Performance Comparison of RFID Tag at UHF ... - Semantic Scholar
JOURNAL OF NETWORKS, VOL. 9, NO. 12, DECEMBER 2014
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Performance Comparison of RFID Tag at UHF Band and Millimeter-Wave Band A. K. M. Baki 1,* and Nemai Chandra Karmakar 2 1. Dept. of Electrical & Electronic Engineering, Ahsanullah University of Science and Technology 141-142 Love Road, Tejgaon, Dhaka 1208, Bangladesh 2. Dept. of Electrical & Computer Systems Engg., Monash University Bldg. 72, Monash University, Clayton Campus, Clayton, VIC 3800, Australia *Corresponding author, Email: [email protected]
Abstract—The ultra high frequency (UHF) band spectrum will likely be congested in near future since the next generation wireless as well as Radio Frequency Identification (RFID) system users will witness the use of UHF band technology with increased demand of bandwidth, bit rate, frequency spectrum and power consumption. The alternate solution is the use of millimeter-wave band technology. It is possible to improve data throughput, range resolution and multi-user capability in mm-wave band RFID system. ‘Higher power reception efficiency and lower side lobe level (SLL) of radiation pattern’ is required for RFID system that will increase the tag range and transmission bit rate. At the same time lower SLL will minimize the interference level. Beam pointing error is another problem of UHF band antenna which reduces the tag range and bit rate. These problems can be minimized by using large number of antenna elements. But with UHF band signal it is practically difficult to construct large array antenna; since the array size becomes tremendously larger with the increase of antenna elements. ‘Higher power reception efficiency and lower SLL’ can practically be obtained by using non-uniform power distribution of large number of antenna elements in millimeter wave band. A new and technically better method of beam forming by implementing the concept of staircase power distribution (SPD) of antenna elements at 60 GHz has been investigated and presented in the paper. The SPD method is compared with Gaussian edge tapering method. It was found that the maximum SLL (MSLL) is the lowest in case of SPD. The beam efficiency of SPD is also equivalent to that of Gaussian edge tapering method. It is easier to fabricate a larger number of antenna elements within smaller area with SPD at 60 GHz system; since the antenna size is smaller and the number of different power distribution in SPD case is less and stepwise uniform. Uniform and less number of different power distribution of SPD also minimizes other technical errors. Index Terms—Adaptive Arrays; Antenna Radiation Pattern; Antenna Tapering; Antenna Theory; Beam Steering; Radio Frequency IDentification
I.
INTRODUCTION
Ultra high frequency band spectrum will likely be congested and the use of millimeter (mm) wave technology in the wireless local area network (WLAN) and Radio Frequency Identification (RFID) systems will
© 2014 ACADEMY PUBLISHER doi:10.4304/jnw.9.12.3215-3220
be witnessed in the near future. The characteristic of mmwave transmission must be considered carefully in particular the strong attenuation at this frequency spectrum. Free-space propagation loss, as an example, at 60 GHz is higher than the one at 5 GHz under the same condition. Other losses and fading factors, such as rain, foliage, scattering, diffraction loss etc., increasingly affect the mm-wave propagation. Directive antennas are best suited to point-to-point applications because the directive antenna pattern improves the channel multipath profile; by limiting the spatial extent of the transmitting and receiving antenna patterns to the dominant transmission path. The antennas used in some applications, such as automatic cruise control (ACC), collision avoidance radar, and RFID reader antenna must have very low side lobes. This is crucial as side lobes lead to false alarms in a collision avoidance radar system. In RFID applications higher side lobes can lead to false tracking of RFID tags. An RFID tag can be identified automatically at a distant location by exchanging information through RFID system. RFID system has different applications such as animal tagging, authenticity verification, inventory tracking and security surveillance [1]. A faster and energy efficient tag reading is needed in some sophisticated applications, particularly for higher number of tags reading [2]. Following frequency bands are generally used in RFID applications: a. b. c. d. e.
Low frequency (LF): 125-134 KHz; High frequency (HF) : 13.56 MHz; Ultra high frequency (UHF): 433 MHz, 860-960 MHz; Microwave (MW) : 2.4 GHz, 5.8 GHz; Millimetre wave (mm-Wave): e.g., 60 GHz and 77 GHz [3];
RFID reader antenna with very low side lobes in its radiation pattern would maximize the received power and minimize interferences. Higher side lobes result in false alarms in RFID applications. Different RFID reader architecture by using phased array/smart antenna concepts is discussed in details in [4]. It is possible to improve data throughput, range resolution and multi-user capability with mm-wave RFID communication without accepting range limiting RF power restrictions [5]. Beam Collection Efficiency (BCE) and Maximum Side Lobe Level (MSLL) are the indices for the evaluation of
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antenna radiation pattern. BCE is the ratio of power flow that is intercepted by the receiving antenna to the whole transmitted power [6]. Suppression of Grating Lobe (GL) and Side Lobe Level (SLL) is necessary for higher BCE and to avoid interferences. When GL appears and SLL increases, the transmitted power is absorbed into these lobes which cause reduction of received power. These also cause higher interference levels. Though array antennas increase the directivity of the antenna system but if all antennas are uniformly excited then the main beam carries only a part of the total power due to the higher SLL. It is possible to increase BCE and reduce SLL if edge tapering concept can be implemented. A better method of power distribution of array antennas by incorporating Isosceles Trapezoidal Distribution (ITD) concept is discussed in [6]. In ITD method, only a few edge antenna elements are tapered. Power levels of the remaining middle antennas are uniform. With ITD, which is also technically better than Gaussian or Dolph-Chebyshev power distribution, it is possible to maintain higher BCE and lower SLL. Another method of ITD with Unequal element spacing (ITDU) to achieve lower Maximum Side Lobe Level (MSLL) and higher beam efficiency (BE) is reported in [7]. It is possible to maintain even higher BE and lower MSLL by incorporating ITDU. Methods of designing transmitter outputs by using on-chip power amplifier (PA) stages in each element need good linearity, high efficiency, high power gain and high output power [8-10]. A four-stage PA with at least 3-dB gain in each stage, with the transistor size doubled in each stage, is discussed in [8-9]. It is possible to design on chip power amplifier stages with variable gains for different antenna elements. This way it would be possible to minimize the SLLs even to lower levels. As a result the BE of the array antenna will increase and interference to other communication systems will decrease. The authors have investigated a comparatively new and technically better method of power distribution of array antenna for 60 GHz system. Instead of using gradual decrement of power distribution of array antenna, the concept of „staircase‟ is implemented and named as Staircase Power Distribution (SPD) [11-12]. Fabrication of array antenna with SPD concept is easier and technically better than other kinds of power distributions; since the number of different power distribution in SPD is least and stepwise uniform. Figure 1 shows a conceptual block diagram of an RFID system. The paper is organized in the following way. General characteristics of radio frequency identification (RFID) tag are discussed in section II. A comparative study of UHF band RFID-system with mm-Wave RFID system is made in section III. A new and technically better method of power distribution of array antenna by implementing SPD concept is described in section IV. A comparative analysis of radiation patterns and power collection efficiency by using SPD and Gaussian edge tapering is made in section V. And finally the conclusion is made in section VI.
