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An integrated MIMO filtenna with wide band-narrow band functionality
Int. J. Electron. Commun. (AEÜ) 110 (2019) 152862
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International Journal of Electronics and Communications (AEÜ) journal homepage: www.elsevier.com/locate/aeue
Regular paper
An integrated MIMO filtenna with wide band-narrow band functionality Sasmita Pahadsingh, Sudhakar Sahu School of Electronics Engineering, KIIT University, India
a r t i c l e
i n f o
Article history: Received 8 April 2019 Accepted 3 August 2019
Keywords: Filtenna ECC RSRR DG MEG
a b s t r a c t In this literature an integrated multiple-input-multiple-output (MIMO) filtenna for both Wideband (WB)Narrowband (NB) functionality is investigated. The approach to the design involves a two element, dual mode MIMO antenna system. In wideband mode two identical circular radiators are used while a pair of rectangular split ring resonator (RSRR) based band pass filters integrated in to the WB antennas are used in narrowband mode. The ground plane is extended in the shape of T shaped stub for the shake of isolation improvement between the ports. All the scattering parameters (S11/S22, S12/S21) of the fabricated antenna prototype are verified in both simulation and measurement for the determination of better antenna performance. Here, a wide impedance bandwidth of 1.2–6 GHz is achieved during wideband mode of operation whereas a frequency response of 3.3–4.0 GHz is found in narrowband mode with isolation in both the cases less than 25 dB and 30 dB respectively. Moreover the performance parameters of MIMO antenna system such as envelope correlation coefficient (ECC), diversity gain (DG) and mean effective gain (MEG) are also evaluated using S matrix to strengthen the design concept. Ó 2019 Elsevier GmbH. All rights reserved.
1. Introduction Electronics and wireless communication system have developed significantly over past decades. The increasing popularity of wireless connectivity demands integration of mobile devices that operates at different standards evolving towards multifunctionality. This leads to the need of miniaturized, more reliable integrated wireless system. In this context, integration of the entire antenna structure on a single substrate is the vision for future wireless system. Therefore several planar integrated antennas including both dual ports [1–5] and single port [6–9] have been addressed in literature. Most of these cases include incorporation of several switching components in the antenna structure where, the switching elements such as PIN diodes, varactor diodes and MEMs are basically integrated into the radiating surface of the antenna. However, such integration techniques may affect the radiation behavior of antenna as they lead to extensive care during the design process due to biasing circuit. At the same time, antennas and filters are considered as the largest key component of integrated wireless system [10,11]. Owing to such facts, one of the effective solutions for this challenging issue is the implementation of filtenna. A filtenna is basically considered as a combination of filter and antenna where the filter is integrated in the feeding line or ground E-mail addresses: [email protected] (S. Pahadsingh), [email protected] (S. Sahu) https://doi.org/10.1016/j.aeue.2019.152862 1434-8411/Ó 2019 Elsevier GmbH. All rights reserved.
plane rather than the radiating surface. This supports minimal fluctuation in radiation behavior as no biasing circuit is required. Meanwhile, the current wireless technologies have faced the challenges of multipath fading caused by reflection and refraction of the communication link. To mitigate such challenges MIMO configuration system with filtenna have been implemented with good inter port isolation. Such system results very low envelope correlation coefficient, which in turn improves the efficiency and reliability of communication link especially under fading environment. [12–16]. The major issue in multiport antenna system is the excellent inter-port isolation, so the antenna element should be placed more than half wavelength. As in typical mobile phone or laptops antennas are distributed around the periphery of device, so maintaining half wavelength does not support always for practical feasibility. In such cases of closely spaced antenna elements, the relative position to each other and ground affects a lot to the radiation behavior. Keeping into eye such issues some recent papers reported in literature in respect of relative distance of different antenna elements [17–19]. In [17] a 3D metamaterial structure is employed to achieve an isolation of 18 dB with inter element antenna array spacing of 0.13k. But the total electrical dimension of the substrate was 60 60 mm2 and operated from 2.35 GHz to 2.45 GHz. To mitigate the mutual coupling an antenna interference cancellation chip was proposed in literature [18] operating near 2.4 GHz with isolation more than 15 dB. Similarly a multilayered EBG is proposed in literature [19] for isolation improvement at operating frequency of 2.55 GHz. The literatures reported in
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[20,21] based on isolation enhancement using neutralization line structure and radial stub resonator respectively. In literature [20] along with isolation of 30 dB in the operating band the gain is also enhanced by using a suspended meta-face. Literature [21] is well suited for UWB MIMO and multiband MIMO with band notched characteristics. Though in literatures [17–21] the inter element spacing between the antennas is maintained very small, but only responsible for narrowband resonance. Moreover, they did not elaborated the performance analysis of MIMO system in detail in diversity environment. But the present design approach emphasizing the idea of both filter and antenna into a single structure for the purpose of both radiation and filtering by utilizing the concept of wideband to narrowband filtenna. So, the first step of the design involves wideband radiator and in the second step filter is embedded in to the wideband to select only the desired frequency of interest in the operating band while rejecting other signals. The potential benefit of this approach includes the elimination of interference between filter and antenna as well as compactness of the system. Considering the above discussed facts, we proposed an antenna solution that addresses important issues faced by modern wireless communication network. Hence, to satisfy the criteria for MIMO filtenna the design approach includes; (i) a pair of wideband MIMO antenna operating at low resonance frequency i.e 1.2 GHz to 6 GHz with good radiation characteristics, (ii) a pair of RSRR based band pass filter integrated in to the wide band antenna for obtaining narrowband response (iii) a T shaped stub in the ground plane for isolation as well as diversity performance improvement in rich multipath fading wireless channel. Here to minimize the multipath reflection and refraction the diversity performances are validated, by evaluating the parameters in terms of ECC, DG and MEG. Another novel feature of this design approach is its compact architecture with both the wideband and narrowband MIMO antennas are packed together on a single substrate. Additionally, a comparative study of channel capacity between SISO (single input, single output) and MIMO system is also investigated in this paper to validate the MIMO operation in the multipath fading scenario. The organization content of the proposed research work includes Section 2 dedicated to the geometry of the antenna design and Section 3 focused on parametric analysis of proposed MIMO filtenna. Section 4 gives the detail explanation of all simulated and measured results while the concluding section reflects all major findings. 2. Antenna configuration and design The complete MIMO filtenna is developed on a single FR4 substrate of dimensions 35 50 1.6 mm3 and dielectric constant of 4.4. Both the wide band antenna and narrowband filtenna are printed on the top of the substrate with defected ground plane on the bottom side. A pair of RSRR based band pass filters is integrated into the wideband antenna for narrowband functionality. The wide band antenna of MIMO system consists of a pair of circular monopole radiating element. The radius of circular radiator is estimated using the relation [22] considering the lower cutoff frequency of wideband spectrum.
