“Architectures and Novel Functionalities for Optical Access OFDM Networks”

A clear increase in the deployment of fibre-to-the-home (FTTH) optical access networks around the world has been experienced in the recent years. FTTH is a flexible, future-proof access technology that enables the provision of Gbit/s data rates per user. Several studies indicate that FTTH will become the key differentiator between competing operators. In addition, FTTH is the only technology capable of creating new revenue streams from high bit-rate applications, e.g. high-definition entertainment (HD-video, HD-games, etc). On the operator side, an important advantage with FTTH is that it permits enhanced operational efficiencies compared with other access technologies. Primarily by reducing maintenance and operating costs. Also, it reduces the operational requirements of the central office equipment, and consequently leads to lower energy consumption. This Ph.D. Thesis aims to extend these advantages beyond the simple FTTH concept by the inclusive integration of the in-building optical distribution path with the final short/medium range wireless link, so providing an integrated end-to-end FTTH network architecture. In this way, the benefits of reduced operational costs and higher efficiencies are fully extended to the end user of the network, the final customer.

In this Thesis, we propose an optical-radio integrated access network architecture employing orthogonal frequency division multiplexing (OFDM) signals for the provision of different services such as Internet, phone/voice, HDTV, wireless and home security. OFDM signals are incorporated in radio standards such as ultra-wide band (UWB), WiMAX, LTE, WLAN, DVB-T or DAB. These formats take advantage of the intrinsic characteristics of OFDM modulation such as relative immunity to multi-path fading. This Thesis includes the proposal and demonstration of multi-standard bi-directional radio-over-fibre transmission of triple-play services (3PLAY) using OFDM-based UWB (for high-definition television), WiMAX (for internet data) and LTE (telephone service) in coexistence over passive optical networks (PON). This technique permits an increase in the overall capacity of the system; providing for triple-play services. Moreover, radio-over-fibre transmission allows using standard equipment at both ends of the architecture like low-cost UWB receivers, mobile devices using LTE and equipment using in-built WiMAX receivers.

Different network architectures for OFDM-based radio-over-fibre transmission are proposed. This permits a cost effective and fully centralised network architecture enabling transmission impairment compensation and network management to be carried out only at the central office. No further compensation, regeneration or format conversion is required along the network. This technique permits the avoidance of many of the conversion stages, so providing cost, space and energy savings.

Also, streaming of audio and video has become one of the main applications of UWB technology. The key market opportunity of UWB radio is the cable replacement in personal area networks using wireless universal serial bus (WUSB) technology. Since UWB offers high capacity and can coexist with other wireless services, UWB is becoming a strong candidate for the connection of high-bitrate demanding devices (e.g. external hard disks) and high-definition video streaming for in-home networks. For these reasons, the viability of UWB-over- fibre transmission is further investigated in this Ph.D. considering the end-to-end architecture comprising the access network, the in-home optical network and the final wireless transmission at customer premises. 

Different photonic techniques to improve the performance of radio-over-fibre transmission are also proposed and investigated. The transmission of OFDM-based signals in polarization multiplexing is investigated for increasing the capacity of the network. This is demonstrated in the case of UWB and WiMAX signals transmission in coexistence. 

For the management of pico-cells, the UWB-over-fibre distribution in in-building optical networks employing different media (single-mode and multi-mode) is investigated. In addition, the optical technique based on photonic analog-to-digital converters is proposed and demonstrated for sensing the high-bandwidth radio spectrum (up to 10 GHz). The localization and management of UWB pico-cells using this technique is also investigated.

As the radio-over-fibre transmission of several wireless services in coexistence through the same optical network requires electro-optic modulators (EOM) with excellent dynamic range and an extensive linear regime, linearization techniques have been also investigated in this Ph.D. Multi-carrier signals with large number of sub-channels like multi-band OFDM-UWB signals have large peak-to-average power ratio (PAPR) which joint with the high bandwidth of each UWB channel (528 MHz) is the main limitation of intensity-modulation systems. Here, we evaluated the performance of new modulators developed by Photline Technologies based on a Y-coupled architecture and a dual-drive modulator in radio-over-fibre applications.  

After this, different architectures for the combined distribution of 3PLAY signals simultaneously in radio-over-fibre networks are proposed and evaluated: (i) a reflective architecture based on a reflective electro-absorption transceiver (R-EAT) to avoid using unacceptably powerful lasers at user premises, and (ii) a bi-directional coarse wavelength division multiplexing (CWDM) architecture using Mach-Zehnder electro-optical modulators with one wavelength for uplink data and another wavelength for downlink information. Within these networks the maximum optical reach is investigated. Also, the integration of the wireless path after the access network and different optical media for in-home optical distribution are investigated. 

Finally, impairment compensation techniques for multi-format OFDM-based radio-over-fibre transmission in optical networks are proposed and studied. An impairment compensation subsystem is proposed based on using extra RF-pilots in the free-spectrum of the multi-OFDM signal to extract the network channel information to be applied on a pre-distortion equalizer. 

The usage of these techniques in the different levels of the optical network is interesting for the proper evolution of the Information Society, which is the main objective of this Ph.D thesis.