# Clustering-Based Channel Allocation Method for Mitigating Inter-WBAN Interference
## 2 Related Works
### Channel Allocation Methods  
Various channel allocation methods have been proposed to mitigate the inter-WBAN interference. According to the operation mode, they can be categorized into the distributed methods [10; 16; 17; 22; 23], the centralized methods [24; 25; 26], and the hybrid methods [27]. In the distributed method, a coordinator in a WBAN decides the channel to use, while a remote server determines the channels for WBANs in the centralized method. In the hybrid method, both WBANs and a server decide the channels interactively. The distributed methods can be classified further according to whether the message exchanges among neighboring WBANs are required or not. In the distributed methods proposed in [16; 17; 22], each coordinator in a WBAN communicates with the coordinators in its neighboring WBANs to collect their state information. Then, by exploiting the collected data, each coordinator determines a channel to use. Since a channel is determined in an iterative manner, it takes time for a coordinator to determine the least congested channel. In addition, the signaling overhead increases sharply as the density of WBANs in an area increases. For example, if a WBAN needs to exchange the message \(n_{s}\) times and the size of a time slot is \(t_{s}\), it takes \(n_{s}t_{s}\) time for a WBAN to determine the least congested channel. In addition, if the message size is \(m_{s}\) and the number of neighboring WBANs of a WBAN is \(n_{n}\), the signaling overhead of a WBAN becomes \(m_{s}n_{n}n_{s}\). Therefore, as the number of WBANs in an area increases, the signaling channel among WBANs becomes congested, which increases the probability that a WBAN is unable to collect necessary messages in time. As a result, the channel selection process may not select the least congested channel. Interms of implementation, the coordinators must be synchronized in a time slot level, which is not easy among autonomous systems such as WBANs. In addition, since a WBAN has to exchange messages with unidentified WBANs, it may cause a security issue. Compared with these methods, our method has the following advantages. In our method, a server determines the channels for the coexisting WBANs. Since a server can determine channels without iteration, our method reduces the channel decision time. In our method, messages are sent from WBANs to a server through a cellular link. Therefore, our method prevents a WBAN from exchanging messages with unidentified WBANs. In addition, since a cellular link is faster and more reliable than an inter-WBAN signaling link, our method is more reliable in that a server can obtain necessary information in time regardless of the number of coexisting WBANs. The methods proposed in [10; 23] operate in a distributed manner without message exchanges among WBANs. In these methods, each WBAN keeps evaluating the quality of a channel by passively monitoring the performance it receives while using the channel. Then, by exploiting a learning process or a pre-configured channel hopping sequences determined by a Latin square, each WBAN selects a channel which is expected to be the best. However, the learning process inevitably involves an exploration phase to find the best channel. Therefore, it takes time for the learning process to find the best channel [10]. During the exploration phase, a WBAN may experience severe interference. In addition, when the environment around a WBAN changes, the exploration phase has to be repeated. In [23], a set of \(K\)-order Latin squares is assumed, and each WBAN randomly selects a Latin square from the set to determine its channel hopping sequence. Since each WBAN is assumed to be composed of the same number of sensor nodes (i.e., \(K\)), their method cannot be applied when WBANs have the different number of sensor nodes, which is common in practice. In addition, since \(K\) is fixed, it cannot easily adapt to the topology changes. Our method is different from these methods in the following aspects. Our method does not require an exploration phase to determine channels.