Brad Brannon Software Defined Radio Pdf
Soft radio runs into hard standardsSoft radio runs into hard standardsBy Brad Brannon, Senior Systems Engineer,Chris Cloninger, Systems Applications EngineerAnalog Devices Inc.Software-defined radio (SDR) makes it very easy to rapidly evolvecommunication services. And it enables new technologies such as adaptiveantenna arrays, which can significantly increase caller capacity withina cell site by utilizing spectral diversity.Software radio is not limited to cellular applications. Already otherwireless applications are being explored, including wireless LAN, Bluetooth-basedpersonal-area networks and even standard commercial FM broadcasts as theymove to new digital formats. In general, any areas where wireless standardsare evolving and hardware may quickly be rendered obsolete are potentialadopters of SDR techniques. However, the way is not at all clear yet.Probably the biggest roadblock is the rigorous expectations placed onthese systems.
Most transmission standards were written with a traditional,fixed-air interface in mind. These standards evolved to avoid the weaknessesand enhance the strengths of traditional architectures. If it is possibleto architect a system that exploits the strengths of software radio, thenfull deployment could occur much sooner.Market gapsEarly on, Analog Devices' engineers recognized that in order for SDRdevelopments to begin, various gaps that existed in the marketplace mustbe filled. These gaps included not only the inadequate performance levelsin mixed-signal ICs, but also the lack of digital equivalents of mixers,synthesizers and filters used in a transceiver signal chain. Thus, AnalogDevices added digital receive and transmit signal processing to its developmentefforts.
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Today, the company's SoftCell chip set consists of high-speed,wide dynamic-range data converters, receive signal processors and transmitsignal processors, all of which grew out of this focused effort. Thesecomponents, coupled with a high-speed digital signal processor (DSP),not only address demanding analog performance requirements, but also providethe necessary versatility to be reprogrammed in order to meet the filteringand modulation requirements of any air standard.The receiver front end of a software-defined radio consists of an analogRF down-converter that converts the desired signal band to a convenientintermediate frequency (IF) for digitization. The down conversion is followedby a high-performance analog-to-digital (A/D) converter such asthe AD6644, which digitizes the IF spectrum. The A/D converter output isprocessed with the AD6624, which is a quad receive signal processor(RSP) that is responsible for tuning and channel filtering.
The outputof the RSP consists of a channel-filtered digital IF signal requiring onlydemodulation that the DSP provides. In the transmit direction, the DSPsends modulated digital data to a transmit signal processor (TSP) likethe AD6622, which modulates the digital carrier.
Data is then convertedto the analog domain using a high-performance digital-to-analog (D/A)converter such as the AD9772A, where it is mixed up to the desiredRF frequency.There are two key specifications for an A/D converter used inwireless applications. The first specification is the signal-to-noiseratio (S/N), which is useful in computing the overall sensitivity ofa receiver. Although A/D converters are voltage devices and NF is a powermeasurement, an equivalent noise figure (NF) can be calculated based ona full-scale referenced S/N.The noise in the signal-to-noise ratio consists of thermal noise andquantization noise (the noise generated when a signal is digitized). Otherfactors that affect A/D converter performance include nonideal quantization(imperfect A/D conversion) and aperture uncertainty (a wideband phase noiseon the clock) in the sample clock.The second specification of interest for an A/D converter is spurious-freedynamic range (SFDR). Spurious response consists of the second, thirdand higher-order harmonics. SFDR primarily determines how large an in-bandor out-of-band interferer can be. If an interferer is sufficiently large,the A/D converter may generate a harmonic that appears as a cochannel interfererof the desired signal.
Spurious performance is very closely related tothe air interface standard and is often the limiting factor in a receiver.For SDR, the distortion (noise and/or spurious) of the A/D converter mustnot impact overall system performance. Therefore, the A/D converter mustbe carefully chosen to meet performance expectations.The receive signal processor is a numeric preprocessor for the DSP.The purpose of the RSP is to replace a local oscillator, quadrature mixer,channel-select filter and data decimation. In a multicarrier application,the RSP replaces the analog selectivity and tuning functions with digitalequivalents. An RSP sets the receiver apart from traditional receiversbecause all channel characteristics are now programmable. This includesdata rate, channel bandwidth and channel shape. In addition to the unlimitedselection of channel characteristics, a digital filter will perform exactlyalike across all boards, unlike analog solutions that always have tolerances.There are several important specifications to consider whenselecting an RSP. First, the device must be capable of handlingthe high data rates required by the interface (A/D converter).
