n (CSI), and the opportunistic approach that use CSI to optimize for spectral usage. Both have merits depending on the objectives. The following subsections give an overview of both approaches. Further details can be found by referring to the research papers directly. Chapter 7 Wireless scheduling and Quality of Service optimization techniques 243 Opportunistic scheduler Opportunistic schedulers are schedulers that take advantage of physical layer (CSI) informa- tion to attempt to optimize to (typically) aggregate (system-wide) wireless capacity, option- ally combined with adherence to minimum QoS. The motivation behind opportunistic scheduling is that since bandwidth is a limited commodity, efficient use of the available resources should be maximized. Research on previous attempts at channel capacity maximi- zation reveals that there are three main ways of achieving this aim. The first is to schedule transmission to a user only when channel conditions are "good" for that user, taking advan- tage of the time-varying channel. The second option is to schedule transmission to a user on the frequency band (subchannels or subcarriers in the OFDMA signal) that yields a good transmission level. This takes advantage of frequency-varying channel conditions. The third alternative is to increase frequency reuse by allocating transmission power to subchannels dynamically, such that the resulting signal-to-noise ratio (SNR) is only just tuned to maintain the transmission in an optimized fashion, minimizing total radiated power, reducing cell size, and increasing frequency reuse. The following paragraphs provide additional details on the various implementations of these types of approaches. The traditional execution of the opportunistic scheduler is to schedule data transmissions to users for which the channel conditions are good. This is known as multiuser diversity. Various research papers investigate the performance benefits of this approach. A landmark paper written by Knopp and Humblet explains that, as the number of user