Habibullah cdmabased

CDMA-Based MAC CDMA-Based MAC Protocol for Wireless Protocol for Wireless

Ad Hoc NetworksAd Hoc Networks

Alaa Muqattash and Marwan KrunzAlaa Muqattash and Marwan Krunz

Presented by: Habibullah PagarkarPresented by: Habibullah Pagarkar for 600.647-Advanced Topics in Wireless Networks.for 600.647-Advanced Topics in Wireless Networks.

JHU. Spring 04JHU. Spring 04

Today’s PresentationToday’s Presentation

IntroductionIntroduction

The Near-Far problemThe Near-Far problem

Protocol designProtocol design

Protocol descriptionProtocol description

SimulationSimulation

Results and EvaluationResults and Evaluation

Conclusion and future workConclusion and future work

Introduction: MotivationIntroduction: Motivation

Mobile Ad hoc NETworks (MANETs)Mobile Ad hoc NETworks (MANETs)

Ability to provide temporary wireless networking Ability to provide temporary wireless networking capability; low throughputcapability; low throughput

Challenge: Increases overall n/w throughput Challenge: Increases overall n/w throughput maintaining low energy consumptionmaintaining low energy consumption Harsh characteristics of channelHarsh characteristics of channel Contention based nature of MACContention based nature of MAC

Focus: CDMA based design of MAC protocol to Focus: CDMA based design of MAC protocol to improve n/w throughputimprove n/w throughput

Intro: CDMA?Intro: CDMA?

Code Division Multiple AccessCode Division Multiple AccessBandwidth = ScarceBandwidth = ScarceTraditional methods: transmit using least Traditional methods: transmit using least b/wb/wEg. TDMA, FDMAEg. TDMA, FDMACDMA based on Spread Spectrum: Each CDMA based on Spread Spectrum: Each user occupies entire available b/w.user occupies entire available b/w.Transmitter B1 bits/s Transmitter B1 bits/s spreadspread with pseudo- with pseudo-random noise (PN) B2 bits/srandom noise (PN) B2 bits/s

Intro: CDMA!Intro: CDMA!

B2/B1 >> 1 (processing gain)B2/B1 >> 1 (processing gain)

PN statistically random but can be exactly PN statistically random but can be exactly reproduced through precise math rulesreproduced through precise math rules

Using locally generated PN receiver Using locally generated PN receiver de-de-spreadsspreads signal; recovers original info signal; recovers original info

Several independently coded signals can Several independently coded signals can occupy the same channel b/w provided occupy the same channel b/w provided each signal has diff PN codeeach signal has diff PN code

Intro: CDMA PropagandaIntro: CDMA Propaganda

3G’s choice3G’s choice6 times capacity of TDMA, FDMA6 times capacity of TDMA, FDMAGraceful signal degradationGraceful signal degradationMulti-path resistanceMulti-path resistanceInterference rejectionInterference rejection802.11 spreads signals with common PN code 802.11 spreads signals with common PN code at physical layerat physical layerThus not allowing concurrent transmissionsThus not allowing concurrent transmissionsDiagramDiagram

Intro: Code Assignment IssuesIntro: Code Assignment Issues

Absence of centralized control (base Absence of centralized control (base station)station)

Code assignment protocol: diff codes to Code assignment protocol: diff codes to diff terminalsdiff terminals

Trivial in small n/wsTrivial in small n/ws

Not feasible for MANET’s time async Not feasible for MANET’s time async systemssystems

Spatial code reuse necessarySpatial code reuse necessary

Intro: Spreading Code protocolIntro: Spreading Code protocol

Which codes to use for packet Which codes to use for packet transmission and monitoring for packet transmission and monitoring for packet receptionreception

3 types3 types Receiver basedReceiver based Transmitter basedTransmitter based HybridHybrid

Intro: Receiver basedIntro: Receiver based

Transmitter uses code of intended receiver to Transmitter uses code of intended receiver to spread packetspread packetIdle node will monitor its own code onlyIdle node will monitor its own code onlyAdvantages:Advantages: Simple receiver circuitSimple receiver circuit

