Syed Muhammad Bilal

Analytical and Experimental Results on System Maximum Reach Increase Through Symbol Rate Optimization

We investigated the reach increase obtained through nonlinearity mitigation by means of transmission symbol rate optimization (SRO). First, we did this theoretically and simulatively. We showed that the nonlinearity model that properly accounts for the phenomenon is the EGN model, in its version that specifically includes four-wave mixing. We then found that for PM-QPSK systems at full C-band, the reach increase may be substantial, on the order of 10-25%, with optimum symbol rates on the order of 2-6 GBd. We show that for C-band PM-QPSK systems over SMF, the potential mitigation due to SRO is greater than that ideally granted by digital backpropagation (the latter applied over a bandwidth of a 32-GBd channel). We then set up an experiment to obtain confirmation of the theoretical and simulative predictions. It consisted of 19 PM-QPSK channels, operating at 128 Gb/s per channel, over PSCF, with span length 108 km and EDFA-only amplification. We demonstrated a reach increase of about 13.5%, when going from single-carrier per channel transmission, at 32 GBd, to eight subcarrier per channel, at 4 GBd, in line with the EGN model predictions. We also extended the theoretical investigation of SRO to PM-16QAM, where we found a qualitatively similar effect to PM-QPSK, although the potential reach increase appears to be typically only about 50% to 60% that of PM-QPSK. Further investigation is, however, in order, specifically to explore the effect on PM-16QAM SRO of the removal of long-correlated phase and polarization noise.

Dual Stage CPE for 64-QAM Optical Systems Based on a Modified QPSK-Partitioning Algorithm

In this letter, a novel two-stage digital feed forward carrier recovery algorithm for 64-ary quadrature amplitude modulation (QAM) is proposed and analyzed. Due to the absence of any feedback loop, the approach shows a high tolerance toward laser phase noise. Different steps involving partition, selection, and rotation of symbols are also discussed. For an OSNR penalty of 1 dB at bit error rate of 10-3, the proposed scheme can tolerate linewidth times symbol duration product equal to 3.3×10-5 and hence can be used with the commercially available state-of-the-art tunable lasers for 64-QAM transmission at 400 Gb/s.

Multistage Carrier Phase Estimation Algorithms for Phase Noise Mitigation in 64-Quadrature Amplitude Modulation Optical Systems

Two novel low-complexity multistage digital feed-forward carrier phase estimation algorithms for 64-ary quadrature amplitude modulation (QAM) are proposed and analyzed by numerical simulations. The first stage is composed of a Viterbi and Viterbi block, based on either the standard quadrature phase shift keying (QPSK) partitioning algorithm using only Class-1 symbols or a modified QPSK partitioning scheme utilizing both Class-1 and outer most triangle-edge symbols. The second stage applies the Viterbi and Viterbi algorithm after a 64-QAM-to-QPSK transformation, while the subsequent stages iterate a maximum likelihood estimation algorithm for phase estimation. All proposed techniques are characterized by a high tolerance to laser phase noise: with an optical signal-to-noise ratio penalty of 1 dB at bit error rate of 10-2, the proposed schemes can tolerate a linewidth times symbol duration product (Δv·Ts) equal to 5.6 × 10-5 and 7.1 × 10-5, respectively. At 32 Gbaud, all of the above linewidth requirements can be met using commercial tunable lasers. The proposed schemes achieve a similar linewidth tolerance with a reduced implementation complexity with respect to algorithms based on the blind phase search method.

Carrier Phase Estimation Through the Rotation Algorithm for 64-QAM Optical Systems

A novel low-complexity two-stage digital feedforward carrier phase estimation algorithm based on the rotation of constellation points to remove phase modulation for a 64-ary quadrature amplitude modulation (QAM) system is proposed and analyzed both experimentally and through numerical simulations. The first stage is composed of a Viterbi and Viterbi (V&V) block, based on either the standard quadrature phase shift keying (QPSK) partitioning algorithm using only Class-1 symbols or a modified QPSK partitioning scheme utilizing both Class-1 and outer most triangle-edge (TE) symbols. The second stage applies the V&V algorithm after the removal of phase modulation through rotation of constellation points. Comparison of the proposed scheme with constellation transformation, blind phase search (BPS) and BPS+MLE (maximum likelihood estimation) algorithm is also shown. For an OSNR penalty of 1 dB at bit error rate of 10-2, the proposed scheme can tolerate a linewidth times symbol duration product (Δν · Ts) equal to 3.7 × 10-5, making it possible to operate 32-GBd optical 64-QAM systems with current commercial tunable lasers.

