A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> Interference and Coexistence Mechanisms <s> Wireless communications technologies can support efficient healthcare services in medical and patient-care environments. However, using wireless communications in a healthcare environment raises two crucial issues. First, the RF transmission can cause electromagnetic interference (EMI) to biomedical devices, which could critically malfunction. Second, the different types of electronic health (e-Health) applications require different quality of service (QoS). In this paper, we introduce an innovative wireless access scheme, called EMI-aware prioritized wireless access, to address these issues. First, the system architecture for the proposed scheme is introduced. Then, an EMI-aware handshaking protocol is proposed for e-Health applications in a hospital environment. This protocol provides safety to the biomedical devices from harmful interference by adapting transmit power of wireless devices based on the EMI constraints. A prioritized wireless access scheme is proposed for channel access by two different types of applications with different priorities. A Markov chain model is presented to study the queuing behavior of the proposed system. Then, this queuing model is used to optimize the performance of the system given the QoS requirements. Finally, the performance of the proposed wireless access scheme is evaluated through extensive simulations. <s> BIB001 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> Interference and Coexistence Mechanisms <s> Public m-health service will enable care-givers to continuously monitor patients' vital signals wherever they are, using Body Area Networks (BANs). To make this service available to a large number of people everywhere, public m-health service should be implemented in a license-free frequency band. However, the coexistence problem inherent in such a medium that may be shared by many co-located systems may cause disruption of BAN transmissions. This paper addresses the above problem by proposing a Centralized BAN Access Scheme (CBAS) to improve service availability and robustness of BANs. CBAS puts most of the complexities in the gateway, which has less energy and resource constraints than the body nodes. The gateway monitors all the available channels and assigns an interference-free channel to each scheduled node. Extensive simulation results show the effectiveness of CBAS in reducing durations of service interruption and thus improving service reliability of a BAN. <s> BIB002 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> Interference and Coexistence Mechanisms <s> We consider reliable communications in Body Area Networks (BAN), where a set of nodes placed on human body are connected using wireless links. In order to keep the Specific Absorption Rate (SAR) as low as possible for health safety reasons, these networks operate in low transmit power regime, which however, is known to be error prone. It has been observed that the fluctuations of the Received Signal Strength (RSS) at the nodes of a BAN on a moving person show certain regularities and that the magnitude of these fluctuations are significant (5 -- 20 dB). In this paper, we present BANMAC, a MAC protocol that monitors and predicts the channel fluctuations and schedules transmissions opportunistically when the RSS is likely to be higher. The MAC protocol is capable of providing differentiated service and resolves co-channel interference in the event of multiple co-located BANs in a vicinity. We report the design and implementation details of BANMAC integrated with the IEEE 802.15.4 protocol stack. We present experimental data which show that the packet loss rate (PLR) of BANMAC is significantly lower as compared to that of the IEEE~802.15.4 MAC. For comparable PLR, the power consumption of BANMAC is also significantly lower than that of the IEEE~802.15.4. For co-located networks, the convergence time to find a conflict-free channel allocation was approximately 1 s for the centralized coordination mechanism and was approximately 4 s for the distributed coordination mechanism. <s> BIB003
Interference-aware mechanisms aim to increase the robustness of WBANs to errors due to strong interference sources operating in the same license-free frequency bands, such as IEEE 802.11 or Bluetooth devices, often present in medical environments. Many existing works embrace the concept of cognition, by dynamically selecting the best channel for data transmission from a pool of available frequency bands, based on specific metrics, such as the channel quality or the interference levels. Coexistence with other wireless technologies that operate in the same frequency bands is also a key issue. Interference-aware channel selection: A cognitive radio approach for mHealth services is considered in  . Two spectrum sensing mechanisms are proposed to detect channels free of primary user activity. Between sensing periods, beacons are transmitted by the coordinator to distribute periodic bandwidth allocations to the nodes for normal traffic, whereas a small time interval is reserved immediately after the beacon transmission for the communication of emergency events. In  , Multi-channel Quality-based MAC (MQ-MAC) is proposed in a scenario where the nodes placed on each patient form a WBAN cluster, and cluster coordinators form a mesh network to forward the aggregated patient data to a monitoring unit. In MQ-MAC, the cluster coordinators create and maintain a table with the link quality of all available channels. For each point-to-point link, the transmitter and the receiver agree on the most suitable channel for their communication and reserve it through a 3-way handshake on the dedicated control channel. This idea is taken one step further in the same work to form the Channel Quality Based Routing (CQBR) scheme, where the route with the best link quality is selected and enable simultaneous transmissions on different channels by neighboring nodes. Co-channel interference provoked by multiple co-located WBANs is also tackled by BANMAC BIB003 , by a fully distributed channel allocation mechanism. Coexistence mechanisms: The Centralized BAN Access Scheme (CBAS), proposed in BIB002 , tackles interference issues in a coexistence scenario with IEEE 802.11 devices. In CBAS the coordinator has cognitive radio capabilities and is able to scan the available spectrum continuously. When a suitable channel is found, the coordinator follows the IEEE 802.11 access rules to reserve the channel for the estimated time required for the data exchange by transmitting a legacy RTS packet. After that, it issues a wakeup command to the next scheduled node on a dedicated secondary control channel, thus waking up the node from its sleep mode and informing it on the data channel selection. Finally, a different interference-related challenge is explored in BIB001 , where the proposed scheme aims to reduce the Electromagnetic Interference (EMI) produced by WBANs, which can lead to the malfunctioning of EMI-sensitive life-supporting medical devices (such as defibrillators) in a hospital environment. A context-aware solution is proposed, in which a coordinating device dynamically determines the transmission power limits for each communication in the WBAN, after consulting an inventory of the location and the EMI constraints of all electronic medical devices in a given setting. The nodes must content for access by transmitting an RTS in the dedicated control channel. The coordinator replies with a negative CTS if no feasible power scheme is available at that specific time due to either EMI constraints or predicted congestion conditions. Otherwise, a positive CTS with the maximum supported power allocation is transmitted, and the node enters a queuing system, waiting for its turn to transmit in the data channel. The presented state-of-the-art approaches for the key MAC layer mechanisms, namely energyefficient channel access, QoS provisioning, reliability, and interference and coexistence are summarized in Table 4 .