Equivalent Isotropic Radiated Power

Description

Equivalent Isotropic Radiated Power (EIRP) is a fundamental concept in radio frequency (RF) engineering and is used extensively in 3GPP specifications for base stations, user equipment, and other radio transmitters. EIRP is defined as the equivalent power radiated from an isotropic directivity device producing the same field intensity at a point of observation as the field intensity radiated in the direction of the same point of observation by the discussed device. In simpler terms, EIRP is the power that an isotropic antenna would need to radiate to produce the same signal strength in a given direction as the actual antenna. It is calculated as the sum of the transmitted power (in dBm) and the antenna gain (in dBi) in that direction, minus any cable losses.

EIRP is a critical parameter for regulatory compliance. Regulatory bodies such as the FCC in the United States, ETSI in Europe, and others set limits on the maximum EIRP that a transmitter can radiate. These limits are designed to prevent interference with other radio services and to ensure the safe use of the radio spectrum. For example, in the 2.4 GHz ISM band, the FCC limits EIRP to 1 watt (30 dBm) for point-to-multipoint systems. In cellular networks, EIRP limits are specified for base stations and UEs to ensure coexistence with other systems and to protect human health (specific absorption rate, SAR).

In 3GPP specifications, EIRP is used in several contexts. For base stations (BS), the maximum EIRP is often specified as a regulatory requirement, and the BS must not exceed it. For UEs, EIRP is used in the definition of power classes and in the calculation of the link budget. The link budget is a systematic accounting of all gains and losses from the transmitter to the receiver, and EIRP is a key component. It determines the coverage and quality of service that can be achieved. EIRP is also used in the coordination of spectrum sharing between different systems, such as between 5G NR and satellite services.

The concept of EIRP is closely related to Effective Radiated Power (ERP), which is defined relative to a half-wave dipole antenna instead of an isotropic antenna. The relationship is EIRP = ERP + 2.15 dB. EIRP is preferred in 3GPP because it is independent of the antenna type and provides a common reference for all systems. EIRP is typically expressed in dBm or dBW. For example, a base station with a transmit power of 40 dBm and an antenna gain of 18 dBi has an EIRP of 58 dBm (or 58 dBm - 30 = 28 dBW).

In the context of 5G NR, EIRP is particularly important due to the use of massive MIMO and beamforming. With beamforming, the antenna gain is not fixed but varies with the direction of the beam. Therefore, EIRP is also directional and must be evaluated in the direction of the beam. The 3GPP specifications for NR base stations (TS 38.104) and UEs (TS 38.101-1) define EIRP limits and measurement methods. For example, the maximum EIRP for a NR base station in frequency range 1 (FR1) is typically limited to 68 dBm per antenna element, but with beamforming, the total EIRP can be higher. The specifications also define how to measure EIRP in the presence of beamforming, using techniques such as beam peak search.

EIRP is also used in the context of satellite communications, where the EIRP of the satellite transponder determines the received signal strength on the ground. In 3GPP, satellite access is being integrated into 5G NR (NTN), and EIRP is a key parameter for link budget calculations between the satellite and the UE. The specifications for NTN (TS 38.108, TS 38.174) define EIRP limits and measurement methods for satellite base stations and UEs.

In summary, EIRP is a fundamental metric that quantifies the radiated power in a given direction, taking into account the antenna gain. It is used for regulatory compliance, link budget analysis, and system coexistence. Its definition has evolved slightly in 3GPP specifications, but the core concept remains the same.

Purpose & Motivation

EIRP exists to provide a standardized way to quantify the radiated power of a transmitter in a given direction, independent of the antenna type. This is essential for regulatory compliance, as regulatory bodies need a common metric to set limits on radiated power to prevent interference and protect health. Without EIRP, it would be difficult to compare different transmitters with different antenna types and to ensure that they comply with the same limits. EIRP also simplifies link budget calculations, as it combines the transmit power and antenna gain into a single figure. This makes it easier for engineers to design and analyze radio systems.

The concept of EIRP was originally developed in the early days of radio engineering and has been adopted by international standards bodies such as ITU and 3GPP. In 3GPP, EIRP is used in specifications for base stations, UEs, and satellite systems. The definition in 3GPP has evolved from a simple gain-times-power formulation to a more precise field-intensity-based definition. The initial definition in Rel-12 was 'in a given direction, the gain of a transmitting antenna multiplied by the net power accepted by the antenna from the connected transmitter.' This was a straightforward definition but did not explicitly reference isotropic directivity. In Rel-13, the definition was changed to the current one, which is based on field intensity and isotropic directivity. This change was made to align with international standards and to provide a more rigorous definition that is independent of the antenna's physical implementation.

The motivation for the change in Rel-13 was to harmonize with ITU-R recommendations and to ensure that EIRP is defined in a way that is applicable to all types of antennas, including active antenna systems (AAS) and beamforming antennas. The field-intensity-based definition is more general and can be applied to any antenna, including those with complex radiation patterns. It also clarifies that EIRP is a directional quantity and must be evaluated in the direction of interest. This is particularly important for 5G NR, where beamforming is used extensively. The change in definition did not alter the fundamental meaning of EIRP but made it more precise and internationally aligned.