Peak-to-average ratio

Crest factor is a measure of a waveform, such as alternating current or sound, showing the ratio of peak values to the average value. In other words, crest factor indicates how extreme the peaks are in a waveform. Crest factor 1 indicates no peaks, such as direct current. Higher crest factors indicate peaks, for example sound waves tend to have high crest factors.

Crest factor or peak-to-average ratio (PAR) is calculated from the peak amplitude of the waveform divided by the RMS value of the waveform:[1]

C = {|x|_\mathrm{peak} \over x_\mathrm{rms}}

The peak-to-average power ratio (PAPR) is a related measure that is defined as the peak amplitude squared (giving the peak power) divided by the RMS value squared (giving the average power):[2]

\mathit{PAPR} = ^2 \over {x_\mathrm{rms}}^2} = C^2

Crest factor and PAPR are therefore dimensionless quantities. While the crest factor is most simply expressed by a positive rational number, in commercial products it is also commonly stated as the ratio of two whole numbers, e.g., 2:1. The PAPR is most used in signal processing applications. As it is a power ratio, it is normally expressed in decibels (dB).

The minimum possible crest factor is 1, 1:1 or 0 dB.

Examples

This table provides values for some normalized waveforms:

Wave type Waveform Peak magnitude (normalized) RMS value Crest factor PAPR (dB)
DC 1 1 1 0.0 dB
Sine wave 1 {1 \over \sqrt{2}} \approx 0.707[3] \sqrt{2} \approx 1.414 3.01 dB
Full-wave rectified sine 1 {1 \over \sqrt{2}} \approx 0.707[3] \sqrt{2} \approx 1.414 3.01 dB
Half-wave rectified sine 1 {1 \over 2} = 0.5[3] 2 \, 6.02 dB
Triangle wave 1 {1 \over \sqrt{3}} \approx 0.577 \sqrt{3} \approx 1.732 4.77 dB
Square wave 1 1 1 0 dB
PWM-Signal
V(t) \ge 0.0V
1 \sqrt{ \frac{t_1}T }[3] \sqrt{ \frac T{t_1}}

20\log\sqrt{ \frac T{t_1}} dB

QPSK 1 1 1 0 dB[4]
8PSK 3.3 dB [5]
π/4DQPSK 3.0 dB [5]
OQPSK 3.3 dB [5]
8VSB 6.5–8.1 dB [6]
64QAM 1 \sqrt{ \frac{3}{7} } \sqrt{ \frac{7}{3} } 3.7 dB[4]
\infty-QAM 1 {1 \over \sqrt{3}} \sqrt{3} 4.8 dB[4]
WCDMA downlink carrier 10.6 dB
OFDM ~12 dB
GMSK 1 1 1 0 dB

Notes: 1. crest factors specified for QPSK, QAM, WCDMA are typical factors needed for reliable communication, not the theoretical crest factors which can be larger.

Digital multimeters

Crest factor is an important parameter to understand when trying to take accurate measurements of low frequency signals. For example, given a certain digital multimeter with an AC accuracy of 0.03% (always specified for sine waves) with an additional error of 0.2% for crest factors between 1.414 and 5, then the total error for measuring a triangular wave (crest factor = 1.73) is 0.03% + 0.2% = 0.23%.

Acoustics

In acoustics, crest factor is usually expressed in decibels. For example, for a sine wave the 1.414 ratio is 20 log(1.414) or 3 dB. Most ambient noise has a crest factor of around 10 dB while impulsive sounds such as gunshots can have crest factors of over 30 dB. (Note the waveform factor of the half wave sine wave rectified signal should be 2.22 not 1.11)

Peak-to-average ratio (PAR) meter

A peak-to-average ratio meter (Par meter) is a device used to measure the ratio of the peak power level to the time-averaged power level in an electrical circuit. This quantity is known as the peak-to-average ratio (p/a r or PAR). Such meters are used as a quick means to identify degraded telephone channels.

Par meters are very sensitive to envelope delay distortion. They may also be used for idle channel noise, nonlinear distortion, and amplitude-distortion measurements.

The peak-to-average ratio can be determined for many signal parameters, such as voltage, current, power, frequency, and phase.

Crest factor reduction

Many modulation techniques have been specifically designed to have constant envelope modulation, i.e., the minimum possible crest factor of 1:1.

In general, modulation techniques that have smaller crest factors usually transmit more bits per second than modulation techniques that have higher crest factors. This is because (1) Any given linear amplifier has some "peak output power"—some maximum possible instantaneous peak amplitude it can support and still stay in the linear range. (2) The average power of the signal is the peak output power divided by the crest factor. (3) The number of bits per second transmitted (on average) is proportional to the average power transmitted (Shannon–Hartley theorem).

Orthogonal frequency-division multiplexing (OFDM) is a very promising modulation technique; perhaps its biggest problem is its high crest factor.[7][8] Many crest factor reduction techniques (CFR) have been proposed for OFDM.[9][10] The reduction in crest factor results in a system that can either transmit more bits per second with the same hardware, or transmit the same bits per second with lower-power hardware (and therefore lower electricity costs[11]) (and therefore less expensive hardware), or both.

Applications

See also

References

 This article incorporates MIL-STD-188).

External links

  • Definition of peak-to-average ratio – ATIS (Alliance for Telecommunications Industry Solutions) Telecom Glossary 2K
  • Definition of crest factor – ATIS (Alliance for Telecommunications Industry Solutions) Telecom Glossary 2K
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