In the past, signals have been hidden by various means, including placing them next to a large signal such as an AM broadcast carrier or using spread spectrum technology to create low probability of detection (LPOD) signals. More recently the development of various digital modulation formats has provided a new means for signals to be hidden in plain sight. The presence of digitally modulated signals is commonplace now, to the point that if a signal looks familiar, such as a CDMA “Bart’s head,” it can easily be ignored as a known signal.
That may be a big mistake as people intent on hiding transmitters may be creating signals that look like CDMA, GSM or LTE signals but have something completely different under the hood. It is becoming necessary to confirm that a signal is actually what it appears to be. To do this requires more than measuring the shape and bandwidth of the signal, but also demodulating the signal to confirm that all the expected signaling is present on the signal. These measurements are easily done with an antenna attached to a handheld signal analyzer such as the BTS Master or Spectrum Master. That is how all the signals shown in this application note were measured.