The main impediments for long distance signal transmission in the fibre optic system, especially the radio over fibre (RoF) system, are the chromatic dispersion and signal power attenuation. Additionally, the power consumption in the laser diode and optical amplifiers affect the signal transmission costs; however, it is lower than in a wireless system. Therefore, decreasing the power consumption and chromatic dispersion and increasing the data bit rate in RoF are the demands for present and future fibre optic system technology. In order to increase the signal transmission distance and improve the frequency spectrum, we study in this paper a mobile Worldwide Interoperability for Microwave Access (WiMAX) signal transmission over RoF via a triple symmetrical dispersion system. The combination of three different fibres - single mode fibre (SMF), dispersion compensating fibre (DCF) and chirped fibre Bragg grating (CFBG) - is used to transmit a 120Mbps mobile WiMAX scalable Orthogonal Frequency Division Multiple Access (OFDMA) signal with 3.5GHz carrier frequency and 20MHz bandwidth over a RoF system. To compensate the dispersion, the SMF and DCF are employed and specifically the high reflector CFBG is applied to reduce signal power loss. In our study, the WIMAX signal is transmitted through a triple symmetrical dispersion system consisting of 2xDCF (20 km) and 2xSMF (100 km) connected to SMF (24 km) and CFBG. Simulation results clearly indicate that the limited signal transmission length and data bit rate in the RoF system, caused by fibre attenuation and chromatic dispersion, can be overcome by the combination of SMF, DCF and CFBG. The transmission distance in the fibre is extended to 792 Km, SNR and OSNR are highly satisfactory; simultaneously the power consumption is decreased.
Abstract— The main impediments for long distance signal transmission in the fibre optic system, especially the radio over fibre (RoF) system, are the chromatic dispersion and signal power attenuation. Additionally, the power consumption in the laser diode and optical amplifiers affect the signal transmission costs; however, it is lower than in a wireless system. Therefore, decreasing the power consumption and chromatic dispersion and increasing the data bit rate in RoF are the demands for present and future fibre optic system technology. In order to increase the signal transmission distance and improve the frequency spectrum, we study in this paper a mobile Worldwide Interoperability for Microwave Access (WiMAX) signal transmission over RoF via a triple symmetrical dispersion system. The combination of three different fibres - single mode fibre (SMF), dispersion compensating fibre (DCF) and chirped fibre Bragg grating (CFBG) - is used to transmit a 120Mbps mobile WiMAX scalable Orthogonal Frequency Division Multiple Access (OFDMA) signal with 3.5GHz carrier frequency and 20MHz bandwidth over a RoF system. To compensate the dispersion, the SMF and DCF are employed and specifically the high reflector CFBG is applied to reduce signal power loss. In our study, the WIMAX signal is transmitted through a triple symmetrical dispersion system consisting of 2xDCF (20 km) and 2xSMF (100 km) connected to SMF (24 km) and CFBG. Simulation results clearly indicate that the limited signal transmission length and data bit rate in the RoF system, caused by fibre attenuation and chromatic dispersion, can be overcome by the combination of SMF, DCF and CFBG. The transmission distance in the fibre is extended to 792 Km, SNR and OSNR are highly satisfactory; simultaneously the power consumption is decreased.
Keywords- Worldwide Interoperability for Microwave Access (WiMAX); Radio over Fibre (RoF); Dispersion Compensating Fibre (DCF); Chirped Fibre Bragg Grating (CFBG); Single Mode Fibre (SMF).
I. Introduction
The communication systems such as wireless broadband and mobile broadband systems offer better client service, by enhancing mobility, accessibility and simplicity of communication between people. Therefore, there has been growing interest in WiMAX systems, WiMAX IEEE 802.16 and 802.16e-2005 mobile [1]. In comparison to Universal Mobile Telecommunication System (UMTS) and Global System for Mobile communications (GSM), WiMAX offers an enlarged significant bandwidth by using the channel bandwidth of 20 MHz and an improved modulation technique (64-QAM).
When equipments are operating with low-level modulation and high-power amplifiers, WiMAX systems are capable to serve larger geographic coverage areas, and they support the different modulation technique constellations, such as BPSK, QPSK, 16-QAM and 64-QAM. WiMAX physical layer consist of OFDM, which offers resistance to multipath. It permits WiMAX to operate in non- line-of-sight environments (NLOS) and is highly understood for alleviating multipath for wireless broadband. WiMAX provides modulation and forward error correction (FEC) coding schemes adapting to channel conditions; it may be changed per user and per frame [2].
