What Are You Missing About MIMO?

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MIMO (Multiple Input, Multiple Output) is a critical component of wireless communication standards. It’s a practical technique for sending and receiving more than one data signal on the same radio channel at the same time via multipath propagation. Multiple antenna systems, known as MIMO systems, boost data throughput without requiring additional bandwidth or transmit power.

While it may not seem like the sexiest of topics…without MIMO, many of the application’s consumers (and those of us who run test labs) take for granted today wouldn’t be possible since the speed of most Wi-Fi networks wouldn’t be able to support it.

MIMO was originally created and field tested in the late 1990s.  Yet its evolution is far from over as it has continued to evolve as wireless standards have been established. This is especially true as the rise of the Internet of Things (IoT) drives connectivity for a greater number of devices and requires additional channels and even higher speeds.

MU-MIMO Scales Up

To meet demand, MU-MIMO (Multiple-User, Multiple Input, Multiple Output) is now being introduced to increase the Wi-Fi transmissions in homes.  Current chips can now support up to four simultaneous streams and provide support for wider channels of up to 160 MHz and 80+80 MHz non-contiguous 11AC, which doubles the capacity over currently available 80 MHz channels.

Ten Critical Steps for Testing

As MIMO evolves and its use propagates, an increasing number of products will need to be tested. Here’s what you should know about MIMO when testing:

  1. Uplink and downlink of cellular systems use MIMO differently. Since you’re dealing with two different bases, antenna configurations are often different creating a typical asymmetric data throughput.
  2. The minimum amount of transmitters and receivers is two. Also keep in mind that the receivers have to be in the same place while the transmitters don’t.
  3. When analyzing signal recovery, remember it’s a two-step process. First, you must recover the channel coefficients and then separate the signals and demodulate.
  4. The demodulator’s tracking processes will remove any phase and small frequency differences, as well as time offsets.
  5. Antenna configuration has massive impact channel path correlation. The BS antenna’s angle of departure is often narrow while the MS angle of arrival is often wide. This allows narrows the range of signals that can be transmitted while increasing the range of signals that can be received.
  6. A high carrier-to-noise ratio is needed. Any signal coupling that’s not completely removed will look like one data stream is interfering with another.
  7. Precoding and Beamforming, though not necessary, can improve MIMO performance if the channel doesn’t move too fast.
  8. Using reference signals, a single-input analyzer can be used to determine cross-channel measurements.
  9. To measure short-term MIMO performance, look at the condition number.
  10. All data streams can be degraded by distortion in one component.

When evaluating equipment to use for measuring MIMO performance, it’s wise to double check that the instrumentation you are using can measure all the mentioned components.

What steps would you add to the test procedures listed above? What test equipment are you using in conjunction with signal generators are you using to test your MIMO and MU-MIMO devices?

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