CS-435 Network Technology & Programming Laboratoryhy435/material/CS435-lecture11.pdf · Network Technology and Programming Laboratory CSD.UoC Stefanos Papadakis & Manolis

Network Technology & Programming LaboratoryCS-435spring semester 2016

Stefanos Papadakis & Manolis SpanakisUniversity of Crete

Computer Science Department

<CS-435> Network Technology and Programming Laboratory

CSD.UoC Stefanos Papadakis & Manolis Spanakis spring 2016

CS-435

• Lecture preview

• Wireless Networking

• Radio Communications Explored



Radio transmission:

two endpoints

Tx Rx



Tx Rx

Signal wave Propagation path

Propagation medium

Signal transformations due to natural

phenomenon; attenuation, external

noise, fading, reflection, diffraction,

refraction, and interference

Radio transmission:

two endpoints



Interference (!)• Interference: anything which alters, modifies, or disrupts a signal during

transmission over a wireless channel.

• Superposition of unwanted signals to a useful signal.

• Examples :

• Electromagnetic interference (EMI): disturbance of an electrical circuit due to

electromagnetic induction or electromagnetic radiation emitted from an external

source

• Co-channel interference (CCI): different radio transmitters using the same

frequency

• Adjacent-channel interference (ACI) (filter interference)

• Inter-symbol interference (ISI): distortion of a signal in which one symbol

interferes with subsequent symbols

• Inter-carrier interference (ICI), caused by Doppler shift in OFDM modulation.

• Conducted interference (noise interference)• …

• Inter/Intra-flow interference refers to the interference between source sharing the

same busy channel of path.



Interference (!)

• Everything on same channel

• sum all powers

• On different channels

• inter-channel power quotient(proportion)



Thermal Noise

• Thermal noise due to agitation of electrons

• Present in all electronic devices and

transmission media

• Cannot be eliminated

• Function of temperature

• Particularly significant for satellite communication



Noise Terminology

• Intermodulation noise – occurs if signals with different frequencies share the same medium• Interference caused by a signal produced at a

frequency that is the sum or difference of original frequencies

• Crosstalk – unwanted coupling between signal paths

• Impulse noise – irregular pulses or noise spikes• Short duration and of relatively high amplitude

• Caused by external electromagnetic disturbances, or faults and flaws in the communications system



Other Impairments

• Atmospheric absorption – water vapor and

oxygen contribute to attenuation

• Multipath – obstacles reflect signals so that

multiple copies with varying delays are

received

• Refraction – bending of radio waves as they

propagate through the atmosphere



SNR / SIR / SINR

• What is interference?

• What is noise?

• noise floor:

• noise factor / noise figure:

• SNR / SINR / SIR:



Sensitivity

• SINR is not the only criterion for reception!

• The Received Signal power must be over a threshold

• Vendors usually provide only RSS thresholds, not SINR



Sensitivity



Rates vs. Sensitivity/SINR

• Different modulation schemes have different

constellations

• Denser constellations carry more bits/point

• higher rate

• increased BER

• needs larger SINR



802.11a/g OFDM



Multipath Propagation



Multipath Propagation

• Reflection - occurs when signal encounters

a surface that is large relative to the

wavelength of the signal

• Diffraction - occurs at the edge of an

impenetrable body that is large compared to

wavelength of radio wave

• Scattering – occurs when incoming signal

hits an object whose size in the order of the

wavelength of the signal or less



Classical two-ray

(ground model)



The Effects of Multipath

Propagation• Multiple copies of a signal may arrive at

different phases

• If phases add destructively, the signal level relative to noise declines, making detection more difficult

• Intersymbol interference (ISI)

• One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit



Types of Fading

• Fast fading

• Slow fading

• Flat fading

• Selective fading

• Rayleigh fading

• Rician fading



Fading in a mobile environment

• The term fading refers to the time variation of

received signal power caused by changes in

the transmission medium or paths.

• Atmospheric condition, such as rainfall

• The relative location of various obstacles

changes over time



The fading channel

• Additive White Gaussian Noise (AWGN) channel thermal noise as well as electronics at the transmitter and receiver

• Rayleigh fading there are multiple indirect paths between transmitter and receiver and no distinct dominant path, such as an LOS path

• Rician fading there is a direct LOS path in additional to a number of indirect multipath signals



Fading: Small and Large scale



Path Loss

• Free Space propagation model:

• Two Ray (Ground Reflection) model:

• Log Distance model



Measured indoor path loss



Measured large-scale path loss



Path Loss Exponent for

Different Environments



Signal Propagation

• Reflection

• Diffraction

• Scattering

• MultiPath

• Fading

• Shadow



Radio Propagation Model

• An empirical mathematical formulation for the:• characterization of radio wave propagation as a

function of : • frequency, distance and other conditions

• A single model developed to • predict the behavior of propagation for similar

links under similar constraints

• formalize the way radio waves are propagated

from one place to another

• Goal : predict the path loss along a link or the

effective coverage area of a transmitter.



