Authors : Lionel M. Ni, Yunhao Liu, Yiu Cho Lau & Abhishek P.Patil.
Prepared by : Farhana Musa
A research team was formed by Lionel M.Ni, Yunhao Liu, Yiu Cho Lau and Abhishek P.Patil, as the main objective of the research is to develop an indoor location-sensing system for various mobile commerce applications. The goal is to implement a prototype indoor location-sensing system using easily accessible wireless devices, which are off-the-shelf products.
To implement the prototype framework, the Spider System manufactured by RF Code 2 was chosen, after looking into the specifications of different available systems. Their active tags have a read range of 150 feet, and this range can be increased to 1000 feet with the addition of a special antenna, should the need arise. Figure 1 shows the RFID readers and tags used in the prototype system, and their relative size compared with a US quarter.
Figure 1. The RFID reader and tag used in the prototype system
System Setup
The prototype environment consists of a sensing network that assists in the location tracking of mobile users or objects within certain accuracy, and a wireless network that enables the communication between mobile devices and the Internet. The sensing network primarily includes the RF readers and RF tags as mentioned earlier. The other major part of the infrastructure is the wireless network that allows wireless communication between mobile devices like PDAs and the Internet. In addition, it also acts as a bridge between the sensing network and the other part of the system.
To be able to track an object’s location, the location information received from the RF readers has to be processed before being useful. After the signal is received by the RF readers, the readers then report the information to “TagTracker Concentrator LI” (a software program/API provided by RF Code, Inc.) via a wired or wireless network. After the information from the readers is processed by the TagTracker Concentrator LI, the processed location information can be buffered locally as a file on the same machine or transmitted via a network socket (configurable in the API).
In addition, the LANDMARC approach increases accuracy without placing more readers by employing the idea of having extra fixed location reference tags to assist in location calibration. These reference tags serve as reference points in the system. This approach has a major advantage, among others, of saving cost. This is because there is no need for a large number of expensive RFID readers, as extra, cheaper RFID tags are used instead.
Experimental results and performance evaluation
A series of experiments was conducted to evaluate the performance of the positioning of the LANDMARC System. In the standard setup, 4 RF readers (n=4) was placed in the lab and 16 tags (m=16) as reference tags while the other 8 tags (u=8) as objects being tracked, as shown in figure 2.
Figure 2. Placement of RF readers and tags (standard placement).
Using this setup, the data are collected via the socket from the TagTracker Concentrator LI in groups of a one-hour period and the system will compute the coordinates of the tracking tags based on each group of data.
I. Influence of the environmental factors
In order to see how well the LANDMARC approach works in different environments, 10 groups of data were collected from midnight to early morning (during which time there is little movement) and another 10 groups of data from 10:00 AM to 3:00 PM (at which time varying level of activities that would result in variations in transmission of the tags). Figure 3 illustrates the comparison.
Figure 3. Cumulative percentile of error distance in the daytime and at night
From the results, there is not much difference in the overall accuracy. This shows that the reference tag approach can successfully offset the dynamics of interference.
II. Effect of the number of readers
One of the problems of using RF to locate objects is the inconsistency of the signal strength reception. This can primarily be due to the environment and the device itself. In most cases,
the environmental factors always have the most impact on the accuracy and maximum detectable range. These include issues like furniture placement, people’s movement, and so on. To better deal with the problem, more RF readers can be added to the system to improve the accuracy. With more RF readers, a better decision can be made for location sensing because more data can be gathered by having extra readers to do the sensing as shown in figure 4.
Figure 4. Cumulative percentile of error distance for 3 and 4 RF readers.
Conclusions
The proposed LANDMARC approach does show that active RFID is a viable cost-effective candidate for accurate indoor location sensing. However, there are three problems that RFID vendors have to overcome in order to compete in a new and growing market.
The first problem is that none of the currently available RFID products provides the signal strength of tags directly. This feature can easily be added as readers do have the signal strength information from tags. Next, the second problem is the long latency between a tracking tag being physically placed to its location being computed by the location server. This can be remedied by the RFID vendors, as they should provide a mechanism to allow users to reconfigure the time interval. Finally, the third problem is the variation in the tags’ signal strength in emitting the RF signal. A possible explanation for this may be due to the variation of the chips and circuits, as well as batteries. If all the above problems can be overcome, the accuracy and latency of LANDMARC System will be greatly improved.
References
[1] Ni, L. M., Liu, Y., Lau, Y. C., & Patil, A. (2004). LANDMARC : Indoor Location
Sensing Using Active RFID. Wireless Networks, 10 (6), 701-710.
[2] RF Code, Inc., http://rfcode.com/ProductsFrame.asp
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