Modulation and demodulation (reminder) analog baseband digital - - PowerPoint PPT Presentation

1

COM-405 Mobile Networks Module A (Part A2) Introduction

• Prof. JP Hubaux

http://mobnet.epfl.ch Note: some of the slides of this and other modules and derived from Schiller’s book

2

Modulation and demodulation (reminder)

synchronization decision digital data analog demodulation radio carrier analog baseband signal 101101001 radio receiver digital modulation digital data analog modulation radio carrier analog baseband signal 101101001 radio transmitter

3

About CSMA/CD

Can we borrow media access methods from fixed networks? Example of CSMA/CD

q Carrier Sense Multiple Access with Collision Detection q 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

q a radio can usually not transmit and receive at the same time q signal strength decreases proportionally to the square of the

distance or even more

q the sender would apply CS and CD, but the collisions happen at the

receiver

q it might be the case that a sender cannot “hear” the collision, i.e.,

CD does not work

q furthermore, CS might not work if, e.g., a terminal is “hidden”

4

Hidden terminals

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

Exposed terminals

q B sends to A, C wants to send to another terminal (not A or B) q C has to wait, CS signals a medium in use q but A is outside the radio range of C, therefore waiting is not

necessary

q C is “exposed” to B

Hidden and exposed terminals

B A C

5

Terminals A and B send, C receives

q signal strength decreases (at least) proportionally to the square of the

distance

q the signal of terminal B therefore drowns out A’s signal q è 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)

q segment space into sectors, use directed antennas q cell structure

TDMA (Time Division Multiple Access)

q 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)

q assign a certain frequency to a transmission channel between a

sender and a receiver

q permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast

hopping (FHSS, Frequency Hopping Spread Spectrum)

CDMA (Code Division Multiple Access)

q assign an appropriate code to each transmission channel (DSSS,

Direct Sequency Spread Spectrum)

q frequency hopping over separate channels (FHSS, Frequency

Hopping Spread Spectrum)

7

Some medium access control mechanisms for wireless

TDMA CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance 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 Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: 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 CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance 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 Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS DSSS

• Used in GSM

Fixed

• Used in Bluetooth

9

f t c k2 k3 k4 k5 k6 k1

Time multiplex

A channel gets the whole spectrum for a certain amount of time. Advantages:

q only one carrier in the

medium at any time Disadvantages:

q precise

synchronization required

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:

q looser coordination q works also for analog signals

Disadvantages:

q wastage of bandwidth

if the traffic is distributed unevenly

q inflexible q guard spaces k2 k3 k4 k5 k6 k1 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:

q more flexibility

But: precise coordination required

t c k2 k3 k4 k5 k6 k1

12

Code multiplex

Each channel has a unique code All channels use the same spectrum at the same time Advantages:

q bandwidth efficient q good protection against interference

and eavesdropping

Disadvantage:

q more complex signal regeneration

Implemented using spread spectrum technology

k2 k3 k4 k5 k6 k1 f t c

13

TDMA/TDD – example: DECT

1 2 3 11 12 1 2 3 11 12 t downlink uplink 417 µs

DECT: Digital Enhanced Cordless Telecommunications TDD: 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 Duplex

15

Some medium access control mechanisms for wireless

TDMA CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance 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 Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS DSSS

• Used in GSM

Fixed

• Used in Bluetooth

16

Mechanism

q random, distributed (no central arbiter), time-multiplex q 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 (1/4)

First transmission Retransmission (if necessary) t0 t0+X t0-X Vulnerable period t0+X+2tprop Time-out t0+X+2tprop+B Backoff period B

• tprop : maximum one-way propagation time between 2 stations
• Information about the outcome of the transmission is obtained after the

reaction time 2 tprop

• B: backoff time

18

Performance of Aloha (2/4)

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 Pr transmissions in 2 second G S k X

[ ] ( ) [ ] [ ] ( )

2 2 2

2 s , 0,1,2,... ! Throughput S: total arrival rate G times the prob. of a successful transmission: .Pr no collision .Pr 0 transmissions in 2 seconds 2 = 0! = Peakvalue at

k G G G

G e k k S G G X G G e Ge G

− − −

= = = = = 1 .5 : 0.184 2 S e = ≈

19

Detail of computation of throughput of previous slide: Define: T : Transmission by a given station A: Absence of transmission by any other station Throughput: 1.Pr(T,A) = N.

i=1 N

∑

Pr(T,A) = N.Pr(A|T).Pr(T) = N.Pr(T).Poisson(0,2G) = G.Poisson(0,2G) = G.e−2G

Performance of Aloha (3/4)

