Optimal design of subcarrier multiplexed lightwave systems employing linearized external modulators

Digital-to-analog converters (DACs) are widely used in bandwidth-limited visible-light communication (VLC) systems for high spectral-efficiency multi-carrier signal generation. To reduce the implementation complexity of the transmitter, a digital 1-bit low-pass delta-sigma modulation (DSM)-enabled DAC-free filter-bank multi-carrier (FBMC) signal generation scheme is proposed and experimentally investigated in a red light-emitting diode-based VLC system. In this scheme, one field programmable gate array gigabit transceiver in combination with a low-pass filter is employed to realize the 609 MHz baseband FBMC signal generation. Besides, a low-complexity fast Walsh-Hadamard transform (FWHT)-based interleaved block precoding (IBP) technique is also proposed to reduce peak-to-average power ratio and achieve a uniform SNR profile, and then improve the bit error rate (BER) performance. The experimental results exhibit that the net data rate of 2.38-Gbit/s 16-ary quadrature amplitude modulation-encoded FBMC signal, generated by the proposed scheme, can transmit 2.3-m free space with the BER less than 3.8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . With the help of the FWHT-IBP technique, about 2-dB improvement in receiver sensitivity is achieved. In addition, the BER performance can provide a slight improvement in the BER performance with the proposed scheme, compared to the conventional DAC-based FBMC-VLC.

This paper investigates the application of an erbium doped fibre amplifier (EDFA) in a coherent receiver using pulse position modulation (PPM) format. Such a combination can exploit the abundant monomode fibre bandwidth to offer improved sensitivity and improved multichannel selectivity through the exploitation of coherent transmission. Original theoretical results are presented for a system operating at a bit rate of 622 Mbit/s and a wavelength of 1.53 /spl mu/m. By using a suboptimal matched filter approach and assuming thermal noise dominant conditions, the results demonstrate that the system offers 7.5 dB sensitivity improvement over a homodyne PPM system and 9.4 dB improvement over PSK homodyne. The system investigated also offers a 10.4 dB sensitivity improvement over an equivalent optically preamplified direct detection (DD) PPM system. It was also demonstrated that by using an optical filter in order to reduce the ASE noise the improvement in receiver sensitivity was only 0.3 dB. This greatly reduces the requirement for optical filters in coherent systems and therefore the full bandwidth of the optical amplifiers can be used.

In optical communication systems, coherent detection permits significant improvements in receiver sensitivity and wavelength selectivity compared with direct detection and allows a variety of modulation/demodulation formats to be used in practical applications. In the 1.3-1.6-µm lightwave band, an ideal coherent receiver requires only 10-20 photons/bit to obtain a bit error rate (BER) of 10−9, while direct detection requires ~1000 photons/bit. Because of its improved wavelength selectivity, the advanced technology of frequency multiplexing with channel spacings of only 100 MHz can be used as in microwave systems instead of the 100 GHz required with conventional optical wavelength-division-multiplexing technology.

The main objective of this paper is to investigate the application of an erbium doped fiber amplifier (EDFA) in a coherent receiver using the pulse position modulation (PPM) format. Such a combination can exploit the abundant monomode fiber bandwidth to offer improved sensitivity and improved multichannel selectivity through the exploitation of coherent transmission. Original theoretical results are presented for a system operating in thermal noise dominant condition at a bit rate of 622 Mbit/s and a wavelength of 1.53 micrometer. By using a suboptimal matched filter the results demonstrate that the system offers 7.5 dB sensitivity improvement over a homodyne PPM system and 9.4 dB improvement over PSK homodyne. The system investigated also offers a 10.4 dB sensitivity improvement over an equivalent optically preamplified direct detection (DD) PPM system. It is also demonstrated that by using an optical filter in order to reduce the ASE noise due to EDFA the improvement in receiver sensitivity was only 0.3 dB. This greatly reduces the requirement for optical filters in coherent systems and therefore the full bandwidth of the optical amplifiers can be utilized.

A mode-adapted semiconductor optical amplifier (SOA) has been fabricated and packaged. At the gain peak, 1500 nm, the fiber to fiber gain was measured to be 32.5 dB. Statistics for eight packaged devices indicate that a fiber-to-fiber gain of 26.3 dB /spl plusmn/ 1.3 dB and a saturation output power of 12.4 dBm /spl plusmn/ 0.4 dBm are typical at a bias of 500 mA for /spl lambda/ = 1550 nm. Polarization sensitivity at 1550 nm was measured to be 1.1 dB /spl plusmn/ 0.4 dB and the transverse electric (TE) polarization state noise figure (NF) was determined to be 7.0 dB /spl plusmn/ 0.5 dB. The coupling loss was 1.3 dB /spl plusmn/ 0.1 dB per facet. This SOA, with a 1.3-nm filter, was used as an optical preamplifier in a 10-Gb/s return-to-zero (RZ) system testbed with a pseudorandom binary sequence (PRBS) of 2/sup 31/ -1. A 14.5-dB improvement in receiver sensitivity was observed at a bit error rate (BER) of 10/sup -11/.

