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Sub-GHz: An emerging WLAN alternative for IoT applications

As the IoT continues to grow, ever-increasing numbers of edge devices are appearing everywhere – in open and public spaces as well as our homes and workplaces. This can mean long-range outdoor networks such as the smart grid and municipal lighting as well as the short distances associated with applications like home automation. All of these devices must be networked into their IoT infrastructure, and for convenience and accessibility most use wireless connectivity, at least into the gateway.

As these devices vary widely in the demands they make on their network and other components, different network standards have evolved, allowing designers to optimise for their particular project requirements. Is it about high bandwidth and performance, or are low cost, small footprint and minimal battery usage the objectives? Smartphones, security cameras and other data-intensive devices are candidates for higher bandwidth protocols like WiFi and Bluetooth, while smart meters and garage door openers, which intermittently transmit small packets of data, do not need high data rates, but benefit from a protocol with reduced power demand and a longer range.

Sub-GHz technology is achieving increasing popularity as its characteristics match these low data rate devices’ needs well. Fig.1 compares some sub-GHz and 2.4 GHz applications and trends.

Fig.1: Wireless frequency trends in consumer, industrial and automotive applications - Image: ©Premier Farnell Ltd

There are, however, a couple of points to consider when making this comparison. The first is that there are many applications that use both technologies in complement. TI’s CC1350 dual-band wireless MCU, for example, supports both Bluetooth Low Energy (BLE) and sub-GHz protocols. One application is within smoke or CO2 detectors that are distributed around a house . The detectors communicate with a gateway over a simple star sub-GHz network, to benefit from the protocol’s coverage, range, reduced attenuation through walls and ability to ‘bend’ round corners. Meanwhile, the BLE channel is used to carry over-the-air firmware upgrades, sent from an ordinary smartphone or pad without needing special tools.

The second point is that when making a detailed comparison to reach a design decision, it is important to cover all the factors that differentiate the protocols. Accordingly, we review these factors below, to provide a better understanding of when to use sub-GHz, and the advantages of doing so. This is followed by an insight into how sub-GHz radio has been developed as an IEEE802.11 standard, and a review of various practical approaches to embedded wireless system design. Finally, some examples of products available from Farnell to help engineers rapidly complete designs are given.

Advantages of sub-GHz radios

Sub-GHz radios offer relatively simple wireless solutions that can operate continuously on battery power alone for up to 20 years. Notable advantages over 2.4GHz radios include :

Licence-free spectrum allocation: Sub-GHz networks conform to IEEE 802.11ah, a new Wi-Fi standard that operates in the sub-one-gigahertz region of the frequency spectrum; specifically in the Industrial, Scientific and Medical (ISM) band, which is licence-free.

Range: The narrowband operation of a sub-GHz radio enables transmission ranges of a kilometre or more. This allows sub-GHz nodes to communicate directly with a distant hub without hopping from node to node, as is often required using a much shorter-range 2.4GHz solution. There are three primary reasons why the sub-GHz range is superior to that of 2.4GHz applications:

As radio waves pass through walls and other obstacles, the signal weakens. Attenuation rates increase at higher frequencies, therefore the 2.4GHz signal weakens faster than a sub-GHz signal.
2.4GHz radio waves also fade more quickly than sub-GHz waves as they reflect off dense surfaces. In highly congested environments, the 2.4GHz transmission can weaken rapidly, which adversely affects signal quality.
Even though radio waves travel in a straight line, they do bend when they hit a solid edge (like the corner of a building). As frequencies decrease, the angle of diffraction increases, allowing sub-GHz signals to bend farther around an obstacle, reducing the blocking effect.

The Friis Equation demonstrates the superior propagation characteristics of a sub-GHz radio, showing that path loss at 2.4GHz is 8.5dB higher than at 900MHz.

This translates into 2.67 x longer range for a 900MHz radio since range approximately doubles with every 6dB increase in power. To match the range of a 900MHz radio, a 2.4GHz solution would need greater than 8.5dB additional power.

Low interference: The airways are crowded with colliding 2.4GHz signals from various sources, such as home and office Wi-Fi hubs, Bluetooth-enabled computer and cell phone peripherals, and microwave ovens. This traffic jam of 2.4GHz signals creates much interference. Sub-GHz ISM bands are mostly used for proprietary low-duty-cycle links and are not as likely to interfere with each other. The quieter spectrum means easier transmissions and fewer retries, which is more efficient and saves battery power.

