Optimal Method for precise measurement of WiMAX channel power (1)

Now, wireless designers are faced with the arduous task of designing world-class products. Many wireless technical standards define the technical indicators of power, but ignore the specific steps of Power Measurement. Therefore, designers often encounter Errors During measurement. Because the ofdm rf waveforms used in IEEE 802.16 are complex, more errors occur when measuring mobile WiMAX signals.

This article describes important information about WiMAX channel power measurement. These basic principles apply to three major WiMAX power measurements: transmit power measurement, adjacent channel power measurement, and spectrum radiation pass/fail tests.

Basic knowledge of Channel Power Measurement

Some Frequency analyzers have a general measurement mode that does not specify the wireless standards they support, so the designer has full control and flexibility to adjust the measurement settings.

Figure 1 shows the square diagram of the traditional analog spectrum analyzer. In the modern spectrum analyzer, most of the signal adjustment and filtering functions executed after the local oscillator are implemented using digital signal processing, their features and behaviors are the same as the results of simulated processing. Therefore, to perform precise measurement, the designer must correctly set and modify the measurement parameters.

Figure 1 Diagram of traditional analog spectrum analyzer

Display Detector

Figure 1 contains an envelope detector and a display detector (geek mode ). The characteristics of the envelope detector are determined by the design of the spectrum analyzer. You cannot change these features. This article will not discuss them any more.

To accommodate more measurement and application software, the modern spectrum analyzer provides a variety of display detection types. Some detectors provide more optimized results for different types of signals. For example, a peak detector can detect the maximum level from a set of sampling data points to provide the best choice for continuous wave (CW) measurements. Later, the maximum level will be displayed as a trace point on the spectrum analyzer. The average value detector is different from the peak value detector. It can detect the average level of the sampling data point and display it as a trace point.

Figure 2 shows the trace points from the positive-peak value detector, average value detector, and negative peak value detector.

The displayed trace point is usually called "bucket ". The time when the spectrum analyzer samples data points is called the "bucket interval ". Figure 2 shows how different detection types affect the display trace points of a given bucket.

Figure 3 shows the positive-peak value detection, average value detection, and negative peak value detection traces of WiMAX burst pulses from top to bottom.

Note: To show the differences between trace lines, set the resolution bandwidth to 3 MHz.

For Noisy Signals or signals that are similar to noise (such as many digital modulation signals), the average value detector can be used to obtain the best results of average power data. The displayed trace points are the average value of the sample data. Figure 3 shows the different trace lines obtained by processing WiMAX signals using different types of detectors.

Resolution bandwidth

Power Measurement usually refers to the measurement within a certain bandwidth. Similarly, the trace points displayed on the spectrum analyzer must be interpreted using the resolution bandwidth (RBW) filter. For continuous wave signals, decreasing RBW will produce a better signal-to-noise ratio, thus making the measurement results more smooth. For Noise-like signals, the expanded RBW can obtain a more average value in the average detection bucket, or use a narrow video bandwidth (VBW) filter to obtain a smoother measurement result. The disadvantage of using a narrower RBW is that it takes a longer measurement time.

When measuring channel power, spectrum analyzer users should set up RBW according to their wireless technical standards. For example, a given standard dedicated spectrum radiation (SEM) test document typically includes the RBW settings required for measurement. Depending on the frequency range of the signal, the RBW setting of the SEM test may change with the change of the frequency within the whole measurement sweep width. If RBW is not specified, it is recommended that you set RBW for its signal characterization. For narrow bandwidth (CW) signals, it is best to use a narrow RBW filter; for noise-like signals (such as WiMAX signals), it is usually best to use a wide RBW.

Average Technology

The modern spectrum analyzer uses a variety of methods to calculate the average value of the signal, making the signal smoother. The method in which sampling is finally displayed as a trace point on the spectrum analyzer depends largely on the average technology. The main parameters involved include scan time and video bandwidth.

Scan time

The modern spectrum analyzer can typically sample millions of times per second. Assume that the average value detection is selected. If the number of scans is increased, the number of data samples that take the average value within a period of time (bucket) before the trace point is displayed increases accordingly. The larger the sampling volume, the more consistent the displayed average point, and the smoother the Trace Line.

Video bandwidth

The VBW filter in the spectrum analyzer can reduce the difference in the measurement level and maintain the same effect when the number of scans increases. Traditional spectrum analyzers and many modern spectrum analyzers perform VBW filtering at the display scale level, which may cause power measurement problems.

The measurement results of the spectrum analyzer can be expressed both in logarithm (dB) and in linearity (voltage linearity. VBW filtering is a process of calculating the average value. Any mean value calculation at these levels may produce errors. For example, an error of-2.51 dB may occur in the calculation of the noise or the logarithm average value similar to the noise signal.

Therefore, it is best to use the VBW filter on the power scale during power measurement. Some modern analyzers, such as the Agilent PSA spectrum analyzer and the X Series Signal Analyzer, can use their VBW filter on the power scale to avoid these substantial errors. Even so, the average detector is an excellent tool for smoothing the results and can be used in all analyzers equipped with VBW detectors.