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Abstract
This paper describes the potential of analog radio-over-fiber (RoF) and intermediate-frequency-over-fiber (IFoF) transmission technology for the future mobile fronthaul (MFH). The background and approach of the analog RoF-based MFH combined with the beamforming technique are presented. We developed an antenna module by integrating sixteen sets of cascaded photodiode, RF amplifier and a patch antenna as a compact wireless transmitter. Three millimeter-wave (MMW) beams were successfully transmitted from the antenna module with beam separation of 30° by photonic controlled beam steering. The experimental results are presented to show the feasibility of a multi-user multiple-input multiple-output (MIMO) scenario.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the rapidly-growing requirement for data streaming from versatile mobile services and high-speed applications, the mobile network system has evolved to fifth-generation (5G) with a maximum throughput of up to 20 Gbps in its final phase [1–3]. Mobile services such as enhanced mobile broadband (eMBB) services, ultra-reliable and low-latency communication (URLLC) services, and massive machine-type communication (mMTC) services will be supported in 5G mobile systems [4]. The centralized radio access network (C-RAN) architecture, which has already been introduced from the current long-term evolution (LTE) system so as to control multiple antenna cells cooperatively, is essential for providing such versatile services in 5G and beyond [5,6].
Mobile fronthaul (MFH), defined as the connection between the central office (CO) and antenna sites, requires high transmission efficiency for a 5G mobile system. As a result, a conventional digital fiber interface such as the common public radio interface (CPRI) based on digital radio-over-fiber (RoF) will be difficult to adapt, since it requires a transmission capacity greater than ten times that of the user throughput [7].
In Ref. [8], a wideband intermediate-frequency over fiber (IFoF) transmission, in which multiple IF channels were frequency division multiplexed (FDM), was experimentally verified to show a notable capacity expansion of analog-based MFH scheme over a single wavelength. The result indicates the potential of FDM as one of the promising multiplexing methods other than those in optical domain such as wavelength division multiplexing (WDM) or space division multiplexing (SDM). However, the paper focused mainly on optical transmission part, and the total system performance was not discussed. Reference [9] reported the transmission performance with IFoF-based MFH of 7-km single-mode fiber (SMF) in conjunction with a wireless link at millimeter-wave (MMW) frequency carrying a 16-QAM signal and achieved a data capacity up to 24 Gbps. In 2018, a real-time 1-GHz IFoF transmission was demonstrated through a 20-km SMF with 28-GHz MMW [10]. In our previous work, where we have focused on achieving a high optical spectral efficiency [11], we demonstrated an MFH system consisting of cascaded IFoF links, analog frequency converters, and channel selectors based on digital signal processing (DSP) over a 20-km SMF. In this work, a single-wavelength optical signal conveyed the eighteen wireless signals with 400 MHz bandwidth and 64-QAM format, which corresponded to a total user throughput of 27 Gbit/s, although the system complexity might be somewhat higher than that with the simple passive WDM scheme used in the previous studies.
Regarding wireless transmission, photonic antennas comprised of a photodiode and an antenna element for downlink [12–17] or an antenna element and an electric-to-optical (E/O) converter for uplink [18,19] have been widely studied under the assumption that they are used in conjunction with analog MFH. Especially, the studies for downlink mainly focused on control schemes of the beam emitted from the photonic array-antenna. Although photonic array-antenna can realize only an analog beamforming function unlike the massive MIMO antenna that can be equipped by a full-digital beamforming function, it is still effective to accommodate multiple users at a single antenna site by discriminating the directions of multiple beams. In order to steer the RF beam, a true time delay corresponding to one wavelength of RF signal should be controlled. Several techniques for controlling the beam in electronic domain [20,21] or in optical domain [12–18,22] have been reported to date. Among them, recently proposed optical devices exploiting a planar lightwave circuit (PLC) are promising considering the compactness and the scalability [14–17]. It is experimentally verified that controlling the phases of optical signals by heating the optical waveguides embedded in the PLC device, a true-time delay or an equivalent effect is induced resulting in the steering of the beam direction. As such, major building blocks for realizing the radio access network based on analog MFH are almost ready in the scale of the final phase in 5G and beyond.
