There are a number of research papers about indoor localization. We could group these indoor localization systems into three general categories: (1) Angle of Arrival Systems (Angle of Arrival System) [5], (2) Time of Arrival Systems (TOA System)[6], (3)Time Difference of Arrival Systems ( TDOA Systems ), (4) Receive Signal Strength Indexing Systems (RSSI Systems) [7][8], RSSI systems could be subdivided into two parts, the one is Triangulation method and the other one is RSSI Fingerprinting method [9]. We will introduce these different methods and their problems such as limitations or disadvantages.
2-1. Angle of Arrival System (AOA System)
In the first place, the Angle of Arrival System is using a special kind of directional antenna to locate wireless devices. And this kind of antennas contains a multiple element array in which the exact location of each wireless devices .The basic assumption is that all of the nodes in the network have Omni directional antenna.
Every wireless devices of Angle of Arrival System is small but capable of receiving separate signals. Angle of Arrival is defined as the angle between the propagation direction of an incident radio frequency signal and some reference direction that is called orientation. Orientation also represents a fixed direction that the Angle of Arrival is measured is represented in degrees in a clockwise direction from the North.
When the orientation is 0o or point to the North, the Angle of Arrival is absolute degree, otherwise, relative degree as the Figure 1 shown. The point P and point Q are given points with coordinates (x1, y1), (x2, y2) respectively. And the unknown point A’s coordinate (x, y) could be calculated by the time difference with the directional antenna to calculate. In other words, Angle of Arrival System also needs to use time difference of arrival concept to determine the distance of A to given points.
Figure 1: Orientation of Angle of Arrival
One common approach to obtain Angle of Arrival measurements is to use an antenna array on each sensor node. Obviously, only one single directional antenna gives only the bearing not the distance between transmitter and receiver. That is to say, several directional antennas could locate well apart are require in Angle of Arrival positioning. We assume that the beacons have no information about their orientations and the unknowns can detect the Angle of Arrival information between neighbor nodes by using one of the above methods. It is possible to calculate the line-of-sight path from transmitter to receiver. Similarly, we can place another wireless device with directional antenna in different place configuration to repeat the same procedure. The intersection point of two line-of-sight paths represents the location of transmitting wireless devices. Angle of Arrival provides localization services especially suitable for wireless devices that are arranged linearly or areas of sparse wireless devices density. For instance, the straight high way in wide field is very suitable for Angle of
Arrival System.
The Disadvantages of Angle of Arrival System
Originally, Angle of Arrival System is used in the localization of the large-scale region, so the default environment between the transmitter and receiver is line-of-sight. It is especially suitable for Angle of Arrival System for the receivers are arranged in a line. But in normal indoor environments, there are many obstacles such as pillars or walls. Angle of Arrival System’s accuracy is highly influenced by the obstructions so that Angle of Arrival is not suitable for such environments. The localization concept is based on the angle of received RF signals. Once the signal is affected by multipath effect or other RF signal noise, the accuracy is also decreased.
In addition, normal wireless local area network devices such as laptops or access points are not equipped with such special directional antenna. In other words, if the purpose of the system is to locate the positions of wireless devices under wireless local area network, the Angle of Arrival System is also not proper to use.
2-2. Time (Difference) of Arrival System (TOA, TDOA System)
Time of Arrival System locates the moving terminals by measuring the wireless signals sent by these moving terminals. The transmitter of TOA system will send two kinds of signals simultaneously to the receivers. One is ultrasonic waves and the other is the radio frequency signals. The speed of the ultrasonic signal is 331+0.6 t, and the measurement unit of the t is centigrade. The Time of Arrival System require the moving terminals send wireless signals with time stamp to ensure the propagation time from the moving terminal to the receiver. Time of Arrival System makes use of the propagation time between the transmitter and receiver times the speed of the signals to evaluate the distance between them.
In traditional Time of Arrival methods could be categorized into two kinds of methods. The first one is One Way Ranging and the other one is Two Way Ranging. In the first kind of method, measuring distance is to use the difference of Time of Flight (TOF) of signals from two asynchronous transceivers. Once the wireless terminals have synchronized their clock to a common clock, these wireless terminals could adapt the second method to measure location. Figure 2 represents the scenario of using Two Way Ranging to measure distance in Time of Arrival System. And the distance between two wireless devices can be depicted as the equations below:
C
Figure 2: Two Way Ranging of TOA System Wireless Device 2
In Two Way Ranging of TOA System, the packet switching between two wireless devices is using Time Multiplexed of Half-duplex. This kind of procedure mainly depends on the mechanism that merges localization and communication to calculate the distance. Requestor usually sends a packet containing time information to responder. After synchronizing time clock with responder, the requester could determine the time of flight by sending a packet to show the completion of synchronization. But these two kinds of methods are easily restricted by the specification of every communication protocols, so the ranges that can be measured by TOA and accuracy are also restricted.
