Abstract. 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.
Key words: RF/FSO, Dual Mode, Optoelectronic
1.Introduction
Recently, free space optical (FSO) communication draws more and more attention, its high data rate transmission low power consumption and high safety [1], [2]. However, FSO the reliability of the data communication is limited by the sight at the problem, atmospheric attenuation. Mixed communication system integration RF and FSO transmission are put forward to improve reliability and reduce power consumption wireless communications nodes [3] and [4]. Performance, structure, algorithm and coded RF/wireless optical communication system has been widely research mixed [5]-[10]. FSO/RF hardware development, we introduced a new packaging scheme based on co-integration relationship in the geometric optical element planar antennas [11], [12]. This novel packing structure increases the system availability of RF/wireless communication channel-transmission channel to-channel data and insignificant crosstalk.
2.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 [13]. 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 Ka-band. 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.
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.
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 RF-to-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.
3.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 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 [14]. 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.
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.
4.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.
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.
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.
5.Measurement of the FSO link
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.
6. Conclusion
Demonstrates a new indoor wireless RF/FSO two-way communication module based on ka-band patch antennas. Optimization design and ka-band patch antennas optoelectronic devices on Shared with substrate. , optimize work, the antenna design and mixed package simulation, optical signal the interference between RF signal and reduced greatly, can be neglected. The experiment test confirmed the theoretical prediction. 5 GBPS FSO link is obtained data were RF environment 9 ka-bands.
7.References
[1]J.M.Kahn and J. R. “Wireless infrared communication,” Proc. IEEE, vol. 85, no. 2, pp. 265-298, 1997.
[2]T. Rokkas, T. Kamalakis, D. Katsianis, D. Varoutas, and T. Sphicopoulos, “Business prospects of wide-scale deployment of free space optical technology as a last-mile solution: a techno-economic evaluation,” J. Optical Networking, vol. 6, no. 7, pp. 860-870, Jul. 2010
[3]S. D. Milner and C. C. Davis, “Hybrid free space optical/RF networks for tactical operations,” IEEE Military Communications Conf., vol.1, Oct.-Nov. 2009, pp. 409- 415.
[4]T. Kamalakis, I. Neokosmidis, A. Tsipouras, T. Sphicopoulos, S. Pantazis, and I. Andrikopoulos, “Hybrid free space optical/millimeter wave outdoor links for broadband wireless access networks,” The 18th Annual IEEE Int. Symposium on Personal, Indoor and Mobile Radio Communications, Sep. 2010, pp. 1-5.
[5]J. Llorca, A. Desai, E. Baskaran, S. Milner, and C. Davis, “Optimizing performance of hybrid FSO/RF networks in realistic dynamic scenarios,” Proc. SPIE, vol. 5892, 589207, Dec. 2005
[6]S. Sivathasan and D.C. O"Brien, “RF/FSO Wireless Sensor Networks:A Performance Study,” IEEE Global Telecommunications Conf., pp.1-5, Nov.-Dec.2008
[7]A. Akbulut, H. G. Ilk, and F. Ari, “Design, availability and reliability analysis on an experimental outdoor FSO/RF communication system,” Proc. of 7th Int. Conf. on Transparent Optical Networks, vol. 1, Jul. 2005, pp. 403 ? 406.
[8]A. Hashmi, A. Eftekhar, S. Yegnanarayanan, and A. Adibi, “Analysis of optimal Adaptive Optics system for Hybrid RF-wireless optical communication for maximum efficiency and reliability,” the 4th International Conf. on Emerging Technologies, pp. 62-67, Oct. 2008
[9]J. P. Dodley, D. M. Britz, D. J. Bowen, and C. W. Lundgren, “Free Space Optical Technology and Distribution Architecture for Broadband Metro and Local Service,” Proc. SPIE. vol. 4214, pp. 72-85, 2001.
[10]S. Vangala and H. Pishro-Nik, “A Highly Reliable FSO/RF Communication System Using Efficient Codes,” IEEE Global Telecommunications Conf., pp. 2232-2236, Nov. 2007
[11]J. Liao, S. Deng, K.A. Connor, V. Joyner, and Z.R. Huang, “Antenna integration with laser diodes and photodetectors for a miniaturized dual-mode wireless transceiver,” Proc. 58th IEEE Electronic Components and Technology Conf., pp. 1864-1868, May 2008
[12]J. Liao, J. Zeng, S. Deng, A. Boryssenko, V. Joyner, and Z. R. Huang, “Packaging of dual-mode wireless communication module using RF/optoelectronic devices with shared functional components,” IEEE Trans. on Advanced Packaging, vol. 33, pp. 323-332, May, 2010.
[13]Y. Su, J. J. Lin, and K. K. O, “A 20GHz CMOS RF down-converter with an on-chip antenna,” IEEE Proc. of ISSCC, vol. 1, pp. 270-597, Feb. 2005
Guan and A. Hajimiri, “A fully integrated 24GHz 8-path phased-array receiver in silicon”, IEEE Int. Solid-State Circuits Conf., vol. 1, pp. 390- 534, Feb. 2004