Introduction to Wireless and Mobile Networking

Lecture 3: Multiplexing, Multiple Lecture 3: Multiplexing, Multiple Access Access and Frequency Reuseand Frequency ReuseAccess, Access, and Frequency Reuseand Frequency Reuse

Hung-Yu WeigNational Taiwan University

Multiplexing/Multiple AccessMultiplexing/Multiple Access

MultiplexingMultiplexing• Multiplexing in 4 dimensions

– space (si) channels ki

– time (t)– frequency (f) c

k2 k3 k4 k5 k6k1

– code (c) t

c

t

c

• Goal: multiple use of a shared medium

s2

s1 f

fof a shared medium

I t t d d d!t

c

• Important: guard spaces needed!s3 f

3

Frequency multiplexFrequency multiplex• Separation of the whole spectrum into smaller

frequency bandsA h l t t i b d f th t f • A channel gets a certain band of the spectrum for the whole time

• Advantages:• Advantages:– no dynamic coordination necessary– works also for analog signals k k k k kkworks also for analog signals

• Disadvantages:

k2 k3 k4 k5 k6k1

f

c

Disadvantages:– waste of bandwidth if the traffic

is distributed unevenly– inflexible– guard spaces

4

t

Time multiplexTime multiplex• A channel gets the whole spectrum for a certain amount

of time

• Advantages:g– only one carrier in the medium at any time– throughput high even for many users

k2 k3 k4 k5 k6k1• Disadvantages:

f

c– Precise synchronization necessary

f

5t

Time and frequency multiplexTime and frequency multiplex

• Combination of both methods• A channel gets a certain frequency band for a certain • A channel gets a certain frequency band for a certain

amount of time• Example: GSM Example: GSM • Advantages:

– better protection against tappingbetter protection against tapping– protection against frequency

selective interferencek2 k3 k4 k5 k6k1

– higher data rates compared tocode multiplex

• but: precise coordinationf

c

• but: precise coordinationrequired

6t

Code multiplexCode multiplex• Each channel has a unique code• All channels use the same spectrum

t th tiat the same time• Advantages:

b d idth ffi i tk2 k3 k4 k5 k6k1

– bandwidth efficient– no coordination and synchronization necessary– good protection against interference and tapping

c

good protection against interference and tapping• Disadvantages:

– lower user data rates– more complex signal regeneration

• Implemented using spread f

spectrum technology

7t

Spread spectrum technologySpread spectrum technologyp p gyp p gy• Problem of radio transmission: frequency

d d t f di i t b d dependent fading can wipe out narrow band signals for duration of the interferenceg

• Solution: spread the narrow band signal into a broad band signal using a special codea broad band signal using a special code

• protection against narrow band interference

interference spread signalpower power

detection atreceiver

signalspreadinterference

8f f

Effects of spreading and interferenceEffects of spreading and interferencep gp gdP/df dP/df

i) ii)user signalbroadband interferencenarrowband interferencef f

sender

dP/df dP/df

narrowband interference

dP/dfdP/df dP/df dP/df

fiii)

fiv)

receiverf

v)

9

Spreading and frequency selective Spreading and frequency selective f dif difadingfadingchannel

quality

1 23

4

5 6 narrowband channels

frequency4

narrow bandsignal

guard space

2

channelquality

22

22

21

spread spectrum channels

frequencyspreadspectrum

10

DSSS (Direct Sequence Spread Spectrum) DSSS (Direct Sequence Spread Spectrum) • XOR of the signal with pseudo-random number

