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In this paper, we propose and demonstrate a novel integrated radio-over-fiber passive optical network (RoF-PON) system for both wired and wireless access. By utilizing the polarization multiplexed four-wave mixing (FWM) effect in a semiconductor optical amplifier (SOA), scalable generation of multi-frequency millimeter-waves (MMWs) can be provided so as to assist the configuration of multi-frequency wireless access for the wire/wireless access integrated ROF-PON system. In order to obtain a better performance, the polarization multiplexed FWM effect is investigated in detail. Simulation results successfully verify the feasibility of our proposed scheme.
©2013 Optical Society of America
1. Introduction
Radio-over-fiber (RoF) technology has revealed great potential for the application of future broadband wireless access networks, due to its seamless integration of the sufficient bandwidth of optical fiber communication and the high mobility of wireless radio communication [1, 2]. In recent years, 60 GHz millimeter-wave (MMW) RoF system has been widely studied because of its globally available 7-9 GHz unlicensed bandwidth and negligible interference with current low radio-frequency (RF) wireless services [3–5]. According to ECMA-387, the unlicensed 8 GHz bandwidth is actually divided into four sub-bands centered at around 58 GHz, 60 GHz, 62 GHz and 64 GHz with frequency separation of about 2 GHz. A mass of schemes have been proposed to generate multiple-frequency MMWs for the 60 GHz RoF systems for the application of multi-services in multi-occasions. For example, ref [6] has proposed an approach to generate MMWs of 20 GHz, 40 GHz and 60 GHz, by using four-wave mixing (FWM) effect in a semiconductor optical amplifier (SOA). But this approach can only generate three different MMWs with large frequency grid, which is not applicable to ECMA-387. What’s more, a method which can provide MMWs around 60 GHz has been introduced in ref [7] by cascading two Mach-Zehnder modulators (MZMs). Nevertheless, this method suffers from severe instability caused by the bias drifts of the modulators.
In order to meet the demand of high-capacity and high-speed information access, seamless integration of the RoF technology with now existing passive optical network (PON) has already been proposed to effectively provide both wired and wireless access services [8, 9]. Many researches focused on the integration of wired/wireless access have been investigated, for example, a long-reach high split-ratio WDM-PON has been introduced to provide both wired and wireless services by using SOA for remote upconversion [10]. However, all of the schemes are proposed to achieve the integration of wired baseband services and wireless RF services at a single and fixed frequency, which is not suitable for the situation where multi-services, multi-occasions or multi-applications should be taken into consideration [11].
In this work, we propose and demonstrate a novel wired/wireless access integrated RoF-PON system with scalable generation of multi-frequency MMWs, enabled by the polarization multiplexed FWM effect in a SOA. By employing polarization multiplexing (PolM) in the FWM effect of a SOA, multiple MMWs around the radio frequency of 60GHz can be generated [12–14]. In our detailed simulation verification, the proposed PolM-FWM effect in a SOA has been thoroughly discussed so as to obtain a better performance. In the proof-of-concept demonstration, 1.25 Gb/s wired access and 1.25 Gb/s wireless access with nine multi-frequency MMWs have been seamlessly integrated for the application of multi-services in multi-occasions.
2. Principle of the proposed wired/wireless access integrated RoF-PON system with scalable generation of multi-frequency MMWs
Figure 1 shows the principle of our proposed wired/wireless access integrated RoF-PON system with scalable generation of multi-frequency MMWs. In the optical line termination (OLT), the input optical signal before SOA is shown in Fig. 1(a). In Pol-Y, an optical carrier at f0 is modulated with wired data. While other n bands on its right side, with a frequency grid of f1 between them, are modulated with wireless data. The wired band and multiple wireless bands are coupled together and aligned in Pol-Y. While in pol-X, two unmodulated pump bands with 60 GHz grid are centered at f0. Through polarization combining, both signals in Pol-Y and Pol-X are combined together to form the polarization multiplexed input signal of SOA. Then the input signal is sent into SOA to perform the FWM effect and the output signal is given in Fig. 1(b). In pol-X, 4n new wireless bands modulated with wireless data are generated on both the left and right sides of Pump 1 and Pump 2, respectively. The frequency interval between these new bands and the pump signal are all f1 for both Pump 1 and Pump 2 cases. Meanwhile, n new wireless bands are generated on the left side of the original wired signal at f0 in pol-Y. Consequently, the original wired band in Pol-Y can be filtered out to provide the wired signal access while the generated multiple wireless bands and two original pumps in Pol-X can be used to generate multiple-frequency MMWs for the wireless signal access.
