ECE 6640Digital Communications

Dr. Bradley J. BazuinAssistant Professor

Department of Electrical and Computer EngineeringCollege of Engineering and Applied Sciences

ECE 6640 2

Chapter 5

5. Communications Link Analysis.1. What the System Link Budget Tells the System

Engineer. 2. The Channel. 3. Received Signal Power and Noise Power. 4. Link Budget Analysis. 5. Noise Figure, Noise Temperature, and System

Temperature. 6. Sample Link Analysis. 7. Satellite Repeaters. 8. System Trade-Offs.

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Sklar’s Communications System

Notes and figures are based on or taken from materials in the course textbook: Bernard Sklar, Digital Communications, Fundamentals and Applications,

Prentice Hall PTR, Second Edition, 2001.

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What is a Link Budget

• An analysis of the entire communications path – signal, noise, interference, ISI contributions, etc. – Include gains and losses

• Link Budget– An estimate of the input to output system performance.– Will the message get communicated?– What trade-offs can be made and what effect will they have?

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The Channel

• The propagation medium of the communicated signal– Between the transmitting device and the receiving device (e.g. RF

antennas, cable modems, fiber optic transceivers)

• For RF we think of “Free Space”– An ideal approximation for near-ground, atmospheric RF

transmissions.– Non-ideal atmospheric impairments include:

• absorption• reflection• diffraction• scattering.

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Error-Performance Degradation

• Established in Chapter 3– Loss of SNR– Intersymbol interference

• For Digital Communications

– The relationship between SNR and Eb/No– SNR relates the average signal power and average noise power– Eb/N0 relates the energy per bit to the noise energy– Loss: refers to a loss in signal energy– Noise: refers to an increase in noise or interference energy

WTNS

RW

NS

NEb

0

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Sources of Signal Loss and Noise

1. Bandlimiting Loss2. Intersymbol Interference (ISI)3. Local Oscillator Phase Noise4. AM/PM Conversion (Amplitude

variations)5. Limiter Loss or Enhancement6. Multiple-carrier Intermodulation

Products (non-linear devices)7. Modulation Loss (message content

power)8. Antenna Efficiency9. Radome Loss and Noise10. Pointing Loss11. Polarization Loss

12. Atmospheric Loss and Noise13. Space Loss14. Adjacent Channel Interference15. Co-channel Interference16. Intermodulation Noise17. Galactic or Cosmic, Star and

Terrestrial Noise18. Feeder Line Loss19. Receiver Noise20. Implementation Loss21. Imperfect Synchronization

Reference

See Figure 5.1, p. 246.

Figure 5.1

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Gains and Losses to be Discussed

• Antenna Efficiency• Pointing• Atmospheric Noise• Space Loss (path loss)• Receiver

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Range Equations

• The power density in a sphere from a “point source” antenna (surface area of a sphere)

• Receiving power collected by an antenna (using the effective area of the receiving antenna so that p(d) can be collected)

sphereofarea

Pr

Prp tt

__4 2

2ert

err r4APArpP

densityfluxpowerincident

extractedpowertotalAer

• Effective Antenna Area

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Antenna Efficiency and Gain

• The ratio of the effective area to the actual area

• Antenna Gain

– From wikipedia: http://en.wikipedia.org/wiki/SteradianSteradians the SI unit of solid angle. It is used to describe two-dimensional angular spans in three-dimensional space, analogous to the way in which the radian describes angles in a plane.

– Note: a sphere has 4 steradians

p

e

AA

steradians4overintensitypoweraverageintensitypowermaximumG

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Effective Radiated Power

• The effective radiated power is the product of the transmitted power and the antenna gain

– The same EIRP can be achieved in many ways

• In terms of received power using effective radiated power

tt GPEIRP

24 rAEIRPP er

r

24 rAGPP er

ttr

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Antenna Gain in terms of Area

• For antennas with a large area as compared to a signal wavelength

• Antenna Reciprocity– For given antenna and carrier wavelength, the transmitting and

receiving antenna gains are identical.

• The effective area of an isotropic antenna (equal transmission in all directions)

2

2

2

44c

fAAG ererr

2

22

2 4441

fcAAG e

err

4

2 r

erGA

Note:

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Antenna Beamwidth

• Since an isotropic antenna is defined as having a gain of 1, the area ratio of the antenna beam pattern from maximum to -3dB to the area of the sphere is often an estimate of the antenna gain.

• For an antenna with a half power beamwidths in two planes the directivity, D, (and gain) are

• For a /4 beamyx

4GD

37.204

44

4GD3

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Received Power in EIRP

• For an isotropic receiving antenna, the received power is

2isotropice

isor r4A

EIRPP

s222

2

isor LEIRP

r4EIRP

r4EIRPP

• Where Ls is called the “free-space” or “path” loss– Note: It is defined based on an isotropic antenna with G=1!

22

s cfr4r4L

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The Friis Transmission Equation

• The received signal power can be defined as

• There is a family of relationships (pick your application)

2rtt

rs

r r4GGPG

LEIRPP

2ertt

r r4AGPP

22erett

22erett

rrf

cAAP

rAAPP

2rett

r r4GAPP

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Path Loss Considerations

• Path Loss is defined using an isotopic receiving antenna

• The received flux density is strictly a function of distance

• For large “effective area” receiving antennas

22

s cfr4r4L

2r4EIRPdp

2er

r r4AEIRPP

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Path Loss Considerations (2)

• The effective area for Gr=1 receiving antennas change with frequency

4f

c

4A

22

er

3 kHz 7.96E+08 meter^2 -18.02 dB3 MHz 7.96E+02 meter^2 41.98 dB3 GHz 7.96E-04 meter^2 101.98 dB

Frequency Area Path Loss 1km

4

2 r

erGA

22

s cfr4r4L

near-fieldnot valid

Frequency-Wavelength

Frequency c/f=lambda c/f=lambda

3.00E+03Hz 1.00E+05m 62.1mi

3.00E+06Hz 1.00E+02m 109.4yd.