In this paper, we report the current statuses of our most recent results of experimental verification for end-to-end downlink transmission system comprised of MFH based on analog optical transmission schemes, which is potentially capable for multi-ten gigabit per second capacity, and a photonic array-antenna that is applicable to MU-MIMO scenario. We first review our achievements on hybrid MFH consisting of the cascaded IFoF and RoF links [23]. Then we describe the comprehensive experimental evaluation results of photonic array-antenna combined with the MFH and wireless MMW transmission with experimentally confirming its fundamental potential for optically controlled beamforming employing a custom-built photonic array-antenna. We report our experimental demonstration of a spatial multiple beam steering system for a multi-user MIMO (MU-MIMO) scenario with more detailed and more advanced experimental data than those reported in Refs. [24] and [25]. Especially, 16-QAM signal transmission results with dual beam is reported, together with the previous results of QPSK signal transmission with triple beam [24]. The remainder of this paper is organized as follows. In Section 2, the architecture of the hybrid IFoF and RoF for MFH is described. Then, Section 3 describes various demonstrations of the MMW beamforming system. In Section 6, the operating principle of beamforming and its implementation is presented. Then, the experimental result of the multiple beam steering proposed in our previous demonstration is also introduced. Finally, before the conclusion section, we discuss the evolution scenario of MFH toward the future Beyond-5G era.
2. IFOF-based network architecture for mobile fronthaul
If simple RoF transmission scheme would be applied to MFH, several issues would arise. One issue is power fading induced by a mismatch of the relative phase between two sidebands caused by accumulated dispersion. The higher the RF frequency, the shorter the fiber length that the power fading appears. Another issue is that if multiple RF channels at the same frequency should be conveyed to the same antenna site, they should be multiplexed in the optical domain (i.e., WDM or space division multiplexing, “SDM”), and consequently, multiple optical transceivers are required. One solution for solving these issues is the employment of an IFoF transmission scheme in conjunction with FDM of multiple RF channels. Since the frequencies of RF signals should be converted to those emitted in the air, RF channels in the IFoF link should be extracted one by one at a relay site before reaching corresponding antennas. Considering the case that small cells are distributed around the relay site with distances ranging several hundred meters, a RoF transmission scheme can be applied for the final link from the relay site to the antenna. We call the MFH architecture consisting of cascaded IFoF link and RoF link as “hybrid” MFH.
Cable television (CATV) is one of the most popular systems based on the IFoF transmission scheme. Before optical fiber transmission, several frequency bands for broadcasting, such as digital terrestrial band and broadcasting satellite band, are frequency-converted for being realigned in the frequency domain in a band-by-band manner. At a set-top box, one of the bands is selected by analog frequency down-conversion, and then one channel is extracted by digital filtering before demodulation of digital symbols. With a similar configuration, we can arrange 5G signals into several groups, i.e., frequency bands. Referring the CATV system architecture, the authors of Ref. [23] have proposed MFH architecture consisting of dual IFoF links and a RoF link as shown in Fig. 1. The main challenge of the work was to convey multiple channels having the total user throughput more than 20 Gbps by a single wavelength, taking the scalability toward Beyond-5G era into account, as discussed in detail in Section 5. We employed 5G-compliant RF signals with the bandwidth and modulation QAM order of 400 MHz (including 20 MHz guard band) and 64, respectively. It should be noted that the spectral efficiency of the 64-QAM format is assumed to be 3.75 bit/s/Hz [26]. As shown in the insets, eighteen channels that correspond to 27 Gbps user throughput were transmitted through the broadband IFoF link of 20 km SMF in the experiment. In the first relay site, the broadband IFoF signal is received by an O/E converter and then sent into the analog band demultiplexer. An analog demultiplexer, consisting of analog filters and frequency converters, performs band selection and frequency down-conversion, if needed, to the same frequency range as the lowest IF band. Next, each IF band is transmitted to the second relay site through a narrow-band IFoF link of 1 km SMF.
At the second relay site, each channel within the narrow-band IFoF signal is extracted and converted to the desired frequency using a channel demultiplexer. Unlike the band demultiplexer, the channel demultiplexer can conduct channel selection by applying DSP implemented on an FPGA board equipped with an ADC at the input and DACs at the output. The waveform of narrow-band IFoF signal is sampled by ADC and each channel at respective IF is extracted by DSP-based filtering. Thanks to its steep roll-off characteristics, we can utilize a 20-MHz guard band that is involved in a 5G signal with a nominal bandwidth of 400 MHz, and no additional guard band is necessary. The reduction of complexity, as well as physical volume, is also expected by eliminating frequency-dependent bulky analog devices such as bandpass filters and local oscillators.
