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A Compact UWB Antenna Design for Indoor Wireless Applications Mohamed Shehata 1 , Mohamed Sameh Said 1 , and Hassan Mostafa 1,2 1 Electronics and Communication Engineering Department, Cairo University, Giza 12613, Egypt. 2 Nanotechnology and Nanoelectronics Program, Zewail City for Science and Technology, Giza 12588, Egypt Abstract—The design of radiation efficient antennas for ultraw- ide band (UWB) signalling is a challenging step in the design and development of UWB communication systems, especially when UWB signals co-exist with previously standardized narrowband (NB) wireless services. This work presents a compact UWB antenna design that is capable of suppressing NB interference (NBI) signals. The immunity of the proposed design to NBI is tested both experimentally and through simulations. More- over, the experimentally measured spectral characteristics are employed in simulating the impact of the proposed UWB antenna on a typical UWB signalling waveform. Both experimental and simulation results confirm the capability of the proposed design to suppress NBI signals at six different frequencies, corresponding to the most frequently used wireless services at these frequencies, while introducing an almost negligible distortion to the UWB signalling waveforms. Index Terms—Microstrip line (MSL), narrowband interference (NBI), ultra wideband (UWB). I. I NTRODUCTION The UWB technology is becoming a promising candidate for high speed–short range applications. Due to the extremely low power dedicated by the FCC for UWB signalling (less than 0.56 mW) [1], the propagation distance of UWB sig- nals are limited to only few meters (typically 4 10 m). Therefore, UWB devices are often designed for high speed indoor communication applications, where it is highly prob- able to encounter the conventional previously standardized narrowband wireless services such as WLAN and Wi-Fi. Generally, UWB signals often show negligible interference on the existing NB wireless standards. However, the reverse situation is not true. Due to their extremely limited PSD, UWB signals usually suffer from the severe NB interference exerted by previously standardized wireless services. Several UWB system designers usually employ UWB antennas that possess quasi-flat frequency responses in their designs such as the commercially available Skycross (SMT-3TO10M-A) antenna (e.g., [2]-[4]). Unfortunately, the problem of co-existing NBI and UWB signals is ignored in the design of these systems, which are not capable of suppressing NBI. Therefore, UWB receivers should possess NBI mitigation capabilities (e.g., [5]). However, NBI mitigation algorithms increase the UWB receiver complexity to maintain an adequate performance in NBI environments. Alternatively, several designs have been reported to demonstrate UWB antennas with NBI suppression capabilities [6]-[12]. The reported designs offer a lot of the de- sired UWB spectral characteristics such as the large impedance bandwidth. However, to the best of the authors’ knowledge, the impact of the band-notched gain-frequency profile of the reported designs on the UWB signalling waveforms has not been investigated. Moreover, a number of drawbacks are still associated with these designs, such as their complicated geometries, large sizes and weak isolation in multiple input - multiple output (MIMO) structures. In this paper, a dual-band notch UWB planar antenna design is presented. The proposed structure is basically a microstrip antenna whose geometry and dimensions are modified to achieve the NBI suppression capability. Simulation results and experimental measurements indicate that the UWB channel frequency response achieved by including the proposed antenna gain profile can effectively suppress NBI signals at the considered frequencies. The rest of this paper is structured as follows. The design approach of the proposed UWB antenna structure is presented in Section II. In Section III, the experimental measurements as well as simulations for the spatio-spectral response of the proposed antenna are presented. Finally, the whole paper is concluded in Section IV. II. ANTENNA DESIGN AND EXPERIMENTAL MEASUREMENTS As mentioned in Section I, a number of previously standard- ized NB wireless services usually co-exist with UWB devices. The frequency ranges allocated for the most common indoor NB wireless services are listed in Table I. TABLE I FREQUENCY RANGES ALLOCATED FOR COMMON I NDOOR NARROWBAND WIRELESS SERVICES Band Designation Allocated Band Indoor Wireless (GHz) (GHz) Service Standard 3.3 3.3-3.4 Mobile WiMAX (100MHz) IEEE 802.16.e-2005 3.5 3.4-3.6 Mobile WiMAX (200MHz) IEEE 802.16.e-2005 3.7 3.4-3.8 Mobile WiMAX (400MHz) IEEE 802.16.e-2005 Fixed WiMAX IEEE 802.16-2004 5.2 5.15-5.35 Wi-Fi (200MHz) IEEE 802.11 5.5 5.47-5.725 Wi-Fi (255MHz) IEEE 802.11 5.8 5.725-5.85 Fixed WiMAX (125MHz) IEEE 802.16-2004 Wi-Fi IEEE 802.11 978-1-5386-7392-8/18/$31.00 ©2018 IEEE 202
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A Compact UWB Antenna Design for Indoor Wireless Applications A Compact UWB Antenna Design for Indoor Wireless Applications Mohamed Shehata 1, Mohamed Sameh Said , and Hassan Mostafa;2

