doc.: IEEE 802.11-11/1544r0

Submission

November 2011

Ron Porat, Broadcom

Initial Proposal for TGaf PHY

Date: 2011-11-08Authors:

Name Affiliations Address Phone email Ron Porat Broadcom 16340 West Bernardo Dr., San

Diego, CA 92127 858-521-5409 rporat@broadcom.com

Vinko Erceg Broadcom 16340 West Bernardo Dr., San Diego, CA 92127

Slide 1

doc.: IEEE 802.11-11/1544r0

Submission

November 2011

Ron Porat, Broadcom

Outline

• Propose initial PHY details for TGaf based on TGac with possible enhancements based on TGah.

Slide 2

doc.: IEEE 802.11-11/1544r0

Submission

November 2011

Ron Porat, Broadcom

Overview

• It was decided to base TGaf PHY on TGac during the May meeting [1]• TGac provides many useful features to TGaf such as

– Support for 4 different BW options (20/40/80/160MHz)– Support for non-contiguous transmission (80+80) – important for cases where spectrum

availability is fragmented such as in large metropolitan areas– DL MU-MIMO – important for increasing network throughput especially if MIMO spatial

multiplexing doesn’t work due to high antenna correlation or one antenna devices – both are expected in 11af

• In order to minimize time to market we envision direct down clocking of 11ac PHY with minimum modifications to accommodate the specific requirements of TGaf

• The following slides provide some specifics on the required modifications

Slide 3

doc.: IEEE 802.11-11/1544r0

Submission

November 2011

Ron Porat, Broadcom

Downclocking (DC) Ratio

• Down clocking of 11ac PHY provides a simple path from operating 20MHz and higher channels to narrower channels. TGaf requires operation within 6 and 8MHz channels in most countries.

• A byproduct of downclocking is an increase in the OFDM symbol time providing increased delay spread immunity which is expected to be required in TGaf due to deployments of AP at higher elevation to cover large outdoor areas (e.g. cellular offload where WiFi antennas are collocated with cellular antennas)

• In [2], delay spread measurements conducted by NTT were performed in the 5GHz band in Tokyo. Delay spread was found to be up to 350nS and the authors conclude that downclocking by 2 will provide sufficient guard interval (1.6uS)

• [3] provides guidelines for simulating 4G cellular networks and the expected channel rms delay spread is compared against current 802.11 channels in the following slide. Center frequency is assumed at 2GHz

Slide 4

doc.: IEEE 802.11-11/1544r0

Submission

November 2011

Ron Porat, Broadcom

Cont. • The 4G Urban Micro/Macro channels exhibit on average 129/365ns RMS delay spread. • Urban Micro deployments are expected with antenna height at or about the height of the

surrounding buildings. • Urban/Suburban Macro deployments are expected with antenna height clearly above the

surrounding buildings.• It is seen that delay spread up to2-3 times of current 802.11 can be expected if WiFi is collocated with MacroCellular deployments

Slide 5

Scenario RMS DS(ns) Scenario RMS DS

(ns)

4G InHLoS 20

NLoS 39 TGn B 15

4G UMi

LoS 65 TGn C 30

NLoS 129 TGn D 50

O-to-I 49 TGn E 100

4G SMaLoS 59 TGn F 150

NLoS 75

4G UMaLoS 93

NLoS 365

4G RMaLoS 32

NLoS 37

UMi – Urban MicroSMa – Suburban MacroUMa – Urban MacroInH – Indoor HotspotRMa – Rural MacroLoS – Line of SightNLoS – Non Line of SightO-to-I – Outdoor to Indoor

doc.: IEEE 802.11-11/1544r0

Submission

November 2011

Ron Porat, Broadcom

Cont. • Ericsson 3G WCDMA measurements [8] in several cities reveal median rms

delay spread of 100nS in most cities and under 200nS in one city• Summary - Based on the rms delay spread numbers shown in the above

references, possible DC ratios that lead to 8MHz channels are 2.5 and 5 (leading to guard interval of 2/4uS respectively).

• While both options are possible there is a preference for using DC ratio 5 for the following reasons:– TGaf will operate at frequencies as low as 500MHz where delay spread may be

longer due to better propagation conditions. – Higher DC ratio naturally provides lower bit rates – with DC ratio of 2.5 the lowest

64FFT bit rate will be 2.6Mbps compared with 1.3Mbps for DC ratio of 5.

• DC ratios higher than 5 cause rapid deterioration in system efficiency due to the increase in fixed overhead arising from longer preambles, beacons, ACK etc. A detailed analysis is provided in Appendix A showing the general negative effect of downclocking on MAC level Tput .

