Roi Rath

Robust Transmission of 29-Gb/s OFDM Signal Over 1-km OM1 MMF Under Center Launching

Center launching is a simple and straightforward method to couple optical signals to a multimode fiber (MMF), and able to preferentially excite a few lower-order modes with reduced modal dispersion. However, system imperfections and mechanical perturbations will excite undesired higher order modes, leading to a selectively faded channel frequency response. For this reason, center launching is conventionally less favorable than offset launching when on-off keying is used. This problem can be circumvented by leveraging on the fine frequency granularity of the orthogonal frequency-division multiplexing (OFDM) signal, which is inherently less sensitive to the frequency fading. We have experimentally demonstrated the transmission of a 40-Gb/s OFDM signal (net data rate of 29.48 Gb/s) over 1-km G62.5/125 MMF without using complicated bit-and-power loading. The channel bandwidth is less than 9.1 GHz. We also demonstrate the robustness of the system against fiber bending, polarization variation, unmatched launch beam spot size, as well as its long-term stability.

On the optimization of link design using nonlinear equalization for 100Gb/s 16QAM transmission

The reduction of the number of EDFAs in a transmission link using nonlinear equalization was investigated for a 25GBaud RZ50-16QAM transmission. Nonlinear equalization was found to reduce the amount of EDFAs by up to 50% in comparison with linear equalization, depending on the type of fiber and the sampling rate of receiver ADC.

Precoding and equalization in digital coherent optical transmission

Digital signal processing techniques offers many advantages, such as the simplification of the transmission channel, flexibility of transceivers, and efficient compensation of almost any transmission impairment. Digital signal processors have been recently introduced in coherent long-haul optical transmission systems, as well as in non-coherent access networks. In this contribution, we review a number of specific signal processing algorithms that are useful in such systems. We focus on advanced equalizers for linear and nonlinear distortions. We examine the Tomlinson-Harashima precoding approach, as well as advanced flexible non-integer fractionally-spaced equalizers in a butterfly structure. For the compensation of fiber nonlinearities, Volterra based equalization is investigated.

Advanced digital signal processing for coherent and non-coherent optical transmission

Digital signal processors (DSP) are recently introduced in coherent long-haul optical transmission systems as well as in non-coherent access networks. This contribution reviews a number of specific signal processing algorithms that are useful in such systems. We focus on advanced equalisers for linear and nonlinear distortions. We examine the Tomlinson-Harashima precoding approach as well as advanced flexible non-integer fractionally-spaced butterfly equalisers. For nonlinearity compensation Volterra based equalization is investigated.

On the performance of digital back-propagation for imperfect knowledge of link design

Digital back-propagation (DBP) was shown to have the potential to become the most promising method for compensation of fiber nonlinearities. However, apart from its high computational effort, the performance of DBP is dependent on the knowledge of the transmission link, i.e. the links dispersion map as well as its power profile. In this contribution, we investigate the performance of DBP under the assumption of an imperfect link-design knowledge, and discuss its implications in an optical network scenario.

Decoding metrics for multistage bit-wise coded modulation in optical communications

A fiber-optic communication system employing a multistage bit-wise coded modulation scheme is considered as it enables the efficient utilization of higher-order modulation formats in combination with binary forward error correction (FEC) coding. Since modern FEC schemes employ soft-decision decoding, soft information on coded bits is required. In this paper, the decoding metric for the multistage bit-wise receiver structure is reviewed. Both numerical as well as experimental results are provided for the proposed decoding metrics using coded transmission over dispersion-unmanaged and dispersion-managed links.