The modernization in wireless and mobile technologies demand for efficient antenna modules which have compact size, easy fabrication, high radiation efficiency, high data-rate capacity, large bandwidth and low power requirement.1–3 The efficacy of well-known Ultra-wideband antennas makes them an attractive choice in recent years. In the year 2002, ultra-wideband communication got unlicensed spectrum ranging from 3.1-10.6GHz from federal communication commission(FCC).4 Due to wide spectrum of such antennas, their effective deployment has seen in many applications such as tracking and navigation systems, cognitive radios, spread-spectrum communication, wireless sensor networks, military applications and soon.5,6 The performance of UWB systems is limited by fading problems which occur because of various phenomenon's such as reflection, refraction and diffraction of electromagnetic waves. Multipath fading degrades the transmission quality of signal in free space. To combat its effect, various diversity/MIMO techniques have reported in the literature. The multiple input multiple output (MIMO) approach is highly suitable in reducing fading effects thereby attaining high data rates, range, robust, secure and reliable communication. In addition, by combining UWB technology and MIMO approach, the system throughput and transmission power can be improved.
Design of compact UWB system with multiple antennas is a challenging task due to space constraints. The closely operating frequencies and space issues results in strong mutual coupling among the antenna elements. The isolation among multiple radiating elements in a system can be enhanced by adopting various decoupling structures including various slits, decoupling strips, different shaped stubs; orthogonal orientation of antenna elements; providing spacing between radiating elements; using slotted-edge-substrate method and soon.7–22 However, various decoupling structures occupy more space, and make the antenna configuration much complex.7–9 It has observed that antenna size increases with greater spacing between radiating elements. Consequently, compactness in antenna size, design complexity minimization and coupling minimization are main concern in MIMO design for UWB communication.
The overlap of existing licensed bands such as WLAN, WiMAX etc. with that of Ultra-wideband resulting in overlapping of electromagnetic-signals. This electro-magnetic interference is another challenge faced by UWB antennas. The remedy of this is to design an antenna with notch behavior. Much UWB-MIMO system with notch behavior has been developed in recent years. A UWB-MIMO antenna with single notch behavior has reported in 10, where L-shaped slits has used to create notch at WLAN band. A dual-port MIMO antenna with arc shaped slot resonators in CPW feed for stop-band is presented in.11 In reference 12, a pair of inverted L-strips are used to provide dual notches at bands ranging from 5.15–5.85 GHz and 6.7–7.1 GHz. UWB-MIMO antenna integrated with EBG structures to attain three stop-bands at Wi-MAX, WLAN and X band reported in reference 13. A compact MIMO with band notch at WLAN and good isolation has reported in.14 UWB-MIMO antenna designs with decoupling resonators15, parasitic decoupling structure attached to ground plane16 and ground stubs17 has reported with very low coupling. A 4x4 UWB-MIMO design consist of slotted circular disc shaped radiators with dual notches reported in reference 18, where large spacing is used for improving isolation among antenna elements which in turn increases the design size to 81x87 mm2. In reference 19, a quad-port MIMO with dual band notches using complimentary split ring resonators and good diversity by slotting substrate was presented. A Koch fractral MIMO design with grounded stubs for isolation and C-shaped slotted radiator for notch characteristics reported in.20 A UWB antenna with LC band stop filter and good isolation presented in reference 21. A MIMO antenna with quasi self-complimentary structure which contributes for wide band characteristics as well as additional decoupling presented in 22, in which notch band is obtained using a bent slit in patch. However, among the existing designs of notched-band UWB-MIMO, it is difficult to maintain compactness, low coupling levels, centre frequency of notches and sharpness of notch-bands. Moreover, most of the reported designs have either one, two or three notches at maximum. So, it is required to design a simple, small and efficient MIMO system with low coupling issues, high isolation, easy fabrication and large number of unwanted frequency suppression to stop interference through numerous existing bands.