Disadvantage:Disadvantage: Primary collision can happenPrimary collision can happen Broadcast requires transmitter to unicast to each Broadcast requires transmitter to unicast to each

receiverreceiver

DiagramDiagram

Intro: Transmitter basedIntro: Transmitter based

A different code is assigned to each nodeA different code is assigned to each node

But, the receiving node must listen to all But, the receiving node must listen to all codescodes

Advantages:Advantages: Avoids Primary CollisionAvoids Primary Collision Simplified BroadcastSimplified Broadcast

Disadvantage:Disadvantage: Increased complexity of the receiverIncreased complexity of the receiver

Intro: Hybrid basedIntro: Hybrid based

Prevalent ApproachPrevalent Approach Fields of the packet are spread using a common codeFields of the packet are spread using a common code Other fields are spread by a receiver or a transmitter Other fields are spread by a receiver or a transmitter

based mechanismbased mechanism

In the reservation based schemes:In the reservation based schemes: a code is used for RTS/CTS a code is used for RTS/CTS Another code for data exchangeAnother code for data exchange

Receivers will listen to the common codeReceivers will listen to the common code If a receiver was intended by the transmitterIf a receiver was intended by the transmitter Switch to own (or transmitter) code to receive the Switch to own (or transmitter) code to receive the

signalsignal

Example: RA-CDMAExample: RA-CDMA

Intro: RA-CDMAIntro: RA-CDMA

Guaranteed free of primary collisionsGuaranteed free of primary collisions

However, non-zero cross-correlation However, non-zero cross-correlation causes multi-access interference; MAIcauses multi-access interference; MAI

Results in secondary collision at receiver Results in secondary collision at receiver (collisions between transmissions using (collisions between transmissions using diff codes)diff codes)

This is known as Near-Far problem; the This is known as Near-Far problem; the bane of MANETsbane of MANETs

The Near-Far problem in RA-The Near-Far problem in RA-CDMACDMA

System is time-sync if signals originate from System is time-sync if signals originate from same transmitter. Eg downlink in cellular CDMAsame transmitter. Eg downlink in cellular CDMA Common time reference, diff receivers, same path Common time reference, diff receivers, same path

and same time delaysand same time delays Complete orthogonal codesComplete orthogonal codes

System is time-async if signals originate from System is time-async if signals originate from multiple transmitters. Eg uplink in cellular or multiple transmitters. Eg uplink in cellular or MANETsMANETs No common time reference, diff transmitters, diff path No common time reference, diff transmitters, diff path

and diff time delaysand diff time delays Not possible to have orthogonal codesNot possible to have orthogonal codes

The Near-Far problem in RA-The Near-Far problem in RA-CDMACDMA

CDMA codes suffer from non-zero cross-corrCDMA codes suffer from non-zero cross-corrReceiver computes cross corr between signal Receiver computes cross corr between signal and local PNand local PNIf PN same message intended for this receiverIf PN same message intended for this receiverElse 0 or non-zero depending if sync or asyncElse 0 or non-zero depending if sync or asyncNear-Far severe consequence of MAI: receiver Near-Far severe consequence of MAI: receiver trying to detect signal of one is closer to anothertrying to detect signal of one is closer to anotherTransmission power equal, closer signalTransmission power equal, closer signal higher powerhigher power incorrect decoding. Collision incorrect decoding. CollisionDiagramDiagram

NFP: ImpactNFP: Impact

d0=distance between receiver and d0=distance between receiver and intended transmitterintended transmitterCalculations show that if there is only 1 Calculations show that if there is only 1 interferer at distance < 0.38 d0 from interferer at distance < 0.38 d0 from receiver, secondary collision will occurreceiver, secondary collision will occurp=probability that terminal is transmitting in p=probability that terminal is transmitting in a given slota given slotL=number of nodes within a circle L=number of nodes within a circle centered at transmitter radius d0 centered at transmitter radius d0

