Packaging of Ka-Band Patch Antenna and Optoelectronic Components for Dual-Mode Indoor Wireless Communication 1
Jun Liao1, Pengfei Wu1, Ali Mirvakili2, Valencia Joyner2, and Z. Rena Huang1 ECSE Department, Rensselaer Polytechnic Institute, 110 8th St, Troy, NY 12180 2 ECE Department, Tufts University, 161 College Ave, Medford, MA, 02155
[email protected]
Abstract In this paper, we present a RF/FSO dual mode communication module formed by a Ka-band patch antenna and optoelectronic components. A LED and a PiN diode are bonded on the patch antenna as the front-end transceiver of the FSO communication channel. The patch antenna is modified to accommodate LED/PiN assembly with minimized crosstalk between the RF channel and the optical channel. Both the simulation and the experimental measurement validate that the interference between the Ka-band RF signals and the visible/infra-red optical signal is negligible. This new module can achieve 5 Gbps end-to-end FSO data transmission with RF channel backup. Introduction Recently, the Free Space Optical (FSO) communication draws more and more attention for its high data rate transmission with low power consumption and high security [1],[2]. However, the reliability of FSO data communication is limited by the line-of-sight alignment issue and atmospheric attenuation. Hybrid communication systems which integrate RF and FSO transmission have been proposed to increase the reliability and reduce power consumption in wireless communication nodes [3],[4]. The performance, architecture, algorithm and coding of RF/FSO hybrid system has been widely studied [5]-[10]. On the RF/FSO hardware development, we introduced a new packaging scheme based on the co-integration of optical elements within the geometry of planar antennas [11], [12]. This novel packaging structure increases the system availability of RF/FSO data transmission channel with negligible channel-to-channel crosstalk. However, our previous work mainly focuses on the RF/FSO module, built upon 10 GHz antenna system. With the recent development of high frequency RF circuits [13], [14] and on-chip antennas [15], [16], a RF/FSO system based on a higher frequency antenna becomes achievable. A Ka-band patch antenna with the radiator dimension of 3.35mm × 2.8mm and resonant frequency of 35 GHz is designed in this paper as a baseline for the miniaturized RF/FSO transceiver. The 35 GHz is chosen to utilize the low atmospheric attenuation of electromagnetic waves in the Ka band. Two rectangular bonding pads are added along the non-radiation edge of the antenna for optical element integration with low crosstalk. Full-wave EM wave simulation tool (HFSS) is used to optimize the radiation property of the revised patch antenna, and analyze the crosstalk between RF channel and FSO channel. A 660 nm LED is assembled on one of the bonding pads of the patch antenna to form the RF/FSO transmitter front-end. The antenna is attached to a 4-layer PCB, incorporating high-speed optical driver circuits for LED modulation. A GaAs PiN photodiode is integrated with a separate patch antenna to form the RF/FSO receiver module.
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The data rate of the RF/FSO transmitter and receiver modules are measured. A 5 Gbps FSO channel is achieved with a commercial transmitter and the RF/FSO receiver. However, the FSO transmitter can only operate up to 20 MHz due to the large rise time (100 ns) of the off-the-shelf LED chip. The inter-channel coupling between RF and FSO link is studied theoretically and experimentally. Simulation results demonstrate that the coupling from RF antenna to optical port is lower than -17 dB, and the optical devices have negligible effect on antenna radiation. Experimental results confirm that the RF signal does not affect the quality of FSO link eye diagram, and the antenna radiation characteristics are almost the same with or without optical device operation. Design of the patch antenna at Ka-Band The patch antenna at Ka-Band is designed on a Duroid 5880 substrate. The thickness of the dielectric material is 0.01 inch, and the dielectric constant is 2.2. The initial patch dimension is derived from the classic patch antenna theory [17]. Then, HFSS software is used to finely tune the parameters to achieve desired return loss and radiation pattern in Ka band. However, there are several challenges in designing high frequency antennas for RF/FSO system integration. The major difference between the traditional patch antenna and the antenna used in this RF/FSO system is the optical device bonding pads. In a 35 GHz RF system, the dimension of the bonding pads is not negligible compared to the patch radiator dimension. The bonding pads should not only provide enough space for optical device assembly, but also be tailored to suppress any additional resonance at Kaband. The second design challenge is that the impedance matching slot, which connects the transmission line and patch radiator, will disturb the TM010 electric field distribution in the antenna substrate. To address the above mentioned design challenges, HFSS is programmed to sweep around the initial patch dimension for an optimized solution. The return loss and radiation pattern are monitored during the iteration process. The final antenna dimension after HFSS optimization is shown in Figure 1. The size of the patch radiator is 3.35mm × 2.8mm. In addition, the optical device bonding pad is 1.2mm × 0.5mm, and the impedance matching slot is 0.8mm × 0.58mm. The distance between the bonding pads and patch radiator is 0.2 mm.
