Mobility management in small cell heterogeneous networks

Introduction In cellular networks, handover refers to the mechanism by which the set of radio links between an active mode mobile device and base station cells is modified. Mobility in the idle mode (when the mobile has no data bearers established and is not transmitting or receiving user plane traffic), termed cell selection/reselection, typically ensures that the UE selects the strongest available cell in preparation for an outgoing or incoming call/data session. Handover reliability is a key performance indicator (KPI) since it directly impacts the perceived quality of experience (QoE) of the end user. In contrast, cell reselection is less important since no bearers are established and suboptimal performance is apparent only on call establishment and as a signaling cost to the network operator. For this reason, the remainder of this chapter focuses on handover. In GSM and LTE the mobile supports only a single radio link such that the handover swaps this link from one cell (the serving cell) to another (the target cell). In WCDMA, however, multiple links (on the same frequency) may be established (known as “soft handover”). Handovers can be classified as: • intra-RAT, meaning within the same radio access technology (RAT), for example, LTE to LTE • intra-frequency (serving and target cells are on the same frequency) • inter-frequency (serving and target cells are not on the same frequency) • inter-RAT • between cells of different RATs. Handover may be triggered for a number of reasons: • to maintain the connectivity of the mobile and support data transfer (often called a “coverage handover”) • to balance the loading of cells with overlapping coverage or to handover a mobile between overlapping cells to ensure data rates demanded by an ongoing service are met (often called a “vertical handover”). Vertical handovers target stationary mobiles, implying that the radio conditions of links to serving and target cells are relatively stable. More challenging are coverage handovers that result from the motion of the mobile, leaving the coverage of the serving cell and entering that of the target cell. Since indoor users are usually stationary, the focus of coverage handovers is on outdoor users, on foot or in vehicles.