Fernando A. Gutiérrez

All-Analogue Real-Time Broadband Filter Bank Multicarrier Optical Communications System

This paper studies the key aspects of an optical link which transmits a broadband microwave filter bank multicarrier (FBMC) signal. The study is presented in the context of creating an all-analogue real-time multigigabit orthogonal frequency division multiplexing electro-optical transceiver for short range and high-capacity data center networks. Passive microwave filters are used to perform the pulse shaping of the bit streams, allowing an orthogonal transmission without the necessity of digital signal processing (DSP). Accordingly, a cyclic prefix that would cause a reduction in the net data rate is not required. An experiment consisting of three orthogonally spaced 2.7 Gbaud quadrature phase shift keyed subchannels demonstrates that the spectral efficiency of traditional DSP-less subcarrier multiplexed links can be potentially doubled. A sensitivity of -29.5 dBm is achieved in a 1-km link.

Optimum Bias Point in Broadband Subcarrier Multiplexing With Optical IQ Modulators

This paper develops a theoretical analysis of the tradeoff between carrier suppression and nonlinearities induced by optical IQ modulators in direct-detection subcarrier multiplexing systems. The tradeoff is obtained by examining the influence of the bias conditions of the modulator on the transmitted single side band signal. The frequency components in the electric field and the associated photocurrent at the output of the IQ modulator are derived mathematically. For any frequency plan, the optimum bias point can be identified by calculating the sensitivity gain for every subchannel. A setup composed of subcarriers located at multiples of the data rate ensures that the effects of intermodulation distortion are studied in the most suitable conditions. Experimental tests with up to five QPSK electrical subchannels are performed to verify the mathematical model and validate the predicted gains in sensitivity.

Impact of band rejection in multichannel broadband subcarrier multiplexing

This paper explores experimentally the impairments in performance that are generated when multiple single-sideband (SSB) subcarrier multiplexing (SCM) signals are closely allocated in frequency to establish a spectrally efficient wavelength division multiplexing (WDM) link. The performance of cost-effective SSB WDM/ SCM implementations, without optical filters in the transmitter, presents a strong dependency on the imperfect sideband suppression ratio that can be directly achieved with the electro-optical modulator. A direct detected broadband multichannel SCM link composed of a state-of-the-art optical IQ modulator and five quadrature phase-shift keyed (QPSK) subcarriers per optical channel is presented, showing that a suppression ratio of 20 dB obtained directly with the modulator produced a penalty of 2 dB in overall performance, due to interference between adjacent optical channels.

Experimental Comparison of FBMC and OFDM for Multiple Access Uplink PON

Filter bank multicarrier (FBMC) is a favourable candidate for next generation optical access networks due to its capability to operate without a cyclic prefix and its high resilience to synchronization errors in the system. In this paper, we analyze the performance of FBMC in upstream passive optical networks (PON), without the employment of inter optical network unit (inter-ONU) frequency guard bands. By using both experimental work and numerical simulations, the impact of relative time offsets on inter-ONU orthogonality is studied. Furthermore, numerical simulations are also used to study the impact of frequency offsets in such systems. Moreover, comparisons are drawn to orthogonal frequency division multiplexing (OFDM)-based upstream PONs. This paper experimentally demonstrates that imperfect time synchronization impacts all subcarriers in an OFDM system, while it affects only the immediately adjacent subcarriers in an FBMC system, which could be corrected for by the restoration of synchronicity between users. A simple feedback mechanism is experimentally demonstrated to automatically restore orthogonality and synchronization between users in order to validate the practical implementation of the scheme.

WDM Orthogonal Subcarrier Multiplexing

Electro-optical transceivers can be implemented employing all-analog signal processing in order to achieve low values of power consumption and latency. This paper shows that the spectral efficiency of such solutions can be increased by combining orthogonal multicarrier techniques and off-the-shelf microwave components. A real-time 108-Gbit/s experiment was performed emulating a wavelength division multiplexing (WDM) system composed of five optical channels. The optical carriers were provided by an externally injected gain switched optical frequency comb. Each optical channel transmitted a 21.6-Gbit/s orthogonal subcarrier multiplexing (SCM) signal that was modulated and demodulated in the electrical domain without the requirement for digital signal processing. The net data rate remained higher than 100 Gbit/s after taking into account forward error correction overheads. The use of orthogonally overlapping subchannels achieves an unprecedented spectral efficiency in all-analog real-time broadband WDM/SCM links.

100 Gbit/s real-time all-analogue filter bank OFDM based on a gain-switched optical comb

A real-time 5×21.6 Gbit/s WDM electro-optical transceiver is presented. Optical carriers were spaced by 20 GHz and each one transmitted four orthogonally overlapping broadband subcarriers. Only analogue electronics were employed, achieving an unprecedented spectral efficiency in DSP-less SCM links.

All-analogue real-time filter bank OFDM over 50 Km of SSMF using a novel synchronization technique

A 21.6 Gbit/s 1.78 bit/s/Hz OFDM signal is transmitted over 50 Km of fiber without using DSP in the transmitter or the receiver. The synchronization scheme only requires one PLL to synchronize all the subcarriers.

FBMC for directly modulated passive optical networks (PON)

In this paper the transmission of FBMC based services for PONs using direct modulation of low-cost Discrete Mode lasers is studied. The FBMC system is composed of 21 overlapping subcarriers each modulated by 16-QAM. We experimentally study the performance of such a directly modulated FBMC system in back-to-back and fibre transmission scenarios up to 75 km of SSMF.

WDM Orthogonal Subcarrier Multiplexing Based on Mode-Locked Lasers

The (de)modulation of broadband orthogonal subchannels relying on all-analogue signal processing potentially achieves high-capacity orthogonal subcarrier multiplexing (OSCM) electro-optical transceivers with low power consumption and latency. Overall transmission rates can be multiplied by employing wavelength division multiplexing (WDM) technology. Mode-locked lasers (MLL) are relevant optical frequency combs, as they produce a high number of comb tones while presenting a small footprint and low power consumption. Unlike baseband transmission, the use of high-frequency subchannels in OSCM systems overcomes the high relative intensity noise that MLL comb tones present at low frequencies. This paper reports a direct-detection real-time all-analogue WDM/OSCM experiment that emulates a 432 Gbit/s (20 × 21.6 Gbit/s) electro-optical transceiver. The 20 optical carriers were generated by a state-of-the-art quantum-dash MLL. The net data rate after considering forward error correction overheads is still higher than 400 Gbit/s. This is the highest capacity achieved in real-time broadband all-analogue WDM/OSCM links to date.