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JOURNAL OF NETWORKS, VOL. 9, NO. 12, DECEMBER 2014
Figure 1. Conceptual block diagram of an RFID system
II.
CHARACTERISTICS OF RADIO FREQUENCY IDENTIFICATION TAG
The power received by a tag can be expressed by the following Friis transmission formula [11]: 𝑃𝑡𝑎𝑔 = 𝑃𝑇 𝐺𝑇 𝑔𝑡 (
𝜆
4𝜋𝑑 𝑡
)2
(1)
where, 𝑃𝑇 is the power transmitted by the reader antenna; 𝐺𝑇 is the gain of transmitting antenna; 𝑔𝑡 is the gain tag antenna; 𝑑𝑡 is the distance between the transmitting antenna and the tag; 𝜆 In equation (1), the term „( )2 ‟ is the free space loss 4𝜋𝑑 𝑡
factor which is a function of operationg frequency and tag distance. Received power by the reader antenna can be expressed as [11]: 𝑃𝑟 = 𝑃𝑇 𝐺𝑇 𝐺𝑅 𝑔𝑡
1
𝜆
( )4 𝜉
𝑑 𝑡2 𝑟𝑏2 4𝜋
(2)
where, 𝐺𝑅 is the gain of reader antenna; 𝜉 is the backscatter efficiency of the tag; 𝑟𝑏 is the distance between the reader antenna and the tag; The sensitivity of the reader, or minimum reader power (𝑃min _𝑟 ), is specified for maximum possible operation range. When 𝑃min _𝑟 and 𝑔𝑡 are fixed, the ranges 𝑑𝑡 /𝑟𝑏 can be controlled by controlling 𝐺𝑇 and/or 𝐺𝑅 . III.
MILLI-METER WAVE BAND FOR RFID SYSTEM
Higher data transfer rate even with gigabit range is achievable [3] at mm-wave (e.g., 60 GHz) band. Signal at mm-wave band can create pencil like main beam with improved gain. Pencil like main beam also occupies smaller surrounding space. Interferences with other communication channels can be minimized with this kind mm-wave signal. The reader can also receive signal through narrower space, thereby reducing the chances of interferences. RFID system at 60 GHz has been reported in [13-14]. The signal at 60 GHz is rapidly absorbed by atmospheric oxygen over long distances. Therefore it can be used for short distance communication and the frequency reuse would be possible. In U. S. A. the maximum limit of power transmission in the 60 GHz band is 40 dBm (10 watt), which is higher than the limit in the UHF band. Beam pointing error is another reason
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of lower received power. For N+1 number of antenna elements, beam pointing error can be expressed as [15]:
2 3 d cos 0 N 3/ 2
rms
Figure 3) with larger number of mm-wave antenna elements. One example of main beam width for two different cases is also mentioned in Table 1. For 9elements array the main beam width is about 20° (Figure 3). This is generally the case for UHF band antenna. On the other hand, the main beam width for 50-elements antenna is 4° which can be the case for a 60 GHz system (Figure 3). „Minimum beam pointing error and narrower main beam‟ with larger number antenna elements will increase the BCE and reduce the interference level in a mm-wave band system.
(3)
where R.M.S. phase error; 2 = Phase constant;
d = Spacing between antenna elements; 0 = Main beam steering angle; Equation (3) shows that if the number of antenna elements is increased then the beam pointing error decreases. It would be possible to create a pencil like beam with higher gain if the number of antenna elements can be increased. Additionally, the use of larger antenna elements will minimize beam pointing error. Figure 2 shows the beam pointing errors with different number of array elements. One is with 9-elements array and the other is with 50-elements array. It is apparent from Figure 2 that the beam pointing error can be brought down near to zero by using even higher number of antenna elements. With UHF band signal, there is a limitation of fabrication using antenna elements larger than 9, since the array size becomes tremendously larger. The nearby transponders also cannot be spatially distinguished at UHF band signal since the reader transmission cannot be efficiently directed. On the other hand radiation from the mm-wave reader can be directed efficiently since larger number of antenna elements can be fabricated on to a smaller area at mm-wave band. Figure 3 shows the radiation patterns for the two different cases, one is with 9 antenna elements and the other is with 50. The simulation was done by using 60 GHz signal. Figure 3 shows that the same transmitted power can be concentrated into smaller spatial area with higher number of antenna elements. This will also help isolate the tag of interest from other tags. Therefore mm-wave antenna would help in finding tag in high-density sensor network such as item level identification. Some advantages of RFID system at mmwave over UHF band is summarized in Table 1. Since the inter-element spacing (generally𝜆/2 ) for 60 GHz signal is much smaller than that of 900 MHz UHF band signal, a huge number of antenna elements can be fabricated within smaller area for 60 GHz band system. A comparison of antenna size for two different cases is mentioned in Table 1. It was mentioned earlier that the beam pointing error can be minimized (shown in Figure 2) and the main beam can be made narrower (shown in AF
( N 1) 2
n ( N 1) 2
1e jn
[( N 1) 2] N s 1 N s 2 ... N sl
n [ ( N 1) 2] N s 1 N s 2 ... N sl
[( N 1) 2] N s 1
n [ ( N 1) 2] N s 1
( l l 1 )e
jn
( 2 1 )e jn
[( N N S 1) 2]
Following are the notations of (4), N = Total number of antenna elements,
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n [ ( N N S 1) 2]
Figure 2. Beam pointing error vs. beam pointing angle with different number of array elements
Radiation patterns with staircase power distribution.
Figure 3. Radiation patterns with two different number of array elements
IV.
Array Factor (AF) of one-dimensional array antenna with staircase power distribution (SPD) can be expressed [11-12] by (4).
[( N 1) 2] N s 1 N s 2
n [ ( N 1) 2] N s 1 N s 2
( A l )e
STAIRCASE POWER DISTRIBUTION OF ARRAY ELEMENTS
( 3 2 )e jn ...