R¼
87:94 pffiffiffiffi in mm f r er
ð1Þ
where ‘‘f r ” is the resonant frequency in GHz and ‘‘er ” is the relative permittivity of the substrate. On the bottom side of the substrate the design of micro-strip ground plane originally started from the dimension of 11 50 mm2, then it is little truncated and defected in the form of rectangular slot for better impedance matching below 2 GHz
in both the pair of wideband antenna. For the purpose of isolation improvement the ground plane is extended further in the form of T shaped stub vertically between the two monopole element. Again to obtain the band pass behavior in the desired band, the dimensions of RSRR based filter were estimated from the center frequency by using the relation [23]
Sr ¼ 2ðg 1 þ g 3 2g 4 Þ ¼
kg c ¼ pffiffiffiffiffiffiffi 2 2f center eeff
ð2Þ
where ‘kg ’ is the guided wavelength, ‘eeff ’ is the effective permittivity of the material, and ‘Sr ’ is the inner perimeter of RSRR which is the function of ring length ‘g1’, ring width ‘g3’ and strip width ‘g4’. The complete geometry of the antenna is shown in Fig. 1. To obtain the dimension of final proposed filtenna step by step design process are conducted using HFSS in both wideband and narrowband mode and are shown in Table 1. 3. Study of MIMO filtenna with parametric analysis In this section rigorous investigation on various wideband and narrowband parameters are carried on using ANSYS HFSS [24] to find the optimal behavior of proposed MIMO filtenna. Here it is important to note that due to the symmetry of the design S11 is similar to S22 and S12 is similar to S21. The wideband antenna performance in terms of different design steps is shown in Fig. 2 and the performance parameters in term of S11/S22, S12/S21 in the range of 1–6 GHz are depicted in Fig. 3. Similarly, in narrowband mode the effects of T shaped grounded stub and RSRR based band pass filter are discussed to study the filtenna behavior. 3.1. Effect of T shaped stub The T shaped ground stub of proposed antenna performs two basic functions such as impedance matching and inter port isolation improvement both in WB and NB mode. A stub in the ground plane acts as a reflector and also preserves the antenna dimension compact in MIMO system. From Fig. 3(a) it is found that without T shaped stub the starting frequency of reflection coefficient (S11/ S22 < 10 dB) is about 1.8 GHz whereas it is lowered down to 1.2 GHz with T stub. The isolation or transmission coefficient is also well maintained below 22 dB through the operating band as seen from Fig. 3(b). Likewise the effect of stub on narrow band antenna performances are also depicted in Fig. 4. Here, in this case the reflection coefficient (S11/S22) of narrowband antenna is not much affected by the presence of stub, rather the transmission coefficient (S12/S21) between the ports is significantly reduced to 30 dB. Hence the T shaped stub plays a vital role in improving the inter port isolation in both the states (WB and NB) which is indeed the main motivation of this approach for diversity performance of MIMO system. 3.2. Effect of RSRR based band pass filter The narrowband functionality is realized by utilizing a pair of rectangular split ring resonator (RSRR) based band pass filter, integrated into the wideband antenna. To aid the better understanding of filter segment the S parameter analysis is also conducted and is depicted in Fig. 5(a). It is evident that the pass band is obtained at 3.9 GHz with transmission coefficient of approximately 1 dB. Here, the split gap (g2) acts as fixed capacitance and allows the filter to perform bandpass behavior. The band pass filter modifies its operating frequency by changing the length, width and split gap of the resonator. These variations of S parameter due to length (g1), width (g4) and split gap (g2) of the resonator are shown in Fig. 5 (b), (c) and (d) respectively. Hence the integration of such type of
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Fig. 1. Complete geometry of the proposed filtenna (a) top view and (b) bottom view.
Table 1 Optimized dimension of antenna. Symbol
Value (in mm)
Symbol
Value (in mm)
Symbol
Value (in mm)
L W R Wg1
35 50 8.5 25
Lg1 Lg2 Lg3 S
11 18 6 3
g4 g1 g2 g3
1 7.5 1 7
RSRR into the wideband monopole antenna is considered as filtenna, where the monopole antenna is responsible for wideband application and band pass filter response is suitable for narrowband. Here it is noticed that the presence of RSRR in wideband antenna element lowers down the narrowband resonance to 3.5 GHz (3.3–4.0 GHz) making it suitable for WiMAX application. Based on the above stated parametric analysis the optimized dimension of proposed MIMO filtenna is illustrated in Table 1. 4. Simulated and measured result analysis 4.1. S parameter The fabricated prototype of the proposed MIMO filtenna for both the states (wideband and narrowband) is depicted in Fig. 6. The S parameters are measured using a vector network analyzer and a comparison with simulated results are shown in Fig. 7. It is found from Fig. 7(a) that in both the cases of simulation and measurement a wide impedance bandwidth of 1.2–6 GHz along with transmission coefficient (S21) well below 25 dB is obtained in
wideband state of proposed antenna. Similarly, in narrowband state (integration of filter into the wideband antenna), the MIMO filtenna performance both in simulation and measurement is shown in Fig. 7(b). It is found that the simulated and measured impedance bandwidth (S11/S22) ranges from 3.3 GHz to 4.0 GHz with mutual coupling less than 30 dB. There are some discrepancies are noticed between simulated and measured S11 and S21 such as in wideband state the measured S11 is slightly wider than the simulated one while mutual coupling is higher in measurement than simulation. Nevertheless, in narrowband state the simulated and measured S11 and S21 are well correspondence to each other. These occurrences of minute difference in wideband state are due to the limitation in fabrication and measurement processes. 4.2. Radiation performance The radiation pattern behavior of proposed MIMO filtenna are simulated and measured independently by exciting port 1 and terminating port 2 and vice versa. The simulated and measured co
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Fig. 2. Different designing steps of wideband antenna.
