The installation of solid-state street lights is picking up pace globally. The benefits of networking these luminaires across the city are well known, and it is expected that connected outdoor lighting will provide the infrastructure for further web-based smart city applications, often as part of the Internet of Things (IoT) movement.

This article addresses the advantages of Power Line Communication (PLC) as a backbone connection technology in networked outdoor lighting applications. While it can (and is often) combined with wireless networking technology, PLCs have distinct advantages when nodes such as street lights are already connected via their own mains network.

Refer to the recently introduced STMicroelectronics STEVAL-IHP007V1 evaluation kit, which is based on the ST7580 single-chip power line communication and networking solution, plus Echelon SmartServer2.0PL networking, control and management equipment.

Control and Connect

Significant energy savings and reduced maintenance costs are key drivers for the growth of connected outdoor lighting installations worldwide. It has been widely reported that public lighting can consume up to 40% of a municipal energy budget. Cost savings can be significant, with estimates ranging from 30% to 70%. These savings are primarily the energy efficiency achieved by installing long-life, dimmable LED technology and, importantly, the ability to monitor the lighting network.

Figure: Connectivity is key to smart street lighting, enabling control and monitoring to provide energy efficiency, easier maintenance and the implementation of safety features. Source: Philips (CityTouch)

In smart cities, connected solid-state street lights can bring additional benefits to utilities, cities and the public. With the addition of network technology and some sensors, street lights are becoming the core of more and more smart city applications, from traffic monitoring and parking management, to environmental and weather reporting, to public safety and security facilities, and even Internet hotspots.

City-wide energy efficiency can make a valuable contribution to reducing CO2 emissions. Public safety improvements are high on most urban agendas, whether it’s simply providing a more appropriate color temperature for nighttime driving, increasing the ability to illuminate accident scenes, monitoring pollution, or even triangulating the location of gunshots. At the same time, it is becoming increasingly feasible to use lighting network infrastructure to provide wider coverage broadband communications to citizens, business and consumer customers. So, thanks to LED technology, the humble lamp post is becoming an important data center.

Regardless of the application, the key requirements are two-way communication, controllability and connectivity. According to market research firm Strategies Unlimited in its recently released “Connected Outdoor Lighting Report: Lighting Analysis and Forecast 2015”, global revenue is expected to grow at an average annual rate of 40% through 2022 (52% CAGR ).

In terms of connectivity, a range of technologies have been adopted, including Ethernet backbones and low-power wireless mesh LAN protocols such as IP6LoWPAN and ZigBee. More recently, Wi-Fi is emerging as a contender. However, there is another solution that is often overlooked in our increasingly wireless world: Power Line Communication (PLC).

The market report highlights connectivity technologies, stating that while wireless communication is widely used, wired communication technology will continue to be a more suitable choice for a range of outdoor applications, especially in tunnels and remote roads. However, whether wired or wireless, a prominent issue is the need for interoperability and compatibility between various network protocols, which will facilitate a more competitive market and spur faster growth.

Benefits of PLCs

PLCs are a solution that should not be overlooked by cities converting to LED street lights. In the IoT space, it has proven to be a popular route for smart grid and smart meter applications. PLCs have been used to connect street lighting networks that have not yet been updated to solid state luminaires. The point is; the technology is proven and available from many vendors.

Leveraging existing powerline infrastructure for communication saves installation time and costs. It can be used in conjunction with RF wireless networks to take advantage of both approaches. Advantages of PLCs over wireless systems include weather resistance, inherent cable-based security, and immunity from interference from other networks. In addition, signal transmission cannot be blocked by obstacles such as trees or buildings; in fact, with PLC, communication is maintained underground through tunnels.

Essentially, PLCs carry data on conductors that are also used for power transmission. It operates by adding a modulated carrier signal to the wiring system. No additional data cables are required. PLC modules can be easily installed in lamp posts, and may be mounted on the bottom of the pole rather than the light fixture itself.

