One of the most exciting developments in radio technology has been the introduction of software-defined radios (SDR).  As of this writing, the cellular communication network currently runs on SDR.  Yet this is not the news we often hear from our wireless carriers or regular news sources. 

Today we will look at what sets SDR apart from hardware-based radios.

What Makes a Radio a Radio?

At its core, the basic principle of wireless communications is to send information from a transmitter to a receiver.  This has been a fundamental role of radio since the earliest days of wireless telegraphy in the late 1800s.  To send a message such as speech to a remote location, the system must address 4 basic challenges:

1. Converting the message to an electronic waveform;
2. Transmitting the resulting waveform through a channel (typically air);
3. Capturing the waveform from the channel;
4. Reconstructing the original message with suitable accuracy

Some communications systems transmit their messages through electrical wire or cable.  But transmitting through air requires an antenna.  The electrical length of an antenna determines the range of frequencies it can handle effectively:

• High frequencies require short antennas;
• Low frequencies require long antennas;

Ultra-low frequencies such as the ones most important to human hearing and speech would require antennas over a kilometer long.

Excessively long antennas are avoidable by combining messages with higher frequencies—typically 100 kHz or higher.  Reformatting the message to move through the channel this way is the process of modulation.  Among other things, one goal of modulation is to combine the message with a signal of a much higher frequency called the carrier.  After the message and carrier are combined, the transmitter will drive the resulting radio frequency (RF) to its antenna where it will be broadcast as electromagnetic radiation.

The modulation process also provides a way to separate RF broadcasts that don’t wish to interfere with each other.  Figure 1 above is a screenshot of the SDR program SDR# (“SDR Sharp”).  The blue areas of this image show part of the commercial broadcast radio band.  The 3 wide, mountain-like peaks represent broadcasts from local music stations in my area.  The tall, narrow peak on the left side is interference.

Using SDR, I was able to tune into music stations, amateur radio bands, and emergency services.  This was possible even though all of those services use different types of modulation.  Fortunately, an SDR can demodulate most any type of radio signal, provided you can find the correct software.  Demodulation is the process of recovering the original message from a captured radio transmission.

In hardware-based radio, both the modulator and the demodulator are built up of electric circuits.  Amplifiers will increase the magnitude of a signal.  Tuning circuits will tune in only the portion of the radio spectrum that is of interest.  Mixers will combine two or more signals by multiplying them together, and so on.  Ultimately, the hardware built into radio is made to perform mathematical operations on signals.

There are numerous forms of modulation.  Each type requires specific circuitry.  Without going into great detail, some of the most influential modulation types of the 20^th century include:

• AM: Amplitude Modulation
• FM: Frequency Modulation
• QAM: Quadrature Amplitude Modulation (Important to analog color television)
• NBFM: Narrow-Band FM
• PM: Phase Modulation
• ASK: Amplitude Shift Keying – A type of digitized AM
• FSK: Frequency Shift Keying – A type of digitized FM
• BPSK: Bi-Phase Shift Keying – A type of digitized PM
• QPSK: Quadrature Phase Shift Keying – A type of digitized PM

…just to name a few.  Building a hardware-based radio to handle them all would be a daunting task, and probably not very economical.  And how would you add new modulation types after your radio has been assembled?

Software Defined Radio

This is where software-defined radio excels.

Ultimately, modulators and demodulators are just sequences of mathematical operations carried out on analog signals by electric circuits.  Software-defined radios carry out their mathematical operations on digital signals using programming languages like C++, Python, and Hardware Description Languages.  The process of going from analog to digital requires an analog to digital converter (ADC).  The ADC is responsible for digitizing a signal so it may be processed by software.  Reversing this process (i.e.: transmitting a software message to the antenna) requires a digital to analog converter (DAC).

In Figure 3 above, the mixer (which multiplies two signals) can be replaced by code within a program to multiply two values in real-time.  Electrical filters can be replaced with digital filters, and so on.