After frequency up-converting to a millimeter-wave band of 28 GHz, the single channel is transmitted to the antenna sites through RoF links and finally emitted from a spot cell antenna. The signal qualities were evaluated by measuring the error vector magnitude (EVM) after 5 m wireless transmission, and all the channels satisfied the criteria for EVM of less than 8%. More detail of the experimental results can be found in Ref. [23].
It is noted here that we have used the measurement equipment including arbitrary waveform generator (AWG) and the real-time spectrum analyzer that are fully compliant for 5G signal testing in all the experiments described in this paper.
We also have to note that the MFH architecture examined in Ref. [23] was the one that pursued an ultimate frequency utilization efficiency in the first link from CO with the state-of-the-art technologies at the time of development. If IFoF and/or RoF-based MFH will be put into practical application, combination with optical multiplexing techniques such as WDM or SDM should be also considered taking into account pros and cons from installation and operation perspectives.
3. Recent progress and demonstration of beamforming technology
In this section, recent progress and various demonstrations of beamforming technology for the 5G mobile fronthaul are introduced.
In previous works, some approaches on how to control the MMW phase in beamforming systems have been actively investigated [27–32]. To perform discrete delay adjustment for each antenna element, one possible method is to employ a micro-electromechanical system with the integrated waveguide delay lines and electrical switch elements [27,28]. However, due to its limited delay range, the discrete delay operation restricts both the maximum beam scanning angle and its resolution [29]. Another approach is to use microwave delay lines as tunable phase shifters to perform beamforming [30]. Compared with the discrete delay approach, phase shifters with continuous tuning capability and flexibility enable a large scanning angle in the beamforming network. The author of [31] employed a 32-element phased-array-antenna and phase control device at the V-band to demonstrate a beamforming system.
However, such electrical implementation relies on high-speed electrical phase shifters, which may limit the tuning carrier frequency range. In contrast, optical phase handling with the RoF technique emerged as a rather cost-effective solution [32]. Since the phase of the MMW signal is controlled optically, optical phase handling can perform a phase shift at any RF frequency without bandwidth limitation, which significantly simplifies the configuration of the antenna site. Furthermore, the integration of both O/E converters and array-antenna is a promising way to reduce the volume of spot cells. In [33], a photonic-integrated array-antenna module was fabricated to transmit a 3.5 Gbps QPSK signal in an RoF-based beamforming system, which however focused only on mono-MMW beamforming in a single transmitted direction. On the other hand, the author of [16] simultaneously transmitted two RF beams in the frequency bands of 17.6 and 26 GHz, for a multi-user MIMO (MU-MIMO) scenario. Even though a multiple MMW beam steering system was demonstrated in their work, multiple beams need to be spatially-multiplexed at the same frequency in actual MU-MIMO systems.
4. Beamforming demonstration for 5G multi-user mimo
This section shows MU-MIMO demonstrations using multiple MMW beamforming in conjunction with IFoF-based MFH. In this demonstration, we simplified the IFoF configuration by eliminating the broadband IFoF link, as shown in Fig. 2. At an antenna site, a beamforming sub-system including a custom-developed photodiode (PD)-integrated phased-array-antenna module with the minimum scale for forming three distinguishable beams, and optical phase shifters (i.e., optical variable delay lines, VDLs) were employed to conduct fundamental experiment of multiple beam steering for the MU-MIMO scenario. The design and basic characteristics of the antenna module are discussed in the following. Then we demonstrate an MU-MIMO system using these key components. It should be noted that the scale of array-antenna (i.e., the number of antenna elements) used in this experiment are minimum for emitting three beams that can be discriminated from each other, although mutual interferences from adjacent beams inevitably deteriorate the signal quality so that the QAM order of the signal should be decreased.
