Null-While-Talk: Interference nulling for improved inter-technology coexistence in LTE-U and WiFi networks

A recent proposal known as unlicensed LTE offers cost-effective capacity extension to the cellular network operators in which LTE operators bundle the unlicensed spectrum in 5GHz UNII bands with their licensed spectrum via carrier aggregation. But, unlicensed spectrum requires coexistence among the networks operating in the same spectrum, e.g., IEEE 802.11 (WiFi) networks at 5GHz. While WiFi implements Listen-Before-Talk (LBT) and is therefore coexistence-friendly, LTE-Unlicensed (LTE-U) lacks such capability as it is not designed with shared spectrum access in mind. Hence, LTE has a potential to seriously harm WiFi. Prior works suggest forcing LTE-U to separate its transmission in either frequency, time, or space, and without directly collaborating with the WiFi networks. Contrary to these schemes, we introduce an explicit cooperation between neighboring LTE-U and WiFi networks. We propose Null-While-Talk(NWT) which suggests that LTE-U BSs employ MIMO signal processing to create coexistence gaps in space domain in addition to the time domain gaps by means of cross-technology interference nulling towards WiFi nodes in the interference range. In return, LTE-U can increase its own airtime utilization while trading off slightly its gain from MIMO. First, we present simulation results indicating that such cooperation offers benefits to both networks, WiFi and LTE-U, in terms of improved throughput and decreased channel access delay. Moreover, we present Xzero which implements NWT in a practical setting where the LTE-U BS lacks channel state and location information about the WiFi stations to be nulled. Xzero overcomes this challenge by performing an intelligent null search guided by constant feedback from the WiFi node on the null directions being tested. Our Xzero prototype implemented on SDR and COTS WiFi hardware shows the feasibility of our proposal.