Software-defined radio is a radio communication system where components that have been traditionally implemented in hardware are instead implemented by means of software on a personal computer or embedded system. While the concept of SDR is not new, the evolving capabilities of digital electronics render practical many processes which were once only theoretically possible. A basic SDR system may consist of a personal computer equipped with a sound card, or other analog-to-digital converter, preceded by some form of RF front end. Significant amounts of signal processing are handed over to the general-purpose processor, rather than being done in special-purpose hardware; such a design produces a radio which can receive and transmit different radio protocols based on the software used. Software radios have significant utility for the military and cell phone services, both of which must serve a wide variety of changing radio protocols in real time. In the long term, software-defined radios are expected by proponents like the SDRForum to become the dominant technology in radio communications.
SDRs, along with software defined. A software-defined radio can be flexible enough to avoid the "limited spectrum" assumptions of designers of previous kinds of radios, in one or more ways including: Spread spectrum and ultrawideband techniques allow several transmitters to transmit in the same place on the same frequency with little interference combined with one or more error detection and correction techniques to fix all the errors caused by that interference. Software defined antennas adaptively "lock onto" a directional signal, so that receivers can better reject interference from other directions, allowing it to detect fainter transmissions. Cognitive radio techniques: each radio measures the spectrum in use and communicates that information to other cooperating radios, so that transmitters can avoid mutual interference by selecting unused frequencies. Alternatively, each radio connects to a geolocation database to obtain information about the spectrum occupancy in its location and, adjusts its operating frequency and/or transmit power not to cause interference to other wireless services.
Dynamic transmitter power adjustment, based on information communicated from the receivers, lowering transmit power to the minimum necessary, reducing the near-far problem and reducing interference to others, extending battery life in portable equipment. Wireless mesh network where every added radio increases total capacity and reduces the power required at any one node; each node only transmits loudly enough for the message to hop to the nearest node in that direction, reducing near-far problem and reducing interference to others. The ideal receiver scheme would be to attach an analog-to-digital converter to an antenna. A digital signal processor would read the converter, its software would transform the stream of data from the converter to any other form the application requires. An ideal transmitter would be similar. A digital signal processor would generate a stream of numbers; these would be sent to a digital-to-analog converter connected to a radio antenna. The ideal scheme is not realizable due to the current limits of the technology.
The main problem in both directions is the difficulty of conversion between the digital and the analog domains at a high enough rate and a high enough accuracy at the same time, without relying upon physical processes like interference and electromagnetic resonance for assistance. Most receivers use a variable-frequency oscillator and filter to tune the desired signal to a common intermediate frequency or baseband, where it is sampled by the analog-to-digital converter. However, in some applications it is not necessary to tune the signal to an intermediate frequency and the radio frequency signal is directly sampled by the analog-to-digital converter. Real analog-to-digital converters lack the dynamic range to pick up sub-microvolt, nanowatt-power radio signals. Therefore, a low-noise amplifier must precede the conversion step and this device introduces its own problems. For example, if spurious signals are present, these compete with the desired signals within the amplifier's dynamic range.
They may block them completely. The standard solution is to put band-pass filters between the antenna and the amplifier, but these reduce the radio's flexibility. Real software radios have two or three analog channel filters with different bandwidths that are switched in and out; 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, Division of E-Systems Inc. to refer to a digital baseband receiver and published in their E-Team company newsletter. A'Software Radio Proof-of-Concept' laboratory was developed by the E-Systems team 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 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 Ferde
Wireless telegraphy means transmission of telegraph signals by radio waves. Before about 1910 when radio became dominant, the term wireless telegraphy was used for various other experimental technologies for transmitting telegraph signals without wires, such as electromagnetic induction, ground conduction telegraph systems. Radiotelegraphy was the first means of radio communication, it continued to be the only type of radio transmission during the first three decades of radio, called the "wireless telegraphy era" up until World War I, when the development of amplitude modulation radiotelephony allowed sound to be transmitted by radio. In radiotelegraphy, information is transmitted by pulses of radio waves of two different lengths called "dots" and "dashes", which spell out text messages in Morse code. In a manual system, the sending operator taps on a switch called a telegraph key which turns the transmitter on and off, producing the pulses of radio waves. At the receiver the pulses are audible in the receiver's speaker as beeps, which are translated back to text by an operator who knows Morse code.
