In a series of blogs on BLE, we have discussed in detail about the Bluetooth technology, its classifications, its popular low-power variant BLE, its 7 layers OSI mapping and responsibilities, multi master and Multi listener concepts. As Bluetooth SIG upgrades the Bluetooth specifications continuously to meet current market use cases and its performance requirements, developers need to know how to make best use of the configuration. Hardware guidelines for performance and battery life improvements are almost done by semiconductor companies. In software, there are certain parameters which need to be fine-tuned based on product use cases. In this blog, we will discuss various parameters related to BLE operations and important considerations while working with BLE devices.
To begin with, some of the important aspects of Bluetooth low energy communication are as follows,
- Advertisement interval
- Connection interval
- Slave latency
- Connection supervision timeout
- Data throughput
Understanding the above aspects will help any developer to lower the device power consumption, increase the speed of connection and improve the reliability of data transmission and reception.
BLE Physical Layer
It is important to know about the BLE physical layer, such that we understand the BLE communication better, because physical layer includes the actual RF radio and is in charge of sending the signals over the air.
Bluetooth Low Energy is similar to classic Bluetooth where both of them use 2.4GHz spectrum but differ from each other with different modulation index. Classic Bluetooth uses 79 channels whereas BLE uses only 40 channels and the channels of both are spaced differently. Due to this, the BLE and classic Bluetooth cannot communicate between them. But there are modules that can support both BLE and classic Bluetooth, which operate by switching its modulation parameters and the channels.
The 2.4GHz spectrum of BLE is divided into 40 channels (0 to 39) which extend from 2402MHz to 2480 MHz with 2MHz spacing. Among the 40 channels, BLE advertisement takes place in 3 Fixed advertising broadcast channels (37, 38 and 39) and data exchange takes place in the remaining 37 adaptive frequency hopped dynamic data channels.
The following image illustrates the channel layout of BLE with 3 advertising and 37 data channels.
BLE - Channel Layout
BLE Communication
BLE communication takes place between a “central device” (for example Android Smartphone or iPhone) and a “peripheral device”. Any BLE communication can happen only with the following two modes,
- Advertisement mode
- Connection mode
BLE advertisement mode by default is unidirectional and will be initiated only by the peripheral device through sending advertisement packets. The peripheral device will broadcast advertisement to every device around it.
Connection can be initiated only by the central device (within the communication range of the peripheral device) to receive more information. Only in connection mode, can both the peripheral and central device send packets.
Connection cannot be made between two devices without using advertisements and central device cannot send any packets to peripheral device without a connection.
BLE device power consuming stages
BLE modules are generally designed to be low power, but the current draw between modules can vary greatly based on the specifics of the device design, chip selection, and software implementation.
In the idle state, the BLE device isn't doing anything, so it's mostly in low power or sleep mode. The current draw can be very low, often in the microamp (µA) range, sometimes even less.
During advertising, the device's radio is on and actively broadcasting packets to let other devices know it's there. This takes more power, but it’s usually done in short bursts. Current draw during these bursts might be in the range of 3 milliamps (mA) to 10 mA, again depending on the specifics of the device and the advertising settings (like advertising power level, interval, and payload size).
During connection, the device is actively communicating with another BLE device. The power usage can vary significantly based on the Bluetooth connection settings, like connection interval and connection event length.
However, it might be in a similar range to the advertising state, perhaps around 5 mA to 30 mA (give or take a few mA) during active communication.
Power consumption optimization at different stages
Optimization is achieved by tuning the below parameters for the two states of Bluetooth LE devices:
Connected State| Connection interval | It can be configured in a range from 7.5 milliseconds to 4 seconds (in multiples of 1.25 milliseconds). |
| Peripheral latency | Peripheral shall skip connection events without the central device dropping the connection. |
| Connection supervision timeout | Define the maximum time between two received Data Packet PDUs before the connection is considered lost. This shall be a multiple of 10 ms in the range of 100 milliseconds to 32.0 seconds. |
Advertising State
| Advertising interval | It can be configured from 20 milliseconds to 10,485.759375 seconds. However, most stacks cap the maximum to a smaller number, usually 10.24 seconds. The longer the interval, the less current the device will consume during the advertising state. This is a more important parameter when working with broadcast-only devices such as beacons. |
| Advertising data length | Another parameter affecting current advertising consumption is how many payload bytes are sent in each advertising packet. Therefore, it may be beneficial in terms of current consumption to only place primary advertising data in the advertising packet and place all secondary data in the scan response packet, as advertising packets are sent much more frequently than scan response packets. |
Bluetooth 5.0 Improvements
Before Bluetooth 5.0 release, 1M PHY has been the defacto transmission mode. It can transmit at 1Mbps. With Bluetooth 5.0 release, 2M PHY has been introduced to increase the throughput and range of communication along with error correction techniques. As 2M PHY can transmit theoretically at 2Mbps, even for applications where no throughput increase is desired, the reduced radio time of 2M PHY still provides valuable power saving.
Conclusion
With Bluetooth 5.x, there are more improvements w.r.t throughput performance and power consumption. It’s vital information for any BLE product developer, both hardware and software, to know how to fine tune the stack for optimal performance as per need. At Embien, our embedded product engineering team has developed specialized low-power optimization expertise and has supported customers to develop and improve performance for more than 50+ BLE products.