Ppt 4

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J.Dąbrowski, Intro to RF Front-End Design1

Introduction to RF Front-End Design

Jerzy DąbrowskiDivision of Electronic Devices

Department of Electrical Engineering (ISY)Linköping University

e-mail: [email protected]


Objectives of the course

• Learn principles of wireless digital communication transceivers

• Gain knowledge of RF front-end circuits• Learn basic design methods and techniques

for RF circuit design in CMOS technology• Understand the related possibilities and

limitations

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Organization of the course

• Lectures 8 x 2h • Laboratory work 3 x 4h (guided by Henrik

Fredriksson and Rashad Ramzan)• Project work (Simulink/Cadence Spectre) • Individual reports• Course books:

– B. Razavi, RF Microelectronics, Prentice-Hall, 1998 – T.H. Lee, The Design of CMOS RF Integrated Circuits,

Cambridge Univ. Press, 1998/2004


Outline of the lecture

• Wireless communication systems today

• Digital communication RF channel

• Digital modulation techniques

• Multiple access techniques

• Digital RF transceiver at glance

• CMOS RF design issues

• Summary

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Wireless Communication Systems Today

WLANBluetooth

DECTPHS

CT1/CT2EDGE, GSM IS-54/IS-95

PDCGPS

Satellite

Paging

10m 100m 1000m 10km 100km 1000km Range

Bit Ratekb/sec

1

10

100

1000

In-door

Cordless

Cellular

4G directionsAlso many other wireless applications and gadgetsUMTS

CDMA2000

Zigbee

10,000

UWB100,000


Overview of PCS Standards

1 W1, 2, 11Mb/s

QPSK25 ppm3000 kHz

2400-2483CDMA802.11b(DSSS)

0.125, 0.25, 0.5, 2W

3840 (max)

QPSK0.1 ppm5000 kHz

1920-1980 (Tx)2110-2170 (Rx)

W-CDMA/ TD-CDMA

WCDMA(UMTS)

1,4,100 mW1000GFSK20 ppm1000 kHz

2400-2483CDMA/ FDMA/FH

Bluetooth

Peak PowerRate(kb/s)

Modulation Technique

FrequencyAccuracy

ChannelSpacing

Frequencyband (MHz)

Access Scheme

Standard

N/A

0.8, 1, 2, 3 W

250 mW

0.8, 2, 5, 8 W

0.8, 2, 5, 8 W

1228

48

1152

270.8

270.8

GMSK90 Hz200 kHz1710-1785 (Tx)1805-1850 (Rx)

TDMA/FDMA/ TDD

DCS-1800

OQPSK

π/4 QPSK

GMSK

GMSK

N/A1250 kHz

824-849 (Tx) 869-894 (Rx)

CDMA/ FDMA

IS-95

200 Hz30 kHz824-849 (Tx) 869-894 (Rx)

TDMA/FDMA

IS-54

50 Hz1728 kHz

1880-1900TDMA/FDMA/ TDD

DECT

90 Hz200 kHz890-915 (Tx)935-960 (Rx)

TDMA/FDMA/ TDD

GSM

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• Tx’s convert BB to RF signals using modulation

• Tx’s must not corrupt one another – division of RF band

• Rx’s select wanted RF signals and retrieve BB by demodulation

• Rx’s must suppress unwanted signals and noise

Tx1BB1

RF1

Tx2BB2

RF2Rx1

BB…

RF1

RF2

RF …

Tx3BB3

RF3

RF communication channel

Rx2BB…

RF1

RF2

RF3


Above 300 GHz< 1 mm

30–300 GHz10 mm – 1 mm11EHFExtremely high frequency

microwave devices, mobile phones3–30 GHz100 mm – 10 mm10SHFSuper high frequency

television broadcasts, wireless LAN300–3000 MHz1 m – 100 mm9UHFUltra high frequency