20

Performance of Aloha (4/4)

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 e S G e S X t ε = − = − +

[ ] [ ]

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 G aloha prop prop G aloha prop

t X B E T X t e X t B B E T X a e a X t a X + + = + + − + + = + + − + + = s the normalized one-way propagation delay

Computation of the average packet transmission time

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Performance of Slotted Aloha

First transmission Retransmission (if necessary) t0=kX (k+1)X Vulnerable period t0+X+2tprop Time-out t0+X+2tprop+B Backoff period

[ ]

• 1

Peakvalue at 1 : 0.368 Average packet delay: / 1 ( 1)(1 2 )

G G slotaloha

S Ge G S e B E T X a e a X = = = ≈ = + + − + +

22

Some medium access control mechanisms for wireless

TDMA CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance 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 Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS DSSS

• Used in GSM

Fixed

• Used in Bluetooth

23

Carrier Sense Multiple Access (CSMA)

q Goal: reduce the wastage of bandwidth due to packet collisions q Principle: sensing the channel before transmitting (never

transmit when the channel is busy)

q Many variants:

q Collision detection (CSMA/CD) or collision avoidance(CSMA/CA) q Persistency (in sensing and transmitting)

Station A begins transmission at t=0

A

Station A captures the channel at t=tprop

A

24

1-Persistent CSMA

q Stations having a packet to send sense the channel

continuously, waiting until the channel becomes idle.

q As soon as the channel is sensed idle, they transmit their

packet.

q If more than one station is waiting, a collision occurs. q Stations involved in a collision perform a the backoff algorithm

to schedule a future time for resensing the channel

q Optional backoff algorithm may be used in addition for fairness

Consequence : The channel is highly used (greedy algorithm).

25

Non-Persistent CSMA

q Attempts to reduce the incidence of collisions q Stations with a packet to transmit sense the channel q If the channel is busy, the station immediately runs the back-off

algorithm and reschedules a future sensing time

q If the channel is idle, then the station transmits

Consequence : channel may be free even though some users have packets to transmit.

26

p-Persistent CSMA

q Combines elements of the above two schemes q Stations with a packet to transmit sense the channel q If it is busy, they persist with sensing until the channel becomes

idle

q If it is idle:

l With probability p, the station transmits its packet l With probability 1-p, the station waits for a random time and senses

again

27

Throughput expression

S = Ge−2G S = Ge−G S = G 1+G + aG 1+G + aG / 2

( )

" # $ %e

−G 1+2a

( )

G 1+ 2a

( )− 1−e−aG

( )+ 1+ aG

( )e

−G 1+a

( )

S = G 1+ a −e−aG " # $ %e

−G 1+a

( )

1+ a

( ) 1−e−aG

( )+ ae

−G 1+a

( )

S = Ge−aG G 1+ 2a

( )+e−aG

S = aGe−aG 1−e−aG + a

Pure ALOHA Slotted ALOHA Unslotted 1-persistent CSMA Slotted 1-persistent CSMA Unslotted nonpersistent CSMA Slotted nonpersistent CSMA

Protocol Throughput

28

Throughput plot

Normalized propagation delay is a =0 .01

29

CSMA/CD (reminder)

Operating principle

q Check whether the channel is idle before transmitting q Listen while transmitting, stop transmission when collision q 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

30

Why CSMA/CD is unfit for WLANs

q Collision Detection requires simultaneous

transmission and reception operations (which a radio transceiver is usually unable to do) è detecting a collision is difficult

q Carrier Sensing may be suitable to reduce

interference at sender, but Collision Avoidance is needed at receiver

q CSMA/CD does not address the hidden terminal

problem

31

CSMA/CA

Is described in the module B devoted to IEEE 802-11 (we’ll see it next week)

32

Some medium access control mechanisms for wireless

TDMA CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance 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 Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS DSSS

• Used in GSM

Fixed

• Used in Bluetooth

33

DAMA - Demand Assigned Multiple Access

Channel efficiency only 18% for Aloha, 36% for Slotted Aloha Reservation can increase efficiency to 80%

q a sender reserves a future time-slot q sending within this reserved time-slot is possible without collision q reservation also causes higher delays q typical scheme for satellite links

Examples for reservation algorithms:

q Explicit Reservation (Reservation-ALOHA) q Implicit Reservation (PRMA) q Reservation-TDMA

