Medical Electronics
Move over ZigBee
Mac Tochigi looks at how design engineers can achieve low power wireless connectivity for battery powered healthcare sensors and considers the benefits of Bluetooth Low Energy over ZigBee
W
ith the increasing adoption of smart phones and tablet computers, consumers are
looking at doing more with them. One particular area that is gaining popularity is the use of these devices to monitor personal health, especially in the area of fitness and in monitoring athletic performance. The need to provide small, lightweight and wireless battery powered sensors presents engineers with a challenge as to how to make this possible. While recent advances in low energy versions of popular communication stacks
ZigBee are also worthy of consideration. The selection criteria for this decision will certainly include factors such as required data rates, power budget, host connectivity, and any potential costs to licence protocol stacks or silicon. In terms of data transfer requirements, portable healthcare appliances tend to have fairly low needs. In general terms, if the data rate needed is likely to be in excess of 3 Mbps it is likely that Wi-Fi would be the only option. Wi-Fi however does tend to be relatively expensive to implement due to the complexity of host
Figure 2: Bluetooth Low Energy Software Architecture
for many years without needing a replacement battery. Data rates in the order of about 250 kbps tend to limit its use for high volumes of data. ZigBee has a network advantage that finds it widely used for home automation and smart metering applications. It can use a mesh network topology that allows it to connect
Bluetooth has a small power consumption profile perhaps not quite as good as ZigBee, but has the benefit of having a much higher data rate up to 3 Mbps. Compared to the original (Classic) Bluetooth the power consumption of BLE can be as low as a 1/10 to 1/20th. The BLE protocol is very simple and it facilitates
802.11a standard. ZigBee and Bluetooth are both confined to the 2.4 GHz ISM frequency band.
ZigBee has many advantages in terms of power consumption. Most of the time a ZigBee-based device is likely to spend in sleep mode and can wake up very quickly to send data. A ZigBee remote controller, for example, can run on a coin cell battery
up to about 100 nodes. By comparison, Bluetooth/BLE does not have this approach and is used on a peer-to-peer communication basis. Figure 1 shows the average power consumption against data rate for Wi-Fi, Bluetooth, Bluetooth Low Energy and ZigBee and from that figure it is easy to see that BLE has a good advantage in terms of low data rates.
Figure 1: Power consumption vs data rate
such as Bluetooth are suitable for these types of applications, engineers are faced with the decision of whether to make a discrete wireless function within their design or, alternatively, design-in one of the compact commercially available wireless modules.
Bluetooth, of course, is not the only wireless connectivity candidate. Wi-Fi and
26 October 2012
interface connectivity required for security and higher level operating system support. Unfortunately, Wi-Fi is also very heavy on power consumption so it tends not to be suitable for long-life battery-powered devices. If frequency were a key criterion, which it tends not to be, Wi-Fi would be the only candidate if there were a need to operate in the 5 GHz spectrum using the
Components in Electronics
Figure 3: Current consumption characteristic of BLE
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