(4)
jn
Ns1, Ns2, Ns3… Nsl etc. are no. antenna elements tapered from each side (starting from edge of the array) for 1st stage, 2nd stage, 3rd stage…….last stage.
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TABLE I. Parameters Array antenna size and fabrication
Number of elements Beam shaping
antenna
Beam pointing angle and beam pointing error Beam Collection Efficiency (BCE)
Bit rate Interference
Frequency reuse Multipath effect Tag/Transponder size
ADVANTAGES OF MM-WAVE RFID OVER UHF BAND RFID SYSTEM.
Advantages of mm-Wave band Antenna size is smaller at mm-Wave. [For example: the array size for 100 elements of 900 MHz UHF band antenna is 16.66 meter. But the array size of 100 elements of 60 GHz band antenna is only 0.25 meter. In both cases the inter-element spacing is 𝜆/2. ] Fabrication is easier from the perspective of antenna size. Less costly from the perspective of number of antenna elements. Higher number of antenna elements can be used at mm-Wave. For UHF band array, antenna size becomes tremendously larger if the number of antenna elements is increased. Better beam shaping is possible at mm-Wave due to larger number of antenna elements which is shown in Figure 3. Tag separation becomes easier in mm-wave band due to comparatively very narrower beam. It can also be inferred from Figure 3. Beam pointing error can be decreased by using increased number of antenna elements with mm-Wave band. A comparative study of beam pointing error is shown in Figure 2. BCE can be increased at mm-Wave due to: Less beam pointing error (BCE is more closely related to the beam pointing error than BE); Larger number of used elements; [The BCE will be higher for mm-wave band antenna, since with larger number of elements it is possible to make the main beam of the radiation pattern narrower. This way the RFID tag will receive more power through the main beam.] Bit rate is higher due to higher bandwidth (according to Shannon channel capacity formula). Interference is less due to narrower main beam and better spatial separation of RFID tag at mm-Wave band. [This scenario can also be inferred from Figure 3. For example the main beam width for 9 elements antenna is about20°. It is the usual case for a UHF band antenna. On the other hand the main beam width for 50 elements antenna is 4°which can be a case for mm-wave band antenna.] Frequency reuse is better due to higher attenuation at 60 GHz. Multipath effect would be less due to higher attenuation at 60 GHz. Smaller at mm-Wave band.
Here last stage is defined as the stage before the middle antenna elements. NS = Ns1 + Ns2+ Ns3+……..+ Nsl = Number of elements tapered from each side, d (sin sin 0 ).
1 , 2 , 3 .............. l are the amplitudes of the antenna elements of 1st stage, 2nd stage, 3rd stage…….last stage. d = spacing between elements (m). 2 = phase constant. A is the amplitude of middle antenna elements. 0 = Direction of beam maximum along the broad side. n = 0, 1, 2……………N. The concept of SPD is shown in Figure 4.
Figure 4. Staircase Power Distribution (SPD) for antenna elements.
BCE for two dimensional antenna rectangular/square shape can be expressed as [6]: BCE2 D
P , x
ry
y
2
d x d y
rx
P , x
ty
y
2
ry ; angle sector due to y dimension of receiving antenna; P x , y is the energy of the radiated electric field. BCE for one dimensional case can be expressed as:
BCE
P r
2
d
P
2
d
w
r is the angle sector due to one dimensional receiving antenna and w is the angle sector 90 .
P is the energy of the one dimensional radiated electric field. Figure 5 shows the normalized BCE for two different beam pointing angles. Uniform power distribution of 200 array elements was used in this case. The BCE was calculated at a distance 10 meter from the reader by assuming 5 cm tag size. The power received can be improved further by implementing the SPD concept, since the BCE is higher with SPD than that of uniform power distribution and will be shown in the following section.
with
d x d y (5)
tx
where, tx ; ty ; are ±90 degree angle sector;
rx ; angle sector due to x dimension of receiving antenna; Figure 5. Beam collection efficiency vs. beam pointing error for different beam pointing angles
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(6)
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RADIATION PATTERNS WITH SPD AND GAUSSIAN POWER DISTRIBUTIONS
Radiation patterns, SLL and Beam Efficiency (BE) for different amplitude distributions (Gaussian and SPD) are compared in this section. BE for two dimensional radiation pattern can be expressed [16] by (7):
BE2 D
P( , )
P( , ) d 2
main _ beam
2
d (7)
4
where, P , is the radiated electric field pattern; BE for one dimensional array can be expressed [7] by (8):
BE1D
P( ) d
m
2
P( )
2
d
Minimum error is introduced with SPD. Therefore SPD of array antenna elements is a good candidate for mmwave RFID applications. MSLL as well as BE for four different beam steering angles and two different power distributions (Gaussian and SPD) are summarized in Table 2 and Table 3 respectively. Table 2 shows that the MSLL for SPD were minimum (-26 dB) for each of the beam steering angles (5°, 15°, 25°, and 35°). Table 3 shows that the BEs for SPD case are also comparable to those of Gaussian edge tapering and for different beam steering angle. The data shown in Table 2 and Table 3 asserts that larger number of SPD array will maximize the power reception and minimize the interference levels in mm-wave band RFID system.
(8)
w
where, m is the angle sector due to one dimensional main beam and w is the observation angle sector of 90 .
P is the one dimensional radiated electric field pattern. Figure 6 shows the amplitude distributions of 25 antenna elements for SPD (10 dB) and Gaussian (10 dB) power distribution. Radiation patterns, MSLL and BE by using 60 GHz signals were compared. Different beam pointing angles were also considered. Figure 7 shows the radiation pattern of 10 dB SPD with 35 degree beam pointing angle. The element spacing was 0.6λ. The performance of SPD can further be improved by considering larger number of antenna elements. In case of Gaussian edge tapering, the number of different power distributions becomes higher with larger number of elements. For example, the required number of different power distribution is 13 for 25 elements of Gaussian edge tapering. It is technically a very difficult task to achieve such a higher number of different power distributions. This also introduces more errors in power distribution. These problems become even worse with the increased number of elements. The number of different power distribution of SPD array (for the case shown in Figure 6) is 4.
Figure 7. Radiation pattern of 25 antenna elements for 10 dB SPD at 35 degree beam pointing angle TABLE II.
MSLL FOR 10 DB GAUSSIAN AND 10 DB SPD EDGE TAPERING
Main Beam Pointing Angle (degree)
Technically it is much easier to implement 4 different power distributions than 13 different power distributions. © 2014 ACADEMY PUBLISHER
15
25
35
-26
-26
-26
-26
-23.69
-23.4
-23.25
-23.25
MSLL Type of Power Distribution
TABLE III.
