Signal Processing Algorithms for Down-Stream Traffic in Next Generation 10 Gbit/s Fixed-Grid Passive Optical Networks

Performance enhanced down-stream signalling for next generation long-reach 10 Gbit/s passive optical networks

By using four O-band directly modulated lasers (DMLs), for the first time a 384 Gb/s (4 × 96 Gbit/s) 8-level pulse amplitude modulation (PAM8) signal is successfully transmitted over a 15 km standard single mode fiber (SSMF) with no optical amplifier. The nonlinear Volterra equalizer is usually used to cope with the distortions induced by the nonlinearity of DML and the bandwidth-limited components. However, the Volterra equalizer would also enhance the noise at high frequency, which is harmful, especially to PAM8 signal because it is more sensitive to noise. Thus, the Volterra equalizer is modified in our scheme by adding a decision feedback process behind. With the help of the modified Volterra equalizer, the enhanced noise at high frequency is effectively eliminated, and a power gain of 0.5 dB and 3.3 dB for 4 × 64 Gbit/s PAM4 signal transmission over 30 km SSMF and 4 × 96 Gbit/s PAM8 signal transmission over 15 km SSMF at the HD-FEC limit can be obtained, respectively. Moreover, the computation complexity of the modified Volterra equalizer could be reduced by 38% compared with the conventional Volterra equalizer.

A 12-channel wavelength division multiplexed (WDM) passive optical network (PON) at 155 Mb/s with 63 km of standard single-mode fiber (SMF) was demonstrated under simulated outside plant temperature conditions. A multifrequency laser (MFL) was used as the transmitter and a waveguide grating router (WGR) as the demultiplexer. A 20/spl deg/C/h temperature change of the WGR could be tracked by changing the MFL temperature and resulted in a power penalty in the detected bit-error rate (BER) of less than 0.75 dB. A scheme to increase the tracking sensitivity by multiple WGR passes is demonstrated.

The enormous growth in data traffic in optical networks has raised the requirement of broadband access networks. Passive Optical Networks (PON) is capable of solving the power, bandwidth and cost problems. Time Division Multiplexed PON (TDM PON) uses a point to multipoint design which is currently being used for data transmission that initiated the requirement of maintaining customer’s security in the downstream direction. These security impairments trigger the need of scrutiny for fault monitoring and eavesdropping in PONs. This paper demonstrates the Wavelength Division Multiplexed PONs (WDM-PON) using scrambling and fault monitoring to provide security which is absent in TDM passive optical networks. Results have been validated at 10 Gbps data rate using OPTISYSTEM12 simulation tool.

A novel low-complexity digital signal processing solution is presented to compensate the impairments of limited bandwidth and nonlinearity. It is based on the equalizer, adaptive noise-whitening postfilter, and maximum likelihood sequence detection (MLSD). The system performance loss is induced by higher noise power at the signal band edges after the equalizer can be mitigated by the postfilter and MLSD. Given the above structure, it is possible to enrich the equalizer by providing nonlinear compensation capability. A memory polynomial equalizer (MPE), instead of Volterra equalizer (VE), is applied as the equalizer to compensate the nonlinearity impairments in order to make a tradeoff between complexity and performance. For the adaption of MPE, the blindly adaptive multistep-size decision-directed least-mean-square algorithm is selected. By using a dual-drive Mach–Zehnder and direct detection, 64-Gb/s single-sideband 4-ary pulse amplitude modulation (SSB-PAM4) transmission over 120-km dispersion-uncompensated standard single-mode fiber (SSMF) with 10-dB bandwidth roughly 13.5 GHz is experimentally demonstrated. Given a 20% overhead soft-decision forward-error correction with bit error rate threshold of $2\times 10^{{-2}}$ , the dispersion-uncompensated SSMF transmission distance can be significantly increased from 40 to 120 km with the proposed receiver side solution. The optical signal noise ratio performances for different fiber reach and receiver structure are investigated. Furthermore, we compare the computational complexity of MPE with VE and conclude that MPE can substantially reduce computation complexity with negligible performance loss.