Propagation Modes

• Ground-wave propagation

• Sky-wave propagation

• Line-of-sight propagation



Ground Wave Propagation• Follows contour of the earth

• Can Propagate

considerable distances

• Frequencies up to 2 MHz

• Example : AM radio



Sky Wave Propagation• Signal reflected from ionized layer of atmosphere back

down to earth

• Signal can travel a number of hops, back and forth

between ionosphere and earth’s surface

• Reflection effect caused by refraction

• Examples

• Amateur radio

• CB radio



Line-of-Sight Propagation



Line-of-Sight Propagation

• Transmitting and receiving antennas must be within line of sight• Satellite communication – signal above 30 MHz

not reflected by ionosphere

• Ground communication – antennas within effectiveline of sight of each other due to refraction

• Refraction – bending of microwaves by the atmosphere• Velocity of electromagnetic wave is a function of

the density of the medium

• When wave changes medium, speed changes

• Wave bends at the boundary between mediums



Fresnel Zone• The area around the visual line-of-sight that radio waves spread out

into after they leave the antenna.

• This area must be clear or else signal strength will weaken.



Fre’s’nell Zone (silent ‘s’) …

• We know that :

• Each wave-front point creates new circular waves

• Microwave beams widen, and

• Waves of one frequency can interfere with each other

• Fresnel zone theory: looks at a line from T to R, and at the

space around that line that contributes to what is arriving at

point R. • Some waves travel directly from T to R, while

• Others travel on paths off axis.

• their path is longer, introducing a phase shift between the direct and indirect beam

• Whenever a phase shift is one full wavelength, you get constructive interference: the

signals add up optimally

• Taking this approach and calculating accordingly, you find that:

• there are ring zones around the direct line T to R which contribute to the signal arriving at point T.



Fre’s’nell Zone (silent ‘s’) …

• There are many possible Fresnel zones, but we are concerned with zone 1.

• If this area were blocked by an obstruction, e.g. a tree or a building, the signal arriving at the far end

would be diminished.

• We need to make sure that these zones be kept free of obstructions

• usually we check that 60 percent of the first Fresnel zone is kept free.

• A formula for calculating the radius of the first Fresnel zone:

• rN is the radius of the zone in meters

• N is the zone to calculate (i.e. N=1)

• d1 and d2 are distances from the

obstructing screen at height h

• λ is the wavelength

• h<<d1,d2 and h>>λ

1 2

1 2

N

N d dr

d d



At the Receiver

• Signal of Interest

• Account path loss + delayed reflections

• Interference

• Transmissions in the same or neighboring

channels/frequencies

• Noise

• Thermal + System noise



Antennas• The antenna provides three fundamental

properties

• Gain

• Direction

• Polarization

• Gain: (pos/neg) increase in power

• Direction: transmission shape/pattern

• Polarization: electric field oscillation axis

orientation



Antennas

• Near field

• Far field / Fraunhofer region



Antennas



Near/Far Field



Omni-directional Antenna

Patterns



Directional Antennas

Patterns



Received Power

• Effective Isotropic Radiated Power



Link Budget

• Predict the wireless link

• Estimate the Received Power =>

• Rate

• Use dB (additions & subtractions)



Link Budget



Link Budget Example

• We want to estimate the feasibility of a 5km link, with one access point

and one client radio.

• The access point is connected to an omnidirectional antenna with

10dBi gain, while the client is connected to a sectorial antenna with

14dBi gain.

• The transmitting power of the AP is 100mW (or 20dBm) and its

sensitivity is -89dBm.

• Cable losses for both the Rx and the Tx are the same at 2 dBm

• The transmitting power of the client is 30mW (or 15dBm) and its

sensitivity is -82dBm.



Link Budget Example (cont.)

• Adding up all the gains and subtracting all the losses for the AP to

client link gives:

20 dBm (TX Power Radio 1)

+ 10 dBi (Antenna Gain Radio 1)


- 2 dBm (Cable loses Rx)

- 2 dBm (Cable loses Tx)

--------------------------------------------------

40 dB = Total Gain

• The path loss for a 5km link, considering free space loss is:

Path Loss = 40 + 20log(5000) = 113 dB

• Subtracting the path loss from the total gain

40 dB - 113 dB = -73 dB

• Since -73dB is greater than the minimum receive sensitivity of the

client radio (-82dBm), the signal level is just enough for the client

radio to be able to hear the access point.

