By: Alviano Burello, Business Development Manager for RF & Wireless, Future Electronics
In the past decade, the technology for connecting devices over unlicensed ISM (Industrial, Scientific and Medical) band frequencies lower than 1GHz has evolved to offer greater capability, but at the expense of greater complexity.
In the 1990s, the limited processing power available from the microcontrollers of the time constrained the designer’s choice of topology; most wireless networks at that time were in point-to-point, bus or star topologies that ran on simple protocol software.
Limitations in the range and number of nodes supported by these technologies, however, led to the introduction of mesh and tree network types. And in recent years, the industry seems to have come to a consensus that ZigBee, an industrystandard mesh networking protocol, offers the best way to implement a multi-node, long-range ISM-band network.
But mesh networking is inherently complex and more difficult to implement than a star network configuration. This is a high price to pay for the extra kilometers of range that ZigBee offers, if in every other respect the features provided by a star network would be adequate for the application. And this is why new technology described in this article that massively extends the range of a star network is exciting wide interest among RF design engineers.
Mesh Networks: Complex by Nature
A star network has an inherently shorter maximum range than a mesh network has. Figure 1 shows that the range of a star network is limited to the distance between one node and another.
A mesh network, by contrast, provides in effect an array of repeaters that can receive and retransmit a signal from any one node to any other node. In a mesh network, traffic routing is dynamic, and signals are passed from one node to the next like the baton in a relay race until they reach their destination. This means that the range of a mesh network can theoretically be made unlimited.
In practice, of course, the software for characterizing multiple nodes, implementing system timing and routing traffic between nodes is highly complex. In fact, the more nodes in the network, the more potential routes for data traffic to take. The software for controlling a mesh network can thus easily become large and difficult to manage.
This in turn has a marked impact on the memory footprint of the hardware supporting a mesh network, and therefore also on Bill-of-Materials (BoM) cost. A complex mesh network protocol such as ZigBee calls for the use of a high-specification microcontroller, as well as requiring large memory resources. On both the hardware and software side, then, a mesh network makes high demands on system resources.
The problem in the past has been that, for OEMs requiring an ISM-band network with a range of several kilometers but only a relatively low data rate, a mesh network has been the only valid topology; star networks were typically limited to an open-space range of around 1km in open space, provided the link used a low bit rate that could support an adequate signal-to-noise ratio (SNR).
Yet for most OEMs it would be far preferable to implement a star network than a mesh network. Because a star network consists of a single master and multiple slave nodes, the routing of traffic is far simpler than in a mesh network. An asynchronous system keeps the control traffic as small as possible, thus making most efficient use of the available duty cycles, and minimizing power consumption, while entailing a low risk of collisions between data frames. In general, the software overhead in a star network is therefore far smaller than that of a mesh network.
In addition, while every node in a mesh network must be intelligent (and therefore complex and expensive), in a star network all the intelligence is concentrated in the controller node; slave nodes are simple and cheap to make.
In fact, a star network can be implemented with just a low-end microcontroller and a small amount of flash memory in the control node. This means it will also consume little power, making it well suited for battery-powered applications.
The constraint that deters designers from implementing a star network has, then, been the range between nodes. This range is strongly affected by the modulation scheme used by the RF transceivers. Sub-1GHz RF transceivers today typically implement Frequency Shift Keying (FSK) or Amplitude Shift Keying (ASK) modulation. Both techniques are sensitive to noise and interference that tend to limit the range that transceivers are able to achieve.
Now a new digital modulation technique introduced by Semtech offers massively extended range between nodes; transmissions can readily be carried as far as 10km in open space at output power levels sanctioned by regulations, for instance up to 14dBm at 868MHz. This increased span between nodes means that systems which previously had to implement a mesh network in order to achieve a range of several kilometers can now instead implement a star network, with all the consequent benefits of this simpler architecture.
Semtech’s new modulation scheme, known as LoRa (for ‘LongRange’), is a completely asynchronous digital modulation scheme. Based on the Direct Sequence Spread Spectrum (DSSS) technique, it is able to vary the length of the chipping code and the bandwidth to match the bit rate required, in a range from 300bits/s to 21kbits/s.
The high performance of the LoRa technology is demonstrated in its ability to receive signals as much as -22dB below the noise floor, coupled with adjacent channel rejection of at least 65dB with a 25kHz offset – some 30dB more than is possible with FSK modulation.
At the same time, Semtech has provided for mutual orthogonality between signals on the same frequency but carrying data at different bit rates – a technique that avoids collisions between the two sets of signals, and provides for a higher aggregate bit rate over any given link.
A number of sub-1GHz transceivers from Semtech implement the LoRa modulation scheme. All offer extremely high sensitivity – up to -148dBm at 169MHz in the case of the multi-frequency SX1276, SX1277 and SX1278 (Figure 2). Peak transmit power at the antenna is rated at 20dBm, giving a huge maximum link budget of 168dBm. The combination of a large link budget, low sensitivity to interference and low collision rate underlies the devices’ ability to achieve a range of several kilometers.
At the same time, the devices are low powered enough to support operation by battery supply. All SX127x LoRa devices, including the SX1272 and SX1273, offer an FSK modulation capability as well as LoRa modulation, to provide for backwards compatibility with legacy devices already installed in the field. They draw just 9.5mA in FSK mode and 10mA in LoRa mode, and operate across a wide voltage range of 1.8V to 3.7V.
Clearly, many OEMs will choose LoRa technology because it allows the use of a star network topology, which benefits from lower BoM cost than an equivalent mesh network system. Semtech’s LoRa devices also enable BoM cost reduction in other respects: for instance, they operate with a standard, low cost quartz crystal with a tolerance of ±30ppm for the PLL reference frequency.
Resurgence of Interest in Star Network Topology
Robust, long-range wireless networking in the unlicensed sub-1GHz wireless frequency bands is finding applications in many market sectors. With the introduction of LoRa technology from Semtech, many such applications can now be implemented as simple, low cost star networks rather than more expensive, complex mesh networks. This will make the economics of wireless networking much more attractive in functions such as automatic meter reading, home automation, sensor networks, the smart grid, car park monitoring and control, security equipment and street lighting. The high link budget and rejection of interference make LoRa devices suitable when long range in open space is required, but also in environments that are hostile to RF transmissions, such as devices inside buildings and underground or in spaces containing high levels of RF noise.
Since widely used FSK modulation technology offers higher data rates – in SX127x devices from Semtech, 300kbits/s compared to the maximum 40kbits/s in LoRa mode – the introduction of LoRa should not be seen as marking the death of the old FSK and ASK modulation schemes. But nevertheless, it is a very large advance in the capability of ISM-band RF technology, and looks set to bring about a resurgence of interest in star networking.
No doubt system designers will also find other interesting ways to make use of the long range offered by Semtech’s new RF technology. A mixed solution is one possibility: a local area network in the star topology, implemented with LoRa modulation, in which the gateway node connects to a wide area network (such as the mobile telephone infrastructure) via a standard protocol such as WM Bus, with the connection to the WAN carried over a GFSK-modulated link. In fact, the LoRa modulation scheme allows the designer to implement a proprietary network protocol as well, so the technology provides almost total freedom to design the appropriate radio network within the constraints of range and SNR.