## 1 Introduction  
A wireless body area sensor network (WBASN) is a wireless sensor network composed of short range, low power, and variable-data-rate sensors placed within, on, or around a human body to monitor biological signals and other functions such as movement patterns [1]. A WBASN system consists of biomedical sensor nodes for monitoring physiological data such as temperature, blood pressure, electrocardiogram (ECG), electroencephalography (EEG), electromyography, and heart rate. Thesesensors may be wearable or implanted devices that collect and transmit vital signals to the coordinator. The collected data can then be forwarded to a hospital for medical purposes. The applications of WBASNs can be categorized into two main fields: (1) medical applications that include remote patient monitoring, rehabilitation, biofeedback, and assisted living and (2) nonmedical applications that include fitness, performance and wellness monitoring, biometrics, and notification management [2]. Of course, for the medical and healthcare applications of WBASNs, the quality and continuity of signals are extremely important. However, radio propagation in WBASNs is dynamic due to the mobility of a human body, and different human tissues may be affected differently by biosensor signals. It is clearly shown in [3] that the quality of signals is affected by the propagation around the human body in accordance with channel models. Also, the effect of different kinds of human tissues to lossy signals is shown in the frequency range between 10 kHz and 1 GHz. In [4], the requirements of WBASNs based on the IEEE 802.15.6 standard are summarized as follows: the bit rate of a link is in the range of 10 kbps to 10 Mbps, packet error rate should be less than 10% for a 256 octet payload for 95% of links, and the time to join or leave a network is less than 3 s. The communication architecture of WBASNs is divided into three tiers: intra-WBASN communication (tier 1), inter-WBASN communication (tier 2), and beyond-WBASN communication (tier 3). Two standards, IEEE 802.15.4 [5] and IEEE 802.15.6 [1], cover WBASN technology. The IEEE 802.15.4 standard defines the physical (PHY) and medium access control (MAC) specification for low-rate wireless personal area networks at short range (up to 100 m) [5]. The IEEE 802.15.6 standard defines the PHY and MAC layers for WBASNs in short-range wireless communication within, on, or around the human body [1]. More specifically, the IEEE 802.15.4 standard is implemented into a specific application of IEEE 24451 standard [6]. The real application is a neurorecording system that aims to collect data of patients. The system consists of electrodes and a wireless module. The electrodes collect EEG signals of human head, and the wireless module uses different communication standards. In this application, three communication standards of IEEE 802.11, IEEE 802.15.1 and IEEE 802.15.4 are used. Furthermore, the MAC protocols of the two standards are compared in [7], where it is concluded that IEEE 802.15.6 is superior to IEEE 802.15.4 in terms of packet loss ratio, average delay, and network throughput. It is also shown that IEEE 802.15.6 carrier sense multiple access with collision avoidance (CSMA/CA) consumes more energy than IEEE 802.15.4 CSMA/CA. In [7], the design challenges of WBASNs are addressed, focusing on the MAC protocol, radio channel, and power consumption. A WBASN is set up for an individual human body, and therefore, in many practical environments multiple WBASNs are unavoidable. Moreover, the human body is mobile, and the sensor devices associated with arms, legs, _et al._, on a human body are also mobile within a limited range. In such operation conditions, intra-WBASN and inter-WBASN interference may severely impair transmission. Therefore, in scenarios involving mobility or overlapped WBASNs, unstable signal integrity may spatially and temporally degrade network performance.