In this article, a novel 4-port UWB MIMO antenna on FR-4 substrate is presented with sharp multiple-band notched behavior (very few in existing literatures). The design consist of identical four modified spinning-top shape radiators (MSTSR) which are orthogonally placed in order to provide polarization diversity. In this way, inter-element isolation amongst the four ports is improved. The four radiators fed through taper off micro strip lines, contributing to ultra-wide band characteristics. In design structure, Notching at X-band satellite communication uplink (Notch#1) is achieved by inserting four spirals coils on the side of semi-circular ground plane. Notch#2 is obtained by etching a circular spiral slot inside the MMSR covering X-band satellite communication downlink. The presented design is very effectively utilizing the spectrum, eliminating the interference from existing bands and supporting various wireless applications. Furthermore, by inserting symmetrical trapezoid rectangular resonators and inverted symmetrical trapezoid rectangular resonators on either side of radiators, notch#3 is obtained at 5.5GHz covering WLAN and notch#4 is obtained at 3.5GHz covering WiMAX band respectively. The detailed description of proposed prototype is presented in preceding sections. Table 1 shows a comparison analysis of the designed antenna with the other reported UWB designs in literature. The designed antenna is advantageous over reported designs in terms of: (1) Relatively small in size(34x34mm2); (2) More monopoles enhance the system throughput and transmission capacity; (3) Multiple stop band property; (4) Stop bands are simply obtained by utilizing simple structures and slot without any significant compromise with size, isolation factor, complexity, and radiation performance; (5) The centre frequencies of stop bands are relatively sharp; (6) the notches has relatively narrow band; (7) Inter-monopole isolation is relatively high without using any complex decoupling structures; (8) Almost all MIMO performance parameters are evaluated; (9) Proposed UWB MIMO with multiple notches design is rare in existing literature.
Table 1
Comparison of proposed design with other literature antennas
Ref.
|
No. of Elements
|
Size
(in mm3)
|
Bandwidth
(in GHz)
|
Notch bands
(in GHz)
|
ECC
|
TARC
(in dB)
|
Channel capacity
(in bits/s/Hz)
|
DG
(in dB)
|
Isolation
(in dB)
|
5
|
1
|
12x19x1.6
|
2.95-12
|
3.3–3.8,
5.1-5.825,
7.25–7.75
|
-
|
-
|
-
|
-
|
-
|
6
|
1
|
20x40x0.762
|
3.1–10.5
|
5.725–5.85,
8.025–8.4
|
-
|
-
|
-
|
-
|
-
|
7
|
2
|
22x31x0.125
|
3–10
|
-
|
< 0.3
|
-
|
< 0.4
|
-
|
> 15
|
8
|
2
|
32x32x0.8
|
3.1–10.6
|
-
|
< 0.04
|
-
|
-
|
-
|
> 15
|
9
|
2
|
25x45x0.8
|
3.1–12
|
-
|
< 0.2
|
-
|
-
|
> 9.5
|
> 15
|
10
|
2
|
38.5x38.5x1.6
|
3.08–11.8
|
5.03–5.97
|
< 0.02
|
-
|
-
|
-
|
> 15
|
11
|
2
|
50x50x1.524
|
2.8–10.8
|
4.75–6.12
|
< .02
|
-
|
-
|
-
|
> 15
|
12
|
2
|
26x28x0.8
|
2.9–10.8
|
5.05–5.86, 6.68–7.43
|
< 0.08
|
<-12
|
|
> 9.95
|
> 15
|
13
|
2
|
64x45x1.6
|
2.5–11
|
3.3 to 3.6,
5 to 6,
7.1 to 7.9
|
< 0.02
|
-
|
-
|
-
|
> 15
|
14
|
2
|
30x30 x1.6
|
3.1–11
|
4.98–5.96
|
< 0.02
|
<-10
|
< 0.35
|
9.51
|
> 20
|
15
|
4
|
40x40x1.6
|
3.1–11
|
-
|
< 0.004
|
-
|
< 0.4
|
-
|
> 20
|
16
|
4
|
40x43x1
|
3.1–10.6
|
-
|
< 0.2
|
-8
|
< 0.3
|
-
|
> 20
|
17
|
4
|
35x35 x0.8
|
3–12
|
-
|
< 0.07
|
-
|
-
|
-
|
> 15
|
18
|
4
|
87x81x1.6
|
3.03–10.74
|
3.87–5.94,
7.04–8.7
|
< 0.1
|
-
|
-
|
-
|
> 20
|
19
|
4
|
58x58x0.8
|
3-13.5
|
3.25–3.75,
5.3–5.7
|
< 0.008
|
-
|
-
|
> 9.9
|
> 22
|
20
|
4
|
45x45x1.6
|
2-10.6
|
5.15–5.85
|
< 0.003
|
-
|
< 0.1
|
-
|
> 17
|
21
|
4
|
50x39.8x1.524
|
2.7–12
|
5.1–5.9
|
< 0.03
|
-
|
-
|
-
|
> 17
|
22
|
4
|
35x35x1
|
3–12
|
5.15–5.825
|
< 0.5
|
-
|
-
|
-
|
> 20
|
Prop. Work
|
4
|
34x34x1.6
|
3-12.9
|
7.9–8.49, 7.36–7.72,
5.3–5.61,
3.4–3.66
|
< 0.016
|
<-6 except at notch bands
|
< 0.1
|
> 9.92
|
> 17
|