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 Analysis And Performance Evaluation Conclusion

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

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

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

MIFS < CIFS < RIFS

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 th

e 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

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

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

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

Analytical Modeling

= of subchannel i

Number of backoff

Throughput

802.11

CSMA/CA

CSMA/CA with RTS/CTS

Model Validation

CSMA/CA vs. OFDMA/CSMA

Maximum Throughput

Effect of Increasing The Load

32 stations

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