  To avoid the foreseeable spectrum crunch, LTE operators have started to
explore the option to directly use 5 GHz unlicensed spectrum band being used by
IEEE 802.11 (WiFi). However, as LTE is not designed with shared spectrum access
in mind, there is a major issue of coexistence with WiFi networks. Current
coexistence schemes to be deployed at the LTE-U BS create coexistence gaps only
in one domain (e.g., time, frequency, or space) and can provide only
incremental gains due to the lack of coordination among the coexisting WiFi and
LTE-U networks. Therefore, we propose a coordinated coexistence scheme which
relies on cooperation between neighboring LTE-U and WiFi networks. Our proposal
suggests that LTE-U BSs equipped with multiple antennas can 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 antenna diversity. The cooperation offers benefits to both
LTE-U and WiFi in terms of improved throughput and decreased channel access
delay. More specifically, system-level simulations reveal a throughput gain up
to 221% for LTE-U network and 44% for WiFi network depending on the setting,
e.g., the distance between the two cell, number of LTE antennas, and WiFi users
in the LTE-U BS neighborhood. Our proposal provides significant benefits
especially for moderate separation distances between LTE-U/WiFi cells where
interference from a neighboring network might be severe due to the hidden
network problem.
