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Chapter 18 High Throughput and
802.11n
802.11 n history
MIMO
HT Channels
HT PHY
HT MAC
HT Operation
Exam Essentials
• Define the differences between MIMO and SISO.
– Understand that SISO devices use only one radio chain,
whereas MIMO systems use multiple radio chains.
• Understand spatial multiplexing.
– Describe how SM takes advantage of multipath and sends
multiple spatial streams resulting in increased throughput.
• Explain MIMO diversity.
– Be able to explain the differences between simple switched
diversity and the advanced diversity used by MIMO antenna
systems. Explain the use of maximal ratio combining with MIMO
diversity.
• Understand transmit beamforming.
– Explain how optional transmit beamforming can be used to steer
beams in an optimal path toward a receiving radio and the
benefts of the beamforming process.
Exam Essentials
• Understand 20 MHz and 40 MHz channels.
– Understand legacy 20 MHz channels, 20 MHz HT
channels, and 40 MHz channels and how they use
OFDM. Explain why 40 MHz channels work best in
the 5 GHz UNII bands. Explain primary and
secondary channels.
• Explain the guard interval.
– Describe how the guard interval compensates for
intersymbol interference. Discuss the use of both 800and 400-nanosecond GIs.
• Understand modulation coding schemes.
– Explain how modulation coding schemes are used to
define data rates and all the variables that can affect
the data rates.
Exam Essentials
• Explain the three HT PPDU formats.
– Describe the differences between non-HT
legacy, HT Mixed, and HT Greenfield.
• Understand HT MAC enhancements.
– Explain how the use of A-MSDU, A-MPDU,
block ACKs, and RIFS are used to increase
throughput at the MAC sublayer. Define the
two new power-management methods used
by HT radios.
• Explain the HT protection modes.
– Describe the differences between protection
modes 0–3. Explain the use of Dual-CTS.
802.11 n HT
• High Throughput technology
– New PHY and MAC specs
– 100 Mbps or greater
• Use MIMO
– Multiple radios and antennas
– USE multipath to advantage
• Different modes of operations
– Co exists with older networks
Pg 589
802.11 n History
• Define data rates with modulation and
coding schemes (MCS)
• Goal is to increase data rates in both 2.4
and 5 ghz
– Potential of 600 mbps
Pg 589
802.11 n Draft
• HT clause 20 radio with MIMO and OFDM
• Must be backward compatible with
– HR-DSSS
– ERP
Pg 590
WiFi Alliance
• 802.11 n draft 2 certified
• Mandatory and tested implementations
• Many vendors had pre 802.11n equipment
Pg 590
MIMO
• Multiple in, Multiple out
• Mutiple radios AND antennas
– Radio Chains
• Also allows for spatial multiplexing
• Transmit beamforming can be used to
“steer” beams for greater coverage
Pg 592
Radio Chains
• Radio chain is single radio and associated
antennas
– Previous technologies were single input single
output
• MIMO has multiple radio chains
– Each radio with own antenna
– 2x3 MIMO 3 radios, with 2 transmitters and 3
receivers
Pg 592
Radio Chains
•Multiple Transmitters provides for more data transmission
through Spatial Multiplexing
•Multiple Receivers gives increased signal to noise ration because
of MIMO antenna diverstiy
•Up to 4x4
•Each Radio chain takes power
•2x2 needs less power than 4x4
Pg 592
Spatial Multiplexing
• Multipath is caused by two or more paths
of same signal arriving in close time, but
out of phase
• MIMO uses the variation in arrivals to
transmit MORE data
Pg 593
Spatial Multiplexing
• MIMO radios transmit multiple radio signals at the same time and
take advantage of multipath.
• Each radio signal is transmitted by a unique radio and antenna of
the MIMO system.
– Each signal is known as a spatial stream,
– each unique stream can contain different data than the other streams
transmitted by one or more of the other radios.
– Each stream will also travel a different path, because there is at least a
half-wavelength of space between the multiple transmitting antennas.
• Multiple streams follow different paths to the receiver because of the
space between the transmitting antennas is known as spatial
diversity.
• Sending multiple independent streams of unique data using spatial
diversity is often also referred to as spatial multiplexing (SM) or
spatial diversity multiplexing (SDM).