Staircase Power Distribution (SPD) Gaussian
BEAM EFFICIENCY (BE) FOR 10 DB GAUSSIAN AND 10 DB SPD EDGE TAPERING
Main Beam Pointing Angle (degree)
5
15
25
35
Beam Efficiency (BE) Type of Power Distribution
Staircase Power Distribution (SPD) Gaussian
VI.
Figure 6. Amplitude distributions of 25 antenna elements for 10 dB Gaussian and 10 dB SPD
5
97.84
97.66
97.23
97.17
98.67
98.65
98.31
97.14
CONCLUSION
A little improvement in RFID-system performance, such as, antenna gain and directivity, can play a significant role in improving the bit error rate, collision mitigation, data rate, interference cancelation, localization of tag and reading range. Milli-meter wave and UHF band signals are analyzed and compared in this paper for Radio Frequency IDentication (RFID)-system. Two types of power distribution of array antenna are also analyzed and compared. The types are Gaussian edge tapering and Staircase Power Distribution (SPD). Maximum power will be transmitted through the main
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beam and less power will be in the Side Lobe Level (SLL) in case of mm-wave band SPD system. SPD system is technically better; since in SPD less and stepwise uniform power distribution is required which will minimize amplitude and other technical errors. Milli-meter wave band array antenna with larger number of elements and SPD is easier to fabricate. Tag separation with SPD array antenna and mm-wave RFID-system will be easier than that of UHF band RFID-system. Exposure level to humans and all other living animals/things as well as interference to/from other communication systems outside the main beam will also be minimum in case of mm-wave band SPD system. Construction of UHF band RFID reader system with larger number of array elements is a difficult job since the array size becomes tremendously larger. This problem can be easily overcome by implementing mm-wave RFID system. ACKNOWLEDGEMENT The work is partly supported by Australian Research Council (ARC) Discovery Project (DP110105606: Electronically Controlled Phased Array Antenna for RFID applications). REFERENCES [1] Want, R. “The Magic of RFID,” ACMQueue, vol. 2, (7), pp. 40-48, Oct. 2004. [2] Klair, D. K. Chin, K. W. and Raad, R. “A Survey and Tutorial of RFID Anti-Collision Protocols,” IEEE Comm. Surveys & Tutorials, vol. 12, no. 3, Third Quarter 2010, pp. 400 – 421. [3] Pursula, Pekka; Karttaavi, T.; Kantanen, Mikko; Lamminen, Antti; Holmberg, Jan; Lahdes, Manu; Marttila, Ilkka; Lahti, Markku; Luukanen, Arttu; Vähä-Heikkilä, Tauno, ‟60-GHz millimeter-wave identification reader on 90-nm CMOS and LTCC’, IEEE Transactions on Microwave Theory and Techniques, vol. 59(2011): 4, pp. 1166-1173, 2011. [4] Nemai Chandra Karmakar, „Recent Paradigm Shift in RFID and Smart Antenna‟, Handbook of Smart Antennas for RFID Systems, John Wiley & Sons, Inc. 2010, pp. 57~82. [5] Carlowitz, C.; Strobel, A.; Schafer, T.; Ellinger, F.; Vossiek, M., "A mm-wave RFID system with locatable active backscatter tag,” Wireless Information Technology and Systems (ICWITS), 2012 IEEE International Conference on, vol., no., pp. 1, 4, 11-16 Nov. 2012
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[6] A. K. M. Baki, N. Shinohara, H. Matsumoto, K. Hashimoto, and T. Mitani, “Study of Isosceles Trapezoidal edge tapered phased array antenna for Solar Power Station/Satellite”, Ieice Trans. Commun., Vol. E90-B, No. 4, pp 968-977, APRIL 2007. [7] A. K. M. Baki, Kozo HASHIMOTO, Naoki SHINOHARA, Tomohiko MITANI, and Hiroshi Matsumoto, “IsoscelesTrapezoidal-Distribution Edge Tapered Array Antenna with Unequal Element Spacing for Solar Power Satellite”, Ieice Trans. Commun., Vol. E91-B, No. 2 February 2008, pp 527-535. [8] Arun Natarajan, Abbas Komijani, Xiang Guan, Aydin Babakhaniand Ali Hajimiri, „A 77-GHz Phased-Array Transceiver With On-Chip Antennas in Silicon: Transmitter and Local LO-Path Phase Shifting‟, IEEE Journal Of Solid-State Circuits, Vol. 41, No. 12, December 2006, pp 2807-2819 [9] Ullrich R. Pfeiffer and David Goren, ‟A 20 dBm FullyIntegrated 60 GHz SiGe Power Amplifier With Automatic Level Control, IEEE Journal Of Solid-State Circuits’, Vol. 42, No. 7, July 2007 pp. 1455-1463 [10] Van-Hoang Do, Viswanathan Subramanian, Wilhelm Keusgen, and Georg Boeck, „A 60 GHz SiGe-HBT Power Amplifier With 20% PAE at 15 dBm Output Power‟, IEEE Microwave And Wireless Components Letters, Vol. 18, No. 3, March 2008, pp. 209-211 [11] A. K. M. Baki, Nemai Chandra Karmakar, Uditha Bandara and Emran Md Amin, „Beam Forming Algorithm with Different Power Distribution for RFID Reader, pages 64~95‟, Book Title: Chipless and Conventional Radio Frequency Identification: Systems for Ubiquitous Tagging, IGI Global, May, 2012, USA, ISBN 978-1-4666-1616-5 (hardcover) [12] A. K. M. Baki, Nemai Chandra Karmakar, ‟60 GHz Array Antenna with New Method of Beam Forming‟, 15th International Conference on Computer and Information Technology, December 2012, pp. 638-641. [13] Karmakar, N. C. „Smart Antennas for Automatic Radio Frequency Identification Readers‟, Chapter XXI, in Handbook on Advancements in Smart Antenna Technologies for Wireless Networks, editior: Chen Sun, Jun Cheng & Takashi Ohira, IGI Global, 2008 pp. 449472. [14] Pellerano, Stefano. Alvarado, Javier. and Palaskas, Yorgos. „A mm-Wave Power-Harvesting RFID Tag in 90 nm CMOS‟, IEEE Journal of Solid-State Circuits, vol. 45, no. 8, August 2010, pp. 1627~1637. [15] Keith, R Carver. Cooper, W. K. and Stutzman, W. L., “Beam-Pointing Errors of Planner-Phased Arrays”, IEEE Trans. On Antenna &Porp, 1973, pp. 199-202. [16] Warren L. Stutzman and Gary A. Thiele, “Antenna Theory and Design”, 2nd edition, John Wiley & Sons, Inc. pp. 29.