Since theA/D converter's sample rate determines the bandwidth that can be processedaccording to the Nyquist theory, the RSP must be capable of handling thesame data rate. The Nyquist theory states that the sample rate must beat least twice the bandwidth of signals being processed to recover theinformation contained therein. In practical systems, the sample rate isfrequently run three times faster than the bandwidth of the signals beingreceived to allow for antialiasing filter response.The next specification of interest is the internal and externalbus widths. They must be wide enough to preserve the signal integrity.Although the A/D converter may only be 14 bits wide, oversampling followedby narrow-band filtering improves the effective S/N (processing gain) ofthe A/D converter by up to 30 dB for some air interface standards.
Thisis the equivalent of 5 more bits. Therefore, internal bus widths must havethe equivalent of at least 19 bits to preserve signal integrity.Since a large portion of any air interface is the channel bandwidthand shape, it is important that the RSP include flexible decimation andfiltering configurations that allow for a wide variety of data rates andfilter bandwidths. Fixed filter widths and shapes should be avoided sincethey limit channel bandwidths and often preclude raised root cosine filtering.Finally, one of the most unique features of an RSP is the abilityto select the desired analog frequency precisely and rapidly.
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Most RSPshave a 32-bit NCO that provides a frequency resolution of about 1 in 4billion. This is usually much more than adequate, and gives great flexibilityin frequency selection. In addition to the flexibility, frequency hoppingis greatly simplified. Since no phase-locked loop (PLL) is used, changingfrequencies is instantaneous. This can be beneficial in applications wherehopping must occur within the guard band of a few symbols.The transmit signal processor is a numeric post-processor for the DSP.The purpose of the TSP is to replace the first local oscillator, quadraturemodulator, channel filtering and data interpolation. Like the RSP, theTSP sets the transmitter apart from traditional designs because all channelcharacteristics are now programmable.
This includes data rate, channelbandwidth and channel shape. Since modulation, channel filtering and otheraspects of the modulation are done digitally, the filters will always performexactly alike across all boards, unlike analog solutions that always havetolerances.There are several specifications that are important when selectinga TSP.
First, the device must be capable of generating dataat the rates required to preserve the Nyquist bandwidth over the spectrumof interest. As with the A/D converter's sample rate, the sample rate ofthe D/A converter determines how much spectrum can be faithfully generatedby the D/A converter. Therefore, the TSP must be capable of generatingdata at least twice as fast as the band of interest and preferably threetimes faster as reasoned earlier for antialiasing filter response.Similar to RSPs, the bus widths are also important, yet for differentreasons. In the transmit direction, there are two different issues. Ifthe TSP is used in a single-channel mode, then the issue is simply quantizationand thermal noise. It is usually not desirable to transmit excess in-bandor out-of-band noise, since this wastes valuable transmitter efficiency.In a multicarrier application, the concern is slightly different. Here,many channels would be digitally summed before reconstruction with a D/Aconverter.
Therefore, each time the number of channels is doubled, an additionalbit should be added so that the dynamic range is not taken from one channelwhen another is added.Finally, the ability to frequency hop is vital. Since a TSP implementsfrequency control with an NCO and a mixer, frequency hopping can be veryfast, allowing the implementation of the most demanding hopping applicationsas found in the GSM specification.Basically, a D/A converter is similar to an A/D converter whenconsidering performance requirements. Therefore, the first specificationof interest is the signal-to-noise ratio. As with an A/D converter,S/N is primarily determined by quantization and thermal noise.
If eitheris too large, then the noise figure of the D/A converter will begin tocontribute to the overall signal chain noise. While noise is not necessarilya concern spectrally, the issue does become important when the D/A converteris used to reconstruct multiple signals. In this case, the D/A converteroutput signal swing ('power') is shared among the carriers. The theoreticalsignal-to-noise ratio of a D/A converter is determined by the same setof equations that govern an A/D converter, and the noise figure can bederived given a specific S/N.All material on this site Copyright © 2001 CMP Media Inc.