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 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.

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 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.

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.