Software-defined radio

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A software-defined radio (SDR) system is a radio communication system which can tune to any frequency band and receive any modulation across a large frequency spectrum by means of a programmable hardware which is controlled by software.

An SDR performs significant amounts of signal processing in a general purpose computer, or a reconfigurable piece of digital electronics. The goal of this design is to produce a radio that can receive and transmit a new form of radio protocol just by running new software.

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.

The hardware of a software-defined radio typically consists of a superheterodyne RF front end which converts RF signals from (and to) analog IF signals, and analog to digital converter and digital to analog converters which are used to convert a digitised IF signal from and to analog form, respectively.

Software-defined radio can currently be used to implement simple radio modem technologies. In the long run, software-defined radio is expected by its proponents to become the dominant technology in radio communications. It is the enabler of the cognitive radio.

Contents

[edit] Operating principles

[edit] Ideal concept

The ideal scheme would be to attach an analog to digital converter to an antenna. A digital signal processor would read the converter, and then software would transform the stream of data from the converter to any other form.

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 practical, however.

[edit] Practical receivers

Current (2003) digital electronics are too slow to receive typical radio signals that range from 10 kHz to 2,4 GHz. An ideal software radio would have to collect and process samples at twice the maximum frequency at which it is to operate. Real software radios solve this problem by using a mixer and a reference oscillator to heterodyne the radio signal to a lower frequency.

The above mixer changes the frequency of the signal. The phase information becomes more difficult to detect in it. Many digital encoding systems depend on phase encoding. The classic solution is to mix and digitize two channels, using a reference oscillator that produces two signals that are the same frequency. However, one of the frequency outputs lags the other by 90 degrees of a cycle. Thus, the two sets of samples provide the needed phase information.

Another related problem is that the information about the bit-timing is lost when the frequency changes. The phase information helps recover that as well.

The sampling works best if it is at a simple multiple of the protocol's symbol rate. Since the distant transmitter and the receiver are linked only by the radio, this means that the sampling speed should somehow adapt to the distant radio's symbol rate. The phase information may therefore be used to adjust the effective sampling rate, as well.

A good software radio must operate at any sample rate within a wide range of rates, in order to be compatible with many protocols, so this adaptive control is crucial. It can be implemented either with a hardware linkage to the converter, or in software.

Any signals above the sampling frequency would "interfere" with the sampling, causing spurious signals to appear in the data stream at a frequency that's the difference between the signal and the sampling frequency. For this reason, a low-pass analog electronic filter must precede the digital conversion step.

Real analog-to-digital converters lack the discrimination to pick up sub-microvolt, nanowatt radio signals. Therefore a low noise amplifier must precede the conversion step. The amplifier introduces its own problems. If spurious signals are present (which is typical), these compete with the desired signals for the amplifier's power. They introduce distortion in the desired signals, or may block them completely. The standard solution is to put a filter between the antenna and the amplifier, but this reduces the radio's flexibility- the whole point of a software radio. Real software radios have two or three analog "channels" that are switched in and out. These contained matched filters, amplifiers and sometimes a mixer.

[edit] Motivation

SDR has generated tremendous interest in the wireless communication industry for the wide-ranging economic and deployment benefits it offers. Following are some of the problems faced by the wireless communication industry due to implementation of wireless networking infrastructure equipment and terminals completely in hardware:

1.Commercial wireless network standards are continuously evolving from 2G to 2.5G/3G and then further onto 4G. Each generation of networks differ significantly in link-layer protocol standards causing problems to subscribers, wireless network operators and equipment vendors. Subscribers are forced to buy new handsets whenever a new generation of network standards is deployed. Wireless network operators face problems during migration of the network from one generation to next due to presence of large number of subscribers using legacy handsets that may be incompatible with newer generation network.

2.The air interface and link-layer protocols differ across various geographies; for example, European wireless networks are predominantly based on GSM, which uses TDMA multiplexing, while many US wireless networks are based on IS-95 or CDMA2000, which use CDMA multiplexing. This problem has inhibited the deployment of global roaming facilities, causing great inconvenience to subscribers who travel frequently from one continent to another. Handset vendors face problems in building viable multi-mode handsets due to high cost and bulky nature of such handsets.

3.Wireless network operators face deployment issues while rolling out new services/features to realize new revenue streams since this may require large-scale customizations on subscribers’ handsets.

[edit] History

One of the first software radios was a U.S. 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 200 MHz. Further, another 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.

[edit] SPEAKeasy phase I

From 1992 to 1995, the goal was to produce a radio for the U.S. Army that could operate from 2 MHz to 2 GHz, and operate with ground force radios (frequency-agile VHF, FM, and SINCGARS), Air Force radios (VHF AM), Naval Radios (VHF AM and HF SSB teleprinters) and satellites (microwave QAM). Some particular goals were to provide a new signal format in two weeks from a standing start, and demonstrate a radio into which multiple contractors could plug parts and software.

The project was demonstrated at TF-XXI Advanced Warfighting Exercise, and met all these goals. There was some discontent with certain unspecified features. Its cryptographic processor could not change context fast enough to keep several radio conversations on the air at once. Its software architecture, though practical enough, bore no resemblance to any other.

