  LTE femtocells have been widely deployed to increase network capacity and to
offload mobile data traffic from macrocells. While cellular users' mobility
behaviors are taken into consideration, a dilemma is formed: Should a User
Equipment (UE) either handover into a femtocell or keep the current connection
with a macrocell? Indeed, various user mobility behaviors may incur significant
signaling overhead and degrade femtocell offloading capability due to frequent
handover in/out femtocells. To address this dilemma, in this paper we propose
an Optimal Threshold Offloading (OTO) algorithm considering the tradeoff
between the signaling overhead and femtocell offloading capability. We develop
an analytical model and define two performance metrics to quantify the
tradeoff. The proposed model not only models user mobility behaviors but also
captures femtocell offloading benefits, and shed light on their fundamental
relationship. The correctness of analytical model and simulation model are
cross-validated by extensive ns2 simulations. Both analytical and simulation
results demonstrate that the OTO algorithm can significantly reduce signaling
overhead at the minor cost of femtocell offloading capability. The results
enable wide applicability in various scenarios, and therefore, have important
theoretical significance. Moreover, the analytical results provide a quick way
to evaluate signaling overhead and offloading capability in LTE networks
without wide deployment, saving on cost and time.
