This tutorial explains the 802.11n concepts in a simple and easy to understand manner. It talks about different types of MIMO improvements (diversity, MRC, spatial multiplexing, space time coding). It also provides a summary of 802.11n packet formats and various capacity/throughput related optimizations proposed in 802.11n (PLCP improvements, BlockAcks, A-MPDU & A-MSDU Aggregation).

1. A Brief Tutorial on IEEE 802.11n
www.airtightnetworks.net

2. Outline
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3. IEEE 802.11
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4 5'
6 %/5 % 7
! * # 8*
#
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4. 802.11 protocol suite
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5. 802.11 MAC and PHY enhancements
Security 802.11i 802.11e QoS
Data link MAC
802.11w 802.11n
Capacity &
PLCP Coverage
Physical 802.11n
PMD
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6. Two-slide primer on 802.11 MAC (1)
6 ( 16 * / 3
) & & 1* # 8* 3
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7. Two-slide primer on 802.11 MAC (2)
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8. Example of DCF CSMA/CA (1)
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9. Example of DCF CSMA/CA (2)
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10. Example of DCF CSMA/CA (3)
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11. Example of DCF CSMA/CA (4)
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12. Example of DCF CSMA/CA (5)
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13. Example of DCF CSMA/CA (6)
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14. Example of DCF CSMA/CA (7)
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15. Motivation for multicarrier modulation
9 & " 1 ) & 3
( : ") & ( 1 3
"
# $ % &
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16. Multicarrier modulation
6 ! : ; ( ") & ! && :
;
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) & : ") & ) & &" ! ) & & :
&
) & - " % & - & - " .*
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17. Orthogonal frequency division
multiplexing (OFDM)
( &/6 #
& & ) &
' , / .) *
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18. OFDM in 802.11
< # 5' & ! =
" - 0 0 12
> = 5'
3 &
3 & ,
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19. 802.11n PHY Enhancements
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20. What is MIMO?
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Tx Rx
# # $ # & & # & & $
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' $ ,
Tx Rx
# ? " ) 1 @ %# @ 3
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21. Spatial diversity
9 & " ( ( & ) & &"
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A ) & & &" ( &
& "
) & & (( &" 1B C8 3
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22. Receiver diversity
r1ej 1s(t) r2ej 2s(t) r3ej 3s(t) rMej Ms(t)
a1e-j 1
a2e-j 2 a3e-j 3 aNe-j M
x x x x
Combiner Output
SNR =
M
( a k rk ) 2
k =1
9 D N0 * ) 7 D M
2
N0 ak
k =1
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23. Receiver diversity: Selection combining
* 7
= k= rk2
Nk
4 , % -
&
Bit stream DSP Radio Radio DSP Bit stream
Tx Rx
" , 5 % (' 6' 4 +
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24. Receiver diversity: Maximum Ratio
Combining (MRC)
! 7 &
& 7
Radio Radio
Bit stream Bit stream
DSP DSP
Radio Radio
Tx Rx
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25. Receiver diversity: Maximum ratio
combining
rk
$ ) & D
N0
rk " ") &D Ek
% 7 D Ek
N0
ME k
* ) ! 7 D
N0
" # :( & 7 ! $ " )
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26. Transmitter diversity: Channel-aware
) & ( & ( ) 1* 3
& 7 %
!
) & ! ! " ) % %# 7 *
! , - , & ,
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27. Transmitter diversity: Channel-unaware
: ) & 1 E*3 & ) )
! , % - &
) & , &
) 5 8 8 , $ &
9 ! & hk = rkej k
7 ! & r(t) = 0.5 (h 1 + h 2 )s(t)
") & ! ( ") & y1 = h1s1 + h2s2
") & ! ") & y2 = -h1s2* + h2s1*
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28. Transmitter diversity: Alamouti scheme
9 ; ( ! ") & !
