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Statistical Analysis for On-Site RF Power Measurement

Parent Category: 2014 HFE

By Tim Holt

Thanks to advances in analog and digital technologies, the multi-function RF power meter bears little resemblance to the earliest versions that appeared in the 1990s. They have all but disappeared as benchtop instruments and today, when accompanied by only a laptop or other computer, the sensor itself incorporates required functionality. However, inline RF power meters (e.g., those using a laptop for display and other functions) designed for troubleshooting the transmit and receive paths of base stations have been hampered by their inability to accommodate complex digital modulation and radio channel access formats for the lack of statistical analysis capability.

Troubleshooting Modern Systems

There are many factors that can degrade the performance of communications systems, including the presence of interfering signals within the system’s operating bandwidth, poor amplifier linearity resulting in distortion and poor waveform fidelity, antenna damage or degradation and transmission line discontinuities causing high VSWR and low loss, and many other anomalies. Networks such as LTE, WiMAX, Next Generation Public Safety Communications (APCO Project 25 Phase 2 and LTE), GSM-R, and military communications systems add a significant layer of complexity to the troubleshooting process when compared to their predecessors as they employ higher-order digital modulation schemes.

Consequently, evaluating the transmission-line and power-related characteristics of such systems in the field requires analysis techniques beyond measurement of traditional modulation parameters such as AM depth or FM modulation. This is because the peak-to-average power ratio of the modulated carrier is a complex function of the data stream content rather than just amplitude and is not constant with time.

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Figure 1 • The Bird Model 7022 combines traditional RF power meter measurements with statistical analysis capability for use with digital modulation schemes.

Analog modulation schemes encode information via amplitude, frequency, or phase using linear modulators, and measuring the envelope power of these signals is straightforward and produces repeatable and predictable results using techniques power meters have always provided. In contrast, modern modulation schemes such as Orthogonal Frequency Division Multiplexing (OFDM) or Code Division Multiple Access (CDMA) combine amplitude and phase modulation to create symbol-based multichannel or multicarrier systems that result in pseudorandom or noise-like power envelopes.

To accurately measure peak and burst average power of time slots associated with TDMA-based and other time-based channel access methods, the power sensor must be able to measure the peak or burst average power of a single time slot within the TDMA frame. These digitally-modulated signals often appear like noise so it is necessary to measure their RF power characteristics using statistical analysis. The instrument must be able to measure and display the complex RF envelope of a carrier signal in terms of probability.

Statistical Analysis

The first inline power meter to incorporate statistical analysis is the Bird Technologies Model 7022 (Figure 1), and it can characterize signals independent of their modulation technique or channel access method. It combines the ability to test waveform properties along with traditional measurements such as forward and reflected average power, burst average power, peak power, VSWR, and return loss.

The instrument can be operated in three modes: conventional, time-domain, and statistical analysis. In conventional mode measurements include forward and reflected average power, VSWR, and return loss like traditional RF power meters, and in time-domain mode time-domain waveform characteristics are measured, with markers to determine average burst power, peak power, and other pulse-related parameters.

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Figure 2 • This screen shows the percentage of time a particular waveform (in this case LTE-TDD) exists at a specific peak-to-average power ratio.  

Finally, in statistical analysis mode, the Model 7022 displays peak-to-average power ratio versus the time in percent that the waveform is at (or exceeds) a specific peak-to-average power ratio. Two movable cursors available within the Windows-based VPM3 software included with instrument can be placed at any point on the curve in order to determine specific values of the waveform peak-to-average ratio and corresponding time.

Statistical Measurements

The most widely used statistical parameter is the Complementary Cumulative Distribution Function (CCDF), which provides an indication of the probability that a measured power level is greater than a specific power level. CCDF measurements require no time synchronization with the waveform to be measured, no specific test or calibration signal, and can be made on “live” signals.

Another, more universal approach to providing statistical measurements is displaying the percentage of time a particular waveform exists at a specific peak-to-average power ratio. Figure 2 illustrates this concept as applied to an LTE-TDD waveform in the VPM3 software. The horizontal axis in the figure represents the peak-to-average power ratio of the waveform being measured and vertical axis represents time in percent. Reading a specific point on the graph provides information about the percentage of time that the signal being measured exhibits a specific peak-to-average power ratio characteristic.

The maximum peak-to-average power ratio of the waveform being measured in this figure is at the point where the curve intersects the horizontal axis. This corresponds to a value of 11.5 dB. The cursors can be placed at any point on the curve in order to determine specific values of the waveform peak-to-average ratio and corresponding time.

The Model 7022’s time-domain mode (Figure 3) is useful for time-varying waveforms such as TDMA-based channel access methods or Time Division Duplex (TDD) formats. A good example of a system employing this technology is a Digital Mobile Radio (DMR) subscriber unit. DMR is an open standard communications format used primarily in land mobile radio systems with Mototrbo being its most prolific implementation. DMR uses a two-slot TDMA format with a 60-ms frame length. At the base station, both time slots are always full but at the subscriber unit only one slot is active, resulting in a 30-ms burst occurring at a 17-Hz rate. Other formats well suited to time-domain mode are TETRA and TETRAPOL, APCO Project 25 Phase 1 and Phase 2, GSM-GSM-R, and LTE-TDD.

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Figure 3 • Time-domain mode is useful for time-varying waveforms such as TDMA-based channel access methods or Time Division Duplex (TDD) formats.

In the analysis of time-domain-based waveforms such as TDMA it may be necessary to measure the peak or burst average power of a single time slot within the TDMA frame. This measurement is easily performed on the VPM3 software by placing movable cursors on either side of the specific time slot to be measured and simply reading the power characteristics from the accompanying table.

More advanced pulse power parameters such as top-level power, minimum power, and other measurements are available by enabling the pulse measurement mode, available on the right side of the display. Several video filter settings (4.5 kHz, 400 kHz, 5 MHz, 20 MHz) are available in the Model 7022 in order to tailor the instrument response to the signal being measured. Several video smoothing settings are available as well.

Summary

Wireless telecommunications is rapidly approaching the day when digital modulation schemes will be universally employed. Over the last 15 years or so, this has transformed not just the transmission systems themselves but the instrument functions required to measure them. Measurement of RF power is a classic example of the transformation, as conventional RF power measurements are no longer useful for characterizing time-based signals characteristic of higher-order modulation schemes. The Bird Model 7022 RF Power Meter with statistical analysis functions is designed to provide the ability to measure digital waveforms as well as all conventional measurements, in a rugged, compact package well suited for use in the field.

About the Author:

Tim Holt is the Director of Systems and Applications Engineering at Bird Technologies Group. He has more than 30 years of experience in the design and application of radio frequency power meters of all types. He can be reached at 440-519-2324 or at tholt@birdrf.com.

 

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