Chapter 18 High Throughput and 802.11n

1
Chapter 18 High Throughput and 802.11n

• 802.11 n history
• MIMO
• HT Channels
• HT PHY
• HT MAC
• HT Operation

2
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.

3
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
  800- and 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.

4
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
  03. Explain the use of Dual-CTS.

5
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
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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
7
802.11 n Draft

• HT clause 20 radio with MIMO and OFDM
• Must be backward compatible with
• HR-DSSS
• ERP

Pg 590
8
WiFi Alliance

• 802.11 n draft 2 certified
• Mandatory and tested implementations
• Many vendors had pre 802.11n equipment

Pg 590
9
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
10
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
11
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
12
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
13
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
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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
15
Spatial Multiplexing

• Each stream can use the same, or different
  modulation techniqus

Pg 593
16
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
17
MIMO Diversity

• MRC is best when going from Non-MIMO to MIMO

Pg 594
18
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
19
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
20
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
21
Transmit Beamforming (TxBF)
Pg 595
22
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
23
HT Channels
Pg 597
24
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
25
40 Mhz Channels

• Works well for 5 Ghz range
• Not as well for 2.4 Ghz

Pg 599
26
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
27
Guard Interval (GI)

• HT can use 400 nanosecond GI
• Increase throughput
• Risk of intersymbol interference
• Look for retransmissions

Pg 602
28
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
29
Modulation and Coding
Pg 603
30
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
31
HT PHY
Pg 605
32
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
33
HT MAC

• New enhancements to MAC for throughput and power
  management
• Frame aggregation
• Power management

Pg 607
34
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
35
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
36
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
37
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
38
HT Power management

• Power Save Multi Poll (PSMP) is an extension of
  the APSD
• Same benefits

Pg 610
39
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
40
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
41
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
42
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
43
Phased Coexistence Operation(PCO)

• Separate timeslots for 20 and 40 Mhz
  transmissions
• No Protection needed
• Could increase jitter-no good for VoWiFi

Pg 613
44
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.

45
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
  800- and 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.

46
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
  03. Explain the use of Dual-CTS.