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Page 1: A Compact UWB Antenna Design for Indoor Wireless Applications A Compact UWB Antenna Design for Indoor Wireless Applications Mohamed Shehata 1, Mohamed Sameh Said , and Hassan Mostafa;2

A Compact UWB Antenna Design for IndoorWireless Applications

Mohamed Shehata1, Mohamed Sameh Said1, and Hassan Mostafa1,21Electronics and Communication Engineering Department, Cairo University, Giza 12613, Egypt.

2Nanotechnology and Nanoelectronics Program, Zewail City for Science and Technology, Giza 12588, Egypt

Abstract—The design of radiation efficient antennas for ultraw-ide band (UWB) signalling is a challenging step in the design anddevelopment of UWB communication systems, especially whenUWB signals co-exist with previously standardized narrowband(NB) wireless services. This work presents a compact UWBantenna design that is capable of suppressing NB interference(NBI) signals. The immunity of the proposed design to NBIis tested both experimentally and through simulations. More-over, the experimentally measured spectral characteristics areemployed in simulating the impact of the proposed UWB antennaon a typical UWB signalling waveform. Both experimental andsimulation results confirm the capability of the proposed design tosuppress NBI signals at six different frequencies, correspondingto the most frequently used wireless services at these frequencies,while introducing an almost negligible distortion to the UWBsignalling waveforms.

Index Terms—Microstrip line (MSL), narrowband interference(NBI), ultra wideband (UWB).

I. INTRODUCTION

The UWB technology is becoming a promising candidatefor high speed–short range applications. Due to the extremelylow power dedicated by the FCC for UWB signalling (lessthan 0.56 mW) [1], the propagation distance of UWB sig-nals are limited to only few meters (typically 4 ∼ 10 m).Therefore, UWB devices are often designed for high speedindoor communication applications, where it is highly prob-able to encounter the conventional previously standardizednarrowband wireless services such as WLAN and Wi-Fi.Generally, UWB signals often show negligible interferenceon the existing NB wireless standards. However, the reversesituation is not true. Due to their extremely limited PSD, UWBsignals usually suffer from the severe NB interference exertedby previously standardized wireless services. Several UWBsystem designers usually employ UWB antennas that possessquasi-flat frequency responses in their designs such as thecommercially available Skycross (SMT-3TO10M-A) antenna(e.g., [2]-[4]). Unfortunately, the problem of co-existing NBIand UWB signals is ignored in the design of these systems,which are not capable of suppressing NBI. Therefore, UWBreceivers should possess NBI mitigation capabilities (e.g.,[5]). However, NBI mitigation algorithms increase the UWBreceiver complexity to maintain an adequate performance inNBI environments. Alternatively, several designs have beenreported to demonstrate UWB antennas with NBI suppressioncapabilities [6]-[12]. The reported designs offer a lot of the de-sired UWB spectral characteristics such as the large impedancebandwidth. However, to the best of the authors’ knowledge,

the impact of the band-notched gain-frequency profile ofthe reported designs on the UWB signalling waveforms hasnot been investigated. Moreover, a number of drawbacks arestill associated with these designs, such as their complicatedgeometries, large sizes and weak isolation in multiple input -multiple output (MIMO) structures. In this paper, a dual-bandnotch UWB planar antenna design is presented. The proposedstructure is basically a microstrip antenna whose geometryand dimensions are modified to achieve the NBI suppressioncapability. Simulation results and experimental measurementsindicate that the UWB channel frequency response achievedby including the proposed antenna gain profile can effectivelysuppress NBI signals at the considered frequencies. The restof this paper is structured as follows. The design approach ofthe proposed UWB antenna structure is presented in SectionII. In Section III, the experimental measurements as well assimulations for the spatio-spectral response of the proposedantenna are presented. Finally, the whole paper is concludedin Section IV.

