We have proposed a novel orthogonal frequency division multiplexing (OFDM) modulated WDM radio over fiber (ROF) system by employing a single photonic crystal fiber (PCF) supercontinuum (SC) multi-wavelength lightwave source. Both wired and wireless applications of ROF access are achieved. In our experiment, we pick out four 40-GHz ROF channels by properly designed fiber Bragg grating (FBG). The 1-Gb/s 16QAM-OFDM downstream signal is demonstrated for both wired and wireless applications over 20-km standard single mode fiber (SMF). A 1-Gb/s OOK upstream signal is also transmitted over 20-km SMF successfully with less than 0.4dB power penalty.
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In the future access services of wireless telecommunication, the millimeter wave (mm-wave) radio over fiber (ROF) system is very attractive to meet the increasing demands of the wide bandwidth, large capacity, low power consumption, as well as system cost-effective. The 40~60GHz mm-wave has recently gain much attention and will probably be the first choice for ROF wireless access systems [1–6]. Furthermore, the dense wavelength division multiplexing (DWDM) technique requires the central office (CO) to support a huge number of base stations (BS). However, in most previous WDM-ROF systems, there must be an individual light source for each link, which results a high-cost and high-maintenance systems, and it is desirable to replace the individual light sources by a multi-wavelength one. Although a multi-wavelength laser is an option for the required source, it is too expensive for practical application. The mode-locked laser diode is cheaper, but the available channel number is limited . A supercontinuum (SC) light source, which is generated by a seed pulse laser and properly designed fiber, can produce a great number of optical carriers by a proper wavelength select device with low cost (such as arrayed waveguide grating ), and its characterization has been studied for multi-wavelength light source in DWDM transmission systems [8–10]. The combination of orthogonal frequency division multiplexing (OFDM) and ROF system has attracted considerable attention for its high spectral efficiency, gigabit broadband and the resistance to chromatic dispersion and polarization mode dispersion [11–14].
In this paper, we propose a novel OFDM modulated WDM-ROF system to provide both wired and wireless applications, where we import the generation of SC light source using the nonlinear dispersion-flattened photonic crystal fiber (PCF). Comparing with the previous SC for DWDM application, PCF can easily generate a stable SC due to its higher nonlinearity and controllable dispersion curve. A wavelength reuse scheme is employed to simplify the BS and reduce power consumption. We experimentally demonstrate the generation and transmission of four 40-GHz ROF channels with 1-Gb/s 16QAM-OFDM signal. The 1-Gb/s OOK upstream is also demonstrated in our experiment.
2. System architecture of WDM-ROF using a SC light source
The proposed architecture is illustrated in Fig. 1 . At the CO, we employ a pico-second pulsed laser with a repetition rate of 10-GHz and an 80 m-long high-nonlinear PCF to generate the SC. The cross section and dispersion curve of the PCF are illustrated in Fig. 2 . It has a three-fold symmetric hybrid core region with a core diameter of 2.1um and the nonlinear coefficient is 11 (W−1·km−1). Besides, it has a small negative dispersion with a variation smaller than 1.5 (ps·nm−1·km−1) between 1500 and 1650 nm and the attenuation is less than 9 dB/km in the range of 1510-1620 nm. The slight negative dispersion values of PCF can prevent soliton propagation and make it attractive for generation of narrow but stable SC. When the incident pulse transmits in the PCF with normal dispersion, the interaction between the self-phase modulation (SPM) and the fiber dispersion itself makes a great contribution to the spectrum broadening. Generally speaking, the spectrum expands rapidly due to the SPM at first; as the pulse propagating, the dispersion broadens the temporal shape. When increasing the input power, the stimulated Raman scattering will lead to an asymmetrical broadening due to the new frequency components on the red side of the spectrum. Then we use N pairs of properly designed fiber Bragg gratings (FBGs) to pick out N dual-peaks for the optical millimeter wave (mm-wave). The N pairs of FBGs have reflective central frequencies fromto, where denoted the difference of their central frequencies which can be ranging from 40GHz to 70GHz according to the demand of the BS. The mm-waves are then allocated to N channels by a demultiplexer (DEMUX). In each channel, a Mach-Zehnder modulator (MZM) is driven by the downstream OFDM signal, thus the ROF signals are generated. Then all ROF signals are combined by a Mux and launched into the optical fiber.
At the remote node (RN), the N channels are firstly separated by a DeMux, and then delivered to different BS. In order to satisfy the various demand of subscribers, we integrating both the wireless and wired access application in the BSs. The subscribers can choose either of the access style according to their demands. In each BS, the received signal is split into two parts by a 3-dB optical coupler: one is to provide light source for uplink connection, and the other is for both wired and wireless applications, which is controlled by an optical switching. For upstream link, either of the dual-peak is filtered out and then re-modulated by the OOK signal through an intensity modulator (IM), and there is no additional light source needed any more. At the CO, we use a low speed avalanche photodiode (APD) for upstream demodulation.
