1 Wireless Basics

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Lecture #1

Networks andNetworks and Wireless NetworksWireless Networks

Dr. Kun YangSchool of Comp. Sci. & Elec. Eng.

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, ,

Monday 16th March 2009

Agenda

Fundamentals of Computer NetworksBasics of Wireless Networks

PHY: Wireless TransmissionMAC: Medium Access ControlCognitive Radio

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A General Communication ModelCarries

data

Trans- Trans-

Source System Destination System

generatesdata to be

transmitted

Converts datainto transmittable

signals

Convertsreceived signal

into data

Takesincoming

data

missionSystem

missionSystem

Transmitter Receiver Destination

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workstation modem modem server Public telephone

Network

Networking

Point to point comm.not usually practical

Devices are too far apartLarge set of deviceswould need impracticalnumber of connections

Solution is acommunications

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Transmission vs. Switching

requires manytransmission links

Centralised switch networkminimises transmission but

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Trade off

ISO OSI (Open SystemsInterconnection) Reference Model

7 applicationapplication-specific protocolsA lication-

3 network

4 transport

5 session

6 presentationdata representation and encoding

dialog and synchronisation

message transfer (and maybe error control,flow control, connection management)

network routing and addressing (and maybe

End-to-end

oriented End-to-endscope

Globalscope

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1 physical

2 data linkdata link control (framing, datatransparency and maybe error control)

mechanical, electrical and optical interface

Hop-by-hop Localscope


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Layer Specific StandardsService definition

(Functional descriptionfor internal use) Addressing (Service

Access Point: SAP)

Protocol Specification

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(precise syntax andsemantics forinteroperability)

What’s a protocol?

A human protocol and a computer network protocol

Hiya

HeyGot theplace?

TCP connectionreq

TCP connectionresponse

Get http://www.essex.ac.uk

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time


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Protocols in OSI RM

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The Full ISO OSI RM

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Layering: Physical Communicationapplicationtransportnetwork

data

link physical

applicationtransportnetwork

link

network link

physical

data

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p ys ca app cat ontransportnetwork

link physical

The OSI Environment

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At Source: each layer takes data from above,adds header information to create new dataunit, and then passes new data unit to layer

below.

At Destination : the oppositeoperation


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Addressing

socket

13Unicast – broadcast - multicast

Network Structure

Administrative boundariesAutonomous system (AS):

intra-domain issuesBorderrouter

internal policyrouting metric?Routing protocols: RIPv2,

OSPFv2Interconnection of ASs:inter-domain issuesInterconnectivityinformation

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Routing protocols: BGPService LevelAgreement/SpecificationBusiness Model


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Network Structure : another viewAccess network:

With dedicated linksLow level of multiplexing

core

low volume of trafficISPsBroadband access: fixed, wireless

Distribution network: (transit)interconnectivity at local levellow multiplexingmedium volume of trafficMetropolitan Area Network

Core(backbone)

metro

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Core network – backbone:international/global interconnectivityhigh volume of traffichigh multiplexing

Network edge: applications and hosts

A packet passes through many networks!

ISP: Internet Service Provider

Tier 1 ISP NAP

Tier-2 ISPTier-2 ISP

localISPWireless

ISPwireless

ISP

localISP Tier 3

ISProut er

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Tier 1 ISP Tier 1 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISP

localISP

localISP

localISP

mobilehosts


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Agenda



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Some slides here pay courtesy to J. Schiller, K.Guild & D. Hunter.

