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doc.:IEEE 802.11-12/0109r0
Submission Laurent Cariou
January 16, 2012
Slide 1
Short Ack
Authors:
Date: 2012-01-16
Name Company Address Phone email Laurent Cariou Orange Labs 4, rue du clos courtel
35512 Cesson Sévigné 33-299124350 [email protected]
m Philippe Christin Orange Labs 4, rue du clos courtel
35512 Cesson Sévigné philippe.christin@orange.
com Getachew Redieteab Orange Labs 4, rue du clos courtel
35512 Cesson Sévigné [email protected]
m
doc.:IEEE 802.11-12/0109r0
Submission Laurent Cariou
January 16, 2012
Scenarios
• 802.11ah is of particular interest for Orange for the Machine-type communication use cases, both for indoor or outdoor.
• On top of that, they are the only current exploitable use cases in Europe.
doc.:IEEE 802.11-12/0109r0
Submission Laurent Cariou
January 16, 2012
Limitations
• With such use cases, the transmitted data packet size are quite similar for all users and are very short (sensor reports).
• Data packet duration is therefore almost equal to ACK duration.
• As ACKs represent almost 50% of the total transmitted packets, a reduction of the ACKs duration would be very beneficial.
– for capacity
– for battery lives (reduction of the awake-state duration in case of sleep mode)
• For that reason, we propose a Short ACK
doc.:IEEE 802.11-12/0109r0
Submission Laurent Cariou
January 16, 2012
Classical ACK
• ACKs are carrying the information:– to signal the identity of the ACK destinator (sendor or the previous packet)
– to signal if the packet has been received (implicit)
L-STF L-LTF L-SIG ACK body frame
Address 3(DA)
Address 4(SA)
Sqce Ctrl
Frame body FCSDurat°/ ID
Address 1RA
Address 2TA
Frame Ctrl
ACK
DATA
doc.:IEEE 802.11-12/0109r0
Submission Laurent Cariou
January 16, 2012
Short ACK
• Reducing the ACK MAC body is not efficient, the best solution is to suppress the body.
• We then have a simple PHY-layer ACK, which can be shortened to the simple transmission of the L-STF.
– at the receiver, the CCA detects a WIFI transmission thanks to the L-STF classical receiving process, and becomes busy.
– the transition of the CCA back to idle after the L-STF duration allows the receiver to detect that it is an ACK (note that the receiver is waiting to receive this ACK)
– This PHY-level ACK detection is then forwarded to the MAC layer.
• The short ACK becomes similar to the NACK which is an implicit PHY-layer “no ACK”: the detection of a NACK is made by the non-reception of the ACK
Slide 5
L-STF
L-STF L-LTF L-SIG ACK body frameACK
Short ACK
doc.:IEEE 802.11-12/0109r0
Submission Laurent Cariou
January 16, 2012
Short ACK
• The information that we loose is the confirmation that the ACK corresponds to the previous packet transmission.
– in case of false-detection, the transmitter will not send the packet again.
• But do we really loose this information?– Not really, because classical protections (including CCA protection) ensures that
only the destinator is allowed to transmit.
doc.:IEEE 802.11-12/0109r0
Submission Laurent Cariou
January 16, 2012
Only some scenarios can be problematic• Sensor network characteristics:
• Great number of sensors
• Power save mode are active: wake-up to receive DTIM field in beacons
• Uniform packet size (and probably reduced number of available MCSs)
• Consequences on transmissions:• Sensors that wake-up will all contend for channel access at the same time
after the beacon
• The probability of sensors having the same backoff is high
• it is therefore likely that two sensors will initiate uplink transmission simultaneously toward the AP, and that the duration of the transmission will be the same
• This leads to the following use cases
doc.:IEEE 802.11-12/0109r0
Submission Laurent Cariou
January 16, 2012
Scenario 1: Intra-BSS collision
AP
STA 1
STA 2Frame 2
t1+DIFS+BO t2
t
t
t
STA 1
STA 2
AP
Frame 1
Frame 1 OK, thus acknowledged
t2+SIFSt1
Ack
Frame 1 is acknowledged
Frame 2 is acknowledged, while it shouldn’t
ACK
doc.:IEEE 802.11-12/0109r0
Submission Laurent Cariou
January 16, 2012
AP 1
Scenario 2: OBSSs with hidden stations
AP 1
STA 1
STA 2Frame 2
t0+DIFS+BO1=t1+DIFS+BO2
t2
t
t
t
STA 1
Frame 1
Frame 2 erroneous, do not acknowledge
t2+SIFSt0
AP 2
STA 2
AP 2
tt1 Frame 1 OK, thus acknowledge
Frame 1 is acknowledged
Frame 2 is acknowledged, while it shouldn’t
Ack
ACK
doc.:IEEE 802.11-12/0109r0
Submission Laurent Cariou
January 16, 2012
Solution for these scenarios• The solution is to improve short ACK with a PHY-layer receiver
identifier (PHY-layer protection)
• This can be done simply by applying time reversal (TR) technique to the short ACK transmission.
doc.:IEEE 802.11-12/0109r0
Submission Laurent Cariou
January 16, 2012
Short ACK with Time Reversal• TR consists in focusing the energy in space and time
toward the destinator by exploiting the previous signal reception.
Slide 11
STA APUL Packet
received channel impulseresponse
STA APDL ACK L-STF
Correlation with the reversechannel impulse response
.
ACK channel impulse response concentrated in timeonly for this STA
ACK received by the intended destinator ACK received by another destinator
doc.:IEEE 802.11-12/0109r0
Submission Laurent Cariou
January 16, 2012
Short ACK with Time Reversal• By doing this,
– the ACK reception sensitivity is improve for the destinator
– the identity of the destinator is implicitly transmitted with the ACK (embedded in the concentrated channel impulse response)
Slide 12
doc.:IEEE 802.11-12/0109r0
Submission Laurent Cariou
January 16, 2012
Conclusion
• Short ACK enables strong capacity and power saving gains
• To solve the destinator information ambiguity with short ACK, time reversal (TR) protection is a simple answer.
Slide 13
