Bluetooth 5.0 Coded PHY for Low-Energy Long-Range Communication

At Simbex, we’ve been developing connected medical devices and connected sports and fitness devices since the early 2000’s. One of our better-known products is the Riddell InSite head impact monitoring system, developed in partnership with Riddell, the market-leading football helmet manufacturer. In this post, we discuss the use of Bluetooth 5 (BLE 5.0) as the communication protocol for the next generation of our InSite product.

We’re all familiar with using Bluetooth to connect our phones to car audio systems or wireless headphones. These problems are well understood, and the solution is clear. But what about maintaining robust communication between an alert monitor carried by a team trainer and 75 or more players wearing InSite helmets? Again, BLE 5 offers a solution with long-range mode, also known as Coded PHY. Some engineers have reported BLE Coded PHY achieving steady device communication at distances of over 1,000 meters depending on power output and the antenna used. Since our InSite system is designed exclusively for use on football fields, our focus is on reliable communication at up to 150 yards.

InSite uses a five channel sensor that fits inside of the Riddell helmet. As shown in the InSite System diagram on the right, player head impact data is processed and captured by an in-helmet electronic unit called the player unit.

Since we first launched InSite in 2014, there have been many advancements in hardware technology like smaller and more accurate sensors, increased compute power of low energy devices, and more robust communications protocols. As we started to conceptualize the next generation of InSite, two important objectives were identified:

1. Expand the InSite feature set and improve usability by leveraging smartphone hardware and software ecosystems.
2. Ensure robust helmet to sideline communication given the continual increase in the level of radio frequency noise at sports venues.

BLE 5 provides a means to achieve both of our objectives. First, Bluetooth is universally supported by smartphones, and native BLE 5 support is quickly becoming the standard for new smartphone models reaching the market in 2020 and 2021. Second, by design, Bluetooth and the BLE 5.0 version are designed to operate in environments with existing 2.4GHz Wi-Fi networks. The BLE protocol uses many discrete channels spaced 1MHz apart of narrow band frequency-hopping and has the capability to skip over any discrete channels perceived to have interference. This helps Bluetooth operate reliably in environments alongside existing Wi-Fi networks.

The playing surface of a football field is 120 yards long by 53 yards wide. Add in the sidelines and space at the ends of the field and the playing surface is about 70 yards wide by 140 yards long. At any given time, the InSite alert monitor needs to be able to receive alerts and HIE data from any player on the field. The distance between the player and trainer can vary from a few yards to well over 100 yards. This scenario presents some real communication challenges for our low power InSite player units. These are:

1. Alert monitor knows about all InSite helmets currently on the field (up to 150 helmets per team).
2. Alert monitor receives alerts from all InSite helmets in less than 30 seconds from the impact event.

InSite helmets don’t need to continuously transmit data back and forth to the alert monitor the way your wireless headphones and phone do. Instead, they “advertise” to the alert monitor at a regular interval, sending the minimum amount of data necessary to update the alert monitor with a player’s status. When a player experiences a significant impact, the player unit modifies the advertisement data to notify the alert monitor that an impact event has occurred and requests a connection to upload the data. When the alert monitor receives this request, it establishes a secure BLE connection with the player unit, downloads the impact data characteristics, and resets the device to start sensing for another impact.

Bluetooth is a full protocol stack. The bottom layer of the stack is called the Physical Layer and is normally referred to as “PHY”. Bluetooth 5 added two new PHY variants to the PHY specification used in Bluetooth 4. Each PHY variant has its own particular characteristics and was designed with specific aims in mind. The three PHYs have been named to allow them to be referenced in specifications, and they are labeled LE 1M, LE 2M and LE Coded.  The first two PHYs are primarily used for streaming data at close range. LE 1M is Bluetooth 4, and LE 2M was introduced in Bluetooth 5 along with LE Coded. The LE Coded PHY allows range to be quadrupled (approximately), compared to Bluetooth 4 and this has been accomplished without increasing the transmission power required.

To achieve the increase in range, LE Coded PHY uses Forward Error Correction (FEC), a particular approach to error correction. It works by adding additional, redundant bits to the transmitted packets. The sole purpose of those bits is to support the application of the FEC algorithm so that error correction can be performed. Using FEC reduces the amount of data that can be transmitted by the player unit. HIE data packets are small compared to streaming music or sending images, so the benefits of Coded PHY far outweigh the cost of reduced transmission volume.

Using BLE Coded PHY, our first generation BLE player units are communicating consistently to an Android smartphone with our alert monitor app at 50, 100, and 150 yards, and in some cases as far as 250 yards. Latency is very low, giving us confidence that alerts will be received by the alert monitor in well under 30 seconds. We are currently pushing the envelope of the BLE 5.0 protocol in our field tests in order to characterize the system’s performance across a range of environments. The results from our fee-air testing show a 95% average success rate (85% lower bound at p=0.001) of receiving an impact alert within 30 seconds from over 100 tests conducted across distances of 50m to 250m. Our next phase in development is optimizing our implementation of the BLE 5.0 protocol to extend these results into high RF environments while still maintaining low-power operation.

Our initial results with BLE 5 have been extremely positive. BLE 5 long-range mode is proving to be a robust solution for on-field communication between InSite smart football helmets and an Android smartphone alert monitor. One of the critical requirements of InSite player units is that the battery must last for the entire football season, so the next step in our development process will be to optimize the power budget of the player unit electronics.