Keysight WLAN Measurement Guide 109
Concepts
WLAN Measurement Concepts
Power Stat CCDF Measurement Concepts
Purpose
Many of the digitally modulated signals now look noise-like in the time and
frequency domain. This means that statistical measurements of the signals can
be a useful characterization. Power Complementary Cumulative Distribution
Function (CCDF) curves characterize the higher level power statistics of a
digitally modulated signal. The curves can be useful in determining design
parameters for digital communications systems.
In mobile communications, battery life is one of the most important
characteristics of the device. The efficiency of the power amplifier is key to
maximizing battery life. The Peak to Average Ratio (PAR) of the signal is a
concern. The PAR is defined as the ratio of the peak envelope power to the
average envelope power of a signal. A signal with a high PAR requires more
headroom in the amplifier, which makes it less efficient. The PAR for the basic
configuration (one DPDCH and one DPCCH) is equal or larger than 3.6 dB
during 0.1 percent of the time.
The power statistics CCDF measurement can be affected by many factors. For
example, modulation filtering, modulation format, combining the multiple
signals at different frequencies, number of active codes, and correlation
between symbols on different codes with spread spectrum systems will all
affect measurement results. These factors are all related to modulation and
signal parameters. External factors such as signal compression and expansion
by nonlinear components, group delay distortion from filtering, and power
control within the observation interval also affect the measurement.
The complementary cumulative distribution function (CCDF) fully characterizes
the power statistics of a signal. It provides PAR versus probability.
shows the CCDF curves for a WLAN 802.11n signal.
CCDF curves can help you in several situations:
— To determine the headroom required when designing a component.
— To confirm the power statistics of a given signal or stimulus. CCDF curves
allow you to verify if the stimulus signal provided by another design team is
adequate. For example, RF designers can use CCDF curves to verify that the
signal provided by the digital signal processing (DSP) section is realistic.
— To confirm that a component design is adequate or to troubleshoot your
subsystem or system design, you can make CCDF measurements at several
points of a system. For example, if the ACLR of a transmitter is too high, you
can make CCDF measurements at the input and output of the PA. If the PA
design is correct, the curves will coincide. If the PA compresses the signal,
the PAR of the signal is lower at the output of the PA.