Choosing a wireless protocol for your IoT design

Operating in the license-free industrial, scientific, medical spectrum (868.2MHz for Europe and 902MHz for the USA), a new ultra-narrow band cellular service called Sigfox provides a viable alternative. 

So which should you choose for your IoT design? Balancing the following points is crucial: 

1. Network Coverage
GSM offers near worldwide coverage, but the mix of signal generations varies. Some countries boast 2.5G and 4G but omit 3G, whilst others exclusively use 3G or 4G. This can make it challenging for OEMs to design a product to cater for all European markets.
Sigfox coverage encompasses France, Spain and the Netherlands and is in development elsewhere in Europe, the USA and South America. As the whole EU uses the same frequency, this makes it easier to design and promote pan European products, subject to coverage being available. Note that OEMs will need to make some slight antenna revisions to cater to the US market.

2. Message Rate
GSM data rates accommodate many different application requirements with theoretical limits of 4G up to 100Mb/s+ to the more modest 2.5G up to 171kb/s. In contrast, Sigfox is suitable for uses that involve sending small amounts of data, such a smart energy meters. It has a data rate of 100b/s, a maximum of 140 transmitted messages per day per device and a maximum customer payload per message of 12-bytes. There’s also a downlink restriction of up to four messages per day and 8-bytes per message.

3. Cost: BOM & airtime
Cost wise, it’s necessary to review both the BOM and airtime. GSM has seen module and airtime costs tumble since 2010. Hence, some 2.5G Quad band GPRS module from u-blox are priced at under £10.00 for a 1,000-piece order. Airtime is typically as little as £0.80 per device per month with 1MB of data. A physical SIM card is required for network access, although if larger volumes are planned, the use of a SIM IC is now possible.

By comparison, Sigfox modem devices are much cheaper. The AX-SIGFOX-1 wireless SoC device from Axsem costs approximately £1.50 per device in 1,000 piece quantities. Airtime costs from £0.30 per device per month and varies depending on the overall number of devices connected and daily message volume. The SIM is embedded within each chip, a plus point from a manufacturing and cost perspective.

4. Battery
Unsurprisingly GSM requires more battery power than Sigfox but this makes it more flexible too. 2.5G typically requires a power supply capable of dealing with 2A peaks with an average transmit connected current of 250mA. Careful design of available device idle modes can drop standby consumption to 1mA.

Sigfox boasts very low power consumption. For example, the Axsem SoC device mentioned previously only draws 10mA at 0dB during either TX or RX. Even if the output power is increased up to 14dB, the modem only demands 45mA at its peak and idles at 950nA.

Wrapping up, your wireless protocol of choice will largely depend on the range and data volume requirements, but it’s clear that GSM is no longer your only cellular option. For large packets of data sent both ways frequently, GSM is the clear winner, but for small amounts of low frequency data from a battery-powered device, Sigfox tops my list.