Radiotelegraphy was used for long distance person-to-person commercial and military text communication throughout the first half of the 20th century. It became a strategically important capability during the two world wars, since a nation without long distance radiotelegraph stations could be isolated from the rest of the world by an enemy cutting its submarine telegraph cables. Beginning about 1908, powerful transoceanic radiotelegraphy stations transmitted commercial telegram traffic between countries at rates up to 200 words per minute. Radiotelegraphy was transmitted by several different modulation methods during its history; the primitive spark gap transmitters used until 1920 transmitted damped waves, which had large bandwidth and tended to interfere with other transmissions. This type of emission was banned by 1930; the vacuum tube transmitters which came into use after 1920 transmitted code by pulses of unmodulated sinusoidal carrier wave called continuous waves, still used today. To make CW transmissions audible, the receiver requires a circuit called a beat frequency oscillator.
A third type of modulation, frequency shift keying was used by radioteletypes. Morse code radiotelegraphy was replaced by radioteletype networks in most high volume applications by World War 2. Today it is nearly obsolete, the only remaining users are the radio amateur community and some limited training by the military for emergency use. Wireless telegraphy or radiotelegraphy called CW, ICW transmission, or on-off keying, designated by the International Telecommunication Union as emission type A1A, is a radio communication method in which the sending operator taps on a switch called a telegraph key, which turns the radio transmitter on and off, producing pulses of unmodulated carrier wave of different lengths called "dots" and "dashes", which encode characters of text in Morse code. At the receiving location the code is audible in the radio receiver's earphone or speaker as a sequence of buzzes or beeps, translated back to text by an operator who knows Morse code. Although this type of communication has been replaced since its introduction over 100 years ago by other means of communication it is still used by amateur radio operators as well as some military services.
A CW coastal station, KSM, still exists in California, run as a museum by volunteers, occasional contacts with ships are made. Radio beacons in the aviation service, but as "placeholders" for commercial ship-to-shore systems transmit Morse but at slow speeds; the US Federal Communications Commission issues a lifetime commercial Radiotelegraph Operator License. This requires passing a simple written test on regulations, a more complex written exam on technology, demonstrating Morse reception at 20 words per minute plain language and 16 wpm code groups. Wireless telegraphy is still used today by amateur radio hobbyists where it is referred to as radio telegraphy, continuous wave, or just CW. However, its knowledge is not required to obtain any class of amateur license. Continuous wave radiotelegraphy is regulated by the International Telecommunication Union as emission type A1A. Efforts to find a way to transmit telegraph signals without wires grew out of the success of electric telegraph networks, the first instant telecommunication systems.
Developed beginning in the 1830s, a telegraph line was a person-to-person text message system consisting of multiple telegraph offices linked by an overhead wire supported on telegraph poles. To send a message, an operator at one office would tap on a switch called a telegraph key, creating pulses of electric current which spelled out a message in Morse code; when the key was pressed, it would connect a battery to the telegraph line, sending current down the wire. At the receiving office the current pulses would operate a telegraph sounder, a device which would make a "click" sound when it received each pulse of current; the operator at the receiving station who knew Morse code would translate the clicking sounds to text and write down the message. The ground was used as the return path for current in the telegraph circuit, to avoid having to use a second overhead wire. By the 1860s, telegraph was the standard way to send most urgent commercial and milita
A digital signal is a signal, being used to represent data as a sequence of discrete values. This contrasts with an analog signal. Simple digital signals represent information in discrete bands of analog levels. All levels within a band of values represent the same information state. In most digital circuits, the signal can have two possible values, they are represented by two voltage bands: one near a reference value, the other a value near the supply voltage. These correspond to the two values "zero" and "one" of the Boolean domain, so at any given time a binary signal represents one binary digit; because of this discretization small changes to the analog signal levels do not leave the discrete envelope, as a result are ignored by signal state sensing circuitry. As a result, digital signals have noise immunity. Digital signals having more than two states are used. For example, signals that can assume three possible states are called three-valued logic. In a digital signal, the physical quantity representing the information may be a variable electric current or voltage, the intensity, phase or polarization of an optical or other electromagnetic field, acoustic pressure, the magnetization of a magnetic storage media, etcetera.
