Wavelength-division-multiplexing passive optical network (WDM-PON) is a promising architecture for next-generation access networks because of its large bandwidth, protocol transparency and scalability. In this paper, we propose a cost-effective, high-speed upstream WDM-PON scheme adopting polarization division multiplexed (PDM) on-off keying (OOK) modulation at the optical network unit (ONU) and digital coherent/self-coherent detection with a novel blind dual-modulus equalization algorithm at the optical line terminal (OLT). As such, the upstream capacity can be directly enhanced at low ONU expenditure, and receiver sensitivity as well as power budget can be also improved. Enabled by the scheme, 40-Gb/s upstream transmission in 80-km WDM-PON is experimentally demonstrated.
© 2015 Optical Society of America
With the rapid development of Internet and proliferation of multimedia-based services, the bandwidth demand of end-users increases constantly, and a high-speed passive optical network (PON) supporting at least 40-Gb/s service for certain subscribers will be in demand in the near future [1,2]. Owing to advantages including large bandwidth, protocol transparency and scalability to support multiple subscribers, wavelength-division-multiplexing (WDM)-PON is deemed promising to cope with the capacity requirement . On the other hand, deploying expenditure is always a critical concern in PON scenario. In upstream WDM-PON transmission, if complex modulation scheme such as orthogonal frequency-division-multiplexing (OFDM) is used [4,5], the overall cost of optical network units (ONUs) would be a great challenge due to the requirement of high-speed digital circuits and high-linearity electronic and/or optical devices. Alternative upstream schemes may rely on cost-effective and mature on-off keying (OOK) transceiving modules, in which the transmission impairments induced by chromatic dispersion (CD) pose a large limitation on achievable data rate and reach . Although the capacity can be doubled by exploiting polarization division multiplexing (PDM) technique, realizing polarization de-multiplexing of PDM-OOK signals using direct detection is not straightforward due to the trouble of tracking the variation of polarization states in transmission link.
With massive production and advanced photonic integration techniques, coherent or self-coherent detection is becoming an attractive choice for upstream WDM-PON [2,7], which can provide higher power budget by improving receiver sensitivity. In this paper, we propose an upstream WDM-PON scheme based on PDM-OOK modulation at the ONU and digital coherent or self-coherent detection at the optical line terminal (OLT). As such, enhanced upstream data-rate can be achieved with simple ONU structure, which is very beneficial for cost reduction and PON deployment. Upon reception, a novel digital signal processing (DSP) method named dual-modulus algorithm (DMA) effectively compensates the transmission impairments and blindly de-multiplexes two polarizations. Furthermore, the scheme is robust against laser phase noise, allowing for the use of cost-effective large-linewidth lasers . Based on the scheme, 20-Gb/s and 40-Gb/s upstream OOK transmissions over 80-km standard single mode fiber (SSMF) are experimentally demonstrated with good receiver sensitivity.
This paper is organized as follows. In Section 2, the architecture of proposed WDM-PON scheme is described. Section 3 shows the principle of DMA-enabled digital coherent detection scheme with simulation results. Experimental results are reported in Section 4. Finally the paper is concluded in Section 5.
2. Architecture of the upstream WDM-PON scheme
Figure 1 shows the architecture of proposed WDM-PON, in which we focus on upstream transmission. At the ONU, the PDM transmitter consists of two conventional optical OOK modules and a polarization beam combiner (PBC), which can be highly economical without requiring additional costly devices. At the OLT, conventional direct-detection OOK receivers are upgraded to dual-polarization digital coherent receivers, which is also compatible with legacy ONUs using single-polarization (SP) OOK modulation . It is worth noting that the architecture can be readily extended to realize colorless WDM-PON by replacing optical OOK modules with reflective modulators, such as reflective semiconductor optical amplifiers (RSOAs) combined with optical circulators . Since the signals generated by RSOAs can be still OOK or PDM-OOK modulation, the OLT-side DMA-based coherent detection is still applicable. On the other hand, for downstream transmission, dispersion pre-compensation combined with judicious modulation format can be considered, which allows for cost-effective direct detection at ONU-side and may avoid costly analog-to-digital conversion (ADC) and DSP .
