley's MIMO RF Test System makes testing much faster and easier at a reasonable price.
If you aren't familiar with Keithley's name being tied to RF test, it's time to get acquainted. The company has slowly built an excellent line of RF test products, including the model 2920 vector signal generator (VSG) and the model 2820 vector signal analyzer (VSA). Add the model 2895 MIMO synchronization unit and powerful MIMO signal analysis software, and you get a complete and very capable MIMO test system.
A brief look
Most wireless applications still use single-input single-output (SISO), where one transmitter (Tx) sends a signal to a single receiver (Rx). This arrangement works fine, but at UHF and microwave frequencies, the signal is subject to the usual noise and interference as well as multipath fading. Reflections from buildings, cars, trees, people, and other obstacles cause multiple but delayed signals to arrive at the receiver, producing signal cancellation. Also, moving objects create Doppler shifts that produce fading.
Multipath interference has another negative effect called intersymbol interference (ISI). The modulated data generates symbol changes in phase and/or amplitude. If the data rate is fast enough, these symbol changes occur at a rate that can cause them to overlap if the same signal is received, but at different times over different paths.
With one symbol interfering with another, data recovery at the receiver produces errors or no usable signal. The problem is usually solved by slowing the data rate so the symbols don't overlap in the presence of multipath effects. Yet it defeats the purpose of the link because high speed is needed or desired. The way to have your cake and eat it too, so to speak, is to use MIMO.
MIMO uses multiple transmitters and receivers, with two transmitters and two receivers (2x2) being the common arrangement (Fig. 1). The data to be transmitted is divided into two parallel streams, and each is used to modulate a transmitter. The signals are also encoded, making them unique so that they can be recovered at the receiver. The two signals are transmitted on the same frequency.
At the other end of the link, each receiver picks up both signals in addition to any multipath signals. Due to the signal decoding and the multiple delayed versions of the signals, it's possible to recover each signal and reconstruct the data through mathematical algorithms.
The receivers recover all individual signals and combine them to create the final highly robust output. MIMO makes the previously undesirable multipath signals useful, greatly improving link reliability. This enables greater transmit range as well as the ability to retain a high data rate in the presence of noise and multipath effects.
The situation improves if you use even more transmitters and receivers. This increases system cost, but with today's small, low-cost transceivers, multiple transceivers are practical and affordable. Many modern WLANs use a 3x2 arrangement. But the complexity grows as the number of transceivers increases. A 4x4 MIMO will produce up to 16 signals to decode in the receiver. Thanks to powerful DSP processors, FPGAs, or ASICs, the process becomes realistic.
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