Fig. 1 Principle of the proposed wired/wireless access integrated RoF-PON system with scalable generation of multi-frequency MMWs.
In order to effectively demonstrate the scalable generation of multiple-frequency MMWs utilizing the polarization multiplexed FWM effect in SOA, two wireless bands are selected from the multiple wireless bands and the two original pumps in Pol-X: one is selected from the left part in Pol-X, while the other is selected from the right part in Pol-X. We assume that the ith band in the left is selected where –n≤i≤n and it can be given as
where A is the amplitude and , are the angular frequency and phase, respectively. f1 is the frequency interval of the multiple wireless bands in Pol-Y. Similarly, we assume the jth band in the right is selected where –n≤j≤n and it can be given asand then these two bands are taken to perform the optical heterodyning in a photodiode (PD) and the current at the output of the PD within its limited bandwidth can be described aswhere R is the responsivity of the PD. Equation (3) shows that a MMW signal with a frequency of around 60GHz has been generated. The frequency range of the MMW signal is [60-2nf1, 60 + 2nf1] with a frequency grid of f1. Thus totally 4n + 1 MMWs with different frequency can be generated by the polarization multiplexed FWM effect in SOA.3. Simulation setup
Figure 2 shows the simulation setup of the proposed wireless/wired integrated ROF-PON system. In order to demonstrate and verify this proposal, VPI transmission-Maker Version 8.3 is used and we assume n = 2 and f1 = 2 GHz in the demonstration. In the OLT, a laser-diode (LD1) at f0 = 193.1 THz with 3 dBm launch power is used as the optical source and it is then divided into two branches. One branch is modulated by a 30 GHz radio frequency (RF) source in an intensity modulator (IM1) to perform the optical carrier suppression (OCS) modulation, so as to generate two first-order sidebands as two pumps with a frequency interval of 60GHz. A polarization controller (PC1) is used to adjust the polarization state of two pumps to Pol-X, as depicted in Fig. 2(a). The other branch is input into IM2 and modulated with 1.25 Gb/s baseband non-return-to-zero (NRZ) data, thus the wired band is generated. Meanwhile, LD2 at 193.102 THz and LD3 193.104 THz both with 3 dBm launch power are firstly coupled together, and then the coupled signal are sent into IM3 and modulated with another 1.25 Gb/s baseband NRZ data, thus two wireless bands are achieved. The wired band and two wireless bands are combined together and a variable optical attenuator (VOA1) is used to adjust the initial power difference (P0) between the wired band and two wireless bands. Then PC2 is utilized to adjust the polarization state of the combined signal to Pol-Y, as illustrated in Fig. 2(b). After that, a polarization beam combiner (PBC) is adopted to generate the polarization multiplexed signal by combining signals in Pol-X and Pol-Y together which is given in Fig. 2(c). The polarization multiplexed signal is taken as the input of SOA to perform the FWM effect. Finally, ten new wireless bands are generated and eight of them are generated in Pol-X, as shown in Fig. 2(d). The SOA output signal from OLT with the power of 13.8dBm is then transmitted over 20km standard single mode fiber (SSMF) and VOA2 which has a 21 dB attenuation to simulate a 128 split. In each ONU, the received signal has a power of −11.2 dBm and a polarization beam splitter (PBS) is used to separate the signals to Pol-X and Pol-Y, respectively. In Pol-X, a de-multiplexer (DeMUX), such as a passive AWG, is utilized to filter out two selected wireless bands with a particular frequency interval fRF, as shown in Fig. 2(e). Then these two wireless bands are combined together by an optical combiner (OC) and sent into PD1 to generate the MMW at the frequency of fRF by optical heterodyning. According to Eq. (3), we can obtain nine different MMWs at the frequency from 52 GHz ~68 GHz with 2 GHz frequency grid. While in Pol-Y, the wired band at 193.1 THz is filtered out by a band-pass filter (BPF) as shown in Fig. 2(f), and then detected by PD2 to obtain the wired data. Therefore, each ONU can simultaneously provide both wired access and multi-frequency MMWs enhanced wireless access. Our simulations mainly focus on the downstream transmission of both wired baseband signal and wireless multiple-frequency MMWs and the implementation of upstream transmission can also been achieved according to [10].
Fig. 2 Simulation setup of the proposed wireless/wired integrated RoF-PON. The insets show the spectra at different stages.
