Software Defined Radio

What is Software Defined Radio?

Software-defined radio (SDR) is a radio communication system where components that have been typically implemented in hardware (e.g. mixers, filters, amplifiers, modulators/demodulators, detectors, etc.) are instead implemented by means of software on a personal computer or embedded system. While the concept of SDR is not new, the rapidly evolving capabilities of digital electronics render practical many processes which used to be only theoretically possible.

The History of SDR

The term “digital receiver” was coined in 1970 by a researcher at a United States Department of Defense laboratory. A laboratory called the Gold Room at TRW in California created a software baseband analysis tool called Midas, which had its operation defined in software.

The term “software radio” was coined in 1984 by a team at the Garland, Texas Division of E-Systems Inc. (now Raytheon) to refer to a digital baseband receiver and published in their E-Team company newsletter. A ‘Software Radio Proof-of-Concept’ laboratory was developed there that popularized Software Radio within various government agencies. This 1984 Software Radio was a digital baseband receiver that provided programmable interference cancellation and demodulation for broadband signals, typically with thousands of adaptive filter taps, using multiple array processors accessing shared memory.

In 1991, Joe Mitola independently reinvented the term software radio for a plan to build a GSM base station that would combine Ferdensi’s digital receiver with E-Systems Melpar’s digitally controlled communications jammers for a true software-based transceiver. E-Systems Melpar sold the software radio idea to the US Air Force. Melpar built a prototype commanders’ tactical terminal in 1990-91 that employed Texas Instruments TMS320C30 processors and Harris digital receiver chip sets with digitally synthesized transmission. That prototype didn’t last long because when E-Systems ECI Division manufactured the first limited production units, they decided to “throw out those useless C30 boards,” replacing them with conventional RF filtering on transmit and receive, reverting to a digital baseband radio instead of the SPEAKeasy like IF ADC/DACs of Mitola’s prototype. The Air Force would not let Mitola publish the technical details of that prototype, nor would they let Diane Wasserman publish related software life cycle lessons learned because they regarded it as a “USAF competitive advantage.” So instead, with USAF permission, in 1991 Mitola described the architecture principles without implementation details in a paper, “Software Radio: Survey, Critical Analysis and Future Directions” which became the first IEEE publication to employ the term in 1992. When Mitola presented the paper at the conference, Bob Prill of GEC Marconi began his presentation following Mitola with “Joe is absolutely right about the theory of a software radio and we are building one.” Prill gave a GEC Marconi paper on PAVE PILLAR, a SPEAKeasy precursor. SPEAKeasy, the military software radio was formulated by Wayne Bonser, then of Rome Air Development Center (RADC), now Rome Labs; by Alan Margulies of MITRE Rome, NY; and then Lt Beth Kaspar, the original DARPA SPEAKeasy project manager and by others at Rome including Don Upmal. Although Mitola’s IEEE publications resulted in the largest global footprint for software radio, Mitola privately credits that DoD lab of the 1970s with its leaders Carl, Dave, and John with inventing the digital receiver technology on which he based software radio once it was possible to transmit via software.

A few months after the National Telesystems Conference 1992, in an E-Systems corporate program review, a vice-president of E-Systems Garland Division objected to Melpar’s (Mitola’s) use of the term “software radio” without credit to Garland. Alan Jackson, Melpar VP of marketing at that time asked the Garland VP if their laboratory or devices included transmitters. The Garland VP said “No, of course not — ours is a software radio receiver”. Al replied “Then it’s a digital receiver but without a transmitter, it’s not a software radio.” Corporate leadership agreed with Al, so the publication stood. Many amateur radio operators and HF radio engineers had realized the value of digitizing HF at RF and of processing it with Texas Instruments TI C30 digital signal processors (DSPs) and their precursors during the 1980s and early 1990s. Radio engineers at Roke Manor in the UK and at an organization in Germany had recognized the benefits of ADC at the RF in parallel, so success has many fathers. Mitola’s publication of software radio in the IEEE opened the concept to the broad community of radio engineers. His landmark May 1995 special issue of the IEEE Communications Magazine with the cover “Software Radio” was widely regarded as watershed event with thousands of academic citations. Mitola was introduced by Joao da Silva in 1997 at the First International Conference on Software Radio as “godfather” of software radio in no small part for his willingness to share such a valuable technology “in the public interest.”

Perhaps the first software-based radio transceiver was designed and implemented by Peter Hoeher and Helmuth Lang at the German Aerospace Research Establishment (DLR, formerly DFVLR) in Oberpfaffenhofen, Germany, in 1988. Both transmitter and receiver of an adaptive digital satellite modem were implemented according to the principles of a software radio, and a flexible hardware periphery was proposed.

The term “software defined radio” was coined in 1995 by Stephen Blust, who published a request for information from Bell South Wireless at the first meeting of the Modular Multifunction Information Transfer Systems (MMITS) forum in 1996, organized by the USAF and DARPA around the commercialization of their SPEAKeasy II program. Mitola objected to Blust’s term, but finally accepted it as a pragmatic pathway towards the ideal software radio. Though the concept was first implemented with an IF ADC in the early 1990s, software-defined radios have their origins in the defense sector since the late 1970s in both the U.S. and Europe (for example, Walter Tuttlebee described a VLF radio that used an ADC and an 8085 microprocessor). About a year after the First International Conference in Brussels. One of the first public software radio initiatives was the U.S. DARPA-Air Force military project named SpeakEasy. The primary goal of the SpeakEasy project was to use programmable processing to emulate more than 10 existing military radios, operating in frequency bands between 2 and 2000 MHz. Another SPEAKeasy design goal was to be able to easily incorporate new coding and modulation standards in the future, so that military communications can keep pace with advances in coding and modulation techniques.