FM and television broadcasts30–300 MHz10 m – 1 m8VHFVery high frequency

Shortwave broadcasts and amateur radio3–30 MHz100 m – 10 m7HFHigh frequency

AM broadcasts300–3000 kHz1 km – 100 m6MFMedium frequency

Navigation, time signals, AM longwavebroadcasting

30–300 kHz10 km – 1 km5LFLow frequency

Military communication3–30 kHz100 km – 10 km4VLFVery low frequency

300–3000 Hz1000 km – 100 km3ULFUltra low frequency

30–300 Hz10,000 km – 1000 km2SLFSuper low frequency

3–30 Hz100,000 km – 10,000 km1ELFExtremely low frequency

< 3 Hz> 100,000 km

Example usesFrequencyWavelengthITU bandAbbrBand name

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Propagation Effects

• Path loss, interferers and external noise

• Multi-path and fading

( ) [ ]( ) [ ]dBm

dBλ/π4log20

RxAntPTxAntTxRx

P

GLGPP

RL

+−+=

=

RTx Rxλ

Power loss in open area

Received power incl. gain of the antennas

Direct path

Tx Rx

Reflective path

Moving objects or Rx/Txresult in signal fluctuations, (different varying paths)

Immobile or mobile object

Wanted signal is corrupted by interferers and noise

intsignoisesig PPSIRPPSNR ==


Digital Tx & Rx

Analog BB

inputADC DSP DAC RF

Front-End

RF part (analog)

RFFront-End

ADC DSP DAC

Analog BB

output

RF part (analog)

BB part (digital)

BB part (digital)

Coding, Interleaving,

Shaping, Modulation

Demodulation, Deinterleaving, Decoding

Upconversion,gain, filtering

Downconversion,gain, filtering

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BB data rate

t

x(t)

t

y(t)Sampling

1/fSNyquist limit

fS > 2Bx

Number of bits per sample: NSampled BB data rate: R = fS N bits/sec

Example: For voice coding B = 3.4 kHz fS = 8 kHz and N = 8 → R = 64 kb/sec.Next, compression with vocoders is used so R = 2.4 .. 9.6 kb/secbut the transmitted data rate would be much higher for system arrangements and extra data needed, e.g. GSM – 270 kb/s, IS-95 (CDMA) – 1.23Mb/s

QuantizationN bits


Shannon limitsInformation capacity: C = 2B log2M [bits/sec]

Channel bandwidth

Number of signal levels transmitted

Bandwidth efficiency: C/B = 2 log2M [bits/sec/Hz]

For 2-levels: C/B = 2, maximum possible to achieve,

1 10 01 1

0 0

Tb2Tb

Low pass channel

Bmin = 1/2Tb

In Rx at least the first harmonic is needed

M = 2

Period = 2Tb

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Shannon limit due to noise

C = 2B log2M [bits/sec]

Information capacity if B or M 000001

010

100101

011

The more levels the more noise harmful

C = B log2(1 + SNR) [bits/sec]

Channel noise limits C, but M is not specified here.

In practice bit rate must be R < C to support transmission with an acceptable error rate

M-ary system


RF systems vs channel capacity

×+=

=

BC

NE

BC

BNRESNR

b

b

02

0

1log

Bit rate R < C for any system

e.g. for GSM: R/B = 270kbps/200kHz = 1.35 @ SNR = 9dB for BER < 10-3

for DECT:R/B = 1152kbps/1728kHz = 0.67@ SNR = 10.3dB for BER < 10-3

GSM

DECTR < C

R > C

Tradeoff between signal BW and power

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Digital modulation schemes

• Frequency shift keying FSK• Phase shift keying PSK, QPSK, …• M-ary QAM• Minimum shift keying MSK• OFDM technique


Basic view on modulation

Modulator

em(t)

x(t) =A0cosω0t

s(t)

3 different parameters available for modulationby the base-band signal

(i.e. the modulating signal)

Base-band signal to be transmittedlow frequency

Sinusoidal carrierhigh frequency

( )φω +⋅= tAtx 00 cos)(

AmplitudeFrequency Phase

Sinusoidal Carrier x(t) :

Angle

Angle modulation more useful in digital communication for its higher immunity to noise and interference

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Frequency shift keying (FSK)

∫bT

0

sFSK(t)Acosω1t

xBB(t)

Modulator Acosω0t

+

01

ω1 ω1ω0

xBB(t)Acosω1t

sFSK(t)

Acosω0t

01

∫bT

0

Coherent detector based on correlation

-Thresholddetector

Tb = n0/f0 = n1/f1

“Orthogonal” frequencies


FSK (cont’d)