Pg 593
Spatial Multiplexing
• Using spatial multiplexing can greatly
increase throughput
– Each transmission is a multiplier of speed
– IF full transmission is received
• You WANT the signals to arrive at different
times
– Take advantage of multipath
Pg 593
Spatial Multiplexing
• Each stream can use the same, or
different modulation techniqus
Pg 593
MIMO Diversity
• Antenna Diversity helps to reduce effect of
multipath
– Single radio with multiple antennas
• MIMO takes advantage of multipath
– Multiple radios with own antennas
– Radio Chains
• Receive Diversity looks for best received signal
– Maximal Ratio Combining will look for best signal by
adding the received information together
Pg 594
MIMO Diversity
• MRC is best when going from Non-MIMO
to MIMO
Pg 594
Transmit Beamforming (TxBF)
• Optional PHY capability in 802.11n
– Phased array or smart antenna
• Switched array
– Fixed beam patterns
• Adaptive Array
– Maneuvers beam to targeted receiver
• Allows transmitter to “focus” signal
– Arrange transmissions to create constructive
multipath
– Transmitter must know details about receiver
Pg 595
Transmit Beamforming (TxBF)
• Emulate a high gain unidirectional antenna
• Results in higher throughput
• Could be used in conjuction with Spatial
Diversity Multiplexing (SDM)
– Restricted to situations with matching
antennas numbers
• Most likely to be used where SDM is not
an option
Pg 595
Transmit Beamforming (TxBF)
• Transmitter (beamformer) will use
sounding frames to gather information
from receiver (beamformee)
• Implicit feedback requires the transmitter
to analyze the receivers stream
• Explicit feedback will have the receiver do
some of the thinking as well
Pg 595
Transmit Beamforming (TxBF)
Pg 595
HT Channels
• OFDM is used in both 2.4 Ghz and 5 Ghz range
– Clause 20 radios-HT
• 802.11a and g use 20 Mhz OFDM channels
– 52 subcarriers with 4 pilot channels
• HT can use 20 or 40 Mhz channels
– 20 Mhz Channel has 56 subcarriers with 4 being
pilots
– Slightly higher througput
Pg 597
HT Channels
Pg 597
40 Mhz Channels
• Creates 114 subcarriers
– Six used for pilot
• Effectively doubles throughput
• Combines two 20 Mhz channels (bonded)
– Primary and secondary channels
• Positive is one channel above
• Negative is one channel below
• Allows use of additional bandwidth
– Reserved space at top of primary and bottom of
secondary
Pg 599
40 Mhz Channels
• Works well for 5 Ghz range
• Not as well for 2.4 Ghz
Pg 599
Guard Interval (GI)
• Each OFDM Symbol contains 288 bits
– 216 of data and 72 error correction
• 800 nanosecond Guard interval between symbols is
designed to counteract intersymbol interference
• Normal delay spread is 50 to 100 nanaoseconds, max of
200
Pg 602
Guard Interval (GI)
• HT can use 400 nanosecond GI
• Increase throughput
– Risk of intersymbol interference
– Look for retransmissions
Pg 602
Modulation and Coding
• Data rates are defined by modulation and coding
scheme (MCS)
– Based on modulation, number of spatial streams
(antennas) channel size and guard interval
• 77 schemes exist
• 8 mandatory modulation schemes
– Like basic/required rates
• Up to 600 mbps
– With 400 ns GI,4 spatial streams and 64-QAM
Pg 603
Modulation and Coding
Pg 603
HT PHY
• The MSDU is data from layer 3-7
• MPDU is MSDU with 802.11 header (layer
2)
• With Physical layer preamble and PHY
header, this is the PPDU
• Preamble is used to synchronize radios
• PHY Header gives info about transmitting
MPDU
• 3 PPDU structures
Pg 605
HT PHY
Pg 605
HT PHY
• Non-HT Legacy
– Same as 802.11a and g formats
• HT Mixed
– Contains non-HT short and long training symbols so
legacy systems can understand
– Also has HT symbols
– Broadcast traffic must go out on 20 Mhz channels for
backward compatibility
• HT Greenfield
– HT only
– optional
Pg 605
HT MAC
• New enhancements to MAC for throughput
and power management
• Frame aggregation
• Power management
Pg 607
A-MSDU
• MSDU aggregation
• Send multiple MSDU with single MAC header
– Creates new MPDU
– Single destination
• Must be same 802.11e service access category
Pg 607
A-MPDU
• MPDU aggregation
• Send multiple MPDU with single PLCP header
– Single PHY preamble and header
• Must be same 802.11e service access category
• Each MPDU has separate encryption
• Less saved overhead
Pg 607
MTBA and RIFS
• Each unicast frame needs acknowledge
ment
• With A-MPDU, each MPDU would need an
ACK
– Multiple traffic ID block acknowledgement
frame (MTBA)
– Similar to the 802.11e ack for frame bursts
• RIFS is a new reduced interframe space of
only 2 nanoseconds
– Only for greenfield
Pg 609
HT Power management
• Basic Power Save
– APs will buffer traffic-legacy power save
• Spatial Multiplexing Power Save (SM power
save)
– Power down all but one radio
– Static-power down all but one.
