  Future mobile networks will exploit unlicensed spectrum to boost capacity and
meet growing user demands cost-effectively. The 3GPP has recently defined a
Licensed-Assisted Access (LAA) scheme to enable global Unlicensed LTE (U-LTE)
deployment, aiming at ($i$) ensuring fair coexistence with incumbent WiFi
networks, i.e., impacting on their performance no more than another WiFi
device, and ($ii$) achieving superior airtime efficiency as compared to WiFi.
In this paper we show the standardized LAA fails to simultaneously fulfill
these objectives, and design an alternative orthogonal (collision-free)
listen-before-talk coexistence paradigm that provides a substantial improvement
in performance, yet imposes no penalty on existing WiFi networks. We derive two
LAA optimal transmission policies, ORLA and OLAA, that maximize LAA throughput
in both asynchronous and synchronous (i.e., with alignment to licensed anchor
frame boundaries) modes of operation, respectively. We present a comprehensive
performance evaluation through which we demonstrate that, when aggregating
packets, IEEE 802.11ac WiFi can be more efficient than 3GPP LAA, whereas our
proposals can attain 100% higher throughput, without harming WiFi. We further
show that long U-LTE frames incur up to 92% throughput losses on WiFi when
using 3GPP LAA, whilst ORLA/OLAA sustain $>$200% gains at no cost, even in the
presence of non-saturated WiFi and/or in multi-rate scenarios.
