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In this paper, we propose a cost-effective wavelength-reused mode-division-multiplexing (MDM) system for high speed symmetrical bidirectional mobile fronthaul application. At the base band unit (BBU) pool, one of the spatial modes is used to transmit signal carrier while the others are used for downstream (DS) signal channels. At the remote radio unit (RRU) side, the signal carrier is split and reused as modulation carrier for all the upstream (US) signal channels after mode demultiplexing. Thanks to the low mode crosstalk characteristic of the mode multiplexer/demultiplexer (MUX/DEMUX) and few-mode fiber (FMF), the signal carrier and each signal channel can be effectively separated. The spectral efficiency (SE) is significantly enhanced when multiple spatial channels are used. Compared with other wavelength reused scheme in which the downstream and upstream be modulated in orthogonal dimension, the modulation format of both directions are independent in the proposed wavelength reused MDM system. Therefore, it can easily achieve symmetrical bidirectional transmission without residual re-modulation crosstalk. The proposed scheme is scalable to multi-wavelength application when wavelength MUX/DEMUX is utilized. With the proposed scheme, we demonstrate a proof of concept intensity modulated 4 × 25-Gb/s 16-QAM orthogonal frequency division multiplexing (OFDM) transmission over 10-km FMF using low modal-crosstalk two-mode FMF and MUX/DEMUX with error free operation. The downstream receiver sensitivity is −21 dBm while the upstream receiver sensitivity is −18 dBm for bidirectional transmission. Due to the Rayleigh backscattering and other spurious reflections, the upstream suffers 2 dB power penalty compared with unidirectional transmission without downstream. To mitigate bidirectional transmission impairments, we propose a simple and effective method to suppress Rayleigh backscattering by shifting the downstream subcarrier frequency. A receiver sensitivity improvement of up to 2.5 dB is achieved for upstream with different downstream power.
© 2016 Optical Society of America
1. Introduction
With the development of LTE-based radio access networks, the cloud radio access network (C-RAN) has gained much attention for its reduced capital expenditures (CAPEX) and operating expense (OPEX) [1–3]. In the C-RAN, remote radio unit (RRU) and base band unit (BBU) are separated. Baseband processing and control/management functions are performed in the common central BBU pool (aggregate BBU) that serves a large group of RRUs [4]. Currently, fronthaul with common public radio interface (CPRI) is specified for the digitalized data connection between RRUs and BBUs [5, 6]. To satisfy upcoming high capacity 5G mobile communication services with multi input multi output (MIMO) antenna, wavelength division multiplexing (WDM) combines with intermediate frequency over fiber (IFoF) technology have been proposed to carry fronthaul traffic due to the advantages of high spectrum efficiency, energy savings and service transparency [7–10]. In the WDM-based fronthaul, colorless operation and cost effectiveness are greatly desired in practical deployment and maintenance at the RRU side [11, 12]. To this end, tunable distributed feedback (DFB) laser, and several wavelength-reused schemes including self-seeded reflective semiconductor optical amplifier (RSOA), reflective electro-absorption modulator with SOA (REAM-SOA) and weak resonant cavity Fabry-Perot laser diode (WRC-FPLD) have been proposed for RRUs [13–19]. Wavelength tunable and stable DFB laser is costly in high-density RRU, especially in dense wavelength division multiplexing (DWDM) fronthaul system. RSOA-based RRU has demonstrated the feasibility in commercial long term evolution (LTE) environment [15, 16]. However, the upstream (US) data rate is limited by its modulation bandwidth and the upstream signal may be deteriorated by spontaneous noise. In the REAM-SOA and WRC-FPLD scheme [17–19], the upstream signal quality depends much on the optical modulation depth of downstream (DS), which may cause severe residual intensity modulation crosstalk. Meanwhile, the bidirectional modulation is format dependent and it is not preferred in future C-RAN fronthaul.