The electrical distribution of high-frequency microwave signals through either free space or transmission lines causes difficulties and costs. Losses increase with frequency in free space, due to absorption and reflection; and in transmission lines impedance rises with frequency, which leads to very high losses. Therefore, expensive regenerating equipment is required to distribute high-frequency radio signals electrically over long distances. The alternative would be to distribute baseband signals, radio frequency (RF) signals or signals at low intermediate frequencies (IF) from the control station (CS) to the base station (BS) and subsequently to the user. The RF or baseband signals are down-converted to the required microwave or mm-wave frequency at each BS, amplified and transmitted. Reverse, from user to the BS, the signals would be up-converted [4].
The radio in millimeter-wave (mm-wave) band is the promising media to transmit the ultra-broadband signal in the wireless telecommunication system and, since the recent ten years, has been developing to a favorite research topic. By optics method the mm-wave signal is readily generated and can be transmitted through a long fiber distance. In the RoF system, the generation of the optical mm-wave is one of the key techniques. Several techniques to generate the optical mm-wave at around 60 to 120GHz have been reported, including direct modulation of laser diode (LD), heterodyne technique with optical phase locking, electrical sub-harmonic injection and external modulation. Of all these techniques, the optical external modulation is an appropriate option to generate the optical mm-wave signal with high spectral purity [5].
RoF systems are analogue fibre optic links, which are used to transmit demodulation signal carrier of radio frequency (RF) directly or indirectly from a CS to a BS through a remote unit antenna (RAU) or radio accesses antenna (RAP) to the client [6].
In order to find leading techniques for the WiMAX network deployment, RoF has been studied extensively in recent years. Many studies have been focused on the fibre [7] [8] [9] [10], the low attenuation (0.2 dB/km), and high performance solution for high-speed fibre based on wireless access.
The utilization of millimeter-wave (MMW) frequency for high-speed wireless access, in future RoF systems, would meet the requirement of high bandwidth and overcome the spectral congestion at low frequency. Surely, the RoF system is the future network technology, which has on one hand the capacity to satisfy the demands for decreasing electromagnetic smoking, wireless traffic, power, noise, cost, and antenna size and, on the other hand, to increase frequency, bandwidth, data rate and capacity also eventually improves the spectral efficiency.
So far, the investigations have been narrowed to resolve the dispersion effect and to control the chromatic dispersion, which is explained in Section II, the signal distortion also reduce the power and increase the transmission distance [11]. In our study, the compensators methods are used to equalize the dispersion slope in a fibre and have been demonstrated in the form of DCF and CFBG. DCF has proved to be effective to overcome chromatic dispersion in high velocity light; with a special design, consisting of a fibre and a negative dispersion slope, it compensates the positive dispersion in SMF. We also use an optical amplifier and a CFBG, due to the high insertion loss of DCF, which is discussed in Section III and IV.
CFBG is a high Bragg reflector placed in a short segment of an optical fibre, used to correct chromatic dispersion. It can be used as a wavelength-specific reflector or as an inline optical fibre. CFBGs are extensively employed for functions such as dispersion compensation, stabilizing laser diodes, and add/drop multiplexing in optical fibre systems. The CFBGs comply with environmental requirements by increased stability and durability (free from rust), they can be highly multiplexed (many sensing points in a single fibre cable), and have the advantage of low power attenuation through transmission over several kilometers [12].
This paper studies methods to increase the WiMAX signal transmission distance through utilizing a RoF system with the aim to reduce power consumption and to obtain satisfactory OSNR, SNR and high quality signal transmission spectrums. The work focuses on WiMAX signals transmitted over RoF by applying SMF, DCF and CFBG. The paper is organized as follows. In Section II, we focus on related work; in Section III, we describe the theory of light dispersion in the fibre optic cable for SMF, DCF and for CFBG. In Section IV, we introduce the system description of WiMAX and RoF system and describe the design of the complete system. We discuss the simulation results in Section V and finally, conclusions are drawn in Section VI.