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Performance Comparison of RFID Tag at UHF Band and Millimeter-Wave Band A. K. M. Baki 1,* and Nemai Chandra Karmakar 2 1. Dept. of Electrical & Electronic Engineering, Ahsanullah University of Science and Technology 141-142 Love Road, Tejgaon, Dhaka 1208, Bangladesh 2. Dept. of Electrical & Computer Systems Engg., Monash University Bldg. 72, Monash University, Clayton Campus, Clayton, VIC 3800, Australia *Corresponding author, Email: [email protected]
Abstract—The ultra high frequency (UHF) band spectrum will likely be congested in near future since the next generation wireless as well as Radio Frequency Identification (RFID) system users will witness the use of UHF band technology with increased demand of bandwidth, bit rate, frequency spectrum and power consumption. The alternate solution is the use of millimeter-wave band technology. It is possible to improve data throughput, range resolution and multi-user capability in mm-wave band RFID system. ‘Higher power reception efficiency and lower side lobe level (SLL) of radiation pattern’ is required for RFID system that will increase the tag range and transmission bit rate. At the same time lower SLL will minimize the interference level. Beam pointing error is another problem of UHF band antenna which reduces the tag range and bit rate. These problems can be minimized by using large number of antenna elements. But with UHF band signal it is practically difficult to construct large array antenna; since the array size becomes tremendously larger with the increase of antenna elements. ‘Higher power reception efficiency and lower SLL’ can practically be obtained by using non-uniform power distribution of large number of antenna elements in millimeter wave band. A new and technically better method of beam forming by implementing the concept of staircase power distribution (SPD) of antenna elements at 60 GHz has been investigated and presented in the paper. The SPD method is compared with Gaussian edge tapering method. It was found that the maximum SLL (MSLL) is the lowest in case of SPD. The beam efficiency of SPD is also equivalent to that of Gaussian edge tapering method. It is easier to fabricate a larger number of antenna elements within smaller area with SPD at 60 GHz system; since the antenna size is smaller and the number of different power distribution in SPD case is less and stepwise uniform. Uniform and less number of different power distribution of SPD also minimizes other technical errors. Index Terms—Adaptive Arrays; Antenna Radiation Pattern; Antenna Tapering; Antenna Theory; Beam Steering; Radio Frequency IDentification
I.
INTRODUCTION
Ultra high frequency band spectrum will likely be congested and the use of millimeter (mm) wave technology in the wireless local area network (WLAN) and Radio Frequency Identification (RFID) systems will
© 2014 ACADEMY PUBLISHER doi:10.4304/jnw.9.12.3215-3220
be witnessed in the near future. The characteristic of mmwave transmission must be considered carefully in particular the strong attenuation at this frequency spectrum. Free-space propagation loss, as an example, at 60 GHz is higher than the one at 5 GHz under the same condition. Other losses and fading factors, such as rain, foliage, scattering, diffraction loss etc., increasingly affect the mm-wave propagation. Directive antennas are best suited to point-to-point applications because the directive antenna pattern improves the channel multipath profile; by limiting the spatial extent of the transmitting and receiving antenna patterns to the dominant transmission path. The antennas used in some applications, such as automatic cruise control (ACC), collision avoidance radar, and RFID reader antenna must have very low side lobes. This is crucial as side lobes lead to false alarms in a collision avoidance radar system. In RFID applications higher side lobes can lead to false tracking of RFID tags. An RFID tag can be identified automatically at a distant location by exchanging information through RFID system. RFID system has different applications such as animal tagging, authenticity verification, inventory tracking and security surveillance [1]. A faster and energy efficient tag reading is needed in some sophisticated applications, particularly for higher number of tags reading [2]. Following frequency bands are generally used in RFID applications: a. b. c. d. e.
Low frequency (LF): 125-134 KHz; High frequency (HF) : 13.56 MHz; Ultra high frequency (UHF): 433 MHz, 860-960 MHz; Microwave (MW) : 2.4 GHz, 5.8 GHz; Millimetre wave (mm-Wave): e.g., 60 GHz and 77 GHz [3];
RFID reader antenna with very low side lobes in its radiation pattern would maximize the received power and minimize interferences. Higher side lobes result in false alarms in RFID applications. Different RFID reader architecture by using phased array/smart antenna concepts is discussed in details in [4]. It is possible to improve data throughput, range resolution and multi-user capability with mm-wave RFID communication without accepting range limiting RF power restrictions [5]. Beam Collection Efficiency (BCE) and Maximum Side Lobe Level (MSLL) are the indices for the evaluation of
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antenna radiation pattern. BCE is the ratio of power flow that is intercepted by the receiving antenna to the whole transmitted power [6]. Suppression of Grating Lobe (GL) and Side Lobe Level (SLL) is necessary for higher BCE and to avoid interferences. When GL appears and SLL increases, the transmitted power is absorbed into these lobes which cause reduction of received power. These also cause higher interference levels. Though array antennas increase the directivity of the antenna system but if all antennas are uniformly excited then the main beam carries only a part of the total power due to the higher SLL. It is possible to increase BCE and reduce SLL if edge tapering concept can be implemented. A better method of power distribution of array antennas by incorporating Isosceles Trapezoidal Distribution (ITD) concept is discussed in [6]. In ITD method, only a few edge antenna elements are tapered. Power levels of the remaining middle antennas are uniform. With ITD, which is also technically better than Gaussian or Dolph-Chebyshev power distribution, it is possible to maintain higher BCE and lower SLL. Another method of ITD with Unequal element spacing (ITDU) to achieve lower Maximum Side Lobe Level (MSLL) and higher beam efficiency (BE) is reported in [7]. It is possible to maintain even higher BE and lower MSLL by incorporating ITDU. Methods of designing transmitter outputs by using on-chip power amplifier (PA) stages in each element need good linearity, high efficiency, high power gain and high output power [8-10]. A four-stage PA with at least 3-dB gain in each stage, with the transistor size doubled in each stage, is discussed in [8-9]. It is possible to design on chip power amplifier stages with variable gains for different antenna elements. This way it would be possible to minimize the SLLs even to lower levels. As a result the BE of the array antenna will increase and interference to other communication systems will decrease. The authors have investigated a comparatively new and technically better method of power distribution of array antenna for 60 GHz system. Instead of using gradual decrement of power distribution of array antenna, the concept of „staircase‟ is implemented and named as Staircase Power Distribution (SPD) [11-12]. Fabrication of array antenna with SPD concept is easier and technically better than other kinds of power distributions; since the number of different power distribution in SPD is least and stepwise uniform. Figure 1 shows a conceptual block diagram of an RFID system. The paper is organized in the following way. General characteristics of radio frequency identification (RFID) tag are discussed in section II. A comparative study of UHF band RFID-system with mm-Wave RFID system is made in section III. A new and technically better method of power distribution of array antenna by implementing SPD concept is described in section IV. A comparative analysis of radiation patterns and power collection efficiency by using SPD and Gaussian edge tapering is made in section V. And finally the conclusion is made in section VI.