Slide 6

doc.: IEEE 802.11-11/1544r0

Submission

November 2011

Ron Porat, Broadcom

Cont. • In the US channelization is 6MHz. While DC ratio of 4 seems like a good

choice by creating 5MHz channels with sufficient delay spread immunity, we propose to use the same DC ratio of 5 for the following reasons:– Current regulations require 55dB signal attenuation at the band edge far exceeding

current 802.11 spectral mask requirement. It is expected that in order to meet those requirement the actual signal BW must be much lower

– One DC ratio is preferable for implementation reasons (note that 11ah converged on one DC ratio as well)

• In addition we note the following: – Many outdoor WiFi systems are currently deployed in many regions (using 11n with DC=1)

[5][6]– Delay spread can exceed the GI without impairing low QAM transmissions

• Hence we conclude that one DC ratio of 5 can provide a good solution for 8MHz TV channels such as in Europe and China and 6MHz channels with very stringent mask requirements such as in the US

Slide 7

doc.: IEEE 802.11-11/1544r0

Submission

November 2011

Ron Porat, Broadcom

Extended Range / Lower Rates • With the current proposal for DC ratio 5 the lowest rate achieved by the

64FFT signal is 1.3Mbps in a 4MHz BW.• Lower bit rates may benefit some applications such as sensors or M2M by

allowing them to be farther from the AP.• We propose to leverage the decision in 802.11ah to include a 32FFT PHY if

providing lower rates for 802.11af is desirable• The addition of an optional 32FFT can be used to operate on narrower

channels (2MHz) or in the case of the US where a PSD limit exists as a DUP mode where the 32FFT waveform is duplicated across the 4MHz BW to provide lower rate (600kbps or 300kbps)

• Current details of the 32FFT PHY frame format are under discussion in TGah and may finalize by the January meeting, after which they may be introduced into TGaf

Slide 8

doc.: IEEE 802.11-11/1544r0

Submission

November 2011

Ron Porat, Broadcom

Frame Format• The proposed frame format is the same as adopted for 11ac with minimum

naming changes (see figure in Appendix B)• The format is shown below where the L- prefix was replaced by O to denote

Omni and the VHT- prefix was removed• The first three omni fields serve as a general ‘control channel’ since they are

also important for the unintended recipients of the packet in order to properly decode the packet length (see discussion in Appendix C)

• Beamforming is allowed starting from the STF1 field • The first two SIG fields in 11ac were merged into one SIG field with possible

TBD modifications to its content.

Slide 9

O-STF O-LTF O-SIG-A DATA(2 symbols) (2 symbols) (3 symbols)

(1 symbol each)

LTF1 … LTFNSTF1 SIGB

(1 symbol)

doc.: IEEE 802.11-11/1544r0

Submission

November 2011

Ron Porat, Broadcom

Support for Two Non Contiguous Channels• Current TGac supports non-contiguous 80+80 channels (using the 256FFT).• In TGaf fragmentation of 6MHz channels in urban areas in the US (and

perhaps other regions) is expected [7]. • In order to provide higher bit rates we propose to add an optional non-

contiguous mode specific to TGaf using the 6MHz channels (64FFT) allowing usage of two non-contiguous 6MHz channels. This mode may replace the inherited non contiguous mode based on 256FFT

• Using the non-contiguous 6MHz mode when two adjacent 6MHz channels are available solves co-existence issues between 6 and 12MHz channels

• Rural areas have a lot of available channels and usage of four contiguous channels (256FFT) is possible.– It is crucial that rural areas can use wider BW as providing broadband to

underserved communities is one of the goals of TVWS

Slide 10

doc.: IEEE 802.11-11/1544r0

Submission

November 2011

Ron Porat, Broadcom

Summary

• TGaf PHY description is initiated in this document and a new section can be created with modifications as described

• More details are expected in the next meeting

Slide 11

doc.: IEEE 802.11-11/1544r0

Submission

November 2011

Ron Porat, Broadcom

Straw Poll 1• Do you support downclocking of 11ac PHY with DC ratio 5 and with naming

changes as in slide 9 for TGaf?

• Y• N• A

Slide 12

doc.: IEEE 802.11-11/1544r0

Submission

References[1] 11-11-1334-02-00af-closing-report-okinawa-september-2011.ppt

[2] 11-03-0522-02-000j-multipath-delay-profiles-in-5ghz-band.ppt[3] Report ITU-R M.2135-1 (12/2009) Guidelines for evaluation of radio interface technologies

for IMT Advanced[4] 3GPP TR 25.996 - Technical Specification Group Radio Access Network; Spatial channel

model for Multiple Input Multiple Output (MIMO) simulations, Section 5[5] http://www.towerstream.com/MobileDataOffload.aspx[6] http://

www.att.com/gen/press-room?pid=21918&cdvn=news&newsarticleid=33200&mapcode=consumer%7Cmk-att-wi-fi

[7] 11-11-0499-03-00af-us-metro-mhzpops.xls[8] How typical is the "Typical Urban" channel model? Ericsson Research

