Darlene M. Neves

Terabit+ (192 × 10 Gb/s) Nyquist shaped UDWDM coherent PON with upstream and downstream over a 12.8 nm band

In this paper, we numerically and experimentally demonstrate a bidirectional Terabit+ ultradense wavelength-division multiplexing (UDWDM) coherent passive optical network with Nyquist shaped 16-ary quadrature amplitude modulation, offering up to 10 Gb/s service capabilities per user/wavelength in a total spectrum of 12.8 nm over 40 km of standard single-mode fiber. This paper first demonstrates the capability of Nyquist pulse shaping to mitigate crosstalk arising from back-reflections and nonlinear effects in UDWDM networks with coherent transceivers. The latter part of the paper experimentally investigates the bidirectional transmission in terms of receiver sensitivity and nonlinear tolerance under different network transmission capacity conditions, e.g., number of users and bit rate per users.

Analysis of Nonlinearities on Coherent Ultradense WDM-PONs Using Volterra Series

This paper addresses transmission aspects on very high aggregate wavelength-division multiplexing based passive optical networks (WDM-PON). The ultradense WDM-PON system evaluated in this paper transports high-order modulation formats such as M-ary phase-shift keying (PSK) and M-ary quadrature amplitude modulation (QAM) in which coherent detection is performed at the optical network unit (ONU), after transmission over 25 km, 60 km, and 100 km of standard single-mode fiber (SSMF). The first part of this work covers the impact on the systems performance of the most relevant fiber nonlinearities such as self-phase modulation (SPM), cross-phase modulation (XPM) and four-wave mixing (FWM), and their interplay between transmission distance and modulation format. Using a Volterra series method allows estimating the error vector magnitude (EVM) of the received constellation related to different fiber nonlinearities. In a 32 × 625 Mbaud system spaced by 3 GHz (0.025 nm), the FWM to XPM ratio (F/X) varied from 25 up to 30 dB for phase-modulated signals (1.25 Gb/s-QPSK and 1.875 Gb/s-8PSK). On the other hand, this ratio ranged from 2.6 to 7.4 dB for amplitude-modulated signals (2.5 Gb/s-16QAM, 3.75 Gb/s-64QAM and 5 Gb/s-256QAM). After applying a frequency-allocation scheme at both transmitter and local oscillator lasers, some of the FWM crosstalk is mitigated by 2.4 to 3.5 dB. This EVM reduction confirms that increasing the transmission distance and the order of the constellation, the systems performance becomes limited by both interchannel FWM and XPM.

A Time Domain Channel Estimation Scheme for Equalize-and-Forward Relay-Assisted Systems

In this paper we propose a scheme with a dedicated relay node for an OFDM-based system. We consider an antenna array at the base station and a single antenna at both the user terminal and the relay node. The relay-assisted protocol considered is equalize-and-forward. The proposed scheme requires the computation of parameters which depend on unknown variables at the destination and we show that under some assumptions the scheme is suitable to provide all those requirements. Also, we show that since the pilots at the relay node are properly designed the TD-MMSE estimator can still provide the required channel estimate. The system performance is evaluated by considering a typical pedestrian scenario based on WiMAX specification. The results show that the degradation in the estimator performance is negligible.

Performance optimization of nyquist signaling for spectrally efficient optical access networks [Invited]

In this work, Nyquist pulse-shaping technology is exploited for future implementations of optical access networks. The paper first presents technical issues regarding digital implementation of Nyquist spectral shaping for low-symbol-rate systems. Second, the potential benefits regarding mitigation of cross talk (linear and nonlinear) of Nyquist pulse shaping in symmetric and bidirectional wavelength-division multiplexing passive optical networks (WDM-PONs) are presented. Lastly, the paper proposes a WDM-PON solution employing self-coherent detection at the optical network unit and heterodyne coherent detection at the optical line terminal. In this proposed architecture, a multi-objective optimization strategy is developed to enhance the transmission performance of the Nyquist-shaped bands in full-duplex mode.

Channel Estimation Schemes for OFDM Relay-Assisted Systems

The aim of this paper is to adapt and evaluate the performance of two classical pilot-aided channel estimation schemes designed for OFDM systems: least square and minimum mean square error, in a relay-assisted scenario. We consider the Amplify-and- Forward, which is the simplest relay protocol for cooperative systems. Also, we consider the use of a single antenna at all the terminals: base station, user terminal and relay node. In our scenario, it is assumed that some of the user terminals deployed in a certain area could act as relaying-able terminals for the communications of other users. In this communication we show that the classical channel estimation techniques designed for conventional wireless communications can be extended for relay-assisted based systems under some modifications.

Nyquist Signaling for Spectrally-Efficient Optical Access Networks

In this work Nyquist technology application to future optical access networks is discussed. Implementation issues regarding low symbol rates are characterized. The potential benefits of Nyquist in symmetric/bi-directional PON are presented by means of parameter optimization.

An iterative pilot-data-aided estimator for SFBC relay-assisted OFDM-based systems

In this article, we propose and assess an iterative pilot-data-aided channel estimation scheme for space frequency block coding relay-assisted OFDM-based systems. The relay node (RN) employs the equalise-and-forward protocol, and both the base station (BS) and the RN are equipped with antenna arrays, whereas the user terminal (UT) is a single-antenna device. The channel estimation method uses the information carried by pilots and data to improve the estimate of the equivalent channels for the path BS-RN-UT. The mean minimum square error criterion is used in the design of the estimator for both the pilot-based and data-aided iterations. In different scenarios, with only one data iteration, the results show that the proposed scheme requires only half of the pilot density to achieve the same performance of non-data-aided schemes.

Density and Guard Band in Migration Scenarios to Coherent Ultra-Dense WDM

In this work the maximum density is numerically investigated giving the compromise of physical impairments and spectral efficiency on coherent ultra-dense wavelength division multiplexing passive optical networks (UDWDM-PON). The required guard band to legacy 1.25-2.5 Gb/s intensity modulation direct detection (IMDD) systems specified closely to Gigabit-capable PON equipments is also analyzed in order to mitigate linear and nonlinear crosstalk. Using 3 GHz frequency grid guaranteed the best performance of the system (32x1.25 Gb/s quadrature phase-shift keying with coherent detection) with respect to the 10% Error Vector Magnitude limit; 12.5 GHz guard band to legacy IMDD system was sufficient to minimize strong linear and nonlinear crosstalk (mostly four-wave mixing) on both systems.

Weighting nonlinearities on future high aggregate data rate PONs

High aggregate data rate coherent UDWDM-PONs related physical challenges are observed by means of Volterra series simulations. ADC resolution and inter-channel nonlinearities impact in high-order modulation formats at 1–10Gb/s per channel and 100km-reach is studied.

Analysis of transmission impairments on Terabit aggregate PONs

We analyze the transmission limitations of coherent optical access networks transporting over Terabit data rates with high-order QAM modulation. Additionally, we experimentally demonstrate the coexistence of 16×1.25 Gb/s-QPSK with 10 Gb/s-NRZ considering guard bands ≤200 GHz.