# Handover-Driven Interference Management for Co-Channel Deployment of Femto- and Macro-Cells
## 2 Motivation
### Why Handover for Interference Mitigation? A main challenge associated with femtocell technology is how to protect macrocell user services against femtocell users' interference when macro- and femtocells use the same frequency band, while exploiting high spatial reuse of spectrum resources within each femtocell. Due to the high cost of the licensed spectrum, operators may wish to allocate both macro- and femtocells the same frequency channel, called _co-channel deployment_. Under this co-channel deployment, however, transmissions of femtocell users (those being served by femtocells) may cause interference to the ongoing transmissionsof macrocell users (those being served by macrocells), and vice versa, in both downlink (DL) and uplink (UL) communications. As briefly reviewed below in this section, there have been numerous research efforts to tackle this problem based on radio resource coordination; these approaches can be classified into orthogonal _resource block_ (RB) assignment (a resource block (RB) is a group of subcarriers and also the minimum scheduling granule in OFDMA systems; for example, in 3 GPP Long-Term Evolution (LTE), 12 contiguous subcarriers compose an RB (5 RBs in a 900 kHz band)), power control or combinations thereof. Figure 1 compares the achieved DL SINR of a macrocell user under different approaches, showing that radio resource coordination can mitigate the interference problem to some degree. As a macrocell user passes by a femto BS located 300 m away, his SINR gets deteriorated considerably. To protect the nearby macrocell users' communications, the femto BS can reduce its transmitting power (power control) or use a different RB from that allocated to the macrocell user (orthogonal RB assignment). However, both approaches, if operated dynamically, require the detailed (e.g., per-RB) and real-time information of the macrocell's resource usage, and, moreover, their achievable gain is limited. Another solution that can be combined with radio resource coordination is to hand the macrocell user over to the interfering femtocell if the femtocell has spare resources to serve the user. Figure 1 shows that the handover approach can not only mitigate the interference problem but also improve the achievable capacity significantly. The performance gain of the handover approach comes from: (1) reduced interference (due to reduced path losses and enabled coordination within the same serving cell); and (2) load-balancing, as explained next. A macrocell user may experience high DL interference from, and produce high UL interference to, a nearby femtocell. If the macrocell user is handed over to the femtocell, he may experience reduced loss of paths to his serving cell (i.e., the femtocell), thus achieving a better SINR with the same radio resources or a target SINR with less radio resources. Moreover, both macro- and femtocell users--previously interfered with by each other--will be served by the same femtocell with their interference coordination enabled. In many cases, femtocells are also likely to be less loaded than macrocells, as they typically serve fewer users. Thus, a macrocell user, if handed over to a femtocell, will likely be allocated more radio resources. However, the standard handover decisions based on the strongest signal power are not suitable for realizing the above benefits, since, due to the overlaid cell deployment, a macrocell's signal strength could still be strong within a femtocell's coverage area. Therefore, the handover-based approach requires a new method of making handover decisions, tailored to femtocell networks. It should determine which user should be handed over to which femtocell and when.