Background & References

Research Papers:

Poster: Frequency Scaling in Time Synchronization for Wireless Sensor Networks (2018)

Farzad Asgarian, Khalil Najafi

Summary: This paper uses a BLE synchronization protocol in which timestamps are sent at initialization (referred to here as synchronous timeslot), and other timestamps are used over time to help re-synchronize the clocks if needed (referred to as synchronous task). The clock frequency of the master (device with the "correct" time) and all nodes is high during synchronous timeslot, while the clock frequency of all nodes is decreased during synchronous tasks in order to save power and radio activity. This technique called frequency scaling is used alongside a synchronization protocol that they developed called BlueSync. In this way, they are able to achieve synchronization with low power usage.

Notes: The main drawback of this approach is that a sensor node would have to run at two clock frequencies, with a low clock frequency being used after the initial synchronization is performed. This might be undesirable for certain applications. We will take frequency scaling and the BlueSync protocol into consideration when deciding on a time synchronization protocol.

Precise synchronization time stamp generation for Bluetooth low energy (2016)

Carl C. Rheinländer, Norbert Wehn

Summary: In this project, hardware timestamps are generated on BLE devices to improve synchronization between devices. They use the BLE SoC nRF51822 by Nordic Semiconductor for this research. Two main principles are used to obtain the hardware timestamp; first they perform a software interrupt to notify the BLE device of activity, and then they generate a hardware timestamp in the form of power consumption by using a current shunt monitor. By monitoring the current across the shunt, they can determine when BLE transmissions were sent or received. They were able to achieve a standard deviation of 0.9 us between clocks of synchronized devices.

Notes: This work achieves a very high accuracy between synchronized BLE devices. They were able to properly obtain a hardware timestamp by exploiting the power consumption of the BLE device, which is impressive. One drawback of this technique is that it requires the user to construct a shunt monitor, which may not be desirable for many applications. In our project, we hope to find another way to obtain hardware timestamps.

CheepSync: A Time Synchronization Service for Resource Constrained Bluetooth LE Advertisers (2016)

Sabarish Sridhar, Prasant Misra, Jay Warrior

Summary: CheepSync is a time synchronization service for BLE devices that is able to handle timing requirements as precise as 10us. This time synchronization is only for BLE advertisement, which is a one-way communication mode that can be held between multiple devices without a formal bonded connection. They use a custom version of the nRF24Cheep beacon for broadcasting and an Android phone as a passive listener.

Notes: CheepSync uses custom hardware for their research, which means that it is not available for public use. In our project, we plan to use widely available, well documented, and commonly used boards with BLE to ensure that time synchronization is available for public use with minimal barriers.

Software Repositories:

Hardware Datasheets:

We will be syncing BLE between three different embedded systems devices. Currently, our top candidates include:

• LaunchPad Development Kit
• SensorTag
• ESP32