• There is 9dB of margin (82dB -73dB)



Link Budget Example (cont.)• Next we calculate the link from the client back to the access

point:

15 dBm (TX Power Radio 2)



- 2 dBm (Cable loses Rx)

- 2 dBm (Cable loses Tx)

--------------------------------------------------

35 dBm = Total Gain

• Obviously, the path loss is the same on the return trip. So our

received signal level on the access point side is: 35 dB - 113 dB =

-78 dB

• The receive sensitivity of the AP is -89dBm, this leaves us 11dB of

margin (89dB -78dB)

• For the case of 802.11b (2,4GHz) E.I.R.P is 20dBm

IS EVERYTHING OK?

ANY PROBLEMS? …. (think about it)



Link Budget (homework)

• Exercise 1:

• 802.11g , 54Mbps => -73dBm sens.

• Tx Power 20dBm

• EIRP 30dBm

• distance covered?

• Exercise 2:

• 802.11g

• 2km distance

• EIRP 20dBm

• achievable rate?



References

(images/material)• Wireless Communications - Principles and

Practice (Second Edition),

by Theodore S. Rappaport



APPENDIX



Algebra• When using Watt:

• multiply, divide

• When using dB/dBm:

• add, subtract

• The decibel (dB) is a logarithmic unit that indicates the

ratio of a physical quantity (usually power or intensity)

relative to a specified or implied reference level

• Decibel suffix:

• dBm: indicates that the reference quantity is one milliwatt

• dBi : dB(isotropic) – the forward gain of an antenna compared

with the hypothetical isotropic antenna, which uniformly

distributes energy in all directions.



Decibel

• Relative measurement unit:

Examples

• Rule of thumb: +10dB <=> x10



Decibel

• Rule of thumb: +3dB <=> x2

• 10 mW + 3 dB = 20 mW

• 100 mW - 3dB = 50 mW

• 10 mW + 10 dB = 100 mW

• 300 mW - 10 dB = 30 mW



Decibel

• From dB to units:

• -3dB = half the power in mW

• +3dB = double the power in mW

• -10dB = one tenth the power in mW

• +10dB = ten times the power in mW



more algebra…



Basic Encoding Techniques

• Digital data to analog signal

• Amplitude-shift keying (ASK)

• Amplitude difference of carrier frequency

• Frequency-shift keying (FSK)

• Frequency difference near carrier frequency

• Phase-shift keying (PSK)

• Phase of carrier signal shifted



Amplitude modulation



Basic Encoding Techniques



Amplitude-Shift Keying

• One binary digit represented by presence of carrier, at constant amplitude

• Other binary digit represented by absence of carrier

• where the carrier signal is Acos(2πfct)



Binary Frequency-Shift

Keying (BFSK)

• Two binary digits represented by two different

frequencies near the carrier frequency

• where f1 and f2 are offset from carrier frequency fc by equal but

opposite amounts



Multiple Frequency-Shift

Keying (MFSK)• More than two frequencies are used

• More bandwidth efficient but more susceptible to error

• f i = f c + (2i – 1 – M)f d• f c = the carrier frequency

• f d = the difference frequency

• M = number of different signal elements = 2 L

• L = number of bits per signal element



Phase-Shift Keying (PSK)

• Two-level PSK (BPSK)

• Uses two phases to represent binary digits




• Differential PSK (DPSK)

• Phase shift with reference to previous bit

• Binary 0 – signal burst of same phase as

previous signal burst

• Binary 1 – signal burst of opposite phase to

previous signal burst




• Four-level PSK (QPSK)

• Each element represents more than one bit




• Multilevel PSK• Using multiple phase angles with each angle having

more than one amplitude, multiple signals elements can be achieved

• D = modulation rate, baud

• R = data rate, bps

• M = number of different signal elements = 2L

• L = number of bits per signal element



Performance

• Bandwidth of modulated signal (BT)

• ASK, PSK BT=(1+r)R

• FSK BT=2DF+(1+r)R

• R = bit rate

• 0 < r < 1; related to how signal is filtered

• DF = f2-fc=fc-f1



Performance

• Bandwidth of modulated signal (BT)

• MPSK

• MFSK

• l = number of bits encoded per signal element

• M = number of different signal elements



Performance

• Bandwidth efficiency ― The ratio of data rate to transmission bandwidth (R/BT)

• For MFSK, with the increase of M, the bandwidth efficiency is decreased.

• For MPSK, with the increase of M, the bandwidth efficiency is increased.



Performance



Performance



Performance

• Tradeoff between bandwidth efficiency and

error performances: an increase in bandwidth

efficiency results in an increase in error

probability.

Documents

CS-435 Network Technology & Programming Laboratoryhy435/material/CS435-lecture11.pdf · Network Technology and Programming Laboratory CSD.UoC Stefanos Papadakis & Manolis