We propose and experimentally demonstrate a full-duplex integrated passive optical network (PON) and indoor optical wireless communication (OWC) system. For the PON part, an external cavity laser (ECL) and a directly modulated laser (DML) are used as optical sources for downlink and uplink, respectively. The OWC subsystem utilizes a visible light-emitting diode (LED) for downlink and an infrared LED (IR-LED) for uplink transmission. To enable high spectral efficiency (SE), high-speed transmission, and flexible multiple access with simplified optical network unit (ONU)-side digital signal processing (DSP), multilevel, multiband quadrature amplitude modulation (QAM) and hybrid 128/64/16 Nyquist single-carrier frequency domain equalization (N-SC-FDE) modulation are implemented in this paper. In terms of 62.5 MHz OWC N-SC-FDE signals, the system successfully delivered a further 140 cm indoor distance after transmitting over a span of 25 km standard single mode fiber (SSMF) together with three subbands of 250 MHz PON signals. As a proof of concept, an integrated system of symmetrical full-duplex 5 Gb/s PON and 250 Mb/s OWC at each direction for three wired users and one wireless user is successfully achieved. The measured bit error ratios (BERs) for each user are all smaller than the 7% pre-forward-error-correction (pre-FEC) limit of 3.8 × 10−3, which shows the promising potential and feasibility of this proposal to extend multiple services from metro areas to suburban areas.

A Bibliometric Analysis and Visualization of Passive Optical Network Research in the Last Decade

Full-duplex bidirectional transmission at 10 Gb/s is demonstrated for extended wavelength division multiplexed passive optical network (WDM-PON) applications, achieving transmission distances up to 25 km of standard single mode fiber (SSMF) when using a low-bandwidth (approximately 1.2 GHz) reflective semiconductor optical amplifier (RSOA) for signal re-modulation at the optical network unit (ONU). The system is assisted by optimum offset filtering at the optical line terminal (OLT)-receiver and the performance is further improved with the use of decision-feedback equalization (DFE). Chromatic dispersion (CD) and Rayleigh Backscattering (RB) effects are considered and analyzed.

We investigate a novel dispersion compensating technique in double sideband (DSB) modulated and directed-detected (DD) passive optical network (PON) systems using super-Nyquist image induced aliasing. We show that diversity is introduced to the higher frequency components by deliberate aliasing using the super-Nyquist images. We then propose to use fractional sampling and per-subcarrier maximum ratio combining (MRC) to harvest this diversity. We evaluate the performance of conventional orthogonal frequency division multiplexing (OFDM) signals along with discrete Fourier transform spread (DFT-S) OFDM and code-division multiplexing OFDM (CDM-OFDM) signals using the proposed scheme. The results show that the DFT-S OFDM signal has the best performance due to spectrum spreading and its superior peak-to-average power ratio (PAPR). By using the proposed scheme, the reach of a 10-GHz bandwidth QPSK modulated OFDM-PON can be extended to around 90 km. We also experimentally show that the achievable data rate of the OFDM signals can be effectively increased using the proposed scheme when adaptive bit loading is applied, depending on the transmission distance. A 10.5% and 5.2% increase in the achievable bit rate can be obtained for DSB modulated OFDM-PONs in 48.3-km and 83.2-km standard single mode fiber (SSMF) transmission cases, respectively, without any modification on the transmitter. A 40-Gb/s OFDM transmission over 83.2-km SSMF is successfully demonstrated.

Fiber-to-the-home (FTTH) or close to it (FTTx) networks are poised to become the next major success story for optical fiber communications. Future FTTx access networks unleash the economic potential and societal benefit by opening up the first/last mile bandwidth bottleneck between bandwidth-hungry end-users and high-speed backbone networks. Owing to their longevity, low attenuation, and huge bandwidth, passive optical networks (PONs) are widely deployed to realize cost-effective FTTx access networks. Fiber has been envisioned for delivering broadband services for over 30 years. However, many roadblocks related to component and installation costs have slowed down the progress toward FTTx since it was first proposed. Currently, FTTH is being installed in many countries, but it still represents only a fraction of all deployed broadband lines (Shumate [2008]). The two major state-of-the-art PON standards IEEE 802.3ah Ethernet PON (EPON) and ITU-T G.984 Gigabit PON (GPON) consist both of a single upstream wavelength channel and a separate single downstream wavelength channel, whereby both channels are operated using time division multiplexing (TDM). EPON and GPON are expected to coexist for the foreseeable future as they evolve into next-generation PONs (NG-PONs) (Effenberger et al . [2007], Kazovsky et al . [2007]). NG-PONs can be categorized into high-speed TDM PON, wavelength division multiplexing (WDM) PON, and long-reach PON (LR-PON) (Lin [2008], Effenberger et al . [2009a]).