Fig. 3. Simulated S-parameters in wideband mode of operation (a) S11/S22 and (b) S12/S21.
Fig. 4. S parameter with and without T stub in narrowband mode.
polarization patterns of wideband antenna at 1.5 GHz, 3 GHz and 5 GHz are depicted in Fig. 8(a), (b), (c), (d) and (e), (f) respectively. The pattern behavior corresponds to almost omnidirectional for H plane and bidirectional for E plane. Similarly, the narrow band antenna radiation performance at 3.5 GHz for H plane and E plane are also presented in Fig. 8(g), (h) respectively. From the radiation plot it is found that the simulated and measured patterns are well correspondence to each other. It is also noticed that by exciting port 2 and terminating port1 the radiation behavior at resonance frequencies remains almost same as the two ports are identical. In order to study the isolation behavior further, the current distributions are presented in Fig. 9 at resonance frequency of 3.5 GHz by exciting the port 1 and terminating port 2–50 O load. It is evident from Fig. 9(a) that without stub (T shaped), a strong current is coupled from port 1 to port 2 through the ground plane ensuing high mutual coupling between the ports. Similarly from Fig. 9(b) it is observed that with T shaped stub current concentration on the left side of stub is little larger while a small concentration is coupled to port 2. Again from Fig. 9(c) it is found that with the integra-
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Fig. 5. S parameter variation due to (a) filter segment, (b) resonator length (g1), resonator width (g4) and split gap (g2).
tion of RSRR in to the feed line, the concentration of current coupled from port 1 to port 2 is much less, hence reducing the interport coupling. Therefore the integration of RSRR filter into the wideband antenna not only performs narrowband resonance but also reducing the interport isolation which is highly essential for diversity performance in MIMO system. The simulated and measured gain performance as well as the simulated efficiency both in wideband and narrowband mode are depicted in Fig. 10. It is found that, the gain variation is within 3 dB for both simulation and measurement in both the modes of filtenna. The efficiency in both the modes is mantained within the range of 0.6–0.7. Further, a comparative study is described in Table 2 with some related literatures in terms of their total electrical dimensions and interport isolation performance. It is found that the electrical dimension of our proposed filtenna is relatively smaller than the others with better inter port isolation at desired band of interest (3.5 GHz). 4.3. MIMO performance analysis 4.3.1. Correlation co-efficient and diversity gain (DG) One important parameter of the MIMO antenna system for diversity performance is ECC (envelope correlation co efficient). ECC relates the correlation of communication channel to each other [25]. For reliable communication in MIMO antenna system it is necessary to achieve low envelope correlation coefficient. It
can be calculated using the S parameter as well as radiated far field patterns and can be defined by Eqs. (3) and (4) as given below. A comparative study of ECC evaluation is presented in Table 3 by considering both the simulated scattering parameter and far field pattern data. It is found that fairly better result is obtained with far field pattern than S parameter at resonance frequencies both in wideband and narrow band state of the proposed antenna.
S S12 þ S S22 2 11 21 ECC ¼ 1 jS22 j2 þ jS12 j2 1 jS11 j2 þ jS21 j2 RR 2 ½F ðh; uÞ F j ðh; uÞdX 4p i 2 RR 2 jF i ðh; uÞj dX 4p F j ðh; uÞ dX 4p
ECC ðqÞ ¼ RR
ð3Þ
ð4Þ
where (i,j)2 fði; jÞj1 i < j 2; i; j 2 N jg. F i ðh; uÞ is the field radiation pattern of antenna when ports i, j are excited and * denotes Hermitian product. The diversity gain is the amount of improvement achieved by the MIMO antenna system and is related to ECC given by (5)
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DG ¼ 10 1 ECC 2
ð5Þ
To mitigate the fading in rich multipath environment, it is essential that the signal received either from the wideband antenna or from the narrowband antenna should satisfy the crite-
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Fig. 6. Fabricated prototype of proposed antenna in wideband mode (a) top view, (b) bottom view, and narrowband mode (c) top view and (d) bottom view.
Fig. 7. Simulated and measured S parameter of proposed MIMO filtenna in (a) wideband state and (b) narrowband state.
rion that, ECC < 0.5 [25] i.e ECC should be ideally zero but practically should be less than 0.5. The ECC and DG of the proposed MIMO filtenna can be calculated using S-parameter and are shown in Figs. 11 and 12 with respect to frequency for both the states (wideband and narrowband). The values of ECC and DG are almost within the desired range both in simulation and measurement. However, there are minute dissimilarities seen between the simulated and measured values of these said parameters in wideband mode, which are basically due to the limitation of fabrication and measurement processes. On the other hand, in narrowband state the ECC and DG values are almost within 0.1 and 9.96 dB respectively for both simulation and measurement. At resonance frequency the ECC is increased to 0.18 and DG is reduced to 9.84 dB. However, these said parameters are comparatively low
enough at the desired band of interest both in wideband mode and narrowband mode for good diversity performance. 4.3.2. Mean effective gain (MEG) The last parameter analyzed in this work is the mean effective gain (MEG). Mathematically, it is defined as the relation given by (6)
MEGi ¼ 0:5lirad
! M X ¼ 0:5 1 Sij
ð6Þ
j¼1
where lirad and M refers to radiation efficiency at port i and no of antenna elements respectively. Here the value of M = 2 due to two port MIMO system.
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Fig. 8. Simulated and measured radiation patterns in wideband mode at (a) 1.5 GHz (£ ¼ 90 ), (b) 1.5 GHz (£ ¼ 0 ), (c) 3 GHz (£ ¼ 90 ), (d) 3 GHz (£ ¼ 0 ), (e) 5 GHz (£ ¼ 90 ), (f) 5 GHz (£ ¼ 0 Þ and narrow band mode at (g) 3.5 GHz (£ ¼ 90 ), (h) 3.5 GHz (£ ¼ 0 ).
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Fig. 9. Simulated surface current distribution at 3.5 GHz (a) without T stub, (b) with T shaped stub and (c) with RSRR based filter.
