1. CDMA-Based MACCDMA-Based MAC
Protocol for WirelessProtocol 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
2. 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
3. Introduction: MotivationIntroduction: Motivation
Mobile Ad hoc NETworks (MANETs)Mobile Ad hoc NETworks (MANETs)
Ability to provide temporary wireless networkingAbility to provide temporary wireless networking
capability; low throughputcapability; low throughput
Challenge: Increases overall n/w throughputChallenge: 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 toFocus: CDMA based design of MAC protocol to
improve n/w throughputimprove n/w throughput
4. Intro: CDMA?Intro: CDMA?
Code Division Multiple AccessCode Division Multiple Access
Bandwidth = ScarceBandwidth = Scarce
Traditional methods: transmit using leastTraditional methods: transmit using least
b/wb/w
Eg. TDMA, FDMAEg. TDMA, FDMA
CDMA based on Spread Spectrum: EachCDMA based on Spread Spectrum: Each
user occupies entire available b/w.user occupies entire available b/w.
Transmitter B1 bits/sTransmitter B1 bits/s spreadspread with pseudo-with pseudo-
random noise (PN) B2 bits/srandom noise (PN) B2 bits/s
5. Intro: CDMA!Intro: CDMA!
B2/B1 >> 1 (processing gain)B2/B1 >> 1 (processing gain)
PN statistically random but can be exactlyPN statistically random but can be exactly
reproduced through precise math rulesreproduced through precise math rules
Using locally generated PN receiverUsing locally generated PN receiver de-de-
spreadsspreads signal; recovers original infosignal; recovers original info
Several independently coded signals canSeveral independently coded signals can
occupy the same channel b/w providedoccupy the same channel b/w provided
each signal has diff PN codeeach signal has diff PN code
6. Intro: CDMA PropagandaIntro: CDMA Propaganda
3G’s choice3G’s choice
6 times capacity of TDMA, FDMA6 times capacity of TDMA, FDMA
Graceful signal degradationGraceful signal degradation
Multi-path resistanceMulti-path resistance
Interference rejectionInterference rejection
802.11 spreads signals with common PN code802.11 spreads signals with common PN code
at physical layerat physical layer
Thus not allowing concurrent transmissionsThus not allowing concurrent transmissions
DiagramDiagram
7. Intro: Code Assignment IssuesIntro: Code Assignment Issues
Absence of centralized control (baseAbsence of centralized control (base
station)station)
Code assignment protocol: diff codes toCode assignment protocol: diff codes to
diff terminalsdiff terminals
Trivial in small n/wsTrivial in small n/ws
Not feasible for MANET’s time asyncNot feasible for MANET’s time async
systemssystems
Spatial code reuse necessarySpatial code reuse necessary
8. Intro: Spreading Code protocolIntro: Spreading Code protocol
Which codes to use for packetWhich codes to use for packet
transmission and monitoring for packettransmission and monitoring for packet
receptionreception
3 types3 types
Receiver basedReceiver based
Transmitter basedTransmitter based
HybridHybrid
9. Intro: Receiver basedIntro: Receiver based
Transmitter uses code of intended receiver toTransmitter uses code of intended receiver to
spread packetspread packet
Idle node will monitor its own code onlyIdle node will monitor its own code only
Advantages:Advantages:
Simple receiver circuitSimple receiver circuit
Disadvantage:Disadvantage:
Primary collision can happenPrimary collision can happen
Broadcast requires transmitter to unicast to eachBroadcast requires transmitter to unicast to each
receiverreceiver
DiagramDiagram
10. 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 allBut, 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
11. 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 transmitterOther 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/CTSa 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 theSwitch to own (or transmitter) code to receive the
signalsignal
Example: RA-CDMAExample: RA-CDMA
12. Intro: RA-CDMAIntro: RA-CDMA
Guaranteed free of primary collisionsGuaranteed free of primary collisions
However, non-zero cross-correlationHowever, non-zero cross-correlation
causes multi-access interference; MAIcauses multi-access interference; MAI
Results in secondary collision at receiverResults in secondary collision at receiver
(collisions between transmissions using(collisions between transmissions using
diff codes)diff codes)
This is known as Near-Far problem; theThis is known as Near-Far problem; the
bane of MANETsbane of MANETs
13. The Near-Far problem in RA-The Near-Far problem in RA-
CDMACDMA
System is time-sync if signals originate fromSystem 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 pathCommon 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 fromSystem is time-async if signals originate from
