# Bluetooth Communication Leveraging Ultra-Low Power Radio Design
## 2 Back-Channel Communication for Energy-Efficient Operation
### Back-Channel Communication Concept
This approach significantly decreases the active power consumption of the receivers without sacrificing the latency in the network. One of the greatest challenges in designing an IoT platform is dealing with the heterogeneity of devices [14, 4]. The proposed back-channel communication technology breaks this barrier to allow heterogeneous ULP IoT devices to interoperate with already existing BLE infrastructure with minimal power consumption [12]. The concept of embedded back-channel communication enables a variety of new applications with interconnecting heterogeneous devices. For example, the proposed scheme allows ULP devices in deep-sleep states that are not BLE compliant to wake up by back-channel communication embedded in standard BLE compliant packets. Back-channel modulation is an entirely software-defined process which is accomplished by the MAC layer. It can be realized within a packet by generating a specific  
Figure 4: Concept of BLE (Bluetooth Low-Energy)-based back-channel communication. sequence of bits in the payload [4] or applying duty cycling at the bit-level [17; 18]. In addition, back-channel communication can be implemented on a packet-level by generating a proper sequence of packets [12; 16]. Prior works [19; 20] demonstrated the back-channel communication concept in the form of packet length modulation or packet interval modulation using multiple consecutive BLE packets as shown in Figure 4. The packet length or interval modulation utilizes an entire packet (or multiple packets) as an on-off-keying information symbol for an energy-detector based receiver. Its bandwidth efficiency, however, is very poor since the signaling requires many consecutive packets. Moreover, this form of back-channel signaling is particularly vulnerable to interferences as the packet length modulation is susceptible to an interferer packet (or any other high power interference signal) that could appear between multiple back-channel signaling packets. The other related technique is hierarchical modulation. In hierarchical modulation, receivers can selectively demodulate high rate (e.g., 16 QAM) or low rate (e.g., 4 QAM) information from a single packet depending on the channel quality. However, this hierarchical modulation scheme does not allow communication among heterogeneous devices using very distinct modulation schemes (e.g., coherent vs. non-coherent). Other work demonstrated intra-packet back-channel modulation schemes and the feasibility of the embedded back-channel signal generation without modifying the existing packet structure. Embedded back-channel signals are all generated by a set of carefully crafted bit sequences within the boundary of the standard-compliant packet structure. More specifically, binary FSK modulated back-channel communication embedded in GFSK BLE packets has been reported. A standard compliant data receiver can demodulate the entire bit sequence including the bits to create a back-channel message. Meanwhile, at the ULP back-channel receiver, only the embedded back-channel message is decodable, not the entire bit sequence. The backchannel signaling must be accomplished within strict constraints of the standard compliant packet structure.