Recently, mode-division-multiplexing (MDM) as an alternative technique for expanding transmission capacity has been widely investigated for high-speed optical transmission and optical access networks [20–22]. Compared with WDM technology, the MDM-based system is naturally colorless and it may be a good candidate for network application scenario that needs colorless operation. In our previous work, we have experimentally verified the feasibility of MDM for passive optical network (PON) [23]. In this paper, we propose MDM system for high speed symmetrical bidirectional mobile fronthaul application. At the BBU pool, one of the spatial modes is used to transmit signal carrier while the others are used for downstream signal channels. At the RRU side, the signal carrier is firstly mode demultiplexed and then selected to be reused as modulation carrier for all the upstream signal channels. Costly tunable lasers are omitted, which greatly reduce the cost of RRUs. When multiple spatial channels are used, the spectral efficiency (SE) is significantly enhanced. Compared with other wavelength reused schemes [15–19], the proposed system can easily achieve symmetrical bidirectional transmission without residual re-modulation crosstalk. The proposed scheme is scalable to multi-wavelength application when wavelength MUX/DEMUX is utilized. With the proposed scheme, we demonstrate a proof of concept intensity modulated 4 × 25-Gb/s 16-QAM orthogonal frequency division multiplexing (OFDM) transmission over 10-km few-mode fiber (FMF) using low modal-crosstalk two-mode FMF and MUX/DEMUX with error free operation. The downstream and upstream receiver sensitivity is −21 dBm and −18 dBm for bidirectional transmission, respectively. What is more, we propose a simple and effective method to mitigate Rayleigh backscattering in bidirectional transmission by shifting the downstream subcarrier frequency. A receiver sensitivity improvement of up to 2.5 dB is achieved for upstream at different downstream signal power. As fast as we know, this is the first time that MDM system is demonstrated for bidirectional fronthaul application.
2. Operation principle
The proposed wavelength reused MDM architecture for bidirectional mobile fronthaul is shown in Fig. 1. For simplification, analog frontend that consists of filtering and analog RF/IF conversion is not depicted here. By utilizing low modal-crosstalk FMF and all-fiber mode MUX/DEMUX, mode is operated as independent dimension and the signal channels and carrier channel can be individually separated without MIMO digital signal processing (DSP) that joints mode processing. At the BBU pool, a transmitted laser is power split to n paths to different BBUs. n-1 paths are respectively intensity modulated by n-1 transmitters from BBU 1 to BBU n-1 while one path as remodulation carrier for upstream. Then a mode MUX combines the modulated downstream signal and the carrier and converts them to specific modes of the FMF. After the FMF transmission, the signals and carrier are mode demultiplexed and converted to the LP01 mode at the RRUs. At each RRU, the downstream signal is sent to the respective receiver for direct detection after single mode circulator (SMC) while the carrier is split by a 1:n-1 splitter and then input to each transmitter as modulation carrier for all the upstream spatial channels. When a large number of modes and long FMF are adopted, the remodulation carrier for upstream suffers a large insertion/transmission power loss. To compensate for the loss of the remodulation carrier, a single-mode-fiber (SMF) erbium-doped fiber amplifier (EDFA) can be utilized at the input of the 1:n-1 splitter at the RRU side. Cost effective semiconductor optical amplifiers (SOAs) can also be considered at the input of each upstream transmitter. Similar to the downstream, the signals from each RRU are combined and converted to the specific modes of the FMF. At the BBU, the upstream signals are mode demultiplexed and then perform direct detection. By utilizing wavelength reused scheme, costly tunable lasers are omitted for upstream. Thanks to the low mode crosstalk of the FMF and mode MUX/DEMUX, the transmitted signal carrier and each channel can be effectively separated. No complicated modulation or residual remodulation crosstalk needs to be considered. What is more, due to the larger effective core area of FMF compared with the SSMF, the power input FMF can be enhanced without additional nonlinear impairments to increase bidirectional power budget. The proposed scheme is scalable to multi-wavelength application when wavelength MUX/DEMUX is utilized, which could effectively extend the scale of fronthaul systems.
In the wavelength reused system, one of the concern issue is Rayleigh backscattering noise and others spurious reflections. Especially for upstream, the signal is susceptible to the Rayleigh backscattering noise. To combat the impairment, we propose digital frequency shifting scheme to mitigate the Rayleigh backscattering noise of upstream and downstream. Figure 2 shows the proposed mitigating Rayleigh backscattering noise scheme in intensity modulation/direct detection OFDM system. The baseband OFDM signals are firstly generated by constellation mapping and inverse fast Fourier transform (IFFT) operation. After N times up-sampling and filtering, the upstream and downstream OFDM signals are up-converted to different radio frequency ƒr1 and ƒr2 by digital I-Q modulation, respectively. Then intensity Mach-Zehnder modulators (MZM) are utilized to convert the OFDM signals to double sideband optical signals. As shown in Figs. 2(a) and 2(c), the OFDM subcarrier of downstream and upstream is spectrally non-overlapping. Therefore, the Rayleigh backscattering noise will not overlap to the OFDM subcarrier. After direct detection by photodetector, the OFDM signals are respectively converted to baseband with center frequency of ƒr1 and ƒr2, as shown in Figs. 2(b) and 2(d). Digital filtering by DSP is performed to the two signals. The out-of-band Rayleigh backscattering noise can be effectively filtered out. The proposed scheme is transparent to signal modulation format and it also applies to single-carrier modulation system such as Nyquist WDM.
