Constant Envelope OFDM Phase Modulation - Dr. James R. Zeidler

11distorts the waveform nonlinearly, and nonlinear distortion results in a loss of subcarrierorthogonality which degrades performance.One such nonlinear device is the transmitter’s power amplifier (PA) which is responsiblefor the system’s operational range [424]. Ideally the output of the PA is equal tothe input times a gain factor. In reality the PA has a limited linear region, beyond whichit saturates to a maximum output level. Figure 1.9 shows a representative input/outputcurve, known as the AM/AM conversion. In the linear region the curve matches theideal, but as the input power increases the PA saturates. The most efficient operatingpoint is at the PA’s saturation point, but for signals with large PAPR the operatingpoint must shift to the left keeping the amplification linear. The average input poweris reduced and consequently this technique is called input power backoff (IBO). To keepthe peak power of the input signal less than or equal to the saturation input level, theIBO must be at least equal to the PAPR. Thus the required IBO for the OFDM signalin Figure 1.8 is 9.5 dB. At this backoff the efficiency of a Class A power amplifier isless than 6%. Such an efficiency is detrimental to mobile battery-powered devices whichhave limited power resources. Moreover, the operational range of the system is reducedby a factor of nine 2 .Max outputOptimumOutput powerLinear regionActualSaturation regionBackoffInput powerAM/AM curveOperating pointsIdeal AM/AMFigure 1.9: Power amplifier transfer function.2 IBO of 9.5 dB corresponds to 10 9.5/10 ≈ 9 times less signal power transmitted in channel; the(theoretical) efficiency of a Class A amplifier is 0.5/(10 9.5/10 ) ≈ 0.06 [374].