II. ANTENNA DESIGN AND EXPERIMENTALMEASUREMENTS

As mentioned in Section I, a number of previously standard-ized NB wireless services usually co-exist with UWB devices.The frequency ranges allocated for the most common indoorNB wireless services are listed in Table I.

TABLE IFREQUENCY RANGES ALLOCATED FOR COMMON INDOOR NARROWBAND

WIRELESS SERVICES

Band Designation Allocated Band Indoor Wireless(GHz) (GHz) Service Standard

3.3 3.3-3.4 Mobile WiMAX(100MHz) IEEE 802.16.e-2005

3.5 3.4-3.6 Mobile WiMAX(200MHz) IEEE 802.16.e-2005

3.7 3.4-3.8 Mobile WiMAX(400MHz) IEEE 802.16.e-2005

Fixed WiMAXIEEE 802.16-2004

5.2 5.15-5.35 Wi-Fi(200MHz) IEEE 802.11

5.5 5.47-5.725 Wi-Fi(255MHz) IEEE 802.11

5.8 5.725-5.85 Fixed WiMAX(125MHz) IEEE 802.16-2004

Wi-FiIEEE 802.11

978-1-5386-7392-8/18/$31.00 ©2018 IEEE 202

Page 2: A Compact UWB Antenna Design for Indoor Wireless Applications A Compact UWB Antenna Design for Indoor Wireless Applications Mohamed Shehata 1, Mohamed Sameh Said , and Hassan Mostafa;2

The starting point of the proposed design is a typical W×Lmm2 ground-free patch antenna with a ∆L × ∆W mm2

rectangular microstrip feed line as shown in Fig 1 (a). Thegeometry of this initial design as well as its dimensions aremodified and optimized through an iterative design-simulate-enhance process based on cooperative simulation using MAT-LAB and the finite element method (FEM), utilized by HFSSsimulation tool until the desired band-notched spectral char-acteristics are achieved. The final geometry resulting from thedesign cycle is a discontinuous coupling of the patch antennaand its microstrip feed line via a single (∆L+∆l)×∆W mm2

rectangular microstrip as shown in Fig. 1 (b). The microstripfeed line occupies an area of w × l mm2. A 3D view ofthe proposed design is illustrated in Fig. 1 (c). The resultingoptimized dimensions of the radiating element of the proposedstructure are as follows: W = 25.5 mm, w = 4mm, L =19.45 mm, l = 8.4 mm, ∆L + ∆l = 8.4mm, as depicted indetail in the design layout of Fig. 1 (d). Fig. 1 (e) illustratesa photograph for one of two fabricated antenna prototypes.Clearly, the size of the proposed design is comparable to sizeof the familiar and commercially available Skycross (SMT-3TO10M-A) antenna, shown in Fig. 1 (f). The proposed designis implemented by depositing a 0.35 mm thickness copperlayer on only one side of a 1.6 mm thickness FR-4 dielectricsubstrate, whose relative permittivity εr = 4.4 , with noground plan on the other side. This compact structure hasthe advantage of being thin enough to be easily integrable tomany of nowadays personal communications equipment suchas laptops and routers. The entire design occupies an area of20 mm × 30 mm. The proposed structure is excited using a50 Ω capacitive compensated coaxial cable by connecting itscentral conductor to the smaller miscrostrip feed line point asdepicted in Fig. 1 (c).

