Bidirectional WDM Multi-Nodes Analog Radio-Over-Fiber Mobile Fronthaul Link Enhanced by Photonic Integrated Devices

A bidirectional wavelength division multiplexing (WDM) analog radio-over-fiber (A-RoF) mobile fronthaul (MFH) link is enhanced using photonic integrated devices. Two key photonic integrated devices are combined in the A-RoF link: an 8-channel InP directly modulated laser (DML) transmitter and a 32×100-GHz silicon array waveguide grating (AWG). The DML transmitter has 8 parallel monolithically integrated distributed feedback lasers, enabling cooperative and reconfigurable downlink analog transmission. Moreover, the 32×100-GHz AWG is featured by low insertion loss (<4.5-dB) and low crosstalk (<−20.4-dB), to achieve a high-density WDM system. In the distributed field experiments, we have successfully demonstrated a bidirectional A-RoF MFH over 10-km standard single mode fiber, providing an 8×5-Gbit/s 4-quadrature amplitude modulation (QAM) orthogonal frequency division modulation (OFDM) downlink transmission and a 3×12-Gbit/s 16-QAM uplink transmission.


I. INTRODUCTION
F UELED by the proliferation of mobile data traffic and the ultra-dense deployment of small cells, centralized/cloud radio access network (C-RAN) with promises of low energy consumption, high spectrum efficiency, flexibility, and scalability has gained more and more momentum to accommodate the fifth generation and beyond (5G/B5G) network visions of ultra-high data rates, seamless coverage as well as green communication [1], [2]. However, the ultra-dense C-RAN imposes great challenges on legacy mobile fronthaul (MFH) links relying on bandwidth-inefficient digital transmission schemes (e.g., Common Public Radio Interface (CPRI)) to connect distributed units (DUs) and remote radio units (RRUs) [3]. These challenges are exemplified by the limited fronthaul capacity and unsatisfied end-to-end latency. Recently, MFH links based on analog radio over fiber or intermediate-frequency over fiber (A-RoF/IFoF) transport solutions have been actively investigated to reap its distinct benefits of high bandwidth efficiency, low latency, and high scalability [4], [5], [6]. Particularly, it has been experimentally shown that the IFoF MFH transmission links can achieve a CPRI-equivalent data rate exceeding Tbit/s [7], [8]. Nonetheless, with the large-scale deployment of a large number of discrete photonic transceivers, wavelength division multiplexers (WDMs), and other devices, these promising solutions also have concerns about cost and power consumption, which is not conducive to the theme of green wireless communication in the future. Therefore, different photonics integrated technologies [9], [10], [11], [12], [13], [14], [15] have been proposed to empower the A-RoF MFH links for targeting an ultra-compact, cost-and energy-efficient solution. These efforts focus on the active monolithically integrated multi-wavelength transmitter or reconfigurable modulator [16], [17], [18], [19], [20], and passive photonic integrated devices (e.g., hybrid wavelength selective switches) [21], [22]. In [17], a reconfigurable WDM-passive optical network (WDM-PON) based on an 8-channel directly modulated laser (DML) module has been proposed to demonstrate a 10-Gbit/s data transmission. Subsequently, the multi-wavelength transmitter monolithically integrated 8-channel distributed feedback (DFB) laser is proposed for achieving an 8×1.04-Gbit/s throughput 5G new radio (NR) A-RoF system [19]. An InP-based monolithic 4-channel widely tunable transmitter integrated with the DFB laser, semiconductor optical amplifier (SOA), and electroabsorbing modulator (EAM) has been designed and fabricated to support 4×10-Gbit/s mobile fronthauling [20]. A siliconphotonics photonic true time delay device [21] based on optical ring resonators is proposed for beamforming in 5G analog MFH. Furthermore, a C-band integrated passive wavelength selective switch with 160 wavelength channels spaced at 25-GHz has been proposed based on the SiPh platform, demonstrating error-free A-RoF transmission of 20-Gbit/s per wavelength channel [22]. However, these efforts only use single active or passive photonic integrated devices, which still suffer from the issue of limited efficiency in cost/power resources for implementing large-scale and large-capacity WDM A-RoF MFH networks. Therefore, in this paper, we have proposed and experimentally demonstrated an all-photonic integrated devices (PIDs) enhanced WDM A-RoF MFH link. The all-PIDs merged with an 8-channel InP-based directly modulated laser (InP-DML) chip and a 32×100-GHz Silicon-based arrayed waveguide grating (Si-AWG) chip are designed and fabricated to enable an ultracompact transmitter and optical de-/multiplexer for boosting the large-capacity distributed multi-node access bidirectional analog transmission. The InP-DML chip is highly integrated, possessing 8 parallel DMLs to achieve a cooperative and reconfigurable downlink signal transmission. The 32×100-GHz Si-AWG chip has been optimized and fabricated to provide a high-performance passive de-/multiplexer with a low insertion loss (<4.5-dB) and low crosstalk (<−20.4-dB). In the field experiments, our proposal has successfully demonstrated the bidirectional analog signal transmission over a 10-km standard single mode fiber (SSMF) link. In the downlink, the 4-quadrature amplitude modulation (QAM) orthogonal frequency division modulation (OFDM) signal with a 2.5-GHz bandwidth and a 4-GHz center frequency is applied to the 8-channel InP-DML chip to simultaneously perform an 8×5-Gbit/s radio-frequency (RF) signal transmission. For the uplink transmission, the 16-QAM single-carrier signals with a symbol rate of 3-GHz and center frequencies of 3, 8, and 12 GHz are respectively received by three-node photonic frontends in RRU and transmitted to the center unit (CU) via a bidirectional Si-AWG chip, achieving a 3×12-Gbit/s data rate uplink communication.

