Multi-input multi-output (MIMO) transmission of two millimeter-wave radio signals seamlessly converted from polarization-division-multiplexed quadrature-phase-shift-keying optical signals is successfully demonstrated, where a radio access unit basically consisting of only optical-to-electrical converters and a radio receiver performs total signal equalization of both the optical and the radio paths and demodulation with digital signal processing (DSP). Orthogonally polarized optical components that are directly converted to two-channel radio components can be demultiplexed and demodulated with high-speed DSP as in optical digital coherent detection. 20-Gbaud optical and radio seamless MIMO transmission provides a total capacity of 74.4 Gb/s with a forward error correction overhead of 7%.
© 2012 Optical Society of America
To enhance network resilience in access networks, including the “last-mile” section, direct and seamless conversion between optical and radio signals is an emerging issue. This is because nowadays our daily life has come to depend on on-line services due to both the popularity of the Internet and the outstanding growth of mobile technology and many people try to access various information through their handhelds or phones anytime and anywhere regardless of whether network is working normally or not. Along with this tendency, at this time, optical fiber communication technology may only be able to aggregate considerable amount of data traffics even in access networks. However, high-speed connections based on an optical fiber such as a fiber to the home cannot always be deployed everywhere because there are various limitations such as geographical condition, economical balance, provider’s strategy, and damage situation in the case of disasters. Therefore, we have to consider radio transmission link for aggregating large network traffics, which has prior characteristics in system deployment, such as flexible arrangement and easy installation. From the technical point of view, it has been desired that two physical characteristics of wired (optical fiber) and wireless links, that is, potentially large capacity of optical fiber links and ad interim deployment nature of radio links, should be synergistically integrated in order to enable agile provision of maximum capacity. Therefore, a high-speed radio transmission link comparable to the optical fiber communication has been in great demand, as it would enable direct and seamless connectivity to the optical network. Moreover, the technology of a high-speed radio link connected to the optical fiber will protect the optical fiber network against the fiber being cut. Especially in Japan, many communication network facilities, such as communications buildings and base transceiver stations for a mobile phone, suffered great damage from the Great East Japan Earthquake of March 11, 2011 [1, 2]. Communications and electric power equipment were also submerged by the tsunami triggered by the earthquake. In addition, washed away or broken telephone poles and bridges due to the tsunami, ground liquefaction, and ground subsidence, causing severance of aerial and trunk cables for not only communication lines but also power lines, leading to an out of service state. Surprisingly, although underground duct lines were water-damaged, most of fiber cables survived. Reuse of surviving optical fibers can thus help recovering communication lines quickly. In this situation, a high-speed radio link would help in quickly deploying a broadband connection with the surviving optical fiber . During disaster recovery, surveillance of damaged structures as well as remote emergency medical care require a broadband connection for transferring high-resolution images and videos. In this scenario, a high-speed radio link with agile deployment capability connected to the surviving optical fiber may be a promising solution; however, conventional fixed wireless access (FWA) equipment does not have enough capacity and connectivity to the optical fiber network.
Millimeter-wave (MMW) radio is a possible candidate for a high-speed radio link based on a radio-over-fiber (RoF) technology: direct and seamless conversion is possible with this technology, particularly with a direct photonic up-conversion technique [4–8]. Moreover, multi-input multi-output (MIMO) technique is imperative for the realization of high capacity and diversity in advanced radio transmission. Meanwhile, an optical digital coherent detection technology with polarization diversity is a promising candidate for a high-speed optical transport system: digital coherent detection with an advanced modulation format such as quadrature phase shift keying (QPSK) and polarization division multiplexing (PDM), which is a kind of 2 × 2 MIMO, can provide an optical transport system in the 100-Gb/s class . For direct conversion between these MIMO-based optical signals and the advanced radio system with MIMO, some type of a media converter with DSP is necessary because the converter should be able to perform MIMO coding and decoding with a serializer/deserializer for optimization of the radiated radio signal form. Therefore, DSP during conversion would increase energy consumption and latency because conventional media conversion is based on a silicon-based very large scale integrated circuit technology. On the other hand, some 2 × 2 MIMO radio transmission systems directly connected to two optical PDM signals at the frequency less than 60 GHz have been reported [10,11]. However, they are difficult to further increase the transmission distance because 60 GHz band with high propagation loss is selected.