multiple transmitters. Eg uplink in cellular ormultiple transmitters. Eg uplink in cellular or
MANETsMANETs
No common time reference, diff transmitters, diff pathNo 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
14. 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-corr
Receiver computes cross corr between signalReceiver computes cross corr between signal
and local PNand local PN
If PN same message intended for this receiverIf PN same message intended for this receiver
Else 0 or non-zero depending if sync or asyncElse 0 or non-zero depending if sync or async
Near-Far severe consequence of MAI: receiverNear-Far severe consequence of MAI: receiver
trying to detect signal of one is closer to anothertrying to detect signal of one is closer to another
Transmission power equal, closer signalTransmission power equal, closer signal
higher powerhigher power incorrect decoding. Collisionincorrect decoding. Collision
DiagramDiagram
15. NFP: ImpactNFP: Impact
d0=distance between receiver andd0=distance between receiver and
intended transmitterintended transmitter
Calculations show that if there is only 1Calculations show that if there is only 1
interferer at distance < 0.38 d0 frominterferer at distance < 0.38 d0 from
receiver, secondary collision will occurreceiver, secondary collision will occur
p=probability that terminal is transmittingp=probability that terminal is transmitting
in a given slotin a given slot
L=number of nodes within a circleL=number of nodes within a circle
centered at transmitter radius d0centered at transmitter radius d0
17. Proposed protocolProposed protocol
Main Goals:Main Goals:
To provide a CDMA-based MAC solution thatTo provide a CDMA-based MAC solution that
addresses near-far problemaddresses near-far problem
A Protocol that can achieve enhanced throughputA 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
18. Proposed protocolProposed protocol
Suppose that A wants to communicate
with B using a given code and C wants to communicate
with D using a different code. Suppsoe that dAB ≈ dCD,
dCB << dAB, and dAD << dCD. Then, the MAI caused by C
makes it impossible for B to receive A’s transmission. Similarily,
the MAI caused by A makes it impossible for D to
receive C’s transmission.
19. ComparisonComparison
MAC ProtocolMAC Protocol
Responsible forResponsible for
minimizing orminimizing or
eliminatingeliminating
collisionscollisions
Even if a terminalEven if a terminal
has an availablehas an available
spreading code, itspreading code, it
may not bemay not be
allowed to transmitallowed to transmit
SS protocolSS protocol
Decides which PNDecides which PN
code used tocode used to
Spread the signalSpread the signal
Doesn’t solveDoesn’t solve
contention on thecontention on the
mediummedium
20. Design GoalsDesign Goals
Asynchronous, distributed, scalable solution forAsynchronous, distributed, scalable solution for
large Networks (Matches MANETlarge Networks (Matches MANET
environment)environment)
Receiver stage shouldn’t be overly complexReceiver stage shouldn’t be overly complex
(Receiver Based spreading code)(Receiver Based spreading code)
Adapt to channel characteristics and mobilityAdapt to channel characteristics and mobility
patternspatterns
Able to coupe with incorrect code assignmentAble to coupe with incorrect code assignment
“code assignment is left to the upper layers”“code assignment is left to the upper layers”
21. Design ArchitectureDesign Architecture
Two Separate Frequency channels (FDM-likeTwo Separate Frequency channels (FDM-like
partitioning) - one for the RTS/CTS and the otherpartitioning) - one for the RTS/CTS and the other
for data exchangefor data exchange
Common Spreading Code for the controlCommon Spreading Code for the control
channelchannel
Receiver Based spreading codes for the dataReceiver Based spreading codes for the data
channelchannel
Codes are not assumed to be orthogonalCodes are not assumed to be orthogonal
Control and data channels are completelyControl and data channels are completely
orthogonalorthogonal
DiagramDiagram
23. Protocol AssumptionsProtocol Assumptions
Control and data channels are completelyControl and data channels are completely
orthogonalorthogonal
Channel gain is stationary for the duration of theChannel gain is stationary for the duration of the
control and data packet Transfercontrol and data packet Transfer
Gain is same in both directionsGain is same in both directions
Data packets between pair of terminals observeData packets between pair of terminals observe
similar gainsimilar gain
The radio stage can provide a feedback to theThe 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”
