Road to High Speed WLAN Xiaowen Wang Introduction 802.11n - - PowerPoint PPT Presentation

Road to High Speed WLAN

Xiaowen Wang

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Introduction

• 802.11n standardization process.
• Technologies enhanced throughput

– Raw data rate enhancement – Overhead management

• Final remarks

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• Jan. 2004, 11n working group started with the goal of reaching network

throughput of over 200Mb/s.

• Since then, several industry consortiums began to work on proposals

for 11n, namely, TGnSync and WWiSE.

• Jan. 2005 3 complete proposals were submitted to 11n by TGnSync,

WWiSE and MITMOT.

• Till July 2005, no one proposal can pass the confirmation vote. The

three parties began the work of merging to a joint proposal.

• EWC (enhanced wireless connection) formed in Oct. 2005 by

members of TGnSync and WWiSE working on the joint proposal.

• Jan. 2006 proposal from EWC was approved as confirmed proposal.
• First letter ballot stated in March 2006.
• Nov. 2006, draft 1.06 was approved.
• Draft 2.0 was approved in March 2007.

Background

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• Highest raw link data rate in 802.11a/g is 54Mb/s

– Coding rate: 3/4 – Modulation: 64QAM – Symbol duration: 3.2µs – GI duration: 0.8µs

• With network efficiency of 90%, the network throughput would be

48Mb/s.

– 16µs training and 4µs signal field – IFS – ACK and other control packages

Starting Point

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Solutions

• Increase raw physical layer link data rate

– Increase coding rate – Increase constellation density – Increase bandwidth – MIMO: spatial multiplexing

• Reduce overhead

– Physical layer signaling, training, MAC layer header, control and signaling package. – Increase data package length.

Data Rate Enhancement

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Advanced Coding

• Goal: achieve higher data rate and enhance performance
• Convolutional code:

– New code rate: 5/6

• LDPC code: optional

– Code rates: ½， 2/3，3/4，5/6.

• Turbo code had been considered, but does not make it to the final draft.
• STBC: 1 spatial stream to 2 Tx antennas: optional
• Data rate increase: x10/9
• Overhead associated: signaling and training

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High Density Constellation

• 256QAM was considered but not accepted in the current draft．
• The SNR requirement is too high, over 30dB required for PER of 10%.
• Possible data increase: x2
• Overhead associated: signaling

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Bandwidth Increase

• Bandwidth is doubled from 20MHz to 40MHz: optional
• One user can have as many as 108 subcarriers.
• Data rate increase: x2.25
• Overhead associated

– Signaling – MAC layer probing

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MIMO

• Spatial multiplexing: map Nsts spatial streams (independent data

streams) to Ntx transmitter antennas.

– Direct mapping: Nsts = Ntx, either identity or CSD matrices – Spatial expansion (optional): Nsts < Ntx, simple CSD or selective diversity – Beamforming (optional): decide the steering matrix on the run according to the channel information by the sounding packet

• Data rate increase: x4
• Overhead:

– Signaling – Training – MAC support

     

1 1

STS TX STS STS

N N N N

k k k

  

 Y Q X

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MAC Layer Procedure of Beamforming

• Implicit feedback

– Use the reciprocity of the channel – Beamformer send training request, beamformee send training back, then beamformer starts beamforming. – Calibration is needed to ensure reciprocity.

• Explicit feedback

– Null sounding packet is sent by the beamformer and then beamformee estimate the channel and quantize and feedback the channel information to the beamformer.

Overhead Management

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OFDM Symbol Level

• GI (guard interval) length

– 11a/g, GI length is ¼ of symbol length – There are 3 different GI legnth

• ¼ of symbol length, 0.8μs
• 1/8 of symbol length, 0.4μs
• ½ of symbol length, 1.6μs
• FFT size of symbol length

– MIT/MOT proposed to double the symbol length to 128, but was not adopted by the current draft. – To realize the efficiency benefit, the MAC layer package has to be certain length which is not the case in practice.

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PPDU Level

• Signal field has to increase to accommodate the more complex PHY.

– Current HT signal field has 2 OFDM symbols transmitted in BPSK. – Legacy signal field is kept for backward compatibility.

• Training requirement is basically proportional to the number of spatial

streams.

– A 4μs HT-STS is added to improve AGC accuracy – Long training design criteria

• All tones of different channel corresponding to different antenna pairs should be

independently trained. The column of U should be orthognal.

• All transmitter chains should be excited all the time.
• No beam should be formed as different transmitter chains transmit correlated

signals.

– CSD*Walsh

TX LTF

N N

LTS LTS LTS



            U 

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Greenfield Operation

• In the HT mixed format, a legacy (11a/g) training and signal field are

transmitted.

– Attempt to reuse the legacy training had been considered.

• AGC accuracy is the reason for adding HT-STF and HT-LTF
• Signal field is kept for backward compatibility.
• Greenfield operation: no legacy transmission exists

– Greenfield condition is assured by MAC protection scheme. – Save 16μs compared to HT-mixed format – Only 2 pilots instead of 4 pilots are used.

• Each HT-LTF is only half the size (one symbol) of the L-LTF, more

sophisticated channel estimation algorithm is required to ensure the performance.

L-STF L-LTF L-signal HT-STF HT-LFT1 HT-LFT1 HT-signal HT-STF HT-LTF HT-LFT2 HT-LTF2 HT-signal

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MAC Level

• Aggregation

– HT PHY increases the signal field and training – HT PHY decreases the data portion – MAC layer data packet size is limited by higher layer (such as TCP/IP) – Aggregate several MAC layer packets and transmit them in one PPDU.

• Block ACK

– One ACK package is at least 24μs no matter how fast the PHY is. – Block ACK is not only a QoS scheme, but also increase the throughput.

• RIFS

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Final Remarks

• 802.11n is the first wireless standard that MIMO takes the center stage.

– Spatial multiplexing – STBC – Transmitter beamforming

• WiFi announced last August, they are going to began certificate pre-

11n products this year. The feature list is still in working, but only baseline system would be certified.

– Convolutional code with rate ½, 2/3, ¾, 5/6. – 2x2 20MHz spatial multiplexing – Greenfield operation – Packet aggregation and block ACK

• Lessons learned from this successful MIMO OFDM practice.

– System becomes more and more flexible. – Control pane becomes more and more a challenge – Cross layer optimization and tradeoff.

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