## 2 Related Works
### Interference Mitigation  
The existing interference mitigation schemes for BSNs mainly fall into several categories: channel hopping, power control, and beacon shifting.  
In IEEE 802.15.6, one of the mechanisms for mitigating inter-user interference in BSNs is channel hopping [21]. The coordinator of a BSN changes its operating channel in the operating frequency band periodically by including the current channel hopping state and next channel hop fields in its beacons. A BSN should choose a channel hopping sequence different from that of its neighboring BSNs. The drawback of channel hopping is that collisions cannot be effectively alleviated within a limited hopping bandwidth regardless of the delay and the energy consumption. [PERSON] _et al_. [23] proposed a channel hopping approach, where each BSN is assigned a different frequency channel at the network initialization phase. This approach allows monitoring as many patients given available channels, but radio channels are hard to be reused in a dynamic way. To increase the number of monitored BSNs and enable them to move freely, an alternative approach is to allocate channels dynamically in small-scale deployments of BSNs.  
Power control is another approach to reduce the interference in multi-user environments [13; 24; 25; 26; 27]. [PERSON] _et al_. [25] proposed a decentralized power control algorithm based on the received signal interference. The transmit power is so determined that the transmitter can sustain a high data rate, while keeping the adverse interference effect on the other neighboring concurrent transmissions minimal. [PERSON] _et al_. [13] proposed a power control approach for interference mitigation, where each BSN measures the interference from other BSNs and then selects a suitable channel and transmission power by utilizing non-cooperative game theory. A major drawback of this method is the long iteration period (more than 20 iterations) to reach the optimal point. As such, the utilized channels and transmission powers may be changed frequently during the long computing period which makes the system unstable. Power control improves spatial utilization of channels, but it may compromise transmission performance with a reduced transmission power [27]. Because of the simple structure of a BSN and the concern for energy conservation, power control is infeasible for BSNs.  
In healthcare applications, the channel utilization of a BSN is usually low for energy conservation [8; 9]. In IEEE 802.15.6 [28], the coordinator of a BSN may transmit its beacons at different time offsets relative to the start of the beacon periods by including a beacon shifting sequence field in its beacons [29; 30]. However, it is challenging to keep a beacon shifting sequence mechanism in the mobile BSN scenario. [PERSON] _et al_. [31] proposed a distributed flexible beacon schedule scheme to reduce the interference. By employing carrier sensing before each beacon transmission, collisions can be avoided if other BSNs attempt to access the channel at the same time. This scheme consumes additional energy in the channel access as multiple carrier sensing iterations are possibly conducted before each beacon transmission.  
In order to develop an effective interference mitigation scheme and interference analysis, it is imperative to have a thorough understanding of the inter-user interference. In this paper, we address the significance of the inter-user interference in a realistic BSN deployment scenario and then provide suggestions on the interference mitigation schemes.