Obviously, power lines can be noisy environments, and designers need to be mindful of possible interference. These can come from a variety of sources, including: large load impedance changes, attenuation of selective PLC carrier frequencies, and noise interference from other devices on the network such as switch-mode power supplies and inverters. However, PLCs often exclusively use the entire PLC spectrum from 50 to 50 kHz. Techniques to address possible interference include the use of OFDM (Orthogonal Frequency Division Multiplexing) technology, which uses multiple carrier frequencies and single-carrier frequency hopping. Another way to address load impedance, line attenuation, and noise interference is to vary the transmit output power and carrier frequency between nodes.

In a typical installation, a local concentrator/gateway transceiver provides two-way communication between multiple nodes, usually using a mesh network topology, over distances of 2 to 5 kilometers, depending on equipment specifications. Data is typically transmitted to secure servers and management systems or control centers using 3G/GSM/mobile networks (Figure 2). Some recently launched smart city infrastructure platforms are now combining PLCs with IPv6/6lowPAN networks, seamlessly combining wired and wireless technologies.

Figure: Typical automated street light network topology based on power line communication. Source: Maxim Integrated.

Two-way communication is important so 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 PLCs are primarily used for data transfer between gateways and luminaires, an auxiliary RF network (ZigBee is popular) can be used within and between luminaires as needed.

Major players in the lighting space, including Everlight, Osram and Philips Lighting, are working with multinational systems companies such as Cisco, Echelon and Paradox Engineering to build and install luminaires, drivers and control centers for a city-wide LED street lighting scheme . programmable logic controller. In the semiconductor camp, companies including STMicroelectronics and Maxim Integrated have developed key components, such as transceivers based on microcontroller cores, optimized for PLC and street lighting applications.

Integrated SoC reduces design time

For example, STMicroelectronics’ ST7580 is a highly integrated and flexible powerline networking system-on-chip (SoC). Combining a high-performance PHY processor core, protocol controller, and fully integrated analog front end and power amplifier for line driving, the device is an efficient single-chip narrowband powerline communication solution. As a fully integrated SoC, it saves design time as well as space and component costs.

Just recently, ST introduced the STEVAL-IHP007V1 evaluation board, which is described as a power line communication module with functions for managing street lighting network applications. It is based on the ST7580 and also integrates the STM32F ARM ® Cortex ® -M3 microcontroller. The module is suitable for EN50065, FCC Part 15 and ARIB compliant applications, supports B-FSK, B-PSK, Q-PSK and 8-PSK modulation, and features dual-channel operation mode, convolutional ECC and signal-to-noise ratio estimation. It also contains a 128-bit AES encryption block for secure communications. Security is becoming a major concern as terrorists increasingly use communication networks for disruptive communications or sabotage.

ST’s integrated power amplifiers deliver up to 1 A RMS output current and simplify the line coupling network for a low-cost BOM. The UART host interface can be used to communicate with an external host, enabling remote configuration and control of the device and its protocol stack. Standard serial communication ports allow designers to integrate the module with lighting driver boards and ballast controllers from ST and other suppliers. Thanks to its inherent scalability and standard interfaces, ST’s smart street lighting systems offer flexibility in network configuration and management.

open standard

Moving up one level in the system solutions hierarchy is Echelon’s SmartServer 2.0PL PROF SR2 network control and management device. The server has both local and remote control capabilities and can be used either standalone or integrated with third-party control systems. It has built-in drivers for a range of industry standard protocols, and built-in web pages for setup, monitoring and data management functions.

Figure: Echelon’s SmartServer approach to connected lighting, combining power line and radio frequency communication technologies.

SmartServer 2.0PL PROF SR2 is a PLC version with built-in street light segment control, and also supports mesh repetition of street light controllers using power line and RF segments. Connect via internal 10/100BaseT Ethernet or external GSM/GPRS or 3G modem. Echelon’s PLC and RF-based open standard approach to outdoor lighting has been deployed in cities worldwide

in conclusion

Powerline communication provides an obvious starting point for creating a network of connected street lights. With performance and security advantages and (usually) 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 shows how a combination of highly integrated and open standards-based devices can provide an efficient, scalable, and cost-effective solution for connected street lighting in smart cities.

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