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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
17
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
21
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)
28
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