Short Ack Date: 2012-01-16 Authors:
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.
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
Classical ACK Frame body FCS Address 4 (SA) Sqce Ctrl Frame Ctrl Address 3 (DA) Address 1 RA Address 2 TA Durat°/ ID Frame Ctrl Receiver Durat° FCS • 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) DATA ACK L-SIG L-STF ACK body frame L-LTF
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 L-SIG L-STF ACK body frame L-LTF ACK L-STF Short ACK
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.
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
Scenario 1: Intra-BSS collision ACK AP STA 2 STA 1 Frame 1 OK, thus acknowledged Ack AP t Frame 1 is acknowledged Frame 1 STA 1 t Frame 2 is acknowledged, while it shouldn’t Frame 2 STA 2 t t1 t1+DIFS+BO t2 t2+SIFS
Scenario 2: OBSSs with hidden stations ACK AP 1 AP 2 STA 1 STA 2 Frame 2 erroneous, do not acknowledge Frame 2 is acknowledged, while it shouldn’t AP 2 t Frame 2 STA 2 t t1 Frame 1 OK, thus acknowledge Ack AP 1 t Frame 1 is acknowledged Frame 1 STA 1 t t0 t2 t2+SIFS t0+DIFS+BO1 =t1+DIFS+BO2
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.
Short ACK with Time Reversal • TR consists in focusing the energy in space and time toward the destinator by exploiting the previous signal reception. received channel impulse response UL Packet STA AP DL ACK STA AP ACK channel impulse response concentrated in time only for this STA ACK received by the intended destinator ACK received by another destinator L-STF . Correlation with the reverse channel impulse response
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)
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.