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The multiplexing transmission has always been a focus of attention for communication technology. In this paper, the radiation characteristics of circular microstrip patch antenna was firstly analyzed based on cavity model theory, and then spiral beams carrying orbital angular momentum (OAM) were generated, using elliptical microstrip patch antenna, with a single feed probe instead of a standard circular patch with two feedpoints. Moreover, by combining the proposed elliptic microstrip patch antenna with Universal Software Radio Peripheral (USRP), a wireless OAM transmission system was established and the real-time transmission of text, image and video in a real channel environment was realized. Since the wireless OAM transmission has the advantage of good safety and high spectrum utilization efficiency, this work has theoretical significance and potential application.
© 2016 Optical Society of America
1. Introduction
Light has wave-particle duality. Therefore optical orbital angular momentum (OAM) has benefited applications ranging from optical manipulation to quantum information processing and optical communication. However, application of OAM at radio frequency are seldom found. In 2012, Tamburini et al. [1] generated OAM using spiral parabolic antenna and achieved the wireless OAM multiplexing transmission on the same frequency for the first time, then the related field has become a research hotspot [2–5]. Unlike the orbital angular momentum of light [6,7], there has always been a deb ate on the study of wireless OAM transmission. For example, Edfors et al. [8] pointed out that the wireless OAM is just a particular case of MIMO and there are no essential differences between them, and thus research on wireless OAM transmission has no significance. Andersson et al. [9] concluded that the channel capacity can't be increased through OAM according to Weyl theory. Meanwhile, they proved that the transmission power of wireless OAM transmission at is 800 times that of the traditional wireless transmission under the same received power and transmission distance, and the face-to-face wireless OAM transmission is not suitable for broadcasting applications. This is due to the fact that amplitude of the fundamental mode with l = 0 is strong at the center, but the amplitude of the mode with l = ± 1 vanishes at the center. Despite the long-standing controversy, research enthusiasm has never been dampened. Recently, Mari et al. [10] experimentally demonstrated the near-field OAM channel is separable, this suggests that OAM can at least be applied to short-range wireless transmission system with high bitrate. Zheng et al. [11] proposed a partial receiving scheme to solve the large-aperture problem of receiving antenna of wireless OAM transmission. Berglind et al. [12] confirmed that metallic waveguides can propagate OAM modes. Therefore, research on wireless OAM transmission has not only theoretical significance but also great application prospects due to its good safety and high spectrum utilization efficiency.
To the best of our knowledge, the current study on the wireless OAM transmission system mainly focus on the generation of OAM beam, and there are few reports related to the experimental research of transmission system. In this paper, a simple elliptical microstrip patch antenna is used to generate and receive the OAM spiral beams in the near-field. Furthermore, we built an integral wireless OAM transmission system based on the proposed antenna and USRP [13], and realized the real-time transmission of text, images and video in a real channel environment.
2. Transmission system and model analysis
Figure 1 shows the schematic diagram of the wireless OAM transmission that consists of the transmitting and receiving ends mainly including USRP, coaxial cable, OAM transmitting and receiving antennas, and personal computer(PC) installed with Labview software. Transmission system is described as follows: in the transmitting end, Labview-based PC program convert the content needs to send into binary data. Then, the data proceeded by burstification and modulation is sent to USRP through Gigabit Ethernet port for conducting digital to analog conversion, up-conversion and other operations. Finally, Radio Frequency (RF) signals are transferred to OAM antenna via Sub Miniature Version A (SMA) connector and coaxial cables, and the spiral beams propagate to the receiving antenna through free space. At the receiving end, the elliptical microstrip patch antenna captures the signal and accomplishes a signal processing procedure substantially opposite to that of the transmitting end, and the transmission content is ultimately displayed on PC.
The patch antenna is a microstrip patch which consists of a metallic patch placed on a grounded dielectric substrate. Considering the model complexity and calculation accuracy, the model analysis is usually carried out based on cavity model theory. The approximate model is constructed by taking the dielectric region between the microstrip patch and the grounded substrate as a dielectric resonator. Resonant frequencies of TMnm modes in the circular disk are given as:
where C is the speed of light, εr is the relative permittivity of the substrate, is the radius of the circular microstrip patch, Xnm is the mth solution of Bessel function of order n, n represents the number of angular mode, and m denotes the number of radial mode [14]. Figure 2(a)-2(c) respectively shows the electric field distributions of three typical modes TM21, TM31 and TM41. As can be seen from Fig. 2, 2n nodes and antinodes of the electric field distributes regularly along the circle, and the modes are mutually orthogonal.For the circular microstrip patch antenna, two feedpoints with proper angular spacing and phase difference can generate phase-continuous modes which are orthogonal to each other [15], so that the circularly polarized waves are produced. The x and y components of the radiated field in such case can be expressed as [16]:
It can be seen from Eq. (2) that the x-component and y-component of the radiated field consists of phase evolutions of the forms and , which are the general characteristic of the spiral beams. According to Eq. (2), the phase distribution of x and y components of radiated field for three kinds of circularly polarized waves can be calculated and shown in Fig. 3. Obviously, the phase distribution exhibits good periodicity and the period is 2, 3 and 4, respectively. It should be pointed out that the results exactly corresponds to the OAM modes 2, 3, and 4.
Fig. 3 Phase distribution of x and y components of the radiated eld in the case of modes (a) 2. (b) 3. (c) 4.
3. Elliptical microstrip patch antenna for generating OAM spiral beams
A simple method for generating OAM is using a circular patch antenna with two feed-points, of which a schematic graph is shown in Fig. 4. The angular between the feed-points is set to be 45°. Simulation based on the High Frequency Structure Simulator (HFSS) is performed to validate the performance of the antenna. The input signals for the two feed-points have the same amplitude and 90° phase difference, while the working frequency is set to be 2GHz. Figure 4(b) depicts the electric field distribution and Fig. 4(c) displays the corresponding far-field phase distribution. It is shown that the simulation results are in good agreement with the theoretical results. It is noted that we can use a power divider [17] to provide the input signals with the same amplitude and 90° phase difference, but the feed distribution network will become complicated.