Frequencies for communication

1 Mm 10 km 100 m 1 m 10 mm 100 m 1 m

optical transmissioncoax cabletwistedpair

VLF = Very Low Frequency UHF = Ultra High FrequencyLF = Low Frequency SHF = Super High FrequencyMF = Medium Frequency EHF = Extra High FrequencyHF = High Frequency UV = Ultraviolet Light

300 Hz 30 kHz 3 MHz 300 MHz 30 GHz

3 THz

300 THz

visible lightVLF LF MF HF VHF UHF SHF EHF infrared UV

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VHF = Very High Frequency

Frequency and wave length: λ = c/fwave length λ , speed of light c ≅3x108m/s, frequency f


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Radio Frequency Spectrum

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Frequencies for mobile communication

VHF-/UHF-ranges for mobile radiosimple, small antenna for carsdeterministic ro a ation characteristics, reliable connections

SHF and higher for directed radio links, satellitecommunication

small antenna, focusinglarge bandwidth available

Wireless LANs use frequencies in UHF to SHF spectrum

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limitations due to absorption by water and oxygen molecules(resonance frequencies)

• weather dependent fading, signal loss caused by heavy rainfall etc.


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Signal propagation ranges

Transmission rangecommunication possible

distance

sender

transmission

detection

low error rate

Detection rangedetection of the signalpossibleno communicationpossible

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interferenceInterference rangesignal may not bedetectedsignal adds to thebackground noise

Receiving power proportional to 1/d²

Signal propagation

Propagation in free space always like light (straight line)Receiving power proportional to 1/d²d = distance between sender and receiver

Receiving power additionally influenced byfading (frequency dependent)

shadowingreflection at large obstaclesscattering at small obstaclesdiffraction at ed es

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reflection scattering diffractionshadowing


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Signal can take many different paths between senderand receiver due to reflection, scattering, diffraction

Multipath propagation

signal at sender signal at receiver

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interference with “neighbor” symbols, Inter SymbolInterference (ISI)

The signal reaches a receiver directly and phase shifteddistorted signal depending on the phases of the different parts

Modulation

Digital modulationdigital data is translated into an analog signal (baseband)differences in s ectral efficienc ower efficienc robustness

Analog modulationshifts center frequency of baseband signal up to the radio

carrierMotivation

smaller antennas (e.g., λ /4)Fre uenc Division Multi lexin

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medium characteristics

Basic schemesAmplitude Modulation (AM)Frequency Modulation (FM)Phase Modulation (PM)


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Goal: multiple useof a shared medium

Multiplexing

k2 k3 k4 k5 k6k1

channels k i

Multiplexing in 4 dimensionsspace (s i)time (t)

s 2

s 1 f

t

c

f

t

c

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code (c)

s 3 f

t

Spread spectrum technology

Problem of radio transmission: frequency dependentfading can wipe out narrow band signals for durationof the interferenceSolution: spread the narrow band signal into a broadband signal using a special code

protection against narrow band interferenceinterference spread

signal

signal

spread

power power

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protection against narrowband interference

Side effects: coexistence of several signals withoutdynamic coordination

receiver

f f


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Signal vs. NoiseThe signals at the input of a receiver can be small (< 1uV).

electronics is significant.

A perfect receiver at room temperature would generate 4 x 10-21 W of noise per Hz of bandwidth.Noise power is proportional to absolute temperature, coolingthe receiver is nice.Real receivers generate 2 –8 times more noise power than this

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in the active devices used to amplify the small signal.The receiver performance is characterised in terms ofsignal to noise ratio (SNR)

Bit Error Rate (BER)

The receiver is trying to determinethe sequence of bits that were sentby the transmitter.

Bit Error Rate (BER)

A low signal to noise ratio (SNR)means that there is a probabilitythat the wrong decision will be

made.Bit error rate (BER) characterisesthe performance of a receiver.The relationship between bit error

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rate and SNR depends on themodulation type.The signal to noise ratio neededfor a particular minimum BER istermed the power efficiency.

SNR


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Payload Bit Error Rate

The physicalradio layercannot

eave oupper layers

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support theselow errorrates.

An Mechanism to Reduce BER: ARQ

ARQ: Automatic Repeat ReQuest: to repeat data that contains errors.Terminal A sends a message to Terminal B.Terminal B receives the packet with one or more errors.Terminal B has the ability to detect that errors are present.Terminal B sends a message to Terminal A requesting re-transmission.