y = [y1 y2*]T
h1 h2 s1
y= = Hs
h2* -h1 *
s2
9 z = HHy = HHHs = (|h12| + |h22|)I2s
z1 = h1*z1 + h2z2 = (|h12| + |h22|)s1
z2 = h2*z1 – h1z2 = (|h12| + |h22|)s2
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29. Transmitter diversity: Alamouti scheme
2 2
(| h 1 | + | h 2 |)E k
7 ! 7 D
2N 0
" D
6 ! " D 1F & 3
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30. Practical significance: array gain and
diversity gain
/ 7 "& & & " 1Pe3 1
SNR
1
/ # ? " ) % Pe (A x SNR)D
# ? ) ) " D# % ! " 6 D#
Diversity
gain
Array
gain
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31. Practical significance: array gain and
diversity gain
Pe
SNR
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32. Spatial multiplexing
# & & ?
) () + ( + ( +
' , $
6 (( )
9 , %
) ; ) ! 7/
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33. Spatial multiplexing
# ? ) ) ) & & ? D) 1# % 3
A ") & ) &) ? 5
9 " ) " & " ) 1# % 3 ! & ) 1# % 3
;
b1 b1 b3 b5 Radio Radio b1 b3 b5 b1
b2 b2
b3 b3
b4 Split DSP DSP Merge b4
b5 b5
b6 Radio b6
Radio
b2 b4 b6 b2 b4 b6
Tx Rx
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34. Spatial multiplexing gain vs. diversity
gain trade-off
0, MN
Diversity gain
1, (M-1)(N-1)
2, (M-2)(N-2)
k, (M-k)(N-k)
Min(M, N), 0
Spatial multiplexing gain
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35. 802.11n channels
4 # 5'
( . - +
, &
5 % 2
& ,
$ &" : ! & & 4 5'
6 , : #!*
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36. 802.11n Modes of Operation
PLCP Enhancements
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37. 802.11n: Modes of Operation
> # :5 % # ? % ( & 1 " 9* 3
# ? ( &
, * & 7- &
" - 2 ' ; #
, * , %
Detection of PPDU, 6 4 5#) % ,
timing & coarse
freq acquisition
' ,, & ( , , 7 +
!
For use of legacy
devices also
Signalling
(See next slide)
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38. L-SIG (MM) & HT-SIG (MM & GF)
Always 6 Mbps Encoded value indicating
Duration of rest of
the packet
L-SIG of Mixed Mode
Refer to next slides
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39. HT-SIG
Field Name Explanation and coding
Modulation
and Coding Index into the MCS table.
Scheme
Set to 0 for 20 MHz or 40 MHz upper/lower
CBW 20/40
Set to 1 for 40 MHz
Length The number of octets of data in the PSDU in the range 0-65535
Set to 1 indicates that channel estimate smoothing is allowed
Smoothing Set to 0 indicates that only per-carrier independent (unsmoothed)
channel estimate is recommended
Not Set to 0 indicates that PPDU is a Sounding PPDU
Sounding Set to 1 indicates that the PPDU is not a sounding PPDU
Reserved Set to 1
Set to 1 to indicate that the PPDU in the data portion of the packet
Aggregation
contains an AMPDU otherwise, set to 0.
Set to a non-zero number, to indicate the difference between the
number of space time streams (NSTS ) and the number of spatial
STBC
streams (NSS) indicated by the MCS.
Set to 00 to indicate no STBC (NSTS = NSS)
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40. HT-SIG
Field Name Explanation and coding
LDPC Set to 1 for LDPC
coding Set to 0 for BCC
Set to 1 to indicate that the short GI is used after the HT training.
Short GI
Set to 0 otherwise
Indicates the Number of extension spatial streams (NESS).
Number of
Set to 0 for no extension spatial stream
extension
Set to 1 for 1 extension spatial stream
spatial
Set to 2 for 2 extension spatial streams
streams
Set to 3 for 3 extension spatial streams
CRC CRC of bits 0-23 in HT-SIG1 and bits 0-9 in HT-SIG2
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41. Modulation & Coding Scheme (MCS)
# * ) 1 ?3
(" ' < = ' < = ! +
, ( . > +
* & ' ' ( ? 3+
# * ? ( ) 0
'
• MCS 0 to 15 at 20 Mhz (at AP)
• MCS 0 to 7 at 20 Mhz (at client STA)
9
• MCS 16 to 76 are optional
• All MCS at 40 Mhz
' @@ @ %
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42. Rate Dependent Parameters (20 MHz and
Mandatory MCS) What is the formula?
