Bernd Nebendahl

Error Vector Magnitude as a Performance Measure for Advanced Modulation Formats

We examine the relation between optical signal-to-noise ratio (OSNR), error vector magnitude (EVM), and bit-error ratio (BER). Theoretical results and numerical simulations are compared to measured values of OSNR, EVM, and BER. We conclude that the EVM is an appropriate metric for optical channels limited by additive white Gaussian noise. Results are supported by experiments with six modulation formats at symbol rates of 20 and 25 GBd generated by a software-defined transmitter.

Real-time Nyquist pulse generation beyond 100 Gbit/s and its relation to OFDM.

Nyquist sinc-pulse shaping provides spectral efficiencies close to the theoretical limit. In this paper we discuss the analogy to optical orthogonal frequency division multiplexing and compare both techniques with respect to spectral efficiency and peak to average power ratio. We then show that using appropriate algorithms, Nyquist pulse shaped modulation formats can be encoded on a single wavelength at speeds beyond 100 Gbit/s in real-time. Finally we discuss the proper reception of Nyquist pulses.

Real-Time Software-Defined Multiformat Transmitter Generating 64QAM at 28 GBd

We demonstrate a software-defined real-time optical multiformat transmitter. Here, eight different modulation formats are shown. Data rate and modulation formats are defined through software accessible look-up tables enabling format switching in the nanosecond regime without changing the transmitter hardware. No data are lost during the switching process. SP-64 quadrature amplitude modulation at 28 Gbd has been generated and tested. This allows us to generate a 336-Gb/s real-time pseudorandom bit sequence in a dual polarization setup.

Single-laser 32.5 Tbit/s Nyquist WDM transmission

Single-laser 32.5 Tbit/s 16QAM Nyquist-WDM transmission with 325 carriers over 227 km at a net spectral efficiency of 6.4 bit/s/Hz is reported.

Polarization demultiplexing in Stokes space

A technique is demonstrated for polarization demultiplexing of arbitrary complex-modulated signals. The technique is based entirely on the observation of samples in Stokes space, does not involve demodulation and is modulation format independent. The data in Stokes space is used to find the best fit plane and the normal to it which contains the origin. This normal identifies the two orthogonal polarization states of transmission and the desired polarization alignment transformation matrix. The technique is verified experimentally and is compared with the constant modulus algorithm.

Single source optical OFDM transmitter and optical FFT receiver demonstrated at line rates of 5.4 and 10.8 Tbit/s

OFDM data with line rates of 5.4 Tbit/s or 10.8 Tbit/s are generated and decoded with a new real-time all-optical FFT receiver. Each of 75 carriers of a comb source is encoded with 18 GBd QPSK or 16-QAM.

512QAM Nyquist sinc-pulse transmission at 54 Gbit/s in an optical bandwidth of 3 GHz.

We demonstrate for the first time transmission of 54 Gbit/s and 48 Gbit/s over 44 km and 150 km, respectively, utilizing an optical bandwidth of only 3 GHz. We used polarization division multiplexed 512QAM and 256QAM modulation formats in combination with Nyquist pulse shaping having virtually zero roll-off. The resulting spectral efficiencies range up to 18 bit/s/Hz and 16 bit/s/Hz, respectively. Taking into account the overhead required for forward error correction, the occupied signal bandwidth corresponds to net spectral efficiencies of 14.4 bit/s/Hz and 15 bit/s/Hz, which could be achieved in a wavelength division multiplexed network without spectral guard bands.

Real-time OFDM transmitter beyond 100 Gbit/s

Real-time OFDM transmitters breaking the 100 Gbit/s barrier require high-performance, usually FPGA-based digital signal processing. Especially the Fourier transform as a key operation of any OFDM system must be optimized with respect to performance and chip area utilization. Here, we demonstrate an alternative to the widely adopted fast Fourier transform algorithm. Based on an extensive yet optimized use of pre-set look-up tables, our FPGA implementation supports fast reconfigurable channel equalization and switching times in the nanosecond range without re-loading any code. We demonstrate the potential of the concept by realizing the first real-time single polarization OFDM transmitter generating a 101.5 Gbit/s data stream by modulating 58 subcarriers with 16QAM.

Performance Monitoring and Measurement Techniques for Coherent Optical Systems

Modern spectrally efficient optical communication systems utilize polarization-multiplexed coherent transmission in complex modulation format. Coherent receivers used in these systems measure the amplitude and phase of the optical signals for both orthogonally polarized components carrying information. Knowledge of the amplitude and phase of the optical field, in combination with digital signal processing, gives the receiver an inherent metrology and performance monitoring capability. As the optical signal propagates from the transmitter over optical fiber to the receiver, a signal transformation and degradation is expected. The receiver observes the properties of the transmitted optical signal as degraded by the impairments of the transmission medium. The details of monitoring optical signal parameters and link impairments are the focus of this paper. The optical signal parameters include polarization state and residual carrier phase; optical link impairments include chromatic dispersion and polarization mode dispersion. Two distinct techniques are presented: one based on Stokes space analysis, and the other on Kalman filtering. The Stokes space techniques are modulation-format independent and do not require demodulation. The Kalman filtering provides optimal estimation of the physical quantities that describe the optical signal and the optical medium.

Pulse-Shaping With Digital, Electrical, and Optical Filters—A Comparison

We investigate the performance of sinc-shaped QPSK signal pulses generated in the digital, electrical, and optical domains. To this end an advanced transmitter with a digital pulse-shaper is compared to analog transmitters relying on pulse-shaping with electrical and optical filters, respectively. The signal quality is assessed within a single carrier setup as well as within an ultra-densely spaced WDM arrangement comprising three channels. An advanced receiver providing additional digital filtering with an adaptive equalization algorithm to approximate an ideal brick-wall Nyquist filter has been used for all schemes. It is found that at lower symbol rates, where digital processing is still feasible, digital filters with a large number of filter coefficients provide the best performance. However, transmitters equipped with only electrical or optical pulse-shapers already outperform transmitters sending plain unshaped NRZ signals, so that for higher symbol rates analog electrical and optical techniques not only save costs, but are the only adequate solution.