Multiband RF Transceiver System Chapter 5 Software-Defined Radio

李健榮助理教授

Department of Electronic EngineeringNational Taipei University of Technology

Outline

• Introduction

• Mobile Generations

• Traditional Hardware-Defined Radio (HDR)

• Ideal Software-Defined Radio (SDR)

• Basic SDRArchitecure

• Hybrid Analog and Digital Radio Architecture

• Wideband Downconversion in the SDR

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Introduction

• In the 20th century, most radios are hardware defined withlittle or no software control (hardware defined radio, HDR).

� Fixed in function for mostly consumer items.

� A short life and are designed to be discarded and replaced.

• SDR uses programmable digital devices (DSPs or FPGAs) toperform the signal processing necessary to transmit andreceive baseband information at radio frequency.

� Offers greater flexibility and potentially longer productlife.

� Can be upgraded very cost effectively with software.

� A major challenge is to equal the efficiencies of hardware solutions.

� The developer will want to be shielded from the details hardware andcomplete all development in a unified environment using a single high-level language.

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Generation of Mobile Communications

• 1980s: 1st generation of mobile cellular

Uses analog modulation techniques to transmit and receive analog voice only information between mobiles and base stations.

• 1990s: 2nd-generation (2G) systems

They were known as “digital” because they encoded voice into digital streams and used digital modulation techniques for transmission.

• 2000s: IMT 2000 standard (defined 3G-compatible systems)

Support up to 2 Mbps data connections. A means to provide new services tocustomers and to provide much needed capacity via better spectrum utilization.

Of the 3G standards, the 3GPP Universal Mobile Telecommunications System(UMTS) is strongest in Europe (not universal). The 3GPP2 CDMA2000 standardand the TDMA-based GSM-EDGE systems will be successful in North and SouthAmerica, while Japan has its own WCDMA system similar to UMTS.

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3G SDR Applications

• All of the 3G systems are potential SDR applications.

• SDR offers the potential to solve many of the problems causedby the proliferation of newair interfaces.

• Base stations and terminals using SDR architectures cansupport multiple air interfaces during periods of transition andbe easily software upgraded.

• Intelligent SDRs can detect the local air interface and adapt tosuit the need; this capability will be valuable for frequent inter-country travelers.

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Traditional Hardware Radio Architecture

• Conventional dual conversion superheterodyne transceiver:

This design has been around since the 1930s. The analog superheterodyne radiohas experienced a marvelously successful history; it was used in 1G mobile phoneterminals (e.g., AMPS) and is sure to endure in lowcost broadcast radios.

De-mod.

Modulator

Basebandanalogreceive

Basebandanalog

transmit

LO1 LO2

IF

IF

LNA

PA

RFcombiner

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Ideal Software Defined Radio (I)

• The analog functions are restricted to those that cannot beperformed digitally.(Antenna, RF filtering, RF combination, receive pre-amplification, transmit power amplification and reference frequency generation)

123

LNA

PA

RFcombiner

Reference LODigital processing resourcese.g.,Digital signal processorsFPGA’sReconfigurable communications processorsMicroprocessorsMemory

Operatingsystems

Drivers Inter-processorcomms

CORBA Resourcemanagement

Managementand control

Basebanddigitaluser data

Hardware

Middleware

Application software

Framework

Digital subsystemAnalog subsystem

API

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Ideal Software Defined Radio (II)

• Analog conversion stage right up as close as possible to theantenna.

• The separation of carriers and up/down frequency conversionto baseband, channel coding and modulation functions areperformed by the digital processing resources.

• Frameworks using an open API into the middleware will makeapplications development more portable, quicker, and cheaper.

• The ideal architecture is commercially feasible for limited lowdata rate HF and VHF radios but is not yet practical for anygeneration of cellular mobile phone technology.

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Basic SDR Architecture

• For 3G mobile and many other multiuser radio technologies,the ideal SDR is not yet a practical or cost-effective reality.

Direct sampling of wideband RF frequencies at high SNR (>90 dB) is not yettechnically possible.

• Decide where the radio stops being hardware defined andwhere it starts being software defined.

• Considering normal commercial requirements (principally costeffectiveness), it is apparent that SDR implementations of 3Gwireless need purpose-built hardware to be successful.

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2G Radio Architecture

• Compared with current generations, 1G and other equivalentanalog radio systems trade off complexity for bandwidthutilization.

That is, they are less complex and consume more bandwidth.AMPS consumes30 kHz for a voice user.

• A major requirement of the 2G standards was to increasebandwidth efficiency in a increase in complexity.

The 2G Groupe Speciale Mobile (GSM) standard achieved this byimplementing a digital standard that allowed for time division multiplexing,multiple access, and other relatively sophisticated techniques to improvesystem capacity.GSM occupies 200 kHz for its 8 voice users. The addedfeatures can produce an approximately 3 to 4 times capacity improvement.

