11n Overview

8/6/2019 11n Overview

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© 2006 Texas Instruments Incorporated

802.11n Overview




Agenda

New Features Overview PHY Features

PPDU Formats

Frame Aggregation

QoS Enhancements

PSMP Operation

HT Signaling

BA Sessions




New Features Overview

MCS ±

Modulation and Coding Scheme Addition 4 tones in 20Mhz OFDM

Up to 72mbps possible with 1 TX stream

MIMO ±

Multiple Input Multiple Output Up to 4 spatial streams in TX and RX

Up to 289mbps in 20Mhz OFDM

40 Mhz channels

Up to 600mbps with 4 spatial streams

May be divided to 2X20Mhz channels





Transmit Beam Forming Stations with multiple antennas can optimize the TX stream sent to

each peer device

Signaling between beam former and beam formee is required

STBC ± Space Time Block Code

Duplicate Coded data transmitted from several antennas in

different times

Single antenna reception is possible

Increases network range





LDPC

± Low Density Parity Check

Better Error Correction then current FEC

Decoding is resource consuming

Antenna Selection

Optimizes the division of the RF stream over multiple antennas

Achieved after peer-to-peer calibration sequence

PPDU Formats

Green Field ± HT Only

Mixed Format ± HT inside Legacy Format





A-MSDU ±

Aggregated MSDUs Several MSDUs grouped inside one MPDU

Single MAC header

A-MPDU ± Aggregated MPDUs

Several MPDUs grouped inside one PPDU

Single PLCP (Multiple MAC headers)

Block Ack Sessions

Required for A-MPDUs

New Types of BA: Compressed, Multi-TID





PSMP ± Power Save Multi-Poll

Synchronize station transmission/reception to AP to improve PS

Designed for better power consumption with multimedia streams

RIFS ± Reduced Interframe Spacing

2us

Maybe used when no SIFS transmission is expected

RDG ± Reverse Direction

Station may allocate remaining TXOP to peer

Transmit Burst Limit

Limit Consecutive HT and/or RIFS packet bursts




PHY Features MCS

Identify the exact coding, modulation, number of spatial streams

Different mappings according to channel width

20Mhz ± MCS from 0 to 31, 8 different MCS per number of streams

40Mhz ±

MCS form 0 to 31, 8 different MCS per number of streams MCS number above 31 used for UEQM

Guard Interval

Two possible values per MCS 400 or 800 ns.

Changes Achievable Rates

New rate adaptation algorithm required




PHY Features




PHY Features

Channel Number 40 Mhz channels identified by primary and secondary channels

Same numbering scheme as today

STBC

Support on Receive Only

MAC feature allows APs to transmit beacons in STBC format on

secondary channel at an offset of half TBTT




PPDU Formats

Maximal PSDU length is 65535 Legacy Format

Current OFDM format

Mixed Field Legacy Preamble

Different meaning of rate and length fields in the Signal

HT Frame without preamble

Green Field

Complete HT frame




Frame Aggregation

A-MPDU

Aggregate many MPDUs to a single receiver

Minimal spacing between MPDUs

Maximal A-MPDU Length

Requires BA session setup

Each MPDU has its own sequence number

Duration in all MPDUs is the same

RA in all MPDUs is the same Multiple TID is possible

Can signal BA request at the end of an A-MPDU

Otherwise BAR needs to be sent




Frame Aggregation




Frame Aggregation

A-MSDU Similar to our legacy 4X feature

Aggregation of Layer 2 packets w/o individual MAC header

New sub-frame header between MSDUs

Only frames with the same TID are allowed




Frame Aggregation




QoS Enhancements

RDG

Signaled in the HT Control field

Allows to transmit and receive in the same TXOP

ACs may change upon reversed direction




QoS Enhancements




QoS Enhancements

RIFS

Usage

Between transmitted packets in TXOP

Not allowed between RDG

Between PSMP DTT

Between packets sent inside a DTT or UTT

RIFS is allowed in the BSS if indicated inside the HT information

element in the beacon




PSMP Operation

Relevant only the stations in 802.11 PS

Started by an PSMP action frame sent in broadcast or

multicast ± Only after DTIM

Limited to ~8ms

Can be extended with another consecutive PSMP

Schedules DTT and UTT per each station with offset from

start of PSMP

DTT ± Downlink transmission times

UTT ± Uplink transmission times




PSMP Operation




HT Signaling

HT Control Field

Link Adapation

Beam Forming Calibration Sequences

Antenna Selection Calibration Sequences

RDG

NDP Announcment

Order bit in FC is set to 1




HT Signaling

HT Information Field

Appears in Beacons and Probe responses

Controls HT operation in the BSS

RIFS Mode

PSMP only BSS

Protection required/allowed: HT, Greenfield, L-SIG

Transmit Burst Limit

Greenfield Protection

Dual CTS Indication

Dual Beacon

Supported MCS bitmap (Size 128 bits, 1 bit per MCS)




HT Signaling




HT Signaling

Includes station HP capabilities

Sent by AP in Probe Response and Beacons

Sent in Association Frames by AP and STA

Sent in Pilot Frames




B A Sessions

11e feature extended in 11n

Allows sending of frames w/o ACK and receiving a BA for multiple

frames instead

Allows sending packets out of order

Better throughput achieved when good radio conditions

Requires ACK tracking and buffering of sent frames

Requires reordering of received packets before they passed to network

stack

BAR ±

Requests ACK for all packets starting from given sequencenumber

Block ACK ± Provides bit mask specifying which packets are ACKd




Q & A

11n Overview

Documents

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