Oluyemi Omomukuyo

Joint timing and frequency synchronization based on weighted CAZAC sequences for reduced-guard-interval CO-OFDM systems.

A novel joint symbol timing and carrier frequency offset (CFO) estimation algorithm is proposed for reduced-guard-interval coherent optical orthogonal frequency-division multiplexing (RGI-CO-OFDM) systems. The proposed algorithm is based on a constant amplitude zero autocorrelation (CAZAC) sequence weighted by a pseudo-random noise (PN) sequence. The symbol timing is accomplished by using only one training symbol of two identical halves, with the weighting applied to the second half. The special structure of the training symbol is also utilized in estimating the CFO. The performance of the proposed algorithm is demonstrated by means of numerical simulations in a 115.8-Gb/s 16-QAM RGI-CO-OFDM system.

Robust Faster-Than-Nyquist PDM-mQAM Systems With Tomlinson–Harashima Precoding

A training-based channel estimation algorithm is proposed for the faster-than-Nyquist polarization division multiplexed m-ary quadrature amplitude modulation (m = 4, 16, 64) systems with Tomlinson-Harashima precoding (THP). This is robust to the convergence failure phenomenon suffered by the existing algorithm, yet remaining format-transparent. Simulation results show that the proposed algorithm requires a reduced optical signal-to-noise ratio to achieve a certain bit error rate in the presence of first-order polarization mode dispersion and phase noise introduced by the laser linewidth.

Robust Frame and Frequency Synchronization Based on Alamouti Coding for RGI-CO-OFDM

We propose an algorithm for carrying out joint frame and frequency synchronization in reduced-guard-interval coherent optical orthogonal frequency division multiplexing (RGI-CO-OFDM) systems. The synchronization is achieved by using the same training symbols (TSs) employed for training-aided channel estimation (TA-CE), thereby avoiding additional training overhead. The proposed algorithm is designed for polarization division multiplexing (PDM) RGI-CO-OFDM systems that use the Alamouti-type polarization-time coding for TA-CE. Due to their optimal TA-CE performance, Golay complementary sequences have been used as the TS in the proposed algorithm. The frame synchronization is accomplished by exploiting the cross correlation between the received TS from the two orthogonal polarizations. The arrangement of the TS is also used to estimate the carrier frequency offset. Simulation results of a PDM RGI-CO-OFDM system operating at 238.1-Gb/s data rate (197.6 Gb/s after coding), with a total overhead of 9.2% (31.6% after coding), show that the proposed scheme has accurate synchronization, and is robust to linear fiber impairments.

A faster-than-Nyquist PDM-16QAM scheme enabled by Tomlinson-Harashima precoding

We present and numerically demonstrate a polarization-division multiplexing 16-ary quadrature amplitude modulation faster-than-Nyquist (FTN-PDM-16QAM) system enabled by the Tomlinson-Harashima precoding (THP). The THP module is located before the polarization separation module and it is shown that the modulo operator of the THP module at the receiver needs to be removed in order to fully optimize the performance of the soft-decision forward error correction (FEC). The impact of various parameters such as the modulo size factor, the 3-dB bandwidth and roll-off factor of the Nyquist filter on the performance of the FTN-PDM-16QAM system are also investigated. In addition, we compare the FTN-PDM-16QAM system with the PDM-16QAM Nyquist system in terms of the system performance and implementation complexity.

Simple sampling clock synchronisation scheme for reduced-guard-interval coherent optical OFDM systems

A simple data-aided scheme for sampling clock synchronisation in reduced-guard-interval coherent optical orthogonal frequency division multiplexing (RGI-CO-OFDM) systems is proposed. In the proposed scheme, the sampling clock offset (SCO) is estimated by using the training symbols reserved for channel estimation, thus avoiding extra training overhead. The SCO is then compensated by resampling, using a time-domain interpolation filter. The feasibility of the proposed scheme is demonstrated by means of numerical simulations in a 32-Gbaud 16-QAM dual-polarisation RGI-CO-OFDM system.

A spectrally-efficient linear polarization coding scheme for fiber nonlinearity compensation in CO-OFDM systems

In this paper, we propose a linear polarization coding scheme (LPC) combined with the phase conjugated twin signals (PCTS) technique, referred to as LPC-PCTS, for fiber nonlinearity mitigation in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. The LPC linearly combines the data symbols on the adjacent subcarriers of the OFDM symbol, one at full amplitude and the other at half amplitude. The linearly coded data is then transmitted as phase conjugate pairs on the same subcarriers of the two OFDM symbols on the two orthogonal polarizations. The nonlinear distortions added to these subcarriers are essentially anti-correlated, since they carry phase conjugate pairs of data. At the receiver, the coherent superposition of the information symbols received on these pairs of subcarriers eventually leads to the cancellation of the nonlinear distortions. We conducted numerical simulation of a single channel 200 Gb/s CO-OFDM system employing the LPC-PCTS technique. The results show that a Q-factor improvement of 2.3 dB and 1.7 dB with and without the dispersion symmetry, respectively, when compared to the recently proposed phase conjugated subcarrier coding (PCSC) technique, at an average launch power of 3 dBm. In addition, our proposed LPCPCTS technique shows a significant performance improvement when compared to the 16-quadrature amplitude modulation (QAM) with phase conjugated twin waves (PCTW) scheme, at the same spectral efficiency, for an uncompensated transmission distance of 2800 km.

Rf-pilot phase noise compensation for long-haul coherent optical OFDM systems

Long-haul optical transmission systems employing coherent optical orthogonal frequency division multiplexing (CO-OFDM) are sensitive to laser phase noise. This causes a common phase rotation and inter-carrier interference. An effective method to compensate for the phase noise is to insert an RF-pilot tone in the middle of the OFDM signal. This RF-pilot is used to reverse the phase distortion at the receiver. This paper presents a performance analysis of the RF-pilot phase noise compensation scheme in a simulated 64 Gb/s CO-OFDM system. The effects of various parameters including laser linewidth, Mach-Zehnder modulator drive power, pilot-to-signal ratio, and fiber launch power are investigated. A comparison with the pilot-aided common phase error compensation method is provided to show the differences in the BER performance with respect to the required overhead.

Training Symbol-Based Equalization for Quadrature Duobinary PDM-FTN Systems

A training symbol-based equalization algorithm is proposed for polarization de-multiplexing in quadrature duobinary (QDB) modulated polarization division multiplexed faster than-Nyquist (FTN) coherent optical systems. The proposed algorithm is based on the least mean square algorithm, and multiple location candidates of a symbol are considered in order to make use of the training symbols with QDB modulation. Results show that an excellent convergence performance is obtained using the proposed algorithm under different polarization alignment scenarios. The optical signal-to-noise ratio required to attain a bit error rate of

Discrete FRFT-Based Frame and Frequency Synchronization for Coherent Optical Systems

2\times 10^{-2}

Bandwidth-efficient synchronization for fiber optic transmission: system performance measurements

is reduced by 1.7 and 1.8 dB using the proposed algorithm, compared with systems using the constant modulus algorithm with differential coding for four-ary quadrature amplitude modulation (4-QAM) and 16-QAM systems with symbol-by-symbol detection, respectively. Furthermore, comparisons with the Tomlinson–Harashima precoding-based FTN systems illustrate that QDB is preferable when 4-QAM is utilized.