## 2 Application Scenarios  
In this section the application scenario is described as below. As shown in Figure 2, there are four active PUs and each one is authorized to use a certain frequency band to communicate. The different types of circles represent the interference ranges of each PU, and six SUs are shown in Figure 2. In this paper there is a channel which is termed a SH and a SU that can communicate in this channel because it is a channel whose authorized PU is currently inactive or the SU is beyond the interference range of that PU.  
A SU can establish the connection with another SU as long as they both have a shared SH in their respective lists of SHs, so it is important for a SU to identify its neighbors during the initial communication used to set up CR sensor networks. In order to fully utilize the primary spectrum and maximize the efficiency of spectrum, underlay and overlay transmissions, which exploit the white and grey spaces respectively, should be used together [1; 10; 11]. However, for spectrum underlay, the SUs need to transmit at low power to avoid any interference with the PUs, whereas the PUs will cause interference with SUs [12]. In consideration of the low power spectrum density of underlay waveforms, the control channel is designed to convey a small amount of control information, which is named as SUCCH, while the spectrum overlay waveform is used to exchange a large amount of data, which is named as SUTCH. Considering the perspective of a SU, the current spectrum usage is depicted in Figure 3.  
Figure 2: The SUs among four PUs.  
Before explaining the protocol used to set up CR sensor networks, it is necessary to discuss the capabilities of the SUs and define some terms that will be used in the coming discussion. A SU can switch between spectra autonomously and sense the spectrum. Each SU identifies itself by using a different Orthogonal Variable Spreading Factor (OVSF) [12] over spectrum underlay. The number of the SUs in the current CR sensor networks is a priori information available to all the SUs.  
The proposed protocol is firstly discussed under a distributed architecture scenario, which is also called Multi-Hop Architecture. Each SU initially starts to send beacons in different OVSF over spectrum underlay to indicate its presence. At the mean time every SU monitors the spectrum underlay by randomly selecting a form of OVSF while initially starting a timer which counts to \(T_{S}\) seconds. If none of those beacons is captured during the \(T_{S}\) seconds, the SU will change to another form of OVSF in the next time slot. If a beacon is received by selecting the current form of OVSF, the SUs will sent a response in the same form which is considered as the task of carrying on the negotiations. After exchanging the control information with each other, the common SH in the two SUs will start to provide service. The procedure is simply illustrated in Figure 4.  
Figure 4: The procedure of network setup between two SUs.  
Figure 3: Spectral occupancy of the hybrid access.  
In Figure 4, \"Request to Send (RTS)\" and \"Clear to Send (CTS)\" exchange messages to reserve a channel for communications in a similar manner that the IEEE 802.11 Distributed Coordination function (DCF) designs the MAC protocol [13]. RTS or CTS carries information about SUs' lists of SH and accesses SUs states.