The best way to accurately measure WiMAX channel power

Wireless designers are now faced with the daunting task of designing world-class products. Many wireless technology standards specify power specifications, but ignore the specific steps for power measurement, so designers often make mistakes when making measurements. Because the OFDM radio frequency waveform used by the IEEE 802.16 standard is inherently very complex, they make more mistakes when measuring mobile WiMAX signals.

This article provides important information about WiMAX channel power measurements. These basic principles apply to the three main WiMAX power measurements: transmit power measurement, adjacent channel power measurement, and spectrum emissions pass/fail testing.

Basics of Channel Power Measurements

Some frequency analyzers have a generic measurement mode that is not specific to the wireless standards they support, giving designers full control and flexibility in adjusting the measurement setup.

Figure 1 shows a block diagram of a traditional analog spectrum analyzer. In modern spectrum analyzers, most of the signal conditioning and filtering functions performed after the local oscillator are implemented using digital signal processing, and their characteristics and behaviors are comparable to those of analog processing. Therefore, to make accurate measurements, designers must correctly set and modify measurement parameters.

Figure 1 Block diagram of a traditional analog spectrum analyzer

Display Detector

Figure 1 includes an envelope detector and a display detector (detector mode). The characteristics of the envelope detector are determined by the design of the spectrum analyzer itself and cannot be changed by the user. They will not be discussed in this article.

To accommodate more measurements and application software, modern spectrum analyzers offer a variety of display detection types. Some detectors provide more optimized results for different types of signals. For example, a peak detector detects the maximum level from a set of sampled data points, making it the best choice for continuous wave (CW) measurements. This maximum level is later displayed as a trace point on the spectrum analyzer. An average detector, on the other hand, is different from a peak detector in that it detects the average level of the sampled data points and displays it as a trace point.

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

The displayed trace points are often referred to as “buckets.” The time that a spectrum analyzer samples the data points is called the “bucket interval.” Figure 2 shows how selecting different detection types affects the displayed trace points for a given bucket.

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

Note: To show the difference in traces, the resolution bandwidth is set to 3 MHz.

For noisy signals or signals that are close to noise (such as many digitally modulated signals), the best results for average power data are obtained using an average detector, where the displayed trace points are the average of the sampled data. Figure 3 shows different trace displays obtained using different types of detectors for a WiMAX signal.

Resolution bandwidth

Power measurements are usually made within a certain bandwidth. Similarly, the trace points displayed on a spectrum analyzer are interpreted through a resolution bandwidth (RBW) filter. For continuous wave signals, reducing the RBW will give a better signal-to-noise ratio, thus making the measurement result look smoother. For signals that are close to noise, increasing the RBW can get a more average value in the average detection bucket, or using a narrow video bandwidth (VBW) filter to get a smoother measurement result. The disadvantage of using a narrower RBW is that the measurement takes longer. [page]

When making channel power measurements, spectrum analyzer users should set the RBW based on their wireless technology standard. For example, the dedicated spectrum emission (SEM) test document for a given standard usually includes the RBW setting required for the measurement. Depending on the frequency range of the signal, the RBW setting for the SEM test may also vary with frequency across the measurement span. If the RBW is not specified, it is recommended that the user set the RBW for their signal characterization. For narrow bandwidth (CW) signals, it is best to use a narrow RBW filter; for signals that are close to noise (such as WiMAX signals), it is usually best to use a wider RBW.

Average value technique

Modern spectrum analyzers use a variety of methods to average a signal to make it smoother. The way the samples are ultimately displayed as trace points on the spectrum analyzer depends largely on the averaging technique, which mainly involves parameters such as sweep time and video bandwidth.

Scan time

Modern spectrum analyzers typically take millions of samples per second. Assuming average detection is selected, if the number of sweeps is increased, the number of data samples averaged over a period of time (bucket) before the displayed trace point will increase accordingly. The larger the number of samples, the more consistent the displayed average points will be, and the smoother the final displayed trace will be.

Video Bandwidth

The VBW filter in a spectrum analyzer reduces the difference in measurement levels and maintains the same effect as the number of sweeps increases. Traditional spectrum analyzers and many modern spectrum analyzers perform VBW filtering at the level of the display scale, which can cause problems in power measurements.

Spectrum analyzer measurements can be expressed either logarithmically (dB) or linearly (voltage linearity). Because VBW filtering is an averaging process, any averaging at these levels may result in errors. For example, a -2.51 dB error may occur in the logarithmic average of a noisy or near-noise signal.

Therefore, when making power measurements, it is best to use the VBW filter on a power scale. Some modern analyzers, such as the Agilent PSA spectrum analyzers and X-Series signal analyzers, can use their VBW filters on a power scale, thus avoiding these substantial errors. Even so, the average detector is still an excellent tool for smoothing the results and is available in all analyzers equipped with a VBW detector.

WiMAX channel power measurement setup

For orthogonal frequency division multiplexing (OFDM) signals such as WiMAX signals, there are some additional requirements for the correct measurement setup. For example, the time division duplex (TDD) signals used in WiMAX have a bursty nature in the time domain. Even during the RF burst pulse or subframe, the power will change. Due to the non-continuous nature of these signals, it is difficult for the spectrum analyzer to capture the entire signal.

Common trigger sources that support self-triggering measurements on modern spectrum analyzers include: external trigger signals provided by the device under test, other synchronization signal sources, or the RF burst amplitude level of the signal. Generally, an external trigger source that can maintain good synchronization with the RF burst is the best trigger mechanism. This is because it can provide the most stable trigger for the instrument to acquire data, supporting reliable and repeatable measurements.

