Yannis Benlachtar

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.

Theoretical and experimental evaluation of clipping and quantization noise for optical OFDM.

Orthogonal frequency division multiplexing (OFDM) has recently gained substantial interest in high capacity optical fiber communications. Unlike wireless systems, optical OFDM systems are constrained by the limited resolution of the ultra high-speed digital-to-analog converters (DAC) and analog-to-digital converters (ADC). Additionally, the situation is exacerbated by the large peak-to-average power ratio (PAPR) inherent in OFDM signals. In this paper, we study the effects of clipping and quantization noise on the system performance. We analytically quantify the introduced distortion as a function of bit resolution and clipping ratio, both at the DAC and ADC. With this we provide a back-to-back signal-to-noise ratio analysis to predict the bit error rate of the system, assuming a fixed received optical power and ideal electrical-optical-electrical conversion. Simulation and experimental results are used to confirm the validity of the expressions.

Experimental investigation of SPM in long-haul direct-detection OFDM systems

We experimentally investigate the effect of self-phase modulation on direct-detection orthogonal frequency division multiplexed (OFDM) transmission at 11.1 Gb/s over 960km and 1600km uncompensated standard single-mode fiber links. We show that for long-haul systems, the penalties due to nonlinear distortion in OFDM systems are comparable to those in links employing electronic predistortion.

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.

Performance and power consumption of digital signal processing based transceivers for optical interconnect applications

We discuss the suitability of digital signal processing (DSP) and optical orthogonal frequency division multiplexing (OFDM) for optical interconnects in applications such as data centers and high performance computing. The performance and power dissipation of this technology is reviewed and compared with singlecarrier binary modulation formats used in parallel fiber and wavelength division multiplexed interconnects. The paper considers the power consumption of the individual sub-systems of an OFDM link, including FFT cores, data converters (DAC, ADC) and forward error correction circuits. The dependence of the error vector magnitude (EVM) on the DAC/ADC resolution is calculated for a 1024 channel 16-QAM OFDM link, and the implications for the design of the OFDM optical interconnects in terms of minimising the power consumption are discussed.

10.7 Gb/s transmission over 1200 km of standard single-mode fiber by electronic predistortion using FPGA-based real-time digital signal processing

A 10.7 Gb/s electronic predistortion transmitter using FPGA-based real time 2 Samples/bit digital signal processing based on 55-tap finite impulse response filters is described. Transmission over 1200 km of standard single-mode fiber without optical dispersion compensation is demonstrated with an OSNR penalty of only 2.5 dB, compared with back-to- back operation.

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.