mobile networks module a (part a2) introduction prof. jp hubaux mobnet.epfl.ch
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Mobile Networks Module A (Part A2) Introduction Prof. JP Hubaux http://mobnet.epfl.ch. Modulation and demodulation (reminder). analog baseband signal. digital data. digital modulation. analog modulation. radio transmitter. 101101001. radio carrier. analog baseband signal. - PowerPoint PPT PresentationTRANSCRIPT
1
Mobile Networks
Module A (Part A2)
Introduction
Prof. JP Hubaux
http://mobnet.epfl.ch
2
Modulation and demodulation (reminder)
synchronizationdecision
digitaldataanalog
demodulation
radiocarrier
analogbasebandsignal
101101001 radio receiver
digitalmodulation
digitaldata analog
modulation
radiocarrier
analogbasebandsignal
101101001 radio transmitter
3
About CSMA/CD
Can we apply media access methods from fixed networks?
Example of CSMA/CD Carrier Sense Multiple Access with Collision Detection send as soon as the medium is free, listen into the medium if a
collision occurs (original method in IEEE 802.3)
Problems in wireless networks a radio can usually not transmit and receive at the same time signal strength decreases proportionally to the square of the
distance or even more the sender would apply CS and CD, but the collisions happen at the
receiver it might be the case that a sender cannot “hear” the collision, i.e.,
CD does not work furthermore, CS might not work if, e.g., a terminal is “hidden”
4
Hidden terminals 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 receive the collision (CD fails) A is “hidden” for C
Exposed terminals B sends to A, C wants to send to another terminal (not A or B) C has to wait, CS signals a medium in use but A is outside the radio range of C, therefore waiting is not
necessary C is “exposed” to B
Hidden and exposed terminals
BA C
5
Terminals A and B send, C receives signal strength decreases proportional to the square of the distance the signal of terminal B therefore drowns out A’s signal C cannot receive A
If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer
Also severe problem for CDMA-networks - precise power control needed!
Motivation - near and far terminals
A B C
6
Access methods SDMA/TDMA/FDMA/CDMA
SDMA (Space Division Multiple Access) segment space into sectors, use directed antennas cell structure
TDMA (Time Division Multiple Access) assign the fixed sending frequency to a transmission channel
between a sender and a receiver for a certain amount of time
FDMA (Frequency Division Multiple Access) assign a certain frequency to a transmission channel between a
sender and a receiver permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast
hopping (FHSS, Frequency Hopping Spread Spectrum)
CDMA (Code Division Multiple Access) assign an appropriate code to each transmission channel (DSSS,
Direct Sequency Spread Spectrum) frequency hopping over separate channels (FHSS, Frequency
Hopping Spread Spectrum)
7
Some medium access control mechanisms for wireless
TDMA CDMAFDMASDMA
Fixed Aloha ReservationsDAMA
MultipleAccess withCollisionAvoidance
Polling
Pure
CSMA
• Used in GSM Slotted
Non-persistent p-persistent CSMA/CA
• Copes with hidden and exposed terminal• RTS/CTS• Used in 802.11 (optional)
MACAW MACA-BI FAMA
CARMA
• Used in 802.11 (mandatory)
• Used in 802.11 (optional)
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS DSSS
• Used in GSM
Fixed
• Used in Bluetooth • Used in UMTS
8
Some medium access control mechanisms for wireless
TDMA CDMAFDMASDMA
Fixed Aloha ReservationsDAMA
MultipleAccess withCollisionAvoidance
Polling
Pure
CSMA
• Used in GSM Slotted
Non-persistent p-persistent CSMA/CA
• Copes with hidden and exposed terminal• RTS/CTS• Used in 802.11 (optional)
MACAW MACA-BI FAMA
CARMA
• Used in 802.11 (mandatory)
• Used in 802.11 (optional)
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS DSSS
• Used in GSM
Fixed
• Used in Bluetooth
9
f
t
c
k2 k3 k4 k5 k6k1
Time multiplex
A channel gets the whole spectrum for a certain amount of time.
Advantages: only one carrier in the
medium at any time throughput high
for many users
Disadvantages: precise
synchronization necessary
10
Frequency multiplex
Separation of the whole spectrum into smaller frequency bands.
A channel gets a certain band of the spectrum for the whole time.
