Overview:

Compared with traditional wireless networks, the complexity of cognitive radio networks is reflected in coexistence with primary-user networks. In order to realize coexistence between cognitive radio networks and primary-user networks, interweave spectrum sharing scheme conducts secondary users in cognitive radio networks to sense the result of spectrum occupancy, use the available spectrum that is not occupied temporarily by primary users and vacate the working channel reclaimed by primary users. This spectrum sharing scheme avoids harmful interference with primary-user networks from secondary users and introduces a new spectrum access paradigm, dynamic spectrum access (DSA), in cognitive radio networks. DSA improves spectrum efficiency by dynamic spectrum utilization, but it brings huge challenges in networking due to dynamic channel availability subjecting to activities of primary users.

As a special multi-channel wireless networks, cognitive radio networks has the same two problems in MAC design as those in traditional multi-channel wireless networks: (1) Access coordination problem; (2) Conflict avoidance and handle problem. Access coordination aims at solving the problem that how a sender can find its intended receiver in multi-channel wireless networks, i.e. how to let the sender and its intended receiver work over the same channel—rendezvous problem. Rendezvous problem is the fundamental and indispensable problem in MAC design of multi-channel wireless networks. In traditional multi-channel wireless networks, the widely used approach to rendezvous problem is predefined dedicated common control channel which is exploited to negotiate data channel by all users. However, it is invalid to predefine a dedicated common control channel in cognitive radio networks, because the dedicated common control channel can not be used for data channel negotiation any more when it is occupied by primary users, and what worsens is that the occupied dedicated common control channel can not be used for negotiating a new common control channel to maintain the normal operation of the cognitive radio networks. Though this centralized scheme is simple to solve rendezvous problem, it brings single point failure in cognitive radio networks. In order to improve robustness against activities of primary users, channel hopping based schemes are proposed as a new class of approaches to rendezvous problem in cognitive radio networks. These new schemes supply secondary users with abundant channel selection flexibility, but they largely increase the difficulty in rendezvous between the sender and its intended receiver. Especially, due to lack of a centralized coordinator managing channel hopping pattern of secondary users in distributed cognitive radio networks, it is very difficult for secondary users to search each other without knowing each other’s channel hopping pattern. To decrease the searching difficulty, the intuitive solution is increasing the radio number of each secondary user even to be equal to the channel number. Secondary users with multiple radios can search their partners simultaneously over multiple channels at a time, which obviously increases searching success probability, but it is impractical in actuality. For secondary users with a single radio in cognitive radio networks, a large number of research works focus on design of channel hopping pattern to guarantee rendezvous between senders and their intended receivers in finite time. The obvious shortcoming of these research works is that the performance of rendezvous is vulnerable to channel number and activities of primary users.

Our research work also focuses on rendezvous problem for secondary users with a single radio in distributed cognitive radio networks, but we do not emphasize on design of more efficient channel hopping patterns. Our basic idea is that a sender can employ multiple radios from its neighbors to realize simultaneous search over multiple channels at a time when it starts to search its intended receiver. As a result, the sender is “equipped” with multiple radios through neighbor cooperation to improve search efficiency and increase rendezvous probability. Our proposed framework virtualizes multiple radios on a secondary sender though neighbor cooperation to improve robustness of current channel hopping pattern against channel number and activities of primary users, which in turn improves time efficiency of rendezvous in cognitive radio networks.

People: Quan Liu

Papers:

Quan Liu, Deming Pang, Gang Hu, Xiaodong Wang, and Xingming Zhou, "A Neighbor Cooperation Framework for Time-efficient Asynchronous Channel Hopping Rendezvous in Cognitive Radio Networks," IEEE International Symposium on Dynamic Spectrum Access Networks (DySPAN), pp.506-516, 2012