Wi-Fi location, AP location

Wi-Fi real-time positioning system

Wi-Fi based Wireless LAN real-time Positioning system (Wi-Fi RTLS) combined with wireless local area network (WLAN), Radio frequency identification (RFID) and real-time positioning and other technologies, widely used in wireless LAN-covered areas, to achieve complex personnel positioning, monitoring and tracking tasks, and accurately search for the target object, to achieve real-time positioning and monitoring of personnel and goods management.

Wireless local Area network (WLAN) Introduction

Wireless LAN (WLAN, also known as Wi-Fi) is not the traditional cable, while providing all the functions of traditional wired LAN, the infrastructure needed by the network is no longer buried in the ground or hidden in the wall, the network can be as you need to move or change. Compared with the wired network, the main advantage of WLAN is that it does not require cabling and is not subject to the constraints of wiring conditions, so it is very suitable for mobile office users. It has expanded from a number of special industries, such as traditional healthcare, inventory control and management services, to a wider range of industries and even into the home and educational institutions.

Wireless LANs are based on the international IEEE 802.11 standard. The standard specifies that the transmitting power of the wireless network must not exceed 100 MW, the actual transmitting power is about 60~70 MW, the transmitting power of the mobile phone is about 200 MW to 1 watts, and the handheld walkie-talkie up to 5 watts. The use of wireless networks is not as direct contact with the human body, the human body is safe.

The extent to which a general WLAN can be covered depends on whether the environment is open or not. If the external antenna is not added, it is about 250 meters in the field of vision, and if it is a semi-open space, there are spaced areas, about 35~50 meters. Coupled with an external antenna, the distance can reach farther, which is related to the antenna gain value, depending on the user's needs.

APs are referred to as access point abbreviations and are generally translated as "wireless access Nodes" or "bridges." It is mainly in the media access control layer Mac plays the wireless workstation and the cable local area Network Bridge. With APS, like the hub of a wired network in general, wireless workstations can be quickly and easily connected to the network.

Working principle

Where the wireless LAN is covered, the location card or wristband worn on the person periodically signals that the wireless LAN access point (AP) receives the signal and transmits the signal to the location server. The location server determines the position of the person according to the signal strength or signal arrival time difference, and displays the specific location through the electronic map.

Figure 1. Working principle of real-time positioning system based on wireless local area network (Wi-Fi)

System Composition

The Wireless LAN real-time Positioning system architecture is as follows:

Figure 2. RTLS System Architecture Diagram

As shown, the wireless LAN real-time Location monitoring System (Wi-Fi RTLS) is mainly composed of a location card or wristband (Wi-Fi Tag), a wireless LAN access point (ap:access points)) and a back-end monitoring Management Center ((Locating server location server) three parts. Wireless LAN access points can use any product that supports 802.11B.

1) The positioning card or wrist strap as a wireless data acquisition module worn on people or items, the system through the tracking of labels to achieve the tracking of personnel and assets. Can be designed according to the needs of different shapes, wristbands, badges, etc. to adapt to different needs.

2) AP uses 2.4GHz band, support 802.11B/g mode, timely capture card or wristband information, transmission to the back-end monitoring Center, the positioning card or wrist strap control management.

3) installed the location Server software system monitoring and management center, mainly to achieve real-time data analysis and processing. Analysis of the management of the positioning card or wristband data, through the control center of the electronic map monitoring and timely display the location of the field location card or wristband, data can be stored in the storage database at the same time, the monitoring personnel can access the storage server through the computer to query the real-time location information of people or items, alarm information, and a certain period of time

Network Structure

The location-based wireless local area network is different from the general communication network, and it is required to receive more than 3 AP signals at any point. The AP is mainly divided into the main communication AP and the location AP, the main communication AP is responsible for the data exchange between the server and the communication AP, all the main communication APs must be connected through the network cable. Locate the AP, which is responsible for collecting the label information and sending the result to the specified server.

