The installation of solid-state street lighting is gathering pace worldwide. The benefits of networking these luminaires, city-wide, are already well appreciated, and the expectation is that connected outdoor lighting will provide the base infrastructure for further network-based smart city applications, often as part of the Internet of Things (IoT) movement.
This article considers the benefits of power line communication (PLC) as the backbone connectivity technology in networked outdoor lighting applications. Although it can be (and is often) combined with wireless networking technologies, PLC has distinct advantages when nodes such as street lamps are already connected by their own mains power network.
Reference is made to the recently launched STMicroelectronics STEVAL-IHP007V1 evaluation kit based on the ST7580 single-chip power line communication and networking solution, plus the Echelon SmartServer2.0PL networking, control and management device.
Control and connectivity
Significant energy savings and reduced maintenance costs are the primary drivers in the growth of connected outdoor lighting installations worldwide. It is widely reported that public lighting can consume as much as 40% of a municipality’s energy budget. Cost savings can be significant, with estimates varying from 30 to 70%. These savings are primarily gained through energy efficiency achieved by installing long-life, dimmable LED technology, and importantly, the ability to monitor and control the lighting network.
Figure 1: Connectivity is key to smart street lighting, enabling control and monitoring to deliver energy efficiency, simpler maintenance and the implementation of safety features. Source: Philips (CityTouch)
In the smart city, connected solid-state street lighting can deliver additional benefits to utilities, cities, and the general public. With the addition of networking technology and a few sensors, street lights are at the heart of an increasing number of smart city applications ranging from traffic monitoring and parking management, through environmental and weather reporting, to public safety and security facilities, and even internet hotspots.
Energy efficiency on a city-wide scale can make a valuable contribution to reducing CO2 emissions. Public safety improvements are high on most city agendas, whether it is simply the provision of a more appropriate color temperature for nighttime driving, the ability to increase lighting at the scene of an accident, monitor pollution, or even triangulate the location of gunshots. Meanwhile, using the lighting network infrastructure to provide broader-reaching broadband communications for civic, commercial and consumer customers is becoming increasingly feasible. As a result, the humble lamppost, thanks to LED technology, is becoming an important data hub.
Whatever the application, the key requirements are two-way communication, controllability and connectivity. According to market research firm, Strategies Unlimited, in its recently published: "Connected Outdoor Lighting Report: Lighting Analysis and Forecast 2015", global revenue is expected to grow on average at 40% annually (52% CAGR) to 2022.
In terms of connectivity, a range of technologies have been employed, including Ethernet backbones and low-power wireless mesh local network protocols such as IP6LoWPAN, and ZigBee. More recently, Wi-Fi is emerging as a contender. However, in our increasingly wireless world, there is another, often overlooked, solution: power line communication (PLC).
The market report highlights connectivity technologies, stating that although wireless communication is widely used, wired communication technologies will continue to be a more appropriate choice for a range of outdoor applications, especially in tunnels and remote stretches of roadways. A highlighted issue, however, whether wired or wireless, is the need for interoperability and compatibility between the various networking protocols, which will facilitate a more competitive marketplace and stimulate more rapid growth.
PLC benefits
PLC is a solution that should not be overlooked by cities converting to LED street lamps. In the IoT sector, it is already proving a popular route in smart grid and smart meter applications. PLC has already been used to connect street lighting networks yet to be updated with solid-state luminaires. The point is; the technology is proven and readily available from a number of vendors.
Taking advantage of the existing power line infrastructure for communication purposes can save installation time and costs. It can be combined with RF wireless networks to take advantage of the benefits of both approaches. Advantages of PLC over wireless systems include: immunity to weather effects, inherent security as it is cable-based and no interference from other networks. In addition, signal transmissions cannot be blocked by obstructions such as trees or buildings; indeed, with PLC, communication is maintained underground through tunnels.
Essentially, PLC carries data on a conductor that is simultaneously used for power transmission. It operates by adding a modulated carrier signal to the wiring system. No additional data cabling is required. PLC modules can easily be installed in lampposts, and may be at the foot of the pole rather than in the luminaire itself.