Figure 3(a) shows the inside of the developed compact antenna module. Assuming to accommodate four users by sixteen elements as the minimum array size, namely, 1×4-array-antenna forms one beam targeting one user, the module was composed of 16-channel high-speed O/E converters, RF chains, and patch antenna elements. In each channel, a 28-GHz RoF signal was fed into the module and then converted to an electrical signal by PD. To adjust the electrical signal level, two RF amplifiers sandwiching an RF attenuator were cascaded in a single RF chain. As a result, the input signal could be amplified by 44 dB. To perform multiple RF beam steering, we arranged the 16 patch antenna elements in a 4×4-array configuration with a total gain of 17 dBi. Each patch antenna element was 3.8 mm wide and 4.8 mm high, with a gap of 5 mm between adjacent antenna elements. All these optical and electrical devices were integrated into a compact module measuring 15 cm × 8 cm × 8 cm. A full-package overview of the developed antenna module is shown in Fig. 3(b). The normalized frequency response of the developed module is shown in Fig. 3(c). From the figure, a 3-dB bandwidth around the 28-GHz region was found to be around 2.8 GHz, which was sufficiently large to support a 400-MHz signal.
Fig. 3. (a) Inside view with device bonding picture, (b) full-package picture and (c) frequency response of developed PD-integrated array-antenna module [25].
First, single user case with one, two and four patch antenna elements was tested with 6-m MMW transmission distance using a horn antenna at the receiver side. Optical signal power fed to each patch antenna element was −3 dBm. Respective emitted MMW powers were −28.1 dBm, −22.6 dBm and −16.9 dBm for one, two and four elements. Corresponding received MMW power before the amplifier was −63.4 dBm, −57.6 dBm and −41.9 dBm, respectively. As for 64-QAM signal, the EVM of the recived signal was 11.1% when using only a single patch antenna, while it was improved to 8.2% and to 6.9% by using two and four patch antenna elements, respectively. The RF spectra and the constellation plots of received signals are illustrated in Fig. 4(a) and (b), respectively. As 3GPP defines the required EVM less than 8% for 64-QAM signal, the criteria is satisfied only when four patch antenna elements are used in the single user case.
Fig. 4. (a) The received RF spectra and (b) constellation plots of wireless MMW signal emitted by one, two and four patch antenna elements.
Figure 5(a) illustrated the EVMs of single channel MMW signal at different air transmission distance which was emitted by four patch antenna elements. The maximum reachable transmission distance of 10.35 m in free space is obtained with the required 3GPP specification of 8%. To investigate the feasibility of the phase control of the antenna, VDLs were adapted. As described in Section 1, these VDLs can be replaced by novel optical devices which may be more sophisticated in future. First, we chose two patch elements from the sixteen elements. Then, the phases were carefully adjusted so that they became in-phase. Twenty-eight GHz tone signals were emitted from the two patches, and we measured the RF power of the signal as received by a horn antenna. We changed the delay time between the two paths to confirm the VDL-based beam control. Note that the horn antenna was placed in front of the antenna module in this case. Figure 5(b) shows the result. It was found that the plot had a cycle of 10.8 mm, corresponding to a time delay of 36 ps. The simulated result is also shown in the figure as a reference. Since the cycle estimated theoretically is 10.71 mm, both the results coincide well. Here, we generalize this discussion.
Fig. 5. (a) The EVMs of 28-GHz MMW transmission with different wireless distances. (b) Variation of received tone
Figure 6 shows the simulation beamforming principle using four antenna elements with the spacing of d between two elements. Each antenna elements emitted the MMW along X-axis with the same amplitude and phase of α1 and φ1, respectively. When the RF receiver is located in far-field region with an angle difference θ between the Y-axis, the phase delay in distance of d sinθ ⁄ λ is observed for the two adjacent antenna elements. To steer the transmission angle of MMW toward the receiver, we can intentionally set the initial phase of φ1+2πd sinθ ⁄ λ, φ1+4πd sinθ ⁄ λ and φ1+6πd sinθ ⁄ λ for the antenna element #2, #3 and #4. Based on this concept, it is known that the phase difference of two adjacent elements; that is, φi - φi-1 = Δφ, in the beamforming condition should be set as [14,25]:
