WWiSE IEEE 802.11n Proposal

1
WWiSE IEEE 802.11n Proposal

• August 13, 2004
• Airgo Networks, Bermai, Broadcom, Conexant,
  STMicroelectronics, Texas Instruments

2
Contributors and contact information

• Airgo Networks VK Jones, vkjones_at_airgonetworks.
  com
• Bermai Neil Hamady, nhamady_at_bermai.com
• Broadcom Jason Trachewsky, jat_at_broadcom.com
• Conexant Michael Seals, michael.seals_at_conexant.
  com
• STMicroelectronics George Vlantis,
  George.Vlantis_at_st.com
• Texas Instruments Sean Coffey, coffey_at_ti.com

3
Contents

• WWiSE approach
• Overview of key features
• Proposal description
• Physical layer design
• MAC features
• Discussion
• Summary

4
The WWiSE approach

• WWiSE World Wide Spectrum Efficiency
• The partnership was formed to develop a
  specification for next generation WLAN technology
  suitable for worldwide deployment
• Mandatory modes of the WWiSE proposal comply with
  current requirements in all major regulatory
  domains Europe, Asia, Americas
• Proposal design emphasizes compatibility with
  existing installed base, building on experience
  with interoperability in 802.11g and previous
  802.11 amendments
• All modes are compatible with QoS and 802.11e
• Maximal spectral efficiency translates to highest
  performance and throughput in all modes

5
Overview of key mandatory features

• The WWiSE proposals mandatory modes are
• 2 transmit antennas
• 20 MHz operation
• 135 Mbps maximum PHY rate
• 2x1 transmit diversity modes
• Mixed mode preambles enabling on-the-air legacy
  compatibility
• Efficient greenfield preambles no increase in
  length over legacy
• Enhanced efficiency MAC mechanisms
• All components based on enhancement of existing
  COFDM PHY
• 2x2 MIMO operation in a 20 MHz channel Goal
  is a robust, efficient, small-form-factor,
  universally compliant 100 Mbps mode that fits
  naturally with the existing installed base

6
Overview of key optional features

• The WWiSE proposals optional modes are
• 3 and 4 transmit antennas
• 40 MHz operation
• Up to 540 Mbps PHY rate
• 3x2, 4x2, 4x3 transmit diversity modes
• Advanced coding Rate-compatible LDPC code
• All modes are open-loop

7
Physical layer design

• Data modes
• Transmitter structure
• PHY rates
• MIMO interleaving
• Preambles
• Short sequences
• Long sequences
• SIGNAL fields

8
Transmitter block diagram
9
Mandatory data modes

• 2 transmitter space-division multiplexing, 20 MHz
• 2 transmitter space-time transmit diversity, 20
  MHz
• 802.11a/g (OFDM) modes
• 64-state BCC

10
2 transmitter SDM, 20 MHz (mandatory)

PHY rate Data carriers Pilots Code rate Cyclic prefix, ns Code Constellation
54 Mbps 54 2 1/2 800 BCC 16-QAM
81 Mbps 54 2 3/4 800 BCC 16-QAM
108 Mbps 54 2 2/3 800 BCC 64-QAM
121.5 Mbps 54 2 3/4 800 BCC 64-QAM
135 Mbps 54 2 5/6 800 BCC 64-QAM

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2x1 modes, 20 MHz (mandatory)
PHY rate Data carriers Pilots Code rate Cyclic prefix, ns Code Constellation
6.75 Mbps 54 2 1/2 800 BCC BPSK
10.125 Mbps 54 2 3/4 800 BCC BPSK
13.5 Mbps 54 2 1/2 800 BCC QPSK
20.25 Mbps 54 2 3/4 800 BCC QPSK
27 Mbps 54 2 1/2 800 BCC 16-QAM
40.5 Mbps 54 2 3/4 800 BCC 16-QAM
54 Mbps 54 2 2/3 800 BCC 64-QAM
60.75 Mbps 54 2 3/4 800 BCC 64-QAM
12
Optional data modes

• 20 MHz
• 3 Tx space-division multiplexing
• 4 Tx space division multiplexing
• 3x2, 4x2, 4x3 space-time transmit diversity
• 40 MHz (all 40 MHz modes optional)
• 1 Tx antenna
• 2 Tx space division multiplexing
• 3 Tx space division multiplexing
• 4 Tx space division multiplexing
• 2x1, 3x2, 4x2, 4x3 space-time transmit diversity
• LDPC code option
• An option in all proposed MIMO configurations and
  channel bandwidths