A. PIDs Enhanced Bidirectional WDM Multi-Node A-RoF MFH Link
Fig. 1 depicts the architecture of the proposed PIDs enhanced bidirectional WDM multi-node A-RoF MFH link. In CU, a multi-channel InP-DML chip is act as a compact transmitter, which can dominate the delivery of the large-capacity downlink signal for distributed multi-node RRUs. Then, the difference wavelength optical modulated signals generated by a multichannel InP-DML chip are multiplexed by a Si-AWG chip. The Si-AWG chip with numerous wavelength channels and ultranarrow spacing enables the bidirectional dense WDM system. Through a long distance MFH link, the multiplexed optical signals are de-multiplexed by a symmetrical Si-AWG chip and delivered to multi-node RRUs. For each RRU, a simple analog photonic frontend without costly coherent detection or baseband process units contributes to green RF wireless communication. For uplink transmission, the uplink user signals received by multi-node RRUs are conveyed to CU via the bidirectional Si-AWG chips. Subsequently, the uplink channels of Si-AWG are connected with the PD array in CU to recover the uplink data. Therefore, compared with the conventional MFH link using a discrete and bulky optical transmitter and WDM devices [23], our approach makes a virtue out of high-density integrated PIDs (InP-DML and Si-AWG chips) to significantly save space and electric power resources for the large-scale MFH link.

