【正文】 ransmission schedule for the subsequent allocation period,possibly starting transmissions using resources (transmission slots in certain channels) meant to be allocated to other the rest of this article we refer to this event as spectrum collision, and refer to the users that failed to retrieve the control information as misinformed users. In general, the chances of having a spectrum collision, and hence the average amount of wasted spectrum resources, increase with the number of misinformed users. For this reason, we want a dissemination scheme in which the retrieval probability Pretr, defined as the probability that a generic user successfully retrieves the control information from all other users, is high. We propose the use of work coding in order to implement a reliable and efficient dissemination scheme for the control information. Network coding is a recently introduced paradigm for data dissemination, according to which the packets generated by multiple sources are jointly coded at intermediate nodes and decoded at the final destination. This coding strategy can be very effective in increasing throughput, reducing delay, and enhancing robustness. In order to have a practical implementation of work coding we refer to , where the authors proposed a distributed scheme for work coding that obviates the need for a centralized knowledge about the encoding and decoding functions, and at the same time allows asynchronous data exchange between nodes. According to this approach each node stores all ining packets in an internal buffer and transmits an encoded packet that contains a random linear bination of all packets in its own transmission time this packet is forwarded to all nodes situated within transmission range. Now, if the encoding vectors are generated randomly and the symbols lie in a finite Galois field of sufficient size, the information will be disseminated to all users with high A Distributed Network Coded Control Channel for Multihop Cognitive Radio Networks 第 12頁 (共 13頁 ) probability . Based on this approach, every time a node receives an encoded packet, it has to know the coefficients used to perform the encoding in order to recover the original information packets. A simple solution consists of appending within each encoded packet the corresponding encoding vector that describes which linear bination of information packets it way, the encoding coefficients needed to decode the information stored in encoded packets can be found within the encoded packets themselves. Any node can thus recover the information packetsgenerated by all nodes simply by inverting the matrix that stores all the coefficients of the packets received during data dissemination. Appending the encoding vectors to thepackets incurs additional overhead, which will need to be accounted for in the determination of the total control overhead of our DSA solution。 for a detailed discussion of this issue, the reader is referred to. Finally, in order to have a practical implementation of work coding, we adopt the buffering model . As discussed in our prior work, work coding vastly outperforms other strategies for the purpose of disseminating the control information in singlehop multichannel works. In other words, the use of work coding in conjunction with a pseudorandom channel switch pattern provides us with a virtual control channel, which allows users to efficiently share control information. This work coded virtual control channel is robust against packet losses and link failures, and, most important, does not require the presence of static spectrum resources dedicated to the exchange of control information. As for the detection of unused spectrum resources suitable for secondary access, we note that the work coded control channel is naturally fit for the implementation of a cooperative primary user detection solution. 4 Conclusions In this article we discuss the main challenges that arise in CANs and present a practical solution to these challenges based on a virtual work coded control channel. We present simulation results that prove how it can achieve effective dissemination of control information and efficient spectrum utilization in several A Distributed Network Coded Control Channel for Multihop Cognitive Radio Networks 第 12頁 (共 13頁 ) scenarios. Our solution is shown to be robust against primary user activity and scalable with respect to the number of secondary users. Future research directions include the integration of more elaborate spectrum allocation,transmission scheduling, and routing strategies in the proposed solution.