Mandatory Greenfield Preamble Month Year doc.: IEEE 802.11-yy/xxxxr0 July 2006 Mandatory Greenfield Preamble Date: 2006-07-17 Authors: Notice: This document has been prepared to assist IEEE 802.11. 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 802.11. Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures <http:// 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 <stuart.kerry@philips.com> 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 802.11 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at <patcom@ieee.org>. Richard van Nee, Airgo Networks John Doe, Some Company

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

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

Preamble Types Mixed Mode 40 us Green- field 28 us .11a Month Year doc.: IEEE 802.11-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

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

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

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

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

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

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

The Lesson Learned from 11b & 11g July 2006 The Lesson Learned from 11b & 11g The 802.11b 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

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