SDR and CR Boost Wireless Communications

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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@

SDR and CR Boost Wireless Communications

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