raphically. However, this just allows interference between cells to be mitigated, all the users belonging to the same operator and receiving the same type of service. A more flexible approach was identified in Mitola and Maguire [7]: cognitive radio (CR). The concept of CR as it has developed is that it allows secondary users (SUs) to sense the "spectrum holes" in the time, space, and frequency domains in an autonomous manner and then make rational use of them in an opportunistic way on the premise of causing limited and tolerable interference to primary users (PUs). The spectrum hole is within the band of frequencies assigned to PUs, but at a particular time and specific geographic location those frequencies are not being utilized by a PU [8]. The concept of CR is attractive because it avoids wasting spectrum that is allocated but unused, at any particular time. The detection of the holes requires spectrum sensing and although a lot of research has been done on CR, providing rapid and reliable sensing results in all circumstances is diffi- cult and sensing is one of the key factors hindering the development of CR. The authors in [9] noted that problems needing to be addressed include low SNR sensing, the hidden node problem, a quality of service (QoS) guarantee, passive device detection, and challenges in wideband spectrum sensing. This concept of allowing SUs to transmit in the holes is termed overlay spectrum sharing [9] (or opportunistic spectrum access [OSA] or dynamic spectrum access [DSA] and is illustrated in Figure 8.3. The overlay spectrum sharing gives PUs the highest priority on occupying the spectrum and SUs are only allowed to use unused spec- trum where there is an absence of PUs SO SUs must employ spectrum sensing and keep monitoring as the occupied spectrum must be released by the SU if a PU starts using it. In the IEEE Standard 802.22 this is provided by either a geo-location database or by spectrum sensing [10]. However, as also shown in Figure 8.3, the situation is much more complex when more 