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SDR And CR Boost Wireless Communications
Oct 12, 2011Lou Frenzel| Electronic Design
Fig 1. A conventional legacy radio receiver (a) uses the standard analog superheterodyne
architecture with analog circuitry performing all functions. A more advanced superheterodyne
receiver (b) uses digital demodulation with DSP.
Softwaredefined radio (SD!) used to be rare and e"otic. #ut today$ most modern radios use
SD!%s architecture and techni&ues. 'ach year with continuing advances in s and other
technologies$ SD! becomes more capable and widespread. n fact$ new techni&ues li*e cognitive
radio (!) are ma*ing SD! more useful and beneficial to wireless communications.
SDR Defined
SD! uses software to perform some of the signal processing in a receiver and transmitter. For
e"ample$ a traditional receiver using the ubi&uitous superheterodyne architecture performs all
signal processing with basic electronic circuits (Fig. 1a). +he superheterodyne downconverts the
input signal to an intermediate fre&uency (F) for demodulation and other processing.
'arly SD! receivers (Fig. 1b) replaced the demodulator with an analogtodigital converter
(AD) after the F stage and performed the demodulation and some filtering in a digital signal
processor (DSP). +oday$ because ADs sample faster$ DSPs can handle more functions.
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+o ma*e DSPs wor*$ the amplitude and phase of the signals both must be *nown. +his has led to
an architecture that divides the received signal into two paths$ one producing an inphase ()
signal and a ,- shifted &uadrature (/) signal. A basic carrier signal has the form0
2 Accos(34fct 5 6)
7here fc is the carrier fre&uency$ 6 is the phase$ and Acis the carrier amplitude. Any of these
may be varied for modulation. For demodulation in the digital domain$ a single signal is
insufficient for e"isting algorithms. +herefore$ the modulated signal is converted into the and
/ signals0
2 (t) cos(34fct) 5 /(t) sin(34fct)
7ith the &uadrature signals$ any variations in amplitude$ fre&uency$ or phase can be detected
and used in a demodulation or other process.
Figure 3shows a modern 8/ SD! receiver. A lownoise amplifier (9:A) usually boosts the
input signal from the antenna before it is applied to the two mi"ers. +he mi"ers develop the
and / signals. #oth receive a local oscillator (9;) signal from the phaseloc*ed loop (P99)
fre&uency synthesi
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After the baseband signals have been filtered in low pass filters to eliminate the sum
components at the output of the mi"ers$ the signals are digiti- @< already use this advanced architecture.
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@any functions are now performed digitally0
Filtering (low pass$ high pass$ band pass$ and band reBect)
@odulation (A@$ F@$ P@$ FSC$ #PSC$ /PSC$ /A@$ ;FD@$ etc.)
Demodulation
'&uali. Esamples8s (see ADs Sample !F DirectlyG atwww.electronicdesign.com). +his dual
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channel$ 13bit AD can be used with a cloc* phase shift that allows interleaved or alternate
channel sampling for even faster conversion. DA sampling speed is closely following this trend.
7hile fast conversion is essential$ the DSP must be fast enough to *eep up. +hat has not been a
problem as most processors have easily *ept pace. SD! is software$ of course$ but you still need
hardware that can be implemented physically in several forms.
For instance$ you can write code to run on a generalpurpose processor (EPP). +his may not be
an optimal approach as some of the algorithms call for math procedures that are aw*ward to
handle on most EPPs. owever$ an ntel or A@D dualcore processor used in most Ps today
does a great Bob in some applications. Some EPPs also have special instructions li*e the multiply
and accumulate (function) that are so commonly used in DSP algorithms.
+hen you can also use a DSP designed specifically to handle signal processing code. t has a
special architecture (usually arvard)$ memory$ and arithmetic logic unit (A9=) instruction sets
that ma*e the DSP fast.
+e"as nstruments% popular line of DSPs has been used for years in SD!$ such as the H--- and
--- series. Analog Devices and Freescale also have generalpurpose DSPs. 9i*e any
processor$ DSPs are fully programmable so they%re very fle"ible in applications where changes$
additions$ and updates may be re&uired in the future. loc* speeds to 1 E< are common in
DSPs today.
@ore and more SD! designs are using FPEAs. +he signal processing algorithms such as the fast
Fourier transform (FF+) can be reduced to digital logic and &uic*ly implemented in an FPEA.
Since the cost of FPEAs has steadily declined$ they have become a maBor alternative to DSPs.
FPEAs are faster than some other processors with some functions but still have the fle"ibility of
reprogramming. Altera and Iilin" support SD! on their FPEAs.
Finally$ hard logic is also common today. 7hen implementing fi"ed standards li*e cellular radio
specifications$ the fle"ibility or reprogramming is not necessary. +herefore$ algorithms can be
implemented in fi"ed onchip logic. t is fast$ uses less chip area$ and can bring about a maBor
decrease in power consumption. Such bloc*s of logic are generally called accelerators.
