Saleem Alreesh

Bandwidth-Variable Transceivers based on Four-Dimensional Modulation Formats

In this invited contribution, we discuss technology options for bandwidth-variable transceivers which are key components for the realization of flexible software-defined optical networking. Bandwidth-variable transceivers enable the software-controlled adaptation of physical layer parameters such as transmitted bit rate, spectral efficiency and transparent reach according to the traffic demands at hand. In particular, we focus on recent advances in four-dimensional modulation formats and in modulation format transparent data-aided digital signal processing. It is argued that four-dimensional modulation formats present an attractive complement to conventional polarization-multiplexed formats in the context of bandwidth-variable transceivers, where they enable a smooth transition with respect to spectral efficiency while requiring marginal additional hardware effort. Results of numerical simulations and experiments supporting this statement are presented. For the cost-efficient hardware implementation of bandwidth-variable transceivers supporting several polarization-multiplexed and four-dimensional modulation formats, digital signal processing algorithms are required which operate format transparent and consume little hardware resources. We discuss data-aided signal processing as one possible option, in particular with respect to carrier frequency recovery and channel estimation in combination with frequency domain equalization.

Experimental demonstration of a format-flexible single-carrier coherent receiver using data-aided digital signal processing

We experimentally demonstrate the use of data-aided digital signal processing for format-flexible coherent reception of different 28-GBd PDM and 4D modulated signals in WDM transmission experiments over up to 7680 km SSMF by using the same resource-efficient digital signal processing algorithms for the equalization of all formats.

Experimental Investigation of 126-Gb/s 6PolSK-QPSK signals

We experimentally generate 28-GBd 6PolSK-QPSK signals by utilizing a high-speed 4-channel DAC and an integrated dual-polarization I/Q modulator. In WDM transmission experiments over up to 4800 km standard single-mode fiber, we compare the performance of 126-Gb/s 6PolSK-QPSK and 112-Gb/s PDM-QPSK signals.

Generation, Transmission, and Detection of 4-D Set-Partitioning QAM Signals

Four-dimensional (4-D) set-partitioning quadrature amplitude modulation (4-D SP-QAM) has emerged as an interesting option for cost- and resource-efficient realization of bandwidth variable transceivers in elastic optical networks. In this invited paper, we review the principles for generation of 4-D SP-QAM signals, and describe options for forward error correction coding of 4-D SP-QAM signals and for realization of the digital signal processing in the coherent receiver. Furthermore, we report on the experimental realization of 4-D 512-ary and 2048-ary SP-QAM signals at a symbol rate of 28 GBd and investigate their performance in a Nyquist-WDM scenario. In transmission experiments over standard single-mode fiber, we compare the reach and spectral efficiency of five-carrier Nyquist-WDM signals modulated by various 4-D SP-QAM formats and polarization-division multiplexed (PDM) QAM formats. Of these modulation formats, the one with the lowest spectral efficiency is 128-ary SP-QAM encoding 7 bits/4-D symbol and the one with the highest spectral efficiency is PDM-64QAM encoding 12 bits/4-D symbol. By switching the modulation format, the spectral efficiency can be optimized for a specific reach with a granularity of 0.56 bit/s/Hz.

Joint-Polarization Carrier Phase Estimation for PS-QPSK Signals

We investigate a carrier phase estimation algorithm for the polarization-switched quadrature phase-shift-keying modulation format. The algorithm, which is based on the conventional block Mth power feed-forward scheme, works jointly on both polarizations and shows a significant performance improvement compared with a polarization-independent scheme. We evaluate the tolerance to laser phase noise in numerical simulations and the tolerance to nonlinear distortions in single-channel and Nyquist wavelength-division-multiplexing transmission experiments over up to 17000 km of ultralarge-effective-area fiber.

Experimental investigation of a four-dimensional 256-ary lattice-based modulation format

A four-dimensional modulation format with 256 point from the D₄-lattice (256-D₄) is compared to PM-16QAM in experiments. 256-D₄ is more sensitive at high OSNR but with a 21.3% overhead turbo product code, PM-16QAM outperforms 256-D₄.

Transmission Performance of 4D 128SP-QAM With Forward Error Correction Coding

The performance of the 4-D 128SP-QAM format is experimentally investigated in a Nyquist-wavelength division multiplexing system for two different forward error correction (FEC) scenarios. Two soft-decisions FEC schemes are experimentally evaluated for 128SP-QAM, namely, a turbo product code (TPC) and a low-density parity-check (LDPC) code. The results show that after full convergence the LDPC code outperforms the TPC with respect to coding gain at a bit error ratio of 10-8. However, it requires more decoding iterations to achieve convergence toward an optimum performance state.

Transmission of 512SP-QAM Nyquist-WDM signals

We report on the experimental realization of 28-GBd four-dimensional 512-ary set-partitioning QAM signals and investigate them in a Nyquist-WDM scenario. In transmission experiments over standard single-mode fiber, we compare them with 28-GBd PDM-16QAM and PDM-32QAM Nyquist-WDM signals.

DAC-Free Ultralow-Power Dual-Polarization 64-QAM Transmission at 32 GBd With Hybrid InP IQ SEMZM and BiCMOS Drivers Module

A hybrid transmitter module comprising a 15-segment InP IQ segmented Mach–Zehnder modulator and two 0.13 μm BiCMOS SiGe:C drivers with integrated 4-bit DAC functionality is described. The high-resolution prototype allows a wide range of modulation formats and, for the first time, ultralow-power dual-polarization (DP) 64-QAM signal generation at record speed of 32 GBd without the usage of external DACs is demonstrated. The DP 64-QAM signal is transmitted error free over 80 km of SSMF with 7.8 pJ/bit/polarization energy consumption. Post-FEC BER results are presented, with more than 226 bits decoded without errors after four decoding iterations. The package footprint is 28 mm × 76 mm. These results evidence the advantage of the proposed arrangement for the next generation low-power high-speed optical transceivers.

High-Speed Ultralow-Power Hybrid Optical Transmitter Module With InP I/Q-SEMZM and BiCMOS Drivers With 4-b Integrated DAC

A hybrid optical transmitter module comprising a 15-segment InP in-phase/quadrature-phase segmented Mach-Zehnder modulator and two SiGe:C BiCMOS drivers featuring integrated 4-b digital-to-analog converter functionality is described. The drivers, fabricated in the 0.13-μm process of IHP, deliver a differential output voltage of 2.5 Vpp across all the 15 segments while dissipating less than 1 W of power each, at the maximum. Clear electrical eye diagrams up to 40-Gb/s from every output are reported. The module allows a wide range of modulation formats, among which up to eight-pulse amplitude modulation and polarization-division-multiplexed (PDM) 64-quadrature amplitude modulation (QAM) back-to-back error-free electro-optical transmission at a record speed of 32 GBd are demonstrated. The 32-GBd PDM 64-QAM signal was transmitted error-free over 80 km of standard single-mode fiber, featuring 7.8-pJ/b energy consumption. The devised hybrid arrangement proves the suitability of SiGe HBT drivers for achieving higher speeds over their CMOS counterparts, with comparable low power dissipation, for advanced optical transceivers.