Occupied Bandwidth

Description

Occupied Bandwidth (OBW) is a standardized method to quantify the spectral footprint of a transmitted radio signal. It is defined as the bandwidth between two frequency points, above and below the carrier frequency, that contain a defined percentage of the total integrated power of the emission. The 3GPP standard, aligning with ITU-R recommendations, typically uses a 99% power containment definition. Measurement is performed using a spectrum analyzer or dedicated test equipment. The process involves centering the measurement on the assigned channel, capturing the power spectral density (PSD) of the transmitted signal, and then integrating the power from the center frequency outward until 0.5% of the total power is found on each side; the frequency difference between these two -26 dB (or 0.5%) points is the 99% OBW. For complex modulated signals like those in LTE and 5G NR, the OBW is closely related to, but not identical to, the channel bandwidth or the transmission bandwidth configuration. It is influenced by factors such as the modulation scheme (e.g., QPSK vs. 256-QAM), the spectrum shaping filter (e.g., the roll-off factor of the pulse shaping filter), and the presence of any additional spectral spreading. OBW is a critical parameter for network planning and regulatory compliance. It ensures that a transmitter stays within its allocated frequency block and does not spill over into adjacent allocations, which could cause interference. For equipment certification, the measured OBW must be less than or equal to the declared channel bandwidth to pass regulatory requirements.

Purpose & Motivation

The concept of Occupied Bandwidth exists to provide an objective, measurable definition of how much spectrum a radio emission actually occupies, which is essential for efficient and fair spectrum management. Early radio regulations needed a way to ensure that transmissions stayed within their licensed bands. Simple definitions based on carrier frequency were insufficient for modern complex modulations with wide spectral skirts. OBW provides a consistent, power-based metric that regulators worldwide (like the FCC and ETSI) can use to set rules. It solves the problem of defining the practical 'edge' of a signal whose power decays gradually rather than abruptly. This allows spectrum administrators to pack channels closer together (minimizing guard bands) while still controlling interference, thereby maximizing the utility of scarce spectral resources. Its formalization in 3GPP from the earliest LTE releases (Rel-8) was necessary to characterize the new OFDMA-based signals, which have a distinct spectral shape defined by the subcarrier spacing and cyclic prefix. Accurate OBW definition was foundational for the spectral coexistence of LTE with legacy systems (like UMTS) and for the subsequent introduction of carrier aggregation, where multiple component carriers with defined OBWs are combined.