34

DAMA / Explicit Reservation

Explicit Reservation (Reservation Aloha):

q two modes:

l ALOHA mode for reservation:

competition for small reservation slots, collisions possible

l reserved mode for data transmission within successful reserved slots

(no collisions possible)

q 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

35

DAMA / Packet reservation (PRMA)

Implicit reservation

q based on slotted Aloha q a certain number of slots form a frame, frames are repeated q stations compete for empty slots according to the slotted aloha

principle

q 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

q 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 D t ACDABA-F ACDABA-F AC-ABAF- A---BAFD ACEEBAFD reservation

36

DAMA / Reservation-TDMA

Reservation Time Division Multiple Access

q every frame consists of N mini-slots and x data-slots q every station has its own mini-slot and can reserve up to k data-slots

using this mini-slot (i.e. x = N * k).

q 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 reservations for data-slots

• ther stations can use free data-slots

based on a round-robin scheme e.g. N=6, k=2

37

Some medium access control mechanisms for wireless

TDMA CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance 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 Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS DSSS

• Used in GSM

Fixed

• Used in Bluetooth

38

MACA - collision avoidance

MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance

q Designed especially for packet radio networks (Phil Karn, 1990) q Principle:

l 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

l CTS (clear to send): the receiver grants the right to send as soon as it

is ready to receive

Signaling packets contain

q sender address q receiver address q packet size

Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC)

39

MACA mitigates the problem of hidden terminals

q A and C want to

send to B

q A sends RTS first q C waits after receiving

CTS from B

The hidden terminal problem might still arise, especially in case of mobility of the nodes

MACA principle

A B C RTS CTS CTS

40

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

41

MACA variant: application in IEEE 802.11

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 for data RTS; RxBusy RTS; CTS data; ACK time-out Data with errors; NAK ACK: positive acknowledgement NAK: negative acknowledgement RxBusy: receiver busy time-out NAK; RTS

42

Some medium access control mechanisms for wireless

TDMA CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance 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 Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS DSSS

• Used in GSM

Fixed

• Used in Bluetooth

43

Spread Spectrum principle

c(t) c(t)

( ) Φ f Φ j f ( ) Φ r f ( ) ~( ) Φ f Φ t f ( )

Synchronization n Pseudo-random code Filter Decoder Coder

Φ s f ( )

f

Φ s f ( )

s

S

Bs

( )

s f

Φ

power density spectrum of the original signal

s

S

power density of the original signal s

B

bandwidth of the original signal

( )

j f

Φ

power density spectrum of the jamming signal

44

Spread Spectrum principle

c(t) c(t)

( ) Φ f Φ j f ( ) Φ r f ( ) ~( ) Φ f Φ t f ( )

Synchronization Pseudo-random code Filter Decoder Coder

Φ s f ( )

f

Φ t f ( )

s s t t

S B S B = Bt

45

Spread Spectrum principle

c(t) c(t)

( ) Φ f Φ j f ( ) Φ r f ( ) ~( ) Φ f Φ t f ( )

Synchronization Pseudo-random code Filter Decoder Coder

Φ s f ( )

j

S

f

Φ j f ( )

B j

46

Spread Spectrum principle

c(t) c(t)

( ) Φ f Φ j f ( ) Φ r f ( ) ~( ) Φ f Φ t f ( )

Synchronization Pseudo-random code Filter Decoder Coder

Φ s f ( )

B j

t

S

j

S

f

Φr f ( )

Bt

47

Spread Spectrum principle

c(t) c(t)

( ) Φ f Φ j f ( ) Φ r f ( ) ~( ) Φ f Φ t f ( )

Synchronization Pseudo-random code Filter Decoder Coder

Φ s f ( )

s

S

f

~( ) Φ f

j j t

B S B Bt Bs

P

signal

Pjamming = Ss Bs S j Bj Bt Bs = Ss Bs S j Bj

Psignal Pjamming

! " # $ % &

• riginal

!

⋅ Bt Bs

Processing gain

!

