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This paper proposes a novel conjugate-driven frequency shift keying (FSK) modulated wavelength division multiplexing passive network (WDM-PON) with variable-rate multicast services. Optical orthogonal frequency division multiplexing (OFDM) is adopted for multicast overlay services with different rate requirements. A differential detection is used for the demodulation of FSK signal, which can eliminate the crosstalk from the OFDM signal. A total 40-Gb/s FSK point to point (P2P) signal and 6.3-Gb/s OFDM overlay with three kinds of variable-rate multicast services are experimentally demonstrated. A physical-layer adaptive identification is proposed for the variable-rate multicast services. After 25km single mode fiber (SMF) transmission, the power penalties of FSK P2P signal and OFDM multicast overlay are 1.3dB and 1.7dB respectively.
©2011 Optical Society of America
1. Introduction
Passive optical networks (PON) have received much interest due to its cost-effective way to provide fiber to the home (FTTH) [1–4]. Several PON technologies have been proposed in previous literatures including time division multiplexing PON (TDM-PON), wavelength division multiplexing PON (WDM-PON) and optical code division multiplexing PON (OCDM-PON) [5–12]. Recently, Gigabit PON (GPON) and Ethernet PON (EPON) based on TDM technology have been standardized and widely deployed due to its simple structure and low cost. However, as the increase of broadband services and the rapid growth of Internet traffic, it is desired to implement 40 + Gb/s fiber access network [7–9]. When the access rate of TDM-PON upgrades to 40Gb/s beyond, the high-speed burst receiving and timing will become an intractable problem. OCDM-PON can provide all-optical access network, but the development of optical coder/decoder for OCDM is immature, which restricts the number of subscribers. WDM-PON has been considered as a promising approach for enhancing optical access network bandwidth [10–12]. It not only has a large capacity, but also ensures the network security as well as easy management.
In the conventional WDM-PON, the on-off keying (OOK) modulation format is adopted for the P2P signal; nevertheless, compared with the constant envelope signal which has a flat top near the central frequency of the spectrum, the OOK signal has a sharper spectral peak and it will suffer more from Rayleigh scattering (RB). Moreover, the inconstant envelope signal is easy to cause cross-channel interference due to the fiber nonlinearity in WDM-PON. Signal with constant envelope is preferred due to its constant optical power during transmission. Frequency shift keying (FSK) and differential phase shift keying (DPSK) modulation formats are both with constant envelope [13]. However, compared with FSK format, the demodulation of DPSK format requires a Mach-Zehnder delay interferometer (MZDI) at the optical network unit (ONU), which is sensitive to the environmental temperature as to affect the performance of DPSK signal. On the other hand, the FSK modulation has higher fiber nonlinear threshold (NLT) than DPSK, which can increase the power budget of the network [14].
In order to enable more flexible data delivery, a robust network architecture where the optical line terminal (OLT) can synchronously transmits P2P signal and multicast signal (e.g. video, HDTV et al.) is highly desired. Several interesting schemes have been proposed to attach the multicast signal onto the WDM-PON [15–18]. The multicast data is either DPSK format or nonreturn-to-zero (NRZ) format, which has a limited capacity for the hundreds of video streams. On the other hand, to enhance the system flexibility of WDM-PON, it is necessary to take account of dynamic bandwidth adjustment for multicast services. Orthogonal frequency division multiplexing (OFDM) has been considered to be one of the strongest candidates for future WDM-PON since it has a unique and inherent capability of high spectral efficiency and easy-realized bandwidth allocation [19–22]. It can realize variable rates for multicast services on different OFDM subcarriers.
In this paper, we propose a DML-based WDM-PON with novel conjugate-driven FSK format for P2P downstream signal and OFDM format for variable-rate multicast services. We further adopt a novel differential detection for FSK demodulation in our scheme, which can eliminate the crosstalk from the OFDM multicast signal. In future WDM-PON, the multicast services shall be dynamically changed according to the demands of subscribers, which require a flexible bit-rate mechanism for multicast services. In our proposed scheme, we further adopt a physical-layer adaptive identification for the variable-rate multicast services, which is labeled through an identify OFDM symbol. It can distinguish different service modulation and rate without high-layer assist. A 4 × 10Gb/s FSK P2P signal and 6.3Gb/s OFDM multicast signal consisting of three services are successfully demonstrated in our experiment. The experimental results show the prospect of our scheme in future optical access network.
