Factors Affecting RFID System Performance and Non-parametric Analysis

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Security is an important issue in RFID systems ^[18]. There are the following security attacks ^[19] to RFID systems:

1. Eavesdropping: The adversary can monitor the wireless unsecured communication easily and collect the information transmitted by a tag.

2. Spoofing: An adversary may replace a valid item with a fake tag or replace the tag of an expensive item with that of a fake tag with data obtained from a cheaper item.

3. Denial of Service: An adversary may initiate a Denial of Service attack (DOS) to bypass or avoid Security systems. A DOS attack is easily carried out by placing a large number of fake tags for identification by a reader.

4. Tampering: Another avenue for attacking an RFID security mechanism might be a physical attack on an RFID tag or a reader to discover the information stored in tag.

5. Man in the middle attack: An adversary can modify the response of the tag to the reader.

6. Replay Attack: The attackers can eavesdrop the response message from the tag, and retransmit the message to the legitimate reader.

7. Data loss The protocol can be damaged by power interruption, hijacking, and database desynchronization.

Security protocols for RFID systems were discussed on the basis of security, implementation cost, and practical implementation possibility. These security protocols include Kill Tag, Hash-Lock, Enhanced Hash Lock, Selective Blocker Tag, Tag Broker Model, and Molnar Wagner controlled delegation ^[19].

4. The RFID System and the Research Method

A BlueBean RFID Development Lab System (with an Alien RFID fixed reader ALR-9900) ^{[13, 14]} was used in experiment in the Automated Identification Technology lab at Mississippi Valley State University, USA. It is shown in Figure 2. The BlueBean RFID Development Lab System mainly includes a BlueBean RFID Development Lab Kit, the Alien RFID Gateway demonstration software ^[14], and a computer. The BlueBean RFID Development Lab Kit mainly includes a portal and an ALR-9900 RFID reader with a pair of antennae. Only one antenna was used in this research. The RFID portal is on wheels for easy transport. Several main parameters and features of the ALR-9900 RFID reader ^[13] are provided in Table 1. Before using the ALR-9900 RFID reader, open the ‘Alien RFID Gateway’ software from the shortcut on the desktop.

The width of the RFID portal center is 1890 mm. The distance between the reader antenna and the vertical pole of the RFID portal is 105 mm. The height of the reader antenna center was 920 mm; the height of the tag center was 965 mm during the experiment.

An RFID tag with a UPM Belt Antenna and an Impinj Monza 5 chip was used for the reading operation of the BlueBean RFID Development Lab System. The protocol support of the tag complies with EPC Class 1 Gen 2 and ISO 18000-6C. The maximum read range can be measured when the tag was moved slowly away from the fixed reader until the reader cannot read the tag.

Figure 2. The BlueBean RFID Development Lab System

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Table 1. Several main parameters and features of the ALR-9900 RFID reader

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There are two main methods of measuring tag performance. The first method uses variable power and fixed distance to the tag. The transmitted power is varied to find the minimum value at which the tag becomes detectable. The second method uses fixed transmitted power and variable distance to the tag. The reader transmits with constant power while the tag is being moved away until it cannot be read ^[1]. The U-test, a non-parametric analysis method, is used in this research to study the difference of the two methods.

The BlueBean RFID Development Lab System provides software-controlled digital attenuation (built into the Alien reader) that reduces the emitted power but not the return signal. Thus, the read range of the reader can be varied by varying the attenuation. The RF attenuation value ranges from 0 (no attenuation, maximum power) up to 150 (maximum attenuation, minimum power), in increments of 10, each “decade” representing an additional 1 dB of RF attenuation ^[15]. The effect of RF attenuation on the response rate is studied in this research. The response rate is the ratio of number of successful reads to the number of read attempts ^[16].

Tag orientation can be changed through rotation. Figure 3 shows the tag orientations and angles for four cases. Only three angles are shown for the four cases; there can be more angles such as 135 and 180. The read range at various angles (0, 45, 90, 135 and 180) were tested in this research.

Figure 3. Tag orientations and angles

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A design of experiments (DOE) method ^{[11, 12]} was used to analyze how various factors impact the maximum read range of the BlueBean RFID Development Lab system. The DOE method helps eliminate unnecessary or undesirable experimentation and provide accurate information. Three independent factors and one dependent variable were used. The dependent variable is the maximum read range. The three independent factors include: the RF attenuation, the tagged material, and the tag orientations angle. The three factors are each tested at two different levels. The levels generally are the minimum and maximum (or lowest and highest) target values for a factor. The factors and the level description are listed below:

The RF attenuation: maximum value when the tag can be read (-1) and 0 value (without attenuation) (+1)

The tagged material: stapler with steel (-1) and book (paper) (+1)

The tag orientations angle: with the least read range (-1) and with the largest read range (+1)

5. Experimental Results and Analysis

5.1. The Effect of RF Attenuation on the Response Rate

Figure 4. The response rate at various RF attenuation values (distance = 1200 mm)

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Figure 5. The response rate at various RF attenuation values (distance = 1260 mm)

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The response rate is the ratio of number of successful reads to the number of read attempts. Figure 4 and Figure 5 show the response rate at various RF attenuation values for two distance data. The distance was measured between the reader antenna and the tag. The tag orientations angle is 0 in Case I. The results indicate that the RF attenuation can decrease the response rate greatly. In Figure 3, the response rate r is 100% when the RF attenuation F is between 0 and 2 dB; then r decreases with the increase of F. When F = 9 dB, r = 0 (the tag cannot be read). Figure 4 shows the situation with the increased distance (1260 mm). The response rate r is not 100% in Figure 4; r decreases with the increase of F; and r = 0 when F gets to 8 dB. It means that the distance can cut the response rate r and the maximum RF attenuation F (the tag cannot be read at the maximum F).