Processing gain: Increase in received signal power thanks to spreading

48

Spread Spectrum principle

c(t) c(t)

( ) Φ f Φ j f ( ) Φ r f ( ) ~( ) Φ f Φ t f ( )

Synchronization Pseudo-random code Filter Decoder Coder

Φ s f ( )

s

S

f

( ) Φ f

j j t

B S B Bs

49

Some medium access control mechanisms for wireless

TDMA CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance 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 Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS DSSS

• Used in GSM

Fixed

• Used in Bluetooth

50

Frequency Hopping Spread Spectrum (FHSS) (1/2)

q Signal broadcast over seemingly random series of

frequencies

q Receiver hops between frequencies in sync with

transmitter

q Eavesdroppers hear unintelligible blips q Jamming on one frequency affects only a few bits q Rate of hopping versus Symbol rate

q Fast Frequency Hopping: One bit transmitted in multiple

hops.

q Slow Frequency Hopping: Multiple bits are transmitted in a

hopping period

q Example: Bluetooth (79 channels, 1600 hops/s)

51

Frequency Hopping Spread Spectrum (FHSS) (2/2)

tb tc Fast Frequency Hopping:

b c

t t >

tb : duration of one bit tc : duration of one chip Chip: name of the sample period in spread-spectrum jargon

52

Some medium access control mechanisms for wireless

TDMA CDMA FDMA SDMA Fixed Aloha Reservations DAMA Multiple Access with Collision Avoidance 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 Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access FHSS DSSS

• Used in GSM

Fixed

• Used in Bluetooth
• Used in UMTS

53

Direct Sequence Spread Spectrum (DSSS) (1/2)

XOR of the signal with pseudo-random number (chipping sequence)

q many chips per bit (e.g., 128) result in higher bandwidth of the signal

Advantages

q reduces frequency selective

fading

q in cellular networks

l neighboring base stations can use the

same frequency range

l neighboring base stations can

detect and recover the signal

l è enables soft handover

Disadvantages

q precise power control necessary q complexity of the receiver user data chipping sequence resulting signal 1 1 1 0 1 0 1 0 1 1 1 1 XOR 1 1 0 1 0 1 1 1 1 = tb tc

tb: bit period tc: chip period

54

Direct Sequence Spread Spectrum (DSSS) (2/2)

X user data chipping sequence modulator radio carrier spread spectrum signal transmit signal transmitter demodulator received signal radio carrier X chipping sequence lowpass filtered signal receiver integrator products decision data sampled sums correlator

55

Categories of spreading (chipping) sequences

q Spreading Sequence Categories

q Pseudo-random Noise (PN) sequences q Orthogonal codes

q For FHSS systems

q PN sequences most common

q For DSSS beside multiple access

q PN sequences most common

q For DSSS CDMA systems

q PN sequences q Orthogonal codes

56

Generating a Pseudo-random Noise chip sequence with a linear feedback shift-register (LFSR)

{ }

1 Pr 0 1 2 1 N = ⎛ ⎞ ⋅ − ⎜ ⎟ ⎝ ⎠

R τ

( )=

1 − 1 N ⎧ ⎨ ⎪ ⎩ ⎪ τ = 0,N, 2N, ...

• therwise

Properties of PN sequences:

q Property 1: In a PN sequence we have: q 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

q Property 3: The periodic autocorrelation of a PN sequence is:

{ }

1 Pr 1 1 2 1 N = ⎛ ⎞ ⋅ + ⎜ ⎟ ⎝ ⎠

{ } { }

1 Pr 0 Pr 1 2 ≈ ≈

3

1 10 10 for n N

−

≥ ⇒ ≤

number of registers: n period:

2 1

n

N = −

57

Orthogonal Codes

q Orthogonal codes

q All pairwise cross correlations are zero q Fixed- and variable-length codes used in CDMA systems q For CDMA application, each mobile user uses one sequence

in the set as a spreading code

l Provides zero cross correlation among all users

q Types

q Walsh codes q Variable-Length Orthogonal codes

58

Walsh Codes

1

1 1 H 1 ⎛ ⎞ = ⎜ ⎟ ⎝ ⎠

1 1 1 1

H H H H H

k k k k k − − − −

⎛ ⎞ = ⎜ ⎟ ⎜ ⎟ ⎝ ⎠

1

1 1 H 1 ⎛ ⎞ = ⎜ ⎟ ⎝ ⎠

q Set of Walsh codes of length n consists of the n rows of an

n x n Hadamard matrix:

q Sylvester's construction: q Every row is orthogonal to every other row and to the logical

not of every other row

q Requires tight synchronization

q Cross correlation between different shifts of Walsh sequences

is not zero

2

1 1 1 1 1 1 H 1 1 1 1 ⎛ ⎞ ⎜ ⎟ ⎜ ⎟ = ⎜ ⎟ ⎜ ⎟ ⎝ ⎠

...

