Rachid Bouziane

Generation of optical OFDM signals using 21.4 GS/s real time digital signal processing

We demonstrate a field programmable gate array (FPGA) based optical orthogonal frequency division multiplexing (OFDM) transmitter implementing real time digital signal processing at a sample rate of 21.4 GS/s. The QPSK-OFDM signal is generated using an 8 bit, 128 point inverse fast Fourier transform (IFFT) core, performing one transform per clock cycle at a clock speed of 167.2 MHz and can be deployed with either a direct-detection or a coherent receiver. The hardware design and the main digital signal processing functions are described, and we show that the main performance limitation is due to the low (4-bit) resolution of the digital-to-analog converter (DAC) and the 8-bit resolution of the IFFT core used. We analyze the back-to-back performance of the transmitter generating an 8.36 Gb/s optical single sideband (SSB) OFDM signal using digital up-conversion, suitable for direct-detection. Additionally, we use the device to transmit 8.36 Gb/s SSB OFDM signals over 200 km of uncompensated standard single mode fiber achieving an overall BER<10(-3).

Optical OFDM for the data center

We investigate the use of orthogonal frequency division multiplexing (OFDM) to increase the capacity of multimode fiber (MMF)-based optical interconnects for data center applications. This approach provides a solution to modulation bandwidth limitations of the lasers, and to the intermodal dispersion of the MMF which leads to frequency-dependent attenuation. Recent studies on adaptively modulated OFDM are reviewed, and new simulation results assessing the capacity of such links for lengths of up to 300 m are presented, assuming the use of 50/125 µm graded-index MMF at a wavelength of 850 nm. The use of coded OFDM as an approach to deal with intermodal dispersion is also discussed.

Signal-signal beat interference cancellation in spectrally-efficient WDM direct-detection Nyquist-pulse-shaped 16-QAM subcarrier modulation

An experimental demonstration of direct-detection single-sideband Nyquist-pulse-shaped 16-QAM subcarrier modulated (Nyquist-SCM) transmission implementing a receiver-based signal-signal beat interference (SSBI) cancellation technique is described. The performance improvement with SSBI mitigation, which compensates for the nonlinear distortion caused by square-law detection, was quantified by simulations and experiments for a 7 × 25 Gb/s WDM Nyquist-SCM signal with a net optical information spectral density (ISD) of 2.0 (b/s)/Hz. A reduction of 3.6 dB in the back-to-back required OSNR at the HD-FEC threshold was achieved. The resulting reductions in BER in single channel and WDM transmission over distances of up to 800 km of uncompensated standard single-mode fiber (SSMF) achieved are presented.

Real-time OFDM or Nyquist pulse generation - which performs better with limited resources?

We investigate the performance and DSP resource requirements of digitally generated OFDM and sinc-shaped Nyquist pulses. The two multiplexing techniques are of interest as they offer highest spectral efficiency. The comparison aims at determining which technology performs better with limited processing capacities of state-of-the-art FPGAs. It is shown that a novel Nyquist pulse shaping technique, based on look-up tables requires lower resource count than equivalent IFFT-based OFDM signal generation while achieving similar performance with low inter-channel guard-bands in ultra-dense WDM. Our findings are based on a resource assessment of selected DSP implementations in terms of both simulations and experimental validations. The experiments were performed with real-time software-defined transmitters using a single or three optical carriers.

Nyquist-shaped dispersion-precompensated subcarrier modulation with direct detection for spectrally-efficient WDM transmission

We report on the first experimental demonstration of 14 Gb/s direct detection single-sideband subcarrier modulated Nyquist QPSK transmission. Using electronic pre-compensation, transmission over 800 km of dispersion-uncompensated standard single-mode fiber was achieved.

Design studies for ASIC implementations of 28 GS/s optical QPSK- and 16-QAM-OFDM transceivers

We designed at the register-transfer-level digital signal processing (DSP) circuits for 21.8 Gb/s and 43.7 Gb/s QPSK- and 16-QAM-encoded optical orthogonal frequency division multiplexing (OFDM) transceivers, and carried out synthesis and simulations assessing performance, power consumption and chip area. The aim of the study is to determine the suitability of OFDM technology for low-cost optical interconnects. Power calculations based on synthesis for a 65 nm standard-cell library showed that the DSP components of the transceiver (FFTs, equalisation, (de)mapping and clipping/scaling circuits) consume 18.2 mW/Gb/s and 12.8 mW/Gb/s in the case of QPSK and 16-QAM respectively.

Real-Time Digital Signal Processing for the Generation of Optical Orthogonal Frequency-Division-Multiplexed Signals

In this paper, we investigate the design of a field-programmable-gate-array (FPGA) based optical orthogonal frequency-division multiplexing (OFDM) transmitter implementing real-time digital signal processing at 21.4 GSample/s. The transmitter was utilized to generate 8.34 Gb/s QPSK-OFDM signals for direct detection. We study the impact of the finite resolutions of the inverse fast Fourier transform cores and the digital-to-analog converters on the system performance. Furthermore, we describe a transmission experiment over 800 and 1600 km of uncompensated standard fiber with negligible optical SNR penalties and bit error rate <; 10-3.

Optimizing FFT Precision in Optical OFDM Transceivers

The effects of different system parameters on the performance of real-time optical orthogonal frequency-division-multiplexing (OFDM) transceivers are investigated. Numerical simulations are carried out to search for the optimum arithmetic precision of the fast Fourier transform and its inverse (FFT/IFFT) which are key components in optical OFDM transceivers.

Design studies for an ASIC implementation of an optical OFDM transceiver

We designed at the register-transfer-level the DSP circuits for a 21.8 Gb/s QPSK-OFDM transceiver, and carried out synthesis and simulations assessing performance, power consumption and chip area, to determine their suitability for low-cost optical interconnects.

Generation and transmission of 85.4 Gb/s real-time 16QAM coherent optical OFDM signals over 400 km SSMF with preamble-less reception

This paper presents a real-time, coherent optical OFDM transmitter based on a field programmable gate array implementation. The transmitter uses 16QAM mapping and runs at 28 GSa/s achieving a data rate of 85.4 Gb/s on a single polarization. A cyclic prefix of 25% of the symbol duration is added enabling dispersion-tolerant transmission over up to 400 km of SSMF. This is the first transmission experiment performed with a real-time OFDM transmitter running at data rates higher than 40 Gb/s. A key aspect of the paper is the introduction of a novel method for OFDM symbol synchronization without relying on training symbols. Unlike conventional preamble-based synchronization methods which perform cross-correlations at regular time intervals and let the system run freely in between, the proposed method performs synchronization in a continuous manner ensuring correct symbol alignment at all times.