-+

sFSK(t)

Envelopedetector

Envelopedetector xBB(t)

01

Thresholddetector

f0

f1

Non-coherent FSK detector (simpler receiver)

In coherent FSK detection oscillator and carrier need synchronizationWhen off-phase by π/2 the correlator outputs 0 instead of 1

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Phase shift keying (PSK)

Acosω0t

sPSK(t)xBB(t)

-11

ϕ ϕ + πModulator

∫bT

0

xBB(t)sPSK(t)

Acosω0t

01Threshold

detector

Tb = n0/f0

Coherent detector based on correlation(non-coherent PSK detection possible only in differential mode)

ϕ + π


Quadrature PSK (QPSK)

Acosωct

sQPSK(t)

Asinωct

Q IxBB(t)

01

Serial-to-parallel

-

+

Output signal takes on 4 values which happen every second input bit.

sQPSK (t) = α1cosωct - α2sinωct

α1,2= ± Ac

Model (constellation):α1

α2

+Ac

+Ac

-Ac

-Ac

π/4

-π/4

3π/4

-3π/4

Modulator

During transitions the phase change is ± π/2 or ± π

± 1

± 1 Required BW is half that of BPSK

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QPSK (cont’d)

∫bT

0

∫bT

0

sQPSK(t)

Thresholddetector

Thresholddetector xBB(t)

01

I QAcosωct

Asinωct

QPSK detector

Advantage of QPSK:As bits are grouped and transmitted in pairs, the bandwidth needed is half compared to binary PSK.

α1

α2

Phase transitions in QPSK


Offset QPSK

Asinωct

Acosωct

Q IxBB(t)

01

-

+Tb

sOQPSK(t)Due to delay by 1 bit we avoid simultaneous transitions of bits in both branches

Advantage: all phase changes at output ±π/2,narrower bandwidth needed, less demands on linearity of PA

α1

α2

Phase transitions in OQPSK

Drawback: cannot be adopted to differential encoding to support non-coherent receptionϕ ϕ + π/2 ϕ - π/2

± 1

± 1

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M-ary QAM

sQAM (t) = αi cosωct - βi sinωct

α

16-ary QAM constellation

(4 bits are encoded)

Combined amplitude and phase modulation

βEven larger throughput but more susceptible to channel noise, higher SNR needed.Also very linear amplifier required


Minimum shift keying

sinω1t

cosω1t

Q IxBB(t)

Tb

sMSK(t)sinωct

cosωct

(+1, -1)

(+1, -1)

Rectangular pulses are replaced by half-sinusoids of ω1= π/2Tb that modulates the carrier of ωc

MSK based on Offset QPSK

Advantage: no abrupt phase changes at the output, signal bandwidth saved and less prone to amplitude variations when limited in band!

Different variants of MSK exist, GMSK, GFSK, …

Pulse shaping by half-sinusoids

−+= ∫∑

∞−

t

mbmMSK dtmTtpbtAts )(ωcos)( c

Gaussian or Raised-cosine most popular

Tb

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OFDM techniques

fc1

f

Tone modulated by rectangular pulse

fc2 fc3

PowerMultiple sub-carriers to transmit signal bits in parallel for very high throughput

Spectra of different sub channels can partly overlap, pulse shaping not necessary

OFDM is usually combined with QAMor PSK

Seri

al t

o pa

ralle

l

Com

bine

r

fc1

fc2

fcN

BB data OFDM modulated signal

Modulator Densely spaced !


Bit error rate

=

0

2

, erfc21

NTA

P bcPSKe

Distance between A1A2 for FSK is smaller than for PSK – so more immune to noise

α1

α2

A1 A2

PSK

α2

=

0

2

, 2erfc

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NTA

P bcFSKe

BER ~ Pe (probability of making an error in detector when transmitting a symbol)

Note that erfc(·) is a descending function, PSK better than FSK

Other coherent QPSK techniques and MSK have similar BER to PSK (for the same power), QAM is much worse, but high throughput

α1A1

A2

FSK

These modelsassume AWGN

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Multiple-Access techniques

• FDMA (Frequency division)• TDMA (Time division)• CDMA (Code division)• Up-link and down-link TDD/FDD


FDMA and TDMA systems

User_1User_2 User_3

∆t ∆t ∆t

User_1User_2 User_3

∆t ∆t ∆t

N time slots

User_1User_2 User_3

∆f ∆f ∆f

N channels in band

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CDMA systemsDirect sequence CDMA

Code sequence (chip)

BB data

1 bit period

Data encoded

ff

User 1Encoding

ff

User 2 ( spectrumspreading )

Decoding for User 1

( spectrumdespreadingby correlation)

f

Signal 1

Signal 2

Signal 1

Signal 2

Coding sequences for different users are orthogonal (e.g. Walsh, Barker), signals overlap in frequency band and in time.