Terminal A re-transmits the errored packet.The new packet is received successfully.

ARQ is only applicable when the channel error rate is very low.If the probability of a bit error occurring in a packet is significant the

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system will be overwhelmed by repeat requests and throughputcollapses.

ARQ results in a throughput reduction even when there are no errors.Terminal A has to idle while it waits for a repeat request from Terminal B.


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Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking, may include header fields

• Error detection not 100% reliable!• protocol may miss some errors, but rarely• larger EDC field yields better detection and correction

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Parity Checking

Single Bit Parity:Detect single bit errors

Single Bit Parity

If the sum of the bits in a packet is even, add an extra ‘0’ to the packet.If the sum of the bits in a packet is odd, add and extra ‘1’ to thepacket.This will only detect a single bit error, if two bits are in error then itwill fail.

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Radio channels can give rise to bursts of errors.More complex error detection schemes are needed.

Cyclic Redundancy Check (CRC) codes have become the standardway of solving the problem.


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Cyclic Redundancy Checkview data bits, D, as a binary numberchoose r+1 bit pattern (generator), G

, , exactly divisible by G (modulo 2)receiver knows G, divides by G. If non-zero remainder:error detected!can detect all burst errors less than r+1 bits

widely used in practice (ATM, HDCL)

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CRC Example

Want:

D.2r XOR R = nG

D.2r = nG XOR Requivalently:

if we divide D .2r by G, weget remainder R

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R = remainder[ ]D.2 rG


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Agenda



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Multiple Access Links

Wireless is Broadcast in nature.

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single shared broadcast channel but two or moresimultaneous transmissions by nodes interference

only one node can send successfully at a time


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Multiple Access protocols

channel, i.e., determine when a node can transmitCommunication about channel sharing must use channelitself!What to look for in multiple access protocols?

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Ideal Multiple Access Protocol

Broadcast channel of rate R bps1. When one node wants to transmit it can send at rate R. 2. When M nodes want to transmit, each can send at

average rate R/M

3. Fully decentralized:no special node to coordinate transmissionsno synchronization of clocks, slots

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. mp e


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Medium Access Control (MAC)MAC is concerned with controlling each terminal’saccess to the radio resource.

revent on o co s ons – oc ng.Fairness in allocating resource

Quality of service (QoS): latency (delay), jitter, loss.Distinction between infrastructure networks and ad-hoc networks

Infrastructure networks can have a centralised view of the

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pro em• Each terminal can be allocated its resource• The infrastructure is responsible for solving the MAC problem

Ad-hoc networks require distributed MAC• Each terminal has to be responsible for its own MAC

Basic Wireless Network Types

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MAC Protocols: a taxonomyThree broad classes:

Channel Partitioning “ ”v e c anne n o sma er p eces me s o s, requency,

code)allocate piece to node for exclusive use

Random Accesschannel not divided, allow collisions“recover” from collisions

“ ”

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tightly coordinate shared access to avoid collisions

Channel Allocation

Fixed Channel AllocationCertain Channels are assigned to each cell.Easy to implement but problems when traffic load changes

Borrowing Channel Allocation

Neigbouring busy cells borrow channels from quiet onesThe borrowed channel may be being re-used in a neighbouringcell.

Dynamic Channel Allocation

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A brain dynamically allocates channels to cells depending ontheir loadWhere is the brain??


