Hybrid ofdma csma based medium access control for next generation wireless lans
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Hybrid OFDMA/CSMA Based Medium Access Control for Next-Generation Wireless LANs. Yaser Pourmohammadi Fallah, Salman Khan, Panos Nasiopoulos, Hussein Alnuweiri IEEE ICC 2008 學生:黃志堅. Outline. Introduction OFDMA System Specifications Hybrid OFDMA/CSMA MAC Design

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Hybrid OFDMA/CSMA Based Medium Access Control for Next-Generation Wireless LANs

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Hybrid ofdma csma based medium access control for next generation wireless lans

Hybrid OFDMA/CSMA Based Medium Access Control for Next-Generation Wireless LANs

Yaser Pourmohammadi Fallah, Salman Khan, Panos Nasiopoulos, Hussein Alnuweiri

IEEE ICC 2008

學生:黃志堅


Outline

Outline

  • Introduction

  • OFDMA System Specifications

  • Hybrid OFDMA/CSMA MAC Design

  • Analysis And Performance Evaluation

  • Conclusion


Introduction

Introduction

  • The efficiency and scalability of WLAN is greatly dependent on multiple access (MA)

  • Address low CSMA\CA MAC efficiency under heavy loading conditions in crowded WLANs

  • Support multimedia traffic and variable length packet transmission in the MAC

  • Use orthogonal frequency division multiple access (OFDMA) to solve


Ofdma system specifications

OFDMA System Specifications


Ofdma system specifications1

OFDMA System Specifications

  • Subchannels formed from grouping subcarriers together

    • may be adjacent

      • higher throughput in fixed, portable and low mobility

    • may be distributed

      • high throughput in mobility applications


Hybrid ofdma csma mac design

HYBRID OFDMA/CSMA MAC DESIGN

  • Their proposed MAC uses a two stage frame delivery process

    • Transmission opportunity Request (TR) phase

    • Scheduled data Transmission (ST) phase

  • OFDMA is used in the TR phase and the ST phase uses controlled CSMA


Mac operation timeline

MAC Operation Timeline

  • MIFS < CIFS < RIFS


To request tr phase 1 2

TO Request (TR) Phase 1/2

  • AP keeps track of the number of associated stations (Q) and active stations (n)

    • first assigns the active stations, then distributes the rest of the stations

  • Assign number of (more than one) station to each subchannel

  • To achieve even higher performance

    • AP can dynamically change the number of subchannels

    • assign stations to different subchannels


To request tr phase 2 2

TO Request (TR) Phase 2/2

  • Each subchannel uses its own separate CSMA/CA procedure

  • After a station transmits a TR message, it will wait for a response from the AP

    • a poll

    • position in the schedule

  • If no response is received, the station will interpret this as a collision (or lost packet)

    • doubles its contention window size

    • selects new random backoff number


Scheduled transmission st phase 1 2

Scheduled Transmission (ST) Phase 1/2

  • All subchannels in the ST phase are assigned to only one station at a given time

  • The schedule and order of access are enforced

    • broadcast messages indicating the schedule

    • explicit poll (TO assignment) messages


Scheduled transmission st phase 2 2

Scheduled Transmission (ST) Phase 2/2


Quality of service and multimedia support

Quality of Service and Multimedia Support

  • To enable QoS and multimedia provisioning for the proposed MAC

    • prioritized random access

    • scheduled guaranteed access

  • Provide priority services, we specify different limits for contention window sizes

  • Also required that the access point schedule TOs for higher priority ahead of the others


Parameters used for simulation

PARAMETERS USED FOR SIMULATION


Analytical modeling

= of subchannel i

Analytical Modeling

Number of backoff


Throughput

Throughput


802 11

802.11

  • CSMA/CA

  • CSMA/CA with RTS/CTS


Model validation

Model Validation


Csma ca vs ofdma csma

CSMA/CA vs. OFDMA/CSMA


Maximum throughput

Maximum Throughput


Effect of increasing the load

Effect of Increasing The Load

32 stations


Conclusion

Conclusion

  • The MAC protocol they proposed combines OFDMA with CSMA/CA mechanisms

  • Compared to CDMA systems, combat fading with less complexity

  • Compared to TDMA systems, it suitable for a combination of data and multimedia traffic

  • Another research subject that can be based on the design of scheduling algorithms


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