Digital signals are used in all digital electronics, notably computing equipment and data transmission. The term digital signal has related definitions in different contexts. In digital electronics a digital signal is a pulse train, i.e. a sequence of fixed-width square-wave electrical pulses or light pulses, each occupying one of a discrete number of levels of amplitude. A special case is a logic signal or a binary signal, which varies between a low and a high signal level. In digital signal processing, a digital signal is a representation of a physical signal, a sampled and quantized. A digital signal is an abstraction, discrete in time and amplitude; the signal's value only exists at regular time intervals, since only the values of the corresponding physical signal at those sampled moments are significant for further digital processing. The digital signal is a sequence of codes drawn from a finite set of values; the digital signal may be stored, processed or transmitted physically as a pulse-code modulation signal.
In digital communications, a digital signal is a continuous-time physical signal, alternating between a discrete number of waveforms, representing a bitstream. The shape of the waveform depends the transmission scheme, which may be either a line coding scheme allowing baseband transmission; such a carrier-modulated sine wave is considered a digital signal in literature on digital communications and data transmission, but considered as a bitstream converted to an analog signal in electronics and computer networking. In communications, sources of interference are present, noise is a significant problem; the effects of interference are minimized by filtering off interfering signals as much as possible and by using data redundancy. The main advantages of digital signals for communications are considered to be the noise immunity to noise capability, the ability, in many cases such as with audio and video data, to use data compression to decrease the bandwidth, required on the communication media.
A waveform that switches representing the two states of a Boolean value is referred to as a digital signal or logic signal or binary signal when it is interpreted in terms of only two possible digits. The two states are represented by some measurement of an electrical property: Voltage is the most common, but current is used in some logic families. A threshold is designed for each logic family; when below that threshold, the signal is low. The clock signal is a special digital signal, used to synchronize many digital circuits; the image shown can be considered the waveform of a clock signal. Logic changes are triggered either by the falling edge; the rising edge is the transition from a low voltage to a high voltage. The falling edge is the transition from a high voltage to a low one. Although in a simplified and idealized model of a digital circuit, we may wish for these transitions to occur instantaneously, no real world circuit is purely resistive and therefore no circuit can change voltage levels.
This means that during a short, finite transition time the output may not properly reflect the input, will not correspond to either a logically high or low voltage. To create a digital signal, an analog signal must be modulated with a control signal to produce it; the simplest modulation, a type of unipolar encoding, is to switch on and off a DC signal, so that high voltages represent a'1' and low voltages are'0'. In digital radio schemes one or more carrier waves are amplitude, frequency or phase modulated by the control signal to produce a digital signal suitable for transmission. Asymmetric Digital Subscriber Line over telephone wires, does not use binary logic.
Amplitude modulation is a modulation technique used in electronic communication, most for transmitting information via a radio carrier wave. In amplitude modulation, the amplitude of the carrier wave is varied in proportion to that of the message signal being transmitted; the message signal is, for example, a function of the sound to be reproduced by a loudspeaker, or the light intensity of pixels of a television screen. This technique contrasts with frequency modulation, in which the frequency of the carrier signal is varied, phase modulation, in which its phase is varied. AM was the earliest modulation method used to transmit voice by radio, it was developed during the first quarter of the 20th century beginning with Landell de Moura and Reginald Fessenden's radiotelephone experiments in 1900. It remains in use today in many forms of communication. AM is used to refer to mediumwave AM radio broadcasting. In electronics and telecommunications, modulation means varying some aspect of a continuous wave carrier signal with an information-bearing modulation waveform, such as an audio signal which represents sound, or a video signal which represents images.