3. Dual-modulus algorithm for PDM-OOK signal
3.1 Technique principle
The DSP method for upstream PDM-OOK signal after coherent detection and ADC is shown in Fig. 2. The resampling and normalization part are similar to those in PDM-QPSK recovery , which aim at providing normalized data with proper sample spacing for subsequent digital equalizer. Then blind equalization is performed to combat linear channel impairments such as CD, polarization-mode-dispersion (PMD) and filtering effect. Here this equalizer (referred to as DMA) is implemented by four butterfly-configured adaptive finite impulse response (FIR) filters. Electronic dispersion compensation (EDC) may be performed prior to equalization  (e. g., using frequency-domain overlap-and-add approach ), so that the DMA equalizer can be mainly used for polarization de-multiplexing. After channel equalization, if OFDM or quadrature amplitude modulation (QAM) signal is to be correctly demodulated, frequency and phase estimation are generally required. Nevertheless, for OOK signal which does not encode data on carrier phase, frequency and phase recovery may not be indispensable. In this case, decision can be performed based on the modulus of recovered data.
The principle of proposed DMA in terms of error signal calculation method for PDM-OOK is shown in Fig. 3. Constant modulus algorithm (CMA)  and cascaded multi-modulus algorithm (CMMA)  are two commonly adopted blind equalization algorithms that exploit knowledge of signal formats. However, for PDM-OOK format, the CMA is not well compatible, because OOK does not present constant data amplitude. On the other hand, CMMA is specifically used to equalize QAM signals, which replaces a single radius of convergence with multiple new radii of convergence and uses cascaded error function. When considering blind equalization of OOK, the origin of coordinates can be taken as one convergence radius (i.e., R1 = 0) together with the other modulus R2 = 1, which is the basic idea behind the proposed DMA. As such, two reference moduli A1 = (R1 + R2)/2 and A2 = (R2-R1)/2 are defined to make the final error of OOK approach zero. The filter tap weight updating criteria are similar to CMMA [13–15], as given below,
Where μ is the convergence parameter and (·)* denotes complex conjugation. The feedback error signal εx,y for filter tap updating of OOK and ex,y for OOK are respectively given by16] or its modified form can be considered.
3.2 Numerical investigation
To investigate the effectiveness of proposed DSP method, simulations of PDM-OOK upstream transmission using DMA-based digital coherent detection are carried out. Figure 4 depicts the block diagram of simulation setup for PDM-OOK upstream transmission. VPItransmissionMaker Optical Systems software platform is employed for front-end and optical channel simulation, while the baseband DSP are performed in Matlab and embedded into VPI modules. The data rate on each polarization is 10-Gbit/s. At the OLT, the EDFA for pre-amplification has a fixed output power of 0-dBm and a noise figure of 5-dB. The 2nd-order Gaussian-shape OBPF has a bandwidth of 50-GHz. The 4th-order Bessel-shape ELPF has a 3-dB bandwidth of 10-GHz. The thermal noise of balanced photodiodes is set to zero. The output power of local oscillator (LO) is set to 10-dBm. In DSP procedure, 9-tap, T/2-spaced DMA equalizer is adopted with fixed step of 0.002.
Figure 5(a) shows theoretical and simulated bit error ratio (BER) curves versus received power of PDM-OOK in BtB case and after 80-km transmission. Theoretical BER is governed by the following equation :18]
The tolerance to laser linewidth of the proposed scheme in the scenario of coherent WDM-PON is also investigated. Figure 5(b) shows BtB and post-transmission performance of BER performance as a function of signal laser and LO linewidth. Note that in this simulation the linewidth of signal laser and LO are always set identical, and received power is fixed at −44 dBm. It is observed that, compared with the result in Fig. 5(a), negligible BER penalty is induced when signal laser and LO possess less than 30-MHz linewidth. This is because the pure intensity-modulated OOK signal is highly tolerant to fluctuation in phase. The results indicate that it would be possible to use cost-effective large-linewidth lasers in the system, such as distributed Bragg reflection (DBR) laser or vertical cavity surface emitting laser (VCSEL) . Meanwhile, it is noted that two lasers combined with PBC may be more suitable for ONU since cost-effective direct-modulated lasers can be introduced. In this case, modified DSP method may be needed to combat the possible frequency difference between the two lasers.
