# Towards Collaborative and Dynamic Spectrum Sharing via Interpretation of Spectrum Access Policies
## 2 Cognitive Policy-Based Radio  
In this section, we describe the concept of a Cognitive Policy-Based Radio (CPBR), starting with brief definitions of related types of radios, based on the IEEE P1900.1 standard [1]. It should be noted that the P1900.1 definitions do not necessarily fully align with the regulatory definitions of the International Telecommunication Union (ITU) or the Federal Communications Commission (FCC). _Hardware Radio_ is \"a type of radio whose all communications functions are entirely implemented in hardware\", but most importantly, changes in communications capabilities can only be achieved by changing the hardware. Thus, a radio is considered to be a Hardware Radio even if some of its functions are implemented in software, but the \"regulated emission or reception parameters cannot be changed in the field, post manufacture, without physically modifying the device.\"  
_Software-Defined Radio_ (_SDR_) is \"a radio in which some or all of the physical layer functions are software defined\", i.e., implemented in software (which implies more than just controlling the functions implemented in hardware). Thus, unlike in a Hardware Radio, regulated parameters in SDR are changeable through software modification. _Policy Based Radio_ (_PBR_) is a type of radio whose behavior is governed by a policy-based control mechanism, with policy defined as a \"set of rules, expressed in a machine-readable format that are independent of the radio implementation regardless of whether the radio implementation is in hardware or software\". Consequently, although PBRs are usually SDRs, this is not a logical requirement for being policy-based. _Cognitive Policy-Based Radio_ (_CPBR_) is a type of radio that is (1) cognitive, i.e., \"aware of its environment and internal state and can make decisions about its radio operating behavior based on that information and predefined objectives\" (c.f. Section 1), and (2) uses a policy-based control mechanism (akin to PBR). Policies in CPBR, expressed in a machine-readable format (referred to as a _policy representation language_), can not be only ingested and displayed but prescribe a behavior that is interpreted and executed by the radio. Access to the Electromagnetic Spectrum (EMS) is one of the aspects that is controlled according to policies it has. In most cases, this means that they dynamically sense the spectrum for current usage and use the parts of it that are not occupied by the Primary Users (PU). As soon as a PU is sensed and there is evidence that transmitting in the PU's band could result in interference to the PU, the CR vacates the current channel and tries to grab a different, unoccupied part of the spectrum. The decisions on whether to use a specific spectrum can be made via the interchange of messages between the collaborating radios, or it can be implemented autonomously by each node following the locally available policies. Policies specify a number of factors that should be taken into consideration by the radios in order to arrive at a spectrum use decision, for example, the location of the device, the frequency and time range of interest or the maximum transmit power. Different policies pertain to different geographical areas and can change dynamically. Therefore, the CR design cannot account for all possible policies and must incorporate a mechanism for interaction with some authority that can dynamically grant or deny access to particular parts of the spectrum. Once a new policy is fetched from an authority, a CPBR radio designates it as an _active policy_ and incorporates it into its control mechanism. In our approach, the Policy Manager (PM) component is in charge of interacting with an authority and maintaining the active policy, while the Policy Engine (PE) component interprets new policies and ensures that the radio operates within the constraints established by the PM. The interaction inside the radio between the main Controller and the PE requires a language for expressing the spectrum access requests and the responses to such requests.