# A Survey on Node Clustering in Cognitive Radio Wireless Sensor Networks
## 3 CR-WSN Characteristics and Architecture
### Classification of CR-WSNs According to Network Topology
#### 3.4.2 Hierarchical Topology  
Here, data aggregation is accomplished by structuring CR-WS nodes into clusters. A node that is responsible for data aggregation and fusion is called a cluster head. Data aggregation and fusion at the CH reduces the number of messages transmitted to the BS, and hence, mitigates energy consumption and conserves bandwidth by reducing the number of nodes taking part in transmissions.  
Conventionally in WSNs, clustering is based on several parameters, such as node deployment, cluster count, inter-cluster communications, node and CH mobility, node types, node roles, node identifiers, node connectivity, neighborhood information, the cluster formation algorithm, the power conservation method, CH roles, and the CH selection method. The classification of clustering methods in WSNs has been well studied [32; 38; 39; 40; 41; 42; 43; 44].  
In addition to the above-mentioned parameters, clustering in CR-WSNs should consider the types of nodes, as described in Section 3.2. Based on the types of nodes deployed, there are two types of hierarchical CR-WSN.  
(a) Hierarchical CR-WSN with All CR-WS Nodes (Hierarchical CR-WSN-all)  
This is similar to traditional hierarchical WSNs, where sensor nodes are organized into clusters, and the CHs serve as relays for transmitting the data. CHs may work for data fusion and reduce data transmission redundancy. CHs are selected from potential cluster head (PCH) nodes. Figure 3 illustrates the states of CR-WS PCH nodes.  
Although clustering algorithms and CH-selection algorithms are similar to traditional WSNs, CR-WSNs consider one or more cognitive parameters, such as the number of idle channels available to a node, and the maximum number of nodes connected to a channel. Although deployment is easy in this topology, it is expensive in terms of energy efficiency and hardware costs.  
(b) Hierarchical CR-WSN with Only CHs as CR-WS Nodes (Hierarchical CR-WSN-few)  
In this type of CR-WSN, WS nodes are organized into clusters, and only CHs are equipped with CR. Other WS nodes go without CR. CHs have dedicated hardware with different transmission ranges, and are generally stationary.  
CHs are responsible for receiving instructions from the BS and the surrounding CHs about the available spectrum, and they allocate channels for inter-cluster and intra-cluster communications. CHs gather spectrum information by sensing, and they make a channel-selection decision by selecting the best available channel for communications with CMs and the BS. CMs select and hop to the channel ordered by the CHs in order to switch on and start communications. Because the CHs have a higher transmission capacity than CM nodes, the minimum requirement for the number of clusters can be derived from the lower bound of the throughput.  
This type of CR-WSN is more reasonable in hardware costs. Because only CHs have CR and serve without changing roles, they require better battery backup. Energy harvesting by the CHs, and  
Figure 3: States of CR-WS potential cluster head (PCH) nodes.  
additional CHs for backup in every cluster, can help prolong network lifetime. In this type of network, CH deployment is difficult.  
To the best of our knowledge, there is no study on CR-WS node-clustering algorithms that considers hierarchical CR-WSN-few along with other cognitive parameters, such as connectivity, coverage, number of CMs, spectrum availability, and, very importantly, PU protection.  
A typical hierarchical network topology for a CR-WSN where only CHs are CR-WS nodes is shown in Figure 4.  
Regardless of the topology used, the BS receives spectrum information from the spectrum database, its own sensing, and the CR-WS nodes.