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JOURNAL OF NETWORKS, VOL. 9, NO. 12, DECEMBER 2014
Figure 1. Conceptual block diagram of an RFID system
II.
CHARACTERISTICS OF RADIO FREQUENCY IDENTIFICATION TAG
The power received by a tag can be expressed by the following Friis transmission formula [11]: 𝑃𝑡𝑎𝑔 = 𝑃𝑇 𝐺𝑇 𝑔𝑡 (
𝜆
4𝜋𝑑 𝑡
)2
(1)
where, 𝑃𝑇 is the power transmitted by the reader antenna; 𝐺𝑇 is the gain of transmitting antenna; 𝑔𝑡 is the gain tag antenna; 𝑑𝑡 is the distance between the transmitting antenna and the tag; 𝜆 In equation (1), the term „( )2 ‟ is the free space loss 4𝜋𝑑 𝑡
factor which is a function of operationg frequency and tag distance. Received power by the reader antenna can be expressed as [11]: 𝑃𝑟 = 𝑃𝑇 𝐺𝑇 𝐺𝑅 𝑔𝑡
1
𝜆
( )4 𝜉
𝑑 𝑡2 𝑟𝑏2 4𝜋
(2)
where, 𝐺𝑅 is the gain of reader antenna; 𝜉 is the backscatter efficiency of the tag; 𝑟𝑏 is the distance between the reader antenna and the tag; The sensitivity of the reader, or minimum reader power (𝑃min _𝑟 ), is specified for maximum possible operation range. When 𝑃min _𝑟 and 𝑔𝑡 are fixed, the ranges 𝑑𝑡 /𝑟𝑏 can be controlled by controlling 𝐺𝑇 and/or 𝐺𝑅 . III.
MILLI-METER WAVE BAND FOR RFID SYSTEM
Higher data transfer rate even with gigabit range is achievable [3] at mm-wave (e.g., 60 GHz) band. Signal at mm-wave band can create pencil like main beam with improved gain. Pencil like main beam also occupies smaller surrounding space. Interferences with other communication channels can be minimized with this kind mm-wave signal. The reader can also receive signal through narrower space, thereby reducing the chances of interferences. RFID system at 60 GHz has been reported in [13-14]. The signal at 60 GHz is rapidly absorbed by atmospheric oxygen over long distances. Therefore it can be used for short distance communication and the frequency reuse would be possible. In U. S. A. the maximum limit of power transmission in the 60 GHz band is 40 dBm (10 watt), which is higher than the limit in the UHF band. Beam pointing error is another reason
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of lower received power. For N+1 number of antenna elements, beam pointing error can be expressed as [15]:
2 3 d cos 0 N 3/ 2
rms
Figure 3) with larger number of mm-wave antenna elements. One example of main beam width for two different cases is also mentioned in Table 1. For 9elements array the main beam width is about 20° (Figure 3). This is generally the case for UHF band antenna. On the other hand, the main beam width for 50-elements antenna is 4° which can be the case for a 60 GHz system (Figure 3). „Minimum beam pointing error and narrower main beam‟ with larger number antenna elements will increase the BCE and reduce the interference level in a mm-wave band system.
(3)
where R.M.S. phase error; 2 = Phase constant;
d = Spacing between antenna elements; 0 = Main beam steering angle; Equation (3) shows that if the number of antenna elements is increased then the beam pointing error decreases. It would be possible to create a pencil like beam with higher gain if the number of antenna elements can be increased. Additionally, the use of larger antenna elements will minimize beam pointing error. Figure 2 shows the beam pointing errors with different number of array elements. One is with 9-elements array and the other is with 50-elements array. It is apparent from Figure 2 that the beam pointing error can be brought down near to zero by using even higher number of antenna elements. With UHF band signal, there is a limitation of fabrication using antenna elements larger than 9, since the array size becomes tremendously larger. The nearby transponders also cannot be spatially distinguished at UHF band signal since the reader transmission cannot be efficiently directed. On the other hand radiation from the mm-wave reader can be directed efficiently since larger number of antenna elements can be fabricated on to a smaller area at mm-wave band. Figure 3 shows the radiation patterns for the two different cases, one is with 9 antenna elements and the other is with 50. The simulation was done by using 60 GHz signal. Figure 3 shows that the same transmitted power can be concentrated into smaller spatial area with higher number of antenna elements. This will also help isolate the tag of interest from other tags. Therefore mm-wave antenna would help in finding tag in high-density sensor network such as item level identification. Some advantages of RFID system at mmwave over UHF band is summarized in Table 1. Since the inter-element spacing (generally𝜆/2 ) for 60 GHz signal is much smaller than that of 900 MHz UHF band signal, a huge number of antenna elements can be fabricated within smaller area for 60 GHz band system. A comparison of antenna size for two different cases is mentioned in Table 1. It was mentioned earlier that the beam pointing error can be minimized (shown in Figure 2) and the main beam can be made narrower (shown in AF
( N 1) 2
n ( N 1) 2
1e jn
[( N 1) 2] N s 1 N s 2 ... N sl
n [ ( N 1) 2] N s 1 N s 2 ... N sl
[( N 1) 2] N s 1
n [ ( N 1) 2] N s 1
( l l 1 )e
jn
( 2 1 )e jn
[( N N S 1) 2]
Following are the notations of (4), N = Total number of antenna elements,
© 2014 ACADEMY PUBLISHER
n [ ( N N S 1) 2]
Figure 2. Beam pointing error vs. beam pointing angle with different number of array elements
Radiation patterns with staircase power distribution.
Figure 3. Radiation patterns with two different number of array elements
IV.
Array Factor (AF) of one-dimensional array antenna with staircase power distribution (SPD) can be expressed [11-12] by (4).
[( N 1) 2] N s 1 N s 2
n [ ( N 1) 2] N s 1 N s 2
( A l )e
STAIRCASE POWER DISTRIBUTION OF ARRAY ELEMENTS
( 3 2 )e jn ...
(4)
jn
Ns1, Ns2, Ns3… Nsl etc. are no. antenna elements tapered from each side (starting from edge of the array) for 1st stage, 2nd stage, 3rd stage…….last stage.
