Co-polarized Transmission

Description

Co-polarized transmission refers to a specific configuration in over-the-air (OTA) testing of user equipment (UE) or base station (BS) where the polarization of the device under test (DUT) transmission antenna is aligned with the polarization of the test antenna. This alignment ensures that the received signal power is maximized, as the polarization mismatch loss is minimized. In 3GPP specifications, such as TS 38.101 (NR UE radio transmission and reception) and TS 38.521 (NR UE conformance testing), co-polarized transmission is one of the test conditions used to evaluate the radio frequency (RF) performance of devices, particularly in millimeter wave (mmWave) bands where polarization effects are significant.

The concept is rooted in antenna theory: electromagnetic waves have a polarization state, typically linear (vertical or horizontal), circular, or elliptical. When the transmitting and receiving antennas have the same polarization, the signal is received with maximum efficiency. Conversely, cross-polarized transmission (orthogonal polarization) results in significant signal loss, which can be used to test diversity and MIMO performance. In co-polarized transmission, the test setup ensures that the DUT and test antenna are aligned in polarization, often by physically rotating the DUT or using a polarization-controlled test antenna.

In the context of 5G NR, co-polarized transmission is particularly important for evaluating the performance of phased array antennas and beamforming. The DUT may have multiple antenna elements with different polarizations; the test procedure specifies which polarization to use for transmission. The test antenna, typically a dual-polarized horn or array, can be configured to match the DUT's polarization. Measurements such as total radiated power (TRP) and effective isotropic radiated power (EIRP) are conducted under co-polarized conditions to ensure accurate characterization.

The introduction of co-polarized transmission in Release 19 reflects the maturation of 5G mmWave testing and the need for standardized methods to assess polarization performance. It is part of a broader set of test conditions including cross-polarized transmission, which together provide a comprehensive evaluation of a device's antenna system. These tests are crucial for ensuring that devices meet regulatory requirements and operate reliably in diverse propagation environments.

Overall, co-polarized transmission is a fundamental concept in OTA testing, enabling repeatable and accurate measurements of RF performance. It helps manufacturers and operators verify that devices can achieve the required data rates and coverage in real-world scenarios.

Purpose & Motivation

The co-polarized transmission test condition exists to provide a standardized and repeatable method for evaluating the RF performance of devices under matched polarization conditions. In mmWave and massive MIMO systems, antenna polarization plays a critical role in link budget and overall system performance. Without a defined co-polarized condition, test results could vary significantly due to polarization mismatch, leading to inconsistent device certification and deployment issues.

Historically, conducted testing (via cables) was sufficient for sub-6 GHz frequencies, but mmWave frequencies require OTA testing due to the integrated nature of antenna arrays. 3GPP thus developed OTA test methodologies, and co-polarized transmission is one of the key conditions to ensure that devices can achieve maximum signal strength when polarization is aligned. This is essential for verifying that a device can meet peak throughput and coverage requirements.

The motivation also includes the need to test beamforming and MIMO capabilities, where polarization diversity is often used. By defining co-polarized and cross-polarized conditions, 3GPP enables a thorough assessment of a device's antenna system, including its ability to handle different polarization states. This helps in identifying design flaws and ensuring interoperability with network infrastructure.