Localization systems nowadays are widely used in many applications, such as many road guide services, navigation services, and wireless network devices tracking service and so on. Localization systems can be roughly categorized into Indoor Localization System and Outdoor Localization System. Outdoor Localization System mainly consists of GSM Localization System and GPS Localization System [1], while according to the principle the Indoor Localization Systems could be categorized to Angle of Arrival System (AOA System), Time (Difference) of Arrival System (TOA, TDOA System),Receive Signal Strength Indication System(RSSI System). For most part, Outdoor Localization System is used for comparatively wide-range area, such as road guide, navigation or airline navigation, which tolerant error range can be larger (20~100 meter or more). But it’s not suitable for indoor environments because the required accuracy of applications is usually higher than the one that we mentioned before. Normally, Wireless Local Area Network (WLAN) is one of the most widely used wireless networks. Since the Angle of Arrival and Time Difference of Arrival methods all need other special hardware which are usually not inexpensive, these methods are somewhat not suitable for normal WLAN localization. Compared to the methods mentioned before, RSSI localization method is relatively better for WLAN environments. We are going to introduce the pros and cons of these method’s design principles and the implementation approaches in the following paragraph.
1-1 Issues of Localization
Generally speaking, the requirements of localization can be categorized into four parts. In the first place, accuracy is one of the most important issues of localization.
According to the different application of the localization, the requirement of accuracy is also different. Beside, the volume of localization devices should also be adequate, or the huge volume of localization devices will lead to much inconvenience. The
localization system should work day and night, in other words, it can work continuously all day long. Furthermore, the refresh rate should also fast enough to meet the needs of the different applications. Last but not the least, the price and easy-to-use are always important issues of localizations.
1-2 Outdoor Localization
Global Positioning System (GPS) is originally developed by the United States Department of Defense uses in the military purpose positioning system, mainly by the outer space part, the ground control part and the user acceptor and so on which are composed of three major parts. The first part is outer space part including highly approximately is 20,200 kilometers, 24 satellites (in 2008 had 29), distributes on the inclination angle 55 degree 6 close circular tracks, the satellite distribution enables on the Earth any place, any time at least to be possible simultaneously to observe to 4 above satellite signals. The second part is ground control part, which distributes by the satellite control center in global composed of certain inspection stations. The user receiver has the main performance, the receive satellite broadcast's signal, and carries on processing to the signal, provides to locates the useful information. As a result of the American armed forces against fear precise localization by the enemy use, will be infiltrated in the initial period localization signal the mixed miscellaneous news to reduce the localization the accuracy, will provide for the folk utilization when its miss distance will be most may to 100 meters, also will be the initial period utilization the reason which will limit. Relieved the signal disturbance completely after 2000, the error only then reduced to 15 meters ~20 meters, but the military purpose and the civil precision still had a dropping variance, the folk use's error still reached about military 10 times. Because the precision increases, makes GPS the civil category to expand, in the past the error was big when only suited for the ships, the large-scale delivery vehicle use, after the error further reduced, continually the common saloon car, even
individual user, entered the acceptable scope. But the user enters the building overhang, the lane to make or in the building, because the satellite signal is weaken causes GPS to locate is unable to display effectiveness, must therefore seek other aiding methods.
1-3 Indoor Localization
With the rapid growth of telecommunication and multimedia, the requirements of localization and navigation are increasing day by day, especially at some complicated indoor environments such as airport lobby, department store, fast food multiple stores, library, basement parking lots etc, which often needs to trace or locate the moving terminals in the position of the indoor space. Nowadays, the most popular and widely used wireless network is wireless local area network, WLAN or Wi-Fi.
For many issues, we need to locate the user’s position. But due to the restrictions of complicated factors such as locate time, locate precision and multiple environments, comparatively well-defined localization techniques nowadays cannot be make good use of it. Consequently, professionals proposed many solutions of indoor localization such as A-GPS [2] localization techniques, Wi-Fi localization (also called WLAN Localization) techniques, and graphical analysis, signal localization, computer vision localization and so forth. These indoor localization technology may induce as a whole is several kinds, namely the GNSS [3] technology (for example pseudo satellite and so on), locates the technology wireless (wireless communication signal, radio frequency wireless label, ultrasonic wave, light track, wireless sensory element localization technology and so on), other localization technology (computer vision, dead-reckoning and so on), as well as GNSS and wireless localization combination localization technology (A-GPS or A-GNSS).