In this study, we demonstrate the proof of concept of seamless conversion of a PDM-QPSK optical signal to an MMW 2 × 2 MIMO radio signal. An RoF transmitter (Tx) forms a “radio-friendly” optical signal with high accuracy based on a precise optical modulation technique. A polarization-diversity optical-to-electrical (O/E) converter equipped with a radio access unit (RAU) can generate two-channel radio signals from the PDM-QPSK signals without any DSP at the RAU. Commonly used optical digital coherent algorithms are applicable to the demodulation of this MIMO radio transmission as well as compensation for transmission impairments of an optical fiber link including a radio link. 2 × 2 MIMO radio under 2.3-m transmission is successfully demonstrated with a power penalty of 3 dB owing to power interference from two antennas. W-band (75–110 GHz) MMW signals can be used for applications that access the network because of its low atmospheric attenuation (less than 1 dB/km) and the available bandwidth of 35 GHz .
2. Concept of Optical and Radio Seamless MIMO Transmission System
Figure 1 shows a schematic illustration of the conventional and proposed optical and radio seamless MIMO transmission systems. In a conventional transmission system, shown in Fig. 1(a), an RAU capable of media conversion comprises a deserializer (Des.) including polarization demultiplexing and demodulation, a MIMO coding block with a DSP, and a radio front-end (FE) consisting of filters and amplifiers, with an antenna pair. From an optical Tx, a PDM signal is transmitted over the optical fiber and then launched into the O/E conversion block. The Des. provides slow-speed electrical signals with large parallelism because the DSP clock speed of around 1 GHz is much slower than the symbol rate of several tens of Gbaud. The Des. and DSP perform impairment compensation for optical fiber transmission and optimization of the signal form for radio radiation. Generally, the bandwidth of the radio signal is much narrower than that for the optical signal, and thus, media conversion with a modulation format conversion is necessary with MIMO signal preparation. In this sense, the DSP functionality would increase the energy consumption of the RAU.
To reduce energy consumption, an optical and radio seamless MIMO transmission system based on a RoF technique, shown in Fig. 1(b), is proposed. The system comprises an RoF Tx, an RAU with a polarization-diversity O/E converter and a radio FE, and a DSP-aided coherent radio receiver (Rx). The PDM RoF signal generated at the RoF Tx is converted to a two-channel radio signal directly at the O/E converter. It should be noted that the signal form including the modulation format does not change. Moreover, the mixture of the polarization of the signal transmitted over the optical fiber may reflect the MIMO channels of the radio signal. The DSP-aided radio Rx can perform demodulation of the 2 × 2 MIMO radio signals with impairment compensation for both the optical and the radio links. In this configuration, DSP is provided only at the Rx, and thus, the number of digital signal processors in the system is reduced as compared to that in the conventional system (Fig. 1(a)).
The carrier frequency of the radio signal should be adequately assigned depending on its application due to a trade-off relationship between the capacity and transmission distance. For example, a microwave (MW) signal can be delivered to a large area with a diameter of larger than 5 km despite the use of an omnidirectional antenna owing to its ultra-low atmospheric attenuation. However, the available bandwidth is much narrower than 1 GHz because of its extremely limited bandwidth due to radio regulations. On the other hand, MMW or higher carrier frequency is a possible candidate for high-capacity radio transmission because its available bandwidth is more than several gigahertz. However, the attenuation coefficient is greater than that of MW signals in general. W-band (75–110 GHz) has a relatively lower attenuation coefficient than 1 dB/km, and thus, it is expected that a W-band power amplifier would easily be able to extend the transmission distance to several kilometers [12, 13].