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To predict the coupling from the antenna port to the optical ports, a gold bonding wire is added on metal pads to the right of the patch radiator, as shown in Figure 1. The antenna fed port is named as port 1. Two 50 ohm optical ports (port 2 and port 3) are assigned along the right edge of the optical device bonding pads. Simulation result in Figure 3(a) shows that the effect of bonding wire to the antenna radiation pattern is negligible. The maximum gain is still along Z direction, and absolute amplitude is 5.938 dBi, which is similar to the radiation pattern in Figure 2(a). In Figure 3(b), the S parameters between different ports are plotted. The RFto-optical coupling (S21 and S31) is less than -17 dB at 35 GHz, which means less than 1.995% RF power will be coupled into optical channel at 35 GHz.
Figure 1: Schematics of the Ka band patch antenna on Duroid 5880 substrate.
The simulated radiation pattern and return loss of the 35 GHz patch antenna are shown in Figure 2. The maximum gain is 6.018 dBi along Z direction. The resonance frequency is around 35 GHz, and the 10 dB bandwidth is around 0.6 GHz.
(a)
(a)
(b) Figure 3: (a) Radiation pattern and (b) S parameters between different ports on the Ka band patch antenna with bonding wires.
(b) Figure 2: (a) Radiation pattern and (b) return loss of the Ka band patch antenna.
Antenna Fabrication and Optical Device Assembly The antenna is fabricated in a standard Class 100 cleanroom. The substrate Duroid 5880 board, which has copper layer on both surfaces, is used in this work. At first, a piece (1.25 inch ×1.25 inch) is cut from a RT/Duroid 5880 board. And then the stand photolithography is used to transfer the pattern from the mask to the top surface of the substrate. The Duroid 5880 sample is then attached on a 4 inch silicon carrier wafer for the spin coating process. Then the HDMS
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and Shipley 1813 photoresist are coated on the top surface of the Duroid 5880 board with an 8 inch headway spin coater. After this process, the sample is soft-baked for 1 minute and is exposed under Karl-Suss aligner. And finally, this sample is developed in PD523AD developer. It is observed under microscope that only antenna part is covered by photoresist. After the photolithography process, the unwanted copper is removed with a PCB etching solution from GM electronics. Kapton tape is used to protect the bottom copper layer (working as ground plane of the antenna) during the wet etching process. After etching, positive photoresist striper PRS-3000 is used to remove the photoresist on the top surface of the antenna. The LED chip used in this work is CC660-30 from Epitex, the size of which is 300 μm by 300 μm. This LED is bonded to the pads on the antenna by conductive epoxy adhesive (SEC 1233), in the same way as we presented in our previous paper [18]. The PiN diode (PX-CK11 from AXT Optoelectronics) has a dimension of 250μm by 460μm. The PiN diode is attached to the antenna and then its anode and cathode are bonded to the optical device bonding pads to the right of the patch radiator. Figure 4 shows the bonding of the PiN diode to the patch antenna.
Wire bonding
PiN
Figure 5: Measured patch antenna return loss
The radiation pattern of the 35 GHz antenna is measured in an anechoic chamber with NSI near field scanner. The antenna under test is placed 1.358 inch away from the probe header (horn antenna on the NSI scanner). The horn antenna is programmed to scan 5.7 inches along the horizontal direction and 5.7 inches along the vertical direction. The scan step in both directions is 0.163 inch. Therefore, there are total 36 × 36 =1296 sampling points. At each sampling point, the near-field electromagnetic field is measured. Discrete Fourier Transform algorithm is employed to derive far field pattern from the near field data. The far field radiation pattern for the 35 GHz patch antenna is shown in Figure 6. Both E plane and H plane fields have maximum gain perpendicular to the antenna substrate.
