Software-defined_radio

# Software-defined radio **Software-defined radio** (SDR) is a radio-communication design in which functions conventionally built from fixed analogue hardware, mixers, filters, modulators, demodulators and detectors among them, are instead carried out by software running on a general-purpose or programmable processor, leaving only an antenna, a radio-frequency front end and an [[Analog-to-digital_converter|analog-to-digital converter]] (and, on transmit, a matching [[Digital-to-analog_converter|digital-to-analog converter]]) as dedicated hardware. This article carries no interactive sketch of its own; its Microsims section below points instead to sketches carried by neighbouring articles that render pieces of the same digitize-then-compute idea. Because the same hardware can be reprogrammed to receive or transmit an entirely different protocol, sometimes called a waveform, a single SDR platform can serve roles that would otherwise each need their own dedicated radio, a flexibility that first attracted militaries and mobile-network operators juggling incompatible standards, and later attracted hobbyists who found that the same idea worked just as well built from a ten-dollar television-tuner chip as from a purpose-built receiver. The approach traces to military research into a single radio able to emulate many otherwise incompatible sets, grew through successive [[United_States_Air_Force|United States Air Force]]-sponsored demonstrations into the general-purpose platforms described in Amateur and home use below, and today underlies many [[Direct-conversion_receiver|direct-conversion]] front ends that hand everything past the antenna to software. ## Operating principles An SDR receiver's task is to move the boundary between hardware and software as close to the antenna as its [[Analog-to-digital_converter|analog-to-digital converter]] allows, so that mixing, filtering, demodulation and decoding, all traditionally analogue tasks, become numerical operations on a stream of samples instead. What that converter can capture is bounded by two of its own numbers: its sample rate, which the sampling theorem requires to exceed twice the highest frequency of interest, or a component above that limit folds back, aliased, onto a lower apparent frequency indistinguishable from a true one there;[^sdr-sampling] and its resolution in bits, which sets the quantization-noise floor against which a weak signal must be read, `SNR ≈ 6.02*N + 1.76` decibels for an N-bit converter digitizing a full-scale sine wave, so each added bit buys roughly 6 decibels of usable dynamic range at the cost of a faster, pricier part.[^sdr-quant] A wideband front end sampling directly at radio frequency asks a great deal of both numbers at once, which is why many practical SDR designs still perform one analogue down-conversion, often to a low or zero intermediate frequency, before digitizing, trading some of the software-defined ideal for a converter within reach of the shelf. On transmit the same trade runs in reverse: a [[Digital-to-analog_converter|digital-to-analog converter]] turns a computed sample stream back into a continuous waveform, a step that is exact only for an idealized reconstruction filter and in practice leaves a staircase that a real converter's zero-order hold approximates, with spectral images above the sample rate that an analogue filter downstream must remove before the signal reaches the antenna.[^sdr-recon] Everything upstream of that last analogue filter and the antenna itself, however, from generating the waveform's [[Signal_modulation|modulation]] to running whatever [[Digital_signal_processing|digital signal processing]] the protocol specifies, can be changed by loading different software rather than by rewiring hardware, which is the entire appeal of the architecture. ## History Software-defined radio grew out of military efforts to replace a rack of separate, single-purpose radios with one reconfigurable platform, and the term itself is generally credited to a mid-1990s paper describing a software radio architecture built around exactly that goal.[^mitola1995] ### SpeakEasy phase I The SpeakEasy program set out to demonstrate a single hardware platform, controlled by software, able to reproduce the waveforms of more than a dozen existing military radios in turn rather than requiring a separate radio for each one.[^cn-speakeasy] Its first phase proved the concept could work at all, at a size, weight and cost that were, by the standards of a purpose-built military radio, considerable, but that were accepted as the price of the flexibility being demonstrated. ### SpeakEasy phase II A second phase pushed the same idea toward something closer to a fieldable radio, adding new waveforms and improving how quickly the platform could be reprogrammed to carry one it had not previously supported, work that fed directly into the larger military software-radio programs described in Military usage below.[^cn-speakeasy] ### 2000s The 2000s carried the same idea out of military laboratories and into general civilian use, as open signal-processing software running on an ordinary computer, paired with increasingly capable and increasingly affordable digitizing hardware, let individual experimenters build receivers, and eventually transmitters, whose entire signal chain past the antenna was ordinary, inspectable code rather than a fixed circuit board.