Spectrum crowding is bad, but I think interference is worse
He’s guilty of some degree of hyperbole, but Lou Frenzel highlights a fundamental issue in his recent column Spectrum Apocalypse: The Coming Death of Wireless. We’re all aware of the limited nature of our shared spectrum resource, and Lou extrapolates to a future in which crowding renders wireless practically unworkable. Fortunately, there are many ways for RF engineers to stave off that dismal future, and Lou summarizes them: modulation schemes and other RF techniques, protocol enhancements, and regulatory steps, and even RF alternatives such as optical.
However, in terms of the day-to-day issues that vex us, I’d argue that interference is more of a headache, and it’s getting worse. As Lou did, I’d like to take the opportunity to summarize the ways to deal with the problem.
In the early days of the vector signal analyzer, we delighted in our newfound ability to completely understand and measure transient signals, especially those that had interference potential because they ventured outside their channel. The VSA’s combination of signal capture+playback, frequency-domain triggering (with pre-trigger delay), and simultaneous multi-domain analysis let us see whatever we wanted, on any time scale. Colorful spectrogram displays made it all clear, as with this marine handheld radio.
A spectrogram display details the behavior of a handheld radio as the transmit button is pressed. The carrier and its sidebands gradually stabilize (top to bottom) over several hundred milliseconds.
The spectrogram above was generated by playing back a gap-free RF capture, and we used the little radio for demonstrations, marveling at how it wandered around in a leisurely but purposeful fashion on its way to its programmed frequency. Unfortunately for other users of the band, this wandering crossed a dozen other channels, creating clicks or pops and sometimes PLL unlocks in other receivers every time the transmit button was pressed.
Fortunately, this interference was an annoyance and not a serious problem, due to the nature of the signal traffic and the low geographic density of transmitters. As far as I can tell, this sort of thing, when it was understood at all, was often just tolerated back then.
These days, the spectrum is infinitely more crowded, and many signals are bursted or otherwise time-varying, so transient interference is a much bigger problem. As a wireless engineer—whether you’re a potential interferer or interferee—you need to reliably detect and accurately measure elusive signals.
As Lou did, I’d like to summarize the techniques, and suggest a sequence:
Assess what you know about the possible interference: If you know its amplitude or frequency or timing, you can proceed to signal capture through VSA software, perhaps with a frequency-specific trigger, negative trigger delay, or both, to catch the beginning of a transient.
Apply real-time spectrum analysis (RTSA): If you suspect interference but know little or nothing about it (or want to ensure you find it if it exists), it’s time for RTSA. This is a scalar-only spectrum measurement and does not provide timing, but will ensure that you spot signals even if you know nothing about them. If timing specifics are important, consider the time-qualified trigger.
Use playback or post-processing: Once the signal in question is in the capture memory, use playback or post-processing in the VSA. Deep capture is available with graphical tools to select only the portion you want for analysis, all relative to timing established by IF magnitude, frequency mask, or time qualification.
Explore the signal during playback: You can easily change the center frequency and span to focus on the frequencies of interest. You can change parameters freely, without the need to repeat the capture, as long as the signal was somewhere in the original capture bandwidth. Repeat as necessary.
Change the analysis and display types: This is the first step to fully understanding the interference. You may want to use time and frequency markers, band-power calculations, or even demodulation to identify the signal and its critical characteristics. Spectrograms and density or persistence displays may reveal important signal behavior—and its relationship to desired signals—at a glance.
All these steps can be performed with signal analyzers that have the processing power and vector architecture to do real-time and vector signal analysis.
No matter what tools and steps are involved, the goal is to know the signal. After all, if you can understand the problem and fix it, you can keep the wireless world running until that spectrum apocalypse hits.