# Bluetooth Communication Leveraging Ultra-Low Power Radio Design
## 2 Back-Channel Communication for Energy-Efficient Operation  
### Synchronization Schemes  
The core operation of a wireless sensor node is to sense different physical phenomena from the ambient environment and share the information securely with other nodes within the network. Therefore, the IoT connectivity landscape is structured in the form of hierarchical layers of networking [11]. Local area networks are utilized to facilitate interaction between different nodes in the form of machine-to-machine (M2M) communication as well as establishing connection with the hub or the access point. Eventually, the access point (AP) sends the aggregated information from the endpoint nodes to the backend analytic engines to handle the event through means of wide area networks, mainly the internet. Based on the targeted application, the wireless connectivity capabilities of each sensor node is determined whether to stay online for continuous data exchange or in a \"bursty\" nature at scheduled/specific events. Many key performance factors regulate the design of the network like the latency budget, power consumption budget and capacity of the network. For short-range networks, usually a star topology is prevalent as seen with Bluetooth and body area networks. In a star topology, a set of endpoints are allowed to communicate only with a single controller (base station/access point), for example a smart phone.  
Reliable network synchronization is needed for wireless sensor networks. It is a prerequisite to establish node-to-node and node-to-AP communication and is commonly performed in a controller-agent manner. However, the synchronization process might dominate power consumption when designing low-power radios for IoT. Two major application-specific synchronization schemes are proposed, resulting in different complexity and power budget requirements as summarized in Figure 3[12]. In the first scheme, both the node and the AP radios keep transmitting the synchronization sequences (beacons) periodically. This approach offers the shortest network latency at the cost of increased power consumption due to the always-on radios. To achieve energy-efficient operation at  
Figure 2: A typical IoT BLE application (**left**), the tradeoff between the required average power consumption and battery’s lifetime in duty-cycled transmission targeting a specific latency (**right**).  
the battery-powered node, the radio is placed in a \"sleep mode\" to preserve the battery's lifetime. A separate ULP wake-up receiver (WuRx) can be designed to off-load the synchronization task from the main radio. The WuRx continuously searches for a pre-defined wakeup signal or the standard beacon from the AP. Once received, the WuRx will wake-up the main radio to perform on-demand high data-rate communication. Alternatively, in the asynchronous scheme, the node's radio is woken up only when needed asynchronously. The AP repeats transmitting the synchronization sequences until it acquires the node.  
The WuRx receiver is an attractive solution to minimize the power consumption of the nodes while not compromising latency or scalability. The average power effectively becomes the active power of the always-on WuRx in addition to the leakage power of the main radio in a deep sleep mode. Since the WuRx and the main radio share the same communication link budget with the AP, both radios need to have similar sensitivity levels in order not to jeopardize the performance of the main radio. The reliability of the WuRx to properly detect the wakeup signaling in the presence of interference in congested bands is also of concern to minimize false alarms. Lastly, it is preferable for the WuRx signal sequence to be compatible with the communication protocol utilized in the network between the main radio and the AP to avoid additional hardware and cost at the AP side.