[^cn-2000s] ## Military usage ### United States Military interest in software-defined radio in the United States grew from the SpeakEasy demonstrations described above into larger acquisition programs, generally grouped under the name Joint Tactical Radio System, aiming to give different branches of the armed forces radios that could interoperate on a common set of [[Communication_protocol|waveforms]], rather than each service, and often each unit, carrying equipment unable to talk to the others.[^cn-jtrs] Incompatible radios between branches, and even between units within the same branch, had long been a recognized problem during joint operations, and a single reprogrammable radio family promised to replace that patchwork with one hardware design carrying whichever waveform a given mission required, loaded as software rather than built into a separate box for each protocol. These programs inherited SpeakEasy's central bet, that a reprogrammable software radio would eventually cost less to field and to upgrade than a fleet of single-purpose sets, and inherited too the difficulty of proving that bet correct on a schedule and a budget a defence-acquisition program can be held to, a tension that stretched schedules, drew sustained oversight scrutiny, and shaped military software-radio efforts for years afterward. ## Amateur and home use Everything the military programs above spent years and large budgets learning, later civilian projects reproduced, piece by piece, at hobbyist prices, once general-purpose computers and cheap wideband digitizing hardware put an entire receiver's signal chain within reach of a personal computer's own processor. ### RTL-SDR RTL-SDR is not a single manufactured product but a repurposing: a family of inexpensive USB television-tuner sticks, sold to receive digital broadcast television, was found to contain a chip able to hand raw digitized radio-frequency samples to a host computer instead of decoding television with them, turning a device costing a small fraction of a purpose-built receiver into a wideband SDR receiver whose real limits are only its digitizer's modest resolution and its tuner chip's own frequency range.[^cn-rtlsdr] The discovery did more than produce one cheap receiver: it put a usable SDR front end within reach of far more experimenters than any earlier hobbyist project had, and much of the software written for other SDR hardware since has had to run on this least-capable, most widely owned front end as a baseline case. ### HPSDR The High Performance Software Defined Radio project is an open, published hardware and software design for an amateur-radio-grade software-defined transceiver, built from separately documented boards rather than a single fixed product, so that a builder or a small manufacturer can assemble, modify or improve any one stage without redesigning the rest.[^cn-hpsdr-etc] Its open documentation let commercial and hobbyist offerings alike build on the same reference design, in the way an open standard is meant to work. ### WebSDR A WebSDR lets many listeners share one physical receiver at once over an ordinary web browser, each tuning independently to a different frequency within the receiver's digitized band, because the server digitizes a wide swath of spectrum once and streams whichever slice of it each visitor has asked for, rather than needing one radio per listener.[^cn-hpsdr-etc] The arrangement turns a single well-placed antenna and receiver into a shared public instrument, letting a listener anywhere hear a part of the spectrum, a particular shortwave band from a particular country, say, that their own location or equipment could never receive directly. ### KiwiSDR A KiwiSDR is a small, self-contained SDR receiver board built specifically to be left running continuously and shared publicly online in the same manner as a WebSDR, packaging the digitizing hardware, the web-serving software and the network connection into one inexpensive unit rather than requiring a separate general-purpose computer alongside a receiver board.[^cn-hpsdr-etc] Numerous KiwiSDR units left online by their owners now form a loose, informal, worldwide network of shared receivers that anyone can tune. ## Other applications Beyond military and amateur use, the same reconfigurable-receiver idea has spread into public-safety and commercial radio systems that must support several incompatible legacy standards during a slow transition to a common one, into cellular-network base stations that can be retuned to a new air interface with a software update rather than a hardware replacement, and into radio-astronomy and spectrum-monitoring instruments that need to digitize a wide, arbitrary swath of spectrum and decide only afterward, in software, what to do with it. A software-defined receiver is also the natural front end for an [[Adaptive_filter|adaptive]] radio system that must sense its environment and change how it transmits or receives in response, since the sensing and the response can both be expressed as code running on the same platform rather than as separate fixed circuits.