# Bluetooth Communication Leveraging Ultra-Low Power Radio Design
## 2 Back-Channel Communication for Energy-Efficient Operation
### Back-Channel Communication Concept  
Much research has been devoted to realizing energy-efficient wireless connectivity considering both the radio architectures and networking topologies. The motivation is to realize lasting operation for both battery-powered and battery-less wireless sensor nodes. According to the ULP radio survey [13], low power operation can be achieved when using simpler modulation techniques like on-off keying (OOK) or frequency-shift keying (FSK) at low data rates. Low complexity modulation schemes eliminate the requirement for having high-accuracy frequency synthesizers (PLL) which typically consume more than 50% of the radio power consumption. Moreover, lower data rates result in a lower SNR requirement and higher sensitivity level which can be achieved with basic radio architectures utilizing passive RF frontends [12]. The majority of previous work in this area utilizes simpler receiver architectures that rely on energy-detection and can achieve sub-10 \(\upmu\)W active power consumption. However, it is difficult for these receivers to operate reliably in the presence of interference. Another challenge is that most of the wireless standards commonly used in IoT connectivity do not support such modulation techniques and data rates. For instance, the BLE standard utilizes Gaussian Frequency Shift Keying (GFSK) modulation with data rates of 1 or 2 Mbps. Although BLE signals are widely used in developed high population density areas, most ULP devices cannot take advantage of BLE connectivity because of their extremely limited power and/or complexity budget. State-of-the-art BLE radios still consume several mWs of active power. One reason behind the high power operation is the interference mitigation techniques to robustly achieve the highest possible sensitivity and adjacent channel rejection (ACR) performance. Efficient  
Figure 3: Different synchronization schemes resulting in different complexity and power budget requirements: (**a**) synchronous; (**b**) wake-up receivers; (**c**) asynchronous. techniques such as asymmetric BLE communications, duty-cycling, wakeup techniques, ring-oscillator-based local oscillator (LO) generation and open loop GFSK modulation have been reported to reduce the power consumption of the radio. Although heavy duty-cycling brings the average power down to sub-100 uW, it comes at the expense of long latency. In this section, we will focus on BLE with back-channel communication. Prior work has reported the idea of back-channel communication, where signals are embedded in standard compliant wireless packets and generated by a standard-compliant transmitter [14, 15, 16, 4]. The basic idea is to design ULP radios that are compatible but not fully-complaint with the BLE standard to encode information in an auxiliary low-complexity and low data rate modality as depicted in Figure 4. Two classes of IoT devices are considered: BLE-enabled (e.g., smart watch) devices where the power consumption is in the range of mW and ULP IoT devices (e.g., temperature sensors) with limited power consumptions of tens of uW. In such scenarios, commercial smart phones can be easily configured to transmit BLE back-channel messages in addition to the fully compliant BLE packets based on the targeted device. In back-channel communication, some BLE standard requirements like data rate can be traded off with power consumption while being able to still communicate properly with already deployed hardware infrastructures. Thus, the major advantage of this technique is that it leverages widespread smart phones to send back-channel signals which can be utilized in different applications scenarios. BLE back-channel communication can be seen as a wakeup mechanism that bridges the gap between ULP and standard compliant BLE radios. The BLE receiver stays in sleep mode until it is woken up through an always-on ULP WuRx listening to the back-channel to realize the concept of BLE on-demand.