Fig. 4 Numerical simulation. (a) A schematic view of the circular patch antenna with two feed-points. (b) Electric field distribution. (c) Far-field phase distribution.
Interestingly, we found that through adjusting the circular patch shape and introducing geometry perturbation, we can get an elliptical microstrip patch antenna that can generate OAM spiral beams with a single feed. Figure 5 shows the top view and side view of the proposed antenna. It is made with an elliptical metallic patch, a dielectric substrate and a microstrip grounded plate. A 50Ω coaxial probe-fed is used at position P. The Dielectric substrate is chosen as FR4_epoxy (εr = 4.6, tanδ = 0.02), and the other geometry parameters are d = 100mm, h = 1.6mm, a = 72mm, b = 63.4mm, x = 9.4mm and y = 22.6mm.
Figure 6 portrays the simulated reflection coefficient (S11). As can be seen from the figure, the return loss in the vicinity of 2GHz is less than −15 dB, which represents the antenna, has an excellent impedance matching performance.
Figures 7(a) and 7(b) depict the electric field distribution at the cross section of the dielectric substrate under elliptical microstrip patch and the far-field phase distribution, respectively. It is seen from Fig. 7(b) that the spiral beams have periodicity and the period is 2. Figure 7(c) shows the 3D radiation pattern. Apparently, electric field vanishes at the center, which is in line with the characteristics of OAM spiral beams. The simulation results agree well with the theoretical analysis, with further confirms the effectiveness of the proposed method.
Fig. 7 Simulation results. (a) electric field distribution. (b) far-field phase distribution. (c) 3D radiation pattern.
4. Experimental results and discussion
We set two antennas face to face and keep them in parallel, as shown in Fig. 1. One functions as transmitting antenna, and the other one acts as receiving antenna. For a variation of transmitting distance, the forward transmission coefficient (S21) between the transmitting and receiving antennas at 2 GHz is simulated, as shown in Fig. 8. Since some fluctuations exist in the simulation results, least square method is adopted to fit the data. The fitting curve is denoted by the red dotted line in Fig. 8. Results show that S21 decreases with the increase of the distance when the distance is less than 4(is free space wavelength), it reaches a relatively stable state between 4and 8, then decreases sharply when the distance is greater than 8, which is in accord with the OAM theory.
Figure 9 shows the wireless OAM transmission system based on elliptical microstrip patch antenna. This system is realized by using the software defined radio USRP as the transmitter and the receiver. In Fig. 9, ① is elliptical microstrip OAM antenna (see real object in Fig. 10), ② is the USRP, ③ is the rack-mount server. LabVIEW2012 software and programs (i.e., Transmit.vi and Receive.vi.) are installed in the two ASUS rack-mount servers (CPU: Intel(R) Core i7-4790 3.6GHz, RAM: 16.0GB, 64-bit operating system). Moreover, the Modulation Toolkit, NI Vision Acquisition Software (With IMAQdx support) and NI Vision Devemlopment Module are needed. In this experiment, USRP N210 (Frequency Range: 50MHz-2.2GHz, Bandwidth: 20 MHz) is used.
The transmission experiment is carried out as the following steps. First, we connect the hardware. In the transmitting end, we connect server host to USRP via Gigabit Ethernet port and USRP's RF1 port to OAM antenna via coaxial cable. In receiving end, we connect server host to USRP via Gigabit Ethernet port and USRP's RF2 port to OAM antenna via coaxial cable. Second, the parameters of USRP are configured. We open the Transmit.vi on the transmitting end server and Receive.vi on the receiving end server, and the two USRPs are set on two different IP addresses. The transmitting format and receiving format is set to “image”, and the remaining parameters are consistent. IQ sampling rate, carrier frequency, gain and modulation mode are 2MHz, 2GHz, 20dB and 8PSK, respectively. Finally, after selecting the picture need to transmit on user interface of the transmitting end, we run the programs, then the received picture will display on the user interface of receiving end immediately.
Figure 11 shows the experimental result of picture transmission, in which Figs. 11(a) and (b) are the transmitted and received picture. It is obvious that the received picture has no distortion compared to the transmitted one. Hence the system reliability at short-distance data transmission is clearly verified. As a matter of fact, we can also achieve the real-time transmission of text and video based on this system. The principle and operation are similar, and it is not included here for brevity.
5. Conclusion
In this paper, resonance characteristic of circular microstrip patch antenna was firstly analyzed based on cavity model theory, and then the electric field distribution and the spiral phase distribution of resonator with two feedpoints for different modes are obtained through theoretical calculation. Mode 2-based modeling and simulation is taken as an example to validate the theoretical analysis. Moreover, OAM spiral beams with a single feedpoint are generated by using the elliptical microstrip patch instead of the circular patch. Finally, we establish a wireless OAM transmission system [18] by combining the presented elliptic microstrip patch antenna with USRP, and realize the real-time transmission of text, images and video in a real channel environment. Since frequency congestion has become a bottleneck of wireless communication, this work may raise the passion of OAM theoretical study and promote the application of wireless OAM multiplexing transmission at radio frequency band.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 61161007, 61261002, 61461052, 11564044), the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20135301110003, 20125301120009), and the Key Program of Natural Science of Yunnan Province (Grant Nos. 2013FA006, 2015FA015). J. J Yang acknowledges support from the Young Academic and Technical Leader Reserve Personnel Training Plan of Yunnan Province (Grant No. 2014HB002).
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