Signal propagation rangesalt.euk.cs.ovgu.de/EuK/lehre/lehrveranstaltungen/ss10/mobkom/C02... · 16...

13 Edgar Nett Mobile Computer Communication SS’10

Signal propagation ranges

distance

sender

transmission

detection

interference

Transmission rangecommunication possible in both directionsrelatively low error rate

Detection rangedetection of the signal possibleno communication possible(error rate too high)

Interference rangesignal may not be detected signal adds to the background noiseand may interfereother transmissions


Signal propagation

Propagation in free space always like light (straight line)Receiving power declines proportional to 1/d2 in vacuum

(d = distance between sender and receiver)Receiving power in real environments additionally affected by

fading due to the atmosphere and large distancesshadowingreflection at large obstaclesrefraction depending on the density of a mediumscattering at small obstaclesdiffraction at edges

scattering diffractionreflectionshadowing refraction


Signals can take many different paths between sender and receiver arriving atdifferent times with different signal strength at the receiver

Multipath propagation

signal at sendersignal at receiver

LOS pulsesmultipathpulses

Run time dispersion (delay spread): signal is dispersed over timesignal is divided into weaker pulsesinterference with “neighbor” symbols, Inter Symbol Interference (ISI)reduction of effective bandwidth


Modulation

Digital modulationdigital data is translated into an analog signal (baseband)basic schemes: ASK, FSK, PSK

Analog modulationshifts center frequency of baseband signal up to the radio carrierMotivation

smaller antennas (e.g., λ/4)Frequency Division Multiplexingmedium characteristics

Basic schemesAmplitude Modulation (AM)Frequency Modulation (FM)Phase Modulation (PM)


Modulation and demodulation

synchronizationdecision

digitaldataanalog

demodulation

radiocarrier

analogbasebandsignal

101101001 radio receiver

digitalmodulation

digitaldata analog

modulation

radiocarrier

analogbasebandsignal

101101001 radio transmitter


Basic Schemes of Digital modulation

Modulation of digital signals known as Shift KeyingAmplitude Shift Keying (ASK):

very simplelow bandwidth requirementsvery susceptible to interference

Frequency Shift Keying (FSK):more robust against interferencesneeds larger bandwidth

Phase Shift Keying (PSK):most robust against interferencemore complex senders and receivers

1 0 1

t

1 0 1

t

1 0 1

t


Goal:multiple use of a shared medium realizedon the physical layer (or on the MAC layer as in the wired case)

Multiplexing can be done regarding 4 dimensions (parameters):

space (si)time (t)frequency (f)code (c)

Example for Space Division Multiplexing (SDM):

channels ki

Multiplexing

k2 k3 k4 k5 k6k1

fs2

s3

s1 f

t

c

t

c

f

t

c


Frequency Division Multiplexing (FDM)

Separation of the whole available spectrum into smaller frequency bandsA channel gets a certain band of the spectrum exclusively for the whole timeAdvantages:

no complex coordination necessarybetween sender and receiver

Disadvantages:waste of bandwidth if channelsare used not permanently, at different timesinflexibleheavy restriction on #of potential senders

k2 k3 k4 k5 k6k1

f

t

c


f

t

c

k2 k3 k4 k5 k6k1

Time Division Multiplex (TDM)

A channel gets the whole frequency spectrum for a certain amount of time

Advantages:only one carrier in themedium at any timethroughput high even for many users

Disadvantages:Precise clock synchronization necessary


Combination of FDM and TDM

A channel gets a certain frequency band for a certain amount of timeExample: GSM for communication with base station

Advantages:better protection against frequency and time selective interferencebetter protection against tapping (security aspect)

Disadvantage:complex coordination necessarybetween sender and receiver

t

f

c

k2 k3 k4 k5 k6k1


Code Division Multiplex (CDM)

Each channel has a unique code

All channels use the same spectrum at the same time

Big Advantage:good protection against interference and tapping

Disadvantages:more complex signal regeneration in the receiver(decoding)signals from the communication partner should arrive with highest signal strengthlower user data rates

k2 k3 k4 k5 k6k1

f

t

c


Spread spectrum technology

Idea: spread the narrow (frequency) band signal into a broad band signal using aspecial code, thus using more (frequency) bandwidth than necessary

Why: being more robust against interferences

Basic principle:

ungespreiztesSignal

gespreiztesSignal

Rauschgrenze

Filter des Empfängers


Using Spreading for all signals of a frequency band

frequency

channelquality

1 23

4

5 6

narrow bandsignal

guard space

narrowband channels due to using FDM

22

22

2

frequency

channelquality

1

spreadspectrum

spread spectrum channels


DSSS (Direct Sequence Spread Spectrum) I

XOR of the signal with pseudo-random number (chipping sequence)Many chips per bit result in higher bandwidth of the signal

AdvantagesMore robust against interferencesSecurity measureMore efficient use of availablebandwidth (senders can use the same frequency range)

