Section: New Results

Cognitive Radio Networks

Multichannel time slot assignment in Cognitive Radio Sensor Networks

Participants : Ons Mabrouk, Pascale Minet.

This is a joint work with Hanen Idoudi and Leila Saidane from ENSI, Tunisia.

The unlicensed spectrum bands become overcrowded causing an increased level of interference for current wireless sensor nodes. Cognitive Radio Sensor Networks (CRSNs) overcome this problem by allowing sensor nodes to access opportunistically the underutilized licensed spectrum bands. The sink assigns the spectrum holes to the secondary users (SUs). Therefore, it must rely on reliable information about the spectrum holes to protect the primary users (PUs). We focused on the MultiChannel Time Slot Assignment problem in CRSN and tackled this problem: first at the level of a cluster (i.e. Intra-cluster multichannel scheduling), second at the level of several clusters coexisting in the same CRSN (i.e. inter-cluster multichannel scheduling).

In 2013, we proposed an Opportunistic centralized TIme slot assignment in COgnitive Radio sensor networks (OTICOR) for the Intra-cluster multichannel scheduling. OTICOR differs from the existing schemes in its ability to allow non-interfering cognitive sensors to access the same channel and time slot pair. OTICOR takes advantages of spatial reuse, multichannel communication and multiple radio interfaces of the sink. We proved through simulations that a smaller schedule length improves the throughput. Applying OTICOR, we showed that, even in the presence of several $PUs$, the average throughput granted to $SUs$ remains important. We also showed how to get the best performances of OTICOR when the channel occupancy by $PUs$ is known.

In 2014, we extended this Intra-cluster multichannel scheduling algorithm by proposing two ways for the sink to determine the available channels and alert the SUs if an unexpected activity of PU occurs. Our objective is to design an algorithm able to detect the unexpected presence of PUs in the multi-hop network while maximizing the throughput. If the estimation of PU presence is accurate, a channel sensing at the beginning of the slotframe is sufficient. This algorithm, called Frame-ICMS (Frame Intra-Cluster Multichannel Scheduling), takes advantage of the slots dedicated to the control period by allowing noninterfering cognitive sensors to access the control/data channel and time slot pair. We showed through simulations that using the control period also for data transmission minimizes the schedule length and maximizes the throughput. However, if the estimation of PU presence is not accurate, channel sensing should be done before each slot. We proposed the Slot-ICMS algorithm.

In 2015, we focused on inter-cluster multichannel scheduling algorithm. We considered the coexistence of different clusters in a same CSRN, each cluster having an intra-cluster multichannel scheduling algorithm. Our goal is to obtain a better scalability without loosing the properties provided by OTICOR:

• collision-free schedule,

• minimized data gathering delays,

• sleeping periods per node to save node's energy.

However, the co-existence of several clusters in the same environment may lead to conflicts in the allocation of time slots and channels between these clusters. To avoid inter-cluster collisions, we donot require that different clusters use different channels, we assign distinct channels only to nodes having one-hop neighbors out of their cluster. Once the problem of inter-cluster collision is avoided, each cluster head schedules the transmissions of its members independently. This whole solution exhibits good performances as shown by the simulation results.