Wireless mesh nets offer bright prospects for lighting control

The following is an abbreviated version of the article.

The full article can be read here.

Smartphones and tablets are becoming omnipresent, and there is rapid growth in user acceptance of these devices as interfaces to anything electronic. That makes smartphones and tablets a natural extension of a lighting control system.

A simple gateway from Ethernet/Wi-Fi/USB to low-power wireless can ensure that users have access to all features of their lighting control system through their smartphone or tablet. Texas Instruments demonstrated the concept at Mobile World Congress this year using the Android-based ZigBee Home Automation lighting application running on a mobile platform.

The low-power requirements of the network and regional nature of RF regulations make it unrealistic to design a wireless lighting network topology that requires all nodes (lights, switches, sensors and remotes) to be in the RF range of a single coordinating node in the network. The solution is an even distribution of routing nodes throughout the building to extend range without increasing power—that is, mesh networking.

Lamps constitute the ideal backbone for a wireless mesh network. A good mesh network is self-forming and self-healing, and can deal effectively with a high number of nodes in diverse topologies.

Compared with a radio at 2.4 GHz, sub-1-GHz radios yield longer range and better penetration for the same amount of power. For that reason, sub-1-GHz solutions are often preferred for outdoor applications such as street and city lighting. There is also generally less interference to deal with in the sub-1-GHz bands, as Wi-Fi gear and microwave ovens do not operate in that range. Radio interference is present in all the open ISM bands, however, and it is thus important that the radio have sufficient output power and a receiver with good selectivity (adjacent channel rejection) and blocking to filter out unwanted signals.

But there is no globally available sub-1-GHz band. That forces development of regionally specific end products—supporting, for example, 868 MHz in Europe or 915 MHz in the United States.

Figure 1. Low-power wireless lighting control. Click on image to enlarge.

Commissioning

Commissioning with traditional lighting is intuitive but expensive; whichever lights are connected after a breaker switch will be controlled by that switch. Wireless adds flexibility in terms of which switches and remotes control which lights, but it also adds complexity in the installation process that must be mitigated by offering an intuitive procedure for connecting lights, sensors and switches.

For professional installations, a USB dongle and a good graphical PC software tool can enable complex connections to be made in an intuitive manner, but such an approach generally requires the services of a trained installer.

User-installed systems, available off the shelf, must be based on simpler, more intuitive methods. One approach, proximity-based commissioning, involves holding the switch or remote control close to the light(s) to be controlled, while pressing a button. The receive signal strength of the packets sent from the light to the switch or remote control is used to determine proximity and qualify the commissioning. The Philips SmartLink system represents a successful implementation of this approach.

Security

Wireless lighting control systems require differing levels of security, depending on the purpose and location of the system. A home lighting system used mainly to set the correct ambiance will have less strict requirements than city lighting or a building’s security lighting would.

Most low-power wireless chip sets today support 128-bit AES encryption of the packets sent over the air, which is generally sufficient to avoid sniffing or injection. Authentication and key exchange when new devices enter the network are challenging and are handled differently depending on the level of security needed and the mechanisms available.

As low-power wireless chip sets have shrunk in size, raised their integration level and dropped in cost, it has become viable to include them in a wide range of lamps to provide direct control of each. Given the long lifetimes of LED- and fluorescent-based lighting, it is practical to integrate the wireless functionality with the light source itself.

Although modern and more efficacious light sources generate less heat than traditional light sources (such as incandescent and high-intensity discharge lamps), LED and CFL lamps contain driver and control electronics and therefore still create high-temperature environments. The heat becomes particularly challenging in compact designs, where the cooling situation surrounding the lamp is generally not controlled.

LEDs must also conduct their heat away (as opposed to radiating heat, as is the case with incandescent and gas discharge technologies). That compounds the problem of keeping the heat away from the driver and control electronics. The low-power wireless and driver control ICs will sustain high temperatures during operation of the lamp.

It is thus important that the ICs support operation at high temperature to ensure correct operation, good RF and high-power-quality performance. Chip sets qualified at 85°C are typically not an option; a 125°C rating is sufficient for most applications, unless a very compact fluorescent design exposes the electronics to even higher temperatures.

The devices and related external components should also be qualified for a high-temperature-operation lifetime that is as long as the expected general lifetime of the bulb.