Fiber Bragg Grating Selection of Frequency Interleaved OFDM Signals in Fiber Supported Microwave Networks

The impact of fiber Bragg grating induced dispersion on the performance of frequency interleaved WDM radio-over-fiber (RoF) links with OFDM signaling is assessed. A comparison is given for both uniform and quarter-cosine apodized gratings. The frequencyinterleaved optical signals are transported over 10-km-long standard single-mode fiber (SMF) in optical double-sideband (ODSB) format and filtered in optical single-sideband (OSSB) format by the FBGs. The use of OSSB signals in the reception aim to avoid the penalties generated by fiber chromatic dispersion. The simulations were performed in VPI and the system performance was evaluated in terms of Error Vector Magnitude (EVM). It is shown that quartercosine apodization of the FBGs improves system performance by 1.0 dB. 1.Introduction As the necessity for fast data rates wireless systems and multimedia services in the local area and the access network increases, the large bandwidths in millimeter-wave (25100 GHz) frequency region have been considered for broadband wireless applications [Y. Chung 2007] as way to resolve the spectral congestion and the scarcity of the transmission bandwidth at lower microwave frequencies. This wide and almost unused wireless bandwidth means an increase in the volume of high-speed data, voice, or image transmission. However, due to the high losses in the atmosphere [Y. Takimoto 1998] the cell size is limited to picocells and a large number of base-stations are required to cover a wide service area. The radio-over-fibre (RoF) technology employs the transmission of RF signals by optical fiber between a central station (CS) and a number of base stations (BSs). In the base stations, the RF signal is transmitted to users by a wireless link. RoF technology can centralise the RF signal processing functions in one shared location, distributing the RF signals to the BSs through optical fibre, which offers low signal loss, simplifying the architecture and reducing the complexity of the BSs. This centralisation of signal processing functions enables equipment sharing, dynamic allocation of resources, and simplified system operation and maintenance. These benefits can translate into major system installation and operational savings [D. Wake 2002], specifically in wide-coverage broadband wireless communication systems. In this context, the combined use of RoF technology and millimeter-waves can provide fast data rates and mobility simultaneously. In this system a central station can transmit the millimeter signal to a remote picocell base station. The application of WDM in RoF networks has many advantages. One of them is the simplification of the network topology by allocating one wavelength for each BSs. Thus, WDM in combination with optical mm-wave transport has been widely studied [R. A. Griffin 1999]-[H. Toda 2003]. To increase the spectral efficiency of the system, the concept of optical frequency interleaving has also been proposed [C. G. Schäffer 2000][H. Toda 2003]. Orthogonal Frequency Division Multiplexing (OFDM) is recognized to be robust against multipath fading and is being used in Digital Video Broadcast (DVB), Digital Audio Broadcast (DAB), Local Area Wireless Networks (e.g. IEEE802.11, MMAC and HIPERLAN/2) and Ultra Wide-Band (UWB) [J. M. B. Oliveira 2007]. However, OFDM is more susceptible to nonlinearities in the optical transmitter and the phase-noise of optical oscillators [T. Berceli 2002]. In this work, we study the performance of OFDM signals, with QPSK modulation and channel equalization in frequency-interleaved WDM-RoF systems with potential application in the access network. The optical channels are transmitted in ODSB format and the wavelength selection is made by a uniform and a quarter-cosine apodized Fiber Bragg grating in OSSB format. 2.Fiber Bragg Gratings FBGs have been proposed to accomplish a large number of tasks in general lightwave communications systems. These tasks include wavelength selection in WDM fiber optic networks, fiber chromatic dispersion compensation and gain equalization in optical amplifiers, among others. Advantages of fiber gratings include low insertion loss, high return loss and potentially low cost. But the most distinguishing feature of fiber gratings is the flexibility they offer for achieving desired spectral characteristics [T. Erdogan 1997]. The FBGs wavelength selection in WDM millimetre-wave fibre-radio systems has been considered by several authors including Kitayama et al. (2002), Castleford et al. (2001), Teixeira et al. (2003), Marra et al. (2004) and Kaszubowska et al. (2004). In this context, various problems of practical interest arise including the effect of grating induced dispersion in the transmission and reflection signals and grating induced crosstalk. The effect of grating dispersion in WDM millimetre-wave fibre-radio is considered in [K. Kitayama 2002]. It is shown that dispersion causes an RF power penalty on signals in the band edge of the FBG. It is also shown that as a consequence dispersion considerably affects the span and channel allocation in WDM fibre-radio systems. To avoid crosstalk between adjacent channels, apodization technique is necessary to reduce the level of the sidelobes in the FBG reflection window response. The quarter-cosine apodization in VPI software can be mathematically given as ( ) ( ) ( ) ( ) ( )       ≤ ≤ −                 − + = − < = 2 2 1 2 1 2 cos 2 1 1