In order to characterize and evaluate the PHY and MAC layers of IBSN, an application scenario with implantable medical devices and sensors is explained. Consider an epileptic patient implanted with a deep-brain stimulator which stimulates the brain with electrical impulses in the event of seizure occurrences. It is possible to detect the occurrence of a seizure beforehand with symptoms that can be measured with implantable sensors such as an EEG sensor, blood flow sensor, and external sensors such as inertial accelerometers. To predict the onset of a seizure, the data from these sensors have to be processed in real time at a base station, and control signals have to be sent to the implanted stimulator to suppress the seizure. The whole process including the processing and communication has to be in real time in order to predict and suppress the seizure on time. The wireless communication between these sensor nodes and medical devices should be highly reliable with negligible delay in order to handle the situation flawlessly. Also, the occurrence of the symptoms is completely random, which means sensing should be done continuously, and occurrence of an event should be predicted locally by the sensor node. Processing the signal locally is out of the scope of this paper, but wireless communication should be efficient not only in terms of performance but also in terms of energy consumption to ensure a long-time operation. As an indication, the battery life time of implantable deep-brain stimulators is typically two to three years [30]. Few off-the-shelf stimulators are equipped with a rechargeable battery that can be recharged via an inductive link [30].
Having short-range communication such as a magnetic-induction system will not serve the purpose of communication with the base station, hence a radio frequency (RF) link with a coverage of at most ten meters and at least two meters is required. To ensure reliable RF com- munication, the underlying MAC protocol is crucial in terms of reliability and energy efficiency. In order to standardize in-body and on-body communication, IEEE 805.14 task group 6 was set.
Network parameter Requirement Frequency of operation 402-405 Mhz
Bandwidth 3Mhz
No. of Channels 10, each channel is 300 Khz bandwidth Power of operation 25µW isotropic radiation power
Interference Accepted Topology Star, P2P Network size 20 nodes max.
Duty cycle 0.1% for non emergency data communication Latency upto 60 ms
Throughput upto 100 Kbps
Table 2.1: Requirements for a MAC protocol in IBSN.
This task group has defined the physical layer properties of body sensor networks. Four types of communication links are foreseen: in-body to in-body communication (Scenario 1 (SC1)), in-body to on-body communication (SC2), on-body to on-body communication (SC3), and on- body to external nodes communication (SC4). Only the first two channel models are considered, where in-body communication is involved, focusing only on the IBSN scenario. In this analysis the characteristics of the proposed Physical (PHY) and MAC layer constraints set by the task
CHAPTER 2. REQUIREMENTS OF PHY AND MAC LAYERS FOR IBSN
group are considered. RF links do not propagate well inside the human body which has to be taken into account at the MAC layer for packet-loss and fading. For in-body communication, a dedicated RF band has been allocated called the Medical Implant Communication Service (MICS) band operating at 402-405 MHz, containing ten channels of 300 KHz bandwidth each with effective isotropically radiated power (EIRP) of 25µW [31]. It is observed that, inside the human body, the MICS band can propagate with less loss and fading than the other frequency bands. The recommended network topology is a star network with a central network controller. However, in a complex application scenario as explained earlier, the need of peer-to-peer (P2P) communication is optimal if the processing can be done locally on the sensor nodes. Scalability is not an issue, since the number of nodes is typically less than fifteen [28] [32] [33]. The data rate of the network can vary for different sensor nodes depending on the type of sensed data. However, in our application scenario, there is no need for high-bandwidth data transmission such as video or audio. For the scenario explained above, a data rate of 20 Kbps is sufficient for reliable transmission of data from stimulator communication, blood flow sensor, inertial sensor, and EEG sensor. The requirements are set based on the recommendations from task group 6 and to meet the applications similar to the scenario explained in this section. Table 2.1 lists the requirements of IBSN.
Chapter 3
Survey of MAC protocols with
and without wake-up radio for
implantable sensor network
In an IBSN, sensor nodes have limited resources such as energy, size of the components (including the sensor, processor and radio), and range of communication. Despite the limited resources available, IBSN applications impose strict requirements for the wireless network in terms of communication reliability, delay, throughput, energy-efficiency and in some applications even Quality of Service(QoS). MAC protocols in wireless networks, aim for minimum delay, maximum throughput and an increased network life-time by controlling the main sources of energy waste such as idle-listening, collisions, over-hearing, and packet lost. There are a number of MAC protocols available for wireless sensor networks, even some are focussed on the IBSN applications. The following section presents a survey on the existing MAC protocols that are optimized for IBSN applications and attempts to find the potential problems in MAC protocols that needs to be solved by further research.
3.1
Features of MAC protocol
The IBSN is a special type of WSN, which varies from WSN in various features such as scalability, reliability, latency and energy-efficiency. As explained in section 2.1, IBSN has three types of communications namely, In-body communication, On-body communication, and Off- body communication. This work focusses on MAC protocols that are available for In-body and On-body sensor networks. The fundamental task of MAC protocol is to avoid collision of data packets and to prevent simultaneous transmissions while preserving maximum throughput, min- imum latency, communication reliability and maximum energy-efficiency [34]. QoS is also an important factor of good MAC protocol. In medical applications a latency of 125 ms of is only allowed, whereas in consumer electronics latency can be less than 250ms [17].Other important features include adaptability to changes in network topology, maximum achievable throughput in different network scenarios, least jitter in heterogeneous traffic, efficient bandwidth utilization with high payload, safety and security. The following table presents the expected values for different features of IBSN as per the IEEE 802.15.6 [17].
As a summary, a good IBSN-MAC should have energy-efficiency, reliability even in heterogen- eous traffic, safety and security in addition to QoS. [34]
Feature of good IBSN MAC Acceptable value for Implanted medical devices Throughput upto 200 KBPS for medical devices
upto 4Mbps for non-medical devices Latency upto 100ms in life critical implants
upto 2 seconds in monitoring medical devices Bandwidth 300KHz MICS band
100MHz in 2.4GHz ISM band 1.74 MHz in 433 MHz ISM band
Duty cycling less than 0.1 % in MICS band medical devices no restriction if Listen before talk is incorporated Interference mitigation CRC, FEC, frequency agility are recommended for
safety purposes.
Table 3.1: Features of MAC protocols as suggested by IEEE 802.15 TG 6 [17]
3.1.1
Attributes of MAC as proposed by IEEE 802.15 TG-6
Wireless Body Area Network (WBAN) has attracted many researchers in academia and industry, because of its great potential to revolutionize the technology for healthcare. Due to its growing requirements, a task group have been set to standardize in-body, on-body and off-body communication [35]. The purpose of the task group is to define new physical and Medium Access Control (MAC) layers optimized for low power in-body/on-body nodes (not limited to humans) to serve a variety of medical and non medical applications. This section will briefly explain the attributes of MAC layer set for in-body and on-body by the task-group and in compliance with MICS band regulations.
IEEE 802.15.6 specification
The IEEE 802.15 task-group 6 [17] suggests that the nodes should be organized into one-hop or two-hop star network. In the case of single-hop star network a single co-ordinator controls the entire operation of the network whereas in the case of two hop star network a relay-capable node may be used to exchange data between hub and the destination-node. The entire physical channel (in time-axis) is divided into super-frame structures. Each super-frame is usually bounded by a beacon period of equal length. In MICS band regulation, the transmission of beacons bounding the super-frame is prohibited. For such non-beacon modes, where beacons are not used, the super-frame boundaries are defined by polling frames.