We propose an enhanced IEEE 802.15.4 gateway for use in a hierarchical network architecture. We perform a system comparison between it and a gateway using a normal sensor mote. This comparison includes a PHY and MAC simulation benefiting from ray trace propagation modeling of a large office building. The proposed enhanced gateway offers a 21.2 dB improvement in receiver sensitivity and an ideal channel equaliser. We analyze these benefits before performing a full system simulation. We report improvements over a classical sensor gateway in all key areas of packet delivery, latency and mote energy efficiency. Just under a six-fold increase in indoors range is also reported. We observe a reduction in performance due to channel distortion and combat it by proposing a fractionally spaced equaliser. We show that for buildings with large RMS delay spreads - greater than 100ns (e.g. conference centres), significant benefit may be derived.

In this paper, we investigate a SOA (semiconductor optical amplifier) preamplifier model by suitable choosing the material and device parameters for the SOA to reduce the amplified spontaneous emission (ASE) noise at low range of 22.3 &#956;W with 0.1 mW input power for PIN receiver. Our proposed SOA optical preamplifier is found to be more relaxed from optical alignment & antireflection coating and eliminate the need of optical filter and give large tolerance of an input light wavelength more than 100 nm. Also we investigate the receiver sensitivity for different bit rate. We show that receiver sensitivity is -69.9 dBm at bit error rate (BER) of 4.6×10^-10 for 10 Gb/s and for the 40 Gb/s bit rate the improved receiver sensitivity is -19.2 dBm with PIN receiver. We also observe the sensitivity of -40.5 dBm at 40 Gb/s with DPSK (differential phase shift keying) receiver. Further the impact of amplified spontaneous emission power, gain variation with input light wavelength & optical gain for TE & TM modes with input power for PIN & DPSK receivers has been illustrated. We show that optical gain of SOA is polarization independent and remain constant 30.06 dB up to gain saturation. We also show that DPSK receiver suffer less from ASE noise. The variation of material loss, length & bias current for our SOA preamplifier to optimize the receiver sensitivity is further illustrated.

Transmission capacity and receiver sensitivity of an intensity-modulation direct detection (IM-DD) optical discrete multi-tone (DMT) system can be improved by using the probabilistically shaping (PS) technique. However, different probabilistic distributions will be required owing to the unbalanced signal-to-noise ratio (SNR) among data-carrying subcarriers (SCs) induced by the imperfect frequency response of optical/electrical devices, which can increase the implementation complexity of the PS-DMT transceiver. In this work, different signal pre-processing schemes including pre-equalization, Walsh–Hadamard transform (WHT)-based full data-carrying SCs precoding (FDSP) and fast WHT-based partial data-carrying SCs precoding (PDSP) are investigated for SNR equalization in a short-reach PS-DMT transmission system. After transmission over 50 km single-mode fiber, the experimental results indicated that three pre-processed signals have almost the same generalized mutual information (GMI) performance and receiver sensitivity improvements. The proposed fast WHT-based PDSP scheme may be a good option for the implementation of the PS-DMT transmission systems with a large SC SNR fluctuation regarding computational complexity.

The present generation of optical transmission systems is based on the 1.3um wavelength range where conventional singlemode fibers have low attenuation and dispersion close to zero. The minimum attenuation of these fibers, however, lies close to 1.55um. At this wavelength transmission systems using semiconductor laser sources with launch powers of 1mW and conventional PIN-FET receivers could potentially operate with inter repeater spacings in excess of 100km. Improvements in transmitter power or receiver sensitivity would increase this spacing still further. The introduction of systems operating at 1.55um, however, has been inhibited by the dispersive nature of conventional silica fiber at this wavelength. Typical dispersion is 18ps/nm Km. The characteristics of the source therefore have an important effect on the overall system performance limiting the distance/bandwidth product that can be achieved. This paper describes the development of a range of 1.55um laser sources. It analyses their characteristics as components and shows their impact on system performance parameters.© (1987) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

