Figure 4 is the same LTE signal shown in two different spectrum-analysis modes. The spectrum on the left is a typical vector signal analyzer (VSA) display providing spectral updates of about 10 times per second.  Note how the signal does not show any anomalies. If the trace were updating live, the user might observe an occasional “bump” in the upper and lower adjacent channel levels indicating that something could be wrong with the signal, though it is impossible to understand the nature of the problem. 

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The signal on the right uses a real-time feature of DPX allowing the user to process more than 48,000 spectrums per second. This dramatic increase in the spectral update rate combined with DPX’s pixel memory buffer allows the user to not only see the transient behavior of the signal, but to also observe the frequency of occurrence as noted by the color grading. DPX enables designers to discover if there are any anomalies with their waveform.

Once discovered, the key to any debug and diagnosis is to capture events of interest. Tektronix now has incorporated versatile triggering to real-time spectrum analysis. Frequency mask triggering allows users to draw a free-form mask on the spectrum display and trigger the instrument when the spectrum of the signal violates this mask. The mask can be created with up to 500 points across the frequency span and up to 80 dB below the reference level.

Masks can be defined graphically or in tabular form. They can be saved independently and re-used at other center frequencies. Masks can also be auto generated using the Auto Draw button in the frequency mask editor menu. This allows users to use a known-good spectrum as a baseline for testing potentially noncompliant waveforms.

Figure 5 shows the steps that LTE design engineers can take to trigger any spectrum violations in the frequency domain. In addition to frequency mask triggering, the hardware offers power-level triggering, either across the entire acquisition bandwidth (function of the span) or in a narrow bandpass centered on center frequency (poor man’s frequency mask triggering). 

Finally, the hardware also features a couple of electrical trigger inputs to trigger on external stimuli. When the RTSA gets triggered from any of these modes, it squirts out a trigger output pulse that can be used to cross-trigger other instruments. This gives the RF anomaly a synchronization that greatly aids in the debug and troubleshooting effort of the system.

Finally, LTE is a highly configurable signal format. Symbol rate, modulation type, frame, and burst configurations can change every frame. Engineers can waste a lot of time configuring the test equipment. LitePoint’s RSALTE software and Tektronix’s RTSAs are integrated to yield one-button measurements so users spend less time configuring test equipment and more time troubleshooting designs.

Circumventing Configuration Complexity

Despite the complexity of WiMAX and LTE, designers are running at full tilt in basestation and handheld mobile development mode. Powerful real-time spectrum analysis tools provide a real advantage, especially those like the Tektronix RSA6100A and RSA3000 series with unique trigger and spectrum display capabilities. The comprehensive, complementary RSALTE software tool adds to that advantage by providing one-button measurements (Fig. 6).

Using Tektronix’s RTSAs and LitePoint’s RSALTE software, designers can examine OFDM DL and SC-FDMA UL signals. A combination of Tektronix hardware features and RSALTE software controls enables users to quickly identify troublesome transients and get a quick handle on probable cause.

The RSALTE display shows power spectral density and mask, symbol constellation, and spectral flatness. For troubleshooting help, the display illustrates phase error versus time, frequency error versus time, complementary cumulative distribution function (CCDF), spectrogram, and EVM of individual subcarriers.
Conclusion

Designers need instruments that can discover and capture the transients for LTE design troubleshooting and analysis. The DPX available from RTSAs offers an intuitive live color view of signal transients changing over time in the frequency domain. It also can instantly display a fault when it occurs. The FMT technology then can be  used to set to trigger on the event in the frequency domain, capture a continuous-time record of changing RF events, and perform time-correlated analysis in all domains. Effective discover, trigger, and capture capability along with the analysis capability that supports the latest March 2008 LTE release are the key considerations for LTE designers.

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