This begs the question: “What does the hardware in a software-defined radio actually do?”

Ideally, a software-defined radio should require no hardware at all except maybe an antenna and the ADC or processor itself.  In practice, an ADC can only deal with a limited range of radio frequencies at a time, so we must include a radio tuner before the radio signal can be digitized properly by the ADC.  Additionally, the voltages coming from a receiving antenna will be quite small (often microvolts), so some low noise amplification will be necessary.  This means a radio signal will pass through filtering and amplification hardware stages before it can reach the ADC and the software program.

With these exceptions, much of what a hardware-based radio does can be relegated to software running within a digital signal processor (DSP).  This makes SDR highly reconfigurable.

Capabilities of Software Defined Radios

The SDR unit featured in these images comes from a family of SDRs called RTL-SDR.  These devices are based on the Realtek RTL2832U, and have a retail cost of around $10 to $40 USD.  Despite being substantially less costly than most lab equipment (and many textbooks), the RTL-SDR featured below is capable of receiving radio emissions in the range of around 24 MHz to 1.8 GHz.

The RTL-SDR is only one family of SDR, and the options seem to grow with each passing year.  In terms of their capabilities, some applications of these systems include…

Receiving Commercial Radio

AM and FM stations are useful reference transmitters for testing your SDR build.  After all, these sites mostly operate 24 hours a day and transmit from fixed positions at predictable power levels.  In the United States, you can use the Federal Communication Commission (FCC) FM Query Broadcast Station Search tool to find technical details on radio stations located inside the U.S.

Decoding Digital Audio

Many of the emergency services have switched over to digital radio systems, such as APCO Project 25 (also simply called P25).  With help from digital decoding programs such as DSDPlus, you can decode this audio.

Receiving Radio Telemetry

When paired with decoding software, SDR can decode transmissions from aircraft and maritime transponders.  This lets you track vehicles in real-time.  It is also possible to receive satellite weather images.

Geolocation of Radio Emissions

Carefully designed SDR networks are capable of tracing radio broadcasts to their origin.  One of the reining methods as of this writing is the Time Difference of Arrival (TDOA) technique.  This method leverages networks of SDR receivers that have been precisely synchronized.

Information Security Auditing

General-purpose SDRs such as the HackRF One have been used in penetration testing of communication systems.  For example: according to the Kansas City Star, hospitals across the country potentially transmit unencrypted potential information in paging systems.  The problem was detected by radio enthusiasts using SDR, who then reported the problem to the press.  Similar trouble has been reported in Canada and the United Kingdom.

Measure RF exposure: SDR has been used in research measuring exposure to radio emissions in real-time.  Provided the resolution and bandwidth of the SDR are appropriate for the signal being studied. 

References

[1]	United States Federal Communications Commission, “FM Query Broadcast Station Search,” United States Federal Communications Commission, [Online]. Available: https://www.fcc.gov/media/radio/fm-query. [Accessed 2 Nov. 2019].
[2]	A. Marso and R. Kelsey, “Does your hospital still use pagers? Your personal information may be at risk,” The Kansas City Star, 24 June 2018.
[3]	Open Privacy Research Society, “Press Release: Open Privacy discovers unencrypted patient medical information broadcast across Vancouver,” Open Privacy Research Society, 9 Sept. 2019. [Online]. Available: Press Release: Open Privacy discovers unencrypted patient medical information broadcast across Vancouver. [Accessed 2 Nov 2019].
[4]	Z. Whittaker, “NHS pagers are leaking medical data,” Techcrunch, 30 Oct. 2019.
[5]	A. C. Bechet, R. Helbet, I. Bouleanu and et. al., “Low Cost Solution Based on Software Radio for the RF Exposure Assessment: A Performance Analysis,” in 11th International Symposium on Advanced Topics in Electrical Engineering (ATEE), Bucharest, Romania, 2019.

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