3. Experiment and discussion
The experiment setup for the proposed scheme is shown in Fig. 3 . The repetition of the pulse laser is 10-GHz, so the SC light source would generate a broadband optical frequency comb with a space of 10-GHz. The optical spectrum of the initial SC is shown in Fig. 4(i) . In our experiment, we use four pairs of FBGs to pick out 4-channel optical mm-waves for the downstream transmission, where = 40GHz. The optical spectrum of the 4-channel mm-waves is shown in Fig. 4(ii), where we can see the space of the dual-peak is 40-GHz. The optical mm-wave carriers are then boosted to 13dBm by an EDFA before entering the MZM for signal modulation. A 1-Gb/s 16QAM-OFDM signal is generated offline and uploaded to the arbitrary waveform generator (AWG) with 10-bits DAC. The 215-1 PRBS with 16QAM coding is mapping into 256 subcarriers, in which 8 pilot sub-carriers are used for channel estimation, 24 subcarriers at low frequency and 24 subcarriers at high frequency unfilled for over-sampling. The length of cyclic prefix is 1/16. We also adopt software up-conversion in programming to eliminate the low frequency damnification and further improve the 16QAM-OFDM signal performance. Training sequence is added every 30 OFDM symbol for channel estimation and synchronization. The electrical spectrum of the signal is shown in Fig. 4(iii), and we can see the central frequency is 500MHz with a bandwidth of 250MHz. The OFDM signal with peak-to-peak voltage of 2V is used to drive the MZM (half wave voltage of 3.5V), and the optical spectrum after the MZM is shown in Fig. 4(a). Thus the 4-channel 16QAM-OFDM modulated ROF signals are generated. Before launched into the 20-km downstream link, the ROF signals are amplified to 16dBm by an EDFA.
After 20-km single mode fiber (SMF) transmission, a tunable optical filter (TOF) is employed to select out the allocated channel as well as suppress the ASE noise. Then the received signal is split by a 3dB optical coupler. One coupler output is delivered to the downstream receiver, while the other is used for 1-Gb/s upstream re-modulation via an intensity modulator (IM). Figure 4(b) and 4(c) show the optical spectra of channel 3 before the optical coupler and after re-modulation respectively.
The downstream receiver includes both wireless and wired applications. The 40-GHz ROF signal is pre-amplified with an EDFA with a small signal gain of 26dB, and an optical switch is employed to select the application style. For wireless application, the 40-GHz ROF signal is directly detected by a 50-GHz PIN PD for O/E conversion and amplified by a narrow band electrical amplifier (EA) centered at 40-GHz. A 40-GHz RF clock and a mixer are used to down-convert the mm-wave signal. Then it is fed into a 20-GHz real time digital sampling scope (TDS) to capture the waveform for offline digital signal processing. For wired application, the downstream signal is fed into a 2.5-GHz APD to execute the O/E conversion. The same mechanism is used to demodulate the 16QAM-OFDM signal. The BER performances of the four channels are almost identical, thus only the BER performance for channel 3 is provided. The BER curves and constellation diagrams for both wireless and wired scenario are illuminated in Fig. 5 . Comparing with the wireless one, the performance of wired application gets 1dB sensitivity improvement at the BER of 10−4. Because the wireless signal is amplified again by a narrow band EA with low noise figure, the wireless signal gets a small different performance (1dB) from the wired signal.
For upstream link, the ROF signal is firstly filtered out either of the dual peak through a TOF, and then the peak is re-modulated by an IM at 1-Gb/s with PRBS length of 231-1. The bandwidth of 16QAM-OFDM is only 250MHz, which is narrow compared with the 1GHz upstream OOK signal, so it is feasible for upstream re-modulation. Furthermore, we adopt an IM with large extinct ratio, which can minimize the effect from downstream signal. After 20-km uplink transmission, a 2.5-Gb/s APD receiver is employed to detect the OOK signal. Figure 6 shows the eye diagrams and measured BER performance of the upstream signal, from which we can see that the power penalty at the BER of 10−9 is less than 0.4dB.
In our experiment, we have used four EDFAs and two TOFs. In the SC-ROF source, there are two EDFAs: one is used as a pump to excitated the nonlinearity in the PCF for SC generation, and the other is used as pre-amplifier for the mm-waves. Compared with the multi-wave laser scheme, it is more cost-effective in practical use. The EDFA at the BS also acts as a pre-amplifier to improve the receive sensitivity of the downstream signal. In the experiment, we adopt two TOF to filter out different channel wavelength. But in practical use, after the channel wavelength allocation by the array waveguide grating, there is no need to use the TOF, which will be much more cost-effective for the BS.
We have proposed and experimentally demonstrated a novel OFDM modulated WDM-ROF system by employing a single SC multi-wavelength lightwave source based on PCF. This scheme also employs wavelength reuse scheme for power consumption. The wired and wireless applications for the downstream signal are demonstrated in our scheme. Both the 1-Gb/s 16QAM-OFDM downstream signal and 1-Gb/s OOK upstream signal present good performance. The results show that the power penalty of the downstream wireless and wired signal at BER of 10−4 are 0.35 and 0.2 dB, respectively. The upstream signal at BER of 10−9 is less than 0.4dB after 20-km SMF transmission.
The financial support from National Basic Research Program of China with No. 2010CB328300, National Natural Science Foundation of China with No. 60677004, 60977046, National High Technology 863 Research and Development Program of China with No. 2009AA01Z220 are gratefully acknowledged. The project is also supported by the Program for New Century Excellent Talents in University of China with No. NECT-07-0111.
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