(chipping sequence)– many chips per bit (e.g., 128) result in higher bandwidth of

the signal• AdvantagesAdvantages

– reduces frequency selective fading user data

tb

– in cellular networks • base stations can use the

same frequency range chipping

0 1 XORtc

same frequency range• several base stations can

detect and recover the signal• soft handover

sequence

resulting

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

• soft handover

• Disadvantages– precise power control necessary

resultingsignal

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

t : bit period

11

precise power control necessary tb: bit periodtc: chip period

DSSSDSSS

user data

spreadspectrumsignal

transmitsignal

Xuser data

chippingsequence

modulator

radiocarrier

signal signal

sequence carrier

transmitter

receivedlowpassfiltered products

d t

sampledsums

correlator

demodulatorsignal

radio

X

chipping

signalintegrator decision

data

carrier sequence

receiver

12

FHSS (Frequency Hopping Spread Spectrum) FHSS (Frequency Hopping Spread Spectrum) • Discrete changes of carrier frequency

– sequence of frequency changes determined via pseudo random b snumber sequence

• Two versions– Fast Hopping: – Fast Hopping:

several frequencies per user bit– Slow Hopping:

several user bits per frequency• Advantages

f l ti f di d i t f li it d t h t – frequency selective fading and interference limited to short period

– simple implementationp p– uses only small portion of spectrum at any time

• Disadvantages

13– not as robust as DSSS– simpler to detect

FHSSFHSS

d t

tb

user data

0 1 0 1 1 t

ft

slowhopping(3 bits/hop)f

f2

f3td

f1

tf

f

td

fasthopping(3 hops/bit)f1

f2

f3

t

tb: bit period td: dwell time

14

FHSSFHSS

d l tuser data

d l t

narrowbandsignal

spreadtransmitsignal

modulator

hopping

modulator

frequency hoppingsequence

transmitter

frequencysynthesizer

receivedsignal data

narrowbandsignal

signaldemodulator

h i

demodulator

f

receiver

hoppingsequence

frequencysynthesizer

15

Cell structureCell structure• Implements space division multiplex: base station covers a

certain transmission area (cell)• Mobile stations communicate only via the base station

• Advantages of cell structures:– higher capacity, higher number of users

l t i i d d– less transmission power needed– more robust, decentralized– base station deals with interference transmission area etc locallybase station deals with interference, transmission area etc. locally

• Problems:– fixed network needed for the base stationsf w f– handover (changing from one cell to another) necessary– interference with other cells

16• Cell sizes from some 100 m in cities to, e.g., 35 km on the

country side (GSM) - even less for higher frequencies

Frequency planning Frequency planning • Frequency reuse only with a certain distance between

the base stationsd d d l f• Standard model using 7 frequencies:

f4f5

f6

f3f2

f5f1

f3f2

f7f4

f1

• Fixed frequency assignment:– certain frequencies are assigned to a certain cellcertain frequencies are assigned to a certain cell– problem: different traffic load in different cells

• Dynamic frequency assignment:– base station chooses frequencies depending on the

frequencies already used in neighbor cells– more capacity in cells with more traffic

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m p y m ff– assignment can also be based on interference measurements

Frequency planningFrequency planningq y p gq y p gf2

f3f2

f3f3f3 f7f2

f12

f3f2

f1

f2

2

f3

f1

f2

f4

f5

f1

f6

3

f2

f4

f5

72

3 cell cluster

f1 f1f3f3 f3

f3f2

f7 f1f3

f5f6 f2

7 cell cluster

f1f1 f1f2f3

f2f3

f2f3

h1h2

h3g1

g2

g3

h1h2

h3g1

g2

g3g1

g2

g3 cell cluster

g3g3 g3 with 3 sector antennas

18

Cell breathingCell breathinggg• CDM systems: cell size depends on current load

Addi i l ffi i h • Additional traffic appears as noise to other users• If the noise level is too high users drop out of g p

cells

19

Clarification on TerminologiesClarification on Terminologies

Similar (but confusing) termsSimilar (but confusing) terms( f g)( f g)• MAC (medium access control) protocol

– A protocol that control which user should access the medium at a given moment

• Multiple Access– Multiple users access network simultaneously– Multiple users access network simultaneously

• Multiplexing– Combine multiple signals/transmissions into 1

transmission

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Similar (but confusing) termsSimilar (but confusing) terms( f g)( f g)• Duplex