2. The principle and architecture
The principle of the proposed architecture is depicted in Fig. 1 . At the OLT side, the N channel of FSK modulated P2P signals are combined with a multiplexer (Mux), then the combined signals are fed into a Mach-Zehnder modulator (MZM) for OFDM multicast modulation. For each channel, two directly modulated lasers (DMLs) at a wavelength space of 25GHz are adopted as the FSK tone source. The P2P signal and its inverted copy are used to drive the DMLs respectively, thus the FSK downstream signal is generated. Compared with the conventional method which adopts one chirped laser [23], the tones of FSK signal in our scheme are more stable. The combined signals are sent into a MZM for OFDM multicast signal adding. According to the bandwidth demand, the various multicast signals such as video or HDTV can be mapped onto different OFDM subcarriers with different modulation format, which is shown in the bottom of Fig. 1. Different multicast services can choose different m-QAM mapping according to their own bandwidth requirements, e.g. 16QAM for higher access rate. Then all the m-QAM mapped service data is modulated onto OFDM subcarriers, which produces a variable access rate. The electrical OFDM baseband signal is up-converted to a RF OFDM signal, and then the RF OFDM multicast signal is laid over the FSK P2P downstream signal through the MZM.
Fig. 1 The principle of proposed FSK modulated WDM-PON architecture with OFDM multicast services (DML: directly modulated laser; LO: local oscillator; MZM: Mach-Zehnder modulator).
After fiber transmission, a de-multiplexer (DeMux) is employed to separate the different channels and deliver them to the ONUs. At the ONU side, the downstream signal is divided into two parts through a 3dB optical coupler, which are used for the demodulation of P2P signal and multicast signal respectively. In our scheme, we adopt a differential detection for FSK signal, which can eliminate the crosstalk from the OFDM signal. The principle of the demodulation is shown as Fig. 2 . The input signal at the ONU can be expressed as
where p(t) and is the conjugate data of two tones of FSK signal, γ is the modulation index of OFDM signal, f1 and f2 are the frequencies of two FSK tones, nASE(t) is the ASE noise and s(t) is the OFDM signal which is given bywhere fRF is the frequency of LO. In Fig. 2, the two FSK tones are separated by a filter consisting of a fiber Bragg grating (FBG) and an optical circulator. Then they are directly detected through the PD respectively. If we ignore the ASE noise, the detected photocurrents of PD1 and PD2 can be expressed asBecause of the conjugate structure of the FSK modulation, we have. So the output P2P data is obtained as
From Eq. (4), we can see that due to the differential detection, the crosstalk from OFDM signal has been successfully eliminated. Moreover, the receive sensitivity of FSK signal has been improved by 3dB. When we adopt the differential detection, the interference from the OFDM signal and signal-noise beating can be eliminated after PD detection. Although it is a slightly more complex receiver structure compared with the single-end FSK receiver, it can enjoy the full advantage of differential detection.
As for the detection of OFDM multicast signal, the f1 + f2 frequency term has been discarded due to the low-pass photo-detection. So the detected photocurrent of OFDM signal can be expressed as
We can see that due to the constant envelope of FSK signal, it will not affect the performance of OFDM multicast overlay.
3. Experimental setup and results
The experimental setup is shown in Fig. 3 , in which we employ four P2P channels. At the OLT, there are four FSK P2P channels combined with a 4 × 1 optical coupler. In each channel, two DMLs are employed as two tones of FSK P2P signal. The wavelengths of tones for four FSK P2P signals are as follows: 1547.12nm and 1547.32nm for channel-1(Ch-1), 1548.32nm and 1548.52nm for Ch-2, 1549.57nm and 1549.77nm for Ch-3, 1550.75nm and 1550.95nm for Ch-4. For each channel, a 10-Gb/s PRBS at word length of 27-1 and its inverted copy are used to drive both the DMLs at the upper and bottom arms, and the output optical powers of the two DMLs are both −2dBm. All the FSK P2P signals are combined through a 3dB optical coupler and then sent for OFDM overlay modulation. The electrical waveforms of two FSK tones and FSK signal of Ch-1 is shown in Fig. 4(a) and (b) . The OFDM overlay signal is generated offline by Matlab programming, and then sent into an arbitrary waveform generator (AWG) at 10-Gsample/s and 8 bit DAC to produce the electrical baseband OFDM waveform. Figure 5 illustrates the generation and demodulation block diagrams for variable-rate multicast service. Because there are various kinds of services in PON and different services have their own access speed requirement. The variable-rate scheme is proposed to adapt the different services’ speed demands in physical layer. In Fig. 5, three different services are firstly mapped into 4QAM, 8QAM and 16QAM modulation format respectively. If there is any bandwidth requirement changes, the OLT can choose an appropriate format for the service, which ensures the variable rate of the multicast services. After parallel-to-serial (P/S), all the service data is modulated onto the OFDM subcarriers through IFFT. In our experiment, a total number of 256 subcarriers are utilized, which includes 29 blank subcarriers and 8 pilot subcarriers. The other 219 subcarriers are averagely distributed to the three services. The total rate of the OFDM multicast signal is 6.3Gb/s. With different m-QAM mapping, the access rate of multicast services is adjustable.
Fig. 3 Experimental setup of the proposed architecture and subcarriers distribution of the channel (AWG: arbitrary waveform generator; SMF: single mode fiber; TDS: real time digital sample scope; BERT: bit error rate tester).