[^cn-otherapps] Secure military and government systems benefit in a related way: a waveform's [[Cryptography|cryptographic]] protection can be updated by loading new software rather than by replacing hardware in the field. ## Microsims This article carries no p5.js sketch, and no three.js companion is planned for it in this run. The sampling, reconstruction and quantization limits described in Operating principles above are the same limits the sketches carried by neighbouring articles render directly, each from a different angle. *Try:* in the [[Radar]] sketch, lower the pulse-repetition frequency until a distant target folds back to a false, short range - the same kind of aliasing an SDR's analog-to-digital converter produces on any frequency component above half its own sample rate. *Try:* in the [[Doppler_effect]] sketch, raise the source speed and read the shifted frequency the observer markers report - exactly the kind of frequency estimate an SDR performs numerically on digitized samples, in software, rather than with a dedicated analogue circuit built for that one task alone. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Software-defined_radio) : [Wikitube](https://en.wikitube.io/wiki/Software-defined_radio) Skeleton mirrored at revision 1362756665. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Direct-conversion_receiver]] - [[Radio_receiver]] - [[Analog-to-digital_converter]] - [[Digital_signal_processing]] - [[Superheterodyne_receiver]] - [[Cryptography]] ## References The sampling theorem, the zero-order-hold reconstruction penalty and the quantizer's SNR formula are standard textbook results and are not separately footnoted beyond the citations below, per the Wikitube style guide §6.1. Page numbers are PDF pages of the open editions linked below. [^sdr-sampling]: Tiberius, C.; Mulder, M. *Engineering Signal Analysis: From Fourier to filtering: Theory*. 2026, pp. 108-113 (PDF pages). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/engineering-signal-analysis-from-fourier-to-filtering-theory . CC BY. [^sdr-recon]: Tiberius, C.; Mulder, M. *Engineering Signal Analysis: From Fourier to filtering: Theory*. 2026, pp. 116-122 (PDF pages). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/engineering-signal-analysis-from-fourier-to-filtering-theory . CC BY. [^sdr-quant]: Dyer, J.; Davis, C. *Measurement and Instrumentation: An Introduction to Concepts and Methods, 1st Edition*. 2020, pp. 39-40 (PDF pages). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/measurement-and-instrumentation-an-introduction-to-concepts-and-methods . CC BY-NC-SA. [^mitola1995]: Mitola, J. "The software radio architecture." *IEEE Communications Magazine*, vol. 33, no. 5, 1995, pp. 26-38. [^cn-speakeasy]: Citation needed: a primary program record (contractor, sponsoring service and dates) for SpeakEasy phases I and II has not been pinned down in this pass. [^cn-2000s]: Citation needed: a primary source naming the specific open hardware and software projects that carried software-defined radio into general civilian use during the 2000s. [^cn-jtrs]: Citation needed: a primary program or oversight-agency record naming the United States military software-defined-radio acquisition programs that followed SpeakEasy, with their dates and outcomes. [^cn-rtlsdr]: Citation needed: a primary source (forum post, paper or changelog) identifying who first demonstrated raw sample output from a DVB-T tuner chip, and when. [^cn-hpsdr-etc]: Citation needed: primary founding records, with creators and years, for the HPSDR, WebSDR and KiwiSDR projects have not been pinned down in this pass. [^cn-otherapps]: Citation needed: a named cellular standard or radio-astronomy instrument documenting software-defined radio's adoption in that setting would support a concrete example here. ## Further reading - Steven Ellingson. *Radio Systems Engineering, Revised First Edition*. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering - Christian Tiberius; Max Mulder. *Engineering Signal Analysis: From Fourier to filtering: Theory*. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/engineering-signal-analysis-from-fourier-to-filtering-theory - Don Johnson. *Fundamentals of Electrical Engineering I*. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/fundamentals-of-electrical-engineering-1 - Michael Stiber; Bilin Stiber; Eric Larson. *Signal Computing: Digital Signals in the Software Domain*. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/signal-computing-digital-signals-in-the-software-domain - Allen Downey. *Think DSP: Digital Signal Processing in Python*. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/think-dsp-digital-signal-processing-in-python - John Dyer; Chad Davis. *Measurement and Instrumentation: An Introduction to Concepts and Methods, 1st Edition*. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/measurement-and-instrumentation-an-introduction-to-concepts-and-methods ## External links This article carries no p5.js sketch and no three.js companion of its own. The live sketches this article's *Try:* lines send the reader to are: - Radar, live: https://editor.p5js.org/sciencenibber/full/40gQ6vCke — editor: https://editor.p5js.org/sciencenibber/sketches/40gQ6vCke - Doppler effect, live: https://editor.p5js.org/sciencenibber/full/ZeLNTrMSo — editor: https://editor.p5js.org/sciencenibber/sketches/ZeLNTrMSo