4. Results and discussion
In order to obtain a better performance for the PolM-FWM effect in SOA, the relationship of the following three key parameters in the demonstration are thoroughly discussed: the optical signal-to-noise ratio (SNR) performance of the generated wireless bands in Plo-X, the initial power difference P0 between the wired band and the wireless bands and the power difference Pdiff between the generated wireless bands and the high-order noise bands. The relationship of SNR and Pdiff versus P0 for different SOA currents is shown in Fig. 3 . In Fig. 3(a), SOA current is set at 400 mA and the SNR increases with the increased P0, while Pdiff decreases when P0 is increased. Two curves intersect at point A and we take this point as the optimized situation to obtain a relatively high SNR while Pdiff is relatively low. So the optimized P0 for 400 mA SOA current is about 12 dB. Similarly, for the cases of SOA currents 350 and 300 mA, the optimized P0 for both cases is about 14 dB. In consequence, for SOA current 300~400 mA, the optimized P0 is 12~14 dB.
Fig. 3 Optical SNR performance of the generated wireless bands in Pol-X and the power difference (Pdiff) between the generated wireless bands and the high-order noise bands versus the initial power difference (P0) between the wired band and the wireless bands for different SOA currents.
Figure 4 illustrates the detailed relationship of SNR and Pdiff versus SOA currents for different P0. In Fig. 4(a), SNR increases with the increase of SOA current while SNR decreases when P0 is enlarged from 12 dB to 14 dB. However, in Fig. 4(b), Pdiff decreases with the increase of SOA current and Pdiff increases when P0 is enlarged from 12 dB to 14 dB.
Fig. 4 The relationship of (a) SNR versus SOA currents for different P0 and (b) Pdiff versus SOA currents for different P0.
According to the analysis above, we set the SOA current as 400 mA and P0 as 13 dB in the simulation demonstration for optimization. The spectrum of SOA input signal is given in Fig. 5(a) and it shows that the power difference between the wired band and two wireless bands is 13 dB. After the PolM-FWM effect in SOA, the spectrum of SOA output signal is shown in Fig. 5(b). As can be seen from Fig. 5(b), the SNR of the generated wireless bands reaches 14.5 dB while the power difference between the generated wireless bands and the high-order noise bands is 12.2 dB.
Figure 6 plots the bit-error rate (BER) performance for 1.25 Gb/s wired access in our proposed wired/wireless integrated ROF-PON system for both back-to-back (B2B) and 20km SSMF transmission. After transmission over 20km SSMF, the receiver’s sensitivity for 1.25 Gb/s wired baseband signal is −20.4dBm and the power penalty at a BER of 10−9 is 0.65 dB. The eye diagrams for B2B and 20km SSMF transmission cases are shown in the inserts (a) and (b) in Fig. 6. The BER performance for 1.25 Gb/s wireless access is also plotted in Fig. 7 . We take 58 GHz and 62 GHz cases for example to verify the performance of the wireless access in our proposed wired/wireless integrated ROF-PON system. Receiver’s sensitivities for 1.25 Gb/s wireless 58GHz and 62GHz after down-conversion are −20.5dBm and −19.8dBm, respectively. The power penalties at a BER of 10−9 after 20km SSMF are 0.4 dB and 0.5dB for 58 GHz and 62 GHz, respectively. Inserts (a)~(d) in Fig. 7 are the corresponding eye diagrams for B2B and 20km SSMF transmission respectively. In this scheme, data rate for both wired baseband signal and wireless multiple-frequency MMWs is limited due to the limited 2 GHz channel separation, but other high-order modulation signal such as M-PSK or M-QAM signals can be utilized to improve the data rate meanwhile occupying a proper bandwidth.
Fig. 7 BER performance for 58 and 62 GHz wireless access after B2B and 20km SSMF and corresponding eye diagrams.
4. Conclusion
We have proposed and verified a novel wired/wireless access integrated RoF-PON system enabled by the polarization multiplexed FWM effect in SOA. In the proposed RoF-PON system, both wired baseband access and wireless access with multi-frequency MMWs can be effectively supported. Detailed investigation about the performance of the polarization multiplexed FWM effect in SOA has been conducted and the optimized initial power difference between the wired band and the wireless bands for different SOA currents has been found to guarantee a better performance of the proposed wire/wireless access integrated ROF-PON system.
Acknowledgments
This work is supported by NSFC No. 61171045. The authors would thank Dr. Xingwen Yi and other anonymous reviewers for improving the clarity and quality of this paper.
References and links
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