Noise alikeImmune to fading


CDMA systems (cont’d)

Frequency-hopping CDMA

Code sequenceof a user

FrequencySynthesizer

BB data

time

f1

f2

f3

t1 t2

More resistant to strong interferers than DS CDMA,since it is similar to FDMA

In CDMA systems power level control of transmitters is critical, feedback is provided by the base station

FH also spreads spectrum

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Up-link and down-link by FDD/TDD/Duplex – ability to transmit and receive simultaneously/

User_1

User_2User_3

∆f ∆f ∆f ∆f

User_1

User_2User_3

∆f ∆f

Reception band Transmission band

FDDuplex

User_1User_2 User_3

∆t ∆t ∆t

User_1User_2 User_3

∆t ∆t ∆t

Reception slots Transmission slotsTDDuplex


RF Transceiver at Glance

RFSection

BasebandSection

• RF Section – analog, high frequencies

• Baseband Section - mostly digital today (DSP), low frequencies

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Digital transmitter at glance

Upconversionand Filtering

PowerAmplifier

Carrier

Modulation & DSPADC

Basebandsignal

Digital baseband section (compression, coding,

shaping, modulation ) RF section (Tx Front-end)(up-conversion, filtering, power gain, power control,matching to antenna)

DAC


Digital receiver at glance

Low NoiseAmplifier

Carrier

DownConversion & filtering

ADC

Basebandsignal

Digital baseband section (equalization, demodulation, decoding, decompression)

RF section (Rx Front-end)

(band selection, matching to antenna, gain, image rejection, down conversion, channel selection)

RFFilter

Demodulator & DSP DAC

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CMOS RF design issues

• Disciplines in RF design• Key goals for ICs for RF transceiver

implementation• Why CMOS technology ?


Disciplines required in RF system design

RF Design

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RF Circuit Design OctagonMulti-objective approach

RF Design

In digital design only one main trade-off between speed and power

Several trade-offs in RF design


Ultimate objective• Single-chip transceiver• Minimum external components• Inductors and capacitors integrated on chip

RFSection

BasebandDSP & Ctrl

Duplexeror switch

Battery or power supply

Crystal

Basebandinput/output

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Bluetooth CMOS TRx from Alcatel (2001)

Low-IF Rx and quadrature TxRF front-end

Layout of single chip TRx (first commercial with integrated BB and ARM processors + memory)


WLAN CMOS TRx from Intel

Intel RFIC transceiver on 0.18 µm TSMC CMOS technology (Taiwan Semiconductor Manufacturing Corporation).

This IEEE 802.11a (in 5 GHz band) transceiver employs a direct-conversion architecture and includes an internal synthesizer. This is Intel's first RFIC used in a WLAN product.

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Why CMOS Technology• Submicron MOSFETs, 180,130, 90 nm today, very fast,

fmax>100GHz, perform well up to 10 GHz or more• Good linearity for higher signal swing• With multiple metal layers good capacitors and inductors

(QL up to 20) can be integrated on a chip• Upper metal layers far from Si substrate – reduce substrate

losses• Lower substrate doping helps to isolate RF blocks and reduce

losses• Large digital bocks (DSP & control) can be integrated on one

chip• CMOS cheaper from other technologies (BiCMOS, GaAs, .. )• Many successful RF CMOS designs performed recently


Summary• Wireless communication systems (mobile, cordless,

WLAN, GPS, … ) are in continuous progress• Wireless communication systems are very complex

multidisciplinary field • Design of RF IC’s is a multi-objective task• CMOS technology proves to be increasingly

competitive for RF IC’s design (even higher frequencies)

• RF CMOS is an attractive research field

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