  Due to the dramatic growth in mobile data traffic on one hand and the
scarcity of the licensed spectrum on the other hand, mobile operators are
considering the use of unlicensed bands (especially those in 5 GHz) as
complementary spectrum for providing higher system capacity and better user
experience. This approach is currently being standardized by 3GPP under the
name of LTE Licensed-Assisted Access (LTE-LAA). In this paper, we take a
holistic approach for LTE-LAA small cell traffic balancing by jointly
optimizing the use of the licensed and unlicensed bands. We pose this traffic
balancing as an optimization problem that seeks proportional fair coexistence
of WiFi, small cell and macro cell users by adapting the transmission
probability of the LTE-LAA small cell in the licensed and unlicensed bands. The
motivation for this formulation is for the LTE-LAA small cell to switch between
or aggregate licensed and unlicensed bands depending on the
interference/traffic level and the number of active users in each band. We
derive a closed form solution for this optimization problem and additionally
propose a transmission mechanism for the operation of the LTE-LAA small cell on
both bands. Through numerical and simulation results, we show that our proposed
traffic balancing scheme, besides enabling better LTE-WiFi coexistence and
efficient utilization of the radio resources relative to the existing traffic
balancing scheme, also provides a better tradeoff between maximizing the total
network throughput and achieving fairness among all network flows compared to
alternative approaches.
