1. Mandatory Greenfield Preamble
Month Year
doc.: IEEE yy/xxxxr0
July 2006
Mandatory Greenfield Preamble
Date:
Authors:
Notice: This document has been prepared to assist IEEE It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.
Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE
Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures < ieee802.org/guides/bylaws/sb-bylaws.pdf>, including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chair as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE Working Group. If you have questions, contact the IEEE Patent Committee Administrator at
Richard van Nee, Airgo Networks
John Doe, Some Company

2. Introduction Greenfield preamble is optional in draft 1.0
July 2006
Introduction
Greenfield preamble is optional in draft 1.0
Many comments to make it mandatory
This presentation summarizes:
Advantages of Greenfield preamble
Disadvantages of Greenfield preamble being optional instead of mandatory
Significant risk of interoperability problems
Richard van Nee, Airgo Networks

3. GF Preamble Benefits More efficient than Mixed-Mode
July 2006
GF Preamble Benefits
More efficient than Mixed-Mode
Higher throughput for the network
More power efficient
Transmitter and Receiver are “on” less of the time
Largest power consumption reduction (as a percentage) occurs for small, periodic packets such as VoIP packets
Better SNR performance because of longer HT channel training
Better SNR in channel estimates
Translates to SNR gain up to 1.5 dB (65 Mbps SIMO)
Ideal for 40MHz transmissions in 2.4GHz
These transmissions need to be protected from legacy devices
Medium is “reserved for 11n” – by definition a “green field”
Data portion of packet is very small making a short preamble more important
Example: 250byte packet uses 48us with MM preamble and 36us with GF preamble
Richard van Nee, Airgo Networks

4. Preamble Types Mixed Mode 40 us Green- field 28 us .11a
Month Year
doc.: IEEE yy/xxxxr0
July 2006
Preamble Types
L-STS
L-LTS
L-SIG
HT-SIG
8 us
4 us
HT-LTS1
HT-STS
HT-LTS2
Mixed Mode
40 us
Green- field
28 us
.11a
Best antenna deployment for one given room based on ray tracing simulations
Three scenarios (drawing + animation)
Key Benefit of Greenfield Preamble: Efficiency Gain of 30% for the Preamble portion of the transmission
Richard van Nee, Airgo Networks
John Doe, Some Company

5. July 2006
Efficiency Gain of GF Preamble vs Packet Size and Link rate in 20MHz modes
Richard van Nee, Airgo Networks

6. July 2006
Efficiency Gain of GF Preamble vs Packet Size and Link rate in 40MHz mode
Richard van Nee, Airgo Networks

7. Network Conditions that Benefit from a Greenfield Preamble
July 2006
Network Conditions that Benefit from a Greenfield Preamble
Greenfield WLAN deployments:
WLAN in 5GHz band; - Band goes virtually unused in major parts of the world
New CE or dedicated Voice Networks
New office building installations where there is no legacy WLAN present
Homes / Buildings with no neighboring networks (stand alone homes)
Mixed Networks (Legacy + .11n) where Medium Access is managed through MAC protection;
OBSS environments; e.g. dense department buildings
Networks with .11b devices present
.11n Devices operating in 40 MHz mode
WLAN networks around the year 2010
Analysts predict that 4-5 years from now all new equipment will be based on .11n, and legacy equipment will be phased out, creating de-facto GF networks
Richard van Nee, Airgo Networks

8. Network Conditions that Benefit from a Greenfield Preamble (2)
July 2006
Network Conditions that Benefit from a Greenfield Preamble (2)
40MHz mode in 2.4GHz band
Legacy 11g devices cannot receive the 40 MHz MM preamble in the extension channel, so MAC reservation mechanisms must be used
Since MAC reservation mechanisms must be used, the GF preamble for 40MHz transmissions can be used to minimize overhead
In the presence of Legacy .11b
Neither the MM-preamble nor the GF-preamble is coexistent with 11b
In a network with an 11b-only station, MAC reservation mechanisms must be used, e.g., CTS-to-self
In this case, the GF preamble can be used and is more efficient
Richard van Nee, Airgo Networks

9. July 2006
Disadvantages of Greenfield Preamble Being Optional instead of Mandatory
No evolution path towards future 11n-GF-only networks
11n networks will always suffer from extra MM overhead even when no legacy devices are around anymore
Interoperability Problems
11n MM-only devices can get false detects on GF packets
Leads to unstable throughput behavior
Richard van Nee, Airgo Networks

10. False Detects by MM-Only Devices
July 2006
False Detects by MM-Only Devices
What happens when a MM-only device receives a GF preamble?
50% of the time, the L-SIG parity bit will pass
If the erroneously detected rate field is unequal to 6 Mbps, the MM receiver thinks it is a legacy packet and will start RX processing based on some random length field
If the erroneously detected rate field is 6 Mbps, then the check for rBPSK between [20,24]us gives indication of HT transmission as opposed to a legacy packet (gives a false result when the SNR is low)
Check CRC-8 in “HT-SIG” between [20,28]us gives indication of a MM preamble (this check gives a false positive with probability 1/256)
If the MM-device believes there is a legacy packet or MM-packet when a GF packet is transmitted, then it
Will hold CCA busy for some random time
Spend a random time on receive processing
Above will lead to unstable throughput behavior
What is needed to detect both MM and GF
An implementation must check for rBPSK in both [16,20]us and [20,24]us instead of once at [20,24]us  Reuse same hardware
Richard van Nee, Airgo Networks

11. The Lesson Learned from 11b & 11g
July 2006
The Lesson Learned from 11b & 11g
The b amendment introduced an optional GF preamble
There were 11b interoperability problems
Some APs denied service to clients which did not support the short preamble, creating confusion for customers who did not understand why some 11b cards did not work
Problem was solved when WiFi made short preamble mandatory
802.11g introduced mandatory GF preamble
11g specifies an optional MM CCK-OFDM preamble
MAC protection used to deal with legacy 11b
making the OFDM GF preamble mandatory rather than optional avoided interoperability problems and maximized throughput in an 11g-only network
Richard van Nee, Airgo Networks

12. Conclusions GF preamble has many advantages
July 2006
Conclusions
GF preamble has many advantages
Less overhead, better throughput
More power efficient
Better SNR performance
Keeping GF optional instead of mandatory has significant disadvantages
Interoperability problems
No evolution path towards future GF only networks
 GF should be made mandatory
Richard van Nee, Airgo Networks