Array Element

Description

An Array Element (AE) is the basic, indivisible radiating component that constitutes an antenna array in a wireless communication system. In the context of 3GPP specifications, particularly for MIMO (Multiple-Input Multiple-Output) and advanced antenna systems (AAS), an AE is defined by its individual radiation pattern, which is typically specified in units of dBi (decibels relative to an isotropic radiator). This pattern describes how the element radiates power as a function of direction in three-dimensional space when excited. Each AE is connected to a dedicated radio transceiver chain, allowing for independent control of the signal's phase and amplitude. The physical arrangement of multiple AEs—such as in a uniform linear array (ULA), uniform planar array (UPA), or more complex conformal arrays—forms the foundational hardware for spatial signal processing.

The operation of an AE is governed by electromagnetic principles. When an electrical signal is fed to the element, it converts the signal into electromagnetic waves that propagate through space. The specific geometry, size, and material of the element determine its intrinsic characteristics, including impedance bandwidth, polarization (e.g., linear, dual, or circular), and the shape of its radiation pattern. In a passive antenna array, AEs are typically passive radiators like dipoles or patches. In an Active Antenna System (AAS), each AE is integrated with an active component (like a power amplifier or low-noise amplifier), forming an Active Antenna Unit (AAU). This integration allows for finer granularity in beam control and improved energy efficiency.

The role of the AE within the network architecture is foundational for advanced radio access techniques. In the Radio Access Network (RAN), particularly in gNBs (for 5G NR) and eNBs (for LTE), the collection of AEs is managed by baseband processing units. By applying complex weight vectors (precoding matrices) to the signals fed to each AE, the system can synthesize a composite radiation pattern for the entire array. This enables key technologies like digital beamforming, where narrow, high-gain beams are electronically steered towards specific User Equipment (UE), and massive MIMO, where a large number of AEs (e.g., 64, 128, or more) serve multiple users simultaneously on the same time-frequency resource. The performance of each individual AE—its gain, efficiency, and pattern consistency—directly impacts the overall array gain, sidelobe levels, and beam-steering accuracy, which are crucial for achieving high data rates, extended coverage, and robust interference mitigation.

From a specification and testing perspective, 3GPP documents such as TR 37.840, TR 38.820, and TR 38.921 define methodologies for characterizing AE patterns. These include conducted and over-the-air (OTA) test requirements to ensure that the assumed AE pattern models used in system simulations and performance requirements (e.g., for radiated power and EIRP) are accurate. The AE pattern is a key input for calculating metrics like Effective Isotropic Radiated Power (EIRP) and Total Radiated Power (TRP). Understanding the AE is therefore essential for radio frequency (RF) engineers, antenna designers, and system integrators working on the physical layer optimization of 4G, 5G, and future 6G networks.

Purpose & Motivation

The concept of the Array Element (AE) exists to provide a standardized, granular building block for designing and analyzing advanced antenna systems. Prior to the widespread adoption of MIMO and beamforming, cellular networks primarily relied on sector antennas with fixed, broad radiation patterns. These traditional antennas offered limited ability to focus energy, resulting in inefficient spectrum use, poor cell-edge performance, and vulnerability to interference. The AE addresses these limitations by enabling a shift from passive, macro-sector coverage to active, user-centric signal delivery. By defining the AE and its properties, 3GPP created a common framework for specifying the performance of antenna arrays, which is essential for ensuring interoperability between base station vendors and for setting realistic system-level performance targets in standards.

Historically, the motivation for formalizing the AE concept grew with the introduction of LTE-Advanced and its emphasis on MIMO. As antenna arrays grew in complexity—from 2x2 MIMO to 8x8 and eventually to massive MIMO with hundreds of elements—it became impractical to specify performance only for the entire integrated antenna. Decomposing the system into its fundamental AEs allows for more flexible and scalable system design. It enables the use of modular AAS architectures where AEs can be combined in different configurations (e.g., different array sizes and geometries) to meet diverse deployment scenarios, from dense urban macro cells to indoor small cells. Furthermore, defining the AE pattern is critical for accurate radio network planning and simulation. System-level simulations that predict coverage, capacity, and interference rely on accurate models of the radiation from each AE to compute realistic beamforming gains and spatial characteristics.

Ultimately, the AE is the enabler of spatial multiplexing and beamforming gains, which are the primary mechanisms for increasing the spectral efficiency and capacity of modern cellular networks. Without a well-defined AE, the consistent implementation and performance verification of technologies like full-dimension MIMO (FD-MIMO), multi-user MIMO (MU-MIMO), and millimeter-wave beam management would be significantly more challenging. It solves the problem of how to precisely control electromagnetic energy in space, turning the antenna from a simple broadcast device into a sophisticated spatial filter that is central to meeting the escalating data rate and connectivity demands of 5G and beyond.