In this sense, the carrier wave, which has a much higher frequency than the message signal, carries the information. At the receiving station, the message signal is extracted from the modulated carrier by demodulation. In amplitude modulation, the amplitude or strength of the carrier oscillations is varied. For example, in AM radio communication, a continuous wave radio-frequency signal has its amplitude modulated by an audio waveform before transmission; the audio waveform modifies the amplitude of the carrier wave and determines the envelope of the waveform. In the frequency domain, amplitude modulation produces a signal with power concentrated at the carrier frequency and two adjacent sidebands; each sideband is equal in bandwidth to that of the modulating signal, is a mirror image of the other. Standard AM is thus sometimes called "double-sideband amplitude modulation" to distinguish it from more sophisticated modulation methods based on AM. One disadvantage of all amplitude modulation techniques is that the receiver amplifies and detects noise and electromagnetic interference in equal proportion to the signal.
Increasing the received signal-to-noise ratio, say, by a factor of 10, thus would require increasing the transmitter power by a factor of 10. This is in contrast to frequency modulation and digital radio where the effect of such noise following demodulation is reduced so long as the received signal is well above the threshold for reception. For this reason AM broadcast is not favored for music and high fidelity broadcasting, but rather for voice communications and broadcasts. Another disadvantage of AM is; the carrier signal contains none of the original information being transmitted. However its presence provides a simple means of demodulation using envelope detection, providing a frequency and phase reference to extract the modulation from the sidebands. In some modulation systems based on AM, a lower transmitter power is required through partial or total elimination of the carrier component, however receivers for these signals are more complex and costly; the receiver may regenerate a copy of the carrier frequency from a reduced "pilot" carrier to use in the demodulation process.
With the carrier eliminated in double-sideband suppressed-carrier transmission, carrier regeneration is possible using a Costas phase-locked loop. This doesn't work however for single-sideband suppressed-carrier transmission, leading to the characteristic "Donald Duck" sound from such receivers when detuned. Single sideband is used in amateur radio and other voice communications both due to its power efficiency and bandwidth efficiency. On the other hand, in medium wave and short wave broadcasting, standard AM with the full carrier allows for reception using inexpensive receivers; the broadcaster absorbs the extra power cost to increase potential audience. An additional function provided by the carrier in standard AM, but, lost in either single or double-sideband suppressed-carrier transmission, is that it provides an amplitude reference. In the receiver, the automatic gain control responds to the carrier so that the reproduced audio level stays in a fixed proportion to the original modulation.
On the other hand, with suppressed-carrier transmissions there is no transmitted power during pauses in the modulation, so the AGC must respond to peaks of the transmitted power during peaks in the modulation. This involves a so-called fast attack, slow decay circuit which holds the AGC level for a second or more following such peaks, in between syllables or short pauses in the program; this is acceptable for communications radios, where compression of the audio aids intelligibility. However it is undesired for music or normal broadcast programming, where a faithful reproduction of the original program, including its varying modulation levels, is expected. A trivial form of AM which can be used for transmitting binary data is on-off keying, the simplest form of amplitude-shift keying, in which ones and zeros are represented by the presence or absence
In radio communications, a radio receiver known as a receiver, wireless or radio is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna; the antenna intercepts radio waves and converts them to tiny alternating currents which are applied to the receiver, the receiver extracts the desired information. The receiver uses electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna, an electronic amplifier to increase the power of the signal for further processing, recovers the desired information through demodulation; the information produced by the receiver may be in the form of sound, moving data. A radio receiver may be a separate piece of electronic equipment, or an electronic circuit within another device. Radio receivers are widely used in modern technology, as components of communications, remote control, wireless networking systems. In consumer electronics, the terms radio and radio receiver are used for receivers designed to reproduce sound transmitted by radio broadcasting stations the first mass-market commercial radio application.