Fig. 10. (a) Simulated and measured gain of MIMO filtenna both in WB and NB mode and (b) simulated efficiency.
Table 2 Comparison with other referred works. References
Design Approach
Electrical dimension
Inter port isolation (dB)
Major findings
[12]
Two port MIMO
58 58 mm2
14
[13] [14]
Two port MIMO Two port MIMO Filtenna
32 56 mm2 80 70 mm2
27 15
[15]
(i) Two port Filtenna
60 60 mm2
20 dB
(ii) Two port MIMO filtenna
80 192
30
Two port MIMO Filtenna
35 50 mm2
35
UWB- 2.8–11 GHz Band Notch-5–6.3 GHz 4G application at 2.6 GHz Band pass resonance at 2.75 GHz and tunability in resonance due to varactor diode. WB(2.24–7.86)GHz Band pass filter-3.41 GHz Filter response at 3.7 GHz Tunabbilty is achieved through varactor diode WB- (1.1–6)GHz Filter response 3.5 GHz
Our Work
Table 3 Comparison of ECC data using radiated field patterns and S-parameter. Frequency (GHz)
ECC (using Simulated S-parameter)
ECC (using Simulated Radiated Field pattern)
1.5 (WB state) 3.5 (WB state) 3.5 (NB state)
0.44 0.25 0.18
0.231 0.068 0.015
The value of power ratio must be less than 3 dB for similar power level at each branch and it is represented as K [26].
K ¼ jMEG1 MEG2 j in dB:
The simulated and measured MEG for both the states (wideband and narrowband) are depicted in Figs. 13 and 14 respectively. Since port 1 and port 2 are identical the ratio of MEG1 and MEG2 is almost unity in both simulation and measurement with power ratio less than 3 dB as well [26]. 4.3.3. Channel capacity analysis In modern wireless communication scenario the main purpose of adopting the MIMO antenna system is to increase the throughput in multipath and fading environment. In this section a comparative analysis of SISO and MIMO system in terms of channel capacity are explained. The channel capacity in MIMO system greatly depends upon the correlation among antenna elements. It is desirable to maintain high isolation and low correlation among
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Fig. 11. Simulated and measured ECC and DG in wideband state (a) ECC and (b) DG.
Fig. 12. Simulated and measured ECC and DG in narrowband states (a) ECC and (b) DG.
Fig. 13. Simulated MEG and power Ratio (a) wideband mode and (b) narrowband mode.
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Fig. 14. Measured MEG and power Ratio (a) wideband mode and b) narrowband mode.
antenna elements to obtain better channel capacity. According to the Shannon’s channel capacity theorem the capacity of a wireless channel can be written as in (7), where C represents the capacity and SNR denotes signal to noise ratio
C ¼ log2 ð1 þ SNRÞ bits=sec=Hz
ð7Þ
Now considering the diversity at both transmitter and receiver, the capacity of N N MIMO system can be calculated as [27] and is given by (8)
SNR HH C ¼ log2 det I þ N
ð8Þ
where I is the N N identity matrix, H is the channel coefficient matrix and H* is the transpose conjugate of H. In this section the channel capacity of proposed two-element MIMO filtenna is calculated by using the normalized S-parameter values at 3.5 GHz as channel matrix. A comparative study of channel capacity using the above relation between Single element SISO and two element MIMO system at the same signal to noise ratio is presented in Fig. 15. In case of MIMO system, there is more than one possible paths exist between transmitter and receiver section that consequently increases number of bits per second. Therefore, from Fig. 15, it can be seen that there is capacity improvement in case of a MIMO system both in simulation and measurement over SISO. With these performance analysis the proposed MIMO system is quite fit for wireless channel in all respect especially in rich multipath fading environment. The overall simulated and measured performance parameters of proposed MIMO filtenna at desired frequency are depicted in Table 4.
5. Conclusion
Fig. 15. Calculated capacity of SISO and MIMO system at 3.5 GHz.
In this paper a compact high isolation dual port MIMO filtenna is presented and verified. The proposed antenna is operated in dual mode (wideband and narrowband) MIMO system. In wideband mode the antenna covers wide frequency band with reasonable inter port isolation. In narrowband mode the dual port MIMO filtenna covers WiMAX frequency band (3.3 GHz- 4 GHz) with improved isolation between ports. Furthermore, it is evident from the performance analysis that proposed MIMO filtenna is beneficial to the advanced wireless applications.
Table 4 Overall simulated and measured results analysis. Wide band Mode
Simulation
Measurement
Narrowband Mode
Simulation
Measurement
S11/S22 (GHz) S12/S21 (dB) (at 3.5 GHz) ECC (at 3.5 GHz) DG (at 3.5 GHz)
1.5–6 22 0.25 9.6
1.1–6 20 0.3 9.5
S11/S22 (GHz) S12/S21 (dB) (at 3.5 GHz) ECC (at 3.5 GHz) DG (at 3.5 GHz)
3.3–4.0 35 0.18 9.8
3.4–4.0 30 0.06 9.9
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Dr. Sasmita Pahadsingh, is an Assistant Professor in School of Electronics Engineering, KIIT Deemed To be University, Bhubaneswar, Odisha, India. She has completed her PhD degree from KIIT University. Her research interests include integrated antenna, frequency reconfigurable antenna, dielectric resonator antenna and MIMO antenna for cognitive radio applications. She has more than 15 years of teaching and research experience and has published more than 15 papers in peer reviewed journals and conference proceedings. She is a member of IEEE, ISTE and Indian Science Congress.
Dr. Sudhakar Sahu is presently working as an Professor, School of Electronics Engineering, KIIT Deemed to be University, Bhubaneswar, Odisha, India. He received the ME degree in Electronics and Tele-communication Engineering from Indian Institute of Engineering Science and Technology (IIEST), Shibpur, West Bengal and PhD (Engineering) degree from Jadavpur University, Kolkata, India. He is the author and co-author of 52 papers in international journals and conference proceedings. His research interests and activities include Metamaterials, UWB antennas, Dielectric Resonator Antenna, Cognitive Radio Antenna, Filters, Computational electromagnetic and application of soft computing techniques in engineering. He is a Senior member of IEEE AP/MTT Society, Fellow IE (India), IET (UK), life member ISTE.
