## 1 Introduction  
Cognitive radio (CR) is a promising technology which can fully use the spectrum by dynamically accessing the primary network. Consequently, dynamic spectrum access technology plays a very significant role and has become a hot research topic. As illustrated in Figure 1, dynamic spectrum access strategies can be classified into three models, e.g., the dynamic exclusive use model, the open sharing model, and the hierarchical model. Among those models, the hierarchical model is a hierarchical access structure for primary users (PUs) and secondary users (SUs), and is the most promising and effective one for current spectrum access policies [1]. The basic idea of the hierarchical model is that the SUs can use the licensed spectrum of PUs, as long as they can limit any interference perceived by PUs. Furthermore, there are two models of the spectrum sharing between PUs and SUs, namely spectrum underlay and spectrum overlay. Spectrum underlay introduces severe constraints on the transmission power of the SUs, therefore, it spreads the transmitted signals over a wide frequency band. The SUs can achieve low data rates with very low transmission power. If the PUs transmit in all the time-slots, the spectrum underlay does not need to detect and perceive the spectrum of the PUs. Spectrum overlay, first presented by [PERSON], can be also regarded as opportunistic spectrum access (OSA). Compared to the spectrum underlay, this model needs to detect and perceive the spectra of the PUs. It finds spatial and temporal spectrum white space for SUs to use, which is also termed as the spectrum holes (SHs). Therefore, this model does not need to obey the severe transmission power constraints of the SUs, and the SUs can achieve high date rates with high transmission power. In most cases, the spectrum overlay and underlay models are used separately. In this paper, a hybrid spectrum access model is proposed to use both the overlay and underlay methods simultaneously to further improve the current spectrum efficiency. The spectrum hole (SH) is a part of the licensed spectrum which is not being used by the owner during a period of time [1]. Among key technologies in CR, the design of the control channel is essential because the SUs need a control channel to coordinate and they have no licensed spectrum to carry the control information. The vulnerabilities resulting from utilizing a dedicated control channel have been well studied. Existing studies of the control channel have shown that using SHs to convey control information is only a basic approach and many shortcomings have been pointed out [2; 3; 4; 5; 6]. Firstly, the SUs may not have a common SH as control channel which would lead to low connectivity of the SUs. Secondly, the arrival of PU is unknown which causes interruptions in the use of the control channel. As the SUs communicate only in the SHs, the SUs need information about those unused bands in which the PUs are inactive. Each SU should maintain a list of SHs which probably will differ from one to another. The SUs can communicate with each other if there is a common SH in their lists. Consequently, there should be a way to pass information about the lists between SUs during the initial communication. Most of the existing MAC protocols of CR sensor networks are focused on avoiding common control channels. However, in this paper, a new method of spreading the power spectrum density in a control channel over an ultra-wide bandwidth is proposed to exploit the underused (gray) spectral regions. Like underlay spectrum sharing, the SUs can always access to the spectrum as long as the interference causing by SUs at the PU receiver can satisfactorily meet the threshold constraint [7]. According to the above analysis and considering the low power spectrum density of underlay waveforms, we propose to design a control channel to convey a small amount of control information, which is termed as SUCCH. At the same time, the spectrum overlay waveform is adopted to exchange a large amount of date, which is named as SUTCH.