4. Experiment setup and results
We experimentally demonstrate upstream PDM-OOK transmission over 80-km optical distribution network (ODN) and the experimental setup is shown in Fig. 6. At the ONU, a 10-Gbit/s pulse pattern generator (PPG) and a 20-Gbit/s PPG are used to generate 215-1 pseudo random binary sequence (PRBS), respectively. A laser with center wavelength of 1549.92-nm is modulated via a Mach-Zehnder modulator (MZM) to generate the optical OOK signal. The PDM is emulated by a polarization beam splitter (PBS), an optical delay line and a polarization combiner (PBC). After PDM, 20-Gb/s and 40-Gb/s upstream data rate are achieved, respectively. The ODN is emulated with 80-km SSMF and a variable optical attenuator (VOA). At the OLT, the pre-amplified PDM-OOK signal first passes an optical band-pass filter with a bandwidth of 50 GHz. The received signal is then sent into a dual-polarization 90-degree optical hybrid with a local oscillator (LO) and interferes with each other before being detected by four balanced detectors (BD). Finally the polarization diverse signals are sampled by a real-time digital storage oscilloscope operating at 50 GS/s and processed offline. The linewidths of signal laser and LO are both about 100-kHz. In DSP procedure, the signal is firstly resampled to 2 samples per symbol and normalized. After CD compensation, four butterfly-configured 13-tap, T/2-spaced adaptive FIR filters are then used for polarization de-multiplexing, the tap coefficients of which are updated by DMA with a convergence parameter of 0.001. Finally, decision and direct error counting are performed to obtain BER. For optical OOK without information encoded on phase, no carrier frequency recovery or phase recovery is used, which can simplify OLT .
Figure 7 shows sample constellation diagrams of two orthogonal polarizations before and after blind DMA equalization for PDM-OOK signal after 80-km SSMF transmission. With the equalization of DMA, data points converge to two radii (0 and 1) finally. Figure 8(a) and 8(b) depict the spectra for 20-Gb/s and 40-Gb/s PDM-OOK upstream signals at ONU, respectively. The insets show corresponding eye-diagrams. Figure 9(a) and 9(b) show the BER performance of 20-Gb/s and 40-Gb/s PDM-OOK upstream signals after 80-km SSMF transmission, respectively. The receiver sensitivity (defined as required received power at target BER of 1e-3) of 20-Gb/s PDM-OOK signals is −36.7dBm in BtB case and −35.5dBm after 80-km transmission. Thus the power penalty caused by 80-km reach ODN is 1.2-dB. In 40-Gb/s PDM-OOK upstream transmission, the receiver sensitivity is −34.2dBm in BtB case and −33.2dBm after 80-km transmission respectively, which results from the higher optical signal to noise ratio (OSNR) requirement upon reception. Compared with simulation results, the penalty in the experiment can be attributed to the imperfectness of available devices, such as PPG and MZM. Also, an electrical or digital low-pass filter at OLT-side with proper bandwidth may further improve the performance. Because the scheme works independently of the generation method of OOK signals, it can be extended to other colorless upstream scheme including RSOA-based OOK modulation. 4-ary pulse amplitude modulation (PAM4) is another possible upstream modulation format that doubles the bit rate by using low-bandwidth components . Compared to PAM4 with direct detection at the same bit-rate, our scheme remains to use simple and mature OOK module at ONU and can support longer reach with good receiver sensitivity by adopting coherent detection. On the other hand, in the context of coherent WDM-PON, ultra-dense (UD-) WDM-PON may be one promising choice to offer enhanced capacity for a large number of end-customers . Compared to coherent UDWDM-PON, our proposal capable of providing high upstream bit rate per λ with simple OOK modules can be more suitable in certain applications such as point-to-point overlay.
In this paper, we propose an upstream WDM-PON scheme using PDM-OOK modulation and DSP-based coherent detection with the DMA equalizer. 20-Gb/s and 40-Gb/s PDM-OOK upstream transmission over 80-km ODN are experimentally demonstrated, achieving receiver sensitivity of −35.5dBm and −33.2dBm, respectively. Moreover, simulations show that this scheme is robust against laser phase noise, which may allow for the use of cost-effective large-linewidth lasers. The scheme can be applied to access scenarios in which symmetric and high-speed data delivery might be preferred, e. g., point-to-point WDM-PON overlay for demanding applications including next-generation mobile backhaul, mobile fronthaul and business applications [22–24].
This work was supported by National Basic Research Program of China (973 Program, No. 2014CB340105 and No. 2014CB340101) and NSFC (No. 61377072, No. 61275071 and No. 61205058). The authors wish to thank Dr. Xiang Liu from Futurewei Technologies for insightful comments.
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