13
Optional modes, common format

Code rate Cyclic prefix, ns Code Constellation
1/2 800 BCC, LDPC 16-QAM
3/4 800 BCC, LDPC 16-QAM
2/3 800 BCC, LDPC 64-QAM
3/4 800 BCC, LDPC 64-QAM
5/6 800 BCC, LDPC 64-QAM
All combinations of 2, 3, 4 transmit antennas and
20/40 MHz offer exactly these 5 modes

All 20 MHz modes have 54 data subcarriers, 2
pilots. All 40 MHz modes have 108 data
subcarriers, 4 pilots
14
Optional mode data rates
20 MHz

Configuration Rate ½, 16-QAM Rate ¾, 16-QAM Rate 2/3, 64-QAM Rate ¾, 64-QAM Rate 5/6, 64-QAM
3 Tx, 20 MHz 81 121.5 162 182.25 202.5
4 Tx, 20 MHz 108 162 216 243 270
40 MHz
Configuration Rate ½, 16-QAM Rate ¾, 16-QAM Rate 2/3, 64-QAM Rate ¾, 64-QAM Rate 5/6, 64-QAM
1 Tx, 40 MHz 54 81 108 121.5 135
2 Tx, 40 MHz 108 162 216 243 270
3 Tx, 40 MHz 162 243 324 364.5 405
4 Tx, 40 MHz 216 364 432 486 540
15

Insert training
FEC encoder, puncturer
Interpol., filtering, limiter
MIMO interleaver
Symbol mapper
Upconverter, amplifier
D/A
IFFT
Add cyclic extension (guard)
16
Preambles

• Mixed-mode preambles
• Capable of operation in presence of legacy 11a/g
  devices
• Ensure correct deferral behavior by devices
  compliant to legacy spec

• Green-field preambles
• Operate in environment or time interval with only
  11n devices on the air
• Applicable in combination with protection
  mechanisms, as in 11g, or in 11n-only BSSs
• Greater efficiency than mixed-mode preambles

Both preamble types are derived from a common
basic structure, providing reuse in algorithms
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Short training sequence

4 Transmitters
3 Transmitters
2 Transmitters
20 MHz STRN 802.11ag short training
sequence 40 MHz mixed mode STRN Pair of
802.11ag short sequences
separated in frequency by 20 MHz 40 MHz green
field STRN Newly defined sequence cs Cyclic
shift
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Long training sequence and SIGNAL-N, green-field,
2 transmitters

20 MHz LTRN 802.11ag long training sequence
with four extra tones, 6.4 usec 40 MHz LTRN
Newly defined sequence, 6.4 usec GI21 GI2 for
LTRN with 1600 ns cyclic shift SIGNAL-N 54
bits, 4 usec
19
Long training sequence and SIGNAL-N, green-field,
3 and 4 transmitters

For 3 transmitters, the first three rows are used
20
Long training and SIGNAL fields, mixed mode, 2
transmitters

21
Long training and SIGNAL fields, mixed mode, 3
and 4 transmitters

For 3 transmitters, the first three rows are used
22
Preamble lengths (20 40 MHz)

All space-time block codes follow the pattern
with the same number of transmit antennas
23
Preamble lengths (20 40 MHz), contd.

Mixed mode
All space-time block codes follow the pattern
with the same number of transmit antennas
24

Insert training
FEC encoder, puncturer
Interpol., filtering, limiter
MIMO interleaver
Symbol mapper
Upconverter, amplifier
D/A
IFFT
Add cyclic extension (guard)
25
Parallel encoders

• For 40 MHz modes with more than two spatial
  streams, two parallel BCC encoders are used

Data payload
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Advanced coding option

• Rate-compatible LDPC code with the following
  parameters
• Transmitter block diagram as for BCC modes,
  except symbol interleaver, rate-compatible
  puncturing, and tail bits are not used

Block length
Information bits
Rate
1944
972
1/2
1944
1296
2/3
1944
1458
3/4
1944
1620
5/6
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LDPC code, contd.

• There is no change required to SIFS or to any
  other system timing parameters when the advanced
  coding option is used
• The block size of 1944 reduces or eliminates the
  need for pad bits at the end of a packet
• Pad bits are eliminated for 2 transmitter
  operation in 20 MHz channels, and 2x1 and 1x1 in
  40 MHz channels
• The four parity check matrices are derived from
  the rate-1/2 matrix via row combining
• The parity check matrices are structured and
  based on square-shaped building blocks of size
  27x27
• The parity check matrices are structured to
  enable efficient encoding

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Insert training
FEC encoder, puncturer
Interpol., filtering, limiter
MIMO interleaver
Symbol mapper
Upconverter, amplifier
D/A
IFFT
Add cyclic extension (guard)
29
MIMO interleaving

