New Bluetooth spec gives the IoT a sense of direction

The recently released Bluetooth 5.1 specification builds in an efficient way of zeroing in on the location of BLE transmitters and receivers.

Mark de Clercq, Dialog Semiconductor
The newest version of the Bluetooth communication standard emerged in January. The most noteworthy feature of Bluetooth 5.1 is an efficient way of discerning the direction of Bluetooth transmitters and receivers. The spec also introduces other advances aimed at speeding communications and boosting energy efficiency.

Makers of Bluetooth ICs have already released hardware that bakes in the features of the new 5.0 spec. Here’s a rundown of the changes Bluetooth 5.1 brings to the table, along with an example of how one supplier has begun applying it.

Bluetooth receivers have used signal strength to estimate distances to Bluetooth transmitters. The 5.1 spec provides two methods for more precise means of pinpointing locations, at the expense of using multiple transmit/receive antennae. To implement the

AoA and AoD — In the angle of arrival (AoA) method implemented in Bluetooth 5.1, a device such as a tag in a real-time locating system (RTLS) transmits a signal from a single antenna. The receiver contains multiple antennas arranged so the receiver sees a phase difference because of the difference in distance from each receiving antenna to the transmitting antenna. In the angle of departure (AoD) method, a device such as a Bluetooth locator beacon transmits a signal via an antenna array. The receiving device, such as a smartphone, uses a single antenna to pick up the signals that are decoded to calculate relative signal direction. This method of direction finding is intended for use in indoor positioning systems such as those for wayfinding.

Angle of Arrival (AoA) method, the receiver must have at least two antennae (at a minimum spacing of 6.2 cm) while the transmitter can have one. With AoA, the receiver uses its multiple received signals to deduce the angle of the source. Similarly, the Angle of Departure (AoD) method requires a transmitter with at least two antennae. The receiving device, which can have a single antenna, picks up multiple signals and from them derives the angle of the source.

One point to note: The range of devices following the 5.1 spec is the same as those of earlier versions. But the accuracy of locating Bluetooth sources using just signal strength falls off severely when obstacles are in the line of sight. The ability to triangulate will likely usher in capabilities such as real-time locating systems and indoor positioning systems.

The direction-finding features use in-phase and quadrature (IQ) sampling to measure the phase of RF that an antenna receives. In AoA, the sampling process is applied to each antenna in the array, one at a time, and in a sequence set by the design of the array. Sampled data is passed up the Bluetooth protocol stack via the Host Controller Interface (HCI) where an algorithm calculates the direction of one device from the other.

BLE tracking — AoA and AoD features in Bluetooth 5.1 are aimed at bringing more precision to real-time locating systems (RTLS) and indoor positioning systems (IPS). Bluetooth RTLS setups for asset tracking employ Bluetooth receivers, often called locators, in fixed locations throughout a facility. The locators connect to a centralized server dubbed a location engine. Bluetooth transmitters commonly called tags are attached to entities the system is tracking. The tags periodically transmit a signal to the location engine which estimates the position of each locator. In an IPS, Bluetooth transmitters referred to as locator beacons reside in fixed locations. Visitors typically use an app on a smartphone to listen for locator beacons. The app uses received signal strength to calculate its real-time position.

Certain parts of the Bluetooth protocol changed to support IQ sampling and the use of IQ samples by higher layers in the stack. The details get to be a bit complicated. For example, at the link layer, there’s a new field called the Constant Tone Extension (CTE) that provides a constant frequency and wavelength signal material against which IQ sampling can be performed.

The 5.1 spec also makes some enhancements to GATT, the Generic Attribute Profile, which defines the way two BLE devices transfer data back and forth using concepts called services and characteristics. The enhancements are generally in the interest of better energy efficiency and faster handshaking between Bluetooth devices. But some of them are in preparation for future enhancements in the works for upcoming spec releases.

Fundamentally, the 5.1 spec defines how BLE clients can skip service discovery on devices where nothing has changed since the last time the two devices communicated. A client may now deduce that a device to which it is connecting is the same type as a previous connection and with an attribute table the client has already cached. If certain details are the same, the client may decide skip parts of the connection protocol because it already has the data it needs.

The classic example of where this might come in handy is that of Bluetooth smart locks, where a smartphone opens the door as its owner approaches. Service discovery need only take place the first time the user opens the smart lock. The user may notice a delay in the door unlocking on the first time through, but from then on, they will experience a near instantaneous response.

Advanced architectures

The new features that Bluetooth 5.1 incorporates put an additional computational load on Bluetooth controllers. Certain architectural features come in handy when handling the extra work. For example, the process of triangulation can employ sophisticated algorithms. So architectural features that include digital signal processing and a large memory are helpful.

Additionally, because triangulation involves signals from multiple antennas, Bluetooth controllers must incorporate a means of handling the multiple RF signals involved. Because the market for single-antenna Bluetooth remains large, controllers implementing the 5.1 standard may handle the need for multiple RF paths by including architectural features for rapidly manipulating an external RF switch, rather than including the switch on the Bluetooth controller IC itself.

dialog block diagram — The SmartBond DA1469x family of Bluetooth controllers from Dialog Semiconductor implements Angle of Arrival and Angle of Departure features of the Bluetooth 5.1 standard. The devices also contain a Sensor Node Controller (SNC) that runs autonomously and independently processes data from sensors connected to its interfaces. Additionally, the DA1469x family has a built-in Power Management Unit that only activates the processing cores on the chip as needed, while also eliminating the need for a separate PMIC.

An example of such a Bluetooth 5.1 controller chip family is the Dialog SmartBond DA1469x family. It is the first wireless multi-core MCU in production based on the ARM Cortex M33 processor. This processor features digital signal processing and floating-point capabilities that aid in triangulation computations. Additionally, these processors have I/O that let them control an external RF switch with a high degree of precision for the purpose of implementing AoA and AoD location. Handling AoA and AoD with an external RF switch, rather than a switch integrated on the Bluetooth chip, keeps down costs of single-antenna Bluetooth apps.

Additionally, the memories on DA1469x devices are expandable, useful for handling complex applications while keeping costs down for more basic uses.

Because it is often advantageous to limit power dissipation in Bluetooth applications, DA1469x chips incorporate a power management function wherein sections of the chip are only active when needed. For example, the chip would power up only the portion of the circuit handling the radio and simple Bluetooth functions when applications don’t involve direction finding. The full Cortex M33 processor would come to life for tasks demanding AoA or AoD.

BLE 5.1 demo board — Visible here is a development board from Dialog Semiconductor configured as a Bluetooth receiver that detects the angle of arrival (AoA) of a Bluetooth transmitter. This particular device is set to measure the angle to the transmitter ten times/sec and display the angle on the LCD.

Another point to note about DA1469x chips is that AoA and AoD functions are available to programmers via APIs. This eliminates the need for detailed knowledge of how to handle raw AoA/AoD data.

There are several other features on DA1469x ICs that don’t specifically relate to AoA/AoD functions, but which are useful for modern IoT needs. These features include a haptic feedback driver, a step motor driver (aimed at handling analog clock movements in hybrid analog/digital clocks), an LCD driver, a high-accuracy ADC for reading sensors, and other specialized analog blocks. Finally, some versions of the IC include built-in charging facilities for lithium-ion and lithium-polymer batteries.

All in all, Bluetooth 5.1 ushers in a number of functions that will better track both people and objects while adding to the user experience.