Clearly, power lines can be noisy environments, and designers need to take care to counter possible interference. These can come from a number of sources including: huge load impedance variations, attenuation on selective PLC carrier frequencies, and noise interference from other equipment on the network, such as switch mode power supplies and inverters. However, PLC often has exclusive use of the entire PLC frequency spectrum from 50 to 50 kHz. Techniques for tackling possible interference include the use of OFDM (orthogonal frequency division multiplexing) technology, which uses multiple carrier frequencies, and single carrier frequency hopping. Another method of tackling load impedance, line attenuation and noise interference is to change the transmit output power and the carrier frequencies between nodes.
In a typical installation, a local concentrator/gateway transceiver provides for two-way communication between a number of nodes, often using a mesh network topology, along a distance of 2 to 5 km, depending on equipment specification. Data is transmitted typically using the 3G/GSM/cell phone network to a secure server and management system or control center (Figure 2). Some recently introduced Smart City infrastructure platforms are now combining PLC with IPv6/6loWPAN networks, seamlessly combining wired and wireless technologies.
Figure 2: A typical automated street light network topology based on power line communication. Source: Maxim Integrated.
Two-way communication is important so that sensor data and lamp status can be transmitted to the control center and on/off and dimming commands can be sent to each lamp driver. While PLC is principally used for data transmission between the gateways and luminaires, supplementary RF networks (ZigBee is popular) can be used within and between the luminaires as necessary.
Major players in the lighting world, including Everlight, Osram and Philips Lighting are joining forces with multinational systems houses such as Cisco, Echelon and Paradox Engineering, to build and install luminaires, drivers and control centers for city-wide LED street lighting schemes based on PLC. Within the semiconductor camp, companies including STMicroelectronics and Maxim Integrated have developed key components such as transceivers based around microcontroller cores, optimized for PLC, and street lighting applications especially.
Integrated SoC cuts design time
The ST7580 from STMicroelectronics, for example, is a highly integrated yet flexible power line networking system-on-chip (SoC). By combining a high-performance PHY processor core, protocol controller and fully integrated analog front end and power amplifier for line driving, the device is effectively a single-chip, narrow band power line communication solution. Being a fully integrated SoC, it can save design time as well as space and component cost.
Just recently, ST has launched the STEVAL-IHP007V1 evaluation board, described as a power line communication module with features to manage street lighting networking applications. Based on the ST7580, it also incorporates an STM32F ARM® Cortex®-M3 microcontroller. Suitable for EN50065, FCC part 15 and ARIB compliant applications, the module supports B-FSK, B-PSK, Q-PSK and 8-PSK modulations and features dual-channel operation mode, convolutional ECC and signal-to-noise ratio estimation. It also incorporates a 128-bit AES encryption block for secure communication. Security is becoming a major concern, as terrorists increasingly exploit communications networks, either for their own subversive communications or for sabotage.
ST’s integrated power amplifier delivers up to 1 ARMS output current and the line coupling network is simplified for a low-cost BOM. A UART host interface is available for communication with an external host, enabling remote configuration and control of the device and its protocol stack. A standard serial communication port allows designers to integrate the module with lighting driver boards and ballast controllers from ST and other vendors. ST’s smart street lighting system thus offers flexibility of network configuration and management due to its intrinsic scalability and standard interfaces.
Open standards
Moving one stage up in the systems solution hierarchy is Echelon’s SmartServer 2.0PL PROF SR2 networking control and management device. Capable of both local and remote control, the server can be used standalone or integrated with a third party control system. It features built-in drivers for a range of industry-standard protocols, and comes with built-in web pages for setup, monitoring and data management functions.
Figure 3: Echelon’s SmartServer approach to connected lighting, combining power line and RF communications technologies.
The SmartServer 2.0PL PROF SR2, is the PLC edition with street light segment control built in, plus support for mesh repeating for street light controllers using both power line and RF segments. Connectivity is via internal 10/100BaseT Ethernet or external GSM/GPRS or 3G modem. Echelon’s open standards approach for outdoor lighting, based on PLC combined with RF, has been deployed in cities worldwide
Conclusion
Power line communication provides an obvious start point for creating a connected street light network. With performance and security advantages and (often) easier installation than RF networks, PLC-based networks installed as the backbone of the system can be used in conjunction with wireless technology to take advantage of both approaches. This article has shown how a mix of highly integrated and open standards-based devices can provide an efficient, scalable and cost-effective solution for connected street lighting in the smart city.