III. SIMULATION AND EXPERIMENTAL MEASUREMENTS

This section is devoted to present the frequency domainsimulation results as well as the experimental measurementsof the antenna design presented in Section I. The spectralcharacteristics of the proposed antenna design are simulatedusing the HFSS software package and are experimentallymeasured in the frequency domain using a 20 GHz vectornetwork analyzer (R & Z ZVB20) by capturing 20,000 pointsover a span of 100 MHz up to 15 GHz. Accordingly, theresolution bandwidth of the VNA measurements is 250 kHz.This resolution bandwidth respects the 1 MHz resolution ofthe FCC spectral measurements regulations [1] and ensuresa maximum uncertainity of less than three decimal digits inGHz frequency measurements.

A. Radiation Characteristics: HFSS Simulation

Figs. 2 (a) and (b) illustrate the 3D side and top views,respectively for the polar plot of the simulated radiation patternusing the HFSS tool at the center frequency of the usefulUWB band, which is f = 6.85 GHz. A maximum and aminimum antenna gain of 2.93 dB and -2.72 dB, respectivelyare observed in both patterns. As clear from Fig. 2, the electricfield radiated by the proposed design is quasi-omnidirectional

Fig. 1. Geometry of the proposed UWB antenna. (a): Basic geometry ofthe microstrip antenna. (b): geometry of the proposed antenna. (c): 3-D viewof the proposed geometry. (d): layout of the proposed geometry with thefinal optimized dimensions. (e): a photograph of the fabricated antenna. (f):a photograph of the Skycross antenna (SMT-3TO10M-A).

in the XoY and the YoZ plans. This quasi-omnidirectionalcoverage enables an almost uniform distribution of the indoorwireless services delivered via the proposed structure.

Fig. 2. Simulated 3D polar plot of the radiation pattern using the HFSSsoftware simulation tool. (a): 3D side view. (b): 3D top view. Both patternsare obtained at f = 3.5 GHz.

B. UWB Channel Model: Experimental Measurement

In this section, the spatio-spectral UWB channel path loss,designated by |H(jω,D)|2, between the TX-RX antenna pairis measured, where D is the TX-RX antenna separation.The experimental setup for the measurement procedure isillustrated in Fig. 3. As depicted in this figure, the UWBchannel frequency response is measured using two similarfabricated prototypes for the proposed antenna.

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Fig. 3. Experimental setup for the spatio-spectral characteristics measure-ments. VNA: Vector network analyzer. AoS: Axis of Symmetry. GND:Ground.

Both antennas are mounted on bases that are 110 cm offthe ground, while connecting the input port of each antennato the VNA via the central conductor of a 50 Ω capacitivecompensated coaxial cable. The two antennas are placed ina face-to-face configuration such that they are aligned alongthe directions of their maximum radiation and are separatedby D = 10, 20 and 30 cm apart. These relatively smallseparation distances ensure line-of-sight (LoS) transmission,while avoiding multipath reflections from the surroundingobjects such as the walls and the windows that often exist ina typical indoor laboratory environment. At each value of D,the channel frequency response is measured for four times toensure its time invariance. Fig. 4 plots the frequency dependentUWB channel path loss, measured by the VNA at TX-RXantenna separation distances as small as 10 cm, 20 cm and 30cm. At these small distances, the large wireless transmissionloss is avoided and hence; the need for an electrical radiofrequency (RF) amplifier. Moreover, multipath propagationphenomena have minor impacts on frequency measurementsat such low transmission distances. Clearly, the UWB channelfrequency response possesses two dips located at the 3.5 GHzand the 5.5 GHz bands, with no fading dips in betweenthese two frequencies. As clear from Fig. 4, the finite stopbandwidths around these two dips cover the frequencies listedin Table I, corresponding to the most frequently encounteredindoor wireless services in a typical indoor environment.

