4 Bluetooth Channel Sounding Flow for Phase-Based Ranging

In the context of CS, an initiator is the Bluetooth device that starts (initiates) the CS procedure and a reflector is the Bluetooth device that responds to (reflects) the CS procedure. The procedure’s operating parameters are exchanged through Link Layer control messages prior to starting the CS procedure.

Figure 4-1 shows the high-level CS flow to perform phase-based ranging between a CS initiator and reflector.

Figure 4-1 High-Level Channel Sounding Phase-Based Ranging Flow

At the beginning of CS subevent, the CS Mode 0 steps are mandatory to provide frequency and timing synchronization between CS initiator and reflector, for the remaining CS steps within that CS sub-event.

Multi-carrier phase ranging involves phase difference measurements at different frequencies, and any relative error (between the initiator and reflector devices) in the generation of the frequencies can disrupt those phase measurements and introduce errors in the overall distance estimation. Therefore, it is important that the initiator and reflector devices keep their carrier frequencies aligned during the entire duration of each individual measurement. Per Bluetooth Channel Sounding draft specification, the initiator device is required to align its timing and carrier frequencies to the reflector. The device uses the initial Mode 0 steps to do this step by estimating and compensating the fractional frequency offset (FFO) between the devices.

The CS Mode 0 steps are then followed by Mode 2 PBR steps which involves both the initiator and reflector devices exchanging the unmodulated CS tones at different channel frequencies.

Out of the 79 designated Bluetooth channels in the 2.4GHz unlicensed ISM band, 72 channels with 1MHz channel spacing can be used for Bluetooth LE, CS PBR (Note: Bluetooth LE advertising channels are not prescribed for CS PBR). Both the initiator and the reflector measure the phase and amplitude of the incoming tone as a function of frequency. Once all the PBR Mode 2 steps are complete, both the initiator and reflector devices can have the phase and amplitude information in the form of in-phase and quadrature-phase (I and Q) measurements. Next, the reflector can communicate this measurement information to the initiator as part of the Bluetooth LE connection events. Note: vice-versa wherein the initiator can send back the measurements to the reflector for further processing is also possible.

Next, the initiator combines the phase and amplitude measurements from both the devices and performs post processing to estimate the distance measurement. The Bluetooth CS does not specify a specific algorithm to compute a distance estimate, but provides some mathematical representation towards distance estimation using the phase measurements. Advanced and efficient post processing algorithms can be used to eliminate multipath and fading effects to provide robust distance estimation values. Additionally, tone quality information can be used filter outliers due to signal interference and noise. Post processing algorithms with varying complexity and efficiency can be used to compute distance approximation with tradeoff consideration with respect to accuracy, power consumption and computational latency requirements.