– TDD (time division duplex)– FDD (frequency division duplex)( q y p )

• Multiple accessTDMA (ti di i i lti l )– TDMA (time division multiple access)

– FDMA (frequency division multiple access)• Multiplexing

– TDM (time division multiplexing)– TDM (time division multiplexing)– FDM (frequency division multiplexing)

22

ExamplesExamplespp• FDMA/FDD

– E.g. AMPS• FDMA/TDD

downlink

li kFDMA/TDD– E.g. CT-2

uplink

tt1 2 3

tt

1 2 3 1 2 31 2 3

23ff

FDD/FDMA FDD/FDMA Ex m l GSMEx m l GSMExample GSMExample GSM

f

124960 MHz

1 200 kHz935.2 MHz

124

20 MHz

915 MHz

t

1890.2 MHz

24

TDD/TDMA TDD/TDMA Ex m l DECTEx m l DECTExample DECTExample DECT

1 2 3 11 12 1 2 3 11 12

417 µs

1 2 3 11 12 1 2 3 11 12

tdownlink uplink

25

Frequency Reuse in Cellular Frequency Reuse in Cellular Frequency Reuse in Cellular Frequency Reuse in Cellular SystemSystemyy

Review: Basic Cellular ConceptReview: Basic Cellular Conceptpp• “Cell”

– Typically, cells are hexagonal– In practice, it depends on available cell sites p , p

and radio propagation conditions• Spectrum reuseSpectrum reuse

– Reuse the same wireless spectrum in other hi l l tigeographical location

– Frequency reuse factor

27

Frequency ReuseFrequency Reuseq yq y• Frequency reuse

– Spatial reuse– Spectral reusep

Cl t• Cluster– A group of cellsg p

• Frequency reuse factor(Total # of channels in a cluster) / (Total # of – (Total # of channels in a cluster) / (Total # of channels in a cell)

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TDMA/FDMA Spatial ReuseTDMA/FDMA Spatial Reuse

A frequency reuse exampleA frequency reuse examplef q y pf q y p• Example

– Frequency reuse factor = 7– Cluster size =7

• QuestionWh h ibl – What are other possible frequency reuse patterns?

30

ClusterCluster• The hexagon is an ideal choice for

ll l b it macrocellular coverage areas, because it closely approximates a circle and offers a y ppwide range of tessellating reuse cluster sizes.sizes.

• A cluster of size N can be constructed if, 2 2 – N = i2 + ij + j2.

– i,j are positive integerj p g• Allowable cluster sizes are

N = 1 3 4 7 9 1231

– N = 1,3,4,7,9,12,…

Determine frequency reuse patternDetermine frequency reuse patternf q y pf q y p• Co-channel interference [CCI]

– one of the major factors that limits cellular system capacity

– CCI arises when the same carrier frequency is used in different cells.

• Determine frequency reuse factorP ti d l– Propagation model

– Sensitivity to CCI

32

Reuse distanceReuse distance• Notations

– D :Reuse distance• Distance to cell using the same frequency

– r : Cell radius– N : Frequency reuse factorN : Frequency reuse factor

• Relationship between D and r– D/r=(3N)^0.5– N = i2 + ij + j2j j

• Proof?

33

L * iL * j

In this case: j=2, i=1

Dr

D

)3/2())((2)()( 222

rL 3=

jiLjLiLDjLiLjLiLD

)5.0(2)3/2cos())((2)()(

222222

222

−⋅⋅⋅−⋅+⋅=

⋅⋅−⋅+⋅= π

Compute D based on “law of cosine”NijjiD

ijjiLD

3)(3/

)(22

2222 ++=

34

law of cosine NijjirD 3)(3/ 22 =++=

Cell splittingCell splittingp gp g• Smaller cells have greater system capacity

– Better spatial reuse• As traffic load grows, larger cells could As traffic load grows, larger cells could

split into smaller cells

35

SectorsSectors• Use directional antenna reduces CCI

Wh ? Thi k b t it!– Why? Think about it!• 1 base station could apply several

directional antennas to form several sectors

• 3-sector cell

36

More about cellularMore about cellular

Cell size & FRFCell size & FRF• Cell size should be proportional to

1/( b ib d it )1/(subscriber density)• Co-channel interference is proportional toCo channel interference is proportional to

