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REFERENCE
TM
Wi-Fi
SERIES
[email protected]
P5.v1
802.11n Principles
2007 Xirrus. All Rights Reserved. Wi-Fi is a trademark of the
Wi-Fi Alliance.
Client Shipments Relative Rate and RangeImproved MAC
Throughput
802.11a/b/g Interoperability
Data Rates
Standardization Timeline
MIMO Signal Processing
Glossary
Channel Bonding
Spatial Multiplexing
M
Rx 1
Rx 2
RxM
MTransmitter
Data Stream Data StreamReceiver
Tx 1
Tx 2
TxN
Spatial Multiplexing transmits completely separate data streams
on different antennas (in the same channel) that are recombined to
produce
new 802.11n data rates. Higher data rates are achieved by
splitting the original data stream into separate data streams. Each
separate streamis transmitted on a different antenna (in the same
channel). MIMO signal processing at the receiver can detect and
recover each stream.
Streams are then recombined which yielding higher data
rates.
Source: DellOro Group 11/06
(Millions)
2 00 5 2 00 6
100
50
02007 2008 2009
802.11a/b/g
802.11a/b/g/n
802.11n
802.11a/g
802.11b
MIMO (Multiple In Multiple Out) Signal Processing uses multiple
antennas and takes advantage of multipath reflections to improve
signalcoherence that greatly increases receiver sensitivity. This
extra sensitivity can be used for greater range or highter data
rates.
The newly enhanced signal is the processed sum of individual
antennas. Signal Processing eliminates nulls and fading that any
one
antenna would see. MIMO Signal Processing is sophisticated
enough to discern multiple spatial streams (see Spatial
Multiplexing).
MIMO Digital Signal Processing
Frequency Across Subcarriers
Attenuation
Antenna 1 Signal
MIMO Processed Signal
Antenna 2 Signal
Antenna 3 Signal
Receiver
802.11nA yet to be released IEEE Standard for wirelessnetworking
that has as target of at least 100Mbps of throughput.
Channel BondingUsing two adjacent channels together as
one to increase data rates.Green Field ModeEliminates support
for 802.11a/b/gdevices when only 802.11n devices are present.
MIMO (Multiple In, Multiple Out)Signal processing thatimproves
both range and rate by receiving and transmittingsignals on
multiple antennas.
MIMO Power Save ModeConserves power consumptionby making use of
multiple antennas (and radios) only when
needed.
Spatial MultiplexingTransmitting two or more separate
datastreams on different antennas at the same time in the same
channel to increase data rate, requires 11n adapters on
bothsides of the link.
Wi-Fi AllianceOrganization that certifies 802.11a/b/g/nproducts
for interoperability.
As of December 2006
Profiles and FinalCertification
Initial Draftand Letter
Ballot
20062003 2004 2005 2007 2008
CommentResolution
UnPlug Fests
BaselineCertification
Proposalsand
Down Selection
Joint ProposalDraft SpecsConfirmed
FirstSponsorBallot
SecondLetterBallot
Final WorkingGroup Approval
RevCom Approval
Q 2 Q 3Q1Q4Q3Q2Q1Q4Q3Q2Q1
TGnFormed
IEEE Publishesthe Final
Specification
Wi-Fi Alliance Estimated Timeline
IEEE Estimated Timeline
Note: Wi-Fi Alliance 802.11n Certification will requiredevices
to support two spatial streams and channel bonding.
N
ACK Frame
N
Data Frame
2
Data Frame
PHY Header
MAC Header
Data Frame Payload
ACK Frame Payload
Frame AggregationMAC data frames are combined and given a single
PHY header
Implicit Block ACK acknowledges all data frames within aggregate
New 802.11n modes are 40% more efficient than legacy modes.
