In everyday life, the questions ―where is my child,‖‖has anyone seen my keys,‖ and
―where is this place‖ are frequently asked. These questions revolve around the need for accurate positioning. As technology advances and global positioning applications mature, site location and information become readily tangible to the general public. Although very convenient, the modern city dweller spends the majority of his time indoors, which stresses the importance of indoor positioning. As such, the challenge would be to establish an indoor positioning method. The medium chosen must be convenient and technologically mature. In the case of GPS, the signals are shielded by the buildings leading to poor recognition of signal. Moreover, the GPS unit carries a high manufacturing price.
Alternatively, the wireless network technology is developing rapidly, and wireless equipment may be found in most indoor settings due to its low cost. This creates an ideal platform to construct an indoor positioning environment.
1.1 Introduction to outdoor location (GPS)
The Global Positioning System (GPS) is a space-based global navigation satellite system (GNSS) that provides location and time information in all weather and at all times and anywhere on or near the Earth when and where there is an unobstructed line of sight to four or more GPS satellites. It is maintained by the United States government and is freely accessible by anyone with a GPS receiver.
A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the Earth. Each satellite continually transmits messages that include:
1. The time the message was transmitted
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2. Precise orbital information (the ephemeris)
3. The general system health and rough orbits of all GPS satellites (the almanac).
The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite. These distances along with the satellites' locations are used with the possible aid of trilateration, depending on which algorithm is used, to compute the position of the receiver. This position is then displayed, perhaps with a moving map display or latitude and longitude; elevation information may be included.
Many GPS units show derived information such as direction and speed, calculated from position changes.
Three satellites might seem enough to solve for position since space has three dimensions and a position near the Earth's surface can be assumed. However, even a very small clock error multiplied by the very large speed of light — the speed at which satellite signals propagate — results in a large positional error. Therefore receivers use four or more satellites to solve for the receiver's location and time. The very accurately computed time is effectively hidden by most GPS applications, which use only the location. A few specialized GPS applications do however use the time; these include time transfer, traffic signal timing, and synchronization of cell phone base stations.
1.2 Introduction to indoor location
Generally, indoor settings are much more confined, and signal transduction is more susceptible to reflections and multi-routing due to walls. This causes confusion and deterioration of the signal. Additionally, the signal time is shorter due to a decreased path in the indoor setting, and leads to lower accuracy when positioning with the time difference method. Positioning with signal strength is also inaccurate due to low
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fluctuations in signal in a smaller setting.
1.2.1 Infrared
Infrared[1] technologies possess the advantages of low cost and high precision, but yield to narrow angles of coverage, in turn demanding more infrared devices in a given establishment. Moreover, the infrared waves may only propagate within sight, limiting the use to a open, spacious setting, greatly limiting the potential of indoor positioning.
1.2.2 Ultrasonic
The ultrasonic[2],[3] positioning method serves as a plausible way to solve the indoor positioning problem. Being low cost and simple, ideally the ultrasonic method may deliver accurate results. The major working principle of the ultrasonic waves rely on reflective metering, or comparing the difference in propagation time of the main wave and reflected wave to accurately position. However, the ultrasonic method is influenced by multi-path and material absorption, therefore reducing range and prowess.
1.2.3 Wireless Communication
Currently, the most utilized Wireless Communication[4],[5] methods are classified as IEEE 802.11b、Bluetooth、and ZigBee(IEEE 802.15.4).
IEEE 802.11b is the most common wireless communication format, prevailing with ample bandwidth, long range, and compatibility with most systems. Nonetheless, the disadvantage is the cost of this method, demanding multiple AP as anchor nodes, and requiring power sockets for the AP units which increase the cost.
Bluetooth possesses short transmission distances and clear fall off of signal, which in turn provides accurate positioning. But these characteristics also indicate a high density of AP as anchor nodes, resulting in high power consumption and cost, rendering this method as ineffective for indoor positioning.
The ZigBee seems to be a prospective candidate due to its low power consumption, often
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running for months or even up to a year with two AAA batteries. When constructing an indoor positioning environment, this proves advantageous because many anchor nodes may be planted at lower cost, and the transmission range of the ZigBee is very adequate for accurate positioning. The major drawback of ZigBee is deficiency in bit rate, but that may be overseen for indoor positioning demands less packets, making ZigBee ideal for this study.
The Table 1 compares the details of IEEE 802.11b、Bluetooth and ZigBee.
Feature(s) IEEE 802.11b Bluetooth ZigBee
Power Sustain hours days months
Complexity Very complex complex Simple
Cost High medium low
Connection Time Up to 3 seconds Up to 10 seconds 30 ms
Range 100m 10m 70m ~ 300m
Compatibility Roaming possible no Yes
Bit Rate 11 Mbps 1 Mpbs 250 Kbps
Table 1 The details of IEEE 802.11b、Bluetooth and ZigBee.
1.3 Introduction to ZigBee (IEEE 802.15.4)
ZigBee[8] is a specification for a suite of high level communication protocols using small, low-power digital radios based on the IEEE 802.15.4-2003standard.ZigBee is a low-cost, low-power, wireless mesh networking standard. First, the low cost allows the technology to be widely deployed in wireless control and monitoring applications. Second, the low power-usage allows longer life with smaller batteries. Third, the mesh networking provides
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high reliability and more extensive range.
The ZigBee network is ideal for text and graphic transfers. ZigBee and Bluetooth both belong to IEEE 802.15 Standard, but the ZigBee demands less power, and utilizes the 128 bit AES encryption technology, providing better security than Bluetooth.
IEEE 802.15.4-2003 Standard Characteristics：
1.This standard specifies operation in the unlicensed 2.4 GHz (worldwide), 915 MHz (Americas) and 868 MHz (Europe) ISM bands. In the 2.4 GHz band there are 16 ZigBee channels, with each channel requiring 5 MHz of bandwidth. The radios use direct-sequence spread spectrum coding, which is managed by the digital stream into the modulator. BPSK is used in the 868 and 915 MHz bands, and OQPSK that transmits four bits per symbol is used in the 2.4 GHz band. The raw, over-the-air data rate is 250 kbit/s per channel in the 2.4 GHz band, 40 kbit/s per channel in the 915 MHz band, and 20 kbit/s in the 868 MHz band. Transmission range is between 10 and 75 meters (33 and 246 feet) and up to 1500 meters for zigbee pro.
2. Low power consumption as a result of lower bit transfer rate (less data trasfer), short execution cycles and Sleep Mode.
3. Users may choose from various topology protocols such as Star, peer-to-peer, or cluster tree according to their needs.
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