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Can we apply media access methods

from fixed networks?Example CSMA/CD

Carrier Sense Multiple Access with Collision Detection sen as soon as t e me um s ree, sten nto t e me um a

collision occurs (original method in IEEE 802.3)

Problems in wireless networkssignal strength decreases proportional to the square of thedistancethe sender would apply CS and CD, but the collisions happenat the receiver

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it might be the case that a sender cannot “hear” the collision,i.e., CD does not workfurthermore, CS might not work if, e.g., a terminal is “hidden”

Hidden terminalsA sends to B, C cannot receive AC wants to send to B, C senses a “free” medium (CS fails)

Hidden and exposed terminals

collision at B, A cannot receive the collision (CD fails)A is “hidden” for C

Exposed terminalsB sends to A, C wants to send to another terminal (not A or B)

B A C

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C has to wait, CS signals a medium in usebut A is outside the radio range of C, therefore waiting is notnecessaryC is “exposed” to B


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Terminals A and B send, C receivessignal strength decreases proportional to the square of the distancethe signal of terminal B therefore drowns out A’s signal

Near and far terminals

canno rece ve

If C for example was an arbiter for sending rights, terminal B A B C

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Also severe problem for CDMA-networks where transmittersshare transmission frequencies and transmission time - precisepower control needed!

Access methods SDMA/FDMA/TDMA

SDMA (Space Division Multiple Access)segment space into sectors, use directed antennas

FDMA (Frequency Division Multiple Access)

assign a certain frequency to a transmission channel between a senderand a receiverpermanent (e.g., radio broadcast), slow hopping (e.g., GSM), fasthopping (FHSS, Frequency Hopping Spread Spectrum)

TDMA (Time Division Multiple Access)assi n the fixed sendin fre uenc to a transmission channel between

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a sender and a receiver for a certain amount of time

The multiplexing schemes are now used to control medium access!


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An FDD/FDMA Example - GSM

f 960 MHz

1

124

20 MHz

200 kHz935.2 MHz

915 MHz

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t

1890.2 MHz

A TDD/TDMA Example: DECT

DECT: Digital Enhanced Cordless Telecommunications

1 2 3 11 12 1 2 3 11 12

tdownlink uplink

417 µs

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Mechanismrandom, distributed (no central arbiter), time-multiplexSlotted Aloha additionally uses time-slots, sending must always

Aloha/slotted aloha

s ar a s o oun ar esAloha

Slotted Aloha

sender A

sender B

sender C

collision

t

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sender A

sender B

sender Ct

DAMA - Demand Assigned Multiple Access

Channel efficiency only 18% for Aloha, 36% for SlottedAloha (assuming Poisson distribution for packet arrivalan pac e engReservation can increase efficiency to 80%

a sender reserves a future time-slotsending within this reserved time-slot is possible withoutcollisionreservation also causes higher delays

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Examples for reservation algorithms:Explicit Reservation (Reservation-ALOHA)Implicit Reservation (PRMA: Packet Reservation MA)


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MACA - collision avoidanceMACA (Multiple Access with Collision Avoidance) uses shortsignaling packets for collision avoidance

RTS re uest to send : a sender re uests the ri ht to send from areceiver with a short RTS packet before it sends a data packetCTS (clear to send): the receiver grants the right to send as soon as it isready to receive

Signaling packets containsender addressreceiver addresspacket size

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

MACA avoids the problem of hidden terminalsA and C want tosend to B

MACA examples: RTS/CTS

A sends RTS firstC waits after receivingCTS from B

MACA avoids the problem of exposed terminalsB wants to send to A, and C

A B C

RTS

CTS(A)CTS (A)

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wants to send to another terminalnow C does not haveto wait because it cannotreceive CTS from A

A B C

RTS

CTSD

RTS

CTS


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Polling mechanismsIf one terminal can be heard by all others, this “central” terminal(a.k.a. base station) can poll all other terminals according to acertain schemeExample: Randomly Addressed Polling

base station signals readiness to all mobile terminalsterminals ready to send can now transmit a random number withoutcollision with the help of CDMA or FDMA (the random number canbe seen as dynamic address)the base station now chooses one address for polling from the list ofall random numbers (collision if two terminals choose the same

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a ressthe base station acknowledges correct packets and continues pollingthe next terminalthis cycle starts again after polling all terminals of the list

