Spread Spectrum MAC Spread Spectrum MAC Protocol with Dynamic Protocol with Dynamic Rate and Collision Rate and Collision Avoidance for Mobile Ad Avoidance for Mobile Ad Hoc NetworkHoc Network

Zaihan Jian, and Mengchu Zhou

Department of Electrical and Computer EngineeringNew Jersey Institute of Technology

IEEE Transaction on Vehicular Technology,Vol. 56, No. 5, September 2007

Outline Outline

IntroductionIntroduction DRCA protocolDRCA protocol Throughput analysisThroughput analysis Evaluation resultsEvaluation results ConclusionConclusion

IntroductionIntroduction

Spread spectrum (SS) has been Spread spectrum (SS) has been used as one of the basic wireless used as one of the basic wireless access technologies access technologies – 2G, IS-95, CDMA2000, and WCDMA2G, IS-95, CDMA2000, and WCDMA

Introduction Introduction

Advantage of SSAdvantage of SS– Concurrent transmission in one Concurrent transmission in one

channel without using TDMA/FDMAchannel without using TDMA/FDMA– SS can provide at most 1.5 times SS can provide at most 1.5 times

capacity compared with TDMA and capacity compared with TDMA and 4.6 times compared with FDMA in 4.6 times compared with FDMA in cellular systemcellular system

– Is feasible to switch from signal to Is feasible to switch from signal to signal for a transmitter or receiversignal for a transmitter or receiver

IntroductionIntroduction

MANETs become extremely useful in MANETs become extremely useful in scenarios where fixed infrastructure iscenarios where fixed infrastructure is infeasible or expensive to builds infeasible or expensive to build

Most proposed MAC protocols for MAMost proposed MAC protocols for MANETs are not SS basedNETs are not SS based– IEEE 802.11 use only one spread codeIEEE 802.11 use only one spread code

Research issues of Research issues of code assignmentcode assignment To assign transmission codes to To assign transmission codes to

network terminals to avoid packet network terminals to avoid packet collision as much as possiblecollision as much as possible

4 kinds of code assignment 4 kinds of code assignment protocolsprotocols– Common-codeCommon-code– Receiver-basedReceiver-based– Transmitter-basedTransmitter-based– HybridHybrid

Collision avoidance in Collision avoidance in SSSS

Collision can be classified into two Collision can be classified into two categoriescategories– Primary collisionPrimary collision: signals with the same : signals with the same

spread code being received at the same spread code being received at the same receiverreceiver

– Secondary collision: collision between two or Secondary collision: collision between two or more transmission that use different codemore transmission that use different code

The paper only focuses on primary The paper only focuses on primary collisioncollision

Bandwidth waste in Bandwidth waste in static code allocation static code allocation casecase

Usable bandwidth = real bandwidth / spreading factor

Spreading factorSpreading factor

Under the white Gaussian noise channelUnder the white Gaussian noise channel

Under the Rayleight fading channel Under the Rayleight fading channel

# of pairs# of pairs 11 22 33 44 55 66 77 88 99 1010

Spreading Spreading factorfactor

11 1212 2323 3434 4545 5757 6868 7979 8989 101011

# of pairs# of pairs 11 22 33 44 55 66 77 88 99 1010

Spreading Spreading factorfactor

11 795795 15815899

23823833

31731788

39739722

47647666

55655611

63563555

71471499

Drawbacks of static Drawbacks of static code assignmentcode assignment

High peak rate for a terminal can be High peak rate for a terminal can be achieved in such contention-based achieved in such contention-based MAC protocolsMAC protocols

In static code assignment SS MAC In static code assignment SS MAC protocol, it is hard to predict which protocol, it is hard to predict which terminal need more bandwidthterminal need more bandwidth– Hard to dynamically assign codeHard to dynamically assign code

Control message Control message formatformat

Operation of DRCAOperation of DRCA

CR: code for RTS and Short Message (SM)CC: code for CTS and ACKCi: code for data transmission

Random backoff,

no carrier sensing

Code for RTS

Code for CTS

Broadcast the selective code,

for avoiding collision

Broadcast the selective code, for code reuse

Throughput analysisThroughput analysis

NN terminals in the system terminals in the system Terminals are directly connected Terminals are directly connected

with each otherwith each other No mobilityNo mobility Slot time t is chosen to Slot time t is chosen to

accomplish RTS-CTSaccomplish RTS-CTS An idle terminal sends out an RTS An idle terminal sends out an RTS

with probability with probability pp

Throughput analysisThroughput analysis

A terminal generates a packet with sA terminal generates a packet with spreading factor preading factor KKe e ,,KKminmin ≦ ≦ KKe e ≦ ≦ KKmaxmax

Channel transmission bit rate: Channel transmission bit rate: m m //KKee

Packet transmission time: Packet transmission time: LL**KKe e //mm Slots of a packet: Slots of a packet: L’L’= = L *KL *Ke e / (/ (mm**tt))

– Packet length is geometrically distributPacket length is geometrically distributed with probability ed with probability qq

ParameterParameter

Transmission probability from state Transmission probability from state aa t to state o state b b ::PPabab

– a/b: a/b transmission pairs is busya/b: a/b transmission pairs is busy– a and b range from 0 to N/2a and b range from 0 to N/2

ii: number of pairs become idle from bus: number of pairs become idle from busy at the beginning of slot fy at the beginning of slot f

N’N’ = = NN - 2 - 2aa + 2 + 2ii– Number of terminals that are available to cNumber of terminals that are available to c

ommunicateommunicate

Parameter Parameter

d: number of successful RTS-CST d: number of successful RTS-CST dialogs in a slotdialogs in a slot– 0 and 10 and 1

d + a – i = bd + a – i = b c: number of RTS transmission at c: number of RTS transmission at

the beginning of a slotthe beginning of a slot d’ = c – dd’ = c – d

– Number of failed RTS transmissionNumber of failed RTS transmission

State transition in State transition in Markov chainMarkov chain

H: event of a transition from state a to H: event of a transition from state a to state bstate b

A: event of exactly one transmission A: event of exactly one transmission occurs and is addressed to an idle terminaloccurs and is addressed to an idle terminal

B: event of one transmission occurs and is B: event of one transmission occurs and is addressed to a busy terminaladdressed to a busy terminal

C: event of zero or more than one C: event of zero or more than one transmission occurtransmission occur

BBii: event that i pairs become idle from : event that i pairs become idle from busybusy

State transition in State transition in Markov chainMarkov chain

State transition in State transition in Markov chainMarkov chain Simplified formSimplified form

Steady state distribution SSteady state distribution Sa a is is given bygiven by

Throughput of DRCA Throughput of DRCA and static allocationand static allocation

Example value of Example value of SSrr

Sr: steady probability of r communication pairs

Simulation parameters in Simulation parameters in multihop environmentmultihop environment

Throughput: 24 pairsThroughput: 24 pairs

Throughput: 9 pairsThroughput: 9 pairs

Throughput differenceThroughput difference

ConclusionConclusion

The paper proposes an SS MAC The paper proposes an SS MAC protocol with dynamic rate and protocol with dynamic rate and collision avoidance call DRCAcollision avoidance call DRCA

Both theoretical and simulation Both theoretical and simulation results show that DRCA outperforms results show that DRCA outperforms static code allocation mechanismsstatic code allocation mechanisms

Thank you!!Thank you!!