Occupied Bandwidth (OBW)

OBW is defined by the ITU-R SM.328 standard as the bandwidth containing 99% of the mean transmitted power. The remaining 0.5% of power is distributed equally (0.25%) in the upper and lower out-of-band regions.

• Measurement Method: Integrate total power across the spectrum, then sweep inward from the band edges until 0.5% of total power remains outside each boundary.
• Contrast with 3 dB Bandwidth: Unlike the simple half-power bandwidth, OBW accounts for spectral regrowth and modulation side-lobes, making it a true measure of real-world spectral occupancy.
• Regulatory Relevance: Spectrum regulators use OBW to define emission masks and license boundaries, ensuring adjacent channel licensees are protected from interference.

While often used interchangeably, Necessary Bandwidth is a theoretical calculation based on modulation type and data rate, whereas Occupied Bandwidth is an empirical measurement of a specific transmitter's actual spectral spread.

• Necessary Bandwidth Formula: For a given modulation scheme (e.g., QPSK), symbol rate, and filtering, the necessary bandwidth is calculated mathematically.
• OBW Variability: OBW is always wider than necessary bandwidth due to non-ideal filter roll-off, IQ modulator impairments, and power amplifier nonlinearity.
• Spectral Regrowth Impact: A nonlinear PA causes spectral regrowth that directly expands the measured OBW, potentially pushing a compliant design out of its licensed channel.

Accurate OBW measurement requires precise power integration, typically performed by a spectrum analyzer or vector signal analyzer in zero-span or swept-frequency mode.

• RMS Detection: Modern analyzers use root-mean-square (RMS) detection to accurately sum the power of noise-like modulated signals, avoiding the errors of peak detection.
• Integration Bandwidth: The analyzer must sweep a span significantly wider than the expected OBW to capture all spectral regrowth components before applying the 99% power threshold.
• Trace Averaging: Multiple sweeps are averaged to reduce measurement variance, but excessive averaging can mask intermittent transient spectral spreading caused by thermal memory effects.

Crest Factor Reduction (CFR) directly influences OBW by limiting signal peaks before the power amplifier, preventing the amplifier from being driven into deep compression.

• Hard Clipping: Abruptly truncates peaks, causing severe spectral regrowth that expands OBW dramatically.
• Peak Windowing: Applies a smooth window to peaks, containing spectral regrowth within a tighter bandwidth and minimizing OBW expansion.
• Trade-off: Aggressive CFR reduces OBW expansion but introduces in-band Error Vector Magnitude (EVM) degradation. The optimal CFR algorithm balances OBW containment against modulation fidelity.

5G New Radio (NR) signals with 100 MHz component carriers and high-order MIMO place extreme demands on OBW characterization.

• Channel Bandwidth vs. OBW: A 100 MHz NR channel has a specified transmission bandwidth configuration, but the actual OBW must remain within the spectral mask limits to avoid adjacent channel interference.
• OBW for Multi-Cluster Signals: Intra-band carrier aggregation creates multi-cluster signals where OBW is defined across the entire aggregated bandwidth, including the gap between clusters.
• mmWave Challenges: At millimeter-wave frequencies, phase noise and PA nonlinearity combine to expand OBW, requiring integrated DPD and CFR solutions to maintain spectral containment.

Monitoring OBW drift over time provides a non-invasive diagnostic of power amplifier degradation and antenna VSWR faults.

• Thermal Runaway Detection: A gradual OBW expansion during continuous transmission indicates rising junction temperatures and worsening PA linearity.
• Load Mismatch: A sudden OBW increase can signal an antenna impedance mismatch, causing the PA to operate outside its designed load-pull contour.
• Preventive Maintenance: Integrating OBW trending into base station self-organizing network (SON) algorithms enables proactive PA replacement before catastrophic failure and regulatory violation.