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Lab 3 – Exploring the RF Spectrum

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Lab 3 – Exploring the RF Spectrum
ECE531 – Software Defined Radio

1 Introduction: Spectrum Exploration
This lab was all about exploring the RF spectrum using software defined radio. To that end, we viewed the bands in which
the signals we use every day reside and analyzed several various modulation techniques. In the first section, we explore
different signals and identify distinct modulation techniques. In the second section, we view the entire spectrum the
Pluto is able to tune to. In the third section, we analyze and (almost) decode the codes that RF transmitting devices use to
unlock cars.
1.1 Signal Identification
The sigidwiki.com signal identification guide is intended to help identify radio signals through example sounds and
waterfall images. Most cataloged signals at sigidwiki are received and recorded using a software defined radio. This can
be a useful resource to identify signals when exploring the RF spectrum.
The following references can also help determine what signals are available in your location:
• http://reboot.fcc.gov/reform/systems/spectrum-dashboard
• http://www.radioreference.com/
Figure 1. Spectrum Exploration using fosphor
Using fosphor, or other spectrum waterfall plot, identify at least four different modulation techniques using received
over-the-air transmissions. (i.e. AM, FM, LTE (OFDM), GSM, etc).
1. Describe the frequency spectrum and time characteristics observed which reveal the modulation.
1.2 IEEE 802.11 Wireless Local Area Networks
Another interesting experiment is using the PlutoSDR to observe the spectrum of wireless local area networks within the
vicinity (WLANs). Most high population density areas employs numerous wireless communication networks for a variety
of applications, such as the university wireless network. Consequently, you can use your PlutoSDR experimentation
platform to plot their magnitude spectrum. IEEE 802.11 [6] is one type of WLAN standard that possesses a list of carrier
frequencies for a collection of Wi-Fi channels. For example, The IEEE 802.11 standard defines Channel 1 of the 2.4 GHz
band to be centered at 2.412 GHz as seen in Figure 4.
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Figure 2. IEEE 802.11 Wi-Fi channels
Specify the CenterFrequency parameter of sdrrx with this carrier frequency, use the FFTs or dsp.SpectrumAnalyzer
objects to plot their magnitude spectrums.
1. Since your AD9364 transceiver supports the 6GHz Band as well, also use the dsp.SpectrumAnalyzer to plot spectrum in
the 5 GHz band.
You might want to turn on “Spectral Averaging” or “PlotMaxHoldTrace” and adjust the amplitude, since the peaks of these
wireless signals could be rather low. It might also be useful to change the BasebandSampleRate parameter of sdrrx to
adjust the frequency resolution for a better graph of the spectrum.
2 Collecting Spectral Data
Utilizing captured data from a file source can make testing much more repeatable when debugging issues during algorithm
development. However, keep in mind that recording complex samples at high sample rates can result in large files very
quickly.
In MATLAB, data samples can be recorded using the comm.BasebandFileWriter system object and read using the
comm.BasebandFileReader system object. For more information on these, see §5.2 in the textbook.
In GNU Radio, the File Sink is available to record complex samples. These samples can then be read using the File Source
block, Inspectrum, or Octave and MATLAB. Inspectrum is a tool for analysing captured signals, primarily from softwaredefined radio receivers [7].
Source files for reading GNU Radio sample files in MATLAB can be found at https://github.
com/gnuradio/gnuradio/tree/master/gr-utils/octave
2.1 Sweeping the Spectrum: Receiving from 60MHz to 6GHz
The PlutoSDR is limited to a maximum bandwidth of 56MHz, but is capable of tuning from 70MHz to 6GHz (with firmware
modification). By performing a frequency sweep with a single SDR, we are able to stitch together multiple sampling
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intervals to get a picture of the wider spectrum. The local oscillator can step by 𝐹𝑠 to get each next adjacent band, as shown
in Figure 5.
Figure 3. Combining individual “tunes” to sweep the spectrum allows full RF spectrum visualization
1. Write a script to capture the entire tunable bandwidth by stitching together multiple intermediate frequencies at
different tuned LO
2. Label some known signals on the resulting spectrum plot
Note: You may wish to use a lower sampling frequency and more LO steps due to the amplitude variation of the Pluto RF
front end.
3 Understanding Your Wireless Devices
RF transmitting devices are typically registered with the FCC for compliance, including emissions test results. A device’s
FCC ID can be used to search the FCC database to determine some details such as manufacturer, frequency and
modulation methods. Internet sites, such as http://fcc.io/, make searching the FCC database very easy.
SDRs and RF scanners make it very easy to record and reply fixed code sequences. For security, devices such as garage door
openers and keyless entry systems utilize rolling codes to prevent replay attacks, where an eavesdropper records the
transmission and replays it at a later time to cause the receiver to ’unlock’.
Figure 4. This garage door opener FCC ID label can be used with http://fcc.io/ to find transmit frequency
1. Find a garage door opener remote control, remote keyfob, or similar RF transmitting device.
My ford fusion hybrid’s FCC ID is M3N-A2C931423
2. Look up the device using its FCC ID. What is the device’s operating frequency? The device’s operating frequency is
314.95 MHz.
3. Use the PlutoSDR to find the signal produced by the device. At what frequency did you find the device? The device
was found at 314.95 MHz, and there was another peak nearby this frequency,
4. Look at the FFT plot of the signal. What modulation is the device using? In Inspectrum, I played with the he
waterfall plot, at different intervals, shows bursts of different frequencies. This tells me the device is using FSK as its
modulation technique. I confirmed this with the device’s user’s manual.
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5. What technique could you use to demodulate and convert to binary data?
6. Visually decode the transmitted bits, does the same sequence of bits repeat while you hold down the button?
Does the sequence change if you release the button and press it again?

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