By Pooria Varahram, Research and Development Principal Engineer at Benetel
The below article is available in full on the 5G Technology World website.
5G’s Orthogonal Frequency Division Multiplexing (OFDM) brings with it a major design challenge, namely an inherent wide dynamic variation between a signal’s peak and the average power. The peak-to-average power ratio (PAPR) leads to inefficient transmission performance. That’s because the OFDM waveform is created by the sum of multiple sinusoidal signals that can exhibit constructive and destructive behaviour. As a result, at some time instances, the ratio between the maximum signal power to its average will become high. A signal with high PAPR will cause a power amplifier (PA) to operate in its non-linear region.
A high PAPR creates two problems. First, out-of-band distortion leads to adjacent-channel interference and spectral emission mask (SEM) violations. Second, in-band distortion degrades the throughput performance. To tackle these problems, a radio’s power amplifier (PA) needs to operate below its 1 dB compression point (P1dB). That backing off improves linearity at the cost of reduced PA efficiency. We need a PAPR reduction technique to let the PA operate at high efficiency while maintaining good linearity.
Several techniques that reduce the PAPR of the OFDM signal have been proposed. There are two main categories for these techniques: distortion-based methods, which result in out-of-band distortion, and distortionless methods. This article defines PAPR in more detail and introduces the techniques to reduce PAPR. The article will also discuss some of the challenges to reduce the PAPR in multi-operator network use cases.
Figure 1. Constructive phenomena between four sinusoids results in high peaks and high PAPR.
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