a higher priority 3G layer if RSCP or Ec/NO is above a threshold B2 (inter-RAT) Ms < b2Threshold1 With Ms = RSRP and Mn = RSCP, coverage handover Mn > b2Threshold2 to a lower priority 3G layer if the serving cell level is below acceptable, but lower priority layer offers sufficient RSCP (coverage) mode equivalent of moving to a higher priority layer whose level or quality is better than a minimum threshold. Such equivalence can be useful in unifying and simplifying the parameter design and optimization of inter-layer mobility thresholds, as will be discussed in the next section. Inter-layer Mobility for Coverage-Limited Network 15.4.1 Basic Concepts In a cellular network each UE should be served by the cell that offers the best service, at least if the mobility parameter design problem is approached purely from UE viewpoint. From network perspective, connecting each and every UE to the best server may not be the best strategy if the target is to maximize network spectral efficiency. The seemingly simple problem of mobility parameter design has received considerable attention in research community where Inter-layer Mobility Optimization Exit point Entry point Higher priority LTE layer Hysteresis LTE entry point Lower priority LTE layer LTE exit point Hysteresis Lowest priority layer (2G or 3G) Loss of network service Figure 15.6 Illustration of inter-layer mobility exit and entry points for a three-layer network it is often cast as a so-called 'user association' problem. The optimum solution to the user association problem is the assignment of each UE to a cell SO that some optimality criterion such as network spectral efficiency is maximized. Various approaches to solve the problem for different optimality criteria can be found in literature, for example, [7, 8], and in many other references listed therein. Especially the introduction of small cells has boosted the efforts on the optimum user association research. However, most of the current research literature on the topic is not very useful in practical daily 