Bidirectional wavelength-division multiplexing transmission over installed fibre using a simplified optical coherent access transceiver.

High-speed broadband services require optical fibres in access networks, in which multiple subscribers are connected to service providers, to satisfy the continuously growing bandwidth demand. The primitive signaling scheme used in access networks enables the use of low-cost equipment but diminishes the bandwidth available to end-users. Thus, current technology will be unable to support future broadband demands. Coherent communication systems offer significantly improved power- and bandwidth-efficiency, but require fundamental simplifications to become economically viable for access networks. Here, we demonstrate a promising simplified coherent receiver exhibiting a robust performance against polarisation fluctuations over an installed fibre network. It enables the realisation of high-order modulation formats and offers high sensitivities, achieving a four-fold increase in the supported number of subscribers and approximately doubling the transmission distance compared to the recently standardized access technology. The proposed solution indicates that digital coherent technology can be feasible and transform the access networks, enabling ubiquitous new services and applications with uncontended, multi-gigabits/user broadband connections.

Blind symbol synchronization for direct detection optical OFDM using a reduced number of virtual subcarriers.

Symbol synchronization constitutes a major component in optical OFDM transceivers. In this paper, we propose reducing the complexity of a blind symbol synchronization technique for direct detection OFDM receivers based on virtual subcarriers by optimizing the number and location of the virtual subcarriers. Compared to the system design in our previous study, this new technique offers a reduction of 92% in the number of virtual subcarriers (from 26 to 2 in a system with 50 data carrying subchannels) resulting in significant savings in complexity with a minimal penalty. Moreover, it offers an increase in the system capacity as more subcarriers can be used to transmit data. The technique was assessed experimentally using a transmission system of direct detection 16-QAM optical OFDM operating at a data rate of 30.65 Gb/s over 23.3 km SSMF with BER of 10(-3). Negligible penalty was observed at high received powers. However, at low received powers, the number of averaging symbols had to be increased in order to improve the robustness of the method.

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

The use of single-sideband subcarrier modulation (SCM) with Nyquist (N) pulse shaping for cost-effective spectrally-efficient wavelength division multiplexed transmission with direct detection is described. Transmission of digitally pre-compensated 7 × 11 GHz-spaced QPSK SCM channels at 14 Gb/s per channel is experimentally demonstrated over distances of up to 800 km of uncompensated standard single-mode fiber (SSMF) (13440 ps/nm chromatic dispersion).

Experimental demonstration of 30 Gb/s direct-detection optical OFDM transmission with blind symbol synchronisation using virtual subcarriers.

The paper investigates the performance of a blind symbol synchronisation technique for optical OFDM systems based on virtual subcarriers. The test-bed includes a real-time 16-QAM OFDM transmitter operating at a net data rate of 30.65 Gb/s using a single OFDM band with a single FPGA-DAC subsystem and demonstrates transmission over 23.3 km SSMF with direct detection at a BER of 10(-3). By comparing the performance of the proposed synchronisation scheme with that of the Schmidl and Cox algorithm, it was found that the two approaches achieve similar performance for large numbers of averaging symbols, but the performance of the proposed scheme degrades as the number of averaging symbols is reduced. The proposed technique has lower complexity and bandwidth overhead as it does not rely on training sequences. Consequently, it is suitable for implementation in high speed optical OFDM transceivers.

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.

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.

Long-haul 10 Gbit/s linear and non-linear IMDD transmission over uncompensated standard fiber using a SQRT-metric MLSE receiver.

We experimentally demonstrated Intensity-Modulated Direct-Detection (IMDD) single-channel 1,040 km linear transmission and 800 km non-linear transmission at 10 Gb/s over standard single-mode (G.652) fiber, without any optical dispersion compensation or mitigation, using a Maximum-Likelihood Sequence-Estimation (MLSE) receiver employing the square-root (SQRT) branch metric with off-line processing. These experiments were designed as to probe the limits of the MLSE approach. They successfully showed that long-haul uncompensated transmission is in principle possible with MLSE, even in the presence of large uncompensated dispersion and strong intra-channel fiber non-linearities, provided that enough complexity can be built into the receiver. In the linear 1,040 km experiment, a Bit Error Rate (BER) of 10(-3) was achieved with an Optical Signal-to-Noise Ratio (OSNR) penalty with respect to back-to-back of 2.9 dB, using two samples per bit and 16,384 trellis states. Several other set-ups were tested as well, including the use of only one sample per bit and fewer trellis states. In the non-linear 800 km experiment, power was ramped up to 12 dBm, exciting substantial Kerr non-linearity, whose induced spectral-broadening exacerbated the effects of the large uncompensated dispersion of the link. Using an MLSE receiver with 1,024 states, we demonstrated a non-linear threshold of 9 dBm. We benchmarked this experiment towards simulations addressing various electrical and optical dispersion compensation strategies. We also carried out an analysis of error run-lengths, on both experiments, which showed that error burstiness may change considerably depending on the number of processor states, OSNR and the amount of non-linearity in the link.