Two cascaded SOAs used as intensity modulators for adaptively modulated optical OFDM signals in optical access networks

Detailed theoretical and numerical investigations of the transmission performance of adaptively modulated optical orthogonal frequency division multiplexed (AMOOFDM) signals are undertaken, for the first time, in optical amplification and chromatic dispersion (CD) compensation free single mode fiber (SMF) intensity-modulated and directdetection (IMDD) systems using two cascaded semiconductor optical amplifiers in a counterpropagating configuration as an intensity modulator (TC-SOA-CC-IM). A theoretical model describing the characteristics of this configuration is developed. Extensive performance comparisons are also made between the TC-SOA-CC and the single SOA intensity modulators. It is shown that, the TC-SOA-CC reaches its strongly saturated region using a lower input optical power much faster than the single SOA resulting in significantly reduced effective carrier lifetime and thus wide TC-SOA-CC bandwidths. It is shown that at low input optical power, we can increase the signal line rate almost 115% which will be more than twice the transmission performance offered by single SOA. In addition, the TCSOA-CC-IM is capable of supporting signal line rates higher than corresponding to the SOA-IM by using 10dB lower input optical powers. For long transmission distance, the TC-SOA-CC-IM has much stronger CD compensation capability compared to the SOA-IM. In addition the use of TC-SOA-CC-IM is more effective regarding the capability to benefit from the CD compensation for shorter distances starting at 60km SMF, whilst for the SOA-IM starting at 90km. © 2014 Optical Society of America.

Fiber nonlinearity-induced penalty reduction in CO-OFDM by ANN-based nonlinear equalization

We experimentally demonstrate ∼2 dB quality (Q)-factor enhancement in terms of fiber nonlinearity compensation of 40 Gb/s 16 quadrature amplitude modulation coherent optical orthogonal frequency-division multiplexing at 2000 km, using a nonlinear equalizer (NLE) based on artificial neural networks (ANN). Nonlinearity alleviation depends on escalation of the ANN training overhead and the signal bit rate, reporting ∼4 dB Q-factor enhancement at 70 Gb/s, whereas a reduction of the number of ANN neurons annihilates the NLE performance. An enhanced performance by up to ∼2 dB in Q-factor compared to the inverse Volterra-series transfer function NLE leads to a breakthrough in the efficiency of ANN.

Multiplier-free blind phase noise estimation for CO-OFDM transmission

We experimentally demonstrate an effective multiplier-free blind phase noise estimation technique for CO-OFDM systems for the first time based on the statistical properties of the received symbols' phases. Our technique operates in polar coordinates, providing very low implementation complexity.

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.

Experimental demonstration of an all-optical interferometric drop, add, and extract multiplexer for OFDM super-channel

The experimental implementation of an all-optical node able of routing a channel contained in an all-optical OFDM super-channel is presented. The extract function is performed through channel selection, reshaping and interferometric suppression.

Comparison of DSP-based nonlinear equalizers for intra-channel nonlinearity compensation in coherent optical OFDM

A novel versatile digital signal processing (DSP)-based equalizer using support vector machine regression (SVR) is proposed for 16-quadrature amplitude modulated (16-QAM) coherent optical orthogonal frequency-division multiplexing (CO-OFDM) and experimentally compared to traditional DSP-based deterministic fiber-induced nonlinearity equalizers (NLEs), namely the full-field digital back-propagation (DBP) and the inverse Volterra series transfer function-based NLE (V-NLE). For a 40 Gb/s 16-QAM CO-OFDM at 2000 km, SVR-NLE extends the optimum launched optical power (LOP) by 4 dB compared to V-NLE by means of reduction of fiber nonlinearity. In comparison to full-field DBP at a LOP of 6 dBm, SVR-NLE outperforms by ∼1 dB in Q-factor. In addition, SVR-NLE is the most computational efficient DSP-NLE.

