SPECTRALLY EFFICIENT MULTICARRIER SYSTEMS FOR FIBER-OPTIC TRANSMISSION

The purpose of this research is to provide a comprehensive study of spectrally efficient multicarrier systems for fiber-optic transmission. Multicarrier optical systems partition a high-data rate digital signal in a wavelength channel into multiple subcarriers. The data rate on each subcarrier can be sufficiently low and thus, the tolerance to transmission impairments can be significantly improved. Although different modulation and detection techniques are used, all the multicarrier systems investigated in this dissertation achieve a high spectral efficiency of 1 Baud/s/Hz. Orthogonal frequency-division multiplexing (OFDM) and Nyquist wavelength-division multiplexing (Nyquist-WDM) are the two basic approaches to achieve the 1 Baud/s/Hz spectral efficiency. OFDM allows spectral overlap of adjacent subcarriers and crosstalk elimination by integration at the receiver; Nyquist-WDM limits the spectral spreading of each subcarrier channel to the symbol rate per subcarrier to avoid spectral overlap. In terms of detection method, both direct detection and coherent detection can be applied in multicarrier systems. This dissertation focuses on the use of three high spectral efficiency optical multicarrier systems. In the theoretical and experimental investigation of a 11.1Gb/s FFT-based OFDM system, a simple dual-drive Mach-Zehnder modulator (MZM) was employed in the transmitter and direct detection in the receiver, which provided an OFDM system implementation with reduced complexity. The data was transmitted through 675km uncompensated standard single-mode fiber (SMF). Next, a 22.2Gb/s digital subcarrier multiplexing based (DSCM-based) OFDM system was used in conjunction with 10 subcarrier channels using QPSK modulation. In this system, an IQ modulator was utilized in the transmitter and coherent detection in the receiver. By using coherent detection, the receiver was able to dynamically select the desired subcarrier channels for detection without changing the system configuration. The present research also explored a 22.2Gb/s 10 subcarrier Nyquist-WDM system with coherent detection, compared its system performance with OFDM systems, and subsequently examined the impact of filter roll-off factor. Finally, a systemic comparison of the three proposed multicarrier systems was performed in terms of their transmission performance and system flexibility. The design tradeoffs were analyzed for different applications and summarized principles for the modern multicarrier fiber-optic system design

Discrete Multitone Modulation for Maximizing Transmission Rate in Step-Index Plastic Optical Fibres

The use of standard 1-mm core-diameter step-index plastic optical fiber (SI-POF) has so far been mainly limited to distances of up to 100 m and bit-rates in the order of 100 Mbit/s. By use of digital signal processing, transmission performance of such optical links can be improved. Among the different technical solutions proposed, a promising one is based on the use of discrete multitone (DMT) modulation, directly applied to intensity-modulated, direct detection (IM/DD) SI-POF links. This paper presents an overview of DMT over SI-POF and demonstrates how DMT can be used to improve transmission rate in such IM/DD systems. The achievable capacity of an SI-POF channel is first analyzed theoretically and then validated by experimental results. Additionally, first experimental demonstrations of a real-time DMT over SI-POF system are presented and discusse

Design and simulation of 1.28 Tbps dense wavelength division multiplex system suitable for long haul backbone

Wavelength division multiplex (WDM) system with on / off keying (OOK) modulation and direct detection (DD) is generally simple to implement, less expensive and energy efficient. The determination of the possible design capacity limit, in terms of the bit rate-distance product in WDM-OOK-DD systems is therefore crucial, considering transmitter / receiver simplicity, as well as energy and cost efficiency. A 32-channel wavelength division multiplex system is designed and simulated over 1000 km fiber length using Optsim commercial simulation software. The standard channel spacing of 0.4 nm was used in the C-band range from 1.5436-1.556 nm. Each channel used the simple non return to zero - on / off keying (NRZ-OOK) modulation format to modulate a continuous wave (CW) laser source at 40 Gbps using an external modulator, while the receiver uses a DD scheme. It is proposed that the design will be suitable for long haul mobile backbone in a national network, since up to 1.28 Tbps data rates can be transmitted over 1000 km. A bit rate-length product of 1.28 Pbps.km was obtained as the optimum capacity limit in 32 channel dispersion managed WDM-OOK-DD system.Comment: Accepted for publication in Journal of Optical Communications - De Gruyte

