We exploit the coherence of frequency combs for high spectral efficiency superchannel transmission via effective sharing of a single pilot tone. By phase-locking the receiver comb to the transmitted pilot tone, carrier offsets are suppressed while both the overhead and complexity associated with the pilot tone are reduced. We form a 55 carrier superchannel using a 25-GHz spaced electro-optic frequency comb seeded by a 100-kHz linewidth laser. At a pilot tone overhead of $ 2%, the reduction in carrier offsets is shown to facilitate blind DSP-based carrier recovery of all 54 $\times$ 24 Gbaud PM-128QAM data channels. The resulting superchannel spectral efficiency is 10.3 bits/s/Hz assuming a 28% overhead for forward error correction. Our results show the potential for optical pilot tones to reduce both overhead and complexity in systems using comb-based superchannels together with high-order modulation formats.

High Spectral Efficiency PM-128QAM Comb-Based Superchannel Transmission Enabled by a Single Shared Optical Pilot Tone

We investigate carrier estimation (CE) for coherent optical receivers where the received signal is impaired by additive white Gaussian noise, laser phase noise, and frequency offset. Best practical 4-, 8-, and 16-point constellations are identified. A generalized differential encoding rule for signal constellations is presented. Performance of our complex-weighted decision-aided maximum-likelihood (CW-DA-ML) phase noise and frequency offset estimator is analyzed at low signal-to-noise ratio (SNR) and the optimal filter lengths are found. CW-DA-ML CE is put in perspective with respect to two fundamental estimators in the literature: (i) differential frequency estimator followed by block Mth power phase estimator (DiffFE-Mth CE), and (ii) fast Fourier transform based frequency estimator followed by block Mth power phase estimator (FFTbE-M th CE), in terms of laser linewidth tolerance, frequency estimation range and speed, SNR threshold, and cycle slip probability. CW-DA-ML CE is 2.5 and 10.5 times faster than DiffFE-M th CE in 4 phase-shift keying and 16 quadrature amplitude modulation signals, respectively, at a 1-dB system penalty for a bit-error rate of 10-3. Our CE has lower cycle slip probability and transmission overhead than DiffFE-Mth and FFTbE-Mth CE. Hence, our CE is shown to be favourable in pilot-assisted (PA) systems. A PA CW-DA-ML CE is introduced and shown to be robust against time-varying frequency offset with minimal training overhead. Analog-to-digital convertor quantization error on our CE performance is also addressed.

https://koasas.kaist.ac.kr/bitstream/10203/192485/1/85564.pdf

On Decision Aided Carrier Phase and Frequency Offset Estimation in Coherent Optical Receivers

We demonstrate a cost-effective, 10-Gb/s full-duplex wavelength-division-multiplexed passive optical network over 20 km single-feeder fiber using a single light source per optical network unit (ONU). We exploit a directly modulated laser (DML) and a reflective semiconductor optical amplifier (RSOA) for downlink and uplink, respectively. In this system, a nonreturn-to-zero (NRZ) downstream signal is first frequency up-converted to around 20 GHz before being fed to a DML for downstream transmission. After transmission over the feeder fiber, a portion of the downstream signal is detected at the ONU and the other portion is fed to the RSOA as seed light. The RSOA is directly modulated by a 10-Gb/s NRZ signal for upstream transmission. To overcome the bandwidth limitation of the directly modulated RSOA, we employ an optical delay interferometer at the upstream receiver. The passive optical device acts as an optical equalizer and enables the 10-Gb/s upstream signal to be accommodated with a 1.3-GHz-bandwith RSOA. We investigate through experiment the crosstalk between the downstream and upstream signals as a function of the downstream subcarrier frequency. Also investigated in this paper is the nonlinear frequency mixing of the downstream and upstream signals in the RSOA.

Symmetric 10-Gb/s WDM-PON Using Directly Modulated Lasers for Downlink and RSOAs for Uplink

We present dual-polarization complex-weighted, decision-aided, maximum-likelihood algorithm with superscalar parallelization (SSP-DP-CW-DA-ML) for joint carrier phase and frequency-offset estimation (FOE) in coherent optical receivers. By pre-compensation of the phase offset between signals in dual polarizations, the performance can be substantially improved. Meanwhile, with the help of modified SSP-based parallel implementation, the acquisition time of FO and the required number of training symbols are reduced by transferring the complex weights of the filters between adjacent buffers, where differential coding/decoding is not required. Simulation results show that the laser linewidth tolerance of our proposed algorithm is comparable to traditional blind phase search (BPS), while a complete FOE range of ± symbol rate/2 can be achieved. Finally, performance of our proposed algorithm is experimentally verified under the scenario of back-to-back (B2B) transmission using 10 Gbaud DP-16/32-QAM formats.

Joint carrier phase and frequency-offset estimation with parallel implementation for dual-polarization coherent receiver.

