Wireless microphone simulation with VESNA

13.05.2013 11:51

It sounds strange, but analog FM wireless microphones (the kind you find in studios, theaters and soon-to-be-restaurants) are one of the obstacles in the efficient re-use of the UHF band that has been freed with the digital switchover. They operate in the same frequency band as the now silenced analogue TV broadcasts, but on the other hand have never been well regulated. While locations and frequencies of TV transmitters are well known, microphones can appear on a lot of different channels and practically in any location.

New devices operating in the TV white-spaces can't be allowed to interfere with them, so they either can't use frequencies where microphones appear (leading to unused spectrum) or they must detect them using spectrum sensing. Reliable and energy-efficient detection of wireless microphones at low signal levels is an open research topic.

To test different ways of detecting microphones, IEEE has come up with simulation waveforms that can be easily reproduced on lab equipment and are reasonably similar to what microphones transmit in real-life usage. They simulate a silent microphone, microphone used with a soft voice and microphone used with a loud voice by frequency modulating a sine wave with different baseband frequencies and deviations (described in this Word document).

For example, here's how a soft voice simulation looks like on a spectrum analyzer when transmitted by a big and expensive Rohde&Schwarz vector signal generator. This is a 3.9 kHz tone modulated using a 15 kHz deviation:

Spectrogram of a wireless microphone simulation using a vector signal generator.

And this is the same signal transmitted with an USRP N210:

Spectrogram of a wireless microphone simulation using USRP N210.

As I mentioned before, VESNA also has some signal generation capabilities. This is how it looks like when the signal is simulated using a direct digital synthesis algorithm and a 4FSK modulator on the CC1101 transceiver:

Spectrogram of a wireless microphone simulation using VESNA SNE-ISMTV.

The order-of-magnitude in price difference in hardware shows itself well enough here as transmission from VESNA obviously differs quite a bit from the desired. But still it's good enough for some detection experiments and of course, you can't have a USRP mounted on every light pole.

Although I haven't yet done detailed measurements, VESNA's transmission actually does seem to comply with FCC's requirement regarding spurious levels for wireless microphones (but not with more stringent European ETSI regulations).

Posted by Tomaž | Categories: Analog

Comments

Hi Avian!
I am a graduate student in BUPT of China and I am working on spectrum sensing with USRP, but I do not have a signal generator to generate wireless microphone signal, so I am curious about the way you generate wireless microphone signal using USRP, can you give me some instructions?
Thank you!

Posted by XuFei

As I mention above, the signal is just an FM carrier modulated with a tone. You can use a cosine source and a frequency modulation block for that in GNU Radio.

Here's a description of more complete FM transmitter.

http://www.opendigitalradio.org/index.php/Simple_FM_transmitter_using_gnuradio

Posted by Tomaž

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