1. ECE 4710: Lecture #36 1 Chapter 8  Chapter 8 : Wired and Wireless Communication Systems  Telephone  Fiber Optic  DSL  Satellite  Digital & Analog TV  Cellular Telephone  Personal Communication Systems (PCS)  Link Budget Analysis and System Design

2. ECE 4710: Lecture #36 2 Link Budget Analysis  BER baseband performance determined by received signal to noise ratio ( S/N )  How do we predict the received signal and noise power?  Link Budget Analysis  Predict received signal power at input to Rx »Depends on Tx output power, channel attenuation (path loss), antenna gains (wireless), etc.  Predict received noise power at input to Rx »Depends on frequency, antenna field of view, temperature, etc.  Predict signal + noise power at detector or MF input in Rx »Depends on Rx gain, noise characteristics, etc. »S/N (or E b /N o ) at detector or MF input determines BER of digital system

3. ECE 4710: Lecture #36 3 Signal Power @ Rx  Signal power at Rx input is a critical parameter in the design of any communication system  For a given Tx power how do we predict the received signal power?  Basic communication system (Tx + Channel + Rx) Receiver

4. ECE 4710: Lecture #36 4 Signal Power @ Rx  Free Space Transmission Channel  Wireless Communication System  Atmosphere (usually) or Outer Space  Power gain of channel  Gain?  Book includes Tx and Rx antennas as part of channel »Not standard perspective but is OK  All channels attenuate the Tx signal and are therefore lossy  do NOT have gain (signal amplification)  Wired Channel  cable attenuation  Wireless Channel  free space path loss

5. ECE 4710: Lecture #36 5 Signal Power @ Rx  Power gain of channel  P Rx is almost always much smaller than P Tx  Example: Cell phone tower P Tx  W while P Rx  nW

6. ECE 4710: Lecture #36 6 Antennas  Antenna Power Gain ( G A )  NOT actual amplification of signal  Power gain relative to “isotropic” antenna  Isotropic antenna »“Iso” = same »Theoretical non-realizable antenna that radiates equal (same) power in all directions (spherical expansion) »Useful reference to compare performance of practical antennas  Power gain

7. ECE 4710: Lecture #36 7 Antennas  Practical antennas  Purpose is to radiate power in specific direction(s)  towards Rx  Focus P Tx in given direction  greater focus  larger “gain”  Larger antenna size (relative to )  greater ability to focus energy in specific direction  larger gain  Antenna is effectively a transducer which takes a time- varying voltage from a circuit and launches a time-varying EM wave in free space »Only time-varying EM waves can effectively propagate large distances

8. ECE 4710: Lecture #36 8 Power Density  Radiated EM wave characterized by power density  Power density = power per unit area ( W / m 2 )  Power density of isotropic antenna at distance d  EIRP = Effective Isotropic Radiated Power  Equal power at any given distance  isotropic  Power decays  1 / d 2 as surface area of sphere expands »Point source of EM energy  Best case free space path gain (loss) is

9. ECE 4710: Lecture #36 9 Power Density  For real antenna the radiated power density is larger than P EIRP for direction of max radiation  antenna gain  FCC specifies EM radiation safety regulations in terms of electric field intensity E ( V / m ) instead of power density ( W / m 2 )  Conversion :  Free-space wave impedance = 377 

10. ECE 4710: Lecture #36 10 Signal Power @ Rx  Radiated power density of real antenna :  Rx antenna at distance d will intercept / capture some of the Tx power density  The amount of power captured at Rx is directly related to Rx antenna size or effective area ( A e )  Larger area = more power captured from Tx density

11. ECE 4710: Lecture #36 11 Signal Power @ Rx  Rx antenna gain is related to effective area by  Thus the signal power at output of Rx antenna is  Link Formula  Friis Transmission Formula

12. ECE 4710: Lecture #36 12  Antenna is a reciprocal element  Gain is the same whether it is transmitting or receiving  G A is linear quantity that is unitless  relative measure between to powers  In decibels  G A (dB) = 10 log ( G A ) Antenna G A & A e

13. ECE 4710: Lecture #36 13 Signal Power @ Rx  Example 1: A PCS cell phone tower transmits at a frequency of 1.9 GHz, has a Tx power of 20 W, and an antenna gain 18 dB. Determine the Rx signal power (in dBm) of a mobile phone at a distance of 3 km assuming the Rx antenna has a gain of 3 dB and has a LOS link to Tx.

14. ECE 4710: Lecture #36 14 Signal Power @ Rx  Example 2: A DirecTV satellite is in geosynchronous orbit above the earth at an altitude of 22,300 miles. The satellite transmits at a frequency of 4 GHz, has a Tx power of 200 W, and uses a dish antenna with a 3 m radius. Determine the Rx signal power at a home Rx that also uses a dish antenna with 0.3 m ( 1 ft ) radius.

15. ECE 4710: Lecture #36 15 Free Space Loss  Recall that book defines where G FS is free space power gain  Using  Then where L FS is the free space loss :

16. ECE 4710: Lecture #36 16 Free Space Loss  Free space loss : in dB :  Best case loss  Free space  free of all matter and particles (vacuum)  Earth’s atmosphere can cause additional loss due to attenuation of EM wave by atmospheric molecules (O 2, H 2 0, etc.) »Only significant for f > 2 GHz and distances > 100 km  Many links are not Line of Sight (LOS)  obstructed (OBS)

17. ECE 4710: Lecture #36 17 Free Space Loss  For obstructed conditions (mobile radio) then useful model is  n is path loss exponent  n = 2  free space or LOS with no atmospheric attenuation  n > 2 for OBS conditions  Mobile radio path loss models use n = 2 – 5 and n in the range of 2.8 - 3.5 is typical

18. ECE 4710: Lecture #36 18 Link Formula  Link formula in simplest form predicts the best case received signal power  Many factors can cause Rx signal power to be lower than simple form of link formula, e.g.  Obstructed link »Building, trees, hills, earth curvature, etc.  Atmospheric attenuation »f > 2 GHz and/or large separation distances  Antenna misalignment (gain is less than max value)