II. RELATED WORK
As mentioned in Section I, RoF technology as a means to deliver WIMAX signals has been studied by a number of researchers. Based on the IEEE 802.16d-2004 specification in the 3.5 GHz band, [13] reported about measured spectra and Error Vector Magnitude (EVM) for different single mode fiber spans up to 5 km.
Also, [14] investigated EVM for RoF WiMAX signal transmission. In this approach, fiber lengths between 0km and 5km for both uplink and downlink instances were investigated. The results show, on the downlink, the EVM measured was better than 3.1% between -3dBm and 10dBm. Lowest EVM was measured at 3dBm.
A hybrid radio on dense-wavelength-division-multiplexing (DWDM) transport system for WiMAX applications is proposed in [15]. The researchers were able to improve the bit error rate (BER) over a large, effective area fiber (LEAF) of 100km.
In [16], a WiMAX-RoF transport system is proposed and achieved satisfactory BER performance over a 120 km SMF length for both, down and up links.
Osadchiy et al [17] proposed a bi-directional WiMAX-over- fiber signal transmission system. The scheme supports signal transmission on a 2.4 GHz carrier at a bit rate of 100 Mb/s downlink and 64 Mb/s uplink for an 80km access fiber link. They also demonstrated a successful transport of 100 Mb/s WiMAX-compliant signals with a 5.8 GHz RF carrier over a 78.8km deployed SMF and a 40km distribution SMF. The results show that the WiMAX signal stayed within 5% RMS EVM after 118.8-km fiber link transmission and air transmission.
III. THEORY AND ANALYSES
Dispersion is a highly important factor due to the effect on the bit rate. There are three types of dispersions: material dispersion, also known as chromatic dispersion, is caused by the fact that the refractive index of the fibre medium varies as a function of wavelengths, waveguide dispersion depends on geometrical characteristics like shape, design and chemical composition of the fibre core and finally, intermodal dispersion, which is related to the fact that the light is not transmitted as a single beam [18].
Accordingly, chromatic dispersion emerges because of variable frequency components and also signals at differing wavelengths move at different velocities due to the refractive index. It has the following units of measurement: ps/nm/km, where nm is the spectral width of the pulse, ps refers to the time spread of the pulse and km refers to the fibre length. The chromatic dispersion of SMF is 16 ps/ nm/km at 1550 nm and 17ps/nm/km at 1552nm, and can be expressed as follows [19]: where L is the fibre length and tg is the time to dispread the distance. For externally modulated sources, transmission distance limited by chromatic dispersion can also be expressed as follows [19]:
illustration not visible in this excerpt
16\D\X2B2 where B is the bandwidth; A is the wavelength and c is the light velocity. Eq. (1) and (2) show the transmission distance of the signal is limited due to the chromatic dispersion in the SMF. To abolish the limitation of signal transmission in the SMF, techniques like DCF and CFBG have been demonstrated to be useful to compensate the accumulated dispersion in the fibre. DCF has a dispersion characteristic that is contrary to that of the transmission fibre. Dispersion compensation is achieved by inserting an open loop of DCF into the transmission path. The total dispersion in the DCF open loop needs to be equal and opposite of the accumulated dispersion in the SMF. This means that, if the SMF has a low positive dispersion, the DCF will have a large negative dispersion. With this technique, the absolute dispersion per length is nonzero at all points along the fibre, whereas the total accumulated dispersion is zero after some distance. The length of the DCF should be minimized as the special fibre used has a higher attenuation than the transmission fibre. The attenuation is around 0.6dB/km at 1550nm compared to 0.2dB/km for SMF [20].
Because of the high power loss in DCF, the CFBG is applied to control and tune the difference in arrival times of the multiple frequency components resulting from a typical dispersion. It has been shown that strong, long, and highly reflective gratings can be used for dispersion in communication links in transmission with negligible loss aspersion, by proper design of the grating. For high-bit-rate systems, higher- order dispersion effects become important, dissipating the advantage of the grating used in transmission. The rules utilized for the design of the grating to compress pulses in a near ideal technique are a compromise between the reduction of higher-order dispersion and pulse recompression. Bandwidths are limited with this configuration by the strength of the coupling constant and length of a realizable uniform period grating.
illustration not visible in this excerpt
Figure 1. The wavelength reflected in CFBG by the Bragg grating referred to Bragg wave is X1. X2, Xi and the unwanted wavelength passes;the spacing decreases along the fibre; accordingly, the Bragg wavelength decreases with distance along the grating length.
