J. Boer, J. Gilbert, E. Perahia, J. Sadowsky, N. Waes, W. Houtum, T. Kunihiro, M. Takagi

1
40 / 20 MHz Interoperability for Robust, High
Performance, and Compatible 802.11n Systems

• Jan Boer, Agere Systems, Inc. jboer_at_agere.com
• Jeff Gilbert, Atheros Communications, Inc.
  gilbertj_at_atheros.com
• Eldad Perahia, Cisco Systems, Inc.
  eperahia_at_cisco.com
• John Sadowsky, Intel Corporation
  john.sadowsky_at_intel.com
• Nico van Waes, Nokia Corporation
  nico.vanwaes_at_nokia.com
• Wim van Houtum, Royal Philips Electronics
  wim.van.houtum_at_philips.com
• Takushi Kunihiro, Sony Corporation
  kuni_at_wcs.sony.co.jp
• Masuhiro Takagi, Toshiba Corporation
  masahiro3.takagi_at_toshiba.co.jp

July 2004
2
Introduction

• In IEEE presentation 802.11-04/0786, we described
  the benefits of 40 / 20 / 10 MHz channelization
  for speed, robustness, and low-cost
• One key design issue is how to coexist and
  interoperate with legacy 20 MHz devices while
  operating in 40 / 20 MHz mode
• Efficiency is critical the legacy interop.
  mechanisms cannot notably degrade performance
  (e.g. 11g)
• This presentation details the 40 / 20 MHz
  PHY-level interoperation mechanisms

3
40 MHz PPDU
Duplicate format legacy compatible preamble
HT Packet Format in 40MHz

• Legacy short and long training and signal fields
  are replicated across the lower and upper
  20MHz channels
• PLCP (signal field) indicates a packet length
  and rate which results in a duration long
  enough to cover the packet exchange
• Much more efficient than RTS/CTS or CTS-only
  protection

4
Legacy SF Coding Modulation
IFFT
upper
sub-channel
BPSK
map
BPSK
lower
map
conv.
24 bit
11a
sub-channel
BPSK
encoder
Interleaver
SF
map
rate 1/2
BPSK
map
BPSK
map
5
Duplicate Receiver

• Duplicate format supports
• simple receiver
• additional frequency diversity gain
• very robust legacy compatible preamble

Combining Equalizer Simple MRC combining Note
If upper sub-channel is not present, combining
weights are zero.
6
20/40 MHz Interoperability

• 20 MHz PPDU ? 40 MHz receiver
• Combine modulation symbols from upper lower
  sub-bands
• 20 MHz PPDU in lower sub-channel
• zero combining weights in upper subchannel
• No loss in performance relative to a 20 MHz
  receiver
• Use differential sub-channel energy to detect 20
  v. 40 MHz signals
• 40 MHz PPDU ? 20 MHz receiver
• One sub-channel is sufficient to decode the
  legacy SF
• Detects only half of the 40 MHz signal? 3 dB
  performance penalty for 20 MHz clients
• But they only have to decode the 6 Mbps legacy SF

7
Conclusions

• Full interoperability between 20MHz and 40MHz
• Use differential sub-channel energy to detect
  20MHz vs. 40MHz signal
• Duplicate legacy compatible preamble in 40MHz
  signal
• 20MHz STA can decode legacy SF
• 40MHz STA can use simple combining scheme to
  decode both 20MHz and 40MHz signals