The most familiar form of radio receiver is a broadcast receiver just called a radio, which receives audio programs intended for public reception transmitted by local radio stations. The sound is reproduced either by a loudspeaker in the radio or an earphone which plugs into a jack on the radio; the radio requires electric power, provided either by batteries inside the radio or a power cord which plugs into an electric outlet. All radios have a volume control to adjust the loudness of the audio, some type of "tuning" control to select the radio station to be received. Modulation is the process of adding information to a radio carrier wave. Two types of modulation are used in analog radio broadcasting systems. In amplitude modulation the strength of the radio signal is varied by the audio signal. AM broadcasting is allowed in the AM broadcast bands which are between 148 and 283 kHz in the longwave range, between 526 and 1706 kHz in the medium frequency range of the radio spectrum. AM broadcasting is permitted in shortwave bands, between about 2.3 and 26 MHz, which are used for long distance international broadcasting.
In frequency modulation the frequency of the radio signal is varied by the audio signal. FM broadcasting is permitted in the FM broadcast bands between about 65 and 108 MHz in the high frequency range; the exact frequency ranges vary somewhat in different countries. FM stereo radio stations broadcast in stereophonic sound, transmitting two sound channels representing left and right microphones. A stereo receiver contains the additional circuits and parallel signal paths to reproduce the two separate channels. A monaural receiver, in contrast, only receives a single audio channel, a combination of the left and right channels. While AM stereo transmitters and receivers exist, they have not achieved the popularity of FM stereo. Most modern radios are "AM/FM" radios, are able to receive both AM and FM radio stations, have a switch to select which band to receive. Digital audio broadcasting is an advanced radio technology which debuted in some countries in 1998 that transmits audio from terrestrial radio stations as a digital signal rather than an analog signal as AM and FM do.
Its advantages are that DAB has the potential to provide higher quality sound than FM, has greater immunity to radio noise and interference, makes better use of scarce radio spectrum bandwidth, provides advanced user features such as electronic program guide, sports commentaries, image slideshows. Its disadvantage is that it is incompatible with previous radios so that a new DAB receiver must be purchased; as of 2017, 38 countries offer DAB, with 2,100 stations serving listening areas containing 420 million people. Most countries plan an eventual switchover from FM to DAB; the United States and Canada have chosen not to implement DAB. DAB radio stations work differently from AM or FM stations: a single DAB station transmits a wide 1,500 kHz bandwidth signal that carries from 9 to 12 channels from which the listener can choose. Broadcasters can transmit a channel at a range of different bit rates, so different channels can have different audio quality. In different countries DAB stations broadcast in either Band L band.
The signal strength of radio waves decreases the farther they travel from the transmitter, so a radio station can only be received within a limited range of its transmitter. The range depends on the power of the transmitter, the sensitivity of the receiver and internal noise, as well as any geographical obstructions such as hills between transmitter and receiver. AM broadcast band radio waves travel as ground waves which follow the contour of the Earth, so AM radio stations can be reliably received at hundreds of miles distance. Due to their higher frequency, FM band radio signals cannot travel far beyond the visual horizon; however FM radio has higher fidelity. So in many countries serious music is only broadcast by FM stations, AM stations specialize in radio news, talk radio, sports. Like FM, DAB signals travel by line of sight so reception distances are
An electronic circuit is composed of individual electronic components, such as resistors, capacitors and diodes, connected by conductive wires or traces through which electric current can flow. To be referred to as electronic, rather than electrical at least one active component must be present; the combination of components and wires allows various simple and complex operations to be performed: signals can be amplified, computations can be performed, data can be moved from one place to another. Circuits can be constructed of discrete components connected by individual pieces of wire, but today it is much more common to create interconnections by photolithographic techniques on a laminated substrate and solder the components to these interconnections to create a finished circuit. In an integrated circuit or IC, the components and interconnections are formed on the same substrate a semiconductor such as silicon or gallium arsenide. An electronic circuit can be categorized as an analog circuit, a digital circuit, or a mixed-signal circuit.
