by Larry Burgess
In the last few years, the smaller size and lower cost of radios has created opportunities for passing more and more information from place to place faster and cheaper. Low data rate scenarios like soil monitoring and crop quality on farms, detection and instant notification of leaks on pipelines, availability of parking spaces, supply chain monitoring, forest fire detection in remote regions, to name just a few, can be realized for far less money or space or power than was possible in the past. In fact, many applications have surfaced recently that occurred to nobody until cheap, low power radio links were possible. What follows is a description of the radio links that are in place and continuing to change and develop to satisfy the needs of the growing IoT industry.
Mismatch of established standards and IoT
There have been several well-known radio standards around for almost 20 years, but they were not well matched with the needed low power, inexpensive, long-range operation radio links suitable for multiple remote devices in the Internet of Things.
Cellular networks are expensive. They started out being focused on voice, which needs to be on for long periods of time compared to short bursts of data. They evolved to serving higher data rates for pictures, streaming video, and millions of twitter messages. Only now are they trying to add some features to accommodate IoT devices, but the technology moves slowly.
Wi-Fi and Bluetooth were developed to serve short-range scenarios. Early Wi-Fi routers served individual homes and small offices. Bluetooth is low power, but short range. It now serves a large market that partially overlaps IoT, but the communication range of these products is still short, consisting of smart home lighting and climate control, health and exercise trackers to name a few applications.
Additionally, Wi-Fi, Bluetooth, and some of the cellular bands work at higher frequencies (above 2 GHz), a frequency range where path loss begins to restrict the useful range of wide area radio networks. As for power consumption, Wi-Fi routers do not need to conserve battery life. Most are powered from power outlets, so there has not been a concerted effort to make WiFi devices low energy users. Cellular consumes, even more, power than Wi-Fi. Bluetooth recently introduced Bluetooth Low Energy (BLE), but its applications are still short range, generally operating at 10 meters or less.
Features of IoT radio links
There are many applications for IoT devices, so the properties listed below do not cover every application. They are aimed at applications with large numbers of sensors and radios, low cost, long range, low to medium data rate, rapid on/off time, and low power drain (non-rechargeable batteries, energy harvesting). The combination of long range and low power is better suited for frequencies lower than those used for Wi-Fi and Bluetooth. In a free space environment, path loss is lower at lower frequencies for a given antenna gain and the scattering and blockage from land and foliage is lower. Most of the long range protocols are in the sub-1 GHz license-free bands, the most prominent of which are a large contiguous band from 902-928 MHz and narrower bands at 868 MHz, 470 MHz, 433 MHz, and 315 MHz. A few protocols use the 2400-2483 MHz band.
Advantages and disadvantages of the license-free bands
The sub 1 GHz bands (902-928 MHz in the US, 868 MHz in Europe and other countries) have lower path loss than the widely-used bands at 2.4 GHz and 5.8 GHz.
One disadvantage of the low-frequency license-free bands is that they do not have the near-global compliance with radio standards like the 2.4 GHz band.
To adapt to different frequency ranges from region to region, advances in chip radio design are producing radios that can support multiple frequency bands, so these radios can be programmed to serve the regions where they are located.
License-free bands have a lot of interference from other users. Unlike licensed bands that belong to one user (or a small number that cooperate with each other), the unlicensed bands may have a large number of users with their radios turning on and off. Each of these radio signals may vary in signal strength, bandwidth, signal duration, etc. The challenge for users in this band is for their signals to get through to each other with so many other users in the band. The signal properties (bandwidth, duration, coding, synchronization) need to work dependably in license-free bands that are crowded with many users and contain a large variation in signal strength, bandwidth, and duration of interfering signals.
Innovative properties of recently developed radio IC’s, modules, and standards
Because of the need to establish reliable communication links in a crowded spectrum and minimize power drain in the radios, device makers and standards bodies have worked to adapt the radio properties to the interference environment in license-free bands. This includes transmitter power adjustment to maintain the radio link without wasting power drain from the battery, data rate adjustments depending on the amount of data needed, duration of the transmitted signal, and modulation to combat interference.
In the license-free bands with a wide spectrum (902-928 MHz and 2400-2483 MHz), some radio systems use spread spectrum techniques like Direct Sequence Spread Spectrum (DSSS), Frequency Hopping, and Chirp (sweeping the frequency up or down). Each of these techniques takes a narrowband signal and spreads it over a much wider frequency so that intermittent narrow band interference block only a small portion of the signal. At the receiver, these spread signals are de-spread, and the information is usually 100% recovered.
Energy harvesting is used in some devices to reduce battery current drain or completely eliminate batteries. The radio products investigated here can be adapted to energy harvesting. Many devices cannot use energy harvesting, however, because this technology provides microwatts of power, which is not enough for many applications.
