【文章內(nèi)容簡(jiǎn)介】 users for medium access, the implementation of a reliable scheme for the detection of unused spectrum, and so on. In this article we discuss these challenges, and we show that, while in the prior literature there are several good solutions that can effectively solve one or some of these issues. After this discussion we present the scheme we have designed in an effort to overe this lack of a plete solution for CANs. Our scheme is based on a virtual control channel which exploits the fact that users visit channels in a pseudorandom fashion and exchange control information whenever they happen to meet in any channel. Efficient dissemination of the control information to all users is achieved by means of work coding . The control information exchanged by users consists of all the information (bandwidth requirements, primary user presence and location, etc.) that is needed to determine channel switch patterns as well as resource allocation for data munication according to a predefined deterministic algorithm. We discuss the performance of the proposed scheme by presenting and discussing simulation results which show that it is an effective solution for the practical realization of CANs. 2 Technical Challenges in Multihop CANs The first issue we encounter in CANs is a chickenegg problem: secondary devices need to coordinate among themselves to perform spectrum access, but they also need to access the spectrum in order to municate and achieve issue is often referred to as the control channel problem,and unfortunately it is often neglected in work related to DSA. The fact is that most DSA related publications focus more on the problem of primary user detection and/or efficient spectrum allocation, and in doing so assume that some control channel A Distributed Network Coded Control Channel for Multihop Cognitive Radio Networks 第 8頁(yè) (共 13頁(yè) ) implementation is available to secondary users. For the practical realization of the control channel, some authors propose to statically allocate some spectrum practice presents two major issues: first, it requires static spectrum regulation, which is exactly what DSA aims at avoiding。 second, the chosen control band could easily bee the bottleneck. This is especially true in multihop scenarios,where the need for control information exchange is potentially very high (., not only for medium access, but also for routing purposes).Some other solutions have been proposed that attempt to solve the first issue by dynamically choosing an unused licensed band to perform secondary user controlmunications。 however, the control bottleneck issue is not addressed by these proposals. Of course, the ideal solution for CANs needs not only to address the issue of the exchange of control information, but also to effectively enable efficient usage of the available spectrum resources. In this respect, it is to be noted that the multiplerendezvous strategies discussed earlier were originally proposed as an extension to singlechannel technologies, most notably IEEE 。 in particular, the advantage seen in these solutions was that just by enabling the use of multiple channels, a significant increase in work capacity could be achieved over the singlechannel case. However, it is to be noted that the capacity limit of multichannel works is still far from being reached by multiplerendezvous schemes,which are more of a practical solution to the problem and do not take a systematic approach to maximizing the channel utilization efficiency. One of the aspects that should be taken into account for an efficient usage of the spectrum is that in a multihop work typically only a subset of the users are in the interference range of a given user. This opens up the possibility of higher spectrum utilization efficiency by means of frequency , in practice this requires more plex spectrum allocation strategies, as well as the availability of more information (., knowledge of the location of each user).Doing this in a distributed fashion is very challenging. Coupled with this problem is the issue of link A Distributed Network Coded Control Channel for Multihop Cognitive Radio Networks 第 10頁(yè) (共 13頁(yè) ) scheduling and routing: traditional ad hoc work routing strategies are not effective in multichannel works, due primarily to the fact that a given link cannot be activated at all times because of the requirement that both the sender and the receiver are on the same channel. Ideally, channel allocation, link scheduling,and routing should be jointly performed in order to maximize spectrum utilization efficiency as well as