As considered for an uplink, implementation of optical transmitter components with a function of the RAU can be easy because required components such as an electrical LO are already equipped at the Rx, that is, it is easy to configure an optical and radio seamless transceiver based on a combination of the RoF Tx and the MMW radio Rx. On the other hand, a radio-to-optical signal conversion at the RAU is an indispensable feature. To solve this issue, the combination of a high-speed optical modulator connected to the antenna unit and an optical filter for suppression of unnecessary optical components can regenerate the baseband optical signal from the received radio signal . The uplink transmission would be feasible with this method as the converted optical signal could be demodulate by the conventional optical coherent detection technique .
3. Experimental Setup
Figure 2 shows the experimental setup for seamless conversion and transmission between a PDM optical signal and an MMW MIMO radio signal. At an RoF Tx, a 92.5-GHz-spaced two-tone optical signal generator consisting of a Mach-Zehnder modulator with an electrical synthesizer operated at 23.125 GHz that outputs only the positive and negative second-order optical components was connected to an arrayed waveguide grating (AWG) to separate the upper-sideband (USB) and lower-sideband (LSB) components [6, 7, 15]. The LSB was launched into an optical in-phase/quadrature (IQ) modulator to generate a QPSK signal with a two-channel pulse pattern generator (PPG) operated at 10 and 20 Gb/s using a pseudo-random bit sequence (PRBS) with a length of 215−1, after an erbium-doped fiber amplifier (EDFA). The optical signal was formed by an optical band-pass filter (OBPF) to suppress the undesired side lobes, taking the following desired radio spectrum into account. The optical QPSK signal was launched into a PDM emulator, which consisted of a 40-m optical delay line corresponding to 4,000 symbols for 20 Gbaud to decrease the pattern correlation between two polarization components, a polarization controller (PC), and a polarization beam combiner (PBC). The LSB component was used as an optical reference for photo-mixing. The LSB and USB components, which were the PDM-QPSK signal and the optical reference signal whose polarization was set at 45° from the x-axis by a PC, respectively, were combined by an optical 3-dB coupler to generate a PDM-QPSK RoF signal. The RoF signal, with its optical power of 8 dBm amplified by the EDFA, was transmitted over a 20-km standard single mode fiber (SMF). At the RAU, the received RoF signal was introduced into a polarization-diversity O/E converter, which comprised two O/E converters followed by a polarization beam splitter (PBS), to split the x-axis- and y-axis-aligned polarization components [5, 10, 11]. The split signals were launched into a uni-traveling-carrier photodiode (UTC-PD), which was utilized as a photo-mixer, and passed through the EDFA and OBPF to generate W-band signals from a beat note between the USB and LSB optical components using a direct photonic up-conversion technique . The W-band signals were radiated from 24-dBi-gain pyramidal horn antennas directly connected to the UTC-PDs for each optical polarization component, independently. A power amplifier (PA) set between the UTC-PD and antenna was also used for evaluating the transmission distance extension. The distance between these Tx antennas was 5 cm. The radio signals were received through free space at a receiver (Rx) located at a distance of 0.9 m without the PA and 2.3 m with the PA. The separation between the Rx antennas, whose gain was the same as the Tx antenna gain, was also set at 5 cm because the 3-dB overlap area of the signals from the Tx antennas was estimated to be 7.6 cmϕ at the Rx antenna facet under the 0.9-m transmission distance condition. At the Rx, the received signals were down-converted by a W-band double-balanced mixer (DBM) connected to an electrical local oscillator (LO) operated at a frequency of 75 GHz without the PA, which was optimized for 20-Gbaud operation, and 77.5 GHz with the PA, respectively. The regenerated intermediate frequency (IF) signals amplified by an IF amplifier were acquired by a real-time digital oscilloscope whose sampling rate and bandwidth were 80 GSa/s and 30 GHz, respectively, and which worked as an analog-to-digital converter (ADC). The digitized signals for the two-channel received components were down-converted to the baseband in the DSP and were then demultiplexed to each polarization component with a butterfly-structured fractionally spaced constant modulus algorithm (FS-CMA). The demultiplexed signals were demodulated with phase noise suppression, re-timing, downsampling, overlap-frequency domain equalization (O-FDE), and a bit error rate (BER) test in a similar manner as in the off-line optical digital coherent technique [6, 7].