3. Experiment setup and results
To verify the feasibility of our proposed scheme, we have experimentally demonstrated a proof of concept MDM system as shown in Fig. 3. Low modal-crosstalk two-mode FMF and MUX/DEMUX are utilized to implement the fronthaul architecture. One LP mode is for signal transmission and the other for carrier. LP11 mode is a mode group which consists of two degenerate modes (LP11a and LP11b), here we only excite one of the two degenerate modes and manually adjust the mode DEMUX to demultiplex the degenerate mode in this experimental setup. Less than 0.1 dB power fluctuation is observed. Bidirectional four wavelength channels with total data rate of 100 Gb/s are demonstrated. For downstream, four lasers with respective center wavelength of 1549.808 nm, 1550.008 nm, 1550.208 nm and 1550.408 nm are combined by a wavelength MUX. The frequency spacing of adjacent two channels is 25 GHz, which is according to ITU-T grid of UDWDM. Then the four lasers are split into two branches by a 1:2 splitter. One branch is modulated by MZM modulation and then recombined with the unmodulated branch at mode MUX. The carriers are converted from LP01 mode to LP11 mode and the modulated signals are LP01 mode for FMF transmission. The signal power into FMF per channel is −3 dBm and the carrier is 0 dBm. We adopt the similar scheme that was shown in our previous work [24] go generate the optical OFDM signal. An arbitrary waveform generator (AWG 7000A) with sampling rate of 25 GS/s generates baseband OFDM signal. The baseband OFDM signal is up-converted to 3.125-GHz by I-Q modulation. The DFT size is 1024, from which 968 subcarriers are used for data transmission, and the cyclic prefix (CP) size is 16. 16-QAM is used as modulation format and the bit rate of a single channel is 25 Gb/s. A MZM is utilized to convert the OFDM signal to double-sideband optical signal. After 10 km FMF transmission, the WDM-MDM signal and carrier are firstly demultiplexed by a mode DEMUX and then sent to wavelength DEMUX to achieve individual signal channel and carrier channel. The signal power input to the receiver for direct detection is −6.2 dBm and the carrier power input to the MZM is −6.8 dBm per channel. The receiver consists of optical pre-amplifier and a PIN photo-diode. Optical pre-amplifier can be omitted by utilizing high receiver sensitivity APD photo-diode. Then the signal is sampled by a realtime digital storage oscilloscope (DSO) operating at 50 GS/s. The sampled OFDM signal is down-converted to baseband and a band-pass finite-impulse-response (FIR) filter is used to eliminate the beating interference effect and other noise. And then the CP is removed after synchronization. Each block is transformed into frequency domain by DFT and least square (LS) equalization is performed. For upstream, the carrier channels are input to the MZMs for upstream modulation, similar as the downstream. The upstream signal power into FMF per channel is −10 dBm. The fabrication parameters of this FMF for transmission are as follows: the core/cladding diameters of the fiber are 13.5-μm and 125-μm, respectively. The refractive index of the core and cladding are 1.446 and 1.440, respectively. The relative index difference is 0.42%. The normalized frequency V is 3.24. Thus, the FMF only supports LP01 and LP11 modes transmission. The attenuation is 0.21-dB/km for both modes and a crosstalk of less than −18 dB between LP01 and LP11 is measured after 10 km FMF and mode MUX/DEMUX. The mode MUX/DEMUX are realized in the form of fused-type coupler and fabricated with a SMF and a FMF by heating and tapering according to phase-matching condition [25, 26]. The optical insertion losses of mode MUX are measured to be 0.3 dB for LP01 mode, 1.8 dB for LP11 mode excitation at the wavelength of 1550 nm. The insertion losses of mode DEMUX are measured to be 0.5 dB for LP01 mode, 2.3 dB for LP11 mode. The crosstalk from LP01 to LP11 is −18.4 dB and from LP11 to LP01 is −26 dB after mode MUX/DEMUX. Compared with using two single mode fibers, no obvious benefit is achieved when only two spatial modes are utilized. However, when the mode number increases, compared to the transmission over multiple parallel SMFs, MDM scheme allows reducing power consumption and infrastructure deployment cost, which offers cost advantages in fronthaul application [27, 28].
In our first bidirectional experiment, the up-converted radio frequency of downstream and upstream is the same, and ƒr1 = ƒr2 = 3.125 GHz. Figures 4(a)-4(b) show the optical spectra of carrier branch and signal branch of downstream. After transmission and demultiplexing, the carrier branch and signal branch of upstream are depicted in Figs. 4(c) and 4(d), respectively. To investigate the performance of mode MUX/DEMUX and FMF, we measure far-field mode patterns at the points A and B in MDM system as shown in Fig. 5. These results show that LP01 mode is successfully converted to LP11 mode and then converted back using mode MUX/DEMUX after 10 km FMF transmission.