Low power: Both power efficiency and system range are functions of the receiver sensitivity plus the transmission frequency. The sensitivity is inversely proportional to channel bandwidth, so a narrower bandwidth creates higher receiver sensitivity and allows efficient operation at lower transmission rates.

In general, all radio circuits running at higher frequencies, including low-noise amplifiers, power amplifiers, mixers and synthesizers, need more current to achieve the same performance as lower frequencies.

Range, low interference and low power consumption are basic advantages of sub-GHz applications over 2.4GHz apps. One disadvantage often cited is that the antennas are larger than those used in 2.4GHz networks. The optimal antenna size for 433MHz applications, for instance, can be up to seven inches. However, antenna size and frequency are inversely proportional. If node size is an important design consideration, developers can Increase the frequency (up to 950MHz) to employ a smaller antenna.

Fig.2: As radio frequencies are increased, antenna sizes reduce accordingly - Image: ©Silicon Labs

Worldwide deployment: Global compliance is an important design consideration for a number of wireless applications. For instance, video game manufacturers who market their products worldwide can use 2.4GHz radios for all their consoles because it is a global ISM allocation. Similarly, wireless applications using the 433MHz band share a global sub-GHz ISM allocation, with Japan being the sole major market exception. In addition, 915MHz is used extensively in North America and Australia, 868MHz is deployed across all of Europe and 315MHz is available in North America, Asia and Japan. Fig. 3 summarises the Industrial, Scientific and Medical (ISM) bands available for sub-GHz use around the world.

Fig.3: Worldwide ISM band allocation for sub-GHz - Image: ©Premier Farnell Ltd

Proprietary solution benefits: Proprietary sub-GHz protocols allow developers to optimize their wireless solution to their specific needs instead of conforming to a standard that might put additional constraints on network implementation.

While standard solutions offer the advantage of vendor-independent interoperable nodes, they normally increase each node’s cost and footprint. For example, a 2.4GHz ZigBee radio node will cost approximately $2.00 (USD) and the software stack will require about 128KB of embedded memory. Conversely, proprietary sub-GHz nodes generally target low-cost systems, with each node costing approximately 30-40% less and requiring 4KB memory for the stack (for example, Silicon Labs’ EZMacPRO®).

With specialized functions and small software stacks, proprietary solutions can achieve smaller die sizes and reduced memory footprints. Additionally, the less complex stacks simplify deployments and reduce maintenance costs. Therefore, proprietary sub-GHz solutions can offer some of the least expensive point-to-point localized networks for applications like garage door openers or home automation systems.

Overall flexibility: The flexibility offered by national regulations in selecting physical layer characteristics such as output transmitted power, modulation scheme, data rate and channel bandwidth, together with the possibility to develop proprietary protocols lets users find the best solution for their needs as well as unmatchable performance and system efficiency at the expense of interoperability and development efforts. Moreover, either a star or mesh network topology can be implemented and, in principle, without any limitations in the number of nodes connected simultaneously .

Sub-GHz radio’s development as an IEEE 802.11 standard

In response to the recognised drawbacks of the existing IEEE 802.11’s 2.4GHz and 5GHz WLAN architectures, the IEEE 802.11 working group initiated a new project, named IEEE 802.11ah, designed to implement an 802.11 standard using sub-GHz licence-exempt bands for cost-effective, large-scale networks. The 802.11ah standard defines new versions of the Physical (PHY) and Media Access Control (MAC) layers intended to increase system throughput .

802.11ah channelization: IEEE 802.11ah defines the channels based on the spectrum available in a given country.

The basic channel width is 1MHz, although it is possible to bond two adjacent channels together into a 2MHz channel to provide higher data throughput capability. Wider channels are available; the widest in the US being 16MHz for the 902 - 928MHz ISM band. This uses the same channel bonding method as adopted for 802.11n and 11ac. Channel widths of 1, 2, 4, 8, and 16 MHz can be used.

Other countries have different spectrum allocations with channels on different frequencies accordingly, but the same basic methods are used, obviously with different limitations on the maximum number of channels that can be bonded together.

