Average power

Description

Average power is a fundamental measurement definition in 3GPP specifications for characterizing the transmitted power of a radio access mode. It specifies that the power is the thermal power measured through a root raised cosine (RRC) filter with a roll-off factor α=0.22 and a bandwidth equal to the chip rate of the radio access mode. The measurement period is defined as a transmit timeslot (or power control group) excluding the guard period, unless otherwise stated. This definition ensures consistent and comparable power measurements across different implementations and test scenarios.

The RRC filter with roll-off α=0.22 is a standard pulse-shaping filter used in WCDMA and other 3GPP radio access technologies. It limits the bandwidth of the transmitted signal to the chip rate while minimizing inter-symbol interference. By specifying the filter characteristics, the measurement captures the actual power within the occupied bandwidth, excluding out-of-band emissions. The bandwidth equal to the chip rate corresponds to the main lobe of the RRC-filtered signal, which contains the majority of the transmitted power.

The measurement period is typically one transmit timeslot, which in WCDMA is 2560 chips (approximately 667 microseconds). Excluding the guard period ensures that the measurement focuses on the active transmission portion, avoiding transient effects at timeslot boundaries. This is particularly important for power control and link adaptation, where accurate power measurements are essential for maintaining signal quality and minimizing interference.

Average power is used in various contexts, including transmitter power specifications, conformance testing, and network planning. It is referenced in specifications such as 25.101 (UE radio transmission and reception), 25.102 (UTRA repeater conformance testing), and 34.124 (E-UTRA, UTRA and GSM/EDGE multi-standard radio base station). The definition has remained largely stable across releases, with minor editorial changes to the wording, reflecting its fundamental role in radio access technology.

In summary, average power provides a standardized method to measure the power of a transmitted signal in 3GPP systems, ensuring interoperability and consistent performance evaluation. It is a key parameter for power control, radio resource management, and compliance testing.

Purpose & Motivation

The concept of average power exists to provide a precise and reproducible definition for measuring the transmitted power of a radio access technology. In early cellular systems, power measurements were often ambiguous, leading to inconsistent test results and difficulties in comparing equipment from different vendors. The 3GPP definition addresses this by specifying the exact filter characteristics and measurement period, eliminating ambiguity.

By defining the measurement through an RRC filter with a specific roll-off and bandwidth, the standard ensures that the measured power reflects the actual signal power within the intended transmission band, rather than total power including out-of-band emissions. This is crucial for regulatory compliance, as it allows authorities to verify that transmitters meet spectral mask requirements. It also facilitates coexistence between different systems by enabling accurate assessment of interference levels.

The exclusion of guard periods from the measurement period prevents transient effects from skewing the results. Guard periods are used to allow for power ramping and to avoid overlap between timeslots; including them would capture power during transitions that are not representative of steady-state operation. This makes the measurement more relevant for assessing link performance and power control accuracy.

Overall, average power was created to solve the problem of inconsistent power measurements, providing a common language for engineers, test houses, and regulators. It has been adopted across multiple 3GPP releases and remains a cornerstone of radio conformance testing.