Wireless Communication

DOI: 10.4324/9781003304036-15

Nearly all wearable devices communicate the sensor data through a wireless connection. There are some devices that use a connector to download data, but that means the data is not available immediately. Wireless communication can send the data in real time. There are many choices for wireless communication. The most used is Bluetooth Low Energy, or BLE. It uses the lowest power, and it can communicate directly to most smartphones. It must be within a few feet of the phone, however, so it will not work unless you take your phone wherever you go. 

Wi-Fi is convenient because it is available on all smartphones as well as hot spots in many places, but it consumes a lot of power, which requires a large battery. Cellular service can connect directly to the Internet rather than pass through a phone. This is a big advantage when a phone is not always being carried. Cellular uses a very large amount of power, however. 

Battery life is a major limitation of wearable devices. To make the device small, the battery must be small; this means it does not last long between charges. A lot of design choices are made to minimize the power consumption. One of the biggest consumers of power is wireless communication. 

One way to limit power is to send the data a very short distance to a smartphone. This is commonly done using Bluetooth Low Energy. It is the lowest power wireless communication for wearable devices, but it only works when the phone is nearby.

Another way to send data is through a gateway that connects to the Internet. Wi-Fi uses hot spots or a phone as the gateway. There needs to be a gateway nearby, but the distance can be hundreds of feet, much more than with BLE. Wi-Fi is not generally a good match for wearable devices, however. It can transmit at very high speeds, but it uses a lot of power. 

For many wearable devices the ideal wireless communication would send data directly to the Internet. Such wireless communication is available. There is a relatively new class of wireless called narrow band or Low Power Wide Area Network (LPWAN). This includes NB-IoT, LTE-M, LoRa, and Sigfox, among others. Narrow band communication transmits several kilometers at low power. The trade-off is a low data rate, but that is not a problem for most wearable devices. They rarely need to send more than a few hundred measurements per second, which is considered slow, and they often send less than one sample per second. 

Table 12.1 compares various wireless standards. The data rate increases from left to right. The communication distance increases from top to bottom, and the power is shown as a number in milliwatts.

One important wireless standard is not shown in this table: NFC or Near Field Communication. It is similar to RFID, which is often used as a substitute for barcodes. NFC requires the transmitter to be within a few centimeters of the wearable device, and it requires no power from the wearable. The device doing the reading provides wireless power. Many smartphones 

Table 12.1 Power—How Much, How Far? 

Source: Voler Systems

Table 12.2 Comparison of Wireless Communication Protocols

Source: Voler Systems

have NFC built in. They are often used to scan a device, such as a glucose monitor. The data is immediately visible on the phone from where it can be transmitted to the Internet to be shared. 

Table 12.2 compares the most popular narrow band wireless communication with two other standards: Bluetooth LE Mesh, which is Bluetooth LE with the ability to communicate data through an array of devices to get greater range, and Zigbee, an older standard that transmits data similar distances to Wi-Fi, but at much lower power. BLE Mesh and Zigbee are not narrow band wireless, and they are uncommon in wearable devices, but they are interesting to compare. LTE-M is also known as Cat-M1, LTE Cat-M1, or eMTC. 

Notice that LTE-M can transmit as fast as 1Mbit/second, which is very fast for wearable devices. It is not advantageous at these speeds, as the power consumption approaches Wi-Fi and cellular. Unlike Wi-Fi and cellular, the power is much lower when the data rate is low. NB-IoT and LoRa are similar. They are very power-efficient when the data rate is very slow. 

Another important differentiation with narrow band wireless is whether it uses a public or private network. A public network, such as the cellular network, has the advantage of easy roaming and no configuration of the network. The penalty is a monthly fee and the reliance on the network to exist. Sigfox has a major disadvantage here, as the network is very limited in the United States. LoRa has a big advantage in being able to run on a private network. This means you can install a network access point or hot spot in a school or factory, and you have LoRa service with no monthly charge. It doesn’t matter that the LoRa public network is very limited in the United States. 

Sigfox is primarily intended for one-way communication, from the wearable device to the Internet. It is popular in France and many European countries, but not in the United States or other countries. The limited public network makes it nearly unusable in most places. In recent years, NB-IoT and LTE-M have become widely available, making them an excellent choice for narrow band wireless. 

NB-IoT has limitations when used in devices that move around, so LTE-M may be a better choice. 

The coverage of LTE-M and NB-IoT around the world is shown here. When it is available in a country, that does not mean it is available everywhere. See Figure 12.1 for detailed coverage in the United States. 

The coverage of NB-IoT in the United States is shown in Figure 12.2. In 2019, Verizon announced that they provided coverage for 92% of the population of the United States. It has grown since then. LTE-M has a similarly high coverage rate. At this high coverage rate, a manufacturer can successfully sell a product that uses it. 

By comparison, Figure 12.3 shows the coverage for public networks for LoRa in the United States by Senet in early 2020. (Coverage data is difficult to get, so it was not possible to compare the coverage at the same points in time.) 

Figure 12.1 NB-IoT and LTE-M Coverage Worldwide. 
Source: GSM Association 

Figure 12.2 NB-IoT and LTE-M Coverage in the USA. 
Source: Verizon 

Figure 12.3 LoRa Public Network Coverage in the USA. 
Source: Senet  

5G cellular has gotten a lot of publicity recently. It has some big improvements over 4G (fourth generation cellular). There are claims that it will transform low data rate communication such as IoT (Internet of Things), including wearable devices. Unfortunately, 5G offers nothing for wearable and IoT devices at this time. The standard is still being developed for these applications. NB-IoT and LTE-M, which are part of 4G, are currently the best way to send data directly to the Internet from a wearable device.