RESEARCH CHALLENGES 27

802.16 standard to improve the PHY-layer characteristics. However, AAS can also act as enablers of Spatial Division Multiple Access (SDMA) schemes. In this way, multiple SSs, separated in space, can simultaneously trasmit or re-ceive on the same subchannel. This, obviously, demands the realization of a scheduling algorithm able to effectively exploit the presence of such beamform-ing capabilities. In this way, through a cross-layer approach, strikbeamform-ing results can be obtained in terms of QoS support. An AAS-aware scheduling could indeed profit from the additional degree of freedom (i.e., the spatial dimen-sion [30]) provided by the underlying PHY techniques. While this may lead to better performance, it also leads to an increase in the complexity of the scheduler itself. Nonetheless, we believe that the use of this and other related multiantenna techniques (e.g., space-time codes) represent a research direc-tion with big potential in terms of throughput optimizadirec-tion. In order to fully take advantage of the power provided by multiple antenna systems, innova-tive QoS-enabled scheduling algorithms, able to work in both space and time dimensions, need to be designed and engineered.

• Opportunistic Scheduling. In wireless networks, channel conditions may vary over time because of user mobility or propagation phenomena. These effects are usually referred to as shadowing and fading, depending on their typical time-scales. They have been traditionally considered as harmful features of the radio interface due to their potentially negative impact on the quality of communication. However, recent research has shown that the time-varying nature of the radio channel can be used for enhancing the performance of data communications in a multiuser environment. Indeed, time-varying channels in multiuser environments provide a form of diversity, usually referred to as multiuser diversity, that can be exploited by an “opportunistic” scheduler, i.e., a scheduler that selects the next user to be served according to the actual channel status [31]. This approach may also be applied, at the cost of some additional complexity and signalling between PHY and MAC, to WiMAX networks. Opportunistic scheduling schemes do not usually apply to flows that require QoS guarantees, due to the unpredictable delays that may come from the channel dynamics. However, their use may actually lead to an enhanced

QoS support. For example, improving the effect of non real-time traffic (i.e., nrtPS and BE traffic) would free some additional resources to higher priority traffic. In this way, opportunistic scheduling schemes may actually help to increase the QoS capabilities of WiMAX networks. Moreover in this case, novel scheduling schemes are required in order to exploit multiuser diversity while providing QoS guarantees to the active traffic flows at the same time. It may be interesting to note that multiple antenna systems can actually be used to build up multiuser diversity by means of random beamforming mechanisms (usually referred to in the literature as “dumb” antennas [32]). While this direction is somehow orthogonal in nature to the one (based on “smart antennas”) outlined above, it could be worth investigating whether these two techniques may be implemented to coexist (for example, in a time-sharing fashion) in order to obtain the advantages of both approaches.

• QoS Support in Mesh-Based Architectures. The techniques we have presented above as research challenges are aimed at providing a better QoS support in PMP architecture. However, they are still subject to the limits imposed by such an architectural choice in terms of service coverage, network capacity and system scalability. One possible solution to overcome such problems could be the adoption of a mesh-based architecture [33]. In mesh topologies, direct communication among neighboring SSs is allowed, so enhancing the network coverage and possibly enabling the deployment of a fully wireless backbone connecting to an Internet gateway. While mesh-based architectures offer in-teresting possibilities thanks to its inherent flexibility, they also present many research challenges to be addressed in terms of medium access control and packet routing. This is even more challenging in the case of QoS support for multimedia flows, where reliable levels of services have to be ensured by means of distributed algorithms. In this framework, a “double cross-layer” approach (where information is shared among PHY, MAC and NET layers) may lead to potentially dramatic performance improvements compared to conventional layered solutions. This clearly entails the definition of radically innovative scheduling protocols, which are able to work in a distributed and collabora-tive way, so cooperating with the routing algorithms in order to provide QoS

REFERENCES 29

guarantees to service flows based on some PHY information. For example, the integration of scheduling and routing protocols can be based on the actual channel conditions, as well as on the level of interference in the network ⁵. The application of these concepts to WiMAX networks is not straightforward, since it would imply some major modifications to the actual standard, in terms of both signalling (necessary for pursuing cross-layer optimization) as well as definition of basic functionalities and interfaces of the routing protocol to be employed.

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