Spectrum sensing is a phrase that is being used a lot at my current job. I have mentioned it before in relation to the experiments in Munich back in February. Let me explain what it means and why it is important.
One possible way of enhancing radio communications in the future is making the receivers and transmitters aware of their environment and capable of adjusting the radio link accordingly. For instance, they could intelligently avoid uncontrollable interference at a specific frequency, cooperatively share a limited part of the spectrum or use frequencies that experience the least fading in the current location. This idea usually falls under the somewhat awkward umbrella of cognitive radio (which sometimes also includes gratuitous applications of strong artificial intelligence and other things not directly related to radio communications).
So broadly speaking, spectrum sensing means measuring the properties of the radio-frequency part of the electromagnetic radiation propagating in an area of interest. In the current real-life usage scenarios you are usually interested in knowing whether there are other third-party transmitters operating in the same part of the spectrum as you. This might be because you don't want them to interfere with your connection. But equally important are cases where you don't want to interfere with them. For instance, recently frequencies where formerly only big, licensed TV operators were allowed to transmit are being opened to general public and consumer devices, with the added catch that these devices must make sure their transmissions will not interfere with licensed users.
The latter use case is especially problematic. If you are only interested in the effect of any third-party transmitters on your radio link, measuring their signal strength at your own antenna is sufficient as the location of the measurement is the same as the point of interest. However electromagnetic field theory says that in general case just by doing measurements at your antenna you can't infer how your transmissions will affect a link between two distant devices in your neighborhood. Empirical rules have been developed though that work in common circumstances with high enough reliability, but they are necessarily hard to satisfy in practice as they require very sensitive spectrum sensing receivers.
How do you detect a transmission? The most simple method is called energy detection - you are simply detecting the received signal level on the antenna and declare a transmission has been detected if the level is high enough above the noise level. Energy detectors work quite similar to classical swept-tuned spectrum analyzers except that they are much simpler and cheaper. Usually an integrated silicon tuner is used. For example Texas Instruments CC2500 is a popular choice for the 2.4 GHz ISM band.
Simple energy detection has one big problem though: you can only detect signals that are significantly above the noise level. For example, in TV band white-spaces FCC requires detection threshold of -114 dBm. At this levels of sensitivity even the unavoidable thermal noise presents major problems. This can be solved though with more advanced methods of detections. For instance, repeating patterns can be still detected when the signal to noise level falls well below unity. And most real-world transmissions include some repetition, so cyclostationary detection doesn't hurt generality much.
In conclusion, to add some practice to all of this theory, here is a spectrum sensing receiver I developed at the Jožef Stefan Institute during the past months. It fits on a VESNA node and is built around the TDA18219HN silicon tuner from NXP. This single chip includes most of the radio-frequency circuitry as well as the intermediate frequency part, a lot of which can be reconfigured through an I2C interface. It's also cheap enough that many such receivers can be used in a sensor network.
The receiver can do energy detection on the VHF and UHF bands with receive bandwidths from 1.7 MHz and 9 MHz and is specifically designed for research into TV white-spaces reuse. In theory it should also be capable of cyclostationary detection using VESNA's CPU, although that has yet to be tested in practice. Here you can see a spectrogram of the UHF TV band that was recorded with it. The central Slovenian DVB-T multiplex can be clearly seen at 562 MHz.
Making this hardware was a lot of fun and I might write a bit more about it in a separate post. There is some ambiguity about the amount of information I can disclose about it though as the documentation for the tuner chip and reference implementation came with some crazy restrictive fine print. However you can already dig through the source code of VESNA spectrum sensing application and my spectrum analyzer Python script (which has been recently updated to work with a properly equipped VESNA in addition to Fun Cube Dongle).