InP/InGaAsP/InGaAs avalanche photodiodes with sepa- rate absorption, grading, and multiplication regions (SAGM-APD's) have been fabricated from wafers grown by chemical beam epitaxy (CBE). These APD's exhibit low dark current ( 500-Mbit/s) lightwave sys- tems experiments have shown that avalanche photodiodes (APD's) can provide several decibels of improvement in receiver sensitivity compared to p-i-n photodiodes ( 11-(5). In long-wavelength (= 1.3- or 1.5-pm) lightwave receivers the highest sensitivities have been achieved with APD's having an InP multiplication region and an 1~,~~Ga,-,.~~As (subscripts deleted in following text) absorption region separated by a transition region consisting of one or more lattice-matched intermediate-bandgap In,Gal -,AsL.P1 -y layers, the separate absorption, grading, and multiplication regions (SAGM)- structure APD ( 11. To operate properly, the carrier concentra- tions and thicknesses of the different layers which comprise this structure must be controlled within very narrow tolerances (6). Although most of the SAGM-APD's that have been reported to date have been grown by liquid-phase epitaxy (LPE), the degree of precision and uniformity necessary to maintain the wafer parameters within the constraints required by this structure are difficult to achieve by this growth technique. Also, LPE is more difficult and more expensive to implement in large-scale production than vapor-phase tech- niques such as hydride vapor-phase epitaxy (VPE), metal- organic chemical vapor deposition (MOCVD), or molecular beam epitaxy (MBE). In the past, however, InP/In,- Gal-,As,P1-, photodetectors grown by MBE and MOCVD have not achieved performance characteristics as good as those grown by LPE due to the high background carrier concentra- tions of the In,Gal-,As,PI-, layers and the difficulty in achieving good InP/In,Gal -,As,P1 ->, interfaces. Recently, InPIInGaAsPiInGaAs SAGM-APD's grown by MOCVD (7) and VPE (8), (9) have been reported. In this paper we report on InP/InGaAsP/InGaAs SAGM-APD's grown by chemical beam epitaxy (CBE) (lo), a crystal growth technique that combines many of the advantages of MOCVD and MBE. A schematic cross section of the back-illuminated InP/

An optical heterodyne communication system is described which employs a separate stable laser local oscillator at the receiver. The theoretical advantage of quantum-limited reception has been realized, demonstrating an improvement in receiver sensitivity of more than 40 dB over that of a conventional photodetector receiver. The fundamental sources of noise in the system are identified as laser oscillator frequency noise, atmospheric phase noise, atmospheric amplitude noise, and quantum noise. The quantitative characteristics of these noise sources are analyzed as they influence the operation of AM and FM laser communications.

We report the use of a 2-m-long Bismuth Oxide fiber with an ultra-high nonlinearity of ~1100 W(-1)km(-1) in a simple 2R regeneration experiment based on self phase modulation and offset filtering. Numerical simulations and experimental results confirm the suitability of this kind of fiber for 2R regeneration. An improvement in receiver sensitivity of more than 5 dB at 10 Gb/s and 2 dB at 40 Gb/s is achieved.

We propose an enhanced reservoir-computing (RC) equalizer that incorporates an expanded output-feedback interval (multi-step feedback) and evaluate its efficacy for intensity-modulation/direct-detection (IMDD) optical transmission systems via numerical simulation. The multi-step feedback mechanism increases the effective feedback depth and memory capacity of the reservoir, enabling richer temporal processing while permitting a substantial reduction in neuron count. In a 40-km IMDD link the proposed RC achieves a 2 dB improvement in receiver sensitivity to single-step feedback mechanism. For longer transmission ≥ 60 km, the proposed architecture outperforms the conventional single-step-feedback RC with markedly fewer reservoir neurons, thereby reducing implementation and computational complexity. These properties indicate that the proposed multi-step-feedback RC is a promising candidate for equalization in short-reach optical transmission systems.

A new version of Absolute Polar Duty Cycle Division Multiplexing transmission scheme over Wavelength Division Multiplexing system is proposed. We modeled and analyzed a method to improve the performance of AP-DCDM over WDM system by using Dual-Drive Mach–Zehnder-Modulator (DD-MZM). Almost 4.1 dB improvement in receiver sensitivity of 1.28 Tbit/s (32 × 40 Gbit/s) AP-DCDM-WDM over 320 km fiber is achieved by optimizing the bias voltage in DD-MZM.

In this study, we propose a novel wavelength shift keying (WSK) technique that is combined with the conventional intensity modulation scheme for the transmission of point-to-point bidirectional data at the rate of 10 Gbps in each direction. We observe that WSK technique has not been investigated for implementation in point-to-point free space optical (FSO) links. Therefore, to the best of our knowledge, our study is the first one to perform this investigation. Our proposed link uses WSK in the downlink direction while we re-use the optical carriers for the transmission of uplink data. The use of WSK in the downlink direction enables us to perform balanced detection at the receiver, resulting in 3 dB improvement in receiver sensitivity compared to simple direct detection. We present bit error rate (BER) results for the signals transmitted in both the directions under different turbulence conditions and FSO link lengths. It was observed that the downlink signals generally perform better compared to the uplink due to the use of balanced detection and higher intensity fluctuations induced over the re-used optical carrier transmitted in the uplink direction.Please check and confirm that all the author names (given name, family name) and initials are correctly identified. The author names and initials are CORRECT.