ZigBee® SoC with positioning function
Today, wireless radio frequency transceivers, microcontrollers, and system-on-chip (SoC) semiconductor devices compatible with IEEE 802.15.4 and suitable for ZigBee have become quite popular. The highly integrated multi-functional SoC solution is an important factor that enables the ZigBee wireless network to be widely used in many applications, including industrial monitoring, home and building automation, sensor networks, and even wireless medical solutions. Adding the positioning function of wireless radio frequency devices to the above solutions has greatly enhanced the value of these networks.
Location function Location data can significantly enhance the value of the information exchanged in many applications. For patient monitoring, asset tracking, inventory control, security, warehousing, manufacturing logistics, and related applications such as highly sensitive services and billing, the positioning function can be said to drive the development of wireless sensor networks. Node or sensor location information is also helpful for wireless network transmission and adjustment. For example, a wireless node may use an existing system such as a global positioning system (GPS) to obtain location information. However, GPS-type systems may be quite expensive, and for low-cost and large-scale simple network applications, they may also be too power-consuming and complicated. In addition, GPS can effectively and accurately perform positioning functions in outdoor environments, but not necessarily indoors.
In many wireless personal area network (WPAN) applications, the attractiveness of a network with a positioning function lies in low-cost sensor devices and automatic operation with low power consumption to achieve a long equipment life. Because ZigBee sensor networks are diverse and expected to be quite common in the future, regional positioning is likely to become one of the most exciting features of this type of wireless system. The concept is that, in a simple and cost-effective way, the positioning function is added to a large number of ZigBee networks, that is, the positioning function is added to the sensor node silicon chip device, which will only slightly increase the complexity and cost. If such a location module is integrated into the chip, and the existing signal indicator in the wireless signal processing process is used to estimate the location, such as the received signal strength indicator (RSSI), the increased size, power consumption, and location estimation can be performed. The complexity is minimized. Radio frequency positioning is a rather complicated process. Through a distributed method (algorithm), the entire process can be divided into multiple manageable work items, which not only can reduce the amount of resources required by the node, but also significantly reduce the location-related compared to the centralized method. Network traffic.
ZigBee SoC with built-in positioning engine
For most ZigBee wireless applications, to reduce the cost and design complexity of the application system without compromising the functions of IEEE 802.15.4 / Zigbee technology, a system-on-chip device with an optimal design is the key. Really implement a system-on-a-chip solution, that is, integrate all operational functions such as wireless radio frequency transceiver, data processing unit, memory and user application functions into a silicon chip, which can achieve high performance, low cost and rapid listing. advantage. The reason why low power consumption can achieve high performance is because the built-in specialized functions of the chip interact closely, which greatly reduces the resources consumed. With the lowest system bill of materials (BOM), smaller size and fewer components, easier assembly and testing, and simple and reliable design, its manufacturing cost can be reduced and the time to market can be accelerated.
Figure 1 is a representative CC2431 of a commercially available silicon chip solution. This is a true system-on-a-chip CMOS device, which can not only exert high performance, but also meet IEEE 802.15.4 / ZigBee operating in the 2.4GHz ISM band The wireless standard requires low cost and low power consumption. Because the 2.4GHz ISM band has the maximum bandwidth and is universally used, it can promote the development of the global market and the flexibility of application design. The 2.4GHz PHY of the IEEE 802.15.4 standard has considerable potential. The CC2431 combines a high-performance 2.4GHz direct sequence spread spectrum (DSSS) radio frequency transceiver with the industry-proven compact high-efficiency 8051 microcontroller, 8KB RAM, 128KB embedded flash memory, and other practical support functions. One of them is the powerful RSSI positioning engine, which is suitable for low-power ZigBee wireless sensor network applications, such as asset tracking, patient monitoring, inventory control, security, and trial networks. One of the main functions of the on-chip positioning engine is the distributed algorithm, for example, the location calculation at each node. Since only the calculated location is transmitted in the process, not the data used for the calculation, performing the location calculation at the node can reduce the network traffic and communication delays that may occur in the centralized algorithm.
Figure 1 ZigBee SoC device with positioning function
Distributed Localization (Distributed LocalizaTIon)
The positioning engine of the CC2431 is a digital hardware block that enables wireless nodes to determine their plane position coordinates in a fast and efficient manner in IEEE 802.15.4 or ZigBee networks. Using the signal values ​​received by multiple reference nodes or other dynamic neighboring nodes of the same engine, the most likely position can be counted, calculated and estimated to complete the positioning of the wireless node.
The design of the positioning engine module makes it easy to use and interface with the built-in microcontroller of the chip. The power consumption of this independent module is quite low and it operates quickly, so it can be used continuously without consuming computing resources of the device.
In the network, a node with a known location is called a reference node, and a node with an unknown location that needs to be calculated is called a blind node. Based on the information received by the closest reference node, CC2431 can use the distributed positioning function to know the location of the node under test.
The network traffic is limited to the nodes covered by the communication range of the (under test) nodes, and does not extend to central nodes that may be far apart. This distributed method can handle a large number of nodes under test in the same network, and the network traffic of the centralized method will increase rapidly due to too many nodes under test. The necessary information exchanged between the reference node and the node under test is the X and Y coordinates of the reference node. The information received by the reference node contains the measured RSSI value, and the positioning engine calculates its own (X, Y) coordinates based on the reference (X, Y) coordinates and the RSSI value.
The RSSI value between the two radio frequencies is deeply affected by the environment (changes). To complement this change, the positioning engine of the CC2431 will collect data from 3 to 16 reference nodes to calculate the location. If the received data comes from more than 16 nodes, the received reference node positions will be sorted and the 16 strongest reference RSSI values ​​will be used.
The actual indoor measurement performed by the ZigBee network using CC2431 is shown in Figure 2.
Figure 2 CC2431 distributed positioning
Conclusion The application of low-power wireless networks not only has considerable business opportunities, but also can improve the safety, comfort and efficiency of daily life. ZigBee adopts the highly stable IEEE 802.15.4 standard with technical quality and industry support, which can promote the popularization of wireless sensor solutions.
The positioning engine integrated in the CC2431 can use the existing ZigBee infrastructure to perform positioning functions, and can perform distributed location estimation with the accuracy of the “indoor intelligence†of the sensor nodes in the ZigBee network. The required funds. Important location information can be used for location-sensitive services, tracking and monitoring, and navigation purposes.
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