Originally posted Jan 20, 2014
Small but significant power anomalies: a case study
After a few somewhat abstract posts, it’s time for something more concrete. I have in mind a signal pair I encountered several years ago, and the signals conspired to hide a subtle but potentially significant problem.
I call the errors “sneaky” due to a kind of symmetry that combined with an average power value—one that was correct—and minimized the visibility of the problem. Seeing through these obscuring factors illustrates some important aspects of power measurement and can help you decide which RF power errors—sneaky or not!—are worth fixing.
The signals in question are the two RF transmitter outputs of an 802.11n WLAN unit in mixed-mode operation. A portion of their RF envelopes, focused on the frame preamble, are shown in the traces below.
RF envelope of two 802.11n MIMO transmitter outputs, with band-power markers. Amplitude anomalies in the HT-LTF portion of the preamble are measured with band-power markers. The upper marker is one symbol long and the bottom marker is two symbols long.
The traces show the RF envelope (log magnitude) of the signal during the preamble and the beginning of payload transmission. Symbol length is 4 µs and preamble elements are usually one or two symbols long. This signal pair is 2×2 MIMO, so the high-throughput long training field (HT-LTF) is two symbols long. In the top trace, the band-power marker measures the first symbol of the HT-LTF; in the lower trace, the marker measures both symbols of the HT-LTF.
Though the content of the preamble and subsequent data transmission changes during the frame, causing the power statistics to change, the transmit power level is generally constant. The anomaly is the power variation during the HT-LTF. Interestingly, the variations are symmetrical, with the first symbol of the HT-LTF having higher-than-average power and the second having lower-than-average power for channel 1 and the opposite for channel 2.
The interesting—and slightly tricky—results of this symmetry are small errors in average power that do not appear in most measurement setups:
- Power of preamble alone or total signal: power reading normal
- Total power of two-symbol preamble segment (HT-LTF): power reading normal
- Power of both signals combined, for all or any symbol or portion of preamble: power reading normal
- Power of one symbol of the two-symbol preamble segment (HT-LTF), measuring one transmitter only: power reading abnormal
Thus, in most of the ways that you would measure signal power, the value looks correct for either signal or both together. Seeing the anomalies clearly requires not just a measurement focused on a specific symbol in the preamble, but on a single transmitter output. Coupling the band-power markers will make it easier to understand time alignment.
In many cases, these specific and revealing measurements will be made because an anomaly appears in an RF envelope measurement, whether made using a VSA or a swept analyzer in zero-span mode. A small anomaly such as this, however, may not be obvious unless the preamble is examined closely.
Why be so concerned about a power error that doesn’t appear in most measurements, doesn’t prevent demodulation, and doesn’t affect total power? The answer: preambles are special because they are the regulating and steering elements of digitally modulated signals (as are mid-ambles and pilot subcarriers). Because preambles guide signal timing, equalization and demodulation, systems are unusually sensitive to problems in this area.
Given the time alignment of these errors, their symmetry and power accuracy overall, it’s logical to suspect that this is an error in the digital baseband, specifically the math for the MIMO channel references.
Another potentially sneaky element comes to mind: The effect of these errors on receivers may vary considerably with signal-to-noise ratio. In good conditions, the channel-estimation processes they drive will be accurate and the separation of MIMO spatial streams will be effective. In poor conditions, the lower amplitudes of HT-LTF symbols may impair channel estimation and thus MIMO stream separation.
Situations like this benefit from a three-step approach to analyzing digitally modulated signals. The first step is spectrum and time-domain or vector measurements to find impairments that may cause problems downstream in demodulation. This is especially important when problems are obscure in demodulation measurements but easier to see in the time or frequency domain, as in this case.
If you’re interested in more detail on the three-step approach, you can view an upcoming webcast live on January 22 or a recording any time afterward. Go to www.keysight.com and search on “Successful Modulation Analysis in 3 Steps Webcast.”