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

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 Eb/No) at detector or MF input determines BER of digital system

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

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

ECE 4710: Lecture #36 5

Signal Power @ Rx

Power gain of channel

PRx is almost always much smaller than PTx

Example: Cell phone tower PTx W while PRx nW

gainpower space free

antennaRx ofgain power

antennaTx ofgain power

output) antennaRx (at Rx intopower signal

antennaTx intopower signal

where

FS

AR

AT

Rx

Tx

ARFSATTx

Rx

G

G

G

P

P

GGGP

P

ECE 4710: Lecture #36 6

densitypower same with theantenna isotropican ofdensity power radiation radiation maximum ofdirection in the antenna actual ofdensity power radiation AG

Antennas

Antenna Power Gain (GA) 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

ECE 4710: Lecture #36 7

Antennas

Practical antennas Purpose is to radiate power in specific direction(s)

towards Rx

Focus PTx 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

ECE 4710: Lecture #36 8

Power Density

Radiated EM wave characterized by power density Power density = power per unit area (W / m2) 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

EIRPTx Pd

P

d

24 radius with sphere of area surface

powerTx

24/1 d

ECE 4710: Lecture #36 9

Power Density

For real antenna the radiated power density is larger than PEIRP 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 / m2)

Conversion :

Free-space wave impedance = 377

377density power

2

E

12

7

0

0

1085.8104

antenna real ofdensity power radiated4

2 d

PGPG Tx

ATEIRPAT

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 (Ae) Larger area = more power captured from Tx density

24

d

PG Tx

AT

where4

22 mAA

d

PGP ee

TxATRx

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

2

22

22 4

44

4 d

GGPGd

PGA

d

PGP ARATTxARTx

ATeTx

ATRx

4or

4 2

2AR

ee

AR

GA

AG

ECE 4710: Lecture #36 12

Antenna is a reciprocal element Gain is the same whether it is transmitting or receiving

GA is linear quantity that is unitless relative measure between to powers

In decibels GA (dB) = 10 log (GA)

Antenna GA & Ae

ECE 4710: Lecture #36 13

dBm 43.6or nW .734 W107.43

30004

)157.0(26320 92

2

RxP

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.

2

2

4

d

GGPP ARATTx

Rx

210 & 6310 10/310/18 ARAT GG

m 157.0109.1

1039

8

f

c

ECE 4710: Lecture #36 14

W p 68.5 W 105.681061.1300,224

)075.0(352185,35200

1223

2

RxP

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.

dB) 45.5 (~ 185,35/(0.075))3(7 22 ATG

2/7dish satellite 4-8 Table AGA

m 075.0/ fc

2rA

dB) 25.5 (~ 9.351/(0.075))3.0(7 22 ARG

ECE 4710: Lecture #36 15

Free Space Loss

Recall that book defines

where GFS is free space power gain

Using

Then

where LFS is the free space loss :

ARFSATTx

Rx GGGP

P

4 then

4 2

2

2

2

ARFSATARAT

Tx

RxARATTxRx GGG

d

GGP

P

d

GGPP

1

4 2

2

FSFS Ld

G

2

24

d

LFS

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 (O2, H20, etc.)

» Only significant for f > 2 GHz and distances > 100 km

Many links are not Line of Sight (LOS) obstructed (OBS)

22 /4 dLFS

dd

LFS

4log20

4log10)dB(

2

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

d

nd

Ln

FS

4log10

4log10)dB(

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)

4 2

2

d

GGPP ARATTx

Rx