For WiMAX downlink signals generated by the base station under test, the base station itself can provide a suitable trigger signal. When performing component analysis, the signal generator that generates the stimulus waveform can provide a usable trigger signal for the spectrum analyzer. When no external trigger source is available, it is best to use the rising edge of the actual RF burst amplitude level to trigger the analyzer. Depending on the architecture used by each analyzer platform, the order of triggering measurements using this method may vary.

Once reliable triggering is achieved, the next consideration is from which time region of interest the channel power measurement should be started. Obviously, the measurement should be made when the signal is actually valid. However, in some cases, it is also necessary (even necessary) to measure during a specific time period within the "valid" portion of the RF burst or subframe. Therefore, it is critical to properly gate the burst region of interest.

Figure 4 The channel power measurement in the figure is gated using a WiMAX downlink burst. The burst consists of a preamble and 8 symbols.

Figure 4 shows the gated region of the WiMAX signal occurring during the second RF burst. Although the first burst causes the trigger, we recommend gating during the second burst. This ensures that any swept LO settling transients generated by the analyzer are removed from the gated region.

Figure 5. The channel power measurement in this figure shows only the gating of the preamble.

With a stable trigger, the spectrum analyzer can start the sweep measurement; if measuring a TDD wireless signal, the precise target area must be gated. For example, some designers may only be interested in the preamble power level or a specific area of ​​a WiMAX signal. Figure 5 shows a gated preamble measurement of a WiMAX signal. This measurement is often called the WiMAX received signal strength indicator (RSSI). In this figure, the gating of the preamble only changes the gating parameters and is necessary to ensure accurate measurements.

Different analyzer platforms

There are several spectrum analyzers available on the market that can perform channel power measurements on WiMAX signals. The Agilent PSA and X-Series analyzer platforms are just one of them. While modifying various parameters on these instruments is critical to obtaining accurate channel power measurements, it is equally important for designers to choose the best measurement solution.

Take the PSA spectrum analyzer and X-Series signal analyzer platforms as examples. Using external triggering, it is very simple to make accurate channel power measurements on a spectrum or signal analyzer platform. However, to trigger measurements using RF bursts, it is helpful to use the following measurement techniques:

Spectrum Analyzer Platform

For GSM waveforms, designers can use the analyzer's trigger feature to change the input signal level of the device while making continuous measurements without having to re-set the spectrum analyzer's trigger level. [page]

Another important technique is the RF burst trigger design, which can trigger using an embedded level 22dB below the peak envelope of the detected signal. This technique is essential for measuring GSM components, but is not suitable for some modern modulation techniques such as OFDM. For OFDM techniques, the high peak-to-average ratio in these signals can cause the relative trigger implementation on the analyzer to trigger falsely.

Figure 6 External configuration of PSA integrated with Agilent 85902A burst carrier trigger (BCT)

In the absence of an external trigger source, an external burst carrier trigger accessory can be used to provide reliable triggering and achieve stable measurements of WiMAX signals. Figure 6 shows the integration of the PSA spectrum analyzer with the Agilent 85092A burst carrier trigger (BCT). When detecting a stable RF burst pulse stream, the BCT samples the RF input signal to be analyzed and provides a TTL output signal synchronized with the burst pulse and an LED status indicator. The integrated instrument can provide the required stable trigger signal, allowing designers to set the gating parameters as described above to make accurate channel power measurements.

Signal Analyzer

Signal analyzers typically use a trigger source that is either externally provided or provided by an internal RF burst carrier trigger unit. Some instruments also have a robust self-trigger capability for situations where no external trigger is available. They eliminate the need for an external burst trigger accessory to provide the most common wireless communications signals, including WiMAX. The RF burst carrier trigger function on the X-Series analyzers is a prime example of a self-trigger capability that includes absolute RF burst amplitude level, cycle timer, and trigger holdoff.

Using absolute RF burst amplitude levels for triggering can solve the problem of false triggering in previous generation spectrum analyzers when measuring OFDM signals with high peak-to-average power ratio (PAPR). When the spectrum analyzer detects that the absolute RF burst level group exceeds a fixed threshold level, the "initial" trigger mechanism will be activated.

The period timer sets an automatic retriggering process for a specified length of time (user adjustable). This process can perform resynchronization after the trigger is established. The user can select the source of this initial synchronization. It is very beneficial to set this parameter equal to the frame period, which will ensure that the measurement is retriggered at each frame boundary, regardless of changes in the external environment or instrumentation.

Figure 7. With sync holdoff turned off, unstable triggering results in inaccurate channel power measurements regardless of gating parameters.

The synchronous holdoff feature improves trigger accuracy by preventing the instrument from starting measurements at invalid portions of the burst. By using a reliable trigger level and measuring at a stable and repeatable measurement point within the RF burst, designers can make accurate and repeatable channel power measurements. Figure 7 shows what might happen if synchronous holdoff is not used.

Summarize

Accurate channel power measurement is an important aspect of wireless device testing. As wireless technology becomes increasingly complex and the time from design to market becomes shorter and shorter, the importance of following prescribed measurement procedures and best practices is increasing. These procedures and methods can provide designers with a feasible solution to save time while ensuring consistency and repeatability of channel power measurements and other related measurements.

When measuring OFDM signals (such as WiMAX), stable triggering and accurate gating parameter settings are also critical. Suitable measurement solutions, such as Agilent PSA or X-Series analyzers, can ensure that designers can quickly and easily perform accurate channel power measurements.