NFP: ImpactNFP: Impact

Proposed protocolProposed protocol

Main Goals:Main Goals: To provide a CDMA-based MAC solution that To provide a CDMA-based MAC solution that

addresses near-far problemaddresses near-far problem A Protocol that can achieve enhanced throughput A Protocol that can achieve enhanced throughput

keeping the same energy requirementkeeping the same energy requirement

Basic ideaBasic idea a distributed admission and feedback among nodesa distributed admission and feedback among nodes

DiagramDiagram

Proposed protocolProposed protocol

Suppose that A wants to communicatewith B using a given code and C wants to communicatewith D using a different code. Suppsoe that dAB ≈ dCD,

dCB << dAB, and dAD << dCD. Then, the MAI caused by Cmakes it impossible for B to receive A’s transmission. Similarily,

the MAI caused by A makes it impossible for D toreceive C’s transmission.

ComparisonComparison

MAC ProtocolMAC Protocol

Responsible for Responsible for minimizing or minimizing or eliminating eliminating collisionscollisions

Even if a terminal Even if a terminal has an available has an available spreading code, it spreading code, it may not be may not be allowed to transmitallowed to transmit

SS protocolSS protocol

Decides which PN Decides which PN code used tocode used toSpread the signalSpread the signal

Doesn’t solve Doesn’t solve contention on the contention on the mediummedium

Design GoalsDesign Goals

Asynchronous, distributed, scalable solution for Asynchronous, distributed, scalable solution for large Networks (Matches MANET large Networks (Matches MANET environment)environment)

Receiver stage shouldn’t be overly complex Receiver stage shouldn’t be overly complex (Receiver Based spreading code)(Receiver Based spreading code)

Adapt to channel characteristics and mobility Adapt to channel characteristics and mobility patternspatterns

Able to coupe with incorrect code assignment Able to coupe with incorrect code assignment “code assignment is left to the upper layers”“code assignment is left to the upper layers”

Design ArchitectureDesign Architecture

Two Separate Frequency channels (FDM-like Two Separate Frequency channels (FDM-like partitioning) - one for the RTS/CTS and the other partitioning) - one for the RTS/CTS and the other for data exchangefor data exchangeCommon Spreading Code for the control Common Spreading Code for the control channelchannelReceiver Based spreading codes for the data Receiver Based spreading codes for the data channelchannelCodes are not assumed to be orthogonalCodes are not assumed to be orthogonalControl and data channels are completely Control and data channels are completely orthogonalorthogonalDiagramDiagram

Design ArchitectureDesign Architecture

Protocol AssumptionsProtocol Assumptions

Control and data channels are completely Control and data channels are completely orthogonalorthogonalChannel gain is stationary for the duration of the Channel gain is stationary for the duration of the control and data packet Transfercontrol and data packet TransferGain is same in both directionsGain is same in both directionsData packets between pair of terminals observe Data packets between pair of terminals observe similar gainsimilar gainThe radio stage can provide a feedback to the The radio stage can provide a feedback to the upper MAC layer (about the interference level) upper MAC layer (about the interference level) “both ways”“both ways”

Protocol DescriptionProtocol Description

Contention based. Uses a variant of RTS/CTS Contention based. Uses a variant of RTS/CTS reservation scheme reservation scheme RTS and CTS are spread using a common code and RTS and CTS are spread using a common code and transmitted over the control channel using fixed power transmitted over the control channel using fixed power PmaxPmaxRTS and CTS are heard by potentially interfering nodes, RTS and CTS are heard by potentially interfering nodes, however, these nodes are allowed to transmit based on however, these nodes are allowed to transmit based on some constraintssome constraintsFor the Data channel, Receive and Transmitter should For the Data channel, Receive and Transmitter should agree on:agree on:

Spreading Code: “code assignment is dealt with at upper layers” Spreading Code: “code assignment is dealt with at upper layers” Transmit Power Transmit Power