DisadvantagesHigh complexity of the receiversPerfect synchronization of sendersand receivers

user data

chipping sequence

resultingsignal

0 1

0 1 1 0 1 0 1 01 0 0 1 11

XOR

0 1 1 0 0 1 0 11 0 1 0 01

=

tb

tc

tb: bit periodtc: chip period


DSSS (Direct Sequence Spread Spectrum) II

Xuser data

chippingsequence

modulator

radiocarrier

spreadspectrumsignal

transmitsignal

transmitter

demodulator

receivedsignal

radiocarrier

X

chippingsequence

lowpassfilteredsignal

receiver

integrator

products

decisiondata

sampledsums

correlator


FHSS (Frequency Hopping Spread Spectrum) I

Discrete changes of carrier frequency (hopping)Hopping sequence determined via pseudo random number sequence

Two versionsFast Hopping: several frequencies per user bitSlow Hopping: several user bits per frequency

Advantagesless complex (especially the receiver)consume less energyuses only small portion of frequency spectrum (bandwidth) at any time

Disadvantagesnot as robust as DSSSsimpler to detect (from security point of view)only supports data rates up to 2 Mbit/s (DSSS up to 11 Mbit/s)


FHSS (Frequency Hopping Spread Spectrum) II

user data

slowhopping(3 bits/hop)

fasthopping(3 hops/bit)

0 1

tb

0 1 1 t

f

f1

f2

f3

t

td

f

f1

f2

f3

t

td

tb: bit period td: dwell time


FHSS (Frequency Hopping Spread Spectrum) III

digitalmodulator

user data

hoppingsequence

analogmodulator

narrowbandsignal

spreadtransmitsignal

transmitter

frequencysynthesizer

receivedsignal

receiver

demodulatordata

hoppingsequence

demodulator

frequencysynthesizer

narrowbandsignal


Summary

Wireless communication exhibits basic differences to wired technologyMuch higher loss rates and changes in transmission characteristicsWell-known (wired) communication protocols cannot be simply adopted

Electromagnetic waves constitute the basis for wireless communicationFrequenciesSignalsModulationAntennasSignal propagation

There is only one medium to be shared in wireless communicationMultiplexingSpread spectrum


Medium Access Control (MAC Layer)


Can we apply media access methods from fixed networks?

Example CSMA/CDCarrier Sense Multiple Access with Collision DetectionSense medium, send if the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3)Based on the fact, that every collision at a receiver can be detected at the sender

Problems in wireless networksSignal strength decreases proportional to the square of the distanceNot all potential senders can “hear” each other

CS might not workCollisions do not happen everywhere

CD might not work


A sends to B, C cannot receive A C wants to send to B, C senses a “free” medium (CS fails)collision at B, A cannot detect the collision (CD fails)A is “hidden” for C

Problem: Hidden and exposed stations

B CAExposed station:B sends to A, C wants to send to another receiver (not A or B)C has to wait, CS signals a medium in usebut A is outside the radio range of C, therefore waiting is not necessaryB is “exposed” to C

Hidden station:


signal strength decreases proportional to the square of the distancethe signal of sender A may be drowned by B’s signalC cannot receive A

Problem: Near and far stations

A B C

A severe problem if CDM is used for medium access control wheresenders can transmit at the same time

precise power control of the sender is needed such that all signals end up at the receiver with about the same signal strength

Stations A and B send, C receives


Access methods SDMA/FDMA/TDMA

SDMA (Space Division Multiple Access)cell structure, size of the cell determined by the range of it is base stationMAC algorithm decides to which cell (base station) a terminal station is associatedStill often several stations located within one cell ---> often used in combination with another access method

FDMA (Frequency Division Multiple Access)assign a certain frequency to a transmission channel between a sender and a receivereither permanent (e.g., radio broadcast), also called pure FDMA orcombining FDMA and TDMA by assigning a sequence of frequencies (e.g FHSS)

or to implement FDD (Frequency Division Duplex) in cell-based mobile radio networks

TDMA (Time Division Multiple Access)assign a transmission channel to each sender for a certain amount of timereceivers can always listen on the same channel frequencyphysical layer not involved (only a matter of digital control software)very flexible ---> all MAC approaches in wired networks based on TDMA


GSM- example for combination of FDM and FDD

f

t

124

1

124

1

20 MHz

200 kHz

890.2 MHz

935.2 MHz

915 MHz

960 MHz


DECT - example for static TDMA + TDD

1 2 3 11 12 1 2 3 11 12

tdownlink uplink

417 µs


Spread spectrum technology

Idea: spread the narrow (frequency) band signal into a broad band signal using aspecial code, thus using more (frequency) bandwidth than necessary

Why: being more robust against interferences

dP/df

fi)

dP/df

fii)

sender

user signalbroadband interferencenarrowband interference

dP/df

fiii)

dP/df

fiv)

receiverf

v)

dP/df

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