Wi-Fi Localization (WLAN Localization)
WLAN Localization, one kind of brand-new information acquisition platform, may realize the complex wide range localization, the monitor and the tracing duty in the widespread application domain. But network node owns localization is the majority application foundations and the premise. The current quite popular Wi-Fi
localization is wireless confined network series IEEE802.11 of standard one kind of localization solution. This system selects the method which the experience test and the signal dissemination model unify, easy to install, needs the very few base depots, can use the same first floor wireless network structure, the system overall accuracy is high. Finland's Ekahau Corporation [4] developed has been able to carry on the indoor localization using Wi-Fi the software. The Wi-Fi cartography's precision probably in 1 meter to 20 meters scopes, overall, it is more precise than the honeycomb network triangulation localization method. But, if locates which Wi-Fi junction point does the reckoning rely on merely in is recent, but is not relies on the synthesis signal strength chart, is very easy in the floor localization to make a mistake. At present, it applies in the small scope indoor localization, the cost is low. But regardless of uses in indoor or the outdoor localization, the Wi-Fi transceiver can only cover radius 90 meter within the regions, moreover very easy to receive other signals the disturbance, thus affects its precision, locator's energy consumption is also high.
Wireless Sensor Network Localization
Wireless Sensor Network (WSN) Localization is increasingly popular. Advances in wireless sensors have made it possible to construct ad hoc networks using cheap wireless sensor nodes. These sensor nodes are composed of a low power process, an acceptable amount of memory, sensor board and network adaptor. Many common applications are emerging. In WSN Localization, wireless sensors could both be transmitter and receiver to locate the position of the users. In these applications, it is necessary to precisely orient these wireless sensor nodes with respect to a global coordinate system so as to report data which is geographically meaningful.
Wireless Sensor Network Localization presents a new problem in system design.
In the first place, the low cost of the wireless sensors nodes facilitates massive scale
and comparatively high parallel performance. On the contrary, each wireless sensor node is likely to have limited resources, such as reliability, power source, and only local communication with the modest number of neighboring nodes.
The goal of WSN Localization System is to determine the physical positions of a group of wireless sensor nodes. And the position of these wireless sensors nodes could be global coordinate through the global position system. Because the low price of sensors, users can equip with the sensor to locate the position of the users. Compared to the wireless local area network, WSN Localization Systems can also be deployed over an area that something need to be monitored in. For instance, a large amount of wireless sensor nodes could be deployed in a battlefield to detect the enemy instead of landmarks.
For outdoor localization methods, such as GSM Localization or GPS Localization methods, they are all not suitable for indoor environments. As the paragraphs mentioned above, the satellite signal is weaken for GPS Localization systems to locate indoor devices. Therefore, we must seek other aiding methods.
There are many different indoor environments, for example, WLAN (Wi-Fi, including 802.11a, 802.11b, 802.11g and 802.11n), WSN (Ad Hoc Sensor Network) and so forth. According to the concept of indoor localization could be categorized into Angle of Arrival, Time (Difference) of Arrival, Received Signal Strength Indication. The RSS based positioning system employs the signal intensity to build the distribution map, and compares the signal intensity of transmitters with receivers to determine the location of the users. Because the previous mentioned indoor localization methods in WLAN based on the RSSI still have some disadvantages. For one thing, RSSI fingerprinting method wastes too much time on training phase. In others words, it is inefficient to pre-collect enough RSSI records to match the most possible region from every divide units of the indoor environment. It needs a great amount of human
resource to measure RSSI in different indoor environments. Nonetheless, RSSI Triangulation method normally could only determine an overlap region of three or more monitor anchor nodes rather than a specific point. Consequently, in average the error distance of RSSI Triangulation method is larger than RSSI Fingerprinting method. On the contrary, RSSI Triangulation method wastes no time on training phase.