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ECE 6640 Digital Communications ECE 6640 Digital Communications Dr. Bradley J. Bazuin Assistant Professor Department of Electrical and Computer Engineering College of Engineering and Applied Sciences 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 2 Sklar’s Communications System Notes and figures are based on or taken from materials in the course textbook: ECE 6640 Bernard Sklar, Digital Communications, Fundamentals and Applications, 3 Prentice Hall PTR, Second Edition, 2001. 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 4 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 5 Error-Performance Degradation • Established in Chapter 3 – Loss of SNR – Intersymbol interference • For Digital Communications E S W b N0 N R – 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 ECE 6640 6 Sources of Signal Loss and Noise 1. Bandlimiting Loss 12. Atmospheric Loss and Noise 2. Intersymbol Interference (ISI) 13. Space Loss 3. Local Oscillator Phase Noise 14. Adjacent Channel Interference 4. AM/PM Conversion (Amplitude 15. Co-channel Interference variations) 16. Intermodulation Noise 5. Limiter Loss or Enhancement 17. Galactic or Cosmic, Star and 6. Multiple-carrier Intermodulation Terrestrial Noise Products (non-linear devices) 18. Feeder Line Loss 7. Modulation Loss (message content 19. Receiver Noise power) 20. Implementation Loss 8. Antenna Efficiency 21. Imperfect Synchronization Reference 9. Radome Loss and Noise 10. Pointing Loss See Figure 5.1, p. 246. 11. Polarization Loss ECE 6640 7 Figure 5.1 ECE 6640 8 Gains and Losses to be Discussed • Antenna Efficiency • Pointing • Atmospheric Noise • Space Loss • Receiver ECE 6640 9 Range Equations • The power density in a sphere from a “point source” antenna (surface area of a sphere) P pr t 4r 2 • Receiving power collected by an antenna (using the effective area of the receiving antenna so that p(d) can be • Effective Area collected) total power extracted Aer P A incident power flux density P pr A t er r er 4r 2 ECE 6640 10 Antenna Efficiency and Gain • The ratio of the effective area to the actual area A e Ap • Antenna Gain maximum power intensity G average power intensity over 4 steradians – From wikipedia: http://en.wikipedia.org/wiki/Steradian Steradians 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 ECE 6640 11 Effective Radiated Power • The effective radiated power is the product of the transmitted power and the antenna gain EIRP Pt Gt – The same EIRP can be achieved in many ways • In terms of received power using effective radiated power A P EIRP er r 4 r 2 A P P G er r t t 4 r 2 ECE 6640 12 Antenna Gain in terms of Area • For antennas with a large area as compared to a signal wavelength Note: 4 A 4 A f 2 G er er 2 r 2 2 G c A r er 4 • 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 4 Aer c Gr 1 2 Ae 2 ECE 6640 4 4 f 13 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 4 D G x y •For a /4 beam 4 43 D G 20.37 4 4 ECE 6640 14 Received Power in EIRP • For an isotropic receiving antenna, the received power is A P EIRP eisotropic riso 4r 2 2 EIRP EIRP Priso EIRP 4 2 r 2 4r 2 L s • Where Ls is called the “free-space” or “path” loss – Note: It is defined based on an isotropic antenna with G=1! 2 2 L 4r 4r f s c ECE 6640 15 The Friis Transmission Equation • The received signal power can be defined as EIRP P G G P G t t r r L r 4r 2 s • There is a family of relationships (pick your use) Pt G t Aer Pr 2 4r Pt Aet Aer Pt Aet Aer Pr 2 2 2 r c r 2 P A G f P t et r r 4r 2 ECE 6640 16 Path Loss Considerations • Path Loss is defined using an isotopic receiving antenna 2 2 L 4r 4r f s c • The received flux density is strictly a function of distance EIRP pd 4r 2 • For large “effective area” receiving antennas EIRPA P er r 4r 2 ECE 6640 17 Path Loss Considerations (2) • The effective area for G=1 receiving antennas change with frequency 2 2 c A f er 4 4 Frequency Area Path Loss 1km 3 kHz 7.96E+08 meter^2 -18.02 dB 3 MHz 7.96E+02 meter^2 41.98 dB 3 GHz 7.96E-04 meter^2 101.98 dB ECE 6640 18 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 power ECE 6640 19 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 20 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) G predemod dB G RF dB G1stMixer dB G IF1dB G 2ndMixer dB G IF2 dB – If the mixers have loss instead of gain (passive mixers) G predemod dB G RF dB L1stMixer dB G IF1dB L2ndMixer dB G IF2 dB Linear gain is multiplicative Gain in dB is additive ECE 6640 21 Noise Figure • The noise figure is a measure of the additional noise that is added by any circuit element. – Effective additional input noise … xt yt Caution, PSin Noise Figure is SNR N F in in often referred to in G P SNRout Sin dB instead as a G N N in amp linear term SNR N N N F in in amp 1 amp SNRout Nin Nin ECE 6640 22 Cascaded Noise Figure • The noise figure is a measure of the additional noise that is added by any circuit element. – Effective additional input noise … xt yt PSin SNR N F in in G G P SNR out 1 2 Sin G 2 G1 Nin Namp1 Namp2 N N N amp2 SNR G G N N N in amp1 G F in 2 1 in amp1 amp2 1 SNRout G1 G2 Nin Nin N 1 N 1 F 1 F 1 amp1 amp2 1 F 1 F 1 F 2 ECE 6640 1 2 1 23 Nin G1 Nin G1 G1 Basic Receiver Bandpass Bandpass Lowpass Amplifier Filter Filter Filter x c t x PreD t x M t Demod • RF Filter removes images • Low Noise Amplifier Tuning • Mixer to IF • IF BPF sets the system BW • Mixer to baseband cos2 f t cos2 f t LO1 LO2 • Baseband LPF to remove mixing products GPreD dB GRF BPF dB GLNA dB G1stMixer dB GBPF dB G2ndMixer dB GLPF dB F1stMixer 1 F1stMixer 1 FBPF 1 FPr eD FRF BPF GRF BPF GAmp GRF BPF GAmp GRF BPF GAmpG1stMixer F 1 F 1 2ndMixer LPF ECE 6640 24 GRF BPF GAmpG1stMixerGBPF GRF BPF GAmpG1stMixerGBPF G2ndMixer Thermal Noise Power • Modeled as additive white Gaussian noise (AWGN) N T B – Where N is the noise power – κ is Boltzmann’s constant – T is absolute temperature in degrees Kelvin – B is the bandwidth in Hertz 228.6 dBW / K Hz T0 290K IEEE ref N0 T0 1.38e 23290 4.00e 21 ECE 6640 N0 204 dBW / Hz 174 dBm / Hz 25 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 input ECE 6640 26 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 27 FM Radio Design Diagram • FM receiver • 200 kHz BW • 12-bit ADC with 10-bit performance • Multiple signal environment • SOI detection threshold ECE 6640 28 Putting It All Together • For dedicated communication systems, link budgets are defined – System Engineer’s responsibility to guarantee that successful communication will occur.
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