Choice of power is critical and represents a trade-off Choice of power is critical and represents a trade-off between link quality and max allowable interferencebetween link quality and max allowable interference

More protocol descriptionMore protocol description

In addition, the protocol incorporates an In addition, the protocol incorporates an Interference Margin Interference Margin into into the power computation. Allows nodes at some distance from a the power computation. Allows nodes at some distance from a receiver to start new transmissions in the futurereceiver to start new transmissions in the futureNodes exploit the knowledge of the power level of the overheard Nodes exploit the knowledge of the power level of the overheard RTS and CTS transmissions to compute this marginRTS and CTS transmissions to compute this marginA transmitter can decide when and at what power it can transmit A transmitter can decide when and at what power it can transmit without disturbing ongoing transmissions in its surrounding and at without disturbing ongoing transmissions in its surrounding and at the same time ensuring enough power at the receiver given the the same time ensuring enough power at the receiver given the current “MAI at the receiver”.current “MAI at the receiver”. Distribute feedbackDistribute feedback to neighbors, through the CTS messages.to neighbors, through the CTS messages.

Channel Access MechanismChannel Access Mechanism

Transmissions that cause neither primary nor Transmissions that cause neither primary nor secondary collisionssecondary collisions

RTS/CTS provide 3 functions:RTS/CTS provide 3 functions: Allow nodes to estimate channel gains between Allow nodes to estimate channel gains between

transmitter and receivertransmitter and receiver A receiver uses CTS to notify its neighbors of the A receiver uses CTS to notify its neighbors of the

additional interference noise “allowable noise rise” it additional interference noise “allowable noise rise” it can accept without impacting its current receptioncan accept without impacting its current reception

Each terminal keeps listening to the control channel Each terminal keeps listening to the control channel regardless of the signal destinationregardless of the signal destination

Protocol recoveryProtocol recovery

When transmission and propagation times When transmission and propagation times of control packets are long high probability of control packets are long high probability of collision of CTS and RTS of another of collision of CTS and RTS of another contending terminal leading to collision contending terminal leading to collision with data packetswith data packets

EgEg

Code AssignmentCode Assignment

N/w topology continuously changingN/w topology continuously changing

Diff to guarantee correct code assignmentDiff to guarantee correct code assignment

Duty of MAC to reduce/eliminate Duty of MAC to reduce/eliminate contention contention

(see previous slide)(see previous slide)

SimulationSimulation

Used CSIM programsUsed CSIM programsFocused on 1 hop throughputFocused on 1 hop throughputData packets have fixed sizeData packets have fixed sizeTransmission periods for RTS, CTS, data, ACK Transmission periods for RTS, CTS, data, ACK in tens of msin tens of msUsed random grid and clustered topologiesUsed random grid and clustered topologiesM=number of mobile hosts. Assume 36 Length 3 M=number of mobile hosts. Assume 36 Length 3 kmkmCA-CDMA 280% throughput increase over CA-CDMA 280% throughput increase over 802.11. Due to simultaneous transmissions802.11. Due to simultaneous transmissionsUses shorter links to save energyUses shorter links to save energy

GraphsGraphs

Conclusion & Future WorkConclusion & Future Work

Conclusion:Conclusion: CA-CDMA is a distributed power control CDMA CA-CDMA is a distributed power control CDMA

based MAC protocol.based MAC protocol. CA-CDMA provides an enhancement for the CA-CDMA provides an enhancement for the

throughput in MANETs through addressing the near throughput in MANETs through addressing the near far problemfar problem

Future Work:Future Work: Combine CA-CDMA with other capacity optimization Combine CA-CDMA with other capacity optimization

schemes. E.g. directional antennasschemes. E.g. directional antennas Multi-rate support is also another opportunity for Multi-rate support is also another opportunity for

capacity optimizationcapacity optimization Devise better schemes for access control over the Devise better schemes for access control over the

control channelcontrol channel

Thank you!Thank you!

PS: I hope I get a good grade!PS: I hope I get a good grade!