NSS = 1
NSS = 2
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43. Rate Dependent Parameters (40 Mhz &
Mandatory MCS)
NSS = 1
NSS = 2
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44. Other Optional MCSs
$ # *
1)
# * D>
• MCS 32
' 0 A ? • Useful under very high noise
$ ( ' ?+ • Lowest rate of 40 Mhz (bpsk)
• 216.7 Mbps (20 Mhz) • 6.7 Mbps max rate
• 450 Mbps (40 Mhz) ' -
# * D4 • Use with
– Tx beamforming
' 3 A? – STBC
$ ( ' ?+ • MCS 33 – 38 (4 SS)
• 288.9 Mbps (20 Mhz) – Max rate 495 Mbps
• 600 Mbps (40 Mhz) • MCS 39 – 52 (4 SS)
– Max rate 495 Mbps
• MCS 53 – 76 (4 SS)
– Max rate 495 Mbps
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45. MAC Enhancements
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46. / )
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47. Motivation
DCF PLCP MPDU1 PLCP ACK DCF PLCP MPDU2 PLCP ACK
SIFS
DCF PLCP MPDU PLCP ACK
) ' 9* %# * !
* ) & ) ! & " 1 ? 3
# ) ' ) ( *
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48. Physical Level Aggregation (A-MPDU)
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- B
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49. Physical Level Aggregation (A-MPDU)
Max Rx Factor(x): 0 to 3 [2^13+x]
Min spacing: 0.25 to 16 usecs
, , G!5 B H 1) & 6F ,
$ , (0 3 7 +
B ,
• 0 indicates no restriction
• Else, ranges from 1/4 to 16 usecs
• Realized by using Delimiters with MPDU length 0
& & , ! 7
F$ !5 B
= '
" 7 7
" 7 ! <
! , & G! * ! < H( , , 6 4 +
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50. A-MSDU
!5 ' B &
! ' B & % & %
F ' B & , ) 6 (= ' +
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51. A-MSDU
' % - , (
!5 B += ' B * ! 7
" 7 ! 7 , - !5 ' B & = '
"!
!5 ' B !I 5 ' B
! !5 ' B & )$ E 5 ' B $ %
%
6 ' B & )$ % %
!' )! !5 ' B $ $
' B , & ( , , +
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52. Block ACK (BA)
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53. Block Ack Packet Exchange
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54. Block ACK Sessions (ADDBA)
Request Response
* " E& * 66 ; 8 8 &
6 & ) ( 6 ( ;8
) 1 ( ? & 3
) 1 3( !
! ;
6 " 7 ! <
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55. Block Ack Parameter Set Field used in
ADDBA Action Management Frames
802.11n
802.11e
E& ) ( &
! ' B & - "!
" 7 ! 7 ! <
• Delayed is sent at a slightly later time after receiving a Block Ack
Req (See next slide)
)6 ) 6 ( 6 ,
= ' +
" 2 &
• Recipient controls the buffers that can be supported
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56. Immediate BlockAck
Delayed BlockAck
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57. Block ACK Sessions (DELBA)
DELBA Parameter set
6 <9E ? & &"
! (+
6 <9E
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58. BlockAckReq (BAR)
802.11n
See Next Slide
802.11e
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59. Fields of BlockAckReq Frame
E & "
E 7 * &
" !9 ! < (1 ) 5 ; "
+ E& & " ( & (! " !+
• Normal ACK 6 1)
• No ACK ; 1) E 7 *
5) 6 & "
• Does BAR consist of • Negotiated between HT
req for different QoS
streams? stations as a part of HT
capabilities
• Support for ACK for • Extensions for using BA with
fragments in BA? 802.11n features such as
) 6 J 6* 4 frame aggregation (A-
• Info about each TID MPDU)
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60. BlockAckReq BAR Info Field
Encoding
-Basic BAR, Compressed BAR
-TID info contains TID for which the
. req has been made
Per TID INFO
-MT BAR
-TID_info contains number of TIDs
-BAR info contains seq number for
that many TIDs
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61. BlockAck frame
E& * )
<? ( ) 1 ? & 3
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62. BA Information for each BA encoding
Basic BA
128 byte bitmap
Compressed BA
Mandatory
8 bit bitmap
No support for fragments
MTBA (repeated
For each TID)
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63. HT Protection Mechanisms
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64. Protection Requirements