where λ and d denote the wavelength of the MMW carrier and the distance between the two patches. The RF beam can be steered toward the desired direction by setting a specific value on Δφ. Since we apply an optical VDL in our proposed system, the phase term can be transferred to delay time Δτ and delay length ΔL. By applying the relationship Δφ = f0Δτ = 2pDL/λ, we can obtain: where c is the speed of light, and f0 the frequency of the MMW carrier. These equations show that steering angles of ±30° can be obtained by precisely setting the optical VDLs to ±2.5 mm, power at 28-GHz with different optical phase shift yielding optical delay times of ±52.4 ps. In Fig. 7, the measured and simulated RF radiation patterns are presented in closed squares and solid lines. Although slight differences are observed in the sidelobes, the pointing directions in the measured patterns coincide well with the simulation results.Figure 8(a) illustrates the experimental setup for the MU-MIMO system using the digital channel demultiplexer and the antenna module. An electrical QPSK or 16QAM signal generated by an AWG was sent to a 10-GHz- class distributed-feedback-laser diode (DFB-LD). The DFB-LD had a 3-dB bandwidth of 7 GHz. After being transmitted over a 20-km SMF, the optical signal was detected and each channel was separated through a developed digital channel demultiplexer. With the in-built ADC and FPGAs, the channel demultiplexer was used as a filter and frequency converter. The separated channels were simultaneously output through three DACs. Before the RoF transmission, three IF channels were upconverted to the 28-GHz band by an analog process and subsequently launched into three lithium-niobate Mach- Zehnder modulators (LN-MZMs). Then, each individual RF signal was later transmitted into a beamforming sub-system, which was composed of cascaded optical splitters, optical VDLs, and the antenna module for multi-beam steering transmission. The alignment of the channels are schematically shown in Fig. 8(b)
Fig. 8. (a) Schematic of experiment setup of the multi MMW beamforming system and (b) alignment of channels [24].
The input modulated optical signal was first separated into four optical paths through a 1-by-4 optical splitter. The phase of each optical path (3×4 channels) could be precisely controlled via the cascaded VDLs. Three MMW beams were simultaneously radiated from the patch antenna to the air. During the experiment, we placed the antenna module on top of the turntable for the measurement of each MMW beam directed toward different angles. The radiated MMW was received by a horn antenna after a 5 m air transmission and finally sent into a spectrum analyzer (SA) to evaluate transmission performance, including EVMs, RF spectra, and constellation plots. As shown in Fig. 9(a), three MMW beams (Ch1, Ch2, and Ch3) were emitted from the antenna module simultaneously at respective steering angles of −30°, 0°, and +30°. At the received angle of −30°, an EVM of 9.5% can be obtained when Ch1 is transmitted alone. However, when considering the case of three-beam transmission, the sidelobes of Ch2 and Ch3 are regarded as noise to Ch1 and degrade its EVM to 17.4%. A similar deterioration can also be observed at the received angles of 0° and +30°. The EVMs of Ch2 and Ch3 were respectively degraded from 8.9% and 9.7% to 17.19% and 17.5% with three RF beams transmitting simultaneously.
Fig. 9. Transmission performance, including EVM, RF spectra and constellation plots, in single-channel and multi-channel at received angles of (a) −30°, 0°, +30° (three beams) [24] and (b) −45°, +45° (two beams).
In contrast, the scenario in Fig. 9(b) illustrated a two MMW-beam transmission at the steering angle of −45° and 45°. Compared to the three-beam case, the interference from the sidelobes of the adjacent RF beam can be decreased for two-beam transmission. With less degradation, the 16-QAM signal carried by two MMW beams can be transmitted simultaneously with the EVM of 12.29% and 12.5% for Ch1 and Ch2, respectively.
Consequently, 16-QAM is available for two-beam case while QPSK for three-beam case. As seen from the constellations in Fig. 9, the limitation to the QAM order is forced by mutual interference among the beams. It means that patch antennas more than 4 per beam (user) is necessary for sharpening the beam to accommodate four users with QPSK or with higher order of QAM signal. Since the antenna was initially designed as the minimum size for MU-MIMO accommodating four users, the extension of its size is one of the future challenges.
5. Conclusion
Analog transmission techniques such as IFoF and RoF are promising approaches to the implementation of high-capacity MFH in mobile network systems of 5G and beyond. For practical wireless application, we have experimentally presented a multiple MMW beam steering by combining a beamforming technique and RoF transmission. MU-MIMO scenario with three MMW beams was demonstrated by using MFH consisting of cascaded IFoF and RoF links and the compact PD-integrated array-antenna module. In conclusion, our proposed system is expected to have sustainable scalability toward the Beyond-5G era.
Funding
Ministry of Internal Affairs and Communications (JPJ000254); National Institute of Information and Communications Technology.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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