sign for RAN Slicing Internet Slice 2 Slice 1 Network slice information Slice 3 broadcast Network slice link Wireless radio link : Slice 1 ((o)) : Slice 2 : Slice 3 Figure 9.6 NS-based mobile network architecture. time Ti, which is the maximum time a UE is allowed to have a transmission rate lower than the minimum threshold. Let T = {T1, T2, , TL} be the set of all service types, and Wi E T be the service type of UE i. We say W = Tn when both zmin and Ti can fulfill the requirement of the service type Tn. We identify a specific NS, say NS j, by the two elements (Tj,Bj), where Tj is the set of service types that NS j can provide, and B is a vector denoting the bandwidth allocation of NS j from all BSs. Let bj h be the k-th element of vector Bj denoting the bandwidth of NS j allocated by BS k. b(k) = when BS k is not in the coverage of NS j. UEs can access the NS via only the covered BSs. In the example of Figure 9.6, UEs can access NS 1 via only BSs 1 and 2. We describe the handoff model from two aspects: handoff trigger condition and handoff cost. Handoff should occur once the QoS of the UE cannot be satisfied [14, 15]. Based on the definition of QoS, the handoff trigger condition for UE n can be written as E [t - TN, t], r (9.18) where r (to) is the achievable transmission rate of UE n at time to. This condition states that UE n cannot achieve the minimum rate requirement ymin in the last Tn time. Once the handoff trigger condition is met, UEs need to select suitable target BSs and NSs. As aforementioned, each type of handoff corresponds to a specific handoff cost. Here we define four handoff costs generated by the four handoff types: (i) CNS, switch NS only; (ii) CBS, switch BS only; (iii) CNS-BS, switch both NS and BS; (iv) CNew, deploy a new NS (this type can be seen as a special handoff in RAN slicing); with the relationship CNS < CBS <CNS-BS < CNew Based on this, we design a handoff mechanism to minimize the overall handoff cost through target BS and NS selections while guaranteeing the QoS of UEs. 9.2 Smar