In illumination systems, both commercial and residential, there is an increasing interest to wirelessly control individual lighting units, such as luminaries, in the system in order to adjust the lighting and its effects. For example, in a typical greenhouse of 100×400 meters dimension, approximately 10,000 to 20,000 lighting units equipped with low-cost wireless nodes are deployed. The nodes may also reside at a location separate from the corresponding lighting unit, although together form the lighting unit or lighting unit node. All of the wireless nodes form a network, such as a mesh network, in which commands can be sent from any wireless control point (or control point node) to any node in the network. Therefore, the control point needs to know the exact location of each lighting unit in the facility. There are several known algorithms/protocols that are currently used to determine the location of each lighting unit in the system. These algorithms or protocols include: Received Signal Strength Index (RSSI), Angle-of-Arrival (AOA) and Time-Difference-of-Arrival (TDOA). Each scheme has its own advantages and drawbacks.
Wireless control of a lighting system provides many advantages in addition to the ability of remotely switching and dimming lighting units in the system. For example, such control provides a convenient way of setting up and making changes to a lighting system and of improving energy utilization. The lighting units may be of any type or combination of types, for example fluorescent, high-intensity discharge (HID), light-emitting diodes (LEDs), incandescent, etc. The ability to utilize a wide variety of products allows the system to provide features, such as emergency lighting control, which can be added without making any wiring changes. Energy utilization by the system can also be regulated by a program which can be readily modified to meet changing demands.
The RSSI based localization protocol benefits from the simplicity achieved by eliminating the need for additional hardware in wireless nodes, such that the system cost, power consumption, and physical size can be greatly reduced. However, the RSSI based localization protocol suffers from shadowing, and multi-path fading in a typical wireless environment, which limits not only the measurement accuracy but also the measurement range. The existing RSSI based localization protocols do not satisfy the large measurement range and high accuracy requirements in lighting unit systems, such as a greenhouse or commercial building.
In AOA based systems, the localization protocol uses directional antennas or antenna arrays to measure the angle or bearing relative to points located at known positions, as illustrated in FIG. 1. The intersection of several measured direction pointers yields a position value. The accuracy of the AOA approach is limited by the possible directivity of the measuring aperture, by shadowing and by multi-path reflections arriving from misleading directions. Additionally, positioning estimation error increases linearly with the localization range.
Radio frequency propagation delay based systems, such as time-of-flight (TOF) systems, rely on the precision of timing between the signal transmitter and the receiver. Therefore, high accuracy synchronization is very important in such systems. By combining at least three distances from three reference nodes, triangulation can be used to estimate a re-locatable station's location. Time Difference of Arrival (TDOA) systems, on the other hand, use the signal arrival time difference to each reference node to calculate the distance difference from the lighting unit node to each of the reference nodes, as illustrated in FIG. 2. Similar to the TOF system, distance-differences from one lighting unit to at least two reference nodes are needed to estimate the location of the lighting unit node. The TDOA approach requires high accuracy synchronization only among the reference nodes. Early-late receivers, hardware counters, and high accuracy clocks are required in the reference nodes. The lighting unit nodes do not require any additional hardware or expensive clocks. This makes the TDOA technique more practical to implement, especially if low-cost radios are employed in the network. Although TDOA systems do not require synchronization between lighting units and reference nodes, their performance is still limited by the narrow bandwidth radios in low-cost nodes.
To address some of the many disadvantages prevailing in the conventional localization based systems, there are at least three challenges to overcome: (1) there is severe multi-path fading, especially in indoor environments, which becomes an impediment to localization accuracy, (2) in order to reduce the number of reference nodes, the measurement range should be large, which further increases the difficulty to improve localization accuracy, and (3) to reduce the system cost, only a low-cost commercial radio is adopted, which has an imprecise clock and narrow bandwidth. Additionally, no hardware modification is allowed in the low-cost radio typically used in each lighting unit node. Each of these challenges requires a solution that provides a position localization system and method with improved accuracy and reduced overhead.