Pump RIN-induced impairments in unrepeatered transmission systems using distributed Raman amplifier.

High spectral efficiency modulation format based unrepeatered transmission systems using distributed Raman amplifier (DRA) have attracted much attention recently. To enhance the reach and optimize system performance, careful design of DRA is required based on the analysis of various types of impairments and their balance. In this paper, we study various pump RIN induced distortions on high spectral efficiency modulation formats. The vector theory of both 1st and higher-order stimulated Raman scattering (SRS) effect using Jones-matrix formalism is presented. The pump RIN will induce three types of distortion on high spectral efficiency signals: intensity noise stemming from SRS, phase noise stemming from cross phase modulation (XPM), and polarization crosstalk stemming from cross polarization modulation (XPolM). An analytical model for the statistical property of relative phase noise (RPN) in higher order DRA without dealing with complex vector theory is derived. The impact of pump RIN induced impairments are analyzed in polarization-multiplexed (PM)-QPSK and PM-16QAM-based unrepeatered systems simulations using 1st, 2nd and 3rd-order forward pumped Raman amplifier. It is shown that at realistic RIN levels, negligible impairments will be induced to PM-QPSK signals in 1st and 2nd order DRA, while non-negligible impairments will occur in 3rd order case. PM-16QAM signals suffer more penalties compared to PM-QPSK with the same on-off gain where both 2nd and 3rd order DRA will cause non-negligible performance degradations. We also investigate the performance of digital signal processing (DSP) algorithms to mitigate such impairments.

Multi-stage CPE algorithms for 64-QAM constellations

We propose and analyze a multi-stage architecture for carrier phase-estimation in 64-QAM systems, based on the cascade of several feed-forward elementary blocks. We outline the beneficial effect of increasing the number of elementary blocks.

Experimental demonstration of fiber nonlinearity mitigation in a WDM multi-subcarrier coherent optical system

Thanks to the use of a digital multi-subcarrier architecture, we experimentally demonstrate a maximum reach increase of 11% for a 19×128Gbit/s PM-QPSK WDM comb with respect to an equivalent single-subcarrier system, achieving 14,100-km transmission over PSCF fiber with EDFA-only amplification.

Blind modulation format identification for digital coherent receivers.

In this paper, a simple novel digital modulation format identification (MFI) scheme for coherent optical systems is proposed. The scheme is based on the evaluation of the peak-to-average-power ratio (PAPR) of the incoming data samples after analog-to-digital conversion (ADC), chromatic dispersion (CD) and polarization mode demultiplexing (PMD) compensation at the receiver (Rx). Since at a particular optical-signal-to-noise ratio (OSNR) value different modulation formats have distinct PAPR values, it is possible to identify them. The proposed scheme and the results are analyzed both experimentally and through numerical simulations. The results demonstrate successful identification among four modulation formats (MF) commonly used in digital coherent systems.

Dual stage carrier phase estimation for 16-QAM systems based on a modified QPSK-partitioning algorithm

Coherent optical communications based on higher order modulation formats are severely affected by the phase noise of transmitter and receiver lasers. In this work, a novel yet simple scheme is presented for carrier phase estimation (CPE) of 16-ary Quadrature Amplitude Modulation (16- QAM) formats, based on a modified QPSK-partitioning algorithm. The proposed algorithm can tolerate a linewidth times symbol duration product (Δv · Ts) of the order of 10-4, with 1 dB penalty at a bit error rate (BER) of 10-3. Tolerance can be further improved by introducing a maximum likelihood estimation (MLE) stage (Δv · Ts ≈ 1.432×10-4). Comparison of the scheme with other proposed algorithms is shown. The obtained results indicate that the presented approach can be used with the commercially available state of the art lasers for 16-ary Quadrature Amplitude Modulation (16- QAM) transmission at 100 Gbps.

Impact of the transmitter IQ-skew in multi-subcarrier coherent optical systems

We show that the impact of non-ideal time synchronization between in-phase and quadrature electrical signals can be critical in multi-subcarrier systems. We propose the use of an 8×8 real-valued butterfly equalizer structure to mitigate it.