# Clustering-Based Channel Allocation Method for Mitigating Inter-WBAN Interference
## 1 Introduction  
As wearable devices become popular, a wireless body area network (WBAN) is attracting a lot of interest not only from academia but also from industry [1, 2]. Especially, as the population of the world older than 65 years old keeps increasing sharply [3], a WBAN is expected to play a pivotal role in supporting customized healthcare services and wellness services [4, 5]. A WBAN is composed of a coordinator node and a set of sensor nodes. Sensor nodes measure the body or environment status and report the measured data to a coordinator. A coordinator processes the data from sensor nodes or sends them to a remote server. A coordinator also orchestrates the packet transmissions within a WBAN. To facilitate the wireless communications between a coordinator and sensor nodes within a WBAN, IEEE has released IEEE 802.15.6 standard [6] and ETSI has released the SmartBAN specifications [7, 8]. These standards specify the frequency bands for a WBAN and allow the sensor nodes to operate in and on a human body with low power and low complexity. The key benefit of a WBAN comes from its ability to support real-time continuous monitoring function without obstructing a user [9, 10]. However, the density of WBANs increases with the popularity of WBANs and the number of channels available for WBANs is limited. Thus, as the number of coexisting WBANs in an area increases, it becomes more likely that multiple WBANs use the same channel at the same time. If more than two WBANs simultaneously send data using the same channel, they interfere with each other's data transmissions. Since the interference between WBANs hinders the data transmission within a WBAN, it may fail the continuous monitoring function of a WBAN. For example, in [11], it is noted that the interference among the coexisting WBANs decreases the data transmission rate within a WBAN by 35%. Therefore, the inter-WBAN interference management problem becomes crucial for a WBAN to function properly even in a highly populated area. The root cause of the inter-WBAN interference is that multiple WBANs within the transmission ranges of each other send data by using the same channel at the same time. To alleviate the amount of inter-WBAN interference, power control methods have been proposed [12; 13]. They attempt to minimize the amount of interference that a WBAN imposes on the other WBANs by controlling the transmission power of each WBAN. Medium access control methods have also been proposed to resolve the inter-WBAN interference problem [14; 15]. They manipulate the transmission opportunities of the coexisting WBANs to avoid the situation where their data transmission times overlap with each other. These methods try to alleviate the interference among the coexisting WBANs that use the same channel. However, as the number of coexisting WBANs increases, these methods decrease the data transmission rate within a WBAN and increase the data transmission delay. The performance of these methods can be improved if the number of coexisting WBANs using the same channel decreases. Inter-WBAN interference can be avoided if the channel used by a WBAN is not the same as those used by its neighboring WBANs. Channel allocation methods have been proposed to assign different channels to the coexisting WBANs [16; 17]. By noting that the amount of interference between two WBANs decreases as the distance between them increases [18], these methods attempt to separate the WBANs using the same channel as far away as possible. Usually, each WBAN determines a channel to use in a distributed manner by exploiting the information received from its neighbors. However, it is difficult to assume that a WBAN exchanges its state information with unidentified WBANs willingly. In addition, a common signaling channel between WBANs can be easily congested as the number of coexisting WBANs increases, which hinders a WBAN from acquiring necessary information in time.