Channel Sounding is an emerging Bluetooth capability which improves the accuracy of distance measurement between two Bluetooth LE devices on the order of tens of centimeters (10 to 30cm accuracy). This milestone improvement emboldens a new set of services and applications that Bluetooth will now be able to deliver including secure access, proximity detection, tracking, and indoor wayfinding.
Bluetooth Channel Sounding, also known as High Accuracy Distance Measurement (HADM), is a step-function improvement to prior distance measurement services including the very crude Received Signal Strength Indicator (RSSI) and the more recent Bluetooth version 5.1 Direction Finding improvements including Angle of Arrival (AoA) and Angle of Departure (AoD). Channel Sounding is expected to be adopted by the Bluetooth SIG in the coming months and a draft specifications is currently available. As background, at the bottom of this page[User1] , we take a look at the capabilities / limitations and evolution from RSSI and AoA / AoD to Channel Sounding.
The Bluetooth system architecture with Channel Sounding
In the Bluetooth system, Channel Sounding introduces hardware changes by means of a new physical layer that uses amplitude-shift keying modulation on 72 physical channels. Channel Sounding radio operations are time-multiplexed in a coordinated fashion with other Bluetooth LE radio operations in the 2.4-GHz band, such as advertising, scanning, and data communication.
A closer look at the Bluetooth LE stack reveals that most of the Channel Sounding feature is included in the link layer, which is a MAC layer protocol above the RF/PHY layer. The link layer handles procedures for advertising, scanning, connection management, asynchronous and isochronous data communication, and data encryption. For Channel Sounding, the link layer is responsible for:
Negotiating and setting up a Channel Sounding procedure between two devices.
Scheduling the Channel Sounding radio operations.
Configuring the RF/PHY layer to perform the required tone exchanges during each radio operation.
Managing Channel Sounding security procedures.
Channel Sounding introduces a new physical layer in the Bluetooth system
A Channel Sounding procedure consists of a series of radio operations known as Channel Sounding events. During a Channel Sounding event, two devices exchange information that’s used to perform distance estimation. This exchange consists of both tones used for distance estimation and packets containing data related to the Channel Sounding procedure. A Channel Sounding event consists of subevents, with each subevent entailing multiple Channel Sounding steps. A Channel Sounding step is a series of tightly synchronized tone transmissions between devices. Dividing a Channel Sounding procedure into events and subevents allows for maximum flexibility in scheduling Channel Sounding radio activity, and therefore optimum coexistence with other timing-sensitive Bluetooth communications.
The high-level communication sequence between two devices performing a Channel Sounding procedure starts with the establishment of a Bluetooth LE connection. The devices then exchange Channel Sounding capabilities and negotiate the configuration. After Channel Sounding security is enabled, the Channel Sounding procedure can start. Following each Channel Sounding subevent, the devices exchange measurement results. This is repeated until the Channel Sounding procedure has gone through all requested physical channels, up to 72 across the entire frequency band. The Channel Sounding procedure is then terminated. Finally, measurement results from the link layer are sent up to an application in the device that typically uses a distance estimation algorithm to calculate the distance between the two devices.
Communication sequence between two devices to perform a Channel Sounding procedure
Channel Sounding doesn’t specify which upper layer software algorithms should be used to calculate distance estimates from the measurement data. This allows companies to tailor their offering to different use cases, adjusting computational complexity to the required estimation accuracy and the expected radio environments for the given use case. Packetcraft and our partners are offering integrated solutions for Bluetooth Channel Sounding from the Bluetooth stack up to the upper layer software algorithms.
Background of Distance Measurement Solutions from RSSI to Distance Finding to Channel Sounding
Received Signal Strength Indication (RSSI)
The oldest method for radio-level distance measurement (ranging) is RSSI. Its basic principle is straightforward: the distance between two radio devices is calculated based on how much the signal between the two – more specifically its amplitude – has decayed over the transmission distance. RSSI can give an indication of the distance between two radio transceivers though it uses coarse estimates. It’s also well established as it’s available in all smartphones. However, it offers very low accuracy, typically three to five meters, and is very susceptible to interference from external factors such as absorption, interference, and diffraction. Just the fact that you’re holding the device in your hand can make a big difference. By using multiple beacons and characterizing the RF environment, RSSI’s accuracy can be enhanced, and it becomes possible to estimate the position in space of a device, but remains insufficient for many applications.
Direction Finding (AoA / AoD)
Direction Finding approaches including AoA and AoD present a meaningful advance in distance ranging accuracy compared with RSSI; however, both techniques don’t directly measure distance but rather estimate the angle of, respectively, incoming and outgoing radio signals. Using trigonometry, distances can then be calculated. These solutions do however require multiple antennas to work. AoD is more complex than AoA and is therefore used much less frequently in direction finding implementations. Angle of Arrival and Angle of Departure are capable of achieving sub-meter accuracy but a lot depends on the practical circumstances. For instance, in indoor locations with a lot of obstacles and reflecting surfaces, multipath propagation hinders reliable measurements. AoA and AoD have two significant issues pertaining to the requirement for multiple antennas, which significantly increases the chip’s footprint and suffers from poor performance in multipath environments.
Bluetooth Channel Sounding introduce a new method to achieve accurate distance estimates, known as Phase-Based Ranging (PBR). In Phase-Based Ranging, a precise estimate of the distance between the two radio devices is accomplished by analyzing the radio signal’s phase. Essentially, the Time of Flight of the radio signal. To measure phase data, both devices alternatingly transmit and receive in a coordinated way and this is repeated multiple times over different frequencies (channels), resolving ambiguities and improving the precision, even in the presence of reflections (multipath).
For more information about Channel Sounding and to discuss early access, or to more generally inquire about our Bluetooth host, controller, and LC3 stacks and software solutions as well as our software test products, please contact Packetcraft at firstname.lastname@example.org.