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Cross-layer design for Multiple access techniques in wireless communications. Daniele Tarchi University of Florence, Italy E-mail: [email protected] Outline. Link Adaptation Adaptive Modulation and Coding VSF-CDMA TETRA Release 2 Ad-Hoc Networks. Link Adaptation.

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Cross-layer design for Multiple access techniques in wireless communications

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Cross layer design for multiple access techniques in wireless communications l.jpg

COST 289 - 5th MCM - Budapest, Hungary

Cross-layer design for Multiple access techniques in wireless communications

Daniele Tarchi

University of Florence, Italy

E-mail: [email protected]


Outline l.jpg

Outline

COST 289 - 5th MCM - Budapest, Hungary

  • Link Adaptation

    • Adaptive Modulation and Coding

    • VSF-CDMA

  • TETRA Release 2

  • Ad-Hoc Networks


Link adaptation l.jpg

Link Adaptation

COST 289 - 5th MCM - Budapest, Hungary

  • 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


Channel capacity l.jpg

Channel capacity

MAI

Attenuation

Noise

Multiple Reflections

Doppler Effect

COST 289 - 5th MCM - Budapest, Hungary

Base Station

UE


Wireless nets scenarios l.jpg

Wireless Nets Scenarios

COST 289 - 5th MCM - Budapest, Hungary

  • Wireless Nets

    • Centralized

    • Distributed

Centralized Systems

Distributed Systems


Ad hoc networks l.jpg

Ad Hoc Networks

COST 289 - 5th MCM - Budapest, Hungary

  • 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


Wireless mac l.jpg

Wireless MAC

Network

LLC

MAC

Physical

COST 289 - 5th MCM - Budapest, Hungary

  • Radio Channel characteristics:

    • Half-duplex

    • Time variant

    • Bursty nature

  • Access techniques:

    • CSMA/CA

    • Control Handshaking

    • Acknowledgements


Drawbacks of mac for distributed networks l.jpg

Drawbacks of MAC for distributed networks

D

A

C

B

B

D

C

A

COST 289 - 5th MCM - Budapest, Hungary

  • Hidden Node

  • Exposed Node

  • Capture Effect

Hidden Node

A

Capture Effect

C

B

D

Exposed Node


Standard ieee 802 11 l.jpg

Standard IEEE 802.11

D

A

B

C

B

A

RTS

Data

CTS

Ack

COST 289 - 5th MCM - Budapest, Hungary

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

RTS

CTS

Data

Ack


Cdma in adhoc networks l.jpg

CDMA in AdHoc Networks

COST 289 - 5th MCM - Budapest, Hungary

  • 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


Proposed protocol for cdma based adhoc nws l.jpg

Proposed Protocol for CDMA based AdHoc Nws

COST 289 - 5th MCM - Budapest, Hungary

  • 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.


Proposed protocol cont d l.jpg

Proposed Protocol (Cont’d)

C (RTS – CTS)

RTS

Cr1 (Data)

CTS

Data

Ack

COST 289 - 5th MCM - Budapest, Hungary

T1

R1

T1

R1


Proposed protocol cont d13 l.jpg

Proposed Protocol (Cont’d)

COST 289 - 5th MCM - Budapest, Hungary


Simulation parameters l.jpg

Simulation Parameters

COST 289 - 5th MCM - Budapest, Hungary

  • 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


Collision probability l.jpg

Collision Probability

COST 289 - 5th MCM - Budapest, Hungary

Low Traffic

Condition

Collision

Avoidance


Throughput l.jpg

Throughput

COST 289 - 5th MCM - Budapest, Hungary

3%

40%


Delay l.jpg

Delay

COST 289 - 5th MCM - Budapest, Hungary


Collision probability18 l.jpg

Collision Probability

COST 289 - 5th MCM - Budapest, Hungary


Comparison with ieee802 11 l.jpg

Comparison with IEEE802.11

COST 289 - 5th MCM - Budapest, Hungary

15%

>50%


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Conclusion & Future Developments

COST 289 - 5th MCM - Budapest, Hungary

  • 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


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