The discrepancy between current-day spectrum allocation and spectrum use suggests that radio spectrum shortage could be overcome by allowing a more flexible usage of the spectrum. Flexibility would mean that radios could find and adapt to any immediate local spectrum availability. A new class of radios that is able to reliably sense the spectral environment over a wide bandwidth detects the presence/absence of legacy users (primary users) and uses the spectrum only if the communication does not interfere with primary users (PUs). It is defined by the term cognitive radio [1] [2] [3]. Cognitive Radio (CR) technology has attracted worldwide interest and is believed to be a promising candidate for future wireless communications in heterogeneous wideband environments.

The original definition of CR is wide, as it envisions the wireless node as a device with cognitive capabilities utilizing all available environmental parameters. According to [1], examples of parameters the CR can exploit are knowledge of time, user location, user preferences, knowledge of its own hardware and limitations, knowledge of the network and knowledge of other users in the network. This initial definition of CR is conceptual, and deviates somewhat from the common contemporary working definition of CR. A sub set of CR that has received a substantial amount of focus is the Spectrum Sensing and Resource Allocation for Cognitive Radio. This is a radio that dynamically monitors activity in its available electromagnetic spectrum and adapts its transmission to available spectral resources. The most common scenario is an unlicensed secondary user (SU) wishing to utilize idle parts of the spectrum when transmission from the licensed PU is absent. It has become a standard practice to simply use the wide term CR also when referring to limited sub definitions such as Spectrum Sensing or Resource Allocation for Cognitive Radio. This is for instance reflected in modern redefinitions. A typical example is this definition of CR from the U.S. National Telecommunications and Information Administration (NTIA) [4] :

Cognitive Radio A radio or system that senses its operational electromagnetic environment and can dynamically and autonomously adjust its radio operating parameters to modify system operation, such as maximize throughput, mitigate interference, facilitate interoperability, access secondary markets.

This definition is a slight misnomer, since it only refers to a more limited adaptive radio, and not to the complete cognitive device, utilizing all available parameters from its environment, as presented by the pioneer Mitola in [1]. However, this redefinition of CR appears to have been widely adopted. To stick with this practice, the NTIA definition of CR will be the working definition in this thesis.

Blind Spectrum Sensing Techniques

CR has been proposed as the means to promote efficient utilization of the spectrum by exploiting the existence of spectrum holes. The spectrum use is concentrated on certain portions of the spectrum while a significant amount of the spectrum remains unused. It is thus key for the development of CR to invent fast and highly robust ways of determining whether a frequency band is available or occupied. This is the area of spectrum sensing for CR which is the first study part in this thesis.

It is stated that current spectrum sensing techniques suffer from challenges in the low signal to noise range (SNR). The reasons for this have to be analyzed. It is suggested that higher order statistics or information theoretic criteria are possible areas to look for a solution to overcome the problem. It is apparent that the problem at hand is wide and challenging. To meet the outlined demands, it is important that the scope is limited to provide a tangible base for the thesis. In addition, blind detection of spectrum holes in the frequency band is a very challenging requirement. As the names imply, blind spectrum sensing algorithms make sensing decisions without any prior knowledge, whereas non-blind approaches utilize some form of a priori knowledge about the underlying signals. Typical known signal features can be modulation type, carrier frequency or pulse shape. Although the importance of blind sensing in the conception of CR devices, only few algorithms exist in the literature. The blind detection is the second challenge to be raised in this part of thesis.

Hence the first step in the research has been to analyze the problem and to decide on the correct approach. The first chapter gives a literature survey on background information and current techniques in spectrum sensing. This chapter analysis also a selection of the conventional approaches to identify problems in the low signal to noise region and to decide on a potential new approach. Alongside the presentation of the survey results, a simultaneous discussion of their relevance is given. A conclusion is made on results that were important enough to pursue further. Based on the findings from the literature survey, two novel detectors are proposed and analyzed.

Resource Allocation Techniques

If the CR can successfully determine with a high degree of certainty that a specific part of the spectrum is idle, it can then transmit on these frequencies without interfering with the licensed owner of the spectrum and thus achieving a better spectral resource efficiency. Therefore, the CR protocol must adapt its signal to fill this void in the spectrum domain. Therefore, a SU device transmits over a certain time or frequency band only when no other user does. The requirement of no interference is extremely rigid to avoid disturbing licensed users. This is exactly the setup in the second part of this thesis where the CR behavior is generalized to allow SUs to transmit simultaneously with PU in the same frequency band. It can be done as long as the level of interference to PUs remains within an acceptable range. It is proposed in this thesis to combine CR with multi-user diversity technology to achieve strategic spectrum sharing and self-organizing communications.