Felix Frey

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.

Generation and Coherent Reception of 107-GBd Optical Nyquist BPSK, QPSK, and 16QAM

We present a 107-GBd coherent optical system for the generation and reception of m-ary phase-shift keyed (PSK) and quadrature-amplitude modulated (QAM) signals, including Nyquist pulse shaping. It is based on phase-stable optical time-division multiplex (OTDM) of two tributaries, which are modulated at half the target symbol rate, optical Nyquist filtering, and a broadband digital coherent receiver. The system is analyzed in back-to-back experiments with binary PSK (BPSK) modulation, quadrature-PSK (QPSK) modulation, as well as with 16-ary QAM. Compared to all-ETDM implementations, the transmitter is based on independent electro-optical modulation of two time-division multiplex tributaries and subsequent optical interleaving after modulation. Although this scheme implies increased hardware complexity, it enables lower penalties with respect to theory compared to all-ETDM experiments presented so far. This is due to the reduced hardware speed requirements. The measured penalties at bit-error ratios of 1 × 10-3 are 0.5, 1.5, and 6 dB for single-polarization BPSK, QPSK, and 16QAM, respectively. Furthermore, the employed phase-stable optical multiplexer allows use of standard receiver DSP at the broadband digital coherent receiver without need for special treatment of the individual OTDM tributaries.

Coherent UDWDM PON with joint subcarrier reception at OLT

In this contribution, we report on the experimental investigation of an ultra-dense wavelength-division multiplexing (UDWDM) upstream link with up to 700 × 2.488 Gb/s polarization-division multiplexing differential quadrature phase-shift keying parallel upstream user channels transmitted over 80 km of standard single-mode fiber. We discuss challenges of the digital signal processing in the optical line terminal arising from the joint reception of several upstream user channels. We present solutions for resource and cost-efficient realization of the required channel separation, matched filtering, down-conversion and decimation as well as realization of the clock recovery and polarization demultiplexing for each individual channel.

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.

Performance Comparison of Different 8QAM Constellations for the Use in Flexible Optical Networks

We investigate the influence of DAC resolution and pulse shaping on the system performance of different 8QAM constellations. Furthermore, we experimentally show that a circular constellation outperforms the commonly used 8QAM constellation by 0.7 dB in terms of OSNR sensitivity at a BER of 3.8·10-3.

Experimental analysis of nonlinear interference noise in heterogeneous flex-grid WDM transmission

We transmit a 28-GBd Nyquist PDM-16QAM channel with WDM-interferers (37.5-GHz grid) over 80-km SSMF spans. Interference noise analysis reveals substantial nonlinear phase noise for PDM-16QAM interferers compared to PDM-4QAM, while Q-penalty is kept small by appropriate CPE.

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.