Access method CDMA

CDMA (Code Division Multiple Access)all terminals send on the same frequency probably at the same time andcan use the whole bandwidth of the transmission channeleach sender has a unique random number, the sender XORs the signalwith this random numberthe receiver can “tune” into this signal if it knows the pseudo randomnumber, tuning is done via a correlation function

Disadvantages:higher complexity of a receiver (receiver cannot just listen into themedium and start receiving if there is a signal)all signals should have the same strength at a receiver

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Advantages:all terminals can use the same frequency, no planning neededhuge code space (e.g. 2 32) compared to frequency spaceforward error correction and encryption can be easily integrated


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Appr oach SDMA TDMA FDMA CDMAIdea segment space into

cells/sectorssegment sendingtime into disjointtime-slots, demanddriven or fixed

segment thefrequency band intodisjoint sub-bands

spread the spectrumusing orthogonal codes

Comparison SDMA/TDMA/FDMA/CDMA

patternsTerminals only one terminal can

be 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

- inflexible antennas uard s ace inflexible com lex receivers needs

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-advantages

,

typically fixed

needed (multipathpropagation),synchronizationdifficult

,

frequencies are ascarce resource

,

more complicated power control for senders

Comment only in combinationwith TDMA, FDMA or CDMA useful

standard in fixednetworks, together with 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

Source: IEEE Comm. Mag.

Wireless vs. Wired

Wireless transmissions are far less robust than wiredNoise

• If the link is too long the noise is bigger than the signal.Interference

• Our own, generated within the network• Other people using the radio spectrum

MobilityWireless systems are inherently broadcast

We may talk about point to point comm but the signals still goever where

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We have to engineer a multiple access scheme so that all terminals canshare the available radio resource

Radio spectrum is an expensive and scarce commodity ->cognitive radio ?The radio channel is challenging - particularly indoors -> femtocell?


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Agenda



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Pondering on Spectrum Usage

The Ofcom Spectrum Vision

Spectrum should be free of technology, policy andusage constraints as far as possible

It should be simple and transparent for licenceholders to change the ownership and use ofspectrum

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and users should feel comfortable that they will notbe changed without good cause.

Source: Ofcom


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Three possible ways of managing Spectrum

Command & Control Zone Market Force Zone Licence-exempt Zone

Authorities:(No variations allowed,

for ~100 years!)

Managedby:

Companies

94%Currentpercentage (UK)

0% 6%

Future

nobody

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Trend:

The regulator cannot know asmuch as the market.

Trading existingspectrum between users

Key area for innovation but do we need more?

What is Cognitive Radio?

“… a radio that is aware of its surroundings and adaptsintelligently….” – Mitola 2000Primary users vs. secondary users

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A Spectrum Usage Test (London)

61Source: Ofcom

Challenges of Cog. Radio

EconomicsCost of CR devices over simpler delivery mechanism likely to be higherNon-real time services for CR not as valuableDepends on spectrum congestion and demand

Hidden node / sharing issues

Ensure CRs do not interfere with each otherMaking sure that they can exist with legacy users and other

Cog. Radio devicesControlling CRs to ensure they have the same spectrum picture

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How will groups of CRs know what’s going on?Devices need to be aware of regulatory constraints

Security and malicious useThe flexibility of CRs enables illegal transmissionsAAA


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Where can I use Cog. Radio?

licence-exempt (LE) bands: all right.and Licenced Bands!: not fair?! restricted to licensees?

Non-time sensitive services, such as downloadingcould be most appropriate.But,

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Broadband wireless servicesMultimedia wireless networking

Conceptual Timeline (IEEE 1900)

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Contact, Q&A

Dr Kun Yang Reader in Pervasive Networks & ServicesSchool of Comp. Science & Electronic Engineering (CSEE),University of Essex, Wivenhoe Park, Colchester,CO4 3SQ, UK

Email: [email protected] : rivatewww.essex.ac.uk ~kun an

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