5G OTA Fundamentals & Key Concepts Influencing OTA Test Chamber and Measurement Decisions
This blog will cover some of the key concepts involved in 5G OTA test and measurement decisions. This will include the dimension of the active radiating antenna module, far field region, quiet zone, link budget calculations and OTA setup calibration. Each of these elements is critical in ensuring accurate and reliable 5G OTA measurements.
Dimension of the Device – The dimension of the mmWave antenna module largely dictates the choice of the OTA test chamber, as it an important factor in determining the far field distance. As per the Fraunhofer equation (below), the larger the antenna aperture the larger the far field distance and the overall chamber footprint.
‘R’ is the far field distance at which the spherical waves appear planar
‘D’ is the dimension of the antenna module
‘ƛ’ is the wavelength
3GPP defines three different device categories based on the antenna configurations. The first category includes devices with only one antenna panel with a dimension within 5 centimeters. The second category includes devices with more than one antenna element, each with a dimension of less than 5 centimeters, and no phase coherence, meaning the antenna panels could be considered completely independent. The third category of devices consists of multiple antenna elements, exhibiting a certain level of phase coherency, necessitating the dimensions of both panels be considered when calculating effective antenna aperture.
DUT Positioning – Once the dimension of the device is determined or the antenna aperture is known, the next consideration is to determine how far the device-under-test (DUT) must be positioned from the measurement antenna, which depends on the behavior of the electromagnetic field in the region surrounding the antenna.
The region surrounding the antenna can be divided into three broad categories.
- Reactive near field – it is the region closest to the antenna, dominated by the stored energy which heavily influences the radiation pattern making it unsuitable to achieve accurate measurements.
- Radiated near field – slightly away from the antenna, the region, though uninfluenced by the reactive electrical components, still poses challenges in making accurate measurements as the beam pattern may get influenced by the physical objects inside the chamber.
- Far field – is the region most suitable for performing mmWave measurements, as the spherical waves propagating out or away from the antenna are considered planar in nature with E & H fields completely orthogonal to each other and to the direction of propagation. The beam pattern in this region is far more stable and predictable, ensuring repeatable and reliable measurements.
Quiet Zone – A volume within the chamber where electromagnetic waves are reflected to the minimum. It is a region of low amplitude and phase variations. Usually, the communication between the measurement horn antenna and the DUT is line of sight (LOS), however in reality, electromagnetic reflections from the walls cause interference with the main LOS wave front. Hence to minimize the amplitude of the reflected wave fronts, RF absorbers are used around the entire perimeter of the 5G OTA chamber to ensure a quality quiet zone.
Link Budget Calculations – This is one of the most critical parameters in ensuring accuracy of measurements. A 5G OTA setup comprises of many elements, including the test instrument, RF cables, connectors, horn antenna, adapters, switches, fixtures, etc., with each element adding a certain level of loss or gain to the signal under measurement. Since mmWave frequencies are quite prone to OTA path loss it becomes crucial to take into account the composites loss or gain of the entire RF transmitter and receiver chain to calculate the actual radiated power and phase generated at the transmitting end, as well as the signal that is measured at the receiving end. This helps ensure enough power is radiated by the transmitter to meet the sensitivity requirements of the receiver.
OTA Setup Calibration – The calibration process is extremely important in ensuring that none of the elements, other than the DUT, in the transmit and receive chain add any level of inaccuracy. The calibration process takes into account the composite loss in the transmit and the receiver chain and aids in measurement accuracy and repeatability. A simple way to perform calibration is by using a known source for signal generation, a well calibrated reference horn antenna with known gain values, a reference cable with known loss and a well calibrated signal analyzer to measure the signal received after loss. The signal received at the analyzer is subtracted from the signal generated to determine the path loss. These offset values are calculated at multiple different frequencies and across both horizontal and vertical polarizations and stored in a table for use when making actual measurements.