TX 0 interleaved bits
Coded bits
TX 1 interleaved bits
Configuration Idepth
108 tones, 1 Tx, 2x1 12
All others 6
Shift of 5 additional subcarriers for each
additional antenna
30

Insert training
FEC encoder, puncturer
Interpol., filtering, limiter
MIMO interleaver
Symbol mapper
Upconverter, amplifier
D/A
IFFT
Add cyclic extension (guard)
31
Space-time block codes and asymmetry

• Simple space-time block codes (STBCs) are used to
  handle asymmetric antenna configurations
• STBC rate always is an integer
• - No new PHY rates result from STBC encoding of
  streams
• Block size is always two OFDM symbols
• STBC encoding follows the stream encoding

32
Space-time block codes

• 2x1
• 3x2

• 4x2
• 4x3

The STBC is applied independently to each OFDM
subcarrier
33

Insert training
FEC encoder, puncturer
Interpol., filtering, limiter
MIMO interleaver
Symbol mapper
Upconverter, amplifier
D/A
IFFT
Add cyclic extension (guard)
34
Power spectral density, 20 MHz

35
Power spectral density, 40 MHz

36
MAC features

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New MAC features

• The WWiSE proposal builds on 802.11e
  functionality as much as possible, in particular
  EDCA, HCCA, and Block Ack
• Goal is backward compatibility and simplicity
• Block Ack is mandatory in the proposal
• Bursting and Aggregation
• MSDU aggregation
• PSDU aggregation
• Increased maximum PSDU length, to 8191 octets
• HTP burst sequence of MPDUs from same
  transmitter, separated by zero interframe spacing
  (if at same Tx power level and PHY configuration)
  or 2 usec (otherwise)

38
New MAC features, contd.

• Block Ack frames ACK policy
• Reduce Block Ack overhead
• Legacy remediation
• N-STA detection/advertisement
• Identification of TGn and non-TGn devices and
  BSSs
• Legacy Protection mechanisms
• Additions to existing protection mechanisms
• 40/20 MHz channel switching
• Equitable sharing of resources with legacy

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Discussion

40
100 Mbps throughput

• See response to CC 27 in 11/04-0877-00-000n
• Efficiency upgrades in 802.11e and further
  enhancements in 11n mean that the 45-50 system
  efficiencies of old 802.11 systems have evolved
  to 75-85 in contemporary systems
• Many such enhancements are commercially available
  in firmware upgrades from multiple vendors
• 100 Mbps throughput is achieved from 135 Mbps PHY
  rate in a variety of setups
• Both EDCA and HCCA allow this efficiency
• 100 Mbps throughput may even be achieved from
  121.5 Mbps PHY rate
• This requires HCCA EDCA does not suffice

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100 Mbps throughput, contd.

• Example scenario
• 4000 byte packets
• HTP burst transmission, 3 packets
• Block ack
• 10 for assorted other users, beacons, etc.

BSS share, etc.
Data payload
Block ack request/ack
20
240
240
4
106
240
4
24
16
32
34
SIFS
DIFS
SIGNAL-N
Preamble
960 usec
42
Robustness of modes

• 2x2 operation achieving 100 Mbps throughput in a
  20 MHz channel is feasible
• Requires high-performance signal processing
• At highest rates, high performance MIMO detection
  and/or advanced coding are required
• 2x3 operation achieving 100 Mbps throughput in a
  20 MHz channel is very feasible
• Achieves throughput targets with MMSE processing
  and BCC
• Balance and approach are up to the implementer
  and beyond the scope of the standard

43
Capacity and operating points, 2x2

• Channel model D, NLOS, half-wavelength spacing
• Curves are envelopes of curves for the 5 rates
• For each constituent curve, capacity is reduced
  by outage

Baseline 108 is a 2 Tx system with 802.11a/g 54
Mbps
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Optionality of 40 MHz

• Reasons why 40 MHz channels are not proposed as
  mandatory
• Limited worldwide applicability
• Europe clause 4.4.2.2 of ETSI EN 301 893 V1.2.3
• Japan ARIB STD-T71
• The repackaging effect
• Halving the number of channels to provide each
  twice the data rate is of questionable value as
  an enhancement
• System and contention overhead
• Double the number of users in a single BSS
  results in increased contention losses two
  separate 20 MHz channels generally provide better
  network capacity, especially with coordinated
  management
• Backward compatibility and interoperability
• In dense legacy network deployments, contiguous
  40 MHz transmission bandwidth may not be
  available or performance may be impaired

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References

• IEEE 802.11/04-0886-00-000n, WWiSE group PHY and
  MAC specification, M. Singh, B. Edwards et al.
• IEEE 802.11/04-0877-00-000n, WWiSE proposal
  response to functional requirements and
  comparison criteria, C. Hansen et al.