Advantages: no dynamic coordination
necessary works also for analog signals
Disadvantages: waste of bandwidth
if the traffic is distributed unevenly
inflexible guard spaces
k2 k3 k4 k5 k6k1
f
t
c
11
f
Time and frequency multiplex
Combination of both methods.
A channel gets a certain frequency band for a certain amount of time.
Example: GSM
Advantages: better protection against
tapping protection against frequency
selective interference
But: precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
12
Code multiplex
Each channel has a unique code
All channels use the same spectrum at the same time
Advantages: bandwidth efficient no coordination and synchronization
necessary good protection against interference
and tapping
Disadvantages: lower user data rates more complex signal regeneration
Implemented using spread spectrum technology
k2 k3 k4 k5 k6k1
f
t
c
13
TDMA/TDD – example: DECT
1 2 3 11 12 1 2 3 11 12
tdownlink uplink
417 µs
DECT: Digital Enhanced Cordless TelecommunicationsTDD: Time Division Duplex
DECT: Digital Enhanced Cordless TelecommunicationsTDD: Time Division Duplex
14
FDMA/FDD – example: GSM
f
t
124
1
124
1
20 MHz
200 kHz
890.2 MHz
935.2 MHz
915 MHz
960 MHz
downlink
uplink
FDD: Frequency Division DuplexFDD: Frequency Division Duplex
15
Some medium access control mechanisms for wireless
TDMA CDMAFDMASDMA
Fixed Aloha ReservationsDAMA
MultipleAccess withCollisionAvoidance
Polling
Pure
CSMA
• Used in GSM Slotted
Non-persistent p-persistent CSMA/CA
• Copes with hidden and exposed terminal• RTS/CTS• Used in 802.11 (optional)
MACAW MACA-BI FAMA
CARMA
• Used in 802.11 (mandatory)
• Used in 802.11 (optional)
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS DSSS
• Used in GSM
Fixed
• Used in Bluetooth
16
Mechanism random, distributed (no central arbiter), time-multiplex Slotted Aloha additionally uses time-slots, sending must always
start at slot boundaries
Aloha
Slotted Aloha
Aloha/slotted aloha
sender A
sender B
sender C
collision
sender A
sender B
sender C
collision
t
t
17
Performance of Aloha (1/3)
First transmissionRetransmission(if necessary)
t0 t0+Xt0-X
Vulnerable period
t0+X+2tprop
Time-out
t0+X+2tprop+B
Backoff period
• tprop : maximum one-way propagation time betwwen 2 stations• Information about the outcome of the transmission is obtained after the reaction time 2 tprop
• B: backoff time
18
Performance of Aloha (2/3)
S: new packets S: throughput of the system
{G
: total load
: arrival rate of new packets
Assumption: Poisson distribution of the aggregate arrival process, with an
average number of arrivals of 2G arrivals/2X seconds
transmissions in 2 seconds
G
S
P k X
2
0
2
2
2, 0,1, 2,...
!Throughput S: total arrival rate G times the prob. of a successful transmission:
no collision 0 transmissions in 2 seconds
2 =
0!
=
Peakvalue at 0.5 :
k
G
G
G
Ge k
k
S GP GP X
GG e
Ge
G
1 0.1842S e
19
Performance of Aloha (3/3)
2
2
Average number of transmission attempts/packet:
attempts per packet
Average number of unsuccessful attempts per packet:
= 1 1
The first transmission requires seconds,
and each subs
G
G
prop
G eS
G eSX t
2
2
equent retransmission requires 2
Thus the average packet transmission time is approx:
( 1)( 2 )
expressed relatively to X:
/ 1 ( 1)(1 2 )
where i
prop
Galoha prop prop
Galoha
prop
t X B
E T X t e X t B
BE T X a e a Xt
a X
s the normalized one-way propagation delay
Computation of the average packet transmission time
20
Performance of Slotted Aloha
First transmissionRetransmission(if necessary)
t0=kX (k+1)X
Vulnerableperiod
t0+X+2tprop
Time-out
t0+X+2tprop+B
Backoff period
-
1Peakvalue at 1 : 0.368
Average packet delay:
/ 1 ( 1)(1 2 )
G
Gslotaloha
S Ge
G S e
BE T X a e a X
21
Some medium access control mechanisms for wireless
TDMA CDMAFDMASDMA
Fixed Aloha ReservationsDAMA
MultipleAccess withCollisionAvoidance
Polling
Pure
CSMA
• Used in GSM Slotted
Non-persistent p-persistent CSMA/CA
• Copes with hidden and exposed terminal• RTS/CTS• Used in 802.11 (optional)
MACAW MACA-BI FAMA
CARMA
• Used in 802.11 (mandatory)
• Used in 802.11 (optional)
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS DSSS
• Used in GSM
Fixed
• Used in Bluetooth
22
Carrier Sense Multiple Access (CSMA)
Goal: reduce the wastage of bandwidth due to packet collisions Principle: sensing the channel before transmitting (never
transmit when the channel is busy) Many variants:
Collision detection (CSMA/CD) or collision avoidance(CSMA/CA) Persistency (in sensing and transmitting)
Station A beginstransmissionat t=0
A
Station A capturesthe channelat t=tprop
A
23
1-Persistent CSMA
Stations having a packet to send sense the channel continuously, waiting until the channel becomes idle.