Figure 3. Wireless LAN Real-time Positioning system (Wi-Fi RTLS) network topology diagram

Functional Characteristics

> Positioning system based on Wi-Fi for Wireless LAN is very popular, inexpensive, easy to build and manage, while positioning network can also be used for internet, voice, video and other purposes;

> real-time personnel or items tracking, at any time to find the location of the person or item, and the path passed in a certain period, and show the route track;

> Regional monitoring: Can set the area range, monitoring the sub-region personnel or items in and out of the situation, non-authorized personnel or items in and out, alarm prompts;

> High positioning accuracy, up to 3-5m (depending on the environment), accurately grasp the position of personnel or items;

> Ultra-Low power consumption, label can replace the battery or charge, and the human body safe, no radiation damage;

> Visual terminal platform to display the location and movement trajectory of people or items in electronic map format,

> can monitor the ambient temperature and other information through sensors, so as to facilitate environmental monitoring and safety precautions.

Positioning Algorithm Introduction

The Wi-Fi location system is a standard-based IEEE 802.11 wireless local Area network (WLAN). The location algorithm is based on the intensity (RSSI) of the received Wi-Fi signal. Where the wireless LAN is covered, the location label periodically signals that the wireless LAN access point (AP) receives the signal and transmits the signal to the location server. The location server determines the position of the label from the AP according to the signal strength, and the distance from the label to at least 3 APs can be used to calculate the location of the label and display the location via an electronic map. This is the traditional triangular positioning principle.

The Wi-Fi signal is reflected before it reaches the receiver, not a straight path, which brings a lot of error to the location calculation, known as Multipath. The traditional triangulation method is not suitable for indoor environments because of the multipath interference caused by obstacles in the interior space, resulting in a large signal change. Using an RF fingerprint identification (fingerprinting) based positioning method. Set multiple sample points within the positioning area to place the locating terminal at a sample point. The scene planning tool can record the signal characteristics emitted by the locating terminal, and establish the signal pattern according to these characteristics and signals of different positions to indicate the location of the locating terminal. Using the signal pattern and the relative position information to establish the database, the positioning system can calculate the position according to the signal characteristics collected in real time. The human body is interfering with the label, the positioning system of the superior frequency technology uses the historical movement trajectory, increases the AP locator, corrects the signal mutation and so on the algorithm to reduce the interference to bring the localization error.

Positioning accuracy and positioning target, the environment and the positioning of the laying density, the fixed can reach 3 meters or even better, positioning people due to human disturbance, accuracy in the radius of 5-10 meters. Positioning accuracy is related to the locator density, environment, positioning object interference, so the accuracy of wireless positioning is not an absolute number. Generally speaking, the accuracy can reach 3 meters-5 meters, sometimes 10 meters or worse. We also have applications that can do 1.5 meters. The higher the density of the locator deployment, the higher the positioning accuracy. Any wireless positioning technology, there will be positioning errors and uncertainties exist. We recommend practical applications, taking into account the characteristics of wireless positioning, through software to solve the uncertainties of precision. The bounce occurs in the positioning because the positioning position is in the middle of several anchor points.

The positioning frequency can be set to 1 seconds or more. The page refresh time is 1 seconds. Network transmission will also take a certain amount of time, positioning algorithm in order to avoid the human body on the positioning accuracy of the interference, the algorithm has made a special adjustment, so there may be 3-6 seconds of delay. The number of AP locators is much, but the AP locator is low cost, can be centrally managed, and can be powered by Poe. The AP Locator wireless transmit function can be banned, so a large number of AP locators do not produce any wireless signal, which is different from the normal AP and does not cause wireless interference.

AP Locator, in addition to the basic functions of a general wireless router, can also be used to scan the surrounding Wi-Fi devices (such as APS, Wi-Fi tags, wi-Fi phones, PDAs, notebooks, etc.), with Wi-Fi identification and positioning capabilities. The wireless real-time Positioning System (RTLS) consists of a single server, multiple AP locators, and multiple tags or terminals.
Network architecture: The server and one or more communication APs are connected via Ethernet, and the AP locator can be wirelessly connected to the communication AP by configuring the client mode. The AP locator can also be configured as AP mode for use as a communications AP (both can be located and communicated), but is connected to the server via Ethernet.
AP Locator has wireless internet access function, AP Locator has scanning Wi-Fi terminal function, Web-based interface management function, can display the Wi-Fi tag alarm function (such as button alarm, low voltage alarm and battery reset alarm, etc.), can judge the distance of the WiFi terminal, stay time; As a WiFi hotspot, to achieve the authentication function (to enforce the Web page registration to the Internet), identify the WiFi phone hardware ID, record WiFi phone stay near the AP time.

Wi-Fi location, AP location