PWS vs CATR: 5G/6G OTA Testing Methods

Introduction : As wireless technology evolves into 5G mmWave and early 6G, the antennas inside smartphones, IoT devices and base stations are required to be tested using Over the Air (OTA) methods. However, accurately measuring an antenna’s true performance requires the device to be exposed to a “plane wave” which are electromagnetic wave fronts are flat and parallel.

In the real world, achieving a natural plane wave requires placing the device at a massive distance from the transmitter; known as the Fraunhofer distance. Because building football field sized anechoic chambers is impractical and expensive, the industry uses specialized technologies to “trick” the device into experiencing far field conditions within a compact laboratory space. The two primary methods for achieving this are CATR (Compact Antenna Test Range) and PWS (Plane Wave Synthesis).

Compact Antenna Test Range (CATR)

This method relies on the principles of microwave optics and physical geometry. In a CATR setup, a feed antenna (like a corrugated horn) transmits spherical waves toward a highly precise, parabolic reflector (often with rolled edges to prevent signal scattering). When the spherical waves hit the parabolic reflector, they bounce off and are transformed into parallel, uniform plane waves. This creates a specific 3D volume in the chamber known as the “quiet zone,” where the Device Under Test (DUT) is placed. The same is shown in the figure-1 below.

Advantages: Because CATR relies on physical geometry and passive reflectors, it can support extremely wide frequency bandwidths.

Disadvantages: While much smaller than a traditional far field chamber, a CATR still requires a relatively large footprint.

Plane Wave Synthesis (PWS)

Plane Wave Synthesis (PWS) represents a leap from optical physics to digital signal processing. Instead of using a physical parabolic mirror to flatten the wave, PWS uses a dense, active array of controlled transmitter elements. This is depicted in the following figure-2.

By digitally manipulating the amplitude and phase of the signal at every individual antenna element in the array, the system effectively synthesizes a uniform plane wave electronically. The wave fronts are perfectly aligned by the time they reach the quiet zone just a short distance away.

Advantages: PWS drastically reduces the required spatial footprint of the testing chamber; often shrinking the required space by a factor of 2 to 5 compared to CATR.

Disadvantages: PWS relies on active electronic components such as phase shifters and other beamforming circuitry. Frequency bandwidth is inherently limited by the capabilities of that specific circuitry, making it less broadband than a passive CATR reflector.

Key differences

Summary: For 5G and 6G OTA testing, the choice between CATR and PWS ultimately comes down to a trade off between measurement bandwidth and laboratory real estate. CATR provides unmatched, broadband frequency coverage through passive microwave optics, whereas PWS leverages active digital signal processing to synthesize plane waves, shrinking the required chamber footprint by up to a factor of five.