As soon as the channel is sensed idle, they transmit their packet.
If more than one station is waiting, a collision occurs. Stations involved in a collision perform a the backoff algorithm
to schedule a future time for resensing the channel Optional backoff algorithm may be used in addition for fairness
Consequence : The channel is highly used (greedy algorithm).
24
Non-Persistent CSMA
Attempts to reduce the incidence of collisions Stations with a packet to transmit sense the channel If the channel is busy, the station immediately runs the back-off
algorithm and reschedules a future sensing time If the channel is idle, then the station transmits
Consequence : channel may be free even though some users have packets to transmit.
25
p-Persistent CSMA
Combines elements of the above two schemes Stations with a packet to transmit sense the channel If it is busy, they persist with sensing until the channel becomes
idle If it is idle:
With probability p, the station transmits its packet With probability 1-p, the station waits for a random time and senses
again
26
Throughput expression
ae
aGeS
eaG
GeS
aeea
eeaGS
eaGeaG
eaGGaGGGS
GeS
GeS
aG
aG
aG
aG
aGaG
aGaG
aGaG
aG
G
G
1
21
11
1
1121
2/11
1
1
1
21
2Pure ALOHA
Slotted ALOHA
Unslotted 1-persistent CSMA
Slotted 1-persistent CSMA
Unslotted nonpersistent CSMA
Slotted nonpersistent CSMA
Protocol Throughput
27
Throughput plot
Normalized propagation delay is a =0 .01
28
CSMA/CD (reminder)
Operating principle Check whether the channel is idle before transmitting Listen while transmitting, stop transmission when collision If collision, one of the 3 schemes above (1-persistent, non-
persistent or p-persistent)
CS: Carrier Sense (Is someone already talking ?)
MA: Multiple Access (I hear what you hear !)CD: Collision Detection (We are both talking !!)
Three states for the channel : contention, transmission, idle
Station
Repeater Terminator
29
Why CSMA/CD is unfit for WLANs
Collision Detection requires simultaneous transmission and reception operations (which a radio transceiver is usually unable to do) detecting a collision is difficult
Carrier Sensing may be suitable to reduce interference at sender, but Collision Avoidance is needed at receiver
CSMA/CD does not address the hidden terminal problem
30
CSMA/CA
Is described in the module devoted to IEEE 802-11
31
Some medium access control mechanisms for wireless
TDMA CDMAFDMASDMA
Fixed Aloha ReservationsDAMA
MultipleAccess withCollisionAvoidance
Polling
Pure
CSMA
• Used in GSM Slotted
Non-persistent p-persistent CSMA/CA
• Copes with hidden and exposed terminal• RTS/CTS• Used in 802.11 (optional)
MACAW MACA-BI FAMA
CARMA
• Used in 802.11 (mandatory)
• Used in 802.11 (optional)
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS DSSS
• Used in GSM
Fixed
• Used in Bluetooth
32
DAMA - Demand Assigned Multiple Access
Channel efficiency only 18% for Aloha, 36% for Slotted Aloha
Reservation can increase efficiency to 80% a sender reserves a future time-slot sending within this reserved time-slot is possible without collision reservation also causes higher delays typical scheme for satellite links
Examples for reservation algorithms: Explicit Reservation (Reservation-ALOHA) Implicit Reservation (PRMA) Reservation-TDMA
33
DAMA / Explicit Reservation
Explicit Reservation (Reservation Aloha): two modes:
ALOHA mode for reservation:competition for small reservation slots, collisions possible
reserved mode for data transmission within successful reserved slots (no collisions possible)