Improving energy efficiency in a wireless sensor network by combining cooperative MIMO with data aggregation

In wireless sensor networks where nodes are powered by batteries, it is critical to prolong the network lifetime by minimizing the energy consumption of each node. In this paper, the cooperative multiple-input-multiple-output (MIMO) and data-aggregation techniques are jointly adopted to reduce the energy consumption per bit in wireless sensor networks by reducing the amount of data for transmission and better using network resources through cooperative communication. For this purpose, we derive a new energy model that considers the correlation between data generated by nodes and the distance between them for a cluster-based sensor network by employing the combined techniques. Using this model, the effect of the cluster size on the average energy consumption per node can be analyzed. It is shown that the energy efficiency of the network can significantly be enhanced in cooperative MIMO systems with data aggregation, compared with either cooperative MIMO systems without data aggregation or data-aggregation systems without cooperative MIMO, if sensor nodes are properly clusterized. Both centralized and distributed data-aggregation schemes for the cooperating nodes to exchange and compress their data are also proposed and appraised, which lead to diverse impacts of data correlation on the energy performance of the integrated cooperative MIMO and data-aggregation systems.

Performance optimisation of the MAC protocol with multiple contention slots in MIMO ad hoc networks

The multiple-input multiple-output (MIMO) technique can be used to improve the performance of ad hoc networks. Various medium access control (MAC) protocols with multiple contention slots have been proposed to exploit spatial multiplexing for increasing the transport throughput of MIMO ad hoc networks. However, the existence of multiple request-to-send/clear-to-send (RTS/CTS) contention slots represents a severe overhead that limits the improvement on transport throughput achieved by spatial multiplexing. In addition, when the number of contention slots is fixed, the efficiency of RTS/CTS contention is affected by the transmitting power of network nodes. In this study, a joint optimisation scheme on both transmitting power and contention slots number for maximising the transport throughput is presented. This includes the establishment of an analytical model of a simplified MAC protocol with multiple contention slots, the derivation of transport throughput as a function of both transmitting power and the number of contention slots, and the optimisation process based on the transport throughput formula derived. The analytical results obtained, verified by simulation, show that much higher transport throughput can be achieved using the joint optimisation scheme proposed, compared with the non-optimised cases and the results previously reported.

Advanced digital signal processing for coherent optical OFDM transmissions

Coherent optical orthogonal frequency division multiplexing (CO-OFDM) has been actively considered as a potential candidate for long-haul transmission and 400 Gb/s to 1 Tb/s Ethernet transport because of its high spectral efficiency, efficient implementation, flexibility and robustness against linear impairments such as chromatic dispersion and polarization mode dispersion. However, due to the long symbol duration and narrow subcarrier spacing, CO-OFDM systems are sensitive to laser phase noise and fibre nonlinearity induced penalties. As a result, the development of CO-OFDM transmission technology crucially relies on efficient techniques to compensate for the laser phase noise and fibre nonlinearity impairments. In this thesis, high performance and low complexity digital signal processing techniques for laser phase noise and fibre nonlinearity compensation in CO-OFDM transmissions are demonstrated. For laser phase noise compensation, three novel techniques, namely quasipilot-aided, decision-directed-free blind and multiplier-free blind are introduced. For fibre nonlinear compensation, two novel techniques which are referred to as phase conjugated pilots and phase conjugated subcarrier coding, are proposed. All these abovementioned digital signal processing techniques offer high performances and flexibilities while requiring relatively low complexities in comparison with other existing phase noise and nonlinear compensation techniques. As a result of the developments of these digital signal processing techniques, CO-OFDM technology is expected to play a significant role in future ultra-high capacity optical network. In addition, this thesis also presents preliminary study on nonlinear Fourier transform based transmission schemes in which OFDM is a highly suitable modulation format. The obtained result paves the way towards a truly flexible nonlinear wave-division multiplexing system that allows the current nonlinear transmission limitations to be exceeded.

Orthogonal Frequency Division Multiplexing Based Air Interfaces and Multiple Input Multiple Output Techniques in Cooperative Satellite Communications for 4th Generation Mobile Systems

