As PHY layer data rates increase, CSMA MAC overheads dominate. The 9 us slot width at 1Gbps data rate can result in MAC efficiency of under 10%. WiFi-Nano proposes a novel speculative transmission based technique that leverages self-interference cancelation and allows for using 800ns slots -- reducing CSMA overheads by an order of magnitude.
Eugenio Magsiretti, Krishna Kant Chintalapudi, Bozidar Radunovic, and Ramachandran Ramjee, WiFi-Nano : Reclaiming WiFi Efficiency through 800 ns slots, in Mobicom, 2011 (Among the 3 best paper award candidates)
Overhead due to CSMA
CSMA allows multiple devices to share the spectrum and co-exist and consequently is perhaps the important mechanism in WiFi. In CSMA, each device generates a random number and counts down to zero before transmitting its packet. Upon sensing an ongoing transmission devices suspend their countdown. However, before decrementing the counter, each device must ensure that no new transmissions were initiated and this defines the slot-width the time that a device must wait before it decrements its counter. WiFi uses a 9us slot width. At high data rates, this slot-width can account for up to 80% of the MAC overhead.
Why slot-width in WiFi is 9us
In order to facilitate CSMA, each transmission in WiFi is preceded by a 4us long preamble. Devices continuously correlate against this preamble to detect an ongoing transmission. Further, devices must allow time for switching between transmit and receive modes and allowances must be made for propagation delays of light.
WiFi-nano uses self-interference cancelation to allow detection of ongoing transmissions even while the device is itself transmitting. This allow for speculative transmissions -- where devices can begin transmitting even before they have ascertained that the medium is idle. However, upon detecting an ongoing transmission, devices can abort their transmissions. In order to enable this, WiFi-Nano uses a novel correlation technique -- the lattice correlator that allows devices to correlate against sub-parts of the preamble. WiFi-nano can now use 800 ns slots, only limited by the speed of propagation of light. Thus, WiFi Nano achieves much higher efficiency WiFi especially at higher PHY data rates.