Out-Of-Band Emission

Description

Out-Of-Band Emission (OOBE) is a fundamental radio frequency (RF) performance parameter for any transmitter, including those in User Equipment (UE) and base stations (gNBs, eNBs) in 3GPP systems. It quantifies the unwanted RF energy a transmitter emits outside its allocated channel bandwidth. OOBE is distinct from spurious emissions, which occur at frequencies farther from the carrier. OOBE arises primarily from two sources: the non-linear amplification of the modulated signal in the power amplifier (PA) and the abrupt switching of the transmitter during power ramping, especially in Time Division Duplex (TDD) systems. The non-linearity of the PA causes spectral regrowth, where the bandwidth of the transmitted signal spreads beyond its intended limits, creating interference in adjacent channels.

In 3GPP specifications, OOBE is rigorously defined and measured. Key specifications like TS 38.101 (UE radio transmission) and TS 38.104 (Base Station radio transmission) define the limits for OOBE through spectrum emission masks (SEM) and Adjacent Channel Leakage Ratio (ACLR) requirements. The Spectrum Emission Mask defines a power spectral density template that the transmitted signal must not exceed when measured outside the assigned channel bandwidth. ACLR is a ratio comparing the power measured in the adjacent channel to the power measured in the assigned channel. It is a critical metric for assessing the potential for a transmitter to interfere with a receiver in the neighboring channel.

Mitigating OOBE is a complex engineering challenge involving both the digital baseband and the RF front-end. In the digital domain, techniques like crest factor reduction (CFR) and digital predistortion (DPD) are employed. CFR reduces the peak-to-average power ratio (PAPR) of the signal, allowing the PA to operate in a more linear region with less spectral regrowth. DPD linearizes the PA by applying an inverse distortion characteristic to the baseband signal, counteracting the PA's non-linearity before amplification. In the RF design, careful filtering and the use of linear PAs are essential. For TDD systems, precise control of the power ramping profile (the shape of the signal turn-on/off) is critical to minimize transient emissions. Compliance with OOBE limits is mandatory for type approval and is vital for ensuring the co-existence of multiple operators and radio access technologies within the same or adjacent frequency bands.

Purpose & Motivation

The specification and strict control of Out-Of-Band Emissions are fundamental to the efficient and fair use of the radio spectrum, which is a finite and shared resource. Without OOBE limits, a powerful transmitter operating on one channel could spill significant energy into neighboring channels, causing severe interference. This would degrade the performance of other users, effectively reducing overall network capacity and quality of service. The problem becomes more acute with the dense frequency reuse and wide channel bandwidths used in modern 4G LTE and 5G NR systems.

Historically, as wireless systems evolved from narrowband analog to wideband digital modulation (like OFDM used in LTE and NR), the potential for spectral regrowth and adjacent channel interference increased significantly. OFDM signals have a high PAPR, making them particularly susceptible to non-linear distortion in power amplifiers. The 3GPP standards for OOBE evolved through successive releases to address these challenges, ensuring that new technologies could be deployed without disrupting existing services. For example, when LTE was deployed in bands adjacent to 2G/3G systems, strict OOBE limits were necessary to protect the legacy networks. Similarly, in 5G NR, especially for mmWave frequencies and wideband carrier aggregation scenarios, controlling OOBE is critical for self-interference within a device's own radios and for coexistence with other services like satellite or radar systems operating in nearby bands. The ongoing evolution of OOBE requirements reflects the continuous push for higher power efficiency (which tends to increase non-linearity) and wider bandwidths, balancing performance with the imperative of spectral cleanliness.