Opportunistic Scheduling in Wireless Networks
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15-10-2010, 04:27 PM

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This article is presented by:Vladimir Vukadinovic and Edouard Drogou
Opportunistic Scheduling in Wireless Networks

In wireless networks, the channel conditions are time-varying due to the fading and shadowing. Different wireless users experience different channel conditions at a given time. This gives raise to the multi-user diversity effect: when there are many users which fade independently, at any one time there is a high probability that some of the users will have a strong channel. By allowing only those users to transmit, the shared channel resource is used in the most efficient manner and the total system throughput is maximized. Such scheduling mechanisms are called opportunistic because they take advantage of favorable channel conditions in assigning time slots to users. If the service requirements of all the users are flexible, such opportunistic scheduling mechanisms can result in higher spectrum utilization, and increased system throughput. Nevertheless, in practice there are several considerations to take into account before realizing such gains. To implement the idea of opportunistic scheduling in a real system, two issues need to be addressed: fairness and users’ service requirements. In reality, channel statistics of different users are not symmetric and, therefore, a scheme designed only to maximize the overall throughput could be very biased, especially where there are users with widely disparate distances from the base station. For example, allowing only users close to the base station to transmit may result in very high throughput, but sacrifice the transmission of other users. Also, a scheduling strategy should not be concerned only with maximizing long-term average throughputs because, in practice, applications may have different utilities and service constraints. For instance, for realtime applications, the major concern is latency: if the channel variations are too slow, a user may have to wait for a long time before it gets the chance to transmit. When designing a scheduling algorithm, the challenge is to address these issues while at the same time exploiting the multi-user diversity gain inherent in a system. Improving the efficiency of spectrum utilization is important, especially to provide high-rate-data service. However, the potential to exploit higher data throughputs in an opportunistic way, introduces the tradeoff problem between wireless resource efficiency and levels of satisfaction among users. The cellular system itself also has to satisfy certain requirements in order to extract the multi-user diversity benefits. The base station has to have access to channel quality measurements: in the downlink, each receiver needs to track its own channel signal-to-noise ratio (SNR) and feed back this information to the base station. The base station has to be able to schedule transmissions among the users on a short timescale as well as to adapt users’ data rates to the instantaneous channel quality. These features are already present in the designs of many 3.5G high data-rate (HDR) systems. This is the reason why the opportunistic scheduling has received lots of attention recently. In this project and implimentation report, we provide a comparative survey of four papers in this area that are listed in the Literature. These papers address the issues that we discussed, either by proposing design principles for opportunistic scheduling, or by analyzing the performance of existing schemes. In the next Section, we provide a brief survey of objectives and contributions for each paper.


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