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TABLE I. Parameters Array antenna size and fabrication
Number of elements Beam shaping
antenna
Beam pointing angle and beam pointing error Beam Collection Efficiency (BCE)
Bit rate Interference
Frequency reuse Multipath effect Tag/Transponder size
ADVANTAGES OF MM-WAVE RFID OVER UHF BAND RFID SYSTEM.
Advantages of mm-Wave band Antenna size is smaller at mm-Wave. [For example: the array size for 100 elements of 900 MHz UHF band antenna is 16.66 meter. But the array size of 100 elements of 60 GHz band antenna is only 0.25 meter. In both cases the inter-element spacing is 𝜆/2. ] Fabrication is easier from the perspective of antenna size. Less costly from the perspective of number of antenna elements. Higher number of antenna elements can be used at mm-Wave. For UHF band array, antenna size becomes tremendously larger if the number of antenna elements is increased. Better beam shaping is possible at mm-Wave due to larger number of antenna elements which is shown in Figure 3. Tag separation becomes easier in mm-wave band due to comparatively very narrower beam. It can also be inferred from Figure 3. Beam pointing error can be decreased by using increased number of antenna elements with mm-Wave band. A comparative study of beam pointing error is shown in Figure 2. BCE can be increased at mm-Wave due to: Less beam pointing error (BCE is more closely related to the beam pointing error than BE); Larger number of used elements; [The BCE will be higher for mm-wave band antenna, since with larger number of elements it is possible to make the main beam of the radiation pattern narrower. This way the RFID tag will receive more power through the main beam.] Bit rate is higher due to higher bandwidth (according to Shannon channel capacity formula). Interference is less due to narrower main beam and better spatial separation of RFID tag at mm-Wave band. [This scenario can also be inferred from Figure 3. For example the main beam width for 9 elements antenna is about20°. It is the usual case for a UHF band antenna. On the other hand the main beam width for 50 elements antenna is 4°which can be a case for mm-wave band antenna.] Frequency reuse is better due to higher attenuation at 60 GHz. Multipath effect would be less due to higher attenuation at 60 GHz. Smaller at mm-Wave band.
Here last stage is defined as the stage before the middle antenna elements. NS = Ns1 + Ns2+ Ns3+……..+ Nsl = Number of elements tapered from each side, d (sin sin 0 ).
1 , 2 , 3 .............. l are the amplitudes of the antenna elements of 1st stage, 2nd stage, 3rd stage…….last stage. d = spacing between elements (m). 2 = phase constant. A is the amplitude of middle antenna elements. 0 = Direction of beam maximum along the broad side. n = 0, 1, 2……………N. The concept of SPD is shown in Figure 4.
Figure 4. Staircase Power Distribution (SPD) for antenna elements.
BCE for two dimensional antenna rectangular/square shape can be expressed as [6]: BCE2 D
P , x
ry
y
2
d x d y
rx
P , x
ty
y
2
ry ; angle sector due to y dimension of receiving antenna; P x , y is the energy of the radiated electric field. BCE for one dimensional case can be expressed as:
BCE
P r
2
d
P
2
d
w
r is the angle sector due to one dimensional receiving antenna and w is the angle sector 90 .
P is the energy of the one dimensional radiated electric field. Figure 5 shows the normalized BCE for two different beam pointing angles. Uniform power distribution of 200 array elements was used in this case. The BCE was calculated at a distance 10 meter from the reader by assuming 5 cm tag size. The power received can be improved further by implementing the SPD concept, since the BCE is higher with SPD than that of uniform power distribution and will be shown in the following section.
with
d x d y (5)
tx
where, tx ; ty ; are ±90 degree angle sector;
rx ; angle sector due to x dimension of receiving antenna; Figure 5. Beam collection efficiency vs. beam pointing error for different beam pointing angles
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(6)
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V.
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RADIATION PATTERNS WITH SPD AND GAUSSIAN POWER DISTRIBUTIONS
Radiation patterns, SLL and Beam Efficiency (BE) for different amplitude distributions (Gaussian and SPD) are compared in this section. BE for two dimensional radiation pattern can be expressed [16] by (7):
BE2 D
P( , )
P( , ) d 2
main _ beam
2
d (7)
4
where, P , is the radiated electric field pattern; BE for one dimensional array can be expressed [7] by (8):
BE1D
P( ) d
m
2
P( )
2
d
Minimum error is introduced with SPD. Therefore SPD of array antenna elements is a good candidate for mmwave RFID applications. MSLL as well as BE for four different beam steering angles and two different power distributions (Gaussian and SPD) are summarized in Table 2 and Table 3 respectively. Table 2 shows that the MSLL for SPD were minimum (-26 dB) for each of the beam steering angles (5°, 15°, 25°, and 35°). Table 3 shows that the BEs for SPD case are also comparable to those of Gaussian edge tapering and for different beam steering angle. The data shown in Table 2 and Table 3 asserts that larger number of SPD array will maximize the power reception and minimize the interference levels in mm-wave band RFID system.
(8)
w
where, m is the angle sector due to one dimensional main beam and w is the observation angle sector of 90 .
P is the one dimensional radiated electric field pattern. Figure 6 shows the amplitude distributions of 25 antenna elements for SPD (10 dB) and Gaussian (10 dB) power distribution. Radiation patterns, MSLL and BE by using 60 GHz signals were compared. Different beam pointing angles were also considered. Figure 7 shows the radiation pattern of 10 dB SPD with 35 degree beam pointing angle. The element spacing was 0.6λ. The performance of SPD can further be improved by considering larger number of antenna elements. In case of Gaussian edge tapering, the number of different power distributions becomes higher with larger number of elements. For example, the required number of different power distribution is 13 for 25 elements of Gaussian edge tapering. It is technically a very difficult task to achieve such a higher number of different power distributions. This also introduces more errors in power distribution. These problems become even worse with the increased number of elements. The number of different power distribution of SPD array (for the case shown in Figure 6) is 4.
Figure 7. Radiation pattern of 25 antenna elements for 10 dB SPD at 35 degree beam pointing angle TABLE II.
MSLL FOR 10 DB GAUSSIAN AND 10 DB SPD EDGE TAPERING
Main Beam Pointing Angle (degree)
Technically it is much easier to implement 4 different power distributions than 13 different power distributions. © 2014 ACADEMY PUBLISHER
15
25
35
-26
-26
-26
-26
-23.69
-23.4
-23.25
-23.25
MSLL Type of Power Distribution
TABLE III.
