مقاله انگلیسی رایگان در مورد پروتکل مسیریابی چند منظوره برای شبکه های رادیویی – الزویر 2018

Cognitive Radio Networks (CRNs) was proposed as a promising solution for the spectrum scarcity problem. This problem is increasing more and more and that is why CRNs are imminent to pervade into all fields of wireless communications. We backup this assertion with the following observations. First, with the inherent inefficiency of the static spectrum licensing policies [1] and proliferation of spectrum accessing mobile and connected devices [2], we are quickly heading towards a wireless spectrum crisis [3]. Second, the spectrum regulatory authorities are now working towards new regulations allowing for wireless spectrum reuse by unlicensed users [4]. These regulations allow the unlicensed secondary users (SUs) to access the spectrum as long as needs of the licensed primary users (PUs) are satisfied. One of the emerging use cases of Cognitive Radios is 5G. 5G networks utilize the concept of Licensed-Assisted- Access (LAA) [5], which basically says that, in order to get very high rates (and due to spectrum scarcity), the network does not just rely on the licensed part of the spectrum, but also uses part of the unlicensed and shared spectrum. However, using the shared spectrum should be done carefully to avoid interference between different parties. For example, 5G should be able to provide cellular access to IoT networks, which are mainly private networks that consist of many devices which should co-exist. Since they are supposed to use the same shared spectrum as that of WiFi, then in such a scenario the WiFi access points could act as the primary users, while the IoT devices are secondary ones. In such a network, powerful nodes (which have multiple antennas for example) should use their capabilities to avoid interfering with the less powerful ones. In order to use the Cognitive Radio (CR) in practice, many challenges need to be considered. Since one of the biggest goals is ensuring the integrity of the PU transmissions, all components of the CR cycle are developed such that this goal is attained [6]. For example, various spectrum sensing and sharing techniques have been developed, each with varying levels of complexity and efficiency[7]. In addition, different spectrum management policies have been envisioned which are better suited for different CR scenarios. For example, an overlay access policy allows a SU to access a spectrum only if a PU is not detected. In contrast, an underlay access policy would allow the coexistence of PU and SU transmissions provided that the level of interference incurred at the PU is not excessive. Developing such systems is therefore by no means an easy feat, and many practical challenges arise which makes designing such systems a challenging task [7, 8].