ECE 4710: Lecture #2 1 Frequency Communication systems often use atmosphere for transmission ...

ECE 4710: Lecture #2 1

Frequency

Communication systems often use atmosphere for transmission “Wireless” Time-varying Electro-Magnetic (EM) Wave Propagation characteristics of EM wave thru atmosphere

are highly dependent on frequency/wavelength

f is frequency is wavelength

c is speed of light = 3 x 108 m/s

)Hz(c

f


Propagation Modes

Three dominate types of propagation modes Ground Wave

» f < 2 MHz» Diffraction causes wave to propagate along Earth

surface» Propagation beyond visual horizon (e.g. AM broadcast

radio) with sufficient Tx power Sky-Wave

» 2 MHz < f < 30 MHz» Refraction/Reflection off ionosphere (50-250 mile alt.)» Intermittent coverage along Earth’s surface


Ground Wave

With sufficient Tx power ground waves can propagate thousands of miles


Sky Wave

International broadcasts (BBC, VOA, etc.) can be heard half-way around the world with modest Tx power

Note that only certain locations on ground can receive Tx

signal


Propagation Modes

Line of Sight = LOS» f > 30 MHz» Signal path must be free from obstructions» Earth’s curvature will determine LOS distance for

antennas mounted on tall towers» LOS distance =

hf : antenna height in feet

hm : antenna height in meters

» Two antenna towers/heights (Tx and Rx)

)km(13.4)miles(2 mfLOS hhD

21 LOSLOST DDD


Line of Sight (LOS)

21 LOSLOST DDD

Short range for reasonable antenna heights h1 = 30 m and h2 = 50 m DT = 52 km or 32 miles !!


Frequency Allocations

Range Designation Propagation Typical Uses

3-30 kHz Very Low Frequency (VLF) Ground Wave,

Low Attenuation

Long Range Navigation & Submarine

Communications

30-100 kHz Low Frequency (LF) Ground Wave,

Medium Attenuation

Long Range Navigation & Radio Beacons

300-3000 kHz

Medium Frequency (MF) Ground Wave & Night Sky Wave

Maritime Radio AM Broadcast Radio

3-30 MHz High Frequency (HF) Ionospheric Sky Wave

Amateur Radio, Military, International Broadcast

(BBC)

30-300 MHz Very High Frequency (VHF) Nearly LOS TV, FM Radio, Mobile Radio

0.3-3 GHz Ultra-High Frequency (UHF) LOS TV, Cellular Phone, Radar,

GPS, PCS, LOS wave

3-30 GHz Super-High Frequency (SHF) LOS,

Atmospheric Atten. Satellite, Radar, Military,

Remote Sensing

30-300 GHz Extremely-High Frequency

(EHF) LOS, Severe

Atmospheric Atten. Radar, Satellite, Military,

Remote Sensing

103-107 GHz Infrared, Visible Light, UV LOS Optical Communication,

Satellite Imaging


LOS Propagation

Why use high frequencies which have smaller propagation distances (LOS)? High carrier frequencies (fc) support larger bandwidth (BW)

signals which leads to higher data rates + more users» Practical Tx/Rx’s can have signal BWs 0.1 fc

» Information data rate Rd BW Antenna size must be at least 10% of for efficient

propagation of EM wave thru atmosphere (~0.5 for RF)

» fc = 10 kHz km antenna height = 3000 m !! Must modulate most baseband signals with high frequency

carrier for wireless transmission to have reasonable antenna size


Information Measure

How is information content measured? Information sent from digital source from the jth message is

where Pj is the probability of transmitting the jth message

Information content will, in general, vary from one message to the next since Pj is usually variable Bit = unit of information and Bit = unit of binary data (0,1) but they are not the same Must use context to determine meaning

)bits(ln)2ln(

1log

)2(log11

log 1010

2 jjj

j PPP

I


Information Measure

Since information content varies from message to message must measure average information

- where m is the number of possible source messages

- H is also called the “entropy” of the source

Rate of Information

)bits(1

log1

21

m

j jj

m

jjj P

PIPH

bits/sT

HRI


Information Measure

Example: 8 digit word (message) with two possible states per

digit (binary). Find the entropy if a) all words equally likely and

b) if half the words have Pj1 = 1/512

a) m = 28 = 256 and since all words equally likely Pj = 1/m = 1/256

b) Note: All Pj = 1 (definition of probability) so 128 Pj1 + 128 Pj2 = 1 Pj2 = (1/128)(1-128 Pj1) = (1/128)(3/4)= 3/512

must have equally likely for average information content = # digits

bits8256log256

1256

1log 22

jj P

mPH

!!881.756.525.21

log1281

log1282

221

21

jj

jj P

PP

PH


Channel Capacity

Ideal channel capacity shown by Shannon to be

Actual channel data rate Rc < C

receiver digital ofpart RF)(not baseband input toat dB)(not ratio noise tosignal

ts)(watts/watlinear is and (Hz)BW channel is where

bits/s1log2

N

SB

N

SBC


Channel Capacity

C B so more bandwidth means higher data rate PSD of rectangular pulse train is (sin x / x)2

As Tb data rate Rc since Rc (Tb )-1 , but B

also !! Increasing signal BW will increase data rate if

everything else remains the same

f

PSD

1 / Ts = FNBW

000 0 01 1 1

Symbol Period = Ts = Tb = Bit Period

Signal BW = Bs 1 / Tb

…


Channel Capacity

C is also S/N Higher signal power means larger channel capacity??? Larger S/N makes it easier to differentiate (detect) multiple

states per digital symbol in presence of noise

higher data rate for same symbol period & bandwidth

vs.00 01 00 10 00 11 00 01

Ts1

0 1 0 1 0 1 0 1

Ts2

Ts1 = Ts2 but R1 = 2R2

**Note that (S /N)1 > (S /N)2 to achieve higher data rate with same bit error probability**


Digital System Performance

Critical Performance Measures: Bit Error Rate (BER) Channel BW = Transmitted Signal BW Received S/ N Signal Power

Channel Data Rate (Rc)

Desire high data rate with small signal BW, low signal power, and low BER!!

Fundamental tradeoff between signal power and BW Example: Error Coding add coding bits to data stream but keep same data

rate

» For same Rc Ts must and BW

» But coding will correct errors allowing weaker signal power for same BER


Error Coding

Errors occur due to corruption of Tx signal by noise and interference in channel

Reduce errors to improve performance Two Coding Types

ARQ: Automatic Repeat Request FEC: Forward Error Correction

ARQ : Add parity bits, Rx detects error, sends request for retransmission of data

FEC: Add coding bits, Rx detects and corrects for some (usually not all) of the errors


Error Coding

ARQ is used often for computer communications (internet) Not possible with one-way communication Not good for systems with large transmission delays Leads to poor data throughput when retransmissions are frequent

FEC is widely used in wireless communication systems Two major types: Block Codes & Convolutional Codes

Coding Performance: measure improvement in S/N before and after coding Lower S/N can achieve same BER for signal with coding compared to

signal without coding “Coding Gain” Coding Threshold: coded signal will have worse performance for S/N

below some threshold value!!


Coding Performance

ECE 4710: Lecture #2 1 Frequency Communication systems often use atmosphere for transmission ...

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