Legacy Operation
High Throughput Operation
Data Frame
1
D ata Frame Blo ck A CK Fr ame
1 12 2N N
ACK Frame
1 2
ACK Frame
N N
Data Frame
SIFS
SIFS
SIFS
RIFS RIFS
Legacy Operation
High Throughput Operation
Data F ra me ACK F ra me
1
1 2
Data Frame Data Frame Data Frame
Block ACKRequestFrame
Block ACKResponseFrame
1 2 N
1 2 2
2
Data F ra me ACK F ra me
N
ACK Frame
RIFS Usage (Reduced Inter-Frame Spacing)
Legacy Only
High Throughput withLegacy Protection
Derive CCADuration fromLength & Rate
Derive CCADuration fromLength & Rate
CCA Duration = (Legacy Length/Legacy Rate)
Legacy Tra ining F ields Legacy Signal F ield HT Signal F ield
HT Tra ining F ields HT PSDU
Legacy Training Fields Legacy Signal Field Legacy PSDU
When an HT frame is transmitted in Mixed Mode, the L-SIG field
provides rate and length values for the transmitted packet.
The L-SIG rate and length values are used by a legacy STA
Station to set the CCA.
The L-SIG rate is set to 6Mbps and the length is spoofed to
cover the remainder of the packet.
The L-SIG field (Legacy Signal Field) sets the Clear Channel
Assessment (CCA) on legacy devices so they do not attempt to
transmit duringa HT (High Throughput) frame transmission.
PHY Level Spoofing (Protects Legacy Devices)
CCA Duration = (Legacy Length/Legacy Rate)
Ch# Ch# 40MHz Channel AllocationExample Channels that can be
Bonded
FrequencyChannel 1 Channel 2 f, (MHz) U.S. EU
36 40 5190 X
44 48 5230 X
52 56 5270 X
60 64 5310 X
100 104 5510 X
108 112 5550 X
116 120 5590 X
124 128 5630 X
132 136 5670 X
149 153 5755 X
157 161 5795 X
40MHz channels are specified by two fields as (Nprimary_ch,
Secondary)
where the first field represents the primary channel number and
thesecond field indicating whether the secondary channel is above
(1) or
below (-1) the primary channel. Channel Bonding wont
increaseaggregate capacity and can make channel planning more
difficult.
40
20 MHz 20 MHz
36
40 MHz
(40, -1)
Standard 802.11 channels areeffectively 20MHz wide.
Channel bonding combinestwo adjacent 20MHz channels
into a single 40MHz channelproviding increased throughput.
E xp ec ted Fi rs t Ge ne ra tio n De vi ce Da ta R at es E xp
ect ed S ec on d Ge ne ra tio n De vi ce Da ta R at es
Optionally
Multiply By 2.077
to Bond
Two 20MHz
Channels
Optionally
Multiply by 1.11 for
Shorter Guard Interval
to Increase
Symbol RateNote: Wi-Fi Alliance 802.11n Certifica-tion will
require devices to support twospatial streams and channel
bonding.
Optionally
Multiply by 2, 3, 4
for the Number of
Additional
Spatial Streams
Choose New Base
Encoding + Modulation
BPSK (6.5)
QPSK (13, 19.5)
QAM-16 (26, 39)
QAM-64 (58.5, 65)
New 11n
Data Rate
Obtaining 802.11nData Rates
x x x =
Expected 802.11n Data Rates
802.11a 802.11gRates
6
9
12
18
24
36
48
54
11n MandatoryData Rates
6.5
13
19.5
26
39
52
58.5
65
With ChannelBonding (40MHz)
13.5
27
40.5
54
81
108
121.5
135
With ShortGuard Interval
15
30
45
60
90
120
135
150
Two SpatialStreams
13
26
39
52
78
104
117
130
With ChannelBonding (40MHz)
27
54
81
108
162
216
243
270
With ShortGuard Interval
30
60
90
120
180
240
270
300
Three SpatialStreams
19.5
39
58.5
78
117
156
175.5
195
With ChannelBonding (40MHz)
40.5
81
121.5
162
243
324
364.5
405
With ShortGuard Interval
45
90
135
180
270
360
405
450
Four SpatialStreams
26
52
78
104
156
208
234
260
With ChannelBonding (40MHz)
54
108
162
216
324
432
486
540
With ShortGuard Interval
60
120
180
240
360
480
540
600
One Spatial Stream Two Spatial Streams Three Spatial Streams
Four Spatial Streams
