# Bluetooth Communication Leveraging Ultra-Low Power Radio Design
## 2 Back-Channel Communication for Energy-Efficient Operation
### ULP Back-Channel BLE RX Prototype  
In order to further clarify the concept of back-channel communication and its implementation in the BLE standard, a BC receiver prototype is presented in this subsection. The receiver consumes 150 \(\upmu\)W and is based on a ring oscillator (RO) for LO generation. The RO-based LO is designed to operate at 0.5\(\times\) the BLE frequency band to reduce the active power consumption [16]. A dual-mixer front-end is hence required to enable the lower frequency for the oscillator. In this receiver, interference rejection is enhanced by (1) using a Zero-IF architecture to eliminate the image problem, (2) utilizing a narrowband low-pass filter at baseband with cutoff frequency of 1 MHz and (3) forming a 2-D backchannel message built on the presence and length of packets in all three advertising channels instead of utilizing a single advertising channel. This improves its resilience to interference. The FSK demodulation is performed by sensing the frequency-hopping sequence on the advertising channels. The frequency-hopping sequence can be defined in any order and still be compliant with the BLE standard. Therefore, this BC receiver can receive a wake-up message from a mobile phone that is sending a BLE compatible message.  
A BLE advertising event includes three packets separated by less than 10 ms as shown in Figure 5. Each of these packets can be transmitted on any of the three BLE advertising channels which are numbered: CH37, CH38 and CH39. These channels' frequencies are 2402, 2426 and 2480 MHz, respectively. The packet duration can be anywhere between 128 to 376 \(\upmu\)s (Figure 5 shows a 300 \(\upmu\)s packet for illustration). The BLE standard specifies the time gap between advertising events to be at least 20 ms, but not more than 10.24 s. On top of this time gap, a pseudo-random delay of up to 10 ms is added to reduce the probability of collisions.  
In the form of back-channel BLE communication used by this receiver, the three BLE advertising channels are scanned for the presence of transmitted energy. This is implemented by scanning the three channels sequentially for energy. To suppress the impact of adjacent channel interference, the bandwidth of the receiver baseband is set to 1 MHz. The receiver's oscillator frequency controller hops the oscillator frequency between all three BLE advertising channels (78 MHz apart).  
This BC-BLE receiver was fabricated in a 65 nm LP CMOS process. At a 112.5 kbps data rate, the measured receiver sensitivity was \(-57.5\) dBm for a 0.1% bit error rate (BER) requirement. To characterize the interference rejection of the receiver, a GFSK-modulated interferer is utilized in measurements. The signal-to interference ratio (SIR) was measured to be \(-4\), \(-20\) and \(-30\) dB for the 1 st, 5 th and 10 th BLE adjacent channels, respectively which is measured when the received signal amplitude is 3 dB higher the sensitivity level. The entire receiver consumes 150 \(\mu\)W total power from two separate supplies: 0.9 V for digital circuits and 1.1 V for analog blocks. About 120 \(\mu\)W (80%) is consumed by the RO-based LO generation including the LO buffers which operate at 1.2 GHz, the digital FLL and the LO frequency dividers. The remaining 30 \(\mu\)W is dissipated in the analog baseband.