Chapter 18 High Throughput and n 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.

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

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

n Draft HT clause 20 radio with MIMO and OFDM Must be backward compatible with –HR-DSSS –ERP Pg 590

WiFi Alliance n draft 2 certified Mandatory and tested implementations Many vendors had pre n 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 n –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 a 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 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 a 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 e service access category Pg 607

A-MPDU MPDU aggregation Send multiple MPDU with single PLCP header –Single PHY preamble and header Must be same e 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 e 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 a/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.