24. Protocol DescriptionProtocol Description
Contention based. Uses a variant of RTS/CTSContention based. Uses a variant of RTS/CTS
reservation schemereservation scheme
RTS and CTS are spread using a common code andRTS and CTS are spread using a common code and
transmitted over the control channel using fixed powertransmitted over the control channel using fixed power
PmaxPmax
RTS 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 onhowever, these nodes are allowed to transmit based on
some constraintssome constraints
For the Data channel, Receive and Transmitter shouldFor 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 PowerTransmit Power
Choice of power is critical and represents a trade-offChoice of power is critical and represents a trade-off
between link quality and max allowable interferencebetween link quality and max allowable interference
25. More protocol descriptionMore protocol description
In addition, the protocol incorporates anIn addition, the protocol incorporates an Interference MarginInterference Margin intointo
the power computation. Allows nodes at some distance from athe power computation. Allows nodes at some distance from a
receiver to start new transmissions in the futurereceiver to start new transmissions in the future
Nodes exploit the knowledge of the power level of the overheardNodes exploit the knowledge of the power level of the overheard
RTS and CTS transmissions to compute this marginRTS and CTS transmissions to compute this margin
A transmitter can decide when and at what power it can transmitA transmitter can decide when and at what power it can transmit
without disturbing ongoing transmissions in its surrounding and atwithout disturbing ongoing transmissions in its surrounding and at
the same time ensuring enough power at the receiver given thethe 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.
26. Channel Access MechanismChannel Access Mechanism
Transmissions that cause neither primary norTransmissions that cause neither primary nor
secondary collisionssecondary collisions
RTS/CTS provide 3 functions:RTS/CTS provide 3 functions:
Allow nodes to estimate channel gains betweenAllow nodes to estimate channel gains between
transmitter and receivertransmitter and receiver
A receiver uses CTS to notify its neighbors of theA receiver uses CTS to notify its neighbors of the
additional interference noise “allowable noise rise” itadditional 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 channelEach terminal keeps listening to the control channel
regardless of the signal destinationregardless of the signal destination
27. Protocol recoveryProtocol recovery
When transmission and propagation timesWhen transmission and propagation times
of control packets are long high probabilityof control packets are long high probability
of collision of CTS and RTS of anotherof collision of CTS and RTS of another
contending terminal leading to collisioncontending terminal leading to collision
with data packetswith data packets
EgEg
28. 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/eliminateDuty of MAC to reduce/eliminate
contentioncontention
(see previous slide)(see previous slide)
29. SimulationSimulation
Used CSIM programsUsed CSIM programs
Focused on 1 hop throughputFocused on 1 hop throughput
Data packets have fixed sizeData packets have fixed size
Transmission periods for RTS, CTS, data, ACKTransmission periods for RTS, CTS, data, ACK
in tens of msin tens of ms
Used random grid and clustered topologiesUsed random grid and clustered topologies
M=number of mobile hosts. Assume 36 Length 3M=number of mobile hosts. Assume 36 Length 3
kmkm
CA-CDMA 280% throughput increase overCA-CDMA 280% throughput increase over
802.11. Due to simultaneous transmissions802.11. Due to simultaneous transmissions
Uses shorter links to save energyUses shorter links to save energy
31. Conclusion & Future WorkConclusion & Future Work
Conclusion:Conclusion:
CA-CDMA is a distributed power control CDMACA-CDMA is a distributed power control CDMA
based MAC protocol.based MAC protocol.
CA-CDMA provides an enhancement for theCA-CDMA provides an enhancement for the
throughput in MANETs through addressing the nearthroughput in MANETs through addressing the near
far problemfar problem
Future Work:Future Work:
Combine CA-CDMA with other capacity optimizationCombine CA-CDMA with other capacity optimization
schemes. E.g. directional antennasschemes. E.g. directional antennas
Multi-rate support is also another opportunity forMulti-rate support is also another opportunity for
capacity optimizationcapacity optimization
Devise better schemes for access control over theDevise better schemes for access control over the
control channelcontrol channel
