1. Analysis on IEEE 802.11n MAC Efficiency
Month Year
doc.: IEEE yy/xxxxr0
September 2007
Analysis on IEEE n MAC Efficiency
Date: 2007/09/13
Authors:
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Minyoung Park, Intel Corp.
John Doe, Some Company

2. Month Year
doc.: IEEE yy/xxxxr0
September 2007
Abstract
This analysis is to understand how efficient current IEEE n MAC protocol is when used for PHY rate of 1Gbps and beyond, and to understand what the challenges are to achieve over 1Gbps MAC throughput with current technology.
Minyoung Park, Intel Corp.
John Doe, Some Company

3. Contention Window (CW)
September 2007
802.11n MAC Protocol
Improving MAC efficiency
Contention Window (CW) pr
RTS pr
CTS pr
DATA pr
ACK
DIFS
SIFS
SIFS
SIFS
preamble CW pr
RTS pr
CTS pr
DATA pr
ACK
DIFS
SIFS
SIFS
SIFS
Block Ack CW pr
RTS pr
CTS pr
DATA …
DATA pr BA
DIFS
SIFS
SIFS
SIFS CW pr
RTS pr
CTS pr
DATA …
DATA pr BA
DIFS
SIFS
SIFS
SIFS pr
DATA …
DATA pr BA
SIFS
SIFS
Minyoung Park, Intel Corp.

4. 802.11n MAC Overhead and Efficiency
September 2007
802.11n MAC Overhead and Efficiency
TXOP
up to 64 subframes
CW (variable)
Actual data
Actual data pr
RTS
SIFS pr
A-MPDU
SIFS pr
A-MPDU
SIFS
DIFS pr
CTS
SIFS pr BA
SIFS pr BA
overhead
overhead
overhead
Total MAC payload (bits)
MAC throughput =
Time consumed transmitting total MAC payload (sec)
MAC throughput
MAC efficiency =
PHY rate
MAC efficiency increases as
TXOP fixed overhead (e.g. DIFS and SIFS)
Number of BAs aggregation overhead
CW (or # of stations) contention overhead
Minyoung Park, Intel Corp.

5. MAC Throughput with Contention
September 2007
MAC Throughput with Contention
Collision overhead
TXOP
CWmax doubled
CW=4
CW=2
CW=8
STA-A
RTS
RTS
SIFS
RTS
SIFS
SIFS
A-MPDU
SIFS
DIFS
DIFS
CTS
DIFS
CTS
SIFS BA
freeze
CWmax doubled
CW=2
CW=10
STA-B
CW=2
RTS
SIFS
A-MPDU
SIFS
RTS
SIFS
RTS
DIFS
CTS
SIFS BA
DIFS
CTS
DIFS
freeze
Aggregate throughput: STA-A + STA-B
overhead
Actual data
Collision overhead
overhead
Actual data
overhead
CW=8
CW=2
RTS
SIFS
A-MPDU
SIFS
RTS
SIFS
RTS
SIFS
SIFS
A-MPDU
SIFS
DIFS
CTS
SIFS BA
DIFS
CTS
DIFS
CTS
SIFS BA
When there is no collision between the stations, the station with the smallest CW will access the channel and thus CW overhead gets smaller as the number of stations increases
Overhead due to collision is shared by all the stations in the network
Minyoung Park, Intel Corp.

6. CSMA/CA Contention Overhead Analysis
September 2007
CSMA/CA Contention Overhead Analysis
Minimum
overhead
Contention overhead = CW overhead + collision overhead
CW overhead decreases as # of STAs increases  STA with smallest CW will access the channel
Collision overhead increases as # of STAs increases
Minyoung Park, Intel Corp.

7. 802.11n MAC Throughput (Single STA)
September 2007
802.11n MAC Throughput (Single STA)
HT 4x4 320MHz TXOP=1, 2, 3ms
HT 4x4 160MHz TXOP=1, 2, 3ms
1Gbps MAC throughput
HT 4x4 80MHz TXOP=1, 2, 3ms
HT 4x4 120MHz TXOP=1, 2, 3ms
BER=1e-5
CW=15
Packet size =1500 Bytes
For 4x4 MIMO, n needs at least >160MHz bandwidth and large TXOP (>1ms)
Minyoung Park, Intel Corp.

8. 802.11n MAC Efficiency (Single STA)
Month Year
doc.: IEEE yy/xxxxr0
September 2007
802.11n MAC Efficiency (Single STA)
HT 4x4 80MHz
HT 4x4 160MHz
HT 4x4 320MHz
HT 4x4 120MHz
BER=1e-5
CW=15
Packet size =1500 Bytes
Efficiency decreases below 50% as PHY rate increases over 3Gbps
Minyoung Park, Intel Corp.
John Doe, Some Company

9. 802.11n MAC Throughput and Efficiency (Multiple STAs)
September 2007
802.11n MAC Throughput and Efficiency (Multiple STAs)
Throughput
Efficiency
PHY rate
Target MAC throughput
802.11n
BER=1e-5
Packet size = 1500 Bytes
TXOP=3ms
Number of STAs: 1~50
Current n requires 2Gbps PHY rate to achieve >1Gbps MAC throughput
Minyoung Park, Intel Corp.

10. MAC Overhead Analysis September 2007
5 STAs (TXOP=3ms)
Varying PHY rate
1040 Mbps (e.g. 4x4 80MHz, TXOP=3ms)
Varying number of STAs
As PHY rate increases, aggregation overhead increases
For a small number of STAs (PHY rate > 1Gbps)
aggregation overhead dominates
For a large number of STAs (PHY rate > 1Gbps)
contention overhead + aggregation overhead dominates
Minyoung Park, Intel Corp.

11. September 2007
Conclusions
The approach to extend n for PHY rate > 1Gbps has MAC efficiency problem
MAC efficiency is ~64% for PHY rate ≈ 1Gbps (5 STAs, TXOP=3ms)
MAC efficiency is ~40% as PHY rate increases to 4Gbps
Contention and aggregation overheads dominate the overall overhead
802.11n needs at least 160MHz BW for 4x4 MIMO to reach 1Gbps MAC throughput
Evolution from n requires higher spectral and MAC efficiency, for example:
PHY rate = 80MHz BW
MAC efficiency = 80%
Performance metric recommended for VHT
Aggregate network throughput (instead of point-to-point throughput)
Minyoung Park, Intel Corp.