WDM PONs based on colorless technology

Experimental results are presented confirming that optical-phase conjugation in a semiconductor-laser amplifier has the potential to compensate for the interplay of chromatic dispersion and nonlinear self-phase modulation, thereby enabling high-capacity high-power transmission over standard singlemode fibres.

The performance of colorless wavelength-division multiplexed passive optical network (WDM-PON) systems suffers from the transmission impairments mainly due to Rayleigh backscattering (RB). In this paper, we propose and demonstrate a single fiber colorless WDM-PON which enhances the tolerance to RB induced noise. The high extinction ratio in both return-to-zero (RZ)-shaped differential phase shift keying (DPSK) downstream (DS) data signal and intensity-remodulated upstream (US) data signal helps to improve the tolerance to RB induced noise. Simulation results show that downstream and upstream signals can achieve error-free performance at 10 Gbit/s with negligible penalty and improve the tolerance to RB induced noise over 25 km standard single-mode fiber.

It has been recently experimentally and theoretically demonstrated that three nonlinear operation regimes can be observed in reflective semiconductor amplifiers in relation to gain saturation and, consequently, in relation to the bleaching of the modulation portion of the injected signal while the injected power increases. For moderately high-injected power, the modulation component of the signal is partially bleached, i.e., the extinction ratio (ER) is reduced. For higher input power, the modulation is exactly cancelled, i.e., the ER is almost 0 dB. Finally a further increase of injected power determines modulation inversion, with consequent eye re-opening. While the first two phenomena can be observed also in SOAs, though usually at higher injected power, the last regime is specific of RSOAs. In highly nonlinear RSOA the simultaneous presence of two waves, the incoming one and the reflected, which locally compete for gain along the device length, defines areas of strong depletion. The RSOA noise reduction capability, which has been widely exploited in passive optical networks (PON) topologies, is achieved by compressing the optical intensity fluctuations associated to the injected signal. This capability relies on the same mechanism governing gain saturation and modulation bleaching. Thus it is not surprising that the same nonlinear regimes observed in relation to gain saturation can be evidenced in relation to relative intensity noise (RIN) compression. In order to highlight this behavior and its consequences on PON design, we experimentally emulated a spectrum-sliced wavelength division multiplexing (WDM) PON topology. The impact on RIN of the injection into the RSOA and subsequent filtering actions operated by arrayed waveguide gratings (AWG) placed at the remote node and at the optical line terminal is evaluated both experimentally and by simulations. RIN measurements are performed in time domain exploiting a real time oscilloscope and in the spectral domain using an electrical spectrum analyzer in different point of the WDM PON topology to enlighten the different contributions to noise. Simulations have been also using an RSOA simple model, which has been realized with the commercial software, OptSim by RSOFT, and has been implemented by matching the data collected from the RSOA gain experimental characterization.

In this paper we evaluate what parameters of the elements of transmission system have high impact on the performance of wavelength-division-multiplexed passive optical network (WDM-PON). The increasing of data rate is one of the solutions for satisfying growing demands on the capacity of transmission system. However, the higher data rate is, the higher the technological issues and expenses on its realization are. That is why the choice of the elements of transmission system with high data rate becomes crucial. For this reason, it is important to investigate what parameters of the elements of transmission system have higher impact on transmission quality. The paper focuses on the eight-channel 40Gbit/s transmission system. WDM-PON systems with Duobinary (DB) and pulse-amplitude-modulation (PAM-4) modulation formats are considered. The paper evaluates how the change of parameters of laser, modulator, fibre and photodiode influences the performance of our investigated WDM-PON. The parameters under research are following — output power, linewidth, relative intensity noise (RIN) and side-mode suppression ratio of laser, extinction ratio, bandwidth and S 21 characteristic of modulator, attenuation, dispersion and nonlinear coefficient of the fibre as well as responsivity, dark current, thermal noise and shot noise of the receiver. As a result, we investigate what parameters of each element of the transmission system are the most significant and should be considered as a primary factor in the design of WDM-PON transmission system. Based on the obtained results we propose possible solutions for the optimal realization of the eight-channel 40Gbit/s WDM-PON transmission systems with PAM4 and DB modulation formats.