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International Journal of Electronics and Communications (AEÜ) journal homepage: www.elsevier.com/locate/aeue
Regular paper
An integrated MIMO filtenna with wide band-narrow band functionality Sasmita Pahadsingh, Sudhakar Sahu School of Electronics Engineering, KIIT University, India
a r t i c l e
i n f o
Article history: Received 8 April 2019 Accepted 3 August 2019
Keywords: Filtenna ECC RSRR DG MEG
a b s t r a c t In this literature an integrated multiple-input-multiple-output (MIMO) filtenna for both Wideband (WB)Narrowband (NB) functionality is investigated. The approach to the design involves a two element, dual mode MIMO antenna system. In wideband mode two identical circular radiators are used while a pair of rectangular split ring resonator (RSRR) based band pass filters integrated in to the WB antennas are used in narrowband mode. The ground plane is extended in the shape of T shaped stub for the shake of isolation improvement between the ports. All the scattering parameters (S11/S22, S12/S21) of the fabricated antenna prototype are verified in both simulation and measurement for the determination of better antenna performance. Here, a wide impedance bandwidth of 1.2–6 GHz is achieved during wideband mode of operation whereas a frequency response of 3.3–4.0 GHz is found in narrowband mode with isolation in both the cases less than 25 dB and 30 dB respectively. Moreover the performance parameters of MIMO antenna system such as envelope correlation coefficient (ECC), diversity gain (DG) and mean effective gain (MEG) are also evaluated using S matrix to strengthen the design concept. Ó 2019 Elsevier GmbH. All rights reserved.
1. Introduction Electronics and wireless communication system have developed significantly over past decades. The increasing popularity of wireless connectivity demands integration of mobile devices that operates at different standards evolving towards multifunctionality. This leads to the need of miniaturized, more reliable integrated wireless system. In this context, integration of the entire antenna structure on a single substrate is the vision for future wireless system. Therefore several planar integrated antennas including both dual ports [1–5] and single port [6–9] have been addressed in literature. Most of these cases include incorporation of several switching components in the antenna structure where, the switching elements such as PIN diodes, varactor diodes and MEMs are basically integrated into the radiating surface of the antenna. However, such integration techniques may affect the radiation behavior of antenna as they lead to extensive care during the design process due to biasing circuit. At the same time, antennas and filters are considered as the largest key component of integrated wireless system [10,11]. Owing to such facts, one of the effective solutions for this challenging issue is the implementation of filtenna. A filtenna is basically considered as a combination of filter and antenna where the filter is integrated in the feeding line or ground E-mail addresses: [email protected] (S. Pahadsingh), [email protected] (S. Sahu) https://doi.org/10.1016/j.aeue.2019.152862 1434-8411/Ó 2019 Elsevier GmbH. All rights reserved.
plane rather than the radiating surface. This supports minimal fluctuation in radiation behavior as no biasing circuit is required. Meanwhile, the current wireless technologies have faced the challenges of multipath fading caused by reflection and refraction of the communication link. To mitigate such challenges MIMO configuration system with filtenna have been implemented with good inter port isolation. Such system results very low envelope correlation coefficient, which in turn improves the efficiency and reliability of communication link especially under fading environment. [12–16]. The major issue in multiport antenna system is the excellent inter-port isolation, so the antenna element should be placed more than half wavelength. As in typical mobile phone or laptops antennas are distributed around the periphery of device, so maintaining half wavelength does not support always for practical feasibility. In such cases of closely spaced antenna elements, the relative position to each other and ground affects a lot to the radiation behavior. Keeping into eye such issues some recent papers reported in literature in respect of relative distance of different antenna elements [17–19]. In [17] a 3D metamaterial structure is employed to achieve an isolation of 18 dB with inter element antenna array spacing of 0.13k. But the total electrical dimension of the substrate was 60 60 mm2 and operated from 2.35 GHz to 2.45 GHz. To mitigate the mutual coupling an antenna interference cancellation chip was proposed in literature [18] operating near 2.4 GHz with isolation more than 15 dB. Similarly a multilayered EBG is proposed in literature [19] for isolation improvement at operating frequency of 2.55 GHz. The literatures reported in
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[20,21] based on isolation enhancement using neutralization line structure and radial stub resonator respectively. In literature [20] along with isolation of 30 dB in the operating band the gain is also enhanced by using a suspended meta-face. Literature [21] is well suited for UWB MIMO and multiband MIMO with band notched characteristics. Though in literatures [17–21] the inter element spacing between the antennas is maintained very small, but only responsible for narrowband resonance. Moreover, they did not elaborated the performance analysis of MIMO system in detail in diversity environment. But the present design approach emphasizing the idea of both filter and antenna into a single structure for the purpose of both radiation and filtering by utilizing the concept of wideband to narrowband filtenna. So, the first step of the design involves wideband radiator and in the second step filter is embedded in to the wideband to select only the desired frequency of interest in the operating band while rejecting other signals. The potential benefit of this approach includes the elimination of interference between filter and antenna as well as compactness of the system. Considering the above discussed facts, we proposed an antenna solution that addresses important issues faced by modern wireless communication network. Hence, to satisfy the criteria for MIMO filtenna the design approach includes; (i) a pair of wideband MIMO antenna operating at low resonance frequency i.e 1.2 GHz to 6 GHz with good radiation characteristics, (ii) a pair of RSRR based band pass filter integrated in to the wide band antenna for obtaining narrowband response (iii) a T shaped stub in the ground plane for isolation as well as diversity performance improvement in rich multipath fading wireless channel. Here to minimize the multipath reflection and refraction the diversity performances are validated, by evaluating the parameters in terms of ECC, DG and MEG. Another novel feature of this design approach is its compact architecture with both the wideband and narrowband MIMO antennas are packed together on a single substrate. Additionally, a comparative study of channel capacity between SISO (single input, single output) and MIMO system is also investigated in this paper to validate the MIMO operation in the multipath fading scenario. The organization content of the proposed research work includes Section 2 dedicated to the geometry of the antenna design and Section 3 focused on parametric analysis of proposed MIMO filtenna. Section 4 gives the detail explanation of all simulated and measured results while the concluding section reflects all major findings. 2. Antenna configuration and design The complete MIMO filtenna is developed on a single FR4 substrate of dimensions 35 50 1.6 mm3 and dielectric constant of 4.4. Both the wide band antenna and narrowband filtenna are printed on the top of the substrate with defected ground plane on the bottom side. A pair of RSRR based band pass filters is integrated into the wideband antenna for narrowband functionality. The wide band antenna of MIMO system consists of a pair of circular monopole radiating element. The radius of circular radiator is estimated using the relation [22] considering the lower cutoff frequency of wideband spectrum.
