Robert Koutsoyannis

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.

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.

Dependence of optical OFDM transceiver ASIC complexity on FFT size

We study the effects of the fast Fourier transform size on the performance and power consumption of real-time optical orthogonal frequency division multiplexing (OFDM) transceivers.

Improving fixed-point accuracy of FFT cores in O-OFDM systems

Optical OFDM communication systems operating at data rates in the 40Gb/s (and higher) range require high-throughput/highly parallel fast Fourier transform (FFT) implementations. These consume a significant amount of chip resources; we aim to reduce costs by improving the systems accuracy per chip-area. For OFDM signals, we characterize the growth of data within the FFT and explore several cost-conscious methods for improving the fixed-point format. Using ASIC synthesis and hardware accurate simulations, we evaluate the corresponding system error and stability of these methods. We introduce Directive Scaling, which provides an average increase in overall accuracy without additional runtime-adaptive mechanisms. ASIC synthesis results show minimal overhead, and we explicitly evaluate and explain the inherent tradeoffs. When applied to an 8-bit IFFT design, our technique improves precision by approximately two bits with just a 4% area overhead, as opposed to the additional 32% area overhead required using standard methods.

Recent progress on real-time DSP for direct detection optical OFDM transceivers

We describe recent developments in real-time digital signal processing for direct detection optical OFDM transceivers, including the optimization of high throughput FFT algorithms and receiver synchronization.

Real-time DSP-based optical OFDM transmission

We present the design of a field programmable gate array (FPGA) based optical orthogonal frequency division multiplexing (OFDM) transmitter operating at 21.4 GS/s and an experimental assessment of its performance in a directly-detected 8.34 Gbit/s QPSK-OFDM configuration over 1600 km of uncompensated standard fiber. We also discuss the suitability of OFDM technology for low-cost, low-power optical interconnects.