2 Basics of Bluetooth Channel Sounding

Bluetooth Channel Sounding uses a known technique called phase-based ranging (PBR) to perform high accuracy distance measurement. In PBR, two devices measure the distance between them by estimating the phase offset or phase difference between a received unmodulated signal and a local oscillator (LO) signal. See Section 8 for the basics of phase-based ranging and the need for multi-carrier phase ranging systems in real world systems. Multi-carrier phase ranging systems measure the phase difference between the received unmodulated signal and the LO signal at multiple RF frequencies to generate measured phase difference vs. frequency curve, that is in turn used to determine the distance between the two devices.

Per Bluetooth CS, if device A (initiator) is measuring the distance to device B (reflector), then, the initiator begins ranging by transmitting an unmodulated tone. The reflector measures the phase of incoming signal relative to the local oscillator and then, transmits an unmodulated tone back to the initiator. Next, the initiator measures the phase of incoming signal relative to the local oscillator. See Figure 2-1.

Figure 2-1 Bluetooth Channel Sounding, Phase Based Ranging

The tone exchange (between the initiator and reflector) and phase measurements (at both the initiator and reflector) are performed at multiple frequencies in the 2.4GHz Bluetooth band (1Mhz frequency steps). With the phase measurements from both initiator and reflector, the differences in the relative phases and the devices’ local oscillators are corrected across all frequencies. After the phase correction, the actual phase offset/shift measured at each frequency is plotted as the measured phase difference vs. frequency curve. In optimal conditions, plotting the phase difference measurements vs. each frequency yields a straight line with a slope that represents the distance between the initiator and the reflector. Since the measured phase differences wrap around 2pi, the phase differences are required to be further straightened to calculate the effective slope and determine the distance between the initiator and reflector.

In the real-world though, the radio signals travel from the initiator’s antenna to the reflector’s antenna by more than one path. This is referred to as multipath propagation and can impact the phase difference measurements and in turn, the ranging resolution and accuracy. In presence of channel impairments, collecting phase measurements at increased the number of frequencies or tones, across multiple antenna paths and using advanced signal processing like IFFT, MUSIC (MUltiple SIgnal Classification) algorithms enable high accuracy distance estimation.

The next section details the channel sounding procedure outlined in Channel Sounding Draft Specification for collecting phase measurements to perform distance measurement.