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Wireless Communication Basics

In a transmitter sending OFDM or multilevel QAM signals, which is the main reason for operating the power amplifier (PA) at a point below where its output saturates (with back-off)?

  1. ATo make the PA's power efficiency higher than when it is driven into saturation
  2. BTo keep the PA from distorting even signals with large peaks, limiting displacement of the signal points and unwanted emissions leaking into adjacent channels
  3. CTo lower the noise figure of the receiver's LNA
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Answer: B. To keep the PA from distorting even signals with large peaks, limiting displacement of the signal points and unwanted emissions leaking into adjacent channels

OFDM and QAM signals vary greatly in amplitude and have a large ratio of peak to average power (PAPR), so the signal is distorted if the PA saturates at the peaks. Distortion displaces the signal points and increases errors, and also spreads unwanted emissions outside the band, interfering with adjacent channels. The output is therefore lowered by an amount matching the PAPR and the PA is used in its linear region, but its power efficiency drops accordingly. To achieve both efficiency and linearity, techniques such as digital predistortion (DPD), which cancels distortion, and envelope tracking, which varies the supply voltage with the signal amplitude, are used.

  • A. PA efficiency is generally highest near saturation, and back-off lowers it.
  • B. Correct. Efficiency is sacrificed to suppress distortion.
  • C. The operating point of the transmitter's PA has nothing to do with the noise figure of the receiver's LNA.
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