Recently, wireless network technology has grown at such a pace that scientific research has become a practical reality in a very short time span. One mobile system that features high data rates and open network architecture is 4G. Currently, the research community and industry, in the field of wireless networks, are working on possible choices for solutions in the 4G system. The researcher considers one of the most important characteristics of future 4G mobile systems the ability to guarantee reliable communications at high data rates, in addition to high efficiency in the spectrum usage. On mobile wireless communication networks, one important factor is the coverage of large geographical areas. In 4G systems, a hybrid satellite/terrestrial network is crucial to providing users with coverage wherever needed. Subscribers thus require a reliable satellite link to access their services when they are in remote locations where a terrestrial infrastructure is unavailable. The results show that good modulation and access technique are also required in order to transmit high data rates over satellite links to mobile users. The dissertation proposes the use of OFDM (Orthogonal Frequency Multiple Access) for the satellite link by increasing the time diversity. This technique will allow for an increase of the data rate, as primarily required by multimedia applications, and will also optimally use the available bandwidth. In addition, this dissertation approaches the use of Cooperative Satellite Communications for hybrid satellite/terrestrial networks. By using this technique, the satellite coverage can be extended to areas where there is no direct link to the satellite. The issue of Cooperative Satellite Communications is solved through a new algorithm that forwards the received data from the fixed node to the mobile node. This algorithm is very efficient because it does not allow unnecessary transmissions and is based on signal to noise ratio (SNR) measures.

Controle adaptativo com desacoplamento aplicado a um sistema de tanques acoplados MIMO

O controle de sistemas MIMO (Multiple Input Multiple Output) é muitas vezes realizado por várias malhas de controladores clássicos que operam com restrições e apresentam baixo desempenho. Técnicas de controle adaptativo são uma alternativa interessante para aumentar o rendimento desses sistemas, como por exemplo os controladores MRAC (Model Reference Adaptive Control), que quando bem projetados, permitem que a dinâmica da planta seja escolhida de maneira a seguir um modelo de referência. O presente trabalho apresenta uma estratégia de desacoplamento para um sistema MIMO de três tanques acoplados e o projeto de um controlador MRAC para o mesmo.

Controle adaptativo com desacoplamento aplicado a um sistema de tanques acoplados MIMO

O controle de sistemas MIMO (Multiple Input Multiple Output) é muitas vezes realizado por várias malhas de controladores clássicos que operam com restrições e apresentam baixo desempenho. Técnicas de controle adaptativo são uma alternativa interessante para aumentar o rendimento desses sistemas, como por exemplo os controladores MRAC (Model Reference Adaptive Control), que quando bem projetados, permitem que a dinâmica da planta seja escolhida de maneira a seguir um modelo de referência. O presente trabalho apresenta uma estratégia de desacoplamento para um sistema MIMO de três tanques acoplados e o projeto de um controlador MRAC para o mesmo.

Sincronização em OFDM

Esta dissertação apresenta o trabalho sobre sincronização de receção para sistemas OFDM. Tendo como objetivo a integração da arquitetura desenvolvida no projeto de investigação \CROWN - Co-operative Radio over Fibre for Wireless Networks" atualmente em curso no Instituto de Telecomunicações. Esta arquitetura de receção foi implementada numa plataforma de desenvolvimento baseada em dispositivos programáveis FPGA, recorrendo as ferramentas de desenvolvimento MatLab, System Generator e ISE. O sistema implementado tem a particularidade de ter um princípio de funcionamento assíncrono e recorre aos algoritmos de Van de Beek [1] e Carlos Ribeiro [2] para proceder a estimação e consequente sincronização. Ambos os algoritmos foram utilizados para estimação do CFO, tendo o algoritmo de Van de Beek sido também utilizado para estimação do início de trama. Foram realizadas análises do desempenho do sistema para diferentes condições, sendo o objectivo de analisar o desempenho dos estimadores implementados. A performance foi então analisada de acordo com BER resultante e do erro de estimação do início de trama e do valor do CFO. Para além da análise individual dos resultados, e também feita uma comparação da precisão de ambos os estimadores.

Artificial neural network nonlinear equalizer for coherent optical OFDM

We propose a novel low-complexity artificial neural network (ANN)-based nonlinear equalizer (NLE) for coherent optical orthogonal frequency-division multiplexing (CO-OFDM) and compare it with the recent inverse Volterra-series transfer function (IVSTF)-based NLE over up to 1000 km of uncompensated links. Demonstration of ANN-NLE at 80-Gb/s CO-OFDM using 16-quadrature amplitude modulation reveals a Q-factor improvement after 1000-km transmission of 3 and 1 dB with respect to the linear equalization and IVSTF-NLE, respectively.