ness should be of high quality to mitigate the hidden zations have involved whether to allow this behavior. If node problem. For example, if a CR wants to use a TV implemented correctly, it is a win-win situation in which band, its sensor should be significantly more sensitive more spectrum is utilized and very little additional in- than a TV set SO that if it detects a TV signal, it will not terference is suffered. Table 2.4b-5 Comparison of variable bit loading algorithms. Method Characteristic Complexity Water filling Original approach, optimal, frequently used for comparison O(N2) Hughes-Hartogs [37] Optimal, loads bits serially based on subcarrier energy level, slow 0(SN²) to converge, repeated sorts Chow [38] Suboptimal, rounds to integer rates using signal-to-noise gap O(N log N +N log S) approximations, some sorting required Lagrange (unconstrained) Krongold [39] Optimal, computationally efficient, efficient table lookup, O(N log N), revised O(N) Lagrange multiplier with bisection search, integer bit loading, power allocation, fast convergence Note: N: number of subcarriers; S: number of bits per subcarrier. Cognitive radio: The technologies required CHAPTER 2.4b transmitters that other nodes are able to see. So while node A may see an open frequency, node B may consider Spectrum Usage Other CR Sensor Policy that frequency to be in use by a distant transmitter node or Reports network L. To get the network started, the CRs must agree on a protocol to find each other. CR Spectrum There are several possible methods by which to do Access this. These methods depend strongly on whether there is an infrastructure in place to help start up a CR network, or whether no infrastructure can be assumed. Sensor "Opportunistic" Reports Use of Spectrum Rendezvous Infrastructure-aided rendezvous (with Look-through) Protocol If we assume that there is an infrastructure component, Figure 2.4b-10 Spectrum access context diagram. A spatially we must also assume it behaves just like the CR and does diverse sensing protocol is r