Section: New Results
Scaling Methods: Cognitive radio networks
Participants : Philippe Robert, Florian Simatos.
Joint work with Ed Coffman, Shuzo Tarumi and Gil Zussman from Columbia University, USA. In light of current and projected demands of wireless communications, techniques are needed to make more efficient use of the wireless spectrum. In one class of applications, certain bands of the wireless spectrum are not licensed to specific users, but instead are reserved for users that compete for channel allocation on a dynamic basis; this dynamic spectrum allocation is also envisioned for cognitive radio networks in which unlicensed users compete for bandwidth within the temporarily unused channels of licensed users. Reconfigurability is a key property of the opportunistic users: Their channels may consist of a number of disjoint sub-bands allocated to them dynamically; a channel is not simply a fixed, single continuous band of frequencies. Exploiting this property leads to new and intriguing fragmentation issues.
In Coffman et al. [20] , we study a baseline mathematical model of these issues and arrive at a number of important insights that need to be borne in mind in system design. We adopt the most basic model in which a spectrum is shared by unlicensed users only, each characterized by a desired total bandwidth and the duration of a time interval over which it is needed. As users come and go, gaps of available bandwidth develop randomly in both size and position. When allocating bandwidth to a user's channel, the spectrum is searched for gaps in a linear scan; gaps are allocated to the channel until the full requested bandwidth has been provided. Fundamental questions include: Is the number of fragments (sub-bands) into which a user's channel is divided a stable process (e.g., can fragmentation increase indefinitely within a continuous model)? Is there a relation between the numbers of users and gaps similar to the 50 storage allocation? Are there normal limit laws similar to those of other fragmentation problems? Affirmative answers to these and similarly posed questions are observed experimentally; the results are rather surprising at first glance, but plausible derivations are given for each.