Time Difference of Arrival System (TDOA System)
Time Difference of Arrival System is to calculating the time difference between some anchor wireless devices to evaluate the distance based on the Time of Arrival method. In traditional, Time Difference of Arrival makes use of One Way Ranging techniques of Time of Arrival. Under this kind of construction, every isochronous anchor wireless devices implement the measuring distance method of Time of Arrival.
Afterwards, we could base on a common reference time of every anchor wireless position terminal and every terminal also knows the distance from the coordinator or a known position to itself. No matter Time of Arrival System or Time Difference of Arrival system, they all needs precisely synchronization clock in their systems, in
other words, once the clock is not meet the precision requirement these system will not work well.
Figure.4 The Working Concept of TDOA
The Disadvantages of Time (Difference) of Arrival System
The accuracy measurements of Time (Difference) of Arrival System are Figure.3 TOA Estimation T1, T2, T3
signal-to-noise ratio, frequency stability of the clock, propagation error of radio frequency wave, the speed of response signal by the data source and so forth.
A common drawback of time-based localization scheme such as Time of Arrival Systems, Time Difference of Arrival and Angle of Arrival System all need expensive hardware of special purpose equipped with the user or network infrastructure. Even if only the insignificantly small time error, which will result in a serious faulty result.
Under the indoor environments, when the diffraction or reflection makes over the phases of the signal therefore indication deviation from line of sight. That is to say, the influence of multipath effect has a great impact to the accuracy of Time-based of Arrival System. In addition, the influence of multichannel effect on the Time of Arrival System is decided by the phases and amplitude of the reflection wave. In general, owing to the variation of environmental factors, the transient time will vary.
2-3 Received Signal Strength Indication System (RSSI System)
Received Signal Strength Indication is a measurement of the power present in a received radio signal. RSSI [8] is generic radio receiver technology metric, which is usually invisible to the user of device containing the receiver, but is directly known to users of wireless networking of IEEE 802.11 protocol family. RSSI normally uses a byte (0~255) to represent signal distribution from the weakest to the strongest, and RSSI’s range could be arbitrary defined by different manufactories. Since the RSSI can aware of a lot of information about the quality of signal, including the relative distance between transmitter and receiver, the propagation model of the radio frequency signal, the situations that affected by other wireless signals and so forth.
a. RSSI Triangulation Method
Compared to the outdoor topology search, indoor topology search needs to locate the wireless devices by using more accurate methods. In order to collect information of the wireless local area network, we develop the indoor topology search system which is used to monitor the wireless network and track the connections between wireless devices. If we want to locate a wireless device, we at least need to place three monitor nodes to calculate the position of the wireless devices. In wireless localization fields, there are two kinds of popular localization methods by receive signal strength. One is triangulation method and another is wireless fingerprinting. In indoor localization system, we deployed three monitor access points (MAP) in a floor which can collect Receive Signal Strength Indication (RSSI). We could translate the received strength to geometric relative distance. In addition, we calculate the coordinate of the wireless devices by many localization methods in the scale of the map. These methods are going to be introduced in the following paragraphs.
Base on the received RSSI from our three or more monitor access points to determine all the wireless devices under specific floor. In the following paragraph, we will use formal form to define the problem with mathematical solution so-called RSSI Triangulation Method to solve the problem.
Figure5. The Scenario of RSSI Triangulation Problem
Triangulation Problem Formulation
Input: Coordinate of Monitor Access Points (MAP) (x1, y1) (x2, y2) (x3, y3)
Receive Signal Strength Index of 3 MAPs RSSI1, RSSI2, RSSI3
Corresponding to distance from goal to 3 MAPs R1, R2, R3
Output: Coordinate of Wireless Devices (x , y)
In the first place, we need to limit the distance between two center points of two circle is between R1+R2 and |R1-R2|, or two circles will have no intersection are even no intersection point.
Limitation : |R2-R1|<= C1C2 <= R1+R2 [Two Circles have 2 intersections]
Equation of three Circles:
The Intersection Line has different meaning of 3 different cases
Use Chroma Formula to solve the intersection (x , y)
b. RSSI Fingerprinting Method
A bunch of radio frequency signals to receiver should be pre-record into data base as the feature of specific region. Just like a person record his fingerprint in the data base. RSSI Fingerprinting Method consists of two phases, training phase and online phase. The training phase is also called offline phase, and it often wastes a lot of time and human resource. Because APs that satisfy the 802.11b/802.11g standards will periodically send a special broadcasting packet called beacon to retrieve the signal strength and store into data base. Generally speaking, there are many methods to match the most possible position according to the pre-collected data, such as neural network, Fuzzy system [10], Subspace technology [11], Hidden Markov Model [12]
and so on. On the contrary, it can eliminate the obstacles or other indoor RF-based signal effects, such as attenuation, multipath effect, and so on.
The most well-known system that is based on the RSSI Fingerprinting Method is the RADAR System [9]. The RADAR operates by recording and processing signal strength information at multiple base stations positioned to provide overlapping coverage in the area of interest. It combines empirical measurements with signal propagation modeling to determine user location and thereby enable location-aware services and applications.