802.11ah PHY / radio interface: 802.11ah uses orthogonal frequency division multiplexing (OFDM) to provide the modulation scheme for the signal. However the 802.11ah physical layer PHY can be split into two categories:

1 MHz channel bandwidth: This mode of operation is aimed mainly at those applications requiring extended range. The narrower bandwidth and slower data rates enable signals at lower signal strengths to be accommodated. Typically these applications may be aimed to IoT or M2M applications where short bursts of data, normally at a low data rate may be required.
As one of the main aims of the 1 MHz channel option is for extended range, a new Modulation and Coding Scheme, MCS index - MCS 10 - is included for long range transmission in addition to the 802.11ac's MCSs. This is effectively a mode of MCS 0 but with a 2x repetition of the data to increase the resilience of the transmission.
Bandwidths of 2 MHz & more: This mode uses bandwidths of 2, 4, 8, or 16MHz. It also uses OFDM, and a design based on a tenth clocking rate of 802.11ac, i.e. symbol length of ten times that in 802.11ac. MIMO is also used within 802.11ah in this mode.

802.11ah MAC: The Media Access Control or MAC layer features several enhanced elements to provide support for large numbers of stations, power saving, and throughput improvements.

Support for a large number of stations: Up to 8191 associated stations can be accommodated, using a hierarchy of 802.11 access point allocate identifiers, called Association Identifiers (AIDs).
Power saving: Power saving is a growing issue, especially for IEEE 802.11ah WLANs used for many IoT and M2M applications. Many of the remote nodes will be battery-powered, and must run for weeks or even years without replacement.

Traffic Indication Map (TIM) stations: These stations remain awake all the time and continually monitor the beacon frames that are sent. It can receive data as soon as it is ready to send.

Non-TIM stations: Non-TIM 802.11ah stations have a doze state. When they are in this state they are unable to receive data, so this is buffered ready for when they become active again.

Throughput enhancements: To best use the available bandwidth, there have been several enhancements to ensure that data is carried as efficiently as possible. This has been achieved with several innovations. These include compact MAC header formats that free up space and improve system efficiency, and MAC mechanisms that eliminate channel access delay and ACK transmission overhead.

Some practical embedded wireless solutions

Different options for implementing practical embedded wireless solutions depend on the choice of IC components, and the software/hardware partitioning . Fig. 4 shows three options, based on the OSI model, for partitioning the software and hardware blocks of an embedded system design.

Fig.4: OSI model of typical embedded system designs - Image: ©Premier Farnell Ltd

SoC: A system-on-chip (SoC) device, such as Silicon Labs’ Si1060 sub-GHz wireless microcontroller (MCU) product, combines an MCU and a transceiver into a single-chip solution that runs the wireless software stack and the application software. The SoC must have enough functionality to support the embedded device; for example the necessary I/O for push-button controls and analogue-to-digital-converters (ADCs) for sensing conditions such as temperature and humidity. In addition, SoCs are typically ultra-low-power, small-form-factor devices that are designed to enable extremely long battery life. This option often provides the most cost-effective solution and smallest physical size.

Typical applications for SoCs include fixed-function devices with a simple user interface or often no user interface at all, such as remote controls, key fobs for locking/unlocking car doors or home door/window security sensors.

MCU + Transceiver: If a suitable SoC is not available, system architectures that combine a separate MCU and wireless transceiver allow developers to select the optimal MCU for the application. MCU options include not only MCU architecture (8-bit versus 32-bit including a choice of ARM Cortex-M cores) but also on-chip features such as an LCD controller, USB support, multiple I/Os, timers, comparators, ADCs, and a range of flash memory sizes. The developer can then add the best transceiver solution to meet the application’s wireless requirements. From an application standpoint, an MCU + transceiver system is similar to an SoC in that the network is typically fairly simple, and there are relatively few timing concerns in the system. The MCU runs all of the software including the application and wireless software. However, the physical layer (PHY) and data link layer are often integrated on the transceiver. The end product will often include a simple user interface for setup and control.

NCP: A network coprocessor (NCP) may be used when the complexity and performance needs of the end product call for a high-end MCU or microprocessor (MPU). Like a wireless SoC, an NCP combines an MCU and transceiver into a single-chip solution, but an NCP does not have the functionality to support the full application. It runs the entire communications stack and interfaces to the host processor via a serial interface such as a UART or SPI. The NCP architecture separates the application’s software complexity from the communications software, which is vital in applications where the network stack timing is critical or there are high throughput requirements. While this approach is the most expensive system solution option, product cost is often offset by reduced development complexity and faster time to market. Common devices suitable for an NCP architecture include gateways, security panels and devices running multiple protocol stacks.

Fig.5 compares the relative merits of these three embedded design approaches.