C. Transmitted and Received UWB Waveform Analysis

The impact of the measured UWB channel frequencyresponse on UWB signals is evaluated via simulations asfollows. A Gaussian-based monocycle pulse is assumed to bea typical UWB signalling waveform and is given by

ψ(t, τg) = (−t/τg2) exp(−t2/τg2);−T/2 ≤ t ≤ T/2 (1)

where τg = 5 ps is the temporal pulse width and T = 1ns is the pulse duration. According to [13], the monocyclewaveform in (1) outperforms other Gaussian-based IR-UWBwaveforms in terms of the spectral efficiency and the maxi-mum wireless reach [14]. Moreover, a monocycle pulse showsthe maximum photo-detected power and signal to noise ratio(SNR) at the received front-end in case of optical [15] andwireless distribution [16], respectively. At the output of the RX

Fig. 4. Measured UWB channel frequency response at D = 10 cm, 20 cmand 30 cm.

antenna side, the received time domain waveform, denoted byy(t,D), is proportional to a linearly distorted form of ψ(t, τg)and is given by

y(t,D) = =−1 Ψ(jω, τg)HDS(jω,D) (t) (2)

where Ψ(jω, τg) is the Fourier transform ofψ(t, τg),=−1. denotes the inverse Fourier transformoperation and HDS(jω,D) is the double-sided UWB channelfrequency response, defined in terms of its single-sidedmeasured counterpart as follows:

HDS(jω,D) =

√1

2(|H(−jω,D)|2 + |H(jω,D)|2)

× exp(−jωD/c) (3)

where c = 3× 108 is the speed of light. Since |H(jω,D)|2consists of a finite set of samples at discrete measurementfrequencies, the Fourier transform operation and its inversein (2) are replaced by the discrete Fourier transform (DFT)operation. Accordingly, (2) is expressed as follows:

y(nTs, D) =1√N

N−1∑k=0

(N−1∑n=0

ψ(nTs) exp(−j2πkn/N)

)× HDS(j2πk∆f,D) exp(j2πkn/N) (4)

where y(nTs, D) is the discrete-time output waveform, Nis the size of the frequency measurements set, Ts = T/N isthe temporal resolution of the UWB waveform, ∆f is the fre-quency resolution, and n and k are time time and frequency in-dexes, respectively. Fig. 5 compares the normalized amplitudetransmitted waveform ψ(nTs, τg) to the received waveformy(nTs) after wireless transmission over a distance of 10 cm,20 cm, and 30 cm. Clearly, the received waveforms preservethe same polarity and a quasi-odd symmetry as the transmittedwaveform. The observed temporal distortion and spread ofthe received waveforms are attributed to the filtering effectoffered by the non-flat UWB channel frequency response.The temporal spread increases with increasing the wireless

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Fig. 5. Simulation of the impact of the measured channel frequency responseon a typical UWB Gaussian monocycle waveform at the receiver side after awireless transmission distance of D = 10 cm, 20 cm and 30 cm.

Fig. 6. Narrow band interference suppression capability of the proposed UWBantenna design as compared to the commercially available Skycross (SMT-3TO10M-A) antenna. Dashed line: applied NBI level of -30 dB.

transmission distance. However, the duration of each of thereceived waveforms does not exceed the time interval allocatedto its transmitted counterpart. Nevertheless, this observationguarantees inter-symbol interference (ISI) free received sig-nalling. Fig. 6 depicts the NBI suppression capability of theproposed UWB antenna design as compared to the Skycross(SMT-3TO10M-A) antenna [2]-[4] at an applied NBI level of-30 dB. Obviously, the Skycross (SMT-3TO10M-A) antennasuppresses the applied NBI power from -30 dB to -55 dB andto -44 dB at frequencies of 3.5 GHz and 5.5 GHz, respectively.On the other hand, the NBI suppression capability of theproposed design outperforms by 3 dB and 6 dB at the samerespective frequencies.

IV. CONCLUSION

In this paper, a compact UWB antenna design with two bandnotches for narrowband interference suppression is presented.The proposed design layout is easily fabricated using theconventional photolithography techniques. The spatio-spectralcharacteristics of the wireless UWB channel, including theproposed antenna, are experimentally measured. Moreover, asemi-analytical approach is followed to study the impact ofthe measured UWB channel frequency response on a typicalUWB signalling waveform. Experimental measurements and

simulation results confirm the NBI interference suppressioncapability of the proposed design, without introducing signif-icant distortion to the UWB signalling waveform.

ACKNOWLEDGMENT

This work was funded by telecommunications regulatoryagency of Egypt (NTRA-Egypt).

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