Breadboards and stripboards are common for testing new designs. They allow the designer to make quick changes to the circuit during development. Analog electronic circuits are those in which current or voltage may vary continuously with time to correspond to the information being represented. Analog circuitry is constructed from two fundamental building blocks: parallel circuits. In a series circuit, the same current passes through a series of components. A string of Christmas lights is a good example of a series circuit: if one goes out, they all do. In a parallel circuit, all the components are connected to the same voltage, the current divides between the various components according to their resistance; the basic components of analog circuits are wires, capacitors, inductors and transistors. Analog circuits are commonly represented in schematic diagrams, in which wires are shown as lines, each component has a unique symbol. Analog circuit analysis employs Kirchhoff's circuit laws: all the currents at a node, the voltage around a closed loop of wires is 0.
Wires are treated as ideal zero-voltage interconnections. Active components such as transistors are treated as controlled current or voltage sources: for example, a field-effect transistor can be modeled as a current source from the source to the drain, with the current controlled by the gate-source voltage. An alternative model is to take independent power sources and induction as basic electronic units; when the circuit size is comparable to a wavelength of the relevant signal frequency, a more sophisticated approach must be used, the distributed element model. Wires are treated as transmission lines, with nominally constant characteristic impedance, the impedances at the start and end determine transmitted and reflected waves on the line. Circuits designed according to this approach are distributed element circuits; such considerations become important for circuit boards at frequencies above a GHz. In digital electronic circuits, electric signals take on discrete values, to represent logical and numeric values.
These values represent the information, being processed. In the vast majority of cases, binary encoding is used: one voltage represents a binary'1' and another voltage represents a binary'0'. Digital circuits make extensive use of transistors, interconnected to create logic gates that provide the functions of Boolean logic: AND, NAND, OR, NOR, XOR and all possible combinations thereof. Transistors interconnected so as to provide positive feedback are used as latches and flip flops, circuits that have two or more metastable states, remain in one of these states until changed by an external input. Digital circuits therefore can provide both logic and memory, enabling them to perform arbitrary computational functions; the design process for digital circuits is fundamentally different from the process for analog circuits. Each logic gate regenerates the binary signal, so the designer need not account for distortion, gain control, offset voltages, other concerns faced in an analog design; as a consequence complex digital circuits, with billions of logic elements integrated on a single silicon chip, can be fabricated at low cost.
Such digital integrated circuits are ubiquitous in modern electronic devices, such as calculators, mobile phone handsets, computers. As digital circuits become more complex, issues of time delay, logic races, power dissipation, non-ideal switching, on-chip and inter-chip loading, leakage currents, become limitations to the density and performance. Digital circuitry is used to create general purpose computing chips, such as microprocessors, custom-designed logic circuits, known as application-specific integrated circuit. Field-programmable gate arrays, chips with logic circuitry
In telecommunication, frame synchronization or framing is the process by which, while receiving a stream of framed data, incoming frame alignment signals are identified, permitting the data bits within the frame to be extracted for decoding or retransmission. If the transmission is temporarily interrupted, or a bit slip event occurs, the receiver must re-synchronize; the transmitter and the receiver must agree ahead of time on which frame synchronization scheme they will use. Common frame synchronization schemes are: Framing bit A common practice in telecommunications, for example in T-carrier, is to insert, in a dedicated time slot within the frame, a noninformation bit or framing bit, used for synchronization of the incoming data with the receiver. In a bit stream, framing bits indicate the end of a frame, they occur at specified positions in the frame, do not carry information, are repetitive. Syncword framing Some systems use a special syncword at the beginning of every frame. CRC-based framing Some telecommunications hardware uses CRC-based framing.
In telemetry applications, a frame synchronizer is used to frame-align a serial pulse code-modulated binary stream. The frame synchronizer follows the bit synchronizer in most telemetry applications. Without frame synchronization, decommutation is impossible; the frame synchronization pattern is a known binary pattern which repeats at a regular interval within the PCM stream. The frame synchronizer aligns the data into minor frames or sub-frames; the frame sync pattern is followed by a counter which dictates which minor or sub-frame in the series is being transmitted. This becomes important in the decommutation stage where all data is deciphered as to what attribute was sampled. Different commutations require a constant awareness of which section of the major frame is being decoded. Asynchronous start-stop Phase synchronization Self-synchronizing code Superframe This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C". J. L. Massey.
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