R¼
87:94 pffiffiffiffi in mm f r er
ð1Þ
where ‘‘f r ” is the resonant frequency in GHz and ‘‘er ” is the relative permittivity of the substrate. On the bottom side of the substrate the design of micro-strip ground plane originally started from the dimension of 11 50 mm2, then it is little truncated and defected in the form of rectangular slot for better impedance matching below 2 GHz
in both the pair of wideband antenna. For the purpose of isolation improvement the ground plane is extended further in the form of T shaped stub vertically between the two monopole element. Again to obtain the band pass behavior in the desired band, the dimensions of RSRR based filter were estimated from the center frequency by using the relation [23]
Sr ¼ 2ðg 1 þ g 3 2g 4 Þ ¼
kg c ¼ pffiffiffiffiffiffiffi 2 2f center eeff
ð2Þ
where ‘kg ’ is the guided wavelength, ‘eeff ’ is the effective permittivity of the material, and ‘Sr ’ is the inner perimeter of RSRR which is the function of ring length ‘g1’, ring width ‘g3’ and strip width ‘g4’. The complete geometry of the antenna is shown in Fig. 1. To obtain the dimension of final proposed filtenna step by step design process are conducted using HFSS in both wideband and narrowband mode and are shown in Table 1. 3. Study of MIMO filtenna with parametric analysis In this section rigorous investigation on various wideband and narrowband parameters are carried on using ANSYS HFSS [24] to find the optimal behavior of proposed MIMO filtenna. Here it is important to note that due to the symmetry of the design S11 is similar to S22 and S12 is similar to S21. The wideband antenna performance in terms of different design steps is shown in Fig. 2 and the performance parameters in term of S11/S22, S12/S21 in the range of 1–6 GHz are depicted in Fig. 3. Similarly, in narrowband mode the effects of T shaped grounded stub and RSRR based band pass filter are discussed to study the filtenna behavior. 3.1. Effect of T shaped stub The T shaped ground stub of proposed antenna performs two basic functions such as impedance matching and inter port isolation improvement both in WB and NB mode. A stub in the ground plane acts as a reflector and also preserves the antenna dimension compact in MIMO system. From Fig. 3(a) it is found that without T shaped stub the starting frequency of reflection coefficient (S11/ S22 < 10 dB) is about 1.8 GHz whereas it is lowered down to 1.2 GHz with T stub. The isolation or transmission coefficient is also well maintained below 22 dB through the operating band as seen from Fig. 3(b). Likewise the effect of stub on narrow band antenna performances are also depicted in Fig. 4. Here, in this case the reflection coefficient (S11/S22) of narrowband antenna is not much affected by the presence of stub, rather the transmission coefficient (S12/S21) between the ports is significantly reduced to 30 dB. Hence the T shaped stub plays a vital role in improving the inter port isolation in both the states (WB and NB) which is indeed the main motivation of this approach for diversity performance of MIMO system. 3.2. Effect of RSRR based band pass filter The narrowband functionality is realized by utilizing a pair of rectangular split ring resonator (RSRR) based band pass filter, integrated into the wideband antenna. To aid the better understanding of filter segment the S parameter analysis is also conducted and is depicted in Fig. 5(a). It is evident that the pass band is obtained at 3.9 GHz with transmission coefficient of approximately 1 dB. Here, the split gap (g2) acts as fixed capacitance and allows the filter to perform bandpass behavior. The band pass filter modifies its operating frequency by changing the length, width and split gap of the resonator. These variations of S parameter due to length (g1), width (g4) and split gap (g2) of the resonator are shown in Fig. 5 (b), (c) and (d) respectively. Hence the integration of such type of
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Fig. 1. Complete geometry of the proposed filtenna (a) top view and (b) bottom view.
Table 1 Optimized dimension of antenna. Symbol
Value (in mm)
Symbol
Value (in mm)
Symbol
Value (in mm)
L W R Wg1
35 50 8.5 25
Lg1 Lg2 Lg3 S
11 18 6 3
g4 g1 g2 g3
1 7.5 1 7
RSRR into the wideband monopole antenna is considered as filtenna, where the monopole antenna is responsible for wideband application and band pass filter response is suitable for narrowband. Here it is noticed that the presence of RSRR in wideband antenna element lowers down the narrowband resonance to 3.5 GHz (3.3–4.0 GHz) making it suitable for WiMAX application. Based on the above stated parametric analysis the optimized dimension of proposed MIMO filtenna is illustrated in Table 1. 4. Simulated and measured result analysis 4.1. S parameter The fabricated prototype of the proposed MIMO filtenna for both the states (wideband and narrowband) is depicted in Fig. 6. The S parameters are measured using a vector network analyzer and a comparison with simulated results are shown in Fig. 7. It is found from Fig. 7(a) that in both the cases of simulation and measurement a wide impedance bandwidth of 1.2–6 GHz along with transmission coefficient (S21) well below 25 dB is obtained in
wideband state of proposed antenna. Similarly, in narrowband state (integration of filter into the wideband antenna), the MIMO filtenna performance both in simulation and measurement is shown in Fig. 7(b). It is found that the simulated and measured impedance bandwidth (S11/S22) ranges from 3.3 GHz to 4.0 GHz with mutual coupling less than 30 dB. There are some discrepancies are noticed between simulated and measured S11 and S21 such as in wideband state the measured S11 is slightly wider than the simulated one while mutual coupling is higher in measurement than simulation. Nevertheless, in narrowband state the simulated and measured S11 and S21 are well correspondence to each other. These occurrences of minute difference in wideband state are due to the limitation in fabrication and measurement processes. 4.2. Radiation performance The radiation pattern behavior of proposed MIMO filtenna are simulated and measured independently by exciting port 1 and terminating port 2 and vice versa. The simulated and measured co
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Fig. 2. Different designing steps of wideband antenna.
