Adaptation Techniques and Enabling Parameter Estimation Algorithms for Wireless Communications Systems - Adaptive Signal Processing in Wireless Communications - page 25

High speed user
Slow
speed
user
Microcell
Overlay macrocell
FIgure 1.11 Illustration of two-layer hierarchical cell structure. High-speed mobiles are
assigned to large cells, and low-speed mobiles are assigned to smaller cells.
CCI and ACI [67]. As a result, for nonuniform traffic conditions, the available channels
can be used more efficiently.
Resource utilization has also evolved by employing a concept called hierarchical cel-
lular structures (HCSs) [68]. The use of HCS has become a major component in third-
generation mobile systems such as UMTS and IMT-2000. In an HCS, various cell sizes
are deployed and small cell clusters are overlaid by larger cells. For example, Figure 1.11
shows a two-layer (e.g., microcell and macrocell) hierarchical system. Microcells increase
capacity within a coverage area, but radio resource management becomes more diffi-
cult. The number of handoffs per cell is increased by an order of magnitude, and the
time available to make a handoff is decreased. HCSs handle this by assigning cells to
the mobiles depending on their speeds (Doppler spread estimate). For example, in the
two-layer structure given in Figure 1.11, low-speed mobiles are assigned to microcells,
whereas high-speed mobiles are assigned to macrocells. Hence, the macrocell-microcell
overlay architecture provides a balance between maximizing the capacity per unit area
and minimizing the number of handoffs [69]. As a result, the risk of call dropping is
reduced, and t here are ot her benefits, like lower handover delays, reduced switching load,
and increased QoS. The HCS can be more than two layers (multilayer HCS). For example,
picocellular layers can also be included in multilayer HCS. Similarly, communication
satellite beams can overlay all the terrestrial layers at the highest hierarchical level.
Recently, dynamic allocation and multitiered design strategies are further general-
ized to take power control, cell handoff, traffic classes (like multimedia), and user pri-
orities into account. Also, there are several studies toward combining link adaptation
schemes with adaptive resource allocation. For example, adaptive modulation (and cod-
ing) can be combined with dynamic channel allocation. Similarly, adaptive modulation
(and coding) can be combined with handover algorithms to introduce more intelligent
handover strategies. All these developments require more sophisticated adaptation of
the network, and they are based on many parameter measurements.