In this simple model, cognitive radio is wrapped around SDR. This model fits well to the fore-mentioned definition of cognitive radio, where the combination of cognitive engine, SDR, and the other supporting functionalities (e.g. sensing) results in cognitive radio. The Cognitive engine is responsible for optimizing or controlling the SDR based on some input parameters such as that are sensed or learned from the radio environment, user’s context, and network condition. The Cognitive engine is aware of the radio’s hardware resources and capabilities as well as the other input parameters that are mentioned above. Hence, it tries to satisfy the radio link requirements of a higher layer application or user with the available resources such as spectrum and power. Compared to hardware radio where the radio can perform only single or very limited amount of radio functionality, SDR is built around software based digital signal processing along with software tunable Radio Frequency (RF) components. Hence, SDR represents a very flexible and generic radio platform which is capable of operating with many different bandwidths over a wide range of frequencies and using many different modulation and waveform formats. As a result, SDR can support multiple standards (i.e. GSM, EDGE, WCDMA, CDMA2000, Wi-Fi, WiMAX) and multiple access technologies such as Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), Orthogonal Frequency Division Multiple Access (OFDMA), and Space Division Multiple Access (SDMA).
The recent boom in the diversity of wireless standards with different options exposes interoperability and multi-mode support issues. SDR has been considered as an inherent solution to address such issues. Although SDR is naturally evolved due to the need to implement radios that can support multiple mode and standards, utilization of SDR in a cognitive radio is not limited to the aforementioned functionalities. SDR is a promising technology to introduce cognition capabilities to cognitive radios. For instance, one of the crucial cognition capabilities of cognitive radios is the dynamic spectrum management system. Spectrum sensing, optimization mechanism to utilize a specific part of the spectrum, and spectrum shaping are the main steps of dynamic spectrum management systems. In the case of spectrum, sensing devices are required to sense the spectrum, which can be either embedded into SDR internally or incorporated to SDR externally. For instance, an antenna can be considered as an internal sensor whereas the video camera can be considered as an external sensor for SDR structures. In other words, SDR can have a structure like a miniature spectrum analyzer in order to provide the spectrum information to cognitive engine. Either the existing receiver front-end of SDR or a designated receiver parallel to the receiver side of SDR can be used to perform spectrum capturing. Captured spectrum is digitized by Analog-to-Digital Converter (ADC) and then the digital samples are sent to digital signal processor for the post-processing.
One of the main characteristics of cognitive radio is the adaptability where the radio parameters (including frequency, power, modulation, and bandwidth) can be changed depending on the radio environment, user’s situation, network condition, geo-location, and so on. SDR can provide a very flexible radio functionality by avoiding the use of application specific fixed analog circuits and components. Therefore, cognitive radio needs to be designed around SDR. In other words, SDR is the core enabling technology for cognitive radio. One of the most popular definitions of cognitive radio, in fact, supports the above argument clearly: “A cognitive radio is an SDR that is aware of its environment, internal state, and location, and autonomously adjusts its operations to achieve designated objectives.”
Even though many different models are possible, one of the simplest conceptual model that describes the relation between cognitive radio and SDR can be described as shown below;
