Advances in cognitive radio networks: A survey <s> B. Spectrum Handoff <s> Recently, secondary usage of spectrum has been considered in order to better exploit spectral resources and overcome the under-utilization of licensed spectrum. Since the licensed user still keeps primary access rights to its spectrum in such a secondary usage scenario, potential secondary users (SUs) have to vacate the spectrum in case the licensed user claims it. In order to maintain the quality of the secondary communication nevertheless, efficient mechanisms for link maintenance are needed. In this paper we present a general model for link maintenance in secondary usage scenarios. We state that the traditional way of adding redundancy to improve the communication not necessarily works in secondary usage scenarios. Furthermore we present performance results of a link maintenance approach applied to a secondary usage system based on opportunistic spectrum sharing, which verifies our assumptions <s> BIB001 </s> Advances in cognitive radio networks: A survey <s> B. Spectrum Handoff <s> Today's wireless networks are characterized by a fixed spectrum assignment policy. However, a large portion of the assigned spectrum is used sporadically and geographical variations in the utilization of assigned spectrum ranges from 15% to 85% with a high variance in time. The limited available spectrum and the inefficiency in the spectrum usage necessitate a new communication paradigm to exploit the existing wireless spectrum opportunistically. This new networking paradigm is referred to as NeXt Generation (xG) Networks as well as Dynamic Spectrum Access (DSA) and cognitive radio networks. The term xG networks is used throughout the paper. The novel functionalities and current research challenges of the xG networks are explained in detail. More specifically, a brief overview of the cognitive radio technology is provided and the xG network architecture is introduced. Moreover, the xG network functions such as spectrum management, spectrum mobility and spectrum sharing are explained in detail. The influence of these functions on the performance of the upper layer protocols such as routing and transport are investigated and open research issues in these areas are also outlined. Finally, the cross-layer design challenges in xG networks are discussed. <s> BIB002 </s> Advances in cognitive radio networks: A survey <s> B. Spectrum Handoff <s> A Markov chain analysis for spectrum access in licensed bands for cognitive radios is presented and forced termination probability, blocking probability and traffic throughput are derived. In addition, a channel reservation scheme for cognitive radio spectrum handoff is proposed. This scheme allows the tradeoff between forced termination and blocking according to QoS requirements. Numerical results show that the proposed scheme can greatly reduce forced termination probability at a slight increase in blocking probability <s> BIB003 </s> Advances in cognitive radio networks: A survey <s> B. Spectrum Handoff <s> With the explosive growth of wireless multimedia applications over the wireless Internet in recent years, the demand for radio spectral resources has increased significantly. In order to meet the quality of service, delay, and large bandwidth requirements, various techniques such as source and channel coding, distributed streaming, multicast etc. have been considered. In this paper, we propose a technique for distributed multimedia transmission over the secondary user network, which makes use of opportunistic spectrum access with the help of cognitive radios. We use digital fountain codes to distribute the multimedia content over unused spectrum and also to compensate for the loss incurred due to primary user interference. Primary user traffic is modelled as a Poisson process. We develop the techniques to select appropriate channels and study the trade-offs between link reliability, spectral efficiency and coding overhead. Simulation results are presented for the secondary spectrum access model. <s> BIB004 </s> Advances in cognitive radio networks: A survey <s> B. Spectrum Handoff <s> Spectrum handoff causes performance degradation of the cognitive network when the primary user reclaims its right to access the licensed spectrum. In a multi-hop cognitive network, this problem becomes even worse since multiple links are involved. Spectrum handoff of multiple links seriously affects the network connectivity and routing. In this paper, we describe a cross-layer optimization approach to solve the spectrum handoff problem with joint consideration of spectrum handoff scheduling and routing. We propose a protocol, called Joint Spectrum Handoff Scheduling and Routing Protocol (JSHRP). This paper makes the following major contributions. First, the concept "spectrum handoff of single link" is extended to "spectrum handoff of multiple links", termed as "multi-link spectrum handoff". Second, we define the problem of coordinating the spectrum handoff of multiple links to minimize the total spectrum handoff latency under the constraint of the network connectivity. This problem is proven to be NP-hard, and we propose both centralized and distributed greedy algorithms to minimize the total latency of spectrum handoff for multiple links in a multi-hop cognitive network. Moreover, we jointly design the rerouting mechanism with spectrum handoff scheduling algorithm to improve the network throughput. Different from previous works in which rerouting is performed after spectrum handoff, our rerouting mechanism is executed before the spectrum handoff really happens. Simulation results show that JSHRP improves the network performance by 50% and the higher degree of interference the cognitive network experiences, the more improvement our solution will bring to the network. <s> BIB005
When the current channel conditions become worse, or the primary user appears and reclaims his assigned channel, secondary users need to stop transmitting data and find other available channels to resume their transmission. This kind of handoff in CR networks is termed as spectrum handoff BIB002 . Since the transmissions of secondary users are suspended during a spectrum handoff, they will experience longer packet delay. Therefore, a good spectrum handoff mechanism should provide with secondary users with smooth frequency shift with the least latency. A good way to alleviate the performance degradation due to long delay is to reserve a certain number of channels for potential spectrum handoff BIB003 . When secondary users need to switch to another frequency, they can immediately pick one channel from the reserved bands. However, if a secondary user reserves too much bandwidth for spectrum handoff, the throughput may be unnecessarily low, because the primary user may not reclaims his licensed band very frequently. Therefore, there is a tradeoff in optimizing the channel reservation. By optimizing the number of channels reserved for spectrum handoff, the blocking probability can be minimized and the secondary users' throughput is maximized. A location-assisted handover algorithm is proposed in  , where the secondary users equipped with the location estimation and sensing devices can report their locations back to the secondary base station. Whenever a handoff becomes a must, secondary users can switch their frequency to one of the candidate channels depending on their locations. A joint spectrum handoff scheduling and routing protocol in multi-hop multi-radio CR networks is proposed in BIB005 , which can minimize the total handoff latency under the constraint on network connectivity. The protocol extends the spectrum handoff of a single link to that of multiple links. In order to achieve a reliable continuous communication among secondary users in the presence of random reclaims from a primary user, secondary users should select their channels from different licensed bands owned by different primary users BIB001 , BIB004 . The multi-band spectrum diversity helps to reduce the impact of the appearance of a primary user and improve the reliability of secondary spectrum access.