Usable range for low data rate radio links.
There is a large variation in the usable range of these radios because the environment in which they are deployed can have a significant effect on the attenuation of the radio signals.
In free space (air to air or ground to air with no trees or buildings nearby), a radio with a half-watt transmitter, which is near the maximum allowed transmit power, can communicate with another radio at a data rate of about 3000 bits/sec that is about 170 miles away and high enough above the horizon (at least 4 miles up) for their antenna beams to overlap.
When both radios in a link are near the ground (which is the case for virtually all the emerging links of IoT), the range is significantly reduced because there is a direct path between the antennas and a reflected path from the ground (think of a mirror). In virtually all scenarios on land (except in the middle of a desert), the reflected wave is almost as strong as the direct wave, and it is close to being 180 degrees out of phase, so there is a significant amount of cancellation from the two paths. Mathematical approximations of ground bounce loss show that the loss increases as the fourth power of the distance, compared to free space, where the loss increases as the square (second power) of the distance. The loss is less when the radios are mounted well above the ground, if possible.
Radios mounted on two towers, each about 30 feet above the ground, need to be within about 15 miles of each other to communicate (compared to 170 miles in free space).
A user on the ground with a radio can communicate with a 30-foot tower-mounted radio up to 5 miles away.
Two users on flat, open ground can communicate with each other up to a range of 1.5 miles.
The usable range inside buildings is much shorter, typically 30 meters, because the transmitters (on keychains or in thermostats) are less powerful and there is much more blockage and scattering than there is outside.
Companies and/or standards that provide radios, IC’s, systems for IoT wireless links.
Table 1 contains information on 11 companies or standards that provide radio systems for IoT applications. The data was compiled from company websites, data sheets, license-free standards like 802.15.4, and related technical articles. Some overall observations from this group of radio systems are
- Most of the systems (9 of 11) work at frequencies below 1 GHz. The frequencies range from the 902-928 MHz band to as low as 109 MHz.
- Although the maximum transmitter power allowed in these bands is +27 to +30 dBm (0.5 to 1 watt), most of the systems in this list work closer to +20 dBm (100 mW).
- The data rate can be as low as 50 to 100 bits per second and as high as 125 kbits per second. The lower the data rate, the longer is the usable distance for the radio link.
- Several of these systems use a form of spread spectrum described earlier, which results in the transmission of a wide bandwidth signal that gets around mostly narrowband signals on the air blocking small parts of its spectrum. The receiver “de-spreads” the desired signal back to the original narrowband signal and spreads the spectrum of the narrowband interfering signals so that they look like wideband noise at a much lower power level than the desired signal.
- The most commonly used radio networks are Star or Mesh networks. There are advantages and disadvantages of each, depending on the characteristics of the application served by the radio system.
Table 1: Power Efficient, License-Free Radio Systems or Standards
|Name||Freq Band(s)||Data Rate and Modulation||Structure||Typical Tx Power|
|SigFox||868 MHz, 902-928 MHz||300 bits/sec BPSK, GFSK||Star (Tower and remotes)||+15 dBm||50 km outdoors|
|NWave||315, 433, 470, 868, 902-928 MHz||100 bits/sec BPSK||Star||+20 dBm||20-30 km rural|
|Symphony Link||902-928 MHz||300-50K bits/sec||Star or Mesh||+23 dBm||8-16 km user on ground to tower|
|Ingenu||2400-2480 MHz||100 bits/sec DPSK with RPMA||Star||+20 dBm||> 500 km line of sight outdoors|
|LoRa||109, 433, 868, 902-928 MHz||300-50k bits/sec Chirp||Star or Mesh||+14 dBm (EU) +27 dBm (US)||10-20 km user on ground to tower|
|Waviot||433, 500, 868-915 MHz||50 Hz BW||Star||+20 dBm||50 km outdoors (tower/tower)|
|Thread||2400-2480 MHz||250 kbits/sec||Mesh||+10 dBm||30 m indoors|
|Zigbee||868, 902-928, 2400-2480 MHz||20-250 kbits/sec BPSK and OQPSK||Star or Mesh||+20 dBm||30 m indoors, 1500 m outdoors|
|EnOcean||315, 868, 902-928 MHz||125 kbits/sec ASK or FSK||Mesh||+10 dBm||30 m indoors 300 m outdoors|
|6LoWPAN||169, 433, 470, 868, 902-928, 2400-2480 MHz||250 to 4M bits/sec ASK, BPSK, O-QPSK, DSSS, Chirp||Mesh||+16 dBm||5-10 km user on ground to tower|
|Z-Wave||868 MHz, 908 MHz||10-100 kbits/sec||Mesh||0 dBm||30 m indoors|