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

Transcription

1 Fiber Bragg Grating Selection of Frequency Interleaved OFDM Signals in Fiber Supported Microwave etworks Diogo Coelho, Henrique M. Salgado 2, Member, IEEE IESC Porto, Rua Dr. Roberto Frias 378, Porto, Portugal 2 IESC Porto, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias 378, Porto, Portugal dcoelho@inescporto.pt, henrique.salgado@inescporto.pt Abstract. 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 0-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.0 db..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 (25-00 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 998] 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

2 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 999]-[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 etworks (e.g. IEEE802., MMAC and HIPERLA/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 997]. 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. (200), 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

3 α α ( z) = z< L( 2 η) π z+ L( η 2) ( z) = cos L( 2 η) z L 2 ηl. () 2 The α parameter is the apodization profile, which describes the refractive index modulation envelope along the grating length z. η is a generalized parameter that specifies the details of the apodization profile, namely, the sidelobe supression. For a uniform FBG α(z) = for L 2 z L 2. 3.Error Vector Magnitude The error vector magnitude (EVM) is a measure used to quantify the performance of a digital radio system. EVM quantifies the error between the transmitted and received symbols and is defined as the root-mean-square value of the difference between a collection of measured symbols, under noise and other distortion parameters, with ideal symbols [R. A. Shofik 2006]. This error is often shown as a percent of the average power per symbols of the constellation. EVM can be mathematically given as [T. Berceli 2002] EVM = n= S n n= S S 0, n 0, n 2 2, (2) where S n is the measured symbol, S 0,n is the ideal symbol and is the number of unique symbols in the constellation. 4. Simulations and Results The schematic of the RoF link simulated is shown in Figure. Three OFDM signals in 56, 57 and 58 GHz are generated in each optical transmitter and multiplexed in the frequency domain. In the first analysis, the OFDM signals uses QPSK modulation and 64 orthogonal subcarriers with channel equalization. The bit rate used was 400 Mbits/s per subscriber, generating a total rate of 2.4 Gbits/s. The subcarrier frequencies were chosen in the GHz window where there is a high atmospheric absorption [M.M. Sayed 2005] enabling the use of picocells for efficient use of the network capacity and management. Figure. Schematic of RoF link simulated

4 The ODSB signals are generated by external modulation of two tunable lasers ( nm or THz and nm or THz) using Mach-Zehnder modulators (MZM with extinction ratio of 2 db). The optical signal with interleaving travels 0 km of single-mode fiber (loss 0.2 db/km and dispersion 6 ps/nm/km) where a fiber Bragg grating (uniform or quarter-cosine apodized), with approximately 60 GHz bandwidth, drops the desire signal in OSSB format. The use of OSSB signals in the reception enables us to avoid the photodetected power penalty due to chromatic dispersion of the transmission fiber. The PI photodetector with.0 A/W responsivity was used in the optical reception. A variable optical attenuator was used in the transmitter to evaluate the performance of the system in terms of EVM for each FBG described before. The first simulation was made with a 0.7 cm long uniform FBG and.45 effective refractive index. Figure 2 plots the optical power transmitted (dbm) versus EVM (%) for the uniform FBG. Figure 2. Optical power (dbm) versus EVM (%) for the uniform FBG In the uniform FBG system simulated, the EVM values are kept below 0% for an injected power at the transmitter of -0 dbm. After that, the simulation with quarter-cosine apodized FBG was realized. A 0.45 cm long quarter-cosine apodized FBG with η = 0. was used. Figure 3 shows the optical power transmitted (dbm) versus EVM (%) for the quarter-cosine FBG. Figure 3. Optical power (dbm) versus EVM (%) for the quarter-cosine FBG with η = 0..

5 The EVM values are kept below 0% at the transmission power of - dbm. As in the previous case channel OFDM 4 is the most affect by the grating induced dispersion. The decrease in the level of the sidelobes in the FBG reflection window for the apodization profile η = 0. results in an improved performance of.0 db. The increase in performance maybe due to a reduction of the grating induced dispersion or the crosstalk effect. This will be the subject of further work, which may lead to an optimized design of the FBG. 4.Conclusions We have successfully demonstrated through simulation the application of the FBG in the selection of frequency-interleaved WDM-RoF links. The subcarrier frequencies allocation was made in the 60 GHz band where there is a high atmospheric absorption. The optical signals were transmitted in ODSB format and dropped by the FBGs in OSSB format to avoid the photodetected power penalty due to chromatic dispersion of transmission fiber. Six OFDM-QPSK signals with channel equalization were transmitted with 400 Mbits/s per subscriber, transported over 0 km of standard singlemode fiber and selected by a uniform and a quarter-cosine apodized FBGs. In the uniform FBG system, the EVM parameter is kept below 0% at the transmitted power of -0.0 dbm. For the apodized systems, this value changes to -.0 dbm. The reduction of the sidelobes gives a better performance. References A. Kaszubowska et al., Multifunctional operation of a fiber Bragg grating in a WDM/SCM radio over fiber distribution system, IEEE Photon. Technol. Lett., vol. 6, pp , Feb A. irmalathas, D. ovak, C. Lim and R. B. Waterhouse, Wavelength Reuse in the WDM Optical Interface of a Millimeter-Wave Fiber-Wireless Antenna Base Station, IEEE Trans. Microwave Theory Tech., vol. 49, no. 0, pp , Oct A. Teixeira et al., Filter optimization for wavelength interleaved radio-over-fiber systems, Proc. SBMO/IEEE Microwave and Optoelectronics Conference, vol. 2, pp , Sep C. G. Schäffer, M. Sauer, K. Kojucharow, and H. Kaluzni, Increasing the channel number in WDM mm-wave systems by spectral overlap, in Int. Topical Meeting Microwave Photonics (MWP2000), Oxford, Sep. 2000, WE2.4, pp C. Lim, A. irmalathas, M. Attygalle, D. ovak, and R. Waterhouse, On the Merging of Millimeter-Wave Fiber-Radio Backbone With 25 GHz WDM Ring etworks, J. Lightwave Technol., vol. 2, no. 0, pp , Oct C. Marra et al., FBG-based optical interface to support a multisector antenna in a spectrally efficient fiber radio system, IEEE Photon. Technol. Lett., vol. 6, pp , Jan D. Castleford et al., Optical crosstalk in fiber-radio WDM networks, IEEE Trans. Microw. Theory Tech., vol. 49, pp , Oct. 200.

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