## 3 Power Efficient Wake-Up Receiver for WBAN  
There are three types of communication in a WBAN: on-demand, emergency, and normal traffic. On-demand traffic is initiated by the coordinator to acquire instantaneous real-time information, mostly for the purpose of diagnostic recommendations. This is further divided into continuous (data stream) and discontinuous communication. Emergency traffic is initiated by the nodes when a critical parameter exceeds a predefined threshold and should be accommodated as soon as possible. This kind of traffic is not generated on regular intervals and is totally unpredictable. Normal traffic is the data traffic in a normal condition with no time critical or on-demand events. This includes unobtrusive and routine signal monitoring. Several techniques have been proposed to minimise the transceiver's duty cycle (the percentage of time during for which it is in an \"active\" state). However, while reducing the duty cycle helps to save power, it severely limits network flexibility. The transceiver's power consumption while listening idly on a channel can approach or even exceed power consumption during data transmission. Therefore, if there is a need for asynchronous events to be initiated by the network coordinator, considerable energy must be spent by individual sensors during idle listening. To avoid wasting energy, a WBAN requires an ultra-low-power wireless receiver which takes over the task of listening for asynchronous signals, allowing the primary transceiver to be shut down. A typical deployment of a dedicated WUR in a wireless sensor would be as an additional circuit to an existing wireless transceiver. It should communicate with the sensor's microcontroller via some standard interface, and ideally re-use the transceiver's antenna and the communication frequency dedicated to the WBAN. A typical application will be in a WBAN network with ranges less than \(2\) m. There will be a dedicated device acting as a Master Node that will act as a network coordinator. This device would send wake up signals, and receive data packets from sensors. The wake up receiver intended for a WBAN is expected to operate in the dense network environment. At any given moment some nodes will be communicating within a WBAN, while some may stay in a sleep mode, monitoring the channel for communication requests. Also, in the vicinity of a network there will be numerous high power transceivers and noise. The WUR must be robust to this ambient traffic and should avoid waking up the sensor on signals intended for the neighbouring nodes, as well as on the ambient noise. From a functional perspective, a WUR is not as restricted as the standard receiver regarding bit error rate and data rate. The metric of interest is the probability of detected wake up signals, because re-sending them increases power consumption for the transmitter as well as the latency. A second metric is the reduction of false alarms. A false alarm is also costly from a power perspective because the sensor is activated needlessly. A low power semi-passive wake up receiver was developed and tested [2]. The receiver application can be seen in Figure 2. It is based on an envelope detector, used for the OOK demodulation. A charge pump (two-stage voltage doubler-multiplier) is used as an OOK signal envelope detector, as first presented in [22]. This demodulated signal is fed to a data slicer, which raises the demodulated digital signal to voltage levels that can be used by later digital circuitry. The comparator threshold is adaptive, rather than constant, and is determined by the wake-up signal strength. This digital signal is then classified using the passive circuitry in order to detect the \"intended\" wake up signal and to reject the random toggling due to noise or nearby wireless communications. The classification is done by detecting the wake-up preamble if it is generated at the expected OOK data rate range. The next part of the circuit is a Pulse-Width Modulation (PWM) decoder and SPI adapter.