Next generation optical access networks

Abstract

This thesis reports some investigations on different aspects that characterize the next generation optical access networks (NGAN) and that contribute in improving the performance of such networks. Due to the increasing demand for high speed transmission services from private and business users, the development of access network is evolving to the use of optical technologies. Passive optical networks (PON) seems to be the most promising solution for NGAN. This kind of networks has a simple topology, where the optical line terminator (OLT) sends the downstream information broadcast to the end users (optical network units ONU). Therefore, data confidentiality is one of the major issues and it is critically important that data transmitted cannot be theft at any point along the communication link by a potential eavesdropper. Optical code division multiple access (OCDMA) seems to have all the advantages to be a valid solution, since many users share the same optical resources by assigning a specific code to each user and the encoded signal shows a noise-like waveform. On the other side, an authorized user can decipher the transmission by the use of the correct code. Various OCDMA schemes had been proposed and demonstrated. When multiple users send simultaneously information with different codes, it is very difficult for an eavesdropper to intercept the correct data, without knowing the code. However, when a single user is active in the network, the security is not guaranteed, because an unauthorized user can easily detect the signal with a standard power detector. To cope with this vulnerability, code switching scheme have been proposed, where both marks and spaces are encoded with two different codes. In this case the information is hidden for an eavesdropper using a simple energy detector, but the system is still vulnerable against a differential detection. In this work, two different solutions have been proposed in order to increase the confidentiality level of an OCDMA network. The first exploits the properties of a multiport encoder/decoder (E/D) with an arrayed waveguide grating (AWG) configuration. This device has input and output ports and by a single input laser pulse a set of phase shifted keyed (PSK) codes is generated. The code cardinality equals the number of the device output ports. It is possible to enlarge the code cardinality and generate by sending laser pulses into n-dimensional codes different encoder input ports at the same time. Consid- ering different configurations, the multiport E/D is able to perform bit and block coding using 1-dimensional and n-dimensional codes. Moreover, to enlarge the cardinality in the case of bit-ciphering, spectral phase codes have been introduced. The security performance of a point-to-point (P2P) transmission have been analyzed in terms of robustness against brute-force code searching, known-plaintext and chosen plaintext attacks. Another possible solution to increase the confidentiality level of an OCDMA transmission and to avoid simple differential detection is to make the transmitted signal fully distorted, by the introduction of scrambling in optical signal processing. The concept of scrambling is borrowed from electronic encoding and wireless communications, where it has been introduced for data protection. It could be performed by adding or modifying components of the original signal, to make the extraction of the original signal difficult from an unauthorized user. Optically, a signal can be scrambled by adding one or more encoders in cascade with the primary OCDMA encoder. A scrambled signal is fully distorted and has a multilevel eye diagram. Therefore, a potential attacker has to try all the possible combinations between the code words to break the transmission. The security performance have been analyzed in terms of robustness against brute-force attack. A P2P transmission and two multi-user configurations have been proposed and numerically simulated by transmissionMaker software. The encoder/decoder devices have been simulated as VPISuper Structured Fibre Bragg Gratings (SSFBGs) that generates 15 chip codes, realizing a simple, feasible and compatible with current optical technologies system. A critical parameter in the next generation optical networks is the spectral efficiency, vi because all the new applications, such ad video conferences or real time games, require high information density. Advanced modulation schemes, such as differential phase shift keying (DPSK) and differential quadrature shift keying (DQPSK), have been applied to increase the spectral efficiency in optical transmissions. On the other hand, OCDMA technique others extraordinary network capabilities, allowing a large number of users to share the same bandwidth. In this case, the spectral efficiency is simultaneous users transmitting at a bit rate η = KB/∆f , B and ∆f where K is the number of is the optical bandwidth. Therefore, the influence of different advanced modulation formats on OCDMA transmission have been investigated in terms of spectral efficiency and bit error rate (BER). In particular, the following modulation formats have been compared: on-off keying (OOK), DUOBINARY, DPSK and DQPSK. Both incoherent and coherent encoding techniques have been analyzed, considering optical orthogonal codes (OOC) and Gold codes respectively. Multiple access interference (MAI) and beat noises have been also taken into account. In the future optical networks, besides high-speed Internet, voice over IP and broadcast video, the service demand will evolve to high bit rate and customization, namely a wide variety demand of quality of service (QoS). Optical Orthogonal Frequency Division Multiplexing (Optical OFDM) is a promising candidate in such networks, thanks to the fact that it meets the twofold requirement of mitigating transmission impairments and providing high bit rates. Moreover, due to the small bandwidth occupied by a single OFDM channel, it is characterized by high spectral efficiency and high tolerance to the fibre dispersions. Applied to PON networks, this technique allows to dinamically allocate different subbands for different services. Traditional OFDM system uses the Fast Fourier Transform (FFT) algorithm to process the signal. Exploiting the properties of the Hartley transform, a real-valued trigonometric transform, a different optical OFDM system has been proposed and numerically simulated, achieving good transmission distances suitable for optical access networks. Instead of the FFT algorithm, the signal processing has been performed by the Fast Hartley Transform (FHT) algorithm that gives a real valued signal. This simplifies the conversion of the OFDM signal into an optical signal and allows to halve the number of the necessary digital-to-analog (DAC) and analog-to-digital (ADC) converters, because vii only the in-phase component has to be processed. Moreover, the frequency separation and orthogonality between the subchannels are kept and, thanks to the kernel of the Hartley transform, each symbol of the data sequence is carried by two symmetrical subbands, increasing the frequency diversity. In order to extend optical transparency to the access networks, providing the same bandwidth of the fibre, optical wireless systems can be a suitable alternative when wired connections cannot be established. In free space optics transmissions light is used as a carrier, but, unlike optical fibres, the light beam is transmitted through the air. A transparent wireless optical system have been experimentally tested, in order to overcome the electricalto-optical-to-electrical (E-O-E) conversion impairments. The optical beam is transmitted directly into the free space channel by a trunked fibre and a system of lens capable to collimate the beam over long distances. At the receiver, the beam is focused into the core of a standard single mode fibre (SMF), by means of a focusing lens and a GRIN lens. Such a system allows an all-optical processing of the transmitted data, the use of Dense Wavelength Division Multiplexing (DWDM) technique, to increase the bit rate, and provides a fast and cost-effective connection for the access segment.