3.00E+09Hz 1.00E-01m 3.9in.

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Atmospheric Attenuation

• As frequencies increase, there is energy absorption based on molecular bonds that the waveforms pass through.

• Attenuation peaks can be identified for H2O and O2.– Frequencies that are better or

worse for wide range or longer distance broadcasting or for short-range, private communications.

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Radio Receiver Consideration

• Receivers collect signals, interference, and noise• Signals-of-Interest (SOI) will require gain and filtering

prior to or as part of the signal processing• The noise collected by the receiver will be processed along

with the signal but will be limited by filtering• The electrical components will add their own noise to the

processed signals.

• Therefore, we need to discuss:– Cascade gain stages– Cascaded noise effects and component noise figures– Bandwidth effects on thermal noise powerECE 6640 21

RFID Receiver Downconversion• ISM Band

Downconversion (902-928 MHz)– Only mixing and filters

shown

• High-side Los– Synthesizer provides center

frequency selection

• IF filter sets bandwidth• LPF for ADC anti-aliasing• Convert to fs/4 for post-

ADC complex processing– Fs > 4 x fmax

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Cascaded Gain

• Multiple the gain (loss) of each stage together– If gain in dB, add the gains (in dB) and subtract the losses (in dB)

– If the mixers have loss instead of gain (passive mixers)

dBGdBGdBGdBGdBGdBG 2IFndMixer21IFstMixer1RFmodprede

dBGdBLdBGdBLdBGdBG 2IFndMixer21IFstMixer1RFmodprede

Linear gain is multiplicativeGain in dB is additive

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Noise Figure

• The noise figure is a measure of the additional noise that is added by any circuit element.– Effective additional input noise …

ampin

Sin

in

Sin

out

in

NNGPG

NP

SNRSNRF

tx ty

in

amp

in

ampin

out

in

NN

NNN

SNRSNRF

1

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Caution, Noise Figure is

often referred to in dB instead as a

linear term

Cascaded Noise Figure

• The noise figure is a measure of the additional noise that is added by any circuit element.– Effective additional input noise …

1

212

11

2

1

1 1111111G

FFFG

FN

NGN

NF

in

amp

in

amp

2amp1ampin12

Sin21

in

Sin

out

in

NNNGGPGG

NP

SNRSNRF

tx ty

in

ampampin

in

ampampin

out

in

NG

NNN

NGGNNNGG

SNRSNRF 1

21

21

2112

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Quick Example

• Amplifier– 20 dB gain– 10 dB Noise Figure

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tx ty

tf2cos 1LO

tx c

Bandpass Filter

tf2cos 2LO

tx M

Amplifier Lowpass Filter

Demod

Bandpass Filter

Tuning

tx eDPr

Basic Receiver

• RF Filter removes images• Low Noise Amplifier• Mixer to IF• IF BPF sets the system BW• Mixer to baseband• Baseband LPF to remove mixing

products

dBGdBGdBGdBGdBGdBGdBG LPFndMixerBPFstMixerLNABPFRFeD 21Pr

ndMixerBPFstMixerAmpBPFRF

LPF

BPFstMixerAmpBPFRF

ndMixer

stMixerAmpBPFRF

BPF

AmpBPFRF

stMixer

BPFRF

stMixerBPFRFeD

GGGGGF

GGGGF

GGGF

GGF

GFFF

211

2

1

11Pr

11

111

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Thermal Noise Power

• Modeled as additive white Gaussian noise (AWGN)

– Where N is the noise power– κ is Boltzmann’s constant– T is absolute temperature in degrees Kelvin– B is the bandwidth in Hertz

BNBTNoisePower 0

HzK/dBW6.228

refIEEEK290T0

21e00.429023e38.1TN 00

Hz/dBm174Hz/dBW204N0 ECE 6640 28

Thermal Noise Temperature

• Once you have determined the relative noise (dB/Hz)

• If the noise temperature is quoted, you now the noise

• Satellite and astronomy always use noise temperatures to describe “objects”.

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00 TN TN

0

0NT

NT

Receiver Operating Characteristics

• Sensitivity – minimum input value• Dynamic Range – usable signal range • Selectivity – filter out adjacent noise and interference• Adjacent Channel Interference (ACI) Rejection

and Image Rejection• Noise Figure

Building a performance diagram for a software radio

Input to ADC inputECE 6640 30

FM Radio Design Diagram

• FM receiver• 200 kHz BW• 12-bit ADC with 10-bit

performance• Multiple signal environment• SOI detection threshold• ROC

– Sensitivity -103 dBm– Dynamic Range 41 dB– Gain 63 dB– NF 10 dB– Selectivity: based on IF filter– ACI: filter attenuation at n

channels away (n x 200 kHz)ECE 6640 31

Putting It All Together

• For dedicated communication systems, link budgets are defined– System Engineer’s responsibility to guarantee that successful

communication will occur.– Examples: WiFi access point locations, satellite communications,

FM radio station coverage areas (and transmitting antenna siting), Cellular Telephone System Base Station Siting, etc.

• If you build it, will it be useful?– Reliability, design margin, upgrades, component replacement

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Table 5.2 Earth Terminal to SatelliteLink Budget

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Satellite Repeater

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Table 5.3 Link Budget Example

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