it is important for all stations to keep the reservation list consistent at any point in time and, therefore, all stations have to synchronize from time to time
Aloha reserved Aloha reserved Aloha reserved Aloha
collision
t
34
DAMA / Packet reservation (PRMA)
Implicit reservation based on slotted Aloha a certain number of slots form a frame, frames are repeated stations compete for empty slots according to the slotted aloha
principle once a station reserves a slot successfully, this slot is automatically
assigned to this station in all following frames as long as the station has data to send
competition for a slot starts again as soon as the slot was empty in the last frame
frame1
frame2
frame3
frame4
frame5
1 2 3 4 5 6 7 8 time-slot
collision at reservation attempts
A C D A B A F
A C A B A
A B A F
A B A F D
A C E E B A F Dt
ACDABA-F
ACDABA-F
AC-ABAF-
A---BAFD
ACEEBAFD
reservation
35
DAMA / Reservation-TDMA
Reservation Time Division Multiple Access every frame consists of N mini-slots and x data-slots every station has its own mini-slot and can reserve up to k data-slots
using this mini-slot (i.e. x = N * k). other stations can send data in unused data-slots according to a
round-robin sending scheme (best-effort traffic)
N mini-slots N * k data-slots
reservationsfor data-slots
other stations can use free data-slotsbased on a round-robin scheme
e.g. N=6, k=2
36
Some medium access control mechanisms for wireless
TDMA CDMAFDMASDMA
Fixed Aloha ReservationsDAMA
MultipleAccess withCollisionAvoidance
Polling
Pure
CSMA
• Used in GSM Slotted
Non-persistent p-persistent CSMA/CA
• Copes with hidden and exposed terminal• RTS/CTS• Used in 802.11 (optional)
MACAW MACA-BI FAMA
CARMA
• Used in 802.11 (mandatory)
• Used in 802.11 (optional)
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS DSSS
• Used in GSM
Fixed
• Used in Bluetooth
37
MACA - collision avoidance
MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance Designed especially for packet radio networks (Phil Karn, 1990) Principle:
RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet
CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive
Signaling packets contain sender address receiver address packet size
Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC)
38
MACA avoids the problem of hidden terminals A and C want to
send to B A sends RTS first C waits after receiving
CTS from B
MACA avoids the problem of exposed terminals B wants to send to A,
C to another terminal now C does not have
to wait for it cannot receive CTS from A
MACA principle
A B C
RTS
CTSCTS
A B C
RTS
CTS
RTS
39
MACA example
A B
C
D
E
A B
C
D
E
A B
C
D
E
RTS CTS
DATA
: blocked from Tx
1 2
3
40
MACA variant: application in IEEE802.11
idle
wait for the right to send
wait for ACK
sender receiver
packet ready to send; RTS
time-out; RTS
CTS; data
ACK
RxBusy
idle
wait fordata
RTS; RxBusy
RTS; CTS
data; ACK
time-out Data with errors; NAK
ACK: positive acknowledgementNAK: negative acknowledgement
RxBusy: receiver busy
time-out NAK;RTS
41
Some medium access control mechanisms for wireless
TDMA CDMAFDMASDMA
Fixed Aloha ReservationsDAMA
MultipleAccess withCollisionAvoidance
Polling
Pure
CSMA
• Used in GSM Slotted
Non-persistent p-persistent CSMA/CA
• Copes with hidden and exposed terminal• RTS/CTS• Used in 802.11 (optional)
MACAW MACA-BI FAMA
CARMA
• Used in 802.11 (mandatory)
• Used in 802.11 (optional)
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS DSSS
• Used in GSM
Fixed
• Used in Bluetooth
42
Polling mechanisms