Spectrally Efficient FDM System with Probabilistic Shaping

This work proposes and explores the use of probabilistic shaping for the non-orthogonal multicarrier spectrally efficient frequency division multiplexing (SEFDM) system. The system design considers the reverse concatenation architecture which cascades the constant composition distribution matching (CCDM) algorithm together with soft-decision forward error correction (SD-FEC)-LDPC code for the probabilistic shaping scheme. The non-orthogonal signalling is implemented by discrete Fourier transform (DFT)-based SEFDM modulation with matched filtering demodulation and advanced interference cancellation detection. The high achievable spectral efficiency, low computation complexity and reliability make SEFDM a good candidate for multicarrier signalling for beyond 5G communications. By adding extra shaping gain and flexibility of rate adaptation, the combination of two capacity-achieving techniques provides significant insight of further performance improvement. In this paper, we investigate the performance of the proposed probabilistically shaped-SEFDM (PS-SEFDM) system with regular QAM constellations. The presented results of the proposed system show less required power and bandwidth saving compared to OFDM when achieving the same error performance and same spectral efficiency

Nonlinear Mixing in Optical Multicarrier Systems

Although optical fiber has a vast spectral bandwidth, efficient use of this bandwidth is still important in order to meet the ever increased capacity demand of optical networks. In addition to wavelength division multiplexing, it is possible to partition multiple low-rate subcarriers into each high speed wavelength channel. Multicarrier systems not only ensure efficient use of optical and electrical components, but also tolerate transmission impairments. The purpose of this research is to understand the impact of mixing among subcarriers in Radio-Over-Fiber (RoF) and high speed optical transmission systems, and experimentally demonstrate techniques to minimize this impact. We also analyze impact of clipping and quantization on multicarrier signals and compare bandwidth efficiency of two popular multiplexing techniques, namely, orthogonal frequency division multiplexing (OFDM) and Nyquist modulation. For an OFDM-RoF system, we present a novel technique that minimizes the RF domain signal-signal beat interference (SSBI), relaxes the phase noise limit on the RF carrier, realizes the full potential of optical heterodyne-based RF carrier generation, and increases the performance-to-cost ratio of RoF systems. We demonstrate a RoF network that shares the same RF carrier for both downlink and uplink, avoiding the need of an additional RF oscillator in the customer unit. For multi-carrier optical transmission, we first experimentally compare performance degradations of coherent optical OFDM and single-carrier Nyquist pulse modulated systems in a nonlinear environment. We then experimentally evaluate SSBI compensation techniques in the presence of semiconductor optical amplifier (SOA) induced nonlinearities for a multicarrier optical system with direct detection. We show that SSBI contamination can be significantly reduced from the data signal when the carrier-to-signal power ratio is sufficiently low

