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234 Tommaso Pecorella, Giada Mennuti into account, whereas, in the second one, other motion components, like Earth rotation and user movements, are considered. The key idea of the algorithm is that, in order to prevent handover failure during a call, bandwidth will be reserved in a particular number S of spot-beams that the call would handover into. In [38], a probabilistic resource reservation strategy for real-time services was proposed. The sliding window concept is adopted to predict the nec-essary amount of reserved bandwidth for a new call in its future handover spot-beams. As for real-time services, a new call request is accepted if the originated spot-beam has available bandwidth and resource reservation is successful in future handover spot-beams. As for non real-time service, new call requests are accepted if the originated spot-beam satisfies its maximum required bandwidth. In [6],[39], a selective look-ahead strategy is proposed where real-time and non-real time service classes are differently treated. Bandwidth allocation only pertains to real-time connection handovers. To each accepted connection, bandwidth allocation is performed in a look-ahead horizon of k cells along its trajectory. This algorithm offers low call dropping probability, i.e., a reliable management of call handovers of and acceptable call blocking probability for new calls. 7.4 Conclusions This Chapter has presented a set of dynamic bandwidth allocation techniques and identified associated research topics. We can conclude this Chapter by highlighting these two types of DBA problems and related techniques: • Handover-constrained techniques, mainly used for LEO satellites, where the main problem is to acquire a resource among a number of different satellites, since the communication lifetime is long enough to require a number of handovers; • Bandwidth-constrained techniques, affecting mainly GEO systems, where the main issue is to cope with the high delay-bandwidth product that makes the reactive approaches unfeasible for delay-constrained traffic types. The problem of multi-tier satellite systems, i.e., satellite systems using a combination of multiple orbital systems, like GEO+LEO, has not been considered, but it could be challenging, due to the multiple use of the different techniques among the various tiers. This problem requires further investigations as it involves also intra-tier and inter-tier routing schemes. Most of the described DBA techniques are inherently satellite-dependent; each satellite system should adapt or implement its own techniques in order to maximize system efficiency. A common theme is that optimizing ‘efficiency’ does not always means maximizing the bandwidth occupancy, but it is Chapter 7: DYNAMIC BANDWIDTH ALLOCATION 235 a concept more related to fulfilling the system goals in terms of QoS, user satisfaction and, ultimately, system capacity to maximize the network operator’s revenue. Hence, one of the possible approaches to further study DBA techniques is to embed a cost-function into the DBA decision process, in order to introduce an abstraction layer between the raw user bandwidth requests and the actual bandwidth allocation decision algorithms. Another topic that needs further investigation is represented by the fair-ness of the proposed techniques. Most techniques that involve terminal-based decisions (like in most DVB-RCS systems) can be heavily affected by fairness issues in a multi-vendor and multi-algorithm environment, thus creating serious issues in real-world deployments. At present, this problem is still an open point and should be addressed either by allowing the centralized decision process to take into account the different behaviors, or by defining some fairness threshold that every user equipment implementation must comply with. We must observe that the first option is not viable in the long-term, as it requires extra-work in the bandwidth allocation decision unit, along with the knowledge of every implementation, and this is not always possible. The second option requires the definition of precise fairness metrics and test suites to certify the user terminal fairness. The DBA implementation is therefore a key element for the efficient oper-ation of many satellite systems. 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