November 2011

Ron Porat, BroadcomSlide 13

doc.: IEEE 802.11-11/1544r0

Submission

November 2011

Ron Porat, Broadcom

Appendix A - Effect of Down clocking on MAC Tput

• In packet based systems such as 802.11, for a given transmission BW, having higher FFT size with a fixed PPDU length in [uS] can reduce the MAC Tput since there is a fixed overhead arising from the preamble and other control/feedback packets– Fixed overhead is in terms of OFDM symbols, not uS,  (e.g., one symbol for LTF), so the fraction of

time used for overhead increases as down clocking/OFDM symbol length increases• We compare 11ac 20MHz with down clocking by 4 to 40MHz with down clocking by 8 – both

result in a theoretical 11af 5MHz BW system– Antenna configuration 4x1 with 3 users co-paired for MU-MIMO

• Tone grouping for feedback - Ng=2– Data assumed to run using MCS9 and control using MCS0– SU Preamble consists of 8 symbols– MU-MIMO packet preamble adds 3 symbols– Slot time = 5+CLOCK_RATIO*4[uS]– SIFS time = CLOCK_RATIO*8[uS]– Feedback at 10msec intervals, uses MCS9– CWMIN=15

• Based on the results we must carefully choose the DC ratio to avoid high MAC Tput loss

Slide 14

doc.: IEEE 802.11-11/1544r0

Submission

November 2011

Ron Porat, Broadcom

SU/MU Results • The tables provide a list of the major contributors to overhead in the system• It is readily seen that downclocking reduces efficiency compared to 11ac and much more with

higher ratio

SU 11af Data/Preamble/Backoff/Difs/Sifs 11af relative to 11ac

DownClk=4, PPDU=1mS 62.2/18.1/11.2/5.2/2.3% 75%

DownClk=8, PPDU=1mS 42.3/28.9/15.7/7.8/3.6% 51%

DownClk=4, PPDU=2mS 77.8/10.6/6.5/3.1/1.3% 86%

DownClk=8, PPDU=2mS 63.3/18.4/10/5/2.3% 70%

MU11af

Data/BAR/Preamble/Backoff/Difs/Sifs/FBP11af relative

to 11acMU Gain vs.

SU

DownClk=4, PPDU=2mS 45/7.1/25.6/5.2/2.4/5.3/1.4% 65% +73%

DownClk=4, PPDU=5mS 65.1/3.9/17.1/3.2/1.5/3.2/1.2% 78% +127%

DownClk=8, PPDU=2mS 26.7/4.1/40.7/6.7/3.3/7.7/2.8% 42% +26%

DownClk=8, PPDU=5mS 42/2.3/31.1/5/2.5/5.8/3.1% 54% +53%

Slide 15

doc.: IEEE 802.11-11/1544r0

Submission

November 2011

Ron Porat, Broadcom

Appendix B – 11ac Frame Format

Slide 16

doc.: IEEE 802.11-11/1544r0

Submission

November 2011

Ron Porat, Broadcom

Appendix C – Omni Preamble • The first portion of the preamble is denoted by O- (for omni) replacing the L- prefix

since although there is no legacy 802.11 systems in this band, it is still important to use only CDD precoding to avoid unintentional beamforming.

• Unintentional beamforming is caused when the transmitter beamforms the preamble for the intended user causing a reduction of power for unintended users in locations ‘spatially orthogonal’ to the intended user.

• This is especially problematic with correlated antennas (expected in TGaf due to low carrier frequency) where users’ spatial signature remains stable across the BW and where one parameter (AoD) dominates the level of interference between users - hence it is more likely for users with AoD separation to see higher power loss with beamforming than with omni transmission such as CDD where signal energy is spread in all directions

• The following slides use the SCM channel model [4] adopted for use in 802.11ah (and in cellular 3G/4G simulations) to demonstrate the undesirable effect of unintentional beamforming on SIG field decoding.

Slide 17

doc.: IEEE 802.11-11/1544r0

Submission

November 2011

Ron Porat, Broadcom

Scenarios Description• We look at scenarios where users are spread in 360 degrees, 60 degrees around the antenna broadside,

60 or 0 degrees around the line connecting the antennas and 60 degrees at the opposite of the broadside

APIntended BFee

Unintended BFee for 60, AoD = 0

Unintended BFee for 60, AoD = 90

Unintended BFee for 60, AoD = 180

Unintended BFee for 360

Slide 18

doc.: IEEE 802.11-11/1544r0

Submission

November 2011

Ron Porat, Broadcom

SCM Suburban Macro Results

• If unintended BFee are concentrated around the broadside (BF 60 with AoD=0 ) there is BF gain – Spatial signature to unintended BFee is

similar to intended BFee • If unintended BFee is 90 degrees away there is a

17-20dB power loss• For users randomly spread across cell (360)

there is a 12 dB loss

• Quantization of the BF feedback (as shown in the black line with circles) doesn’t reduce loss

We compare SIG field PER assuming CDD and BF for the intended user.Severe reduction of some users’ ability to decode the SIG field when BF is applied is seen causing fluctuation in the specific users that will be able to decode the SIG field depending on whether the packet is BF (and to which specific user) or not.

0 5 10 15 20 25 30 35

10-2

10-1

SIG PER SCM Suburban Macro

[dB]

CDDBF 360BF 60 AoD=0BF 60 AoD=90BF 0, AoD=90BF 60, AoD=90 w/Q

Slide 19