Polarization of the antennas for 2 × 2 MIMO radio plays an important role in space division multiplexing (SDM). The orthogonally aligned polarization condition has low cross-talk with a highly directive horn antenna: the interference between two-channel signals becomes negligible. However, full-duplex transmission based on a polarization division duplex (PDD) is not compatible with the PDM technique. On the other hand, parallel antenna polarizations increase the interference, resulting in the generation of some performance penalties. We also evaluate the two conditions to check the availability of PDD transmission with 10-Gbaud QPSK signals.
4. Demonstration and Results
The optical spectra of the Tx signals are shown in Fig. 3(a). An optical reference component at a wavelength of 1549.87 nm and a PDM-QPSK-modulated component at 1550.62 nm are clearly observed. The observed separation between these components is equivalent to the operational frequency of the two-tone generator of 92.5 GHz. The spectrum structure sharply cut at the side lobe is successfully provided by the OBPF, whose bandwidth is approximately 50 GHz, for suppression of the imaging component generated by the DBM. Figure 3(b) shows the fast Fourier transformed (FFTed) power spectra of the received IF signals after the ADC. It should be noted that the spectra were observed without the PA between a UTC-PD and an antenna under an orthogonal polarization antenna configuration. The observed IF spectrum for 20 Gbaud in the middle panel of Fig. 3(b) was similar to an estimated IF spectrum with the observed optical spectrum (Fig. 3(b) top). It should be noted that the minimum required bandwidth for a 20-Gbaud signal is 20 GHz. In this experiment, the edge frequencies of that bandwidth are 7.5 and 27.5 GHz, respectively, because the center frequency of the radio signal of 92.5 GHz corresponded to an IF frequency of 17.5 GHz. The ratio between the desired signal and the imaging component at the IF frequency of 7.5 GHz is estimated to be greater than 20 dB in the optical region. Therefore, a 20-Gbaud signal can be demodulated with sufficient suppression of the imaging components [16, 17]. Obviously, a 10-Gbaud signal is well acquired without any imaging components.
4.1. Orthogonally Aligned Antenna Polarization Condition
First, we evaluated the 2 × 2 MIMO seamless conversion from the PDM-QPSK signals with a different antenna polarization configuration, i.e., the orthogonally aligned antenna polarization condition. The antenna was set to a vertically aligned (V) polarized configuration: the other antenna was horizontally (H) aligned. Under this configuration, the cross-talk between these differently aligned antennas was approximately −30 dB, although the signal overlap at the facet of the received antenna was as described above.
The demodulated constellation diagrams and BER curves are shown in Fig. 4. For a 10-Gbaud signal, clear separations of the QPSK symbols are shown. It should be noted that the output radio power of each UTC-PD was approximately −11 dBm, corresponding to the optical power launched into the BS of −4 dBm. This indicates that the total output power of radio signals was approximately −8 dBm—the sum of the powers of the two antennas. The square bundled symbol constellation for the 20-Gbaud signal and the difference between the slopes of the BER curves for 10 Gbaud and 20 Gbaud can be caused by the bandwidth issue of the ADC described above and the bandwidth of the W-band components such as the DBM. Degradation can be reduced by the development of high-speed and broad-bandwidth ADCs and DBMs . The observed BERs for both 10 and 20 Gbaud transmission were within the 7% FEC limit of the BER of 2 × 10−3: the resultant bit rates for 10 and 20 Gbaud were 37.2 and 74.4 Gb/s, respectively [18, 19].
