Over-The-Air Active Antenna System Base Station

Description

OTA AAS BS (Over-The-Air Active Antenna System Base Station) is a category of base station defined by 3GPP that integrates an active antenna system (AAS) and is tested over the air (OTA) rather than through conducted RF interfaces. It was introduced in Release 15 as part of the ongoing evolution of base station specifications to support advanced antenna technologies, particularly for 5G NR and enhanced LTE. The term specifically applies to AAS base stations that have a certain minimum number of transceiver units (TUs) per cell: at least 8 TUs for E-UTRA or MSR (Multi-Standard Radio) and at least 4 TUs for UTRA. These base stations have only a radiated RF interface, meaning they do not provide conducted RF ports for testing; instead, their RF characteristics are assessed via OTA measurements.

The OTA AAS BS is a subset of the broader AAS BS category. AAS base stations are characterised by the integration of an antenna array with active transceiver units, allowing for advanced beamforming and spatial multiplexing. The OTA AAS BS designation is used to differentiate those AAS base stations that meet specific transceiver unit thresholds and are subject to OTA requirements as defined in 3GPP specifications such as TS 37.105 (Active Antenna System (AAS) Base Station (BS) transmission and reception) and TS 37.145 (Active Antenna System (AAS) Base Station (BS) conformance testing).

The OTA requirements for AAS BS are necessary because traditional conducted testing methods are not feasible for integrated active antenna systems. In a conducted test, the RF signal is measured at a physical port, but in an AAS, the antenna elements and transceivers are tightly integrated, and there is no accessible conducted port. Therefore, OTA testing is used to evaluate the radiated performance of the base station, including parameters such as equivalent isotropically radiated power (EIRP), spurious emissions, and receiver sensitivity. The OTA requirements ensure that the base station meets regulatory and performance standards when deployed in the field.

The introduction of OTA AAS BS in Release 15 coincided with the first phase of 5G NR specifications, which placed a strong emphasis on advanced antenna systems for higher frequency bands (e.g., mmWave) where beamforming is essential. The OTA AAS BS category provides a clear regulatory and testing framework for such base stations, enabling vendors and operators to demonstrate compliance with 3GPP requirements. It also facilitates global harmonisation of testing methodologies for active antenna systems.

In subsequent releases, the definition of OTA AAS BS has remained largely unchanged, indicating stability in the requirements and testing approach. The specifications continue to be updated to address new frequency bands and deployment scenarios, but the core concept of an OTA-tested AAS base station with defined transceiver unit thresholds remains. The OTA AAS BS is a key enabler for the deployment of advanced antenna systems in 5G and beyond, ensuring that performance and compliance can be verified in a consistent manner.

Purpose & Motivation

The OTA AAS BS concept was created to address the testing and compliance challenges posed by active antenna systems (AAS) in modern base stations. Traditional base station testing relied on conducted RF interfaces, where measurements are made at physical ports. However, AAS integrates the antenna array with the transceiver units, eliminating conducted ports and making conducted testing impossible. OTA testing is therefore necessary to evaluate the radiated performance of the base station.

The OTA AAS BS category defines specific thresholds for the number of transceiver units per cell, distinguishing these base stations from other AAS base stations that may have fewer transceiver units and different testing requirements. This differentiation ensures that the appropriate OTA requirements are applied, balancing test complexity with the need to verify performance. The thresholds (≥8 TUs for E-UTRA/MSR, ≥4 for UTRA) were chosen based on the capabilities of advanced antenna systems and the need to support beamforming and spatial multiplexing.

The introduction of OTA AAS BS in Release 15 was motivated by the deployment of 5G NR in higher frequency bands, where AAS is essential for coverage and capacity. The OTA requirements provide a standardised way to demonstrate compliance with regulatory limits and 3GPP performance targets, facilitating global market access for AAS base stations. They also enable operators to compare different vendors' products on a level playing field. Overall, the OTA AAS BS concept supports the widespread adoption of advanced antenna technologies by providing a clear and consistent testing framework.