Fig. 3. Simulated S-parameters in wideband mode of operation (a) S11/S22 and (b) S12/S21.
Fig. 4. S parameter with and without T stub in narrowband mode.
polarization patterns of wideband antenna at 1.5 GHz, 3 GHz and 5 GHz are depicted in Fig. 8(a), (b), (c), (d) and (e), (f) respectively. The pattern behavior corresponds to almost omnidirectional for H plane and bidirectional for E plane. Similarly, the narrow band antenna radiation performance at 3.5 GHz for H plane and E plane are also presented in Fig. 8(g), (h) respectively. From the radiation plot it is found that the simulated and measured patterns are well correspondence to each other. It is also noticed that by exciting port 2 and terminating port1 the radiation behavior at resonance frequencies remains almost same as the two ports are identical. In order to study the isolation behavior further, the current distributions are presented in Fig. 9 at resonance frequency of 3.5 GHz by exciting the port 1 and terminating port 2–50 O load. It is evident from Fig. 9(a) that without stub (T shaped), a strong current is coupled from port 1 to port 2 through the ground plane ensuing high mutual coupling between the ports. Similarly from Fig. 9(b) it is observed that with T shaped stub current concentration on the left side of stub is little larger while a small concentration is coupled to port 2. Again from Fig. 9(c) it is found that with the integra-
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Fig. 5. S parameter variation due to (a) filter segment, (b) resonator length (g1), resonator width (g4) and split gap (g2).
tion of RSRR in to the feed line, the concentration of current coupled from port 1 to port 2 is much less, hence reducing the interport coupling. Therefore the integration of RSRR filter into the wideband antenna not only performs narrowband resonance but also reducing the interport isolation which is highly essential for diversity performance in MIMO system. The simulated and measured gain performance as well as the simulated efficiency both in wideband and narrowband mode are depicted in Fig. 10. It is found that, the gain variation is within 3 dB for both simulation and measurement in both the modes of filtenna. The efficiency in both the modes is mantained within the range of 0.6–0.7. Further, a comparative study is described in Table 2 with some related literatures in terms of their total electrical dimensions and interport isolation performance. It is found that the electrical dimension of our proposed filtenna is relatively smaller than the others with better inter port isolation at desired band of interest (3.5 GHz). 4.3. MIMO performance analysis 4.3.1. Correlation co-efficient and diversity gain (DG) One important parameter of the MIMO antenna system for diversity performance is ECC (envelope correlation co efficient). ECC relates the correlation of communication channel to each other [25]. For reliable communication in MIMO antenna system it is necessary to achieve low envelope correlation coefficient. It
can be calculated using the S parameter as well as radiated far field patterns and can be defined by Eqs. (3) and (4) as given below. A comparative study of ECC evaluation is presented in Table 3 by considering both the simulated scattering parameter and far field pattern data. It is found that fairly better result is obtained with far field pattern than S parameter at resonance frequencies both in wideband and narrow band state of the proposed antenna.
S S12 þ S S22 2 11 21 ECC ¼ 1 jS22 j2 þ jS12 j2 1 jS11 j2 þ jS21 j2 RR 2 ½F ðh; uÞ F j ðh; uÞdX 4p i 2 RR 2 jF i ðh; uÞj dX 4p F j ðh; uÞ dX 4p
ECC ðqÞ ¼ RR
ð3Þ
ð4Þ
where (i,j)2 fði; jÞj1 i < j 2; i; j 2 N jg. F i ðh; uÞ is the field radiation pattern of antenna when ports i, j are excited and * denotes Hermitian product. The diversity gain is the amount of improvement achieved by the MIMO antenna system and is related to ECC given by (5)
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DG ¼ 10 1 ECC 2
ð5Þ
To mitigate the fading in rich multipath environment, it is essential that the signal received either from the wideband antenna or from the narrowband antenna should satisfy the crite-
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Fig. 6. Fabricated prototype of proposed antenna in wideband mode (a) top view, (b) bottom view, and narrowband mode (c) top view and (d) bottom view.
Fig. 7. Simulated and measured S parameter of proposed MIMO filtenna in (a) wideband state and (b) narrowband state.
rion that, ECC < 0.5 [25] i.e ECC should be ideally zero but practically should be less than 0.5. The ECC and DG of the proposed MIMO filtenna can be calculated using S-parameter and are shown in Figs. 11 and 12 with respect to frequency for both the states (wideband and narrowband). The values of ECC and DG are almost within the desired range both in simulation and measurement. However, there are minute dissimilarities seen between the simulated and measured values of these said parameters in wideband mode, which are basically due to the limitation of fabrication and measurement processes. On the other hand, in narrowband state the ECC and DG values are almost within 0.1 and 9.96 dB respectively for both simulation and measurement. At resonance frequency the ECC is increased to 0.18 and DG is reduced to 9.84 dB. However, these said parameters are comparatively low
enough at the desired band of interest both in wideband mode and narrowband mode for good diversity performance. 4.3.2. Mean effective gain (MEG) The last parameter analyzed in this work is the mean effective gain (MEG). Mathematically, it is defined as the relation given by (6)
MEGi ¼ 0:5lirad
! M X ¼ 0:5 1 Sij
ð6Þ
j¼1
where lirad and M refers to radiation efficiency at port i and no of antenna elements respectively. Here the value of M = 2 due to two port MIMO system.
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Fig. 8. Simulated and measured radiation patterns in wideband mode at (a) 1.5 GHz (£ ¼ 90 ), (b) 1.5 GHz (£ ¼ 0 ), (c) 3 GHz (£ ¼ 90 ), (d) 3 GHz (£ ¼ 0 ), (e) 5 GHz (£ ¼ 90 ), (f) 5 GHz (£ ¼ 0 Þ and narrow band mode at (g) 3.5 GHz (£ ¼ 90 ), (h) 3.5 GHz (£ ¼ 0 ).
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Fig. 9. Simulated surface current distribution at 3.5 GHz (a) without T stub, (b) with T shaped stub and (c) with RSRR based filter.
