Listen (on the frequency domain) before you talk

Abstract

Conventional WiFi networks perform channel contention in time domain. This is known to be wasteful because the channel is forced to remain idle, while all contending nodes are backing off for multiple time slots. This paper proposes to break away from convention and recreate the backing off operation in the frequency domain. Our basic idea is to pretend that OFDM subcarriers are integer numbers, and thereby, view today's random backoff process as equivalent to transmitting on a randomly chosen subcarrier. By employing a second antenna to listen to all the subcarriers, each node can determine whether its chosen integer (or subcarrier) is the smallest among all others. In fact, each node can even determine the rank of its chosen integer, enabling the feasibility of a TDMA-like schedule from every round of contention. We develop these ideas into a Time to Frequency (T2F) protocol and prototype it on a small testbed of 8 USRPs. Experiments confirm its feasibility, along with promising throughput gains of more than 35% at high bit rates. A fuller design and thorough evaluation of T2F is a topic of ongoing work.

Polling in the frequency domain: a new MAC protocol for industrial wireless network for factory automation

“…In SMACK (Dutta et al, 2009), OFDM subcarriers are used to acknowledge broadcast messages simultaneously to achieve reliable broadcast while reducing MAC overhead at the same time. FICA (Tan et al, 2010), T2F (Sen et al, 2010) and Back2F (Sen et al, 2011) treat OFDM subcarriers as integer numbers, and instead of picking a random back-off duration in time, nodes contend the channel by signalling on a randomly chosen subcarrier. By utilising information on OFDM subcarriers, REPICK (Feng et al, 2012) uses reversed contention and piggy-backed ACK in frequency domain to reduce the overhead in traditional 802.11 MAC protocol.…”

“…While in subcarrier detection, the AP does not need to recover the data transmitted by subcarriers, which make it easily to be implemented in hardware and achieve more accurate subcarrier detection. Experimental results in USRP/GNURadio platforms show 95% accuracy in subcarrier detection can be achieved (Sen et al, 2010(Sen et al, , 2011. In these experiments, the self-signal from the transmitting antenna may decrease the subcarrier detection accuracy.…”

Use your frequency wisely: Explore frequency domain for channel contention and ACK

“…With self-interference cancellation technologies, the listening antenna can detect which subcarriers are activated by nearby nodes when the transmission antenna is sending packets concurrently. The feasibility of the use of the listening antenna has been verified by system implementations in existing works such as [10] [11] [12].…”

“…4, S , N 1 and N 2 choose contention subcarrier 14, 23 and 35 respectively. The transmission on the contention subcarrier can be implicitly synchronized and detected by the listening antenna [10] [11]. The node with the smallest contention subcarrier ID wins.…”

“…Recently, some works propose to use physical layer technologies to improve the MAC layer efficiency [3] [4]. Within this category, some works, for example, FICA [2], T2F [10], Back2F [11], propose to conduct channel contention in the frequency domain. They treat the OFDM (Orthogonal Frequency Division Multiplexing) subcarriers used at the physical layer as integer numbers and use these subcarriers for channel contention to reduce the backoff time.…”