Staircase Power Distribution (SPD) Gaussian
BEAM EFFICIENCY (BE) FOR 10 DB GAUSSIAN AND 10 DB SPD EDGE TAPERING
Main Beam Pointing Angle (degree)
5
15
25
35
Beam Efficiency (BE) Type of Power Distribution
Staircase Power Distribution (SPD) Gaussian
VI.
Figure 6. Amplitude distributions of 25 antenna elements for 10 dB Gaussian and 10 dB SPD
5
97.84
97.66
97.23
97.17
98.67
98.65
98.31
97.14
CONCLUSION
A little improvement in RFID-system performance, such as, antenna gain and directivity, can play a significant role in improving the bit error rate, collision mitigation, data rate, interference cancelation, localization of tag and reading range. Milli-meter wave and UHF band signals are analyzed and compared in this paper for Radio Frequency IDentication (RFID)-system. Two types of power distribution of array antenna are also analyzed and compared. The types are Gaussian edge tapering and Staircase Power Distribution (SPD). Maximum power will be transmitted through the main
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beam and less power will be in the Side Lobe Level (SLL) in case of mm-wave band SPD system. SPD system is technically better; since in SPD less and stepwise uniform power distribution is required which will minimize amplitude and other technical errors. Milli-meter wave band array antenna with larger number of elements and SPD is easier to fabricate. Tag separation with SPD array antenna and mm-wave RFID-system will be easier than that of UHF band RFID-system. Exposure level to humans and all other living animals/things as well as interference to/from other communication systems outside the main beam will also be minimum in case of mm-wave band SPD system. Construction of UHF band RFID reader system with larger number of array elements is a difficult job since the array size becomes tremendously larger. This problem can be easily overcome by implementing mm-wave RFID system. ACKNOWLEDGEMENT The work is partly supported by Australian Research Council (ARC) Discovery Project (DP110105606: Electronically Controlled Phased Array Antenna for RFID applications). REFERENCES [1] Want, R. “The Magic of RFID,” ACMQueue, vol. 2, (7), pp. 40-48, Oct. 2004. [2] Klair, D. K. Chin, K. W. and Raad, R. “A Survey and Tutorial of RFID Anti-Collision Protocols,” IEEE Comm. Surveys & Tutorials, vol. 12, no. 3, Third Quarter 2010, pp. 400 – 421. [3] Pursula, Pekka; Karttaavi, T.; Kantanen, Mikko; Lamminen, Antti; Holmberg, Jan; Lahdes, Manu; Marttila, Ilkka; Lahti, Markku; Luukanen, Arttu; Vähä-Heikkilä, Tauno, ‟60-GHz millimeter-wave identification reader on 90-nm CMOS and LTCC’, IEEE Transactions on Microwave Theory and Techniques, vol. 59(2011): 4, pp. 1166-1173, 2011. [4] Nemai Chandra Karmakar, „Recent Paradigm Shift in RFID and Smart Antenna‟, Handbook of Smart Antennas for RFID Systems, John Wiley & Sons, Inc. 2010, pp. 57~82. [5] Carlowitz, C.; Strobel, A.; Schafer, T.; Ellinger, F.; Vossiek, M., "A mm-wave RFID system with locatable active backscatter tag,” Wireless Information Technology and Systems (ICWITS), 2012 IEEE International Conference on, vol., no., pp. 1, 4, 11-16 Nov. 2012
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[6] A. K. M. Baki, N. Shinohara, H. Matsumoto, K. Hashimoto, and T. Mitani, “Study of Isosceles Trapezoidal edge tapered phased array antenna for Solar Power Station/Satellite”, Ieice Trans. Commun., Vol. E90-B, No. 4, pp 968-977, APRIL 2007. [7] A. K. M. Baki, Kozo HASHIMOTO, Naoki SHINOHARA, Tomohiko MITANI, and Hiroshi Matsumoto, “IsoscelesTrapezoidal-Distribution Edge Tapered Array Antenna with Unequal Element Spacing for Solar Power Satellite”, Ieice Trans. Commun., Vol. E91-B, No. 2 February 2008, pp 527-535. [8] Arun Natarajan, Abbas Komijani, Xiang Guan, Aydin Babakhaniand Ali Hajimiri, „A 77-GHz Phased-Array Transceiver With On-Chip Antennas in Silicon: Transmitter and Local LO-Path Phase Shifting‟, IEEE Journal Of Solid-State Circuits, Vol. 41, No. 12, December 2006, pp 2807-2819 [9] Ullrich R. Pfeiffer and David Goren, ‟A 20 dBm FullyIntegrated 60 GHz SiGe Power Amplifier With Automatic Level Control, IEEE Journal Of Solid-State Circuits’, Vol. 42, No. 7, July 2007 pp. 1455-1463 [10] Van-Hoang Do, Viswanathan Subramanian, Wilhelm Keusgen, and Georg Boeck, „A 60 GHz SiGe-HBT Power Amplifier With 20% PAE at 15 dBm Output Power‟, IEEE Microwave And Wireless Components Letters, Vol. 18, No. 3, March 2008, pp. 209-211 [11] A. K. M. Baki, Nemai Chandra Karmakar, Uditha Bandara and Emran Md Amin, „Beam Forming Algorithm with Different Power Distribution for RFID Reader, pages 64~95‟, Book Title: Chipless and Conventional Radio Frequency Identification: Systems for Ubiquitous Tagging, IGI Global, May, 2012, USA, ISBN 978-1-4666-1616-5 (hardcover) [12] A. K. M. Baki, Nemai Chandra Karmakar, ‟60 GHz Array Antenna with New Method of Beam Forming‟, 15th International Conference on Computer and Information Technology, December 2012, pp. 638-641. [13] Karmakar, N. C. „Smart Antennas for Automatic Radio Frequency Identification Readers‟, Chapter XXI, in Handbook on Advancements in Smart Antenna Technologies for Wireless Networks, editior: Chen Sun, Jun Cheng & Takashi Ohira, IGI Global, 2008 pp. 449472. [14] Pellerano, Stefano. Alvarado, Javier. and Palaskas, Yorgos. „A mm-Wave Power-Harvesting RFID Tag in 90 nm CMOS‟, IEEE Journal of Solid-State Circuits, vol. 45, no. 8, August 2010, pp. 1627~1637. [15] Keith, R Carver. Cooper, W. K. and Stutzman, W. L., “Beam-Pointing Errors of Planner-Phased Arrays”, IEEE Trans. On Antenna &Porp, 1973, pp. 199-202. [16] Warren L. Stutzman and Gary A. Thiele, “Antenna Theory and Design”, 2nd edition, John Wiley & Sons, Inc. pp. 29.