• Acts like an 802.11a/g station
• Tell AP when powered down or up
– Dynamic allows power up much faster
• AP can trigger the client to wake up with a RTS
• Client sends CTS when powered up
Pg 610
HT Power management
• Power Save Multi Poll (PSMP) is an
extension of the APSD
– Same benefits
Pg 610
HT Operation
• 20, 40 or 20/40
• APs can also support HT and non-HT in
same cell
– RTS and CTS as well as Phased Coexistence
Pg 611
20/40 Channel operation
• 20 for legacy
• 40 for HT
• The HT access point must declare 20 or 20/40
support in the beacon management frame
• Client stations must declare 20 or 20/40 in the
association or reassociation frames.
• Client stations must reassociate when switching
between 20 and 20/40 modes.
• If 20/40-capable stations transmit by using a
single 20 MHz channel, they must transmit on
the primary channel and not the secondary
channel.
Pg 611
HT Protection
• Mode 0-Greenfield-HT Only-no protection
• Mode 1-HT nonmember-All stations are HT
– If non-HT client/AP is heard, but not part of BSS
• Interference
• Mode 2-HT 20 Mhz-all stations must be HT and
are with a 20/40 AP
– If a 20 MHZ HT stations joins, 40 Mhz will protect to
prevent that station from transmitting
• Mode 3-HT Mixed-when one or more non-HT
stations join an HT service set
– 20 or 20/40
Pg 612
Dual CTS Protection
• When using protection, station will send
RTS
– AP will send two CTS, one on 20 Mhz and
one on 40 Mhz
• AP will send two CTS to self
– One 20 Mhz and one 40 Mhz
Pg 613
Phased Coexistence Operation
(PCO)
• Separate timeslots for 20 and 40 Mhz
transmissions
• No Protection needed
– Could increase jitter-no good for VoWiFi
Pg 613
Exam Essentials
• Define the differences between MIMO and SISO.
– Understand that SISO devices use only one radio chain,
whereas MIMO systems use multiple radio chains.
• Understand spatial multiplexing.
– Describe how SM takes advantage of multipath and sends
multiple spatial streams resulting in increased throughput.
• Explain MIMO diversity.
– Be able to explain the differences between simple switched
diversity and the advanced diversity used by MIMO antenna
systems. Explain the use of maximal ratio combining with MIMO
diversity.
• Understand transmit beamforming.
– Explain how optional transmit beamforming can be used to steer
beams in an optimal path toward a receiving radio and the
benefts of the beamforming process.
Exam Essentials
• Understand 20 MHz and 40 MHz channels.
– Understand legacy 20 MHz channels, 20 MHz HT
channels, and 40 MHz channels and how they use
OFDM. Explain why 40 MHz channels work best in
the 5 GHz UNII bands. Explain primary and
secondary channels.
• Explain the guard interval.
– Describe how the guard interval compensates for
intersymbol interference. Discuss the use of both 800and 400-nanosecond GIs.
• Understand modulation coding schemes.
– Explain how modulation coding schemes are used to
define data rates and all the variables that can affect
the data rates.
Exam Essentials
• Explain the three HT PPDU formats.
– Describe the differences between non-HT
legacy, HT Mixed, and HT Greenfeld.
• Understand HT MAC enhancements.
– Explain how the use of A-MSDU, A-MPDU,
block ACKs, and RIFS are used to increase
throughput at the MAC sublayer. Define the
two new power-management methods used
by HT radios.
• Explain the HT protection modes.
– Describe the differences between protection
modes 0–3. Explain the use of Dual-CTS.