CCDF Measurement: Advantages and Disadvantages

Introduction

CCDF (Complementary Cumulative Distribution Function) measurement is a critical tool for analyzing the peak to average power ratio (PAPR) in RF signals. This technique helps evaluate signal behavior, detect distortions, and optimize system performance in wireless communication. CCDF measurement is widely used in power amplifier testing and waveform analysis. Understanding its advantages, like detailed PAPR analysis, and disadvantages, such as complexity in setup, is essential for RF engineers. This guide delves into the benefits and limitations of CCDF measurement to enhance your RF system design and evaluation.

What is CCDF measurement?

CCDF measurement is used to understand the performance of an amplifier by measuring its linearity. For each power level, it depicts the amount of time a signal spends above the average power level. CCDF is the short form of complementary cumulative distribution function. CCDF curve is plotted based on time domain vector data.

A CCDF curve will have “dB above average power” on the x-axis, whereas the y-axis indicates the percent of time the signal power is at or above the power specified by the x-axis.

The CCDF curve displays the probability that the signal power is above the average power. The figure shows a CCDF probability curve for QPSK modulated and 16QAM modulated signal vectors. As shown, 16QAM will have more average power compared to the QPSK vector.

The following steps are followed to plot the CCDF from baseband IQ data vectors:

• Step-1: Arrange the IQ vector data in ascending order as per their magnitude or absolute value.
• Step-2: Determine the average value “X” of all the IQ data samples.
• Step-3: Take the ratio of the step-1 vector values with the average value “X” determined in step-2.
• Step-4: Take the logarithm with base-10 of the values obtained in step-3 and multiply it with numeric 10.
• Step-5: Plot the values obtained in step-4. This gives us the CCDF curve.

Benefits or Advantages of CCDF measurement

Following are the benefits or advantages of CCDF measurement :

• It provides a more accurate measurement than CF (Crest Factor) measurement. Using the CCDF of the CF adds the probability of occurrence to the measurement.
• CCDF is more helpful to analyze the effects of amplifier compression on BER (Bit Error Rate) or EVM (Error Vector Magnitude).
• It displays the probability of the signal power which will be above the average power.

Drawbacks or Disadvantages of CCDF measurement

Following are the drawbacks or disadvantages of CCDF measurement :

• It is not easy to analyze the results obtained by the CCDF measurement method.
• It requires a VSA (Vector Signal Analyzer) to acquire the baseband IQ data for further analysis and plotting of the data. This adds cost to the test system.

Conclusion

CCDF measurement plays a vital role in understanding signal characteristics and optimizing RF system performance. While its precision and detailed insights are significant advantages, the complexity of setup and interpretation can be challenging. By mastering CCDF analysis, engineers can enhance signal efficiency and ensure reliable communication systems, balancing its benefits and drawbacks effectively.

Refer MATLAB source code and Labview VI code used for CCDF measurement.