If one terminal can be heard by all others, this “central” terminal (e.g., base station) can poll all other terminals according to a certain scheme all schemes known from fixed networks can be used (typical
mainframe - terminal scenario)
Example: Randomly Addressed Polling base station signals readiness to all mobile terminals terminals ready to send can now transmit a random number without
collision with the help of CDMA or FDMA (the random number can be seen as a dynamic address)
the base station now chooses one address for polling from the list of all random numbers (collision if two terminals choose the same address)
the base station acknowledges correct packets and continues polling the next terminal
this cycle starts again after polling all terminals of the list
43
ISMA (Inhibit Sense Multiple Access)
Current state of the medium is signaled via a “busy tone” the base station signals on the downlink (base station to terminals)
if the medium is free or not terminals must not send if the medium is busy terminals can access the medium as soon as the busy tone stops the base station signals collisions and successful transmissions via
the busy tone and acknowledgements, respectively (media access is not coordinated within this approach)
mechanism used, e.g., for CDPD (Cellular Digital Packet Data) Similar approach was proposed
for Packet Radio Networks(Kleinrock + Tobagi, 1975)
44
Spread Spectrum principle
c(t) c(t)
( ) f
j f( )
r f( )
~( ) f
t f( )
Synchronizationn
Pseudo-random code
Filter Decoder Coder
s f( )
f
s f( )
sS
Bs
( )s f power density spectrum of the original signal
sS power density of the original signal
sB bandwidth of the original signal
( )j f power density spectrum of the jamming signal
45
Spread Spectrum principle
c(t) c(t)
( ) f
j f( )
r f( )
~( ) f
t f( )
Synchronization Pseudo-random
code
Filter Decoder Coder
s f( )
f
t f( )
s st
t
S BS
B
Bt
46
Spread Spectrum principle
c(t) c(t)
( ) f
j f( )
r f( )
~( ) f
t f( )
Synchronization Pseudo-random
code
Filter Decoder Coder
s f( )
jS
f
j f( )
B j
47
Spread Spectrum principle
c(t) c(t)
( ) f
j f( )
r f( )
~( ) f
t f( )
Synchronization Pseudo-random
code
Filter Decoder Coder
s f( )
B j
tS
jS
f
r f( )
Bt
48
Spread Spectrum principle
c(t) c(t)
( ) f
j f( )
r f( )
~( ) f
t f( )
Synchronization Pseudo-random
code
Filter Decoder Coder
s f( )
sS
f
~( ) f
jj
t
BSB
Bt
Bs
signal s s s s t
jjamming j j sj s
ProcessingPt signal gainPjamming original
P S B S B BBP S B B
S BB
Processing gain: Increase in received signal power thanks to spreading
49
Spread Spectrum principle
c(t) c(t)
( ) f
j f( )
r f( )
~( ) f
t f( )
Synchronization Pseudo-random
code
Filter Decoder Coder
s f( )
sS
f
( ) f
jj
t
BSB
Bs
50
Some medium access control mechanisms for wireless
TDMA CDMAFDMASDMA
Fixed Aloha ReservationsDAMA
MultipleAccess withCollisionAvoidance
Polling
Pure
CSMA
• Used in GSM Slotted
Non-persistent p-persistent CSMA/CA
• Copes with hidden and exposed terminal• RTS/CTS• Used in 802.11 (optional)
MACAW MACA-BI FAMA
CARMA
• Used in 802.11 (mandatory)
• Used in 802.11 (optional)
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS DSSS
• Used in GSM
Fixed
• Used in Bluetooth
51
Frequency Hopping Spread Spectrum (FHSS) (1/2)
Signal broadcast over seemingly random series of frequencies
Receiver hops between frequencies in sync with transmitter
Eavesdroppers hear unintelligible blips Jamming on one frequency affects only a few bits Rate of hopping versus Symbol rate
Fast Frequency Hopping: One bit transmitted in multiple hops.
