Though WiMAX is the most promising technology for enabling BWA systems to be widely deployed, many issues need to be addressed to make it effectively support the requirements and constraints of end-users' multimedia flows. To do so, according to the discussion mentioned previously, an efficient QoS-enabled scheduling algorithm has to be designed and implemented. In this section, we point out and briefly describe the most promising, as well as challenging, directions in such a field, by outlining a research roadmap for QoS provisioning in WiMAX networks. As we considered the scheduling algorithm in isolation in the last section, we shall now present cross-layer approaches, in which performance improvements are obtained by making an appropriate use of information which comes from the lower or upper layers.
Though WiMAX is the most promising technology for enabling BWA systems to be widely deployed, many issues need to be addressed to make it effectively support the requirements and constraints of end-users' multimedia flows. To do so, according to the discussion mentioned previously, an efficient QoS-enabled scheduling algorithm has to be designed and implemented. In this section, we point out and briefly describe the most promising, as well as challenging, directions in such a field, by outlining a research roadmap for QoS provisioning in WiMAX networks. As we considered the scheduling algorithm in isolation in the last section, we shall now present cross-layer approaches, in which performance improvements are obtained by making an appropriate use of information which comes from the lower or upper layers.§ Multiantenna architectures for WiMAX networks. In recent years, intensive research efforts have led to the development of spectrally efficient multi-user transmission schemes for wireless communications based on the use of multiple antenna systems. The use of multiple antennas in combination with appropriate signal processing and coding is indeed a promising direction which aims to provide a high-data rate and a high-quality wireless communications in the access link. In this sense, multiantenna systems can be seen as a way to enhance the cell capacity while offering a better and more stable link quality at the same time. On the other hand, antenna arrays can be used also to achieve beam-forming capabilities, with a remarkable improvement in terms of network performance. Adaptive Antenna Systems (AAS) are encompassed by the IEEE 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 receive on the same subchannel. This, obviously, demands the realization of a scheduling algorithm able to effectively exploit the presence of such beam-forming capabilities. In this way, through a cross-layer approach, striking 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 dimension) provided by the underlying PHY techniques. Although 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 direction with big potential in terms of throughput optimization. To fully take advantage of the power provided by multiple antenna systems, innovative 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 multi-user environment. Indeed, time-varying channels in multi-user environments provide a form of diversity, usually referred to as multi-user diversity, that can be exploited by an "opportunistic" scheduler, that is, a scheduler that selects the next user to be served according to the actual channel status . This approach may also be applied, at the cost of some additional complexity and signaling 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-realtime 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 to exploit multi-user 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 multi-user diversity by means of random beamforming mechanisms (usually referred to in the literature as "dumb" antennas ). Although this direction is somehow orthogonal in nature to the one (based on "smart antennas") outlined before, it could be worth investigating whether these two techniques may be implemented to coexist (e.g., in a timesharing fashion) to obtain the advantages of both approaches.