Fig. 10. (a) Simulated and measured gain of MIMO filtenna both in WB and NB mode and (b) simulated efficiency.
Table 2 Comparison with other referred works. References
Design Approach
Electrical dimension
Inter port isolation (dB)
Major findings
[12]
Two port MIMO
58 58 mm2
14
[13] [14]
Two port MIMO Two port MIMO Filtenna
32 56 mm2 80 70 mm2
27 15
[15]
(i) Two port Filtenna
60 60 mm2
20 dB
(ii) Two port MIMO filtenna
80 192
30
Two port MIMO Filtenna
35 50 mm2
35
UWB- 2.8–11 GHz Band Notch-5–6.3 GHz 4G application at 2.6 GHz Band pass resonance at 2.75 GHz and tunability in resonance due to varactor diode. WB(2.24–7.86)GHz Band pass filter-3.41 GHz Filter response at 3.7 GHz Tunabbilty is achieved through varactor diode WB- (1.1–6)GHz Filter response 3.5 GHz
Our Work
Table 3 Comparison of ECC data using radiated field patterns and S-parameter. Frequency (GHz)
ECC (using Simulated S-parameter)
ECC (using Simulated Radiated Field pattern)
1.5 (WB state) 3.5 (WB state) 3.5 (NB state)
0.44 0.25 0.18
0.231 0.068 0.015
The value of power ratio must be less than 3 dB for similar power level at each branch and it is represented as K [26].
K ¼ jMEG1 MEG2 j in dB:
The simulated and measured MEG for both the states (wideband and narrowband) are depicted in Figs. 13 and 14 respectively. Since port 1 and port 2 are identical the ratio of MEG1 and MEG2 is almost unity in both simulation and measurement with power ratio less than 3 dB as well [26]. 4.3.3. Channel capacity analysis In modern wireless communication scenario the main purpose of adopting the MIMO antenna system is to increase the throughput in multipath and fading environment. In this section a comparative analysis of SISO and MIMO system in terms of channel capacity are explained. The channel capacity in MIMO system greatly depends upon the correlation among antenna elements. It is desirable to maintain high isolation and low correlation among
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Fig. 11. Simulated and measured ECC and DG in wideband state (a) ECC and (b) DG.
Fig. 12. Simulated and measured ECC and DG in narrowband states (a) ECC and (b) DG.
Fig. 13. Simulated MEG and power Ratio (a) wideband mode and (b) narrowband mode.
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Fig. 14. Measured MEG and power Ratio (a) wideband mode and b) narrowband mode.
antenna elements to obtain better channel capacity. According to the Shannon’s channel capacity theorem the capacity of a wireless channel can be written as in (7), where C represents the capacity and SNR denotes signal to noise ratio
C ¼ log2 ð1 þ SNRÞ bits=sec=Hz
ð7Þ
Now considering the diversity at both transmitter and receiver, the capacity of N N MIMO system can be calculated as [27] and is given by (8)
SNR HH C ¼ log2 det I þ N
ð8Þ
where I is the N N identity matrix, H is the channel coefficient matrix and H* is the transpose conjugate of H. In this section the channel capacity of proposed two-element MIMO filtenna is calculated by using the normalized S-parameter values at 3.5 GHz as channel matrix. A comparative study of channel capacity using the above relation between Single element SISO and two element MIMO system at the same signal to noise ratio is presented in Fig. 15. In case of MIMO system, there is more than one possible paths exist between transmitter and receiver section that consequently increases number of bits per second. Therefore, from Fig. 15, it can be seen that there is capacity improvement in case of a MIMO system both in simulation and measurement over SISO. With these performance analysis the proposed MIMO system is quite fit for wireless channel in all respect especially in rich multipath fading environment. The overall simulated and measured performance parameters of proposed MIMO filtenna at desired frequency are depicted in Table 4.
5. Conclusion
Fig. 15. Calculated capacity of SISO and MIMO system at 3.5 GHz.
In this paper a compact high isolation dual port MIMO filtenna is presented and verified. The proposed antenna is operated in dual mode (wideband and narrowband) MIMO system. In wideband mode the antenna covers wide frequency band with reasonable inter port isolation. In narrowband mode the dual port MIMO filtenna covers WiMAX frequency band (3.3 GHz- 4 GHz) with improved isolation between ports. Furthermore, it is evident from the performance analysis that proposed MIMO filtenna is beneficial to the advanced wireless applications.
Table 4 Overall simulated and measured results analysis. Wide band Mode
Simulation
Measurement
Narrowband Mode
Simulation
Measurement
S11/S22 (GHz) S12/S21 (dB) (at 3.5 GHz) ECC (at 3.5 GHz) DG (at 3.5 GHz)
1.5–6 22 0.25 9.6
1.1–6 20 0.3 9.5
S11/S22 (GHz) S12/S21 (dB) (at 3.5 GHz) ECC (at 3.5 GHz) DG (at 3.5 GHz)
3.3–4.0 35 0.18 9.8
3.4–4.0 30 0.06 9.9
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Dr. Sasmita Pahadsingh, is an Assistant Professor in School of Electronics Engineering, KIIT Deemed To be University, Bhubaneswar, Odisha, India. She has completed her PhD degree from KIIT University. Her research interests include integrated antenna, frequency reconfigurable antenna, dielectric resonator antenna and MIMO antenna for cognitive radio applications. She has more than 15 years of teaching and research experience and has published more than 15 papers in peer reviewed journals and conference proceedings. She is a member of IEEE, ISTE and Indian Science Congress.
Dr. Sudhakar Sahu is presently working as an Professor, School of Electronics Engineering, KIIT Deemed to be University, Bhubaneswar, Odisha, India. He received the ME degree in Electronics and Tele-communication Engineering from Indian Institute of Engineering Science and Technology (IIEST), Shibpur, West Bengal and PhD (Engineering) degree from Jadavpur University, Kolkata, India. He is the author and co-author of 52 papers in international journals and conference proceedings. His research interests and activities include Metamaterials, UWB antennas, Dielectric Resonator Antenna, Cognitive Radio Antenna, Filters, Computational electromagnetic and application of soft computing techniques in engineering. He is a Senior member of IEEE AP/MTT Society, Fellow IE (India), IET (UK), life member ISTE.