Slow Frequency Hopping: Multiple bits are transmitted in a hopping period
Example: Bluetooth (79 channels, 1600 hops/s)
52
Frequency Hopping Spread Spectrum (FHSS) (2/2)
tb
tc
Fast Frequency Hopping: b ct ttb : duration of one bit
tc : duration of one chip
53
Some medium access control mechanisms for wireless
TDMA CDMAFDMASDMA
Fixed Aloha ReservationsDAMA
MultipleAccess withCollisionAvoidance
Polling
Pure
CSMA
• Used in GSM Slotted
Non-persistent p-persistent CSMA/CA
• Copes with hidden and exposed terminal• RTS/CTS• Used in 802.11 (optional)
MACAW MACA-BI FAMA
CARMA
• Used in 802.11 (mandatory)
• Used in 802.11 (optional)
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS: Frequency-Hopping Spread SpectrumDSSS: Direct Sequence Spread SpectrumCSMA: Carrier Sense Multiple AccessCA: Collision AvoidanceDAMA: Demand-Assigned Multiple AccessMACA-BI: MACA by invitationFAMA: Floor Acquisition Multiple AccessCARMA: Collision Avoidance and Resolution Multiple Access
FHSS DSSS
• Used in GSM
Fixed
• Used in Bluetooth • Used in UMTS
54
Direct Sequence Spread Spectrum (DSSS) (1/2)
XOR of the signal with pseudo-random number (chipping sequence) many chips per bit (e.g., 128) result in higher bandwidth of the signal
Advantages reduces frequency selective
fading in cellular networks
base stations can use the same frequency range
several base stations can detect and recover the signal
soft handover
Disadvantages precise power control necessary complexity of the receiver
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
55
Direct Sequence Spread Spectrum (DSSS) (2/2)
Xuser data
chippingsequence
modulator
radiocarrier
spreadspectrumsignal
transmitsignal
transmitter
demodulator
receivedsignal
radiocarrier
X
chippingsequence
lowpassfilteredsignal
receiver
integrator
products
decisiondata
sampledsums
correlator
56
Categories of spreading (chipping) sequences
Spreading Sequence Categories Pseudo-random Noise (PN) sequences Orthogonal codes
For FHSS systems PN sequences most common
For DSSS beside multiple access PN sequences most common
For DSSS CDMA systems PN sequences Orthogonal codes
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Generating a Pseudo-random Noise chip sequencewith a linear feedback shift-register (LFSR)
1Pr 0
12 1
N
R 1
1N
0,N, 2N, ...
otherwise
Properties of PN sequences: Property 1: In a PN sequence we have:
Property 2: For a window of length n slid along output for N (=2n-1) shifts, each n-tuple appears once, except for the all zeros sequence
Property 3: The periodic autocorrelation of a PN sequence is:
source: Wikipedia, http://en.wikipedia.org/wiki/LFSR
1Pr 1
12 1
N
1
Pr 0 Pr 12
3110 10for n
N
number of registers: n
period: 2 1nN
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Orthogonal Codes
Orthogonal codes All pairwise cross correlations are zero Fixed- and variable-length codes used in CDMA systems For CDMA application, each mobile user uses one
sequence in the set as a spreading code Provides zero cross correlation among all users
Types Walsh codes Variable-Length Orthogonal codes
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Walsh Codes
1
1 1H
1 0
1 1
1 1
H HH
H H
k k
k
k k
1
1 1H
1 0
Set of Walsh codes of length n consists of the n rows of an n x n Hadamard matrix:
Sylvester's construction:
Every row is orthogonal to every other row and to the logical not of every other row
Requires tight synchronization Cross correlation between different shifts of Walsh sequences
is not zero
2
1 1 1 1
1 0 1 0H
1 1 0 0
1 0 0 1
...
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Typical Multiple Spreading Approach
Spread data rate by an orthogonal code (channelization code) Provides mutual orthogonality among all users in the same
cell
Further spread result by a PN sequence (scrambling code) Provides mutual randomness (low cross correlation)
between users in different cells
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CDMA (Code Division Multiple Access)
Principles all terminals send on the same frequency and can use the whole
bandwidth of the transmission channel each sender has a unique code The sender XORs the signal with this code the receiver can “tune” into this signal if it knows the code of the sender tuning is done via a correlation function
Disadvantages: higher complexity of the receiver (receiver cannot just listen into the
medium and start receiving if there is a signal) all signals should have approximately the same strength at the receiver
Advantages: all terminals can use the same frequency, no planning needed huge code space (e.g., 232) compared to frequency space more robust to eavesdropping and jamming (military applications…) forward error correction and encryption can be easily integrated
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CDMA: principle (very simplified)
Ak
X AsAd
Bk
X BsBd
As + Bs
Ak
X
Bk
X
C+D
C+D
Ad
Bd
C+D: Correlation and DecisionC+D: Correlation and Decision
Spreading Despreading
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CDMA: example
Sender A sends Ad = 1, key Ak = 010011 (assign: „0“= -1, „1“= +1)
sending signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1)
Sender B sends Bd = 0, key Bk = 110101 (assign: „0“= -1, „1“= +1)
sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1)
Both signals superimpose in space interference neglected (noise etc.) As + Bs = (-2, 0, 0, -2, +2, 0)
Receiver wants to receive signal from sender A apply key Ak bitwise (inner product)
Ae = (-2, 0, 0, -2, +2, 0) Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6
result greater than 0, therefore, original bit was „1“ receiving B
Be = (-2, 0, 0, -2, +2, 0) Bk = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e. „0“
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Spreading of signal A
data Ad
signal As
key sequence Ak
1 0 1
10 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1
01 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0
Real systems use much longer keys resulting in a larger distance between single code words in code space.
