MIMO in Practice: Polarization, Data Streams and Real Wi-Fi Performance

MIMO technology in modern Wi-Fi systems is often associated with higher data rates and multiple spatial streams. In practice, real performance depends not only on the access point specification, but also on the propagation environment, antenna configuration, polarization and signal separation between streams.

The following observations and tests show why indoor and outdoor MIMO systems should not be treated in the same way.

Indoor vs Outdoor MIMO Behavior

Indoor vs outdoor MIMO: multipath-rich environments versus line-of-sight propagation and dual polarization.

Indoor Environment

In indoor environments, multipath propagation plays a dominant role. Signals are reflected by walls, ceilings, metal objects and equipment, creating multiple propagation paths.

In such conditions, omnidirectional antennas are typically used, and polarization diversity is not critical, because reflections naturally randomize polarization.

Using directional antennas indoors can reduce the number of propagation paths and limit MIMO performance.

Outdoor Environment

In outdoor installations, especially in line-of-sight conditions, the number of reflections is limited. The propagation path becomes more stable and predictable.

In these conditions, two orthogonal polarizations — vertical (V) and horizontal (H) — allow transmission of two independent data streams.

For this reason, 2x2 MIMO with V/H polarization is typically the most efficient configuration for point-to-point (PTP) and point-to-multipoint (PTMP) links.

Polarization and Stream Separation

Vertical and horizontal polarizations provide the best isolation between data streams.

Proper separation between streams should be around 18–20 dB.

Other polarization types (such as ±45°) are only variations of V/H and may provide lower isolation.

Laboratory Test Setup

To eliminate the influence of reflections and environmental conditions, a controlled test setup was used. Two access points were connected using RF coaxial cables with signal attenuators.

This configuration simulates ideal propagation conditions and allows evaluation of the real capabilities of the radio devices.

Measured Results

• 1 stream: ~320 Mbit/s
• 2 streams: ~400 Mbit/s
• 4 streams: ~424 Mbit/s

In direct RF cable connection:

• 2 streams: ~520 Mbit/s
• 4 streams: ~664 Mbit/s

Ethernet 1 Gbit/s reference: ~920 Mbit/s

Field Test Results

Outdoor tests were performed using directional MIMO V/H antennas.

To achieve high modulation (256-QAM), a signal level of approximately -61 dBm is required.

Conclusions

• Indoor MIMO depends on multipath.
• Outdoor MIMO depends on polarization separation.
• 2x2 MIMO is often the most efficient in real deployments.
• More antennas do not guarantee higher throughput.
• Real performance is always lower than theoretical values.