Software defined radio (SDR)(also known as “software radio”) is a radio communicationsystem where components that have been traditionally implemented in hardware(e.g.mixers, filters, amplifiers, modulator/demodulators, detectors etc.
) are instead implementedby means of software on a personal computer or embedded system.3.1 SDR ConceptPrior to the proliferation of digital signal processing technology in radio systems mosttransceiver functions were implemented in analog circuitry. This confined the capabilitiesof the transceiver to the limitations of the analog technology. Some complex communicationsalgorithms were simply impossible to implement with analog components for a givenproject budget. Since analog circuitry is specified for a certain function it is difficult tomultitask, thus analog systems tend to be physically large and power hungry. Although itis true that in some applications such as filtering, it can outperform its digital counterpart,an analog systems performance is jeopardized by environmental variations.Digital signal processors (DSPs), field programmable gate arrays (FPGAs), and microprocessorsallow analog circuits such as filters, equalizers, and phase-locked loops (PLL)to be packed into one chip, consuming a fraction of the power, area, and cost.
This has ledto the implementation of sophisticated signal processing algorithms such as convolutionalencoding, interleaving, and dynamic power control in small hand-held devices such as cellular phones.Todays transceivers consist of a radio-frequency (RF) front end, and a baseband processingsection. The RF front-end is a loose term referring to the analog circuitry betweenthe antenna and the data converters. The main functions of the RF front end are to modulateand demodulate the carrier with and from the data, respectively. Base band signal processing,voice processing, user interface, power management, and networking functions aredone by a combination of analog and digital chips. Mixed signal (analog and digital) chipdesign, which would allow the integration of many of the current analog and digital functionalitiesinto one chip, is a popular concept in todays wireless industry.
Texas Instrumentsfor example, has announced that by 2004 it will introduce a one-chip GSM phone.Software radio strives to pack as much of the transceivers functionality into a programmablesignal processor as possible. A block diagram of an ideal software radio isshown in Figure 3.1 where the data converters are placed very close to the antenna.Fig.
3.1: In an ideal software radio the RF front end is eliminated.In this system, the RF front end is eliminated and the DSP is tasked with the modulationand demodulation, in addition to the baseband signal processing. Thus, if the DSP isprogrammable, the characteristics of the radio can be significantly defined by the softwarethat it runs on. A designer can alter the performance of the radio simply by reprogrammingthe DSP.This concept has far-reaching implications in the wireless communications industry.
Base station transceiver equipment at cell sites will no longer become obsolete with changesin wireless standards. Thus, migration to newer and more powerful systems will be inexpensive.Wireless switches, access points, and routers will no longer have to be replacedwith system upgrades, but reprogrammed. Satellites and other spacecraft can be reprogrammedfrom Earth to alter their transmission and reception characteristics, thus makingthem more powerful and flexible.Interoperability is another potential benefit of software radio systems. Different wirelesscommunication systems operating on different standards can communicate with eachother. This has been a major focus of the US military because different battlefield units usedifferent communication systems.Software radios can drastically reduce time to market because software modificationscan be done at a fraction of the time of hardware modifications.
Complex 3G handsetstake many months to design and implement. Any errors in this process can lead to a delayof many months for the product to reach market. Software radio systems cut down thiscorrection time significantly.New software features and upgrades can be downloaded to the handset automaticallyor on demand, thus greatly enhancing the degree and quality of services available to cellularcustomers. Handsets will not become obsolete as often as they do now with changingstandards, thus saving customers money.
Signal processing algorithms such as filtering, encoding/decoding, equalization, andmodulation/demodulation can be adaptively altered remotely. For example, in currentCDMA systems the base station controls the power emissions of the handsets to minimizethe near-far effects, as well as multiuser interference. This can be applied to all parametersof the handset, and as a result, transmission quality and capacity can increase. The conceptof cognitive radio, which seeks to make radio systems intelligent and adaptive to theirenvironments, is a future goal for software radios.
3.2 SDR AchitectureA traditional or typical receiver, besides the classic demodulation, performs threeother operations: (1) carrier frequency tuning to select the desired signal, (2) filter to separateit from others received, and (3) amplification to compensate transmission losses. Mosttraditional receivers have used conventional heterodyne schemes for almost a century. Thesuperheterodyne internals blocks are shown in Figure 3.
2.Fig. 3.2: Superheterodyne Receiver’s Interbnal Block.In the previous scheme, after the signal enters through the antenna, it is typicallyamplified by an RF stage that operates only in the frequencies of interest region. Then, thesignal is passed to the mixer which receives the local oscillator contribution by its otherinput.
The local oscillators frequency is set by the radios tuning control. The mixer isin charge of translating the signal to the Intermediate Frequency (IF).Typically, the oscillators frequency is set to a value that ensures that its differencefrom the desired signals frequency is equal to the IF. The next stage is a bandpass filter thatattenuates every signal except a specific portion of the spectrum. The bandwidth of thisstage limits the band width of the signal thats being received.
At the end, the demodulator recovers the original modulating signal from the IF amplifiersoutput employing one of several alternatives. Further processing of the signal dependson the purpose for which the receiver is intended.