Ad+Ak
1
-1
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Spreading of signal B
signal As
As + Bs
1 0 0
00 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 1
11 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1
data Bd
signal Bs
key sequence Bk
Bd+Bk
1
-1
1
-1
2
-2
0
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Despreading of signal A
Ak
(As + Bs) * Ak
correlatoroutput
decisionoutput
As + Bs
0 1 0
1 0 1data Ad
Note: the received signal is invertedNote: the received signal is inverted
2
-2
0
1
-1
0
2
-2
0
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Despreading of signal B
correlatoroutput
decisionoutput
Bk
(As + Bs) * Bk
As + Bs
0 1 1
1 0 0data Bd
Note: the received signal is invertedNote: the received signal is inverted
2
-2
0
1
-1
0
2
-2
0
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Despreading with a wrong key
decisionoutput (1) (1) ?
wrong key K
correlatoroutput
(As + Bs) * K
As + Bs
2
-2
0
1
-1
0
2
-2
0
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Aloha has only a very low efficiency, CDMA needs complex receivers to be able to receive different senders with individual codes at the same time
Idea: use spread spectrum with only one single code (chipping sequence) for spreading for all senders accessing according to aloha
SAMA - Spread Aloha Multiple Access
1sender A0sender B
0
1
t
narrowband
send for a shorter periodwith higher power
spread the signal e.g. using the chipping sequence 110101 („CDMA without CD“)
Problem: find a chipping sequence with good characteristics
1
1
collision
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Comparison SDMA/TDMA/FDMA/CDMAApproach SDMA TDMA FDMA CDMAIdea segment space into
cells/sectorssegment sendingtime into disjointtime-slots, demanddriven or fixedpatterns
segment thefrequency band intodisjoint sub-bands
spread the spectrumusing orthogonal codes
Terminals only one terminal canbe active in onecell/one sector
all terminals areactive for shortperiods of time onthe same frequency
every terminal has itsown frequency,uninterrupted
all terminals can be activeat the same place at thesame moment,uninterrupted
Signalseparation
cell structure, directedantennas
synchronization inthe time domain
filtering in thefrequency domain
code plus specialreceivers
Advantages very simple, increasescapacity per km²
established, fullydigital, flexible
simple, established,robust
flexible, less frequencyplanning needed, softhandover
Dis-advantages
inflexible, antennastypically fixed
guard spaceneeded (multipathpropagation),synchronizationdifficult
inflexible,frequencies are ascarce resource
complex receivers, needsmore complicated powercontrol for senders
Comment only in combinationwith TDMA, FDMA orCDMA useful
standard in fixednetworks, togetherwith FDMA/SDMAused in manymobile networks
typically combinedwith TDMA(frequency hoppingpatterns) and SDMA(frequency reuse)
still faces some problems,higher complexity,lowered expectations; willbe integrated withTDMA/FDMA
In practice, several access methods are used in combinationExample :SDMA/TDMA/FDMA for GSM and IS-54
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References
T. Rappaport: Wireless Communications, Principles and Practice (2nd edition), Prentice Hall, 2002
M. Schwartz: Mobile Wireless Communications, Cambridge University Press, 2005
J. Schiller: Mobile Communications (2nd edition), Addison-Wesley, 2004
Leon-Garcia & Widjaja: Communication Networks, McGrawHill, 2000