Figure 4: PiN diode is bonded to the Patch Antenna
The patch antennas are then attached to the PCB board. The RF/FSO transmitter is a 4-layer FR4 PCB board which integrates the optical driving circuits (SY88922V) and coplanar waveguides for impedance matching. The RF/FSO receiver PCB board integrates a high-speed transimpedance preamplifier (MAX3864), which amplifies the photocurrent signal and produces a differential voltage signal at the output. Measurement of the Patch Antenna at Ka-Band The return loss of the antenna on the RF/FSO transceiver board is measured by network analyzer, and shown in Figure 5. The measured return loss of the fabricated antenna is not fully aligned with the simulation data for several reasons. Firstly, there is subtle electromagnetic coupling between the antenna and the surrounding electrical elements (wires, metal pads and PCB). All those components could affect the impedance of the patch antenna. Secondly, the impedance mismatch introduced by the solder joint between K-connector and patch antenna has more significant effect at higher frequency range. Although the impedance of the antenna at 35 GHz is affected by the abovementioned factors, the measured return loss is still acceptable for RF/FSO application. The resonance frequency is around 35 GHz, and the 10 dB bandwidth is close to the simulation data.
Figure 6: measured patch antenna radiation pattern at 35 GHz.
The radiation pattern of the antenna on RF/FSO receiver board is also measured. As shown in Figure 7, there is no significant difference when optical device is turned on or off. However, the E plane field around -30o direction degrades. The distortion is due to the electromagnetic interaction between patch radiator and the surrounding radiating elements, such as the bonding wires, power supply pins, PCB traces, etc. This radiation pattern distortion will not affect the RF performance too much because the maximum gain direction is still along 0o direction. In addition to that, the RF link does not require perfect alignment as the optical link.
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(a) (a)
(b) Figure 9: 20Mbps eyediagrams for the FSO link when the patch antenna is fed with (a) 0 dBm and (b) 9 dBm RF power at 35 GHz
(b) Figure 7: Measured radiation pattern of the antenna on RF/FSO receiver at 35 GHz when (a) optical device is turned on, and (b) optical device is turned off
Measurement of the FSO link (a) RF/LED transmitter board.
First, the FSO link between the RF/LED transmitter board and a high-speed silicon photodetector is measured. The differential input of the LED driving circuit is fed with 214 -1 pseudorandom bit sequences (PRBS) generated by Agilent 81110A pulse pattern generator. The measured eye diagrams of the FSO data channel when the RF power is 0dBm and 9dBm are shown in Figure 8 and Figure 9, respectively. Although the theoretical speed could reach 100Mbps for this transmitter board, the actual data speed only reached 20Mbps due to the large rise time of the off-the-shelf LED chip. Moreover, it is shown from the eyediagrams that the cross talk between the RF signal and the optical signal is negligible. (b) RF/PiN receiver board The FSO channel of the RF/PiN receiver board is measured with a high speed commercial transceiver module, which is driven by a 27 -1 PRBS signal. A multimode fiber is used to guide the output beam of the commercial transmitter. The laser optical signal coming out of the fiber is collimated and focused to the RF/FSO receiver by two lenses. The schematic of the measurement is shown in Figure 10.
(a)
Figure 10: Schematic of the test for FSO link
(b) Figure 8: 10Mbps eyediagram for the FSO link when the patch antenna is fed with (a) 0 dBm and (b) 9 dBm RF power at 35 GHz
The signal from the RF/FSO receiver board is connected to the digital sampling oscilloscope (Agilent DCA-J 86100C). The waveform of the FSO link at 5Gbps with 0dBm and 9dBm Ka-band RF signal is shown in Figure 11. And also the eye diagram for the corresponding FSO links is shown in Figure 12. It could be drawn from these results that the Kaband RF signal has almost no interference with the optical signal.
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optical signal and the RF signal is greatly reduced and could be neglected. Experimental measurement confirms the theoretical prediction. A 5Gbps FSO data link is achieved in a 9dBm Ka-band RF environment. To our best knowledge, this is the first Ka-band RF/FSO hybrid communication module that has ever reported. Acknowledgments This work was supported in part by the National Science Foundation Smart Lighting Engineering Research Center (EEC-0812056) and New York State (NYSTAR C090145), and in part by NSF ECCS division (0824068).
(a)
(b) Figure 11: 5 Gbps transient waveform for the FSO link when the patch antenna is fed with (a) 0 dBm and (b) 9 dBm RF power at 35 GHz
(a)
(b) Figure 12: 5 Gbps eye diagram for the FSO link when the patch antenna is fed with (a) 0 dBm and (b) 9 dBm RF power at 35 GHz
Conclusions The paper demonstrates a new RF/FSO dual-mode indoor wireless communication module based on Ka-band patch antenna. An optimized Ka-band patch antenna is designed and integrated with optoelectronic devices on shared substrate. With the optimization work during the antenna design and hybrid package simulation, the interference between the
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