1. doc.:IEEE 802.11-12/0109r0 Submission Laurent Cariou January 16, 2012 Slide 1 Short Ack Authors: Date: 2012-01-16

2. 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.

3. 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

4. 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 FCS Durat °/ ID Address 1 RA Address 2 TA Frame Ctrl ACK FCS Durat ° Receiver Frame Ctrl DAT A

5. 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-LTF L-SIG ACK body frame ACK Short ACK

6. 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.

7. 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

8. doc.:IEEE 802.11-12/0109r0 Submission Laurent Cariou January 16, 2012 Scenario 1: Intra-BSS collision AP STA 1 STA 2 Frame 2 t1+DIFS+BOt2 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

9. 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 2 Frame 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 t t1 Frame 1 OK, thus acknowledge Frame 1 is acknowledged Frame 2 is acknowledged, while it shouldn’t Ack ACK

10. 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.

11. 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 AP UL Packet received channel impulse response STA AP DL ACK L-STF Correlation with the reverse channel impulse response. ACK channel impulse response concentrated in time only for this STA ACK received by the intended destinator ACK received by another destinator

12. 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

13. 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