d frequency components. In all implementations, CFR is a nonlinear function that introduces unwanted out-of- band signals. These typically are filtered out in remainder (digital) processing stages that are executed after CFR operation, including DUC that includes a filtering operation. CFR approaches are iterative where the signal can be processed in multiple "passes." Expect CFR to reduce the PAPR of the signal by ~ 3 dB for a typical 4G/LTE or 5G/NR signal. Digital predistortion There are two reasons to include DPD in the system: reduce unwanted (or out-of-spec) out- of-band emissions and improve the DC power efficiency of the RF subsystem. Unwanted emissions are defined as the ACLR. They are caused by spectral regrowth due to the nonlinearity of the PA. One mechanism to lower ACLR is by reducing the PA output power as compared to its peak capabilities, or "back-off," but this comes at a financial cost associated with more expensive components and DC power consumption. Chapter 2 System components, requirements, and interfaces 81 To look at the requirement for DC power reduction, we include the following chart which is strictly theoretical in that it calculates the associated DC power consumption for a PA as a function of the target output power of that PA, given a set of assumed PA efficiency levels. For example, 24 dBm output power (or 250 mW) corresponds to 2.5 W DC is the PA is oper- ating at a 10% efficiency level. The chart is simply conveying this data rather than represent- ing actual PA measurements. We include it to explain the reason for inclusion of DPD. As a rule of thumb, we assume that a PA without DPD operates at 10%, whereas inclusion of CFR can improve efficiency to 15%-20% (3 dB reduced back-off as we discussed before) and inclusion of both CFR and DPD can achieve efficiency levels of 40%-50% or even better for state-of-the-art. However, inclusion of DPD comes at a cost in terms of complexity and DC power con- sumption for the digital algorithm to implement DPD. The DC power consumed by the DP