Bluetooth Low Energy (also known as BLE) was introduced in 2010, ushering in a new era of connectivity, especially between smart devices and smartphones. BLE is a new wireless protocol that shares some features with classic Bluetooth. It takes some of the same concepts that Bluetooth pioneered, such as simple connectivity and fast data transfer, but simplifies them to enable low-power, reliable, and fast data transfer between devices.
This article aims to cover many of the BLE basics you need to know when developing or designing a product. Before discussing BLE, it’s important to note its naming: Bluetooth Smart was the original marketing name for Bluetooth Low Energy. The Bluetooth Smart name and logo didn’t catch on—most people, including us, just called it Bluetooth Low Energy or BLE. It was also confusing from a marketing perspective—is there a Bluetooth that isn’t smart? To make matters more confusing, there are also “smart-ready” devices that are often referred to as “dual-mode,” meaning they support both Bluetooth Low Energy and Bluetooth Classic (for making calls with a cell phone, for example). While commonly referred to as BLE, the Bluetooth SIG discourages the use of this name because BLE is not a trademark they own. Therefore, the official name is Bluetooth LE.
Bluetooth is one of the most popular wireless protocols and it has been used in smartphones, computers, and other devices for more than a decade. Most of us are familiar with Bluetooth and how it allows us to connect headphones and make calls using our phones. The explosion of Bluetooth devices and new use cases made the Bluetooth SIG and other companies realize that Bluetooth was using too much power in some applications and took too long to connect. For example, a keychain finder using Bluetooth would not run for very long and could take a long time to connect, which was frustrating for users.
Starting with the iPhone 4s, Apple began supporting BLE in its devices, which opened the way for a large number of small battery-powered devices. Before BLE, using classic Bluetooth could be cumbersome for users and developers. Classic Bluetooth was complex, used a lot of power, and required an authentication chip on the iPhone, which was expensive, making these products too expensive. For transmitting data, Apple devices can support BLE without this requirement, so the cost is lower. One of the most powerful aspects of BLE is its extensibility and how it allows any developer who has an idea and needs to exchange information to do so, rather than the rigid structure of classic Bluetooth.
Wireless Protocol Comparison
Bluetooth and BLE are great protocols that can simplify connecting your products. However, it’s important to understand where they stand compared to other wireless technologies. WiFi, Zigbee, and other protocols work better in certain applications, but BLE doesn’t always work.
Zigbee, WiFi, and Bluetooth LE all use the 2.4GHz ISM band, but their capabilities vary greatly. While the BLE radio is a short-range device that consumes less power than Zigbee and WiFi, it has evolved in the Bluetooth 5.0 specification to allow up to 20dBm output power as well as LE long range. This means it has a very long range. In fact, we have been able to achieve real-world distances of over 1km. BLE's low peak current consumption is critical when choosing a battery. There is absolutely no way WiFi, with its peak TX current of 200mA or more, can run on a coin cell battery. However, the BLE radio is designed for coin cell operation.
BLE Physical Layer
Bluetooth LE introduces a completely new radio with similar modulation to Bluetooth Classic's 1Mbps, but with some differences. Let's take a closer look at the BLE physical layer. The physical layer refers to the radio itself:
BLE uses the same 2.4GHz ISM band as classic Bluetooth and Wi-Fi because it is “unlicensed” and can be used without an FCC license and is available worldwide. The band starts at 2400MHz and goes all the way to 2483.5MHz. The Bluetooth LE specification divides the band into 40 1MHz channels spaced 2MHz apart. This is half the number of channels in classic Bluetooth, but it helps simplify some radio designs. 3 One of the channels is called “broadcast” and is used only by devices to send beacon packets (called advertising packets). These packets contain information that allows other devices to connect, but can also provide information about the device.
To avoid interference from Wi-Fi and other sources, advertising channels are specifically placed in the lower, upper, and middle parts of the band. For example, Wi-Fi can occupy bandwidth from 20MHz to 40MHz. By spreading out the advertising channels, Wi-Fi stations are less likely to interfere with all of them. For example, if channel 38 and its surrounding channels are interfered with, there are 2 other broadcast channels 37 and 39 that will not be affected.
BLE radios transmit using a modulation scheme of either 1Mbps or 2Mbps, which is suitable for radios that support Bluetooth 5.0. Most, if not all, available radios are 5.0 compatible, but some older devices are not. The 2Mbps PHY (physical layer) allows for faster data transmission, but usually at the expense of range. Bluetooth 5.0 also introduces CODED phys. Transmissions using coded PHY use 1Mbps modulation, but they use bits for redundancy (called coding in wireless lingo). This allows for error correction, effectively improving the signal over noise.
One thing that is often not explained is some of the tradeoffs. 1Mbps is the standard modulation used by most broadcasts. Using 2Mbps requires the radio to negotiate. In order for this to work, both ends of the connection need to support Bluetooth 5.0. The 2Mbps PHY is required for Bluetooth 5.0 support, but Bluetooth long range support is optional. Smartphones support 2Mbps PHY, but we haven't seen one that supports LE long range. Part of the reason is that long range is often an industrial requirement.
Bluetooth 5.0 also supports output power up to +20dBm, which is the maximum limit you can reach. Previously, +10dBm was the limit, but most BLE radios support up to around 4dBm, with a few exceptions. Such high output power enables BLE to cover very long ranges. BLE's flexibility in speed and output power allows for optimized application design and makes BLE powerful enough to accommodate many use cases.
To build a BLE product, you don't need to know much about the physical layer, but you do need to know a little about RF. Like all wireless transmitters, BLE devices send and receive radio frequency signals, which requires antennas and a carefully designed RF design. Antenna design is a complex topic, but virtually any 2.4GHz antenna can be used. PCB and chip antennas are among the most common antennas.