Fig. 4 The electrical waveforms of FSK tones: (a) respective tones; (b) FSK signal (resolution: 100ps/div).
Fig. 5 The block diagrams of variable-rate multicast services: (a) OFDM generation; (b) OFDM demodulation (P/S: parallel-to-serial; IFFT: inverse fast Fourier transform).
The cyclic prefix of OFDM data symbol is 1/16 symbol time. A signalling symbol is attached every 100 OFDM data symbols, which indicates the mapping formats of three different services. The identify symbol is also OFDM modulated signal, which denotes different mapping formats by changing the value of each subcarrier. At the ONU, the receiver will extract the mapping information from the signalling symbol before constellation de-mapping. If the mapping format is changed, the OLT and ONU can automatically adapt the variety through the identify symbol, which is transparent to the media access control (MAC) layer. The baseband OFDM signal from the AWG is mixed with 7.5GHz sine wave through an electrical mixer. Figure 6(a) shows the electrical spectrum of RF OFDM signal, where we can see that the effective bandwidth of the signal is about 2.1GHz. The RF OFDM signal is used to drive the MZM with 1.7V corresponding to half-wave voltage of 3.5V. Then the FSK/16QAM-OFDM downstream signal is launched into a commercial erbium-doped optical fiber amplifier (EDFA) before entering the 25-km single mode fiber (SMF) link and the launched power is set to 5dBm. After transmission, a tunable optical filter (TOF) is employed to select out one of the signal channels for demodulation. The optical spectra at the corresponding points in Fig. 3 are illustrated in Fig. 6(b)-(e).
Fig. 6 Spectra at the corresponding points in Fig. 2: (a) electrical spectrum of RF OFDM signal; (b) before EDFA; (c) after TOF; (d) FBG transmitted light; (e) FBG reflected light. (resolution: 0.01nm)
At the ONU, the FSK/OFDM signals are divided into two parts for FSK and OFDM demodulation respectively. For the FSK P2P signal, a 25GHz IL is adopted to separate the two tones of the signal. Thus the FSK modulation signal is converted into a simple intensity modulation signal which can be detected directly by a single photodiode (PD). We use commercial balanced PD with 3dB bandwidth of 10-GHz to execute the differential detection, and the detected electrical signal is sent to the bit error rate tester (BERT) to test the performance. Figure 7 shows the eye diagrams and average measured BER of the four FSK P2P signals with and without transmission. The power penalty is about 1.3dB at the BER of 10−9, which is mainly caused by the fiber dispersion during transmission. From the eye diagrams, we can see that there is little influence of the OFDM signal. Figure 7 also shows the BER curve of single wavelength. Compared with the single channel, there is about 0.3dB power penalty in the WDM case, which mainly comes from the interference of neighbor channel.
Fig. 7 The measured BER curves and eye diagrams of FSK P2P signal before and after transmission (resolution: 50ps/div).
For the OFDM overlay signal, since the bandwidth of the OFDM signal is about 2.1GHz, the optical signal can be directly sent into a 2.5GHz commercial PD for O/E conversion. After down-converted with a 7.5GHz LO, the baseband OFDM signal is sampled by a 20-GSa/s Tektronix6804B real time digital sample scope (TDS) to capture the waveform of the electrical OFDM signal for offline processing. The measured average BER curves and constellations are shown in Fig. 8 and Fig. 9 respectively. The power penalty is about 1.7dB before and after transmission. Due to fiber dispersion, the two tones would suffer from fading effect, which deteriorates the performance of the OFDM signal. A BER below 10−3 which satisfies the FEC limit can be obtained after 25km SMF transmission. It represents an excellent performance where BER of 10−9 or less can be obtained with the help of FEC.
Fig. 9 The measured constellations of three kinds of multicast services before and after transmission.
4. Conclusion
We have proposed and experimentally demonstrated a novel 40Gb/s conjugate-driven FSK modulated WDM-PON architecture with variable-rate multicast overlay. In this scheme, the 4 × 10-Gb/s FSK modulated P2P signals and 6.3-Gb/s variable OFDM overlay signal with three services are evaluated for a 25-km SMF link. For FSK demodulation, a differential detection is adopted to eliminate the crosstalk from the OFDM overlay signal. We propose a physical-layer adaptive identification for the variable-rate multicast services through signalling symbol. The power penalties of the WDM FSK P2P signals and OFDM overlay signal are 1.3dB and 1.7dB respectively. Our experimental results show a promising solution for next generation access network.
Acknowledgments
The financial supports from National Basic Research Program of China with No. 2010CB328300, National Natural Science Foundation of China with No. 61077050, 61077014, 60932004, BUPT Young Foundation with No.2009CZ07 are gratefully acknowledged. The project is also supported by the BUPT Excellent Ph.D. Students Foundation with No.CX201014 and the Open Foundation of State Key Laboratory of Optical Communication Technologies and Networks (WRI) with No.2010OCTN-02.
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