Fig. 4 Optical spectra for (a) carrier branch of downstream (b) signal branch of downstream (c) carrier branch of upstream (d) and signal branch of upstream.
Fig. 5 Output mode intensity profiles for (a) LP01 input FMF (b) LP01 output FMF (c) LP11 input FMF (d) LP11 output FMF.
Figure 6(a) shows the BER versus the received power of downstream with and without upstream transmission. The four WDM channels have similar performances and we only depict channel 2 as reference. The receiver sensitivity of downstream is −21 dBm. Figure 6(b) shows the upstream performance. The upstream has slightly inferior performance with receiver sensitivity of −18 dBm due to the transmission impairments such as mode crosstalk and ASE induced OSNR degradation. Compared with unidirectional transmission without downstream, a power penalty of 2 dB is observed for upstream because of Rayleigh backscattering noise. No obvious power penalty is observed for downstream because of the weak Rayleigh backscattering noise caused by the upstream signal (which has relatively small power). Increasing symbol rate and adopting higher order modulation format can achieve higher single channel data rate. Combined with multi-wavelength and more spatial modes, higher data rate symmetrical bidirectional transmission can be achieved. When the number of the mode increases in the proposed scheme, low modal crosstalk between the mode groups and the degenerate modes in one mode groups is essential to support more ONUs. To extend the scheme to higher mode groups, elliptical-core FMFs that break the circular symmetry can be used [30]. The MUX/DEMUX can also be realized by asymmetric fused coupler to excite and extract specific mode in one mode groups [31]. Meanwhile, when the number of mode is high, the carrier has to be split to support more users and higher order spatial mode will suffer more loss. Higher launch power at the BBU or power amplification at the RRU may be needed.
To suppress the bidirectional Rayleigh backscattering noise, a simple scheme by shifting the up-converted subcarrier frequency of downstream is demonstrated. Specifically, we halve the sampling number of radio frequency and correspondingly the up-converted subcarrier frequency of downstream is changed from 3.125 GHz to 6.25 GHz while the up-converted subcarrier frequency of upstream remains 3.125 GHz. The optical and electrical spectra of upstream and downstream are shown in Figs. 7(a) and 7(b), respectively. From the electrical spectra, we can see the overlapping part of upstream and downstream is reduced. Figure 8 shows the BER characteristics of upstream when the up-converted RF of downstream changes to 6.25 GHz. We can see 1 dB power penalty improvement is achieved because of reduced Rayleigh backscattering noise.
Fig. 7 (a) Optical spectra of upstream and downstream with frequency shift (b) Electrical spectra of upstream and downstream with frequency shift.
What is more, we investigate the effectiveness of the Rayleigh backscattering noise mitigation scheme with different downstream power. We first measure the influence of increased up-converted RF and power to the BER performance of downstream, as shown in Fig. 9. From the results, we can see at the same input power, BER performance is slightly deteriorated with RF of 6.25 GHz compared with RF of 3.125 GHz. Thus reducing overlapping by increasing radio frequency can alleviate the Rayleigh backscattering, however the overlarge up-converted subcarrier frequency will reduce the chromatic dispersion tolerance. For nonlinear tolerance, we can note that the power penalty is negligible when input power increase from −2 dBm to 4 dBm due to the large effective core area of FMF compared with SMF [31]. Thus, the power input FMF can be enhanced to increase bidirectional power budget.
Fig. 9 BER characteristics versus the received power of downstream with different up-converted RF and downstream power.
Finally, we investigate the effectiveness of the Rayleigh backscattering noise mitigation scheme with different downstream power. We measure the upstream receiver sensitivity versus the different downstream power, as shown in Fig. 10. From the result, we can see that a receiver sensitivity improvement of 2.5 dB is achieved when the downstream power is 2 dBm. The out-of-band Rayleigh backscattering noise can be effectively filtered out with the proposed scheme.
4. Conclusions
A cost effective wavelength reused MDM system for high speed symmetrical bidirectional mobile fronthaul application is proposed in this paper. Using the proposed scheme, we demonstrate a proof of concept intensity modulated 4 × 25-Gb/s 16-QAM OFDM transmission over 10-km FMF using low modal-crosstalk two-mode FMF and MUX/DEMUX with error free operation. Meanwhile, we propose a simple and effective method to suppress Rayleigh backscattering noise by shifting the downstream subcarrier frequency. A receiver sensitivity improvement of up to 2.5 dB is achieved for upstream at different downstream signal power.
Funding
National Basic Research Program of China (973 Program, No. 2014CB340105 and 2012CB315606), the National Natural Science Foundation of China (NSFC) No.61505002, 61377072 and 61275071), China Postdoctoral Science Foundation (CPSF, 2015M580926, 2016T90015).
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