5.2. Two Methods of Measuring Tag Performance

The first method uses fixed distance to the tag and finds the minimum transmitted power (the maximum RF attenuation) at which the tag becomes detectable. The second method uses fixed transmitted power (fixed RF attenuation) while the tag is being moved away until it cannot be read. In the experiment, tag orientations angle is 0 in Case I. Figure 6 and Figure 7 show the results of the first method and the second method, respectively.

Figure 6. Obtained maximum RF attenuation F (given the read range R) using the first method

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Figure 7. Obtained read range R (given the RF attenuation F) using the second method

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Figure 6 and Figure 7 indicate that there is difference between the two experimental methods. The authors conducted a comparative study. The read range R (the maximum transmission range) is given by the Friis free space equation ^[6]:

(1)

where β is the wave length; is the transmitted power from the reader; is the gain of transmitter antenna; is the gain of the receiver tag; is the power transmission coefficient; and is the minimum threshold power of the reader. is equal to the maximum power of the reader at the antenna minus the RF attenuation value. Let

(2)

The parameter S is used to compare the two experimental methods. S values are calculated according to the data in Figure 6 and Figure 7. The authors used m as the unit of R and dB as the unit of to simplify the calculation. The following null hypothesis is formulated:

There is no statistically significant difference in S between the two experimental methods.

The outcome is: the hypothesis is accepted or rejected at α=0.05. α is the level of significance.

The authors used a non-parametric method to test the hypothesis. It is called the U test, the Wilcoxon test, or the Mann-Whitney test, named after the statisticians who contributed to its development. The major advantage of non-parametric methods is that no specific assumptions (such as normal distribution) about the population or the sample are required. Therefore, non-parametric methods can be used under more general conditions ^[17]. The U test is illustrated as follows:

Suppose that is the sum of the ranks of the values of the first method; is the sum of the ranks of the values of the second method. and are the first sample size and the second sample size, respectively. The statistic U is decided based on the following and statistics:

(3)

(4)

Let U be the smaller of the values of and . The U test has the following criterion:

Reject the null hypothesis if , where is given in Table VII ^[17]. for ,, and α=0.05. The U test results are shown in Table 2. Because 56 exceeds 28, the null hypothesis cannot be rejected. In other words, there is no significant difference between the two experimental methods.

Table 2. The U test for the between the first method and the second method

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5.3. The Effect of the Tag Orientations Angle on the Read Range

The read ranges at various tag orientation angles for Case I, Case II, Case III, and Case IV were tested. The testing results are shown in Tables 3-6.

Table 3. Read ranges at various tag orientation angles for Case I

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In Case I, the angle with least read range is 90; the angle with largest read range is 45

Table 4. Read ranges at various tag orientation angles for Case II

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In Case II, the angle with least read range is 90; the angle with largest read range is 45.

Table 5. Read ranges at various tag orientation angles for Case III

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In Case III, the angle with least read range is 0; the angle with largest read range is 135.

Table 6. Read ranges at various tag orientation angles for Case IV

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In Case IV, the angle with least read range is 0; the angle with largest read range is 135.

5.4. DOE Analysis

A two-level factorial DOE methodology was used to study the effects of various factors on the maximum read range of the BlueBean RFID Development Lab System. The selected factors are: the RF attenuation, the tagged material, and the tag orientations angle. They are denoted by F, M, and A, respectively. The maximum read range is denoted by R. Each of the three factors has two levels. The DOE approach using a fractional factorial design based on the orthogonality principle requires 2³= 8 tests. Table 7 shows the three factors, the orthogonal table, and experiment arrangement for testing the maximum read range of the BlueBean reader ALR-9900 based on the orthogonality principle. R₁, R₂ …, and R₈ represent the maximum read ranges in eight tests, respectively.

Table 7. Three factors, orthogonal table, and experiment arrangement

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The main effects ^{[11, 12]} of the three factors (F, M, and A) on the maximum read range R can be calculated according to the formulas (5) - (7) ^[11]. , , and are main effects representing the effect of the RF attenuation, the tagged material, and the tag orientations angle on the read range, respectively.

(5)

(6)

(7)

Table 8. Three factors, orthogonal table, and experiment results for Case I

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Table 8- Table 15 shows read ranges at various parameters; main effects and normalized main effects of the RF attenuation, the tagged material, and the tag orientations angle on the read range for Case I, Case II, Case III, and Case IV. Table 8, Table 10, Table 12, and Table 14 are the tables (including the three factors, orthogonal table, and measured read range values) for Case I, Case II, Case III, and Case IV, respectively. In Table 9, Table 11, and Table 15, the RF attenuation is the most influential factor on the read range while the tagged material is the second most important dominant factor; and the tag orientations angle has the least impact. In Table 13, The tagged material is the most influential factor on the read range while the RF attenuation is the second most important factor; and the tag orientations angle has the least influence. All these results based on main effect analysis indicate that the RF attenuation is the most powerful factor on the read range and that the tag orientations angle has the least influence for Case I, Case II, and Case IV. The tagged material is the most influential factor on the read range and the tag orientations angle has the least influence for Case III.

Table 9. Main effects and normalized main effects for Case I

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Table 10. Three factors, orthogonal table, and experiment results for Case II

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Table 11. Main effects and normalized main effects for Case II

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Table 12. Three factors, orthogonal table, and experiment results for Case III

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Table 13. Main effects and normalized main effects for Case III

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Table 14. Three factors, orthogonal table, and experiment results for Case IV

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Table 15. Main effects and normalized main effects for Case IV

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