B. Key PIDs: 8-Channel InP-DML and 32-Channel Si-AWG Chips
To satisfy the transmission capacity and power efficiency for large-scale A-RoF MFH links, we make great efforts to design and fabricate the high-density integrated PIDs: 8-channel InP-DML and 32-channel Si-AWG chips. Fig. 2(a) shows the schematic diagram of the proposed direct modulation multichannel cooperative reconfigurable microwave photonics transmitting chip. This multi-channel chip highly integrates 8 parallel DMLs array in a WDM mode to implement the electro-optical conversion of analog RF signal. Different from the conventional photonic integrated DML chip [16], our proposed chip enables the external light injection lock function to generate the multichannel coherent optical signal for ensuring the stable-phase transmission of multi-node A-RoF MFH link.
Moreover, we have designed the on-chip tunable microring resonator to dynamically select the available wavelength channel, which is realized by thermally tuning SOA and phase shifter to adjust the filtering characteristics of the microring resonator.  The design layout of the proposed 8-channel parallel DML chip based on the InP platform is shown in Fig. 2(b). The two DFBL regions denote two sets of four parallel integrated DMLs. Fig. 3(a) gives the simulation design layout of our proposed photonic integrated rectangular-type AWG chip with 32 channels and 100 GHz spacing based on a silicon-on-insulator (SOI) platform. Theoretically, more channel number can be expected for the AWG, i.e., 40 channels (100 GHz wavelength spacing) in C-band. However, according to (1) and (2), the footprint and the number of the arrayed waveguide will expand greatly as the number of AWG channels increases, eventually leading to the accumulation of phase errors [24], [25].
where m is the order of the phased array, λ 0 is the central wavelength, n c and n g are effective refractive index and the group index of the arrayed waveguides, Δλ is the channel spacing of the AWG, and N ch is the output number of the AWG, d o is the output waveguide spacing, d a is the arrayed waveguide spacing, and n s is the effective refractive index of free propagation region (FPR).  Therefore, a balance must be maintained between fabrication tolerance and design parameters to ensure high performance AWG chip. Thus, we design the taper connector based deep etched structure (see Fig. 3(b)) to connect the FPR and nanowire waveguide array to relieve the phase mismatch between them. In addition, the sidewall roughness of the nanowire waveguide  array can result in the increase in insertion loss and crosstalk between AWG channels. To improve the performance of the AWG chip, we have designed and fabricated several rounds to optimize the waveguide sidewall inclination and roughness. Fig. 3(c) presents the optimized sidewall inclination profile.

A. Measured Performances of 8-Channel InP-DML and 32-Channel Si-AWG Chips
The 8-channel DML chip is fabricated on the InP material platform and its footprint is about 6×8 mm 2 and the packaged module is 5.0×3.5×1.5 cm 3 . Fig. 4 depicts the scanning electron microscope (SEM) image of the tape-out 8-channel InP-DML chip, as well as the packaged external pictures. The central wavelengths of the 8 channels InP-DML chip can be tuned by changing the independent bias voltage. The microring resonator can act as a notch filter and achieve wavelength tuning through thermal tuning, thereby enriching the operating modes (or wavelength) of the InP-DML chip. Fig. 5(a) shows the image of the 32×100-GHz rectangulartype AWG chip fabricated on the SOI platform, whose footprint is less than 1.4×1.0 mm 2 . The optical transmission responses of 32 channels of the Si-AWG chip are tested by an optical spectrum analyzer (OSA, YOKOGAWA-AQ6370D) with a 0.02-nm resolution, as shown in Fig. 5(b). And insertion loss and crosstalk of each channel are respectively measured. The insertion loss is no more than 4.5-dB, and the crosstalk is lower than −20.4-dB.