-Protection may be required if Non-HT stations are present or Non-
greenfield stations are present
-Types of protection that an HT station provides
-RTS/CTS using a legacy rate
-CTS to self using a legacy rate
- Transmit 1st frame in a backward compatible mode
-1st frame Tx using a Non-HT preamble and then switch to
HT mode
-1st frame Tx using a MM preamble and then switch to
Greenfield operation
-Setting of L-SIG values in preamble to protect the current
transmission
-L-SIG TxOP (See next slide)
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65. L-SIG TxOP Protection
& - 1 ) ')! ( 1) & 6F +
, ' B ( , !) !K ! < 9 ) ' . ) ' + , , % #5' 6C
#5' 6C - & % ! E %
* 51 ) ' ) ! D 7 E % % , , , L
! & 1 )5 $ )$
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66. HT Parameter Negotiation
Information Elements
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67. Advertising HT Capabilities using MAC
Frames
HT Capability Information Element (E.g., Beacon, Probe Response)
Refer to next slides
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68. HT Capabilities Info
Subfield Definition Encoding
Indicates support for receiving LDPC coded Set to 0 if not supported
LDPC coding capability packets Set to 1 if supported
Set to 0 if only 20 MHz operation is
supported
Indicates which channel widths the STA Set to 1 if both 20 MHz and 40 MHz
Supported channel width set supports operation is supported
Set to 0 for Static SM Power Save
mode
Set to 1 for Dynamic SM Power Save
mode
Set to 3 for SM enabled
Indicates the Spatial Multiplexing (SM) Power
SM Power Save Save mode. The value 2 is reserved
Indicates support for the reception of PPDUs Set to 0 if not supported
Greenfield with HT Greenfield format. Set to 1 if supported
Indicates Short GI support for the reception of Set to 0 if not supported
Short GI for 20 MHz 20 MHz packets Set to 1 if supported
Indicates Short GI support for the reception of Set to 0 if not supported
Short GI for 40 MHz 40 MHz packets Set to 1 if supported
Indicates support for the transmission of Set to 0 if not supported
Tx STBC PPDUs using STBC Set to 1 if supported
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69. HT Capabilities Info
Subfield Definition Encoding
Set to 0 for no support
Set to 1 for support of one spatial
stream
Set to 2 for support of one and two
spatial streams
Indicates support for the reception of PPDUs Set to 3 for support of one, two and
Rx STBC using STBC three spatial streams
Set to 0 if not supported
Set to 1 if supported
Support indicates that the STA is able
Indicates support for HTdelayed BlockAck to accept an ADDBA request for HT-
HT-delayed BlockAck operation. delayed Block Ack
Indicates maximum AMSDU length. See 9.7b Set to 0 for 3839 octets
Maximum A-MSDU length (A-MSDU operation). Set to 1 for 7935 octets
In Beacon, Measurement Pilot and
Probe Response frames:
Set to 0 if the BSS does not allow use
of DSSS/CCK in 40 MHz
Set to 1 if the BSS does allow use of
DSSS/CCK in 40 MHz
Otherwise:
Set to 0 if the STA does not use
DSSS/CCK in 40 MHz
Indicates use of DSSS/CCK mode in a 40 MHz Set to 1 if the STA uses DSSS/CCK
DSSS/CCK Mode in 40 MHz capable BSS operating in 20/40 MHz mode. in 40 MHz
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70. HT Capabilities Info
Subfield Definition Encoding
In Beacon, Measurement Pilot and
Probe Response frames transmitted
by an AP.
Set to 0 if the AP does not support
PSMP operation
Set to 1 if the AP supports PSMP
operation
In Beacon frames transmitted by a
non-AP STA:
Set to 0
PSMP support Indicates support for PSMP operation. See
Set to 0 by an AP if the AP allows use
of 40 MHz transmissions in
neighboring BSSs.
Set to 1 by an AP if the AP does not
allow use of 40 MHz transmissions in
neighboring BSSs.
Set to 0 by a STA to indicate to its
When sent by an AP, indicates whether other associated AP that the AP is not
BSSs receiving this information are required to required to restrict the use of 40 MHz
prohibit 40 MHz transmissions. transmissions within its BSS.