The basic arrangement of the radio receiver used an antenna feeding an amplifier and down-converter (see mixer) feeding an automatic gain control, which fed an analog to digital converter that was on a computer VMEbus with a lot of digital signal processors (Texas Instruments C40s). The transmitter had digital to analog converters on the PCI bus feeding an up converter (mixer) that led to a power amplifier and antenna. The very wide frequency range was divided into a few sub-bands with different analog radio technologies feeding the same analog to digital converters. This has since become a standard design scheme for wide band software radios.

[edit] SPEAKeasy phase II

The goals were to get a more quickly reconfigurable architecture (i.e. several conversations at once), in an open software architecture, with cross-channel connectivity (the radio can "bridge" different radio protocols). The secondary goals were to make it smaller, weigh less and cheaper.

The project produced a demonstration radio only fifteen months into a three year research project. The demonstration was so successful that further development was halted, and the radio went into production with only a 4 MHz to 400 MHz range.

The software architecture identified standard interfaces for different modules of the radio: "radio frequency control" to manage the analog parts of the radio, "modem control" managed resources for modulation and demodulation schemes (FM, AM, SSB, QAM, etc), "waveform processing" modules actually performed the modem functions, "key processing" and "crytographic processing" managed the cryptographic functions, a "multimedia" module did voice processing, a "human interface" provided local or remote controls, there was a "routing" module for network services, and a "control" module to keep it all straight.

The modules are said to communicate without a central operating system. Instead, they send messages over the PCI computer bus to each other with a layered protocol.

As a military project, the radio strongly distinguished "red" (unsecured secret data) and "black" (cryptographically-secured data).

The project was the first known to use FPGAs (field programmable gate arrays) for digital processing of radio data. The time to reprogram these is an issue limiting application of the radio.

[edit] Joint Tactical Radio System (JTRS)

The JTRS is a U.S. and allied program (NATO participates) to produce radios which provide flexible and interoperable communications. Examples of radio terminals which require support include hand-held, vehicular, airborne and dismounted radios, as well as base-stations (fixed and maritime).

This goal is achieved through the use of SDR systems based on an internationally endorsed open Software Communications Architecture (SCA). This standard uses CORBA on POSIX operating systems to coordinate various software modules. The SCA documentation is freely available at the JTRS website.

The program is providing a flexible new approach to meet diverse warfighter communications needs through software programmable radio technology. All functionality and expandability is built upon the Software Communications Architecture (SCA).

The SCA, despite its military origin, is under evaluation by commercial radio vendors for applicability in their domains.

[edit] Amateur software radios

A typical amateur software radio, such as the FlexRadio SDR-1000 or the homemade design described in the ARRL Handbook (1999), uses a direct conversion receiver. The conversion is to the audio frequency band, which is sampled by a standard (or enchanced) PC sound card. A fast PC operates custom (usually amateur-written) software as the signal processor.

Uses include every common amateur modulation: morse code, single sideband modulation, frequency modulation, radioteletype, slow-scan television, and packet radio. Amateurs also experiment with new modulation methods: for instance, the DREAM open-source project decodes the COFDM technique used by Digital Radio Mondiale.

More recently, the GNU Radio Universal Software Radio Peripheral (USRP) uses a USB 2.0 interface, a programmable FPGA, and a high-speed set of ADC/DACs, combined with reconfigurable free software. Its sampling and synthesis bandwidth is a thousand times that of PC sound cards, which enables an entirely new set of applications.

On the low-end (and low-cost): the SoftRock kit gives an easy entry into direct conversion shortwave receiver with software-defined demodulation.

[edit] Software Defined Radio & RFID Technology

As well as transmitting audio information, SDR may have value in the emerging field of Radio Frequency Identification (RFID), where devices operate on various frequencies using various communication protocols. In 2001, the Massachusetts Institute of Technology's Auto-ID Center sponsored research by ThingMagic Corporation to develop an RFID reader that used SDR. A direct descendant of this device is now available commercially, and other companies are developing RFID readers that use SDR.

See also digital radio, PACTOR, AMTOR

[edit] UCLA SDR

In 2006 UCLA Integrated Circuits and Systems Lab. (ICSL) introduced a practical software defined radio receiver which can tune to any channel in 800MHz to 6GHz. In contrast to Mitola's analog to digital converter (ADC) based SDR front-end, UCLA SDR uses a highly clock programmable signal conditioner at the front-end, which brings the wanted channel as well as the blockers within the dynamic range of today's ADC circuits. The results were published in International Solid-State Circuits Conference (ISSCC) 2006.

[edit] SDR Vendors

[edit] References

    The papers presented at the SDR Forum 2004 and 2005 Technical Conferences are now available on their website.

    These are useful books:
    Software defined radio : architectures, systems, and functions. Dillinger, Madani, Alonistioti. Wiley, 2003. 454 pages. ISBN: 0470851643 ISBN-13: 9780470851647
    Cognitive Radio Technology. Bruce Fette. Elsevier Science & Technology Books, 2006. 656 pags. ISBN: 0750679522 ISBN-13: 9780750679527
    Software Defined Radio for 3G, Burns. Artech House, 2002. ISBN:1-58053-347-7

    [edit] External links

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