@any cellphone basestation s li*e +%s +@S>3-+1? systemonachip (So) are e"amples
that use accelerators. Figure Hshows the 1? with its A!@ EPP and four " generalpurpose
floatingpoint DSP cores. :ote the accelerator logic on the right. @ost of the 9ayer 1 accelerators
use DSP algorithms for the many SD! functions.
Real SDR Transceivers
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@any SD!s have been developed for the military under the Joint +actical !adio System (J+!S).
+his =.S. Department of Defense program aims to develop a complete line of SD! radios for
voice$ data$ and video that can be used to form ad hoc networ*s on the battlefield. +he program
has been around since the late 1,,-s$ with good progress over the years.
+he whole basis of J+!S is the Software ommunications Architecture (SA). +his open
architecture platform standard defines how the hardware and software wor* together. ;ne of
the primary obBectives is to develop software that is fully transferrable between different
hardware platforms$ ma*ing all military radios multifunctional and interoperable.
+he latest version$ designated SA 3.3.3$ was recently made available to further improve the
programmer%s ability to ma*e the software more fle"ible and scalable. alled SA :e"t$ the
software helps ma*e programs smaller and re&uire less testing.
SA does not have specific provisions for cognitive features. #ut over the past few years$ the =.S.
Defense Advanced !esearch ProBects Agency (DA!PA) has been testing cognitive enhancements
to SA li*e Dynamic Spectrum Access that will hopefully be available in the coming ne"t
generation of J+!S radios.
+he +hales ommunications A:8P!1?K J+!S 'nhanced @ultiband (J'@) nter8ntra +eam
!adio$ which is a J+!S radio$ covers all the F$ F$ and =F military fre&uencies from >- to
H13 @< (Fig. ). Power output can be selected from -.1 to H 7. A wide range of modes and
waveforms is available.
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Cognitive Radio
ognitive radio (!) e"pands the definition of SD! to include features that ma*e a radio
intelligent. +he 7ireless nnovation Forum defines ! as !adio in which communication
systems are aware of their environment and internal state and can ma*e decisions about their
radio operating behavior based on that information and predefined obBectives. +he
environmental information may or may not include location information related to
communicant systems.G
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!s are sometimes called adaptive radios that automatically adBust their behavior or operations
to achieve specific obBectives. +hey can sense$ learn$ and adapt. +hey have an internal memory
that stores instructions for various situations. Stored *nowledge about their own capabilities
ma*es it possible for these radios to ma*e their own decisions.
A ! can also access e"ternal databases for additional decisionma*ing intelligence. t senses by
listening to a channel assessing the presence of other signals$ their characteristics$ and the noise
bac*ground. +he ! learns from its e"perience as well. 7ith all of the *nowledge it has or can
access$ the ! becomes a superintelligent radio.
+he transmitter (+I) and receiver (!I) are full fre&uencyagile SD!s with a mi" of applicable
waveforms and all the related SD! hardware and software (Fig. L). A separate cognitive
processor engine runs the cognitive aspects of the radio. t gets inputs (@) from the !I and +I
to monitor their condition and parameters. t uses these inputs along with others to ma*e
decisions.
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;ther inputs can come from policy instructions stored in memory that define ways to operate
under different conditions. '"ternal databases may also be accessed. Some ! units get location
information by EPS. Decisions are then made$ and controls () are issued to the radios to
achieve the desired result.
!s are also a good e"ample of artificial intelligence (A) in action. A is a collection of software
that is used to store and use *nowledge to solve problems. t can use standard algorithms but it
can also draw on several A techni&ues such as e"pert systems$ natural language processing$
neural networ*s$ fu
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;ne important aspect of ! is dynamic spectrum access (DSA)$ which allows the ! to tune to a
channel in the fre&uency spectrum for its operation after it decides that the channel is unused. A
DSA radio uses unused spectrum$ producing greater efficiency of limited spectrum space.
A cognitive transceiver essentially tells the SD! what to do in the way of fre&uency of operation$
modulation$ power level$ protocol$ and other factors and ma*es corresponding adBustments
automatically. A ! is software that monitors the SD! and delivers commands and control
instructions as needed.
!s primarily see* to solve two maBor wireless problems0 limited spectrum and interoperability
between different radios or wireless systems. A ! can find open spectrum and use it. t also can
change its waveforms or protocols to adapt to radios of a different nature$ ma*ing
communications possible or more reliable.
+here are also several different classifications of !s. For e"ample$ a policybased radio is
programmed with a predefined set of capabilities li*e waveforms and procedures. +he radio is
used by selecting one of several different preprogrammed fi"ed functions. +he fi"ed functions
are loaded during manufacturing$ selected by the user$ or downloaded over the air.
Another ! form is a fully reconfigurable radio. +his fully generic transceiver can operate over a
wide fre&uency and power range. +his type of radio can be fully reconfigured on the fly for new
applications or communications conditions.
CR Examples
+he "E +echnology "@a" carrierclass ! system for mobile communications uses the
unlicensed industrial$ scientific$ and medical (S@) band spectrum in the ,-3 to ,3K@