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Typical Multiple Spreading Approach

q Spread data rate by an orthogonal code

(channelization code)

q Provides mutual orthogonality among all users in the same

cell

q Further spread result by a PN sequence

(scrambling code)

q Provides mutual randomness (low cross correlation)

between users in different cells

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CDMA (Code Division Multiple Access)

Principles

q all terminals send on the same frequency and can use the whole

bandwidth of the transmission channel

q each sender has a unique code q The sender XORs the signal with this code q the receiver can “tune” into this signal if it knows the code of the sender q tuning is done via a correlation function

Disadvantages:

q higher complexity of the receiver (receiver cannot just listen into the

medium and start receiving if there is a signal)

q all signals should have approximately the same strength at the receiver

Advantages:

q all terminals can use the same frequency, no planning needed q huge code space (e.g., 232) compared to frequency space q more robust to eavesdropping and jamming (military applications…) q forward error correction and encryption can be easily integrated

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CDMA: principle (very simplified)

Ak

X

As Ad Bk

X

Bs Bd As + Bs Ak

X

Bk

X

C+D C+D

Ad Bd

C+D: Correlation and Decision

Spreading Despreading

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CDMA: example

Sender A

q sends Ad = 1, key Ak = 010011 (assign: „0“= -1, „1“= +1) q sending signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1)

Sender B

q sends Bd = 0, key Bk = 110101 (assign: „0“= -1, „1“= +1) q sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1)

Both signals superimpose in space

q interference neglected (noise etc.) q As + Bs = (-2, 0, 0, -2, +2, 0)

Receiver wants to receive signal from sender A

q apply key Ak bitwise (inner product)

l Ae = (-2, 0, 0, -2, +2, 0) Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6 l result greater than 0, therefore, original bit was „1“

q receiving B

l 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 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 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 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 data Bd signal Bs key sequence Bk Bd+Bk 1

• 1

1

• 1

2

• 2

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Despreading of signal A

Ak (As + Bs) * Ak correlator

• utput

decision

• utput

As + Bs 1 1 1 data Ad

Note: the received signal is inverted

2

• 2

1

• 1

2

• 2

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Despreading of signal B

correlator

• utput

decision

• utput

Bk (As + Bs) * Bk As + Bs 1 1 1 data Bd

Note: the received signal is inverted

2

• 2

1

• 1

2

• 2

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Despreading with a wrong key

decision

• utput

(1) (1) ? wrong key K correlator

• utput

(As + Bs) * K As + Bs 2

• 2

1

• 1

2

• 2

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Comparison SDMA/TDMA/FDMA/CDMA

Approach SDMA TDMA FDMA CDMA Idea

segment space into cells/sectors segment sending time into disjoint time-slots, demand driven or fixed patterns segment the frequency band into disjoint sub-bands spread the spectrum using orthogonal codes

Terminals

• nly one terminal can

be active in one cell/one sector all terminals are active for short periods of time on the same frequency every terminal has its

• wn frequency,

uninterrupted all terminals can be active at the same place at the same moment, uninterrupted

Signal separation

cell structure, directed antennas synchronization in the time domain filtering in the frequency domain code plus special receivers

Advantages very simple, increases

capacity per km² established, fully digital, flexible simple, established, robust flexible, less frequency planning needed, soft handover

Dis- advantages

inflexible, antennas typically fixed guard space needed (multipath propagation), synchronization difficult inflexible, frequencies are a scarce resource complex receivers, needs more complicated power control for senders

Comment

used in all cellular systems standard in fixed networks, together with FDMA/SDMA used in many mobile networks typically combined with TDMA (frequency hopping patterns) and SDMA (frequency reuse) higher complexity

In practice, several access methods are used in combination Example: SDMA/TDMA/FDMA for GSM

Orthogonal Frequency Division Modulation (ODFM) (In a nutshell)

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Credit: http://www.csie.ntu.edu.tw

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For Next Week

• Read the Web site Mobnet.epfl.ch
• Review the lecture; get prepared for the quiz !! (Starts at 13:15)
• Get your clicker
• Visit the Library at the Rolex Learning Center, room RLC D1 210,

Monday to Friday 8:00 – 18:00

• The app on smartphone or tablet is not allowed
• For all clicker-related issues, please contact Alexandra:

alexandramihaela.olteanu@epfl.ch

• Try to solve the homework (will be online very soon)

We help you as of 15:00 on it

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References

q T. Rappaport: Wireless Communications, Principles and

Practice (2nd edition), Prentice Hall, 2002

q M. Schwartz: Mobile Wireless Communications, Cambridge

University Press, 2005

q J. Schiller: Mobile Communications (2nd edition), Addison-

Wesley, 2004

q Leon-Garcia & Widjaja: Communication Networks, McGrawHill,

2000