B Field Demonstrations for the A-RoF MFH Link
A proof-of-concept experiment is carried out based on the setup shown in Fig. 6. In the indoor lab, the bias voltage and microring resonators of the 8-channel InP-DML chip are adjusted to activate optical coherent carriers with appropriate wavelengths to precisely align the chosen channels 10-17 of the Si-AWG chip for downlink A-RoF MFH link. Fig. 7(a) shows the measured optical spectra of the fabricated 8-channel InP-DML chip, of which the wavelengths are 1551.2 nm, 1552.2 nm, 1553.1 nm, 1554 nm, 1554.9 nm, 1555.7 nm, 1556.6 nm, and 1557.5 nm, respectively. The optical spectra of 8-channel optical signals multiplexed by the Si-AWG chip are measured and shown in Fig. 7(b).
In the experiment, the RF OFDM signal is formed offline for the downlink transmission. The baseband OFDM signal has a fast Fourier transform (FFT) size of 2048, wherein 80 subcarriers are modulated in the 4-QAM format. Since the subcarrier spacing is specified as 15.62-MHz, the obtained OFDM signal occupies a bandwidth of 2.5-GHz due to the Hermitian symmetry processing. The baseband OFDM signal is up-converted to the RF band centered at 4-GHz. A commercial-available high-speed arbitrary waveform generator (Keysight M9502A) running at a sampling rate of 32 GSa/s is leveraged to implement the digital-to-analog (DAC) conversion for the RF OFDM signal.
The output of DAC is applied to 8 channel DMLs through the electrical amplifier for generating the analog microwave photonics signal. The analog microwave photonics signals with different wavelengths are multiplexed by the Si-AWG chip. Through an optical circulator (OC) and 10-km downlink MFH link, the multiplexed optical signal is transmitted to three-node RRUs located on the roof of the college building, as shown in Fig. 6. Here, an erbium-doped fiber amplifier (EDFA) is used to compensate the fiber transmission loss. And the amplified optical signal is divided into three parts via an optical divider (OD) and sent to three analog photonic frontends in three RRUs. In each RRU, the analog RF signal recovered by a photodetector (PD) is amplified by a power amplifier (PA) and emitted into free space for downlink wireless communication. Simplistically, the output of the PD is received by a real-time oscilloscope (OSC, LeCroy WaveMaseter 813Zi-A) with a sampling rate of 40 GSa/s for analog-to-digital (ADC) conversion. Meanwhile, the same RF OFDM signals are respectively applied to the 8 channels of the InP-DML chip to analyze the transmission performance of the downlink A-RoF MFH link. Fig. 7(c) shows the measured power spectrum density (PSD) of the received OFDM signal at the output of PD. Fig. 7(d) demonstrates the measured bit error ratio (BER) performance versus the different received optical power from the PD for the 8 channels PIDs-enhanced A-RoF MFH link. It can be seen from Fig. 7(d), the received optical power margins to keep a BER below the 7% pre-forward error correction (Pre-FEC) threshold (3.8 × 10 −3 ) should be over −12-dBm with the 8×5-Gbit/s signal transmission for 8 channels A-RoF MFH downlink. Two insets in Fig. 7(d) indicate the constellation diagrams of the demodulated 4-QAM signal corresponding to the received optical powers of −16 dBm and −6 dBm, respectively.
For the uplink transmission, the center wavelengths of three DMLs in the three photonic frontends are separately 1546.9 nm, 1550.0 nm, and 1558.0 nm, responding to channels 5, 9, and 18 of the Si-AWG chip in the indoor lab. Fig. 8(a) gives the measured optical spectra of the outputs of three DMLs. Subsequently, the 16-QAM single carrier signals with a symbol rate of 3-GBaud/s and the center frequencies of 3, 8, and 12 GHz generated by the arbitrary waveform generator are respectively applied to three photonic frontends in three RRU for the uplink transmission, as shown in Fig. 6. Three uplink optical signals from three RRUs are then coupled by an OD and conveyed to the indoor lab via the 10-km SSMF. After the de-multiplexing of the Si-AWG chip in the indoor lab, the output optical spectra of channels 5, 9, and 18 are shown in Fig. 8(b). The de-multiplexing uplink optical signals are received by the PD array and digitized by a real-time OSC in the receiver . Fig 8(c)-(e) give the measured PSD of three uplink signals at the output of the PD array. Furthermore, Fig. 8(f) shows the measured BER performances as a function of different received optical power before the PD array for three RRUs uplink transmission. In this figure, one can find that the received optical powers should be over −12-dBm to meet the BER threshold for the 3×12-Gbit/s high-quality uplink transmission of three RRUs. Moreover, two insets in Fig. 8(f) respectively give the constellation diagrams of the demodulated 16-QAM signal corresponding to the received optical powers of −16 dBm and −4 dBm.

V. CONCLUSION
We have proposed and experimentally validated a largecapacity bidirectional WDM multi-node A-RoF MFH link enhanced by photonic integrated devices. In our proposal, a highdensity photonic integrated 8 channels parallel InP-based DML chip is designed and fabricated to enable an ultra-compact transmitter. Moreover, a high-performance 32×100-GHz siliconbased AWG chip with a low insertion loss (<4.5-dB) and low crosstalk (<−20.4-dB) is developed for the large-capacity bidirectional WDM multi-node MFH links. In field experiments, the proposed system can achieve 40-Gbit/s downlink and 36-Gbit/s uplink transmissions, under a 10-km analog SSMF link. Therefore, our proposal is expected to offer a PIDs-enhanced alternative for achieving ultra-compact and low power consumption large-scale A-RoF MFH link and accommodate the visions of future green 6G and beyond wireless networks.