What is RTL-SDR?

I highly recommend using the cheapest, most common and generally best performing dongle at the moment, the Rafael Micro R820T/2. It can be bought for about $20 USD.  All references and information on this site, will reference this dongle.

RTL-SDR is a very cheap software defined radio that uses a DVB-T TV tuner dongle based on the RTL2832U chipset. With the combined efforts of Antti Palosaari, Eric Fry and Osmocom it was found that the signal I/Q data could be accessed directly, which allowed the DVB-T TV tuner to be converted into a wide-band software defined radio via a new software driver.

Essentially, this means that a cheap $20 TV tuner USB dongle with the RTL2832U chip can be used as a computer based radio scanner. This sort of scanner capability would have cost hundreds or even thousands of dollars just a few years ago. The RTL-SDR is also often referred to as RTL2832U, DVB-T SDR, RTL dongle or the “$20 Software Defined Radio”.

There are many other software defined radios better than the RTL-SDR, but they all come at a higher price. Currently we think that the Airspy ($199) and SDRPlay ($149) SDR’s are the best low cost RX only SDR’s. Then there are the HackRF ($300USD) and BladeRF SDRs ($420 and $650), which can both transmit and receive.

What to listen to?

By installing and using RTL-SDR hardware and software you have now entered the long established world of scanning and receive only HAM radio. The twist is that instead of using expensive purpose built hardware you are using a PC (which you no doubt already own) and a $20 USB dongle. It is now possible to listen to:

  • FM: both narrow band and wide-band. The former is used on two way radio systems such as emergency services and private radio networks (like couriers and taxis) and UHF CB and the latter is the usual broadcast FM the likes of which you have in the kitchen and car. Aircraft and boats and ships also use narrow band FM which you can use RTL-SDR to listen to. The SDR# software can receive both narrowband and wideband FM and the latter do stereo FM too!
  • AM: Most AM transmissions are below the bottom frequency of RTL-SDR dongles. You will need a translator to get these frequency bands.
  • Upper/Lower Sideband (USB/LSB). See AM above.
  • CW: Continuous wave for morse code enthusiasts.
  • With GNURadio you can receive and demodulate digital modes such as pagers (POCSAG), ADS-B (aircraft positions), AIS (ship positions), AP25 and TETRA (digital trunk radio) and many others.
  • GPS reception is currently being worked on but should be do-able.
  • Satellite reception including receiving ham transmissions from the International Space Station are possible to. I have seen some screen shots of someone using RTL-SDR and a 2.5m dish to track the carrier signal on deep space robots such as Voyager and the Mars missions.
  • Frequency range: 24 – 1766 MHz  (Can be improved to ~13 – 1864 MHz with experimental drivers)

What are some RTL-SDR Radio Scanner Applications?

The RTL-SDR can be used as a wide band radio scanner. Applications include:

  • Listening to unencrypted Police/Ambulance/Fire/EMS conversations.
  • Listening to aircraft traffic control conversations.
  • Tracking aircraft positions like a radar with ADSB decoding.
  • Decoding aircraft ACARS short messages.
  • Scanning trunking radio conversations.
  • Decoding unencrypted digital voice transmissions.
  • Tracking maritime boat positions like a radar with AIS decoding.
  • Decoding POCSAG/FLEX pager traffic.
  • Scanning for cordless phones and baby monitors.
  • Tracking and receiving meteorological agency launched weather balloon data.
  • Tracking your own self launched high altitude balloon for payload recovery.
  • Receiving wireless temperature sensors and wireless power meter sensors.
  • Listening to VHF amateur radio.
  • Decoding ham radio APRS packets.
  • Watching analogue broadcast TV.
  • Sniffing GSM signals.
  • Using rtl-sdr on your Android device as a portable radio scanner.
  • Receiving GPS signals and decoding them.
  • Using rtl-sdr as a spectrum analyzer.
  • Receiving NOAA weather satellite images.
  • Listening to satellites and the ISS.
  • Radio astronomy.
  • Monitoring meteor scatter.
  • Listening to FM radio, and decoding RDS information.
  • Listening to DAB broadcast radio.
  • Use rtl-sdr as a panadapter for your traditional hardware radio.
  • Decoding taxi mobile data terminal signals.
  • Use rtl-sdr as a high quality entropy source for random number generation.
  • Use rtl-sdr as a noise figure indicator.
  • Reverse engineering unknown protocols.
  • Triangulating the source of a signal.
  • Searching for RF noise sources.
  • Characterizing RF filters and measuring antenna SWR.

Furthermore, with an upconverter or direct sampling mod to receive HF signals the applications are expanded to:

  • Listening to amateur radio hams on SSB with LSB/USB modulation.
  • Decoding digital amateur radio ham communications such as CW/PSK/RTTY/SSTV.
  • Receiving HF weatherfax.
  • Receiving digital radio mondiale shortwave radio (DRM).
  • Listening to international shortwave radio.
  • Looking for RADAR signals like over the horizon (OTH) radar, and HAARP signals.

Note that not all the applications listed may be legal in your country. Please be responsible.

Most of the information provided for the RTL-SDR dongle and related software, were provided by one of the best sources on SDR and RTL-SDR –  I highly recommend buying your SDR dongle via