– 1/Dr– r

– 1/N^0.5

– Path-loss model• Total system capacity is proportional toTotal system capacity is proportional to

– 1/NN F f t

38

• N : Frequency reuse factor

Example: N=7Example: N=7pp• Frequency reuse factor N=7

– N = i2 + ij + j2

– (i,j)=(1,2) or (2,1)( ,j) ( , ) ( , )• Other commonly used patterns

N 3– N=3• (1,1)

– N=4• (2,0); (0,2)

• N=1 is possible– CDMA

39

– CDMA

Compute total system capacityCompute total system capacityp y p yp y p y• Example

– Total coverage area = 100 mile2 = 262.4 km2

– Total 1000 duplex channelsp– Cell radius = 1km – N=4 or N=7– N=4 or N=7

• What’s the total system capacity for N=4 and N=7?

r

22 6233 rrA40

6.22

rrA ==

Compute total system capacityCompute total system capacityp y p yp y p y• # of cells = 262.4/2.6=100 cells• # of usable duplex channels/cell

– S=(# of channels)/(reuse factor)S (# of channels)/(reuse factor)– S4=1000/4=250

S 1000/7 142– S7=1000/7=142• Total system capacity (# of users could be y p y (

accommodated simultaneously)– C=S*(# of cells)C=S (# of cells)– C4=250*100=25000

C 142*100 1420041

– C7=142*100=14200

Evolving deploymentEvolving deploymentg p yg p y

Early stage Intermediate stage Late stage

•Multiple stages of deployment

42•Deployment evolves with subscriber growth

Practical deployment issuesPractical deployment issuesp yp y• Location to setup antenna

– Antenna towers are expensive– Local people do not like BSsp p

• Antenna/BS does not look like antenna/BS

• Antenna• Antenna– Omni-directional– Directional antenna

43

Wireless QoSWireless QoSQQ• Quality of Service (QoS)

– Achieving satisfactory wireless QoS is an important design bj i

g y p gobjective

• Quality measures– Channel availability (wireless network is available when users need nn y (w n w w n u n

it)• Blocking probability• Dropping probability

– Coverage: probability of receiving adequate signal level at different locations

– Transmission quality: fidelity/quality of received signals• BER• FER

• Application-dependentpp p– Voice– Data– Multimedia

44

Multimedia

Wireless QoSWireless QoSQQ• Admission control

– Blocking– Poor reception qualityp q y

• Co-channelsF f t– Frequency reuse factor

– Cell planning• Frequency planning

45

WorstWorst--Case CCI on the Forward ChannelCase CCI on the Forward Channel• Co channel interference [CCI] is one of the prime • Co channel interference [CCI] is one of the prime

limitations on system capacity. We use the propagation model to calculate CCI.

• There are six first-tier, co-channel BSs, two each at f , ,(approximate) distances of D-R, D, and R+D and the worst case (average) Carrier-to-(Co-Channel)

f [CC ] iInterference [CCI] is

1 β−

ΛR

)()(2 βββ −−− +++−=Λ

RDDRD

Worst case CCIon the forward channel

46R= cell radius

OverlayOverlayyy• Dual-mode or dual-frequency phones

O l diff t i l – Overlay different wireless access technologies

Different technologies• Different technologies• Same technology operating in different bands

Increase system capacity• Increase system capacity– Reduce blocking

• Example:– GSM 900/1800– TDMA+CDMA

47

Overlaid cellsOverlaid cells

48

HandoffHandoffffff• Handoff threshold: typically, -90~-100 dBm

(1 10 W)(1~10uW)• Need to prevent from “ping-pong” effectNeed to prevent from ping pong effect

49