Bin Zhen

IEEE body area networks and medical implant communications

IEEE 802.15 established a new working group, wireless body area networks (WBAN), to develop short range wireless technology in and around human body recently. This paper investigates networking issues in implant communications of WBAN. The object is better understanding of medical implant sensor networks and how to start WBANs work. We applied IEEE 802.15.4b and 802.15.4a-chirp spread spectrum (CSS) for implant communications. We found two issues: clear channel assessment of implant devices and adjacent-channel interference from free space signals. Both of them can be attributed to the rapid attenuation of electromagnetic wave through tissues. Therefore the carrier sense multiple access mechanism and transmit mask of 802.15.4b cannot be directly adopted. The modulation of 802.15.4a-CSS is a good reference to WBAN. Besides, a simple two-hop protocol which uses a body surface forwarder was presented for long distance wireless implant communications.

IEEE Body Area Networks for Medical Applications

IEEE 802.15 working group established a new study group, body area network (SG-BAN), to develop short range wireless communication technology in and around human body recently. Wearable healthcare is an important category of potential applications of BAN. This paper introduces the IEEE SG-BAN and investigates the performance of 802.15.4b and 802.15.4a for wearable healthcare applications in the heterogeneous networks environment. The object is better understanding of healthcare requirement of BAN and how and where BAN works to be started. In addition to the scalability issue, we found asymmetric clear channel assessment (CCA) abilities among different types of wireless technology. The asymmetric CCA has significant impact on system performance.

Characterization and Modeling of Dynamic On-Body Propagation at 4.5 GHz

On-body communication channels are of increasing interest as more and more wireless devices are wearable in medical, military, and personal communications. This letter presents an experimental investigation into the dynamic on-body channel with body movements. Based on statistical analyses of level crossing rate and fading duration, a three-state Fritchman model that considers both channel dwelling time and channel quality is proposed to describe the burst characteristics of on-body fading. The dynamic on-body channels are classified into unstable error-free state, constant error-free state, and error state. Parameters for the Fritchman model are estimated from the measured data.

Hybrid Unified-Slot Access Protocol for Wireless Body Area Networks

Wireless body area network (WBAN) solution is an emerging new technology to resolve the small area connection issues around human body, especially for the medical applications. Based on the integrated superframe structure of IEEE 802.15.4, a modified medium access control (MAC) protocol, named Hybrid Unified-slot Access (HUA) protocol for WBANs is proposed with focus on the simplicity, dependability and power efficiency. Considering the support to multiple physical layer (PHY) technologies including ultra-wide band (UWB), the slotted ALOHA is employed in the contention access period (CAP) to request the slot allocation. Mini-slot method is designed to enhance the efficiency of the contention. Moreover, sufficient slot allocation in the contention-free period (CFP) makes HUA adaptive to the different traffic including the medical and non-medical applications. Simulation results show that the protocol effectively decreases the probability of collision in a CAP and extends the CFP slots to support more traffic with quality of service (QoS) guarantee.

Clock Offset Compensation in Ultra-Wideband Ranging

Accurate and low-cost sensor localization is critical for deployment of wireless sensor networks. Distance between Ultra-wideband (UWB) sensor nodes can be obtained by measuring round trip flying time through two-way ranging (TWR) transaction. Because of difficulties in synchronization and channel estimate, the response delay of UWB node is the order of milliseconds. Comparing with the nanosecond propagation delay, relative clock offset between UWB nodes introduces big error in TWR. This paper presents the management of relative clock offset in TWR transaction. The relative clock offset between sensors is estimated by comparing the claimed and real frame duration. Simulation in the UWB channel model shows the relative clock offset after compensation can be reduced to less than ±2 ppm.

Scalable and robust medium access control protocol in wireless body area networks

Wireless body area network (WBAN) solution is an emerging technology to resolve the small area connection issues around human body, especially for the medical applications. Based on the integrated superframe structure of IEEE 802.15.4, this paper proposes a modified medium access control (MAC) protocol for WBAN with focus on the simplicity, dependability and power efficiency. Considering the support to multiple physical layer (PHY) technologies including ultra-wide band (UWB), the slotted ALOHA is employed in the contention access period (CAP) to request the slot allocation. Mini-slot method is designed to enhance the efficiency of the contention. Moreover, sufficient slot allocation in the contention-free period (CFP) makes the proposed protocol adaptive to the different traffic including the medical and non-medical applications. Simulation results show that the protocol effectively decreases the probability of collision in a CAP and extends the CFP slots to support more traffic with quality of service (QoS) guarantee.

Clock management in Ultra-wideband Ranging

The distance between two asynchronous impulse ultra-wideband (UWB) sensors can be obtained from round trip flying time through two-way ranging (TWR) transaction. The clock difference causes error in the flying time measurement account for the order of 10 nanoseconds propagation delay. This paper presents a clock management between UWB sensors in the TWR transaction. The ranging responder estimates the clock of ranging initiator by a delay-locked-loop circuit during reception of ranging frame. When replying the response, the ranging responder locks its clock frequency to the estimated clock of ranging initiator. Consequently the duration of TWR are measured by the same clock. The simulation over the UWB channel models results shows that the relative clock offset can be reduced to less than plusmn1.2 ppm in the worst channel mode.

Characterization and modeling of dynamic on-body propagation

On-body communication channels are of increasing interest as more and more wireless devices are wearable in medical, military, and personal communication applications. This paper presents an experimental investigation of the dynamic feature of on-body channel due to the movement of human body. Statistical analyses, including level crossing rate, fading duration, and fading interval were conducted. Based on the analyses, a three-state Fritchman model that considers the channel dwelling time is proposed to describe the dynamic characteristic of on-body channel. The channel is classified as unstable good state, constant good state, and bad state. The parameters of the Fritchman model are estimated from the measured data.

A Body Surface Coordinator for Implanted Biosensor Networks

Biomedical sensors can be implanted and networked in the human body for health monitor, diagnosis, treatment or as a prosthetic device. Life time and heat generated by implant due to communication and circuitry power consumption are big concerns. This paper presents a new network architecture for long range communications in the implanted sensor networks. We measured the on-body propagation around body surface and in-body propagation through tissue in the frequency ranges of 403MHz and 2.45GHz. We have found that the in-body path loss is more than that of on-body. To exploit the path loss difference, we propose to introduce a body surface coordinator which has more resources to the implanted sensor networks. Instead of only routing data among implants, the coordinator on the body surface can forward data from one implant to another over long distance more safely and efficiently.

UWB Ranging and Crystal Offset

Distance between a pair of UWB sensor nodes without common time reference can be obtained by measuring round trip time. Because of difficulties in synchronization and channel estimate, the response delay of impulse UWB node is the order of milliseconds. Comparing with the nanosecond propagation delay, relative crystal offset between UWB nodes introduces big error in two way ranging. This paper estimates relative crystal offset through measuring the ranging frame duration in the counting window after synchronization. Simulation shows the relative crystal offset can be reduced to less than plusmn2 ppm