Fig.5: Design considerations summary - Image: ©Premier Farnell Ltd

Sub-GHz development kits

Leading suppliers like Farnell offer a wide choice of development kits and evaluation boards to simplify and reduce the learning curve associated with embedded wireless system development; we give some examples below.

LaunchXL-CC1310 - Development Kit, SimpleLink™ CC1310 Sub-1 GHz Wireless MCU, Long Range Connectivity

The LAUNCHXL-CC1310 is a SimpleLink CC1310 sub-1GHz wireless microcontroller (MCU) LaunchPad development kit. It is the first LaunchPad kit with a sub-GHz radio which offers long-range connectivity, combined with a 32-bit ARM cortex-M3 processor on a single chip. The CC1310 wireless MCU contains a 32-bit ARM cortex-M3 processor that runs at 48MHz as the main processor and a rich peripheral feature set that includes a unique ultra-low power sensor controller. This sensor controller is ideal for interfacing external sensors and for collecting analogue and digital data autonomously while the rest of the system is in sleep mode.

The kit’s PCB has an integrated PCB trace broad band antenna, which supports the 868MHz ISM band for Europe and the 915MHz ISM band for US. An on-board emulator gets users started with instant code development in the CCS cloud.

Fig. 6: SimpleLinkLaunchXL-CC1310 development kit PCB

CC1310DK - Evaluation Board, SimpleLink™ Sub-GHz CC1310 MCU, 2 x SmartRF06, 2 x Whip Antenna

The CC1310DK is a SimpleLink sub 1GHz CC1310 development kit. The SimpleLink sub-GHz CC1310 development kit includes all the hardware users need to start evaluating with the SimpleLink ultra-low power wireless MCU platform. The bundle comprises a CC1310 evaluation module kit and a SmartRF06 evaluation board. The CC1310 evaluation module kit contains two boards with the wireless MCU and RF layout using the 7x7mm package, external bias and differential RF output (variant 7XD). The SmartRF06 evaluation board is the mother board for the CC1310 evaluation module. It is specifically designed for running radio performance tests and for developing software. The CC1310 devices on evaluation boards are preprogrammed with an easy to use test application which can be used to test the practical range of the radio.

Fig.7: CC1310 evaluation module kit

EV-ADF70301-915AZ - Development Kit, ADF7030-1 Sub-GHz Radio Transceiver IC, High Performance

The EV-ADF70301-915AZ is a daughter board with an onboard ADF7030-1 high performance, sub-GHz RF transceiver. The ADF7030-1 is a fully integrated radio transceiver achieving high performance at very low power. This IC is ideally suited for applications that require long range, network robustness and long battery life. The ADF7030-1 daughter board plugs into the P1, P2 and P3 headers on the ADuCM3029 mother board.

The Analog Devices ADuCM3029 is an Ultra-Low Power ARM Cortex-M3 MCU with Integrated Power Management and 256 KB of Embedded Flash Memory.

Fig.8: EV-ADF70301-915AZ - Development Kit

MRF89XAM8A-I/RM - Transceiver Module, Surface Mount, Ultra Low Power, 868 MHz

The MRF89XAM8A-I/RM is a MRF89XAM8A ultra low power sub-GHz surface mount transceiver module with integrated crystal, internal voltage regulator, matching circuitry and PCB antenna PICtail/PICtail Plus daughter board. The MRF89XAM8A module operates in the European 863MHz to 870MHz frequency band and is ETSI compliant. The integrated module design frees the integrator from extensive RF and antenna design, and regulatory compliance testing, allowing faster time-to-market. The daughter board can be plugged into multiple demonstration and development boards. For example, it is suitable for 8-bit microcontroller development using the PIC18 explorer board or for 16-bit or 32-bit microcontroller development using the explorer 16 development board.

Fig.9: MRF89XAM8A-I/RM - Transceiver Module

Conclusion

Sub-GHz proprietary solutions are widely used for the wireless connection of nodes in home networks and building automation systems as well as in industrial process applications. Real-time monitoring and control of thousands of nodes enables process optimization, more efficient resource management, prevents breakdowns and saves energy in Smart Factories.

Sub-GHz solutions are also used in the implementation of Smart City infrastructures where each wireless node is part of a network. Nodes are monitored and controlled, and their data can be used for managing light, parking and traffic systems; saving energy and improving the quality of life. Thanks to its wireless coverage range, efficiency and flexibility, sub-GHz technology is one of the building blocks for enabling IoT growth, even if it requires an internet gateway for connecting to the IoT.