Active transmitter unit

Description

An Active transmitter unit is a fundamental concept in 3GPP radio equipment specifications, particularly for defining the capabilities and performance of base stations (BS) and integrated access and backhaul (IAB) nodes. According to the definition, it is a transmitter unit which is ON, and has the ability to send modulated data streams that are parallel and distinct to those sent from other transmitter units to one or more TAB connectors at the transceiver array boundary or to the radiated interface boundary for IAB-MT type 1-O. This definition is used in specifications such as TS 37.105, TS 37.145, TS 38.104, TS 38.141, TS 38.174, and TS 38.194.

The concept is closely tied to the architecture of Active Antenna Systems (AAS) and advanced antenna arrays. In a traditional base station, the transmitter unit is connected to an antenna connector, and the number of active transmitter units determines the number of transmit chains. In AAS, the transceiver array boundary (TAB) connector is a logical interface between the radio distribution network and the antenna array. Each active transmitter unit can drive one or more TAB connectors, and the signals transmitted from different active transmitter units are parallel and distinct, meaning they carry different data streams or different spatial layers. This is essential for multi-antenna techniques such as MIMO, beamforming, and spatial multiplexing.

The definition has evolved to accommodate new technologies. In Rel-13 and Rel-14, the definition referred to 'TAB connectors at the transceiver array boundary' for BS type 1-H. In Rel-15, the definition was expanded to include 'a BS type 1-C antenna connector, or to one or more BS type 1-H TAB connectors at the transceiver array boundary', reflecting the introduction of BS type 1-C (conducted) and BS type 1-H (hybrid) in 5G NR. In Rel-16, the definition was further updated to include 'IAB type 1-H TAB connectors' and 'radiated interface boundary for IAB-MT type 1-O', aligning with the introduction of Integrated Access and Backhaul (IAB) in 5G.

The active transmitter unit is a key parameter for regulatory and performance requirements. For example, the maximum output power, unwanted emissions, and EVM (Error Vector Magnitude) requirements are often specified per active transmitter unit. The number of active transmitter units determines the number of spatial layers that can be transmitted, which directly impacts the peak data rate and spectral efficiency. In IAB nodes, the active transmitter units are used for both access and backhaul links, and the definition ensures that the performance requirements are applied consistently across different node types.

The concept is also used in conformance testing. Test setups for BS type 1-O (radiated) and BS type 1-H (hybrid) use the active transmitter unit as a reference point for measuring radiated power and emissions. The definition ensures that the test procedures are aligned with the actual capabilities of the equipment, regardless of whether the transmitter is connected to a conducted interface or a radiated interface. This is particularly important for millimeter-wave frequencies, where conducted testing is impractical and radiated testing is required.

In summary, the Active transmitter unit is a foundational concept for defining and testing the radio performance of advanced base stations and IAB nodes in 3GPP. Its definition has evolved to keep pace with new architectures such as AAS, 5G NR, and IAB, ensuring that the specifications remain relevant and technically accurate.

Purpose & Motivation

The Active transmitter unit concept exists to provide a clear and standardized way to define the capabilities and performance requirements of radio equipment in 3GPP. Before its introduction, the term 'transmitter unit' was used, but it did not adequately capture the distinction between transmitter units that are actively transmitting and those that are not, nor did it account for the parallel and distinct data streams that are essential for multi-antenna techniques. The Active transmitter unit definition solves this by specifying that the unit must be ON and capable of sending parallel, distinct data streams, which is critical for MIMO and beamforming.

The motivation for this concept came from the introduction of Active Antenna Systems (AAS) in Rel-13. AAS integrates the radio and antenna into a single unit, and the traditional antenna connector is replaced by the transceiver array boundary (TAB) connector. This required a new way to define transmitter units that are not directly accessible via a conducted connector. The Active transmitter unit definition provided a logical reference point for performance requirements, enabling consistent testing and certification of AAS equipment.

As 5G NR and IAB were introduced, the definition was further refined to include new node types such as BS type 1-C, BS type 1-H, and IAB-MT type 1-O. This ensured that the specifications could accommodate the diverse architectures of 5G, from traditional macro cells to small cells and relay nodes. The Active transmitter unit concept thus plays a crucial role in enabling the deployment of advanced antenna systems and ensuring interoperability and performance across different vendors and network operators.