hese subcarriers are orthogonal and the QAM symbol that modulates each subcar- rier can be safely recovered at the receiver end avoiding interference from neighbor subcarriers. One of the drawbacks of this OFDM modulation technique is the high peak-to- average power ratio (PAPR). Because of the independent modulation of each subcarrier, chance dictates that for some specific moments in time, a high number of subcarriers are co-summed in phase, thus causing a large peak in the time-domain signal that needs to be transmitted over the power amplifier (PA). These peaks, depending on the number of sub- carriers, modulation pattern, etc. may fall in the range of 8. 12 dB with respect to the aver- age power level of the transmitted signal. The PA in the transmitter needs to be designed to accommodate this PAPR. If we do not have any advanced techniques available, this can only be done by overdimensioning the PA target output by the amount of PAPR (8. 12 dB as we said before). This overdimensioning Chapter 2 System components, requirements, and interfaces 79 Frequency Domain Time Domain Crest Factor Digital Pre Subcarrier Power Modulation Reduction Distortion Mapping Amplifier (CFR) (DPD) FIGURE 2-32 Generic picture of an OFDM transmitter. OFDM, orthogonal frequency division multiplexing. can costly both financially (through deployment of more expensive components) as well as (DC) power consuming. Hence, it is ideally avoided. We discuss two relevant system compo- nents: CFR to reduce the PAPR and DPD to be able to operate the PA at higher power levels and thus be able to accommodate a higher PAPR. Crest factor reduction The point of CFR is to reduce the peaks in the time-domain OFDM signal and thus the PAPR, where we note that the CF (based on amplitude) is related directly to the PAPR (based on power) as: CF = PAPR. Typically, CFR is applied to the baseband signal before other filtering, up-conversion, and DPD stages are executed. CFR can be implemented by multiple techniques, some of which (from Refs. 5,6) are discuss