Università degli Studi di Firenze 08 July 2004 COST 289 - 5th MCM - Budapest, Hungary 1 Cross-layer design for Multiple access techniques in wireless communications

08 July 2004

COST 289 - 5th MCM - Budapest, Hungary

1

Università degli Studi di Firenze

Cross-layer design for Multiple access techniques in wireless

communications

Daniele TarchiUniversity of Florence, Italy

E-mail: [email protected]

08 July 2004


2Università degli Studi di Firenze

Outline

• Link Adaptation Adaptive Modulation and Coding VSF-CDMA

• TETRA Release 2• Ad-Hoc Networks

08 July 2004



Link Adaptation

• The received signal can be received affected by several type of interference Wireless channel is often affected by multipath-

fading effect A CDMA signal is affected by Multiple Access

Interference• This means that the useful signal strength

varies during the transmission due to the user mobility or number of users within the cell

• The Link Adaptation concept aims to adapt some transmission parameter to the actual channel state with the goal of respect some transmission constraint such as the QoS or the Bit Error rate

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Noise

Multiple ReflectionsDoppler Effect

Attenuation MAI

Base Station

UE

Channel capacity

IN

REC CS

02 1log

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Centralized SystemsDistributed Systems

Wireless Nets Scenarios

• Wireless Nets Centralized Distributed

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Ad Hoc Networks

• The Ad Hoc Networks are: Distributed Wireless With peer-to-peer

connections Without fixed

infrastructure• Advantages:

Absence of infrastructures

Low cost Flexibility and

adaptability• Disadvantages

Distributed systems Routing and access

techniques

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Wireless MAC

• Radio Channel characteristics: Half-duplex Time variant Bursty nature

• Access techniques: CSMA/CA Control

Handshaking Acknowledgemen

ts

Physical

MAC

LLC

Network

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Hidden Node

Exposed Node

Capture Effect

CDB

A

CDB

AHidden Node

Exposed Node

Capture Effect

C

D

B

A

Drawbacks of MAC for distributed networks

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AckData

It is foreseen a CSMA/CA technique with a 4-way handshaking

A B

RTS

CTS

Data

Ack

RTS

CD

BA CTS

Standard IEEE 802.11

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CDMA in AdHoc Networks

• In literature there are four CDMA approaches in Ad-Hoc networks: Common Code All the nodes have a common

code. Receiver-Based Code Each node has its own

Receiver-Based Code. Transmitter-Based Code Each node has its

own Transmitter-Based Code. Pairwise-Based Code Each pair of nodes has

assigned a unique code.

• We have selected herein the Receiver-based Code technique

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Proposed Protocol for CDMA based AdHoc Nws

• In the proposed MAC protocol it is foreseen to adapt the spreading factor for the RTS/CTS packets to the network load.

• Starting from the smallest spreading factor (SF=8), whenever a collision occurs SF is doubled; this leads to: Higher time occupation by RTS/CTS packets, i.e., higher

channel occupancy by each terminal during the contention phase;

Lower MAI with data packet, i.e., higher number of transmitting users.

• The proposed protocol adapts the SF in order to maximize the net throughput taking into account the channel state in terms of interference from other terminals.

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R1T1

RTS

CTS

Data

Ack

Proposed Protocol (Cont’d)

T1 R1

Cr1 (Data)

C (RTS – CTS)

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Proposed Protocol (Cont’d)

no data received

Rx RTS

Rx NACK

Rx ACK

Rx CTS

no CTS received

data message arrived

Send RTS

Tx/RxData

Idle

Send CTS

Backoff

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Simulation Parameters

• Chip Time, Tc=2.5¢10-07 s

• RTS and CTS packets length , Lc=20 Byte

• Data packet length Ld=1460 Byte

• Truncated Pareto traffic model

• Mean number of packets per message, N=25

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Collision Probability

0,00

0,01

0,02

0,03

0,04

0 10 20 30 40 50 60 70 80 90 100

Connectivity [%]

Co

llis

ion

Pro

bab

ilty

Low TrafficCondition

CollisionAvoidance

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Throughput

0,60

0,65

0,70

0,75

0,80

0,85

0,90

0,95

1,00

0 0,1 0,2 0,3 0,4 0,5

Arrival Rate [msg/s]

Th

rou

gh

pu

t

SF 8

SF 16

SF 32

Variable SF

3%

40%

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Delay

0,000,050,100,150,200,250,300,350,400,450,50

0 0,1 0,2 0,3 0,4 0,5


De

lay

[s

]

SF 8

SF 16

SF 32

Variable SF

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Collision Probability

0,000,050,100,150,200,250,300,350,400,450,50

0 0,1 0,2 0,3 0,4 0,5


Co

llis

ion

Pro

ba

bili

ty

SF 8

SF 16

SF 32

Variable SF

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Comparison with IEEE802.11

0,000,100,200,300,400,500,600,700,800,901,00

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7

Arrival Rate

Th

rou

gh

pu

t

IEEE 802.11

Proposed MACProtocol

15%

>50%

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Conclusion & Future Developments

• Conclusions It has been proposed a MAC protocol that exploits the

CSMA/CA technique and the CDMA in order to allow multiple communications at the same time

The Spreading Sequences length is selected in an a adaptive way following the network loading condition

The proposed protocol allows a higher throughput of about 3% for low message arrival rate and about 40% of higher arrival rate for a target throughput

Also in comparison to the classical IEEE 802.11 MAC technique higher performance are allowed

• Future Developments User mobility and power control User priority and type of traffic priority in order to

respect QoS constraints

Documents

Università degli Studi di Firenze 08 July 2004 COST 289 - 5th MCM - Budapest, Hungary 1 Cross-layer design for Multiple access techniques in wireless communications