Optical multicarrier sources for spectrally efficient optical networks

During the last 30 years the capacity of commercial optical systems exceeded the network traffic requirements, mainly due to the extraordinary scalability of wavelength division multiplexing technology that has been successfully used to expand capacity in optical systems and meet increasing bandwidth requirements since the early 1990’s. Nevertheless, the rapid growth of network traffic inverted this situation and current trends show faster growing network traffic than system capacity. To enable further and faster growth of optical communication network capacity, several breakthroughs occurred during the last decade. First, optical coherent communications, which were the subject of intensive research in the 1980’s, were revived. This triggered the employment of advanced modulation formats. Afterwards, with the introduction of orthogonal frequency division multiplexing (OFDM) and Nyquist WDM modulation techniques in optical communication systems, very efficient utilisation of the available spectral bandwidth was enabled. In such systems the spectral guard bands between neighbouring channels are minimised, at the expense of stricter requirements on the performance of optical sources, especially the frequency (or wavelength) stability. Attractive solutions to address the frequency stability issues are optical multicarrier sources which simultaneously generate multiple phase correlated optical carriers that ensure that the frequency difference between the carriers is fixed. In this thesis, a number of optical multicarrier sources are presented and analysed, with special focus being on semiconductor mode-locked lasers and gain-switched comb sources. High capacity and spectrally efficient optical systems for short and medium reach applications (from 3 km up to 300 km), based on optical frequency combs as optical sources, advanced modulation formats (m-QAM) and modulation techniques (OFDM and Nyquist WDM) have been proposed and presented. Also, certain optoelectronic devices (i.e. semiconductor optical amplifier) and techniques (feed-forward heterodyne linewidth reduction scheme) have been utilised to enable the desired system performance

Bandwidth Compressed Waveform for 60 GHz Millimeter-Wave Radio over Fiber Experiment

A bandwidth compressed waveform termed spectrally efficient frequency division multiplexing (SEFDM) is experimentally demonstrated in a 60-GHz millimeter-wave (mm-wave) radio-over-fiber scenario to increase transmission data rates without changing signal bandwidth and modulation format. Experimental results show the advantages of SEFDM and confirm that the bit rate of SEFDM signals can be substantially higher than that of orthogonal frequency-division multiplexing (OFDM) signals. Experimentally, a 2.25 Gbit/s 4QAM OFDM signal is transmitted through 250 m of OM-1 multi-mode fiber and then it is optically up converted to 60 GHz band at the photodiode before delivery to a mm-wave antenna for transmission over a 3 meter wireless link. The work demonstrates that when the OFDM signal is replaced by an SEFDM signal using the same modulation format and occupying the same bandwidth, the bit rate can be increased, by a factor of up to 67%, to 3.75 Gbit/s at the expense of a 3-dB power penalty. Additionally, a bandwidth compressed 4QAM SEFDM is shown to outperform an 8QAM OFDM of the same spectral efficiency, thereby verifying that a lower order modulation format may replace a higher order one and achieve performance gain

Experimental Validations of Bandwidth Compressed Multicarrier Signals

We comprehensively summarize experimental validations 1 of bandwidth compressed multicarrier waveforms for future 5th generation (5G) applications. The proposed waveforms are derived from an existing non-orthogonal multicarrier concept termed spectrally efficient frequency division multiplexing (SEFDM) where sub-carriers are non-orthogonally packed at frequencies below the symbol rate. This improves the spectral efficiency at the cost of self-created inter carrier interference (ICI). In this work, experiments are reported and testing is carried out in three scenarios including long term evolution (LTE)-like wireless link; millimeter wave radio-over-fiber (RoF) link and optical fiber link. In the first scenario, for a given 25 MHz bandwidth, the SEFDM testbed can provide 70 Mbit/s gross data rate while only 50 Mbit/s can be achieved for an OFDM system occupying the same bandwidth. For the millimeter wave experiment, occupying a 1.125 GHz bandwidth, the gross bit rate for OFDM is 2.25 Gbit/s and with 40% bandwidth compression, 3.75 Gbit/s can be achieved for SEFDM. Two experimental optical fiber links are described in this work; a 10 Gbit/s direct detection optical SEFDM system and a 24 Gbit/s coherent detection SEFDM system. The LTE-like signals and millimeter wave technologies are well suited to provide last mile communications to end users as both can support mobility in wireless environments. The lightwave signals delivered by optical fibers would offer higher data rates and support long-haul communications. The reported techniques, used individually or combined, would be of interest to future wireless system designers, where bandwidth saving is of importance, such as in 5G networks, aiming to provide high capacity and high mobility, simultaneously while saving spectrum