Out-of-Band Emission

The primary metric for quantifying out-of-band emissions, ACLR measures the ratio of transmitted power within the assigned channel to the power leaking into an adjacent or alternate channel. For 3GPP 5G NR, the minimum requirement is typically -45 dBc for adjacent channels and -50 dBc for alternate channels, though practical systems often target -55 dBc or better to ensure compliance with regional regulatory masks. ACLR is measured using a root-raised cosine filter matched to the channel bandwidth, and poor ACLR directly correlates to reduced network capacity through increased interference.

A spectral emission mask defines the maximum permitted power at specific frequency offsets from the carrier, creating a regulatory envelope that out-of-band emissions must not exceed. Unlike ACLR, which provides a single ratio, the SEM specifies limits at multiple offset frequencies—typically extending from the channel edge to several times the channel bandwidth. For example, 3GPP TS 38.104 defines SEM limits for 5G base stations at offsets of ±1 MHz, ±5 MHz, ±10 MHz, and beyond, with progressively tighter requirements closer to the carrier. Violating the SEM results in failed regulatory certification.

Out-of-band emissions originate from intermodulation distortion (IMD) generated when multiple signal components pass through a nonlinear power amplifier. The most problematic are third-order intermodulation products (IM3), which fall close to the original carrier frequencies and are difficult to filter. For two tones at frequencies f₁ and f₂, IM3 products appear at 2f₁ - f₂ and 2f₂ - f₁. In wideband modulated signals, these products manifest as spectral regrowth—a broadening of the transmitted spectrum that directly causes adjacent channel interference. Higher-order products (IM5, IM7) contribute to far-out emissions.

The bandwidth expansion factor quantifies how much wider the predistorted signal must be compared to the original signal to effectively cancel out-of-band emissions. For a power amplifier with P-th order nonlinearity, the predistorted signal bandwidth must be at least P times the original signal bandwidth. For example, linearizing a 100 MHz 5G carrier with significant fifth-order distortion requires a DPD signal bandwidth of approximately 500 MHz. This expansion drives requirements for high-speed digital-to-analog converters and wideband observation receivers in the DPD feedback path.

Out-of-band emissions are strictly regulated by bodies including the FCC (United States), ETSI (Europe), and ITU (international) to prevent harmful interference between wireless systems. Key regulatory frameworks include:

• FCC Part 27: Governs miscellaneous wireless communications services
• ETSI EN 301 908: Harmonized standard for IMT cellular networks
• 3GPP TS 38.104: Base station radio transmission and reception Failure to meet these limits can result in fines, license revocation, or market exclusion. In shared spectrum environments like CBRS (3.5 GHz), out-of-band emission control is critical for coexistence with incumbent users.

Spectral regrowth is the physical phenomenon where a band-limited signal passing through a nonlinear device generates frequency components outside its original bandwidth. This occurs because nonlinear transfer functions create amplitude modulation-to-amplitude modulation (AM-AM) and amplitude modulation-to-phase modulation (AM-PM) distortion, which mathematically corresponds to a convolution in the frequency domain. The result is a broadening of the power spectral density that spills into adjacent channels. Digital predistortion works by applying an inverse nonlinearity that pre-expands the signal bandwidth in a controlled manner, such that after passing through the PA, the original band-limited spectrum is restored.