4.2. Parallel-Aligned Antenna Polarization Condition
A PDD system is a possible candidate for full-duplex radio communication especially in an FWA [20, 21]. An effective spectral efficiency would be larger than that for conventional time division duplex and frequency division duplex systems because one polarization can be used for only one way in orthogonally aligned polarization configuration. To evaluate the capability of MIMO transmission under a parallel-aligned antenna polarization condition, we investigate the effect of imperfection of signal separation, comparing with that in the case of the orthogonally aligned polarization condition. Radiated signal from two parallel-aligned antennas at the transmitter-side RAU can induce the interference at the receiver-side RAU. Thus, we used a set of PAs between a UTC-PD and an antenna at the RAU, with a transmission distance of 2.3 m, to evaluate the interference effect.
The observed BERs for the orthogonally aligned antenna polarization and parallel-aligned conditions are shown in Fig. 5(b). For the orthogonally aligned condition, which indicated that one antenna was set to V and the other to H, similar behavior as that shown in Fig. 4(a) was observed. In Fig. 4(a), the output power of each UTC-PD was estimated to be less than −21 dBm, corresponding to the optical power of −15 dBm. The estimated propagation loss difference between 0.9 m and 2.3 m using a Friis propagation equation and the gain of the PA were approximately 8 dB and approximately 12 dB, respectively. Thus, the output power of −4 dBm with a PA gain of 12 dB and a propagation loss difference of 8 dB is consistent with the output power without the PA of less than −21 dBm. In Fig. 5(b), the observed BERs were within the FEC limit, and thus, 37.2-Gb/s QPSK transmission was demonstrated under an error-free condition with an FEC overhead of 7%. The observed power penalty between the orthogonally and parallel-aligned conditions was approximately 3 dB. This value seems to be consistent with the estimation owing to the radio power interference between two radio antennas for the RAU. Therefore, 2 × 2 MIMO radio transmission under the parallel-aligned antenna polarization condition would be capable of doubling the capacity using a PDM in the radio section without an SDM configuration as well as for full-duplex communication with the PDD.
To increase a total capacity, application of higher-order multi-level modulation such as a quadrature amplitude modulation (QAM) and an orthogonal frequency division multiplexing (OFDM) technique is a possible candidate because the bandwidth of the electrical components of a W-band mixer, an amplifier and an ADC was limited. However, in the MIMO system, a signal cross talk from the other channels might degrade an effective resolution of the ADC as the high resolution is required for demodulation of QAM and OFDM signals. Therefore, the orthogonally aligned antenna configuration is suitable because of its low cross talk feature in the radio section. In addition, transmission distance is an important issue for an access network including an FWA configuration. Numerical and experimental evaluation with a high-gain antenna were reported with the possible transmission distance longer than 1 km . Thus, the high-gain antenna with a high-power amplifier set at the RAU will help an extension of the transmission distance to realize the high-capacity FWA.
We demonstrated 74.4-Gb/s optical and radio seamless MIMO transmission with an FEC overhead of 7%. 10–20-Gbaud DP-QPSK optical and 2 × 2 MIMO radio signals were transmitted over 20-km SMF and 0.9–2.3-m free space, respectively. A commonly used optical digital coherent detection technique for polarization diversity such as the butterfly-structured CMA can demodulate 2 × 2 MIMO radio signals with a high symbol rate of up to 20 Gbaud. This DSP-aided optical digital coherent detection technique is applicable for “last-mile” high-speed radio transmission seamlessly connected to optical access networks as well as to a 40-Gb/s-class full-duplex link.
Y. Yoshida, Y. Yasumura, and K. Kitayama are thankful for the financial support received from the “Agile Deployment Capability of Highly Resilient Optical and Radio Seamless Communication Systems” program of the commissioned research of the National Institute of Information and Communications Technology (NICT).
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