Nebojsa Stojanovic

LDPC convolutional codes using layered decoding algorithm for high speed coherent optical transmission

We successfully realized layered decoding for LDPC convolutional codes designed for application in high speed optical transmission systems. A relatively short code with 20% redundancy was FPGA-emulated with a Q-factor of 5.7dB at BER of 10-15.

Measurement of the dispersion tolerance of optical duobinary with an MLSE-receiver at 10.7 Gb/s

We experimentally demonstrate a significant improvement in the dispersion tolerance of optical duobinary modulation when employing an MLSE instead of a standard receiver. We show that the improvement critically depends on the MLSE design.

Directly PAM-4 Modulated 1530-nm VCSEL Enabling 56 Gb/s/

We demonstrate a 56 Gb/s four-level pulse-amplitude modulation (PAM-4) transmission using direct detection and a long-wavelength 18-GHz bandwidth vertical-cavity surface-emitting laser as directly modulated light source for short-reach inter- and intra-connects in datacenters and short-reach networks. Error-free transmission over 2 km at 7% hard-decision forward-error correction threshold is achieved by applying powerful equalization schemes at the receiver side. Three equalization schemes, i.e., a maximum likelihood estimation (MLSE), a feed-forward equalizer (FFE), and a combination of the FFE and the MLSE are thoroughly investigated, and the performance comparison between them is carried out.

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We investigate the performance of linear equalization combined with MLSE in long-haul transmission. In the nonlinear regime with high launch powers Q-factor improvement up to 1.4 dB is demonstrated with a quaternary state model.

Data-Center Interconnects

We investigate the systematic influence of optical polarization effects and chromatic dispersion (CD) on digital timing phase estimation in receivers with coherent demodulation. It is shown by analysis and by simulation that half-Baud differential group delay (DGD) in combination with certain states of polarization (SOP) attenuates the clock tone such that the timing phase can not be estimated. Furthermore, the fundamental limitation of CD to digital timing recovery is analyzed. Proposals for mitigation schemes with low complexity and robust timing phase estimation are discussed.

MLSE-based nonlinearity mitigation for WDM 112 Gbit/s PDM-QPSK transmissions with digital coherent receiver

Direct detection systems with advanced modulation schemes are of great importance in metropolitan networks, because of their low cost and low power requirements. In particular, PAM-4 has attracted considerable attention, but has significant transmission distance limitations in the C-band. To extend its reach, we used a dual drive Mach-Zehnder modulator to generate a chromatic dispersion (CD) pre-compensated signal with an extra (j-1) multiplication to align the optical carrier and the modulated optical signal; by doing so, we achieved successful 128 Gbit/s transmission over an 80 km SSMF link, the longest reported reach of single lane 100 Gbit/s PAM-4 signals over DCF-free links. Synchronized bandwidth pre-compensation was also used, to reduce the influence of bandwidth-limitations.

Impact of optical channel distortions to digital timing recovery in digital coherent transmission systems

We present a method for clock extraction in polarization multiplexed coherent optical systems using frequency domain timing phase detector. The phase detector input signal is pre-processed using a simplified polarizer transfer function enabling the maximum clock tone quality.

Transmission of single lane 128 Gbit/s PAM-4 signals over an 80 km SSMF link, enabled by DDMZM aided dispersion pre-compensation

This paper presents the investigation on the design of automatic gain control (AGC) circuits assumed to be robust against both bias and swing variations in the received unipolar digital signal. We have derived a small signal AGC general model that includes the impact of all relevant AGC components on the AGC transfer function. This model has been used for the calculation of AGC loop parameters of the specified AGC transfer function. The AGC robustness against signal level transients has been improved by introducing the second-order AGC loop. In ac-coupled receivers, the order and corner frequency of high-pass filter (HPF) placed in front of the AGC have been investigated. The HPF has been designed so that the input signal bias variations caused by signal level transients are compressed in acceptable limits, and so that bit-pattern bias variations do not significantly decrease the receiver performance. Signal swing and bias variations become very critical in optical systems using unipolar modulation format. Since the AGC design depends on its application, as an example, we have presented the AGC design concept in dispersion compensating 10.7 Gb/s intensity-modulated direct-detection (IM-DD) nonreturn-to-zero (NRZ) optical receiver using maximum-likelihood sequence estimation (MLSE).

A circuit enabling clock extraction in coherent receivers

The quality of clock tone in many practical phase detectors depends on the width of the non-zero portion of the signal spectrum. In Nyquist systems that deploy the raised-cosine filter, most of the known timing extraction methods completely fail when the roll-off factor (ROF) approaches the zero value. In this letter, we present a novel digital phase detector that is based on two samples per symbol with a performance that is practically independent of the shape of the Nyquist filter and modulation formats. Excellent timing-error detector performance is demonstrated even in extremely band-limited systems - the so-called faster than Nyquist systems (FTN).

On the Design of AGC Circuits in IM-DD NRZ Optical Transmission Systems

We experimentally demonstrate Tx DSP-free 112-Gbit/s PAM-4 transmission over 20 km SMF using commercially available 1298 nm 25-GHz TOSA and ROSA devices. Superior performance is achieved by using timing recovery supported DSP at the receiver.