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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.
ECE 6640 3
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.
ECE 6640 4
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?
ECE 6640 5
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.
ECE 6640 6
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
ECE 6640 7
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
ECE 6640 8
ECE 6640 9
Gains and Losses to be Discussed
• Antenna Efficiency• Pointing• Atmospheric Noise• Space Loss (path loss)• Receiver
ECE 6640 10
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
ECE 6640 11
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
ECE 6640 12
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
ECE 6640 13
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:
ECE 6640 14
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
ECE 6640 15
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
ECE 6640 16
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
ECE 6640 17
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
ECE 6640 18
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.
ECE 6640 19
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.
ECE 6640 20
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
ECE 6640 22
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
ECE 6640 23
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
ECE 6640 24
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
ECE 6640 25
Quick Example
• Amplifier– 20 dB gain– 10 dB Noise Figure
ECE 6640 26
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
ECE 6640 27
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”.
ECE 6640 29
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
ECE 6640 32
Table 5.2 Earth Terminal to SatelliteLink Budget
ECE 6640 33
Satellite Repeater
ECE 6640 34
Table 5.3 Link Budget Example
ECE 6640 35