When sent by a STA, indicates whether the AP Set to 1 by a STA to indicate to its
associated with this STA is required to prohibit associated AP that the AP is required
40 MHz transmissions by all members of the to restrict the use of 40 MHz
Forty MHz Intolerant BSS. transmissions within its BSS.
L-SIG TXOP protection Indicates support for the LSIG TXOP Set to 0 if not supported
support protection mechanism
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71. Example Packet Trace Snippet of a Dlink
AP
5 * & " ( H
!" # $&
% $'
() * '
+ ' ( * ,,-. / !"
0# (1 " 2 2 &
34 5
( ! ( 5 ' 6 6
.0 ',
# 0 ',
+ % !"
% 7 !"
() & . ) (1 % 8 #&
++ 0 $9 &
+ ' %7 !" !"
(, $ & 5
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72. HT Capabilities IE: Supported MCS Set
7? # * E ) D ( # *
? # * 6 ( D ) ? 87 ? # * ; &
A 4 ) ? )
? ; &) & 1 & 3) " ) "
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73. HT Extended Capabilities
4 ' $
! & - ; 3 2
!- G 5 $ H
' & 7
' % ' & 7
9 9 9 %
4 - 7 &
, ! " ) & ) ( ! (
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74. HT Information Element
-Prev IE was about capabilities
-This IE is about actual state of BSS at any point of time (more dynamic)
-See next slide for more
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75. HT Information Element
96 '
' 7 ,
' F , ( -
- ' )!( 0 '6 '+
3 +
)$ &
"
• Does AP Tx beacon in " C 96 ' 7
secondary channel? 4 % , "''
E # *
)'
• Mandatory MCS for all
STAs in BSS ' )' ')" ; ,
• Similar to Basic rates of ')!
.11a/b/g "'' #5' 6C ) I 4
-Phased Coexistence (PCO Parameters)
-PCO Active
-PCO phase (20 or 40 Mhz switch)
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76. HT Information element
Operating mode
-Set to 0
Protection
-All STAs in BSS are 20/40 Mhz HT
-All STAs in a 20 MHz HT BSS are 20 Mhz
HT -Required for Operating mode 1 & 3
-Set to 1 (non-member protection)
-Some members on the channel (maybe
outside BSS) are non-HT -Protection mechanisms discussed
-Set to 2 earlier can be used
-At least one 20 Mhz only STA in a HT
BSS
- Set to 3
- Operating mode can also be
-MM (at least one legacy STA is present in updated dynamically based on BSS
BSS) constitution
Non-GF STAs present
-Set to 0
-All associated STAs in BSS are GF
stations
-Set to 1
-Some non-GF STAs present in a BSS
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77. Channel Switch & Extended Channel
Switch Elements
* &
6 %
• Useful for 40 Mhz transmission
• 0 indicates no sec channel, 2 is reserved
• 1 means secondary is above primary, 3 means below
" & 9
- (! , +
<? * &
'- - 2 (3 2+ ,
" & 9
- (! , +
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78. Quick Overview of advanced .11n features
Optional and/or not yet available today
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79. HTControl
Parameters used for channel measurement procedures and antenna selection
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80. HT Control: Link adaptation
Field Meaning Definition
TRQ Sounding Request Set to 1 to request the responder to transmit a
sounding PPDU.
When set to 0, the responder is not requested to
transmit a sounding PPDU.
MAI MCS request or Antenna When set to 14, the MAI field contains an Antenna
Selection Indication Selection Indication
(ASELI).
Otherwise the MAI field is interpreted
MFSI MFB Sequence Identifier Set to the received value of MSI contained in the
frame to which the MFB information refers.
Set to 7 for unsolicited MFB
MFB/ASELC MCS Feedback and When the MAI field is set to the value ASELI, this field
Antenna Selection is interpreted as defined in Figure n4 (ASELC
Command/Data subfield) and Table n3 (The ASEL Command and
ASEL Data parts of the ASELC subfield).
Otherwise, this field contains recommended MCS
feedback.
A value of 127 indicates that no feedback is present.
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81. RDP Exchange
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82. Thank you
I % JK
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