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Hybrid Radio Architecture (I)

• The analog fixed function HDR survived right through to the1960s and 1970s, making its way into color televisiontransmission, private mobile radio, and even parts of 1Gcellular mobile radio.

• The complexity of a color television receiver and a 1G mobileterminal stretched this analog technology to the absolute limit.

Analog circuits consume more space and power and are more subject toperformance variations as a result of environmental factors (e.g., temperature).

• The emergence of low-cost ADCs, DACs, and DSPs in the1980s and the need for more efficient RF bandwidth utilizationshifted radio architecture development away frompurelyanalog to hybrid analog and digital systems.

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Hybrid Radio Architecture (II)

• Ppopular with early 1990s hybrid radios (e.g., 2G BTS). IFA1, typically 140 or 70 MHz

Each filter ensures that acceptable selectivity and imagerejection are achieved.

IFA2, typically 10.7 MHz. De-interleaving and error correction (e.g., Viterbi decoder)

Modulationandfilter

Demodulationand

equalization

Channeldecoding

Voicedecoding

Networkinterface

voice packetextraction

Voiceencoding

Channelencoding

DAC

FRx

PA

RF combiner

Digitalinformation

bits

LNA

IFA1 IFA2 LP

ADC

LO2Rx

LO2Tx

LO1Rx

LO1Tx

LO3Rx

LO3Tx

fs

Per carrier analog Rx chain Per baseband Rx channel

Per carrier analog Tx chain Per baseband Tx channel GSM 900 BTS:RX: 880–915 MHzTX: 925–960 MHz.

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Hybrid Radio Architecture (III)

• The single carrier case is expanded to a multicarrier systembyadding RF carrier transmit and receive chains.

Channeldecoding

Voicedecoding

Demodulationand filter

Networkinterface

voice packetextraction

Multichannelinformationdigital bits

Voiceencoding

Channelencoding

Modulationand filter

ADC

DAC

FRx IFA1 IFA2 IFA3

LNA

PA

fsLO2Rx

LO2Tx LO3Tx

LO3RxLO1Rx1

LO1Rx2

LO1RxN

LO1Tx1LO1Tx2

LO1TxN

RF combiner

FTx

Tx chain 1

Tx chain 2

Tx chain N

Rx chain 1

Rx chain 2

Rx chain N

Baseband 1 Rx

Baseband 2 Rx

Baseband N Rx

Baseband 1 Tx

Baseband 2 Tx

Baseband N Tx

Multicarrier 1990s digital radio

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Basic SDR Block Diagram

Wideband capability, designed to replace many narrowband analog receive or transmit frequency conversion chains.

Networkinterface

voice packetextraction

Multichannelinformationdigital bits

Channelencoding

Modulationand filter

Vocoding

Channeldecoding

Demodulationand filter

Interpolationfilter

Interpolationfilter

InterpolationFilter

decimation

InterpolationFilter

decimation

ADC

DAC

RF combiner

LNA

PA

FRx IFA1 IFA1

BP1 BP2 BP3IFo

IFo

fs

LORX1 LORX2

LOTX1 LOTX1

FTx

DSUM

NCO

NCO

Analog Rx chain

Analog Tx chain

Wideband analog front end

Down conversion

Up conversion

Hardware defined subsystem Software defined subsystem

Digital frequency conversion and baseband processing resources

Carrier 1

Carrier 2Carrier N

Q

I

Q

I

(A)

(C)

(B)

Digital IF

The hardware subsystemdetails some lower-levelphysical components(PA, LNA, ADC…)

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Wideband Downconversion

• The wideband front end converts or shifts an entire segment ofspectrum to a suitable intermediate frequency—IFD, “thedigital IF”—prior to digitization.

A segment of the GSM 900-MHz band

A popular choice for IFD is 70 MHz due to theCOTS availability of satellite/ microwavefrequency converters.

The spectrum of the required shifted tobaseband by software prior to demodulation.

Carriers CarriersN 2 1 1 2 N

N 2 1 1 2 N

A(f)

A(f)

+f (MHz)~ 900 MHz0

0

0

-f (MHz)

-f (MHz)

-f (MHz)

+f (MHz)

+f (MHz)

(A)

(B)~ 70 MHz = IF

e.g., carrier 2

~ 70 MHz

RF

IFD

BB

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Summary

• In this chapter, the concepts of HDR, SDR, and the hybridanalog and digital radio architecture were introduced.

• The HDR is often fixed in function but offering higherperformance than SDR solution.

• The SDR offers greater flexibility and can be upgraded verycost effectively with software. A major challenge is to equalthe efficiencies of hardware solutions.

• The SDR may be a good solution for wideband or multi-band/multi-mode applications.

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