## 4 Literature Review: Other Research on CWSN
### Spectrum Management and Channel Selection/Assignment  
Figure 6 shows the logical framework of spectrum management. With the distinct QoS requirements of different applications, the immense need of a spectrum allocation scheme can fulfill the flexible bandwidth requirements.  
[PERSON] _et al._[85] proposed a centralized spectrum allocation scheme for CR-WSNs. Wireless sensor node request a spectrum resource to the coordinator. The dedicated coordinator allocates the spectrum resources to the sensors. The central coordinator is responsible for fairness, efficient spectrum utilization and spectrum handoff. Modified game theory is used for spectrum allocation.  
A channel allocation scheme for CR-WSNs is proposed in [86]. The authors proposed two tier sensor networks, the lower tier has the WSNs and the upper tier has the CR nodes responsible for monitoring the environment. The CR nodes in the upper tier compete for spectrum to relay their messages to the sink node. The authors incorporated two constrained Markov decision processes: one for accurately detecting an event while satisfying the delay, PFA and congestion constraints by adjusting the detection criterion; and the other for allocating the best available spectrum to the SU on a priority basis.  
[PERSON] _et al._[87] proposes a scheme for data channel assignment, where each wireless sensor node selects a channel considering the channel utilization and network connectivity of the PU. The channel information is obtained from a gateway node or a sensor node closer to the gateway than the current node. The data can be delivered using multiple channels.  
In this scheme, a gateway node broadcasts a channel assignment request message (CAR) using the control channel. The message consists of a gateway address, channel number and available bandwidth. Upon the reception of a CAR message, each node calculates the available channel list using the information contained in the message. If a node has a reliable direct link to the gateway, the node adds all candidate channels announced by the gateway. Otherwise, the node adds a channel to its available channel list only when there is a reliable link to the upstream node, which uses the same channel. When a wireless sensor node is not a reliable neighbor of the gateway, the node assigns itself a channel  
Figure 6: Logical framework of spectrum management.  
upon the reception of a channel information notification (CIN) message. Once the working channel at a wireless sensor node is determined, the wireless sensor node broadcasts a CIN message using the control channel and then switches to the data channel.  
[PERSON] _et al._[88] examined the channel assignment problem in a cluster-based multi-channel CR-WSNs considering energy consumption. An R-coefficient was proposed to estimate the predicted residual energy using sensor information (current residual energy and expected energy consumption) and the behavior of the PU on the channel. Based on the R-coefficient, three channel assignment approaches are provided: random pairing, greedy channel search and optimization-based channel assignment. However, this scheme does not guarantee fairness among sensors.  
[PERSON] _et al._[11] proposed an energy-efficient channel management scheme for CR-WSNs. In this scheme, CR wireless sensor node adaptively selects its operation mode among channel sensing, channel switching and data transmission/reception, according to the channel sensing outcome. The authors proposed the scheme based on the partially observable Markov decision process framework, considering that the sensing outcome can be erroneous due to noise uncertainty.