Due to the non-rectangular distribution of the constellation points, traditional fast Fourier transform based frequency offset estimation (FFT-FOE) is no longer suitable for 32-QAM signal. Here, we report a modified FFT-FOE technique by selecting and digitally amplifying the inner QPSK ring of 32-QAM after the adaptive equalization, which is defined as QPSK-selection assisted FFT-FOE. Simulation results show that no FOE error occurs with a FFT size of only 512 symbols, when the signal-to-noise ratio (SNR) is above 17.5 dB using our proposed FOE technique. However, the error probability of traditional FFT-FOE scheme for 32-QAM is always intolerant. Finally, our proposed FOE scheme functions well for 10 Gbaud dual polarization (DP)-32-QAM signal to reach 20% forward error correction (FEC) threshold of BER=2×10-2, under the scenario of back-to-back (B2B) transmission.

Feed-forward frequency offset estimation for 32-QAM optical coherent detection.

We present a symbol-by-symbol coherent optical receiver, which employs a novel, complex-weighted, decision-aided, maximum-likelihood (CW-DA-ML) carrier phase and frequency offset estimator. The CW-DA-ML carrier estimator uses a CW transversal filter to generate a carrier reference phasor, and the filter weights are automatically adapted on-line by linear regression on the observed signals. A complete modulo-R reduced frequency offset estimation (FOE) range of ±R/2 is achieved, independent of modulation format, where R is the symbol rate. Carrier phase and frequency tracking is achieved rapidly. The acquisition speed of frequency offset in quaternary phase-shift keying (4-PSK) signals is more than 5 times faster than that of differential FOE. A constant penalty of approximately 1 dB at bit-error rate of 10−4 is demonstrated for all frequency offsets in 4-PSK signals with laser-linewidth-symbol-duration product of 8×10−5.

https://www.researchgate.net/profile/Adaickalavan_Meiyappan/publication/232086595_A_complex-weighted_decision-aided_maximum-likelihood_carrier_phase_and_frequency-offset_estimation_algorithm_for_coherent_optical_detection/links/547925be0cf205d1687f7dbc.pdf

A complex-weighted, decision-aided, maximum-likelihood carrier phase and frequency-offset estimation algorithm for coherent optical detection

We present a low-complexity carrier estimator with an effective filter length that automatically adapts according to the signal-to-noise ratio, laser-linewidth-symbol-duration product, nonlinear phase noise, and modulation format. Laser-linewidth and frequency-offset tolerances are studied. The filter length of a carrier estimator is shown to affect the cycle slip probability besides the bit-error rate (BER) performance. Considering that forward-error-correction codes are not robust to burst errors and phase slips, we demonstrate that filter-length optimization is necessary to avoid spectral-efficiency reduction in pilot assisted systems and potential system failures in differential encoding systems. Our estimator achieves a lower cycle slip probability and a greater nonlinear phase noise tolerance than DiffFE-MPE, DiffFE-BPS, and complex-weighted decision-aided maximum-likelihood (CW-DA-ML) estimator. DiffFE-MPE and DiffFE-BPS refer to the differential frequency estimator (DiffFE) followed by block Mth power phase estimator (MPE) and blind phase search (BPS), respectively. For a 4100 km quaternary phase-shift keying transmission at a BER of 2.5 ×10- 2, our estimator achieves a cycle slip probability of 2.9 ×10- 7 compared to 5.6 ×10- 6, 5.3 ×10- 6, and 3.2 ×10- 7 for DiffFE-MPE, DiffFE-BPS, and CW-DA-ML, respectively.

A Low-Complexity, Low-Cycle-Slip-Probability, Format-Independent Carrier Estimator with Adaptive Filter Length

We propose and demonstrate 40-km upstream transmission of a 2-Gb/s 6-GHz binary phase-shift keyed radio signal using a directly modulated reflective semiconductor optical amplifier (RSOA). A delay interferometer, acting as an optical equalizer, compensates for limited RSOA modulation bandwidth and simultaneously performs single sideband filtering to relieve dispersion-induced radio-frequency (RF) fading effect. Furthermore, extended transmission distance of up to 40 km is shown to be made possible by reduced Rayleigh backscattering-induced crosstalk.

6-GHz Radio-Over-Fiber Upstream Transmission Using a Directly Modulated RSOA

Adaptive decision-aided maximum-likelihood (DA ML) carrier phase estimation (PE) is shown to be more robust to frequency offset, with better frequency-offset and laser-linewidth tolerance compared to conventional DA ML carrier PE.

Adaickalavan Meiyappan

Papers

On Decision Aided Carrier Phase and Frequency Offset Estimation in Coherent Optical Receivers

A complex-weighted, decision-aided, maximum-likelihood carrier phase and frequency-offset estimation algorithm for coherent optical detection

A Low-Complexity, Low-Cycle-Slip-Probability, Format-Independent Carrier Estimator with Adaptive Filter Length

6-GHz Radio-Over-Fiber Upstream Transmission Using a Directly Modulated RSOA

Performance of decision-aided maximum-likelihood carrier phase estimation with frequency offset