1 Mobile Networks Module A (Part A2) Introduction Prof. JP Hubaux

Mobile Networks

Module A (Part A2)

Introduction

Prof. JP Hubaux

http://mobnet.epfl.ch

Modulation and demodulation (reminder)

synchronizationdecision

digitaldataanalog

demodulation

radiocarrier

analogbasebandsignal

101101001 radio receiver

digitalmodulation

digitaldata analog

modulation

radiocarrier

analogbasebandsignal

101101001 radio transmitter

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”

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

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

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)

Some medium access control mechanisms for wireless

TDMA CDMAFDMASDMA

Fixed Aloha ReservationsDAMA

MultipleAccess withCollisionAvoidance

Polling

• 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

• 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 DSSS

• Used in GSM

• Used in Bluetooth • Used in UMTS

TDMA CDMAFDMASDMA

Polling

MACAW MACA-BI FAMA

FHSS DSSS

• Used in GSM

• Used in Bluetooth

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

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

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

k2 k3 k4 k5 k6k1

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

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

FDMA/FDD – example: GSM

20 MHz

200 kHz

890.2 MHz

935.2 MHz

915 MHz

960 MHz

downlink

uplink

FDD: Frequency Division DuplexFDD: Frequency Division Duplex

TDMA CDMAFDMASDMA

Polling

MACAW MACA-BI FAMA

FHSS DSSS

• Used in GSM

Mechanism random, distributed (no central arbiter), time-multiplex Slotted Aloha additionally uses time-slots, sending must always

start at slot boundaries

Slotted Aloha

Aloha/slotted aloha

sender A

sender B

sender C

collision

sender A

sender B

sender C

collision

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

S: new packets S: throughput of the system

: 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

2, 0,1, 2,...

!Throughput S: total arrival rate G times the prob. of a successful transmission:

no collision 0 transmissions in 2 seconds

Peakvalue at 0.5 :

S GP GP X

1 0.1842S e

Average number of transmission attempts/packet:

attempts per packet

Average number of unsuccessful attempts per packet:

The first transmission requires seconds,

and each subs

G eSX t

equent retransmission requires 2

Thus the average packet transmission time is approx:

( 1)( 2 )

expressed relatively to X:

/ 1 ( 1)(1 2 )

where i

Galoha prop prop

Galoha

E T X t e X t B

BE T X a e a Xt

s the normalized one-way propagation delay

Computation of the average packet transmission time

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 )

Gslotaloha

BE T X a e a X

TDMA CDMAFDMASDMA

Polling

MACAW MACA-BI FAMA

FHSS DSSS

• Used in GSM

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

Station A capturesthe channelat t=tprop

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).

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.

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

Throughput expression

eaGeaG

eaGGaGGGS

2Pure ALOHA

Slotted ALOHA

Unslotted 1-persistent CSMA

Slotted 1-persistent CSMA

Unslotted nonpersistent CSMA

Slotted nonpersistent CSMA

Protocol Throughput

Throughput plot

Normalized propagation delay is a =0 .01

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

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

CSMA/CA

Is described in the module devoted to IEEE 802-11

TDMA CDMAFDMASDMA

Polling

MACAW MACA-BI FAMA

FHSS DSSS

• Used in GSM

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

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

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

AC-ABAF-

A---BAFD

ACEEBAFD

reservation

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

TDMA CDMAFDMASDMA

Polling

MACAW MACA-BI FAMA

FHSS DSSS

• Used in GSM

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)

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

CTSCTS

MACA example

RTS CTS

: blocked from Tx

MACA variant: application in IEEE802.11

wait for the right to send

wait for ACK

sender receiver

packet ready to send; RTS

time-out; RTS

CTS; data

RxBusy

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

TDMA CDMAFDMASDMA

Polling

MACAW MACA-BI FAMA

FHSS DSSS

• Used in GSM

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

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)

Spread Spectrum principle

c(t) c(t)

j f( )

r f( )

~( ) f

t f( )

Synchronizationn

Pseudo-random code

Filter Decoder Coder

s f( )

( )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

c(t) c(t)

j f( )

r f( )

~( ) f

t f( )

Synchronization Pseudo-random

s f( )

t f( )

c(t) c(t)

j f( )

r f( )

~( ) f

t f( )

s f( )

j f( )

c(t) c(t)

j f( )

r f( )

~( ) f

t f( )

s f( )

r f( )

c(t) c(t)

j f( )

r f( )

~( ) f

t f( )

s f( )

~( ) f

signal s s s s t

jjamming j j sj s

ProcessingPt signal gainPjamming original

P S B S B BBP S B B

Processing gain: Increase in received signal power thanks to spreading

c(t) c(t)

j f( )

r f( )

~( ) f

t f( )

s f( )

TDMA CDMAFDMASDMA

Polling

MACAW MACA-BI FAMA

FHSS DSSS

• Used in GSM

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)

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

Fast Frequency Hopping: b ct ttb : duration of one bit

tc : duration of one chip

TDMA CDMAFDMASDMA

Polling

MACAW MACA-BI FAMA

FHSS DSSS

• Used in GSM

• Used in Bluetooth • Used in UMTS

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 1 0 1 0 1 01 0 0 1 11

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

tb: bit periodtc: chip period

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

Xuser data

chippingsequence

modulator

radiocarrier

spreadspectrumsignal

transmitsignal

transmitter

demodulator

receivedsignal

radiocarrier

chippingsequence

lowpassfilteredsignal

receiver

integrator

products

decisiondata

sampledsums

correlator

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

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

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

Pr 0 Pr 12

3110 10for n

number of registers: n

period: 2 1nN

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

Walsh Codes

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

1 1 1 1

1 0 1 0H

1 1 0 0

1 0 0 1

Typical Multiple Spreading Approach

Spread data rate by an orthogonal code (channelization code) Provides mutual orthogonality among all users in the same

Further spread result by a PN sequence (scrambling code) Provides mutual randomness (low cross correlation)

between users in different cells

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

CDMA: principle (very simplified)

X AsAd

X BsBd

As + Bs

C+D: Correlation and DecisionC+D: Correlation and Decision

Spreading Despreading

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“

Spreading of signal A

data Ad

signal As

key sequence Ak

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.

Spreading of signal B

signal As

As + Bs

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

Despreading of signal A

(As + Bs) * Ak

correlatoroutput

decisionoutput

As + Bs

1 0 1data Ad

Note: the received signal is invertedNote: the received signal is inverted

Despreading of signal B

correlatoroutput

decisionoutput

(As + Bs) * Bk

As + Bs

1 0 0data Bd

Note: the received signal is invertedNote: the received signal is inverted

Despreading with a wrong key

decisionoutput (1) (1) ?

wrong key K

correlatoroutput

(As + Bs) * K

As + Bs

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

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

collision

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

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

1 Mobile Networks Module A (Part A2) Introduction Prof. JP Hubaux

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