The reflected wavelength in CFBG amends with the grating period, because the spacing of the grating varies and is designed for a desired wavelength. The different wavelengths reflected from the grating will be subject to different delays; if the injected light wavelength differs from the grating resonant wavelength, the light is not reflected. As shown in Figure 1 , the chirp in the period can be related to the chirped bandwidth Achirp of the fibre grating which is presented in the following equation [11]:
illustration not visible in this excerpt
The reflection from a chirped grating is a function of wavelength, and therefore, light entering into a positively chirped grating (increasing period from input end) suffers a delay in reflection that is approximately [11].
illustration not visible in this excerpt
where A0 is the Bragg wavelength at the center of the chirped bandwidth of the grating, and vg is the average group velocity of light in the fibre. By introducing a maximum delay of 2Lg/vg between the shortest and the longest reflected wavelengths, the effect of the chirped grating is that it disperses light. This dispersion is of importance since it can be used to compensate for chromatic dispersion in optical fibre transmission systems. The figure of merit is a high-length grating with a bandwidth important feature of a dispersion-compensating device as at 1550 nm, the group delay t in reflection is ~10 nsec/m. Several parameters affect the performance of the CFBGs for dispersion compensation: the insertion loss due to reflectivity & 100%, dispersion, bandwidth, and polarization mode- dispersion, deviations from linearity of the group delay also group delay ripple. Ignoring the first and the last two parameters momentarily, we consider the performance of a chirped grating with linear delay characteristics, over a bandwidth of AX chirp.
The dispersion coefficient Dg [ps/nm/km] for the linear CFBG is given by the following simple expression [8]: where n is the average mode index, c is the light velocity, AX is the difference in the Bragg wavelengths at the two ends of the grating. Eq. (5) represents that Dg of a chirped grating is ultimately limited by the bandwidth AX; the increase in the transmission distance will be possible only, if the signal bandwidth is reduced.
IV. System Discription OF Wimax over RoF
An important difference between fixed and mobile WiMAX is the physical layer. Mobile WiMAX uses OFDMA as its physical layer transmission scheme instead of plain Orthogonal Frequency Division Multiplexing (OFDM). OFDMA can also be used as a multiple access mechanism when groups of data subcarriers, called sub channels, are allocated to different users. Mobile WiMAX also introduces more scalability into the actual physical layer parameters. cyclic prefix durations and channel bandwidths in multiple OFDMAs, which have different amounts of subcarriers, are utilized to allow the wireless link design to be optimized according to the environment where the system is deployed.
Figure 2 illustrates the schematic simulation setup of WiMAX over RoF, including the dispersion model techniques SMF, DCF and CFBG. In this simulation, the BS deployed the data of mobile WiMAX IEEE 802.16e-2005 to the fibre system as a RF signal; firstly, to the RAU antenna as an electrical signal; subsequently, converted to the fibre optic signal by modulating the RF to the laser beam, which a laser diode has injected into the SMF; this modulation operation arises in the Mach Zehnder Modulator (MZM).
In future work, it would be possible to use a WiMAX Femtocell instead of a BS because it has several advantages. It can be use in microcell, pico-cell area and indoor; the typical cell radius ranges between 50-100m[17]. The proposed scheme would not require to be changed, because, at the end of fibre, the optical signal is converted to the electrical RF signal, which would radiate via Femtocell to micro or pico-cell. The WiMAX transmission signal is centered at 3.5 GHz; comprising 128 subcarriers and 64QAM (6 bit-per-symbol) modulates each; the bandwidth is 20MHz, and the transmitted bit rate is 120Mbps. The important component in WiMAX is the scalable orthogonal frequency division multiplexing (S-OFDMA). In the basic version of OFDMA, one sub-carrier is assigned to each user. The spectrum of each user is quite narrow, which makes OFDMA more sensitive to narrowband interference. The core of an orthogonal multi-carrier transmission is the Fast Fourier transform (FFT) respectively, inverse FFT (IFFT) operation; synchronization and channel estimation process together with the channel decoding play an important role. To ensure a low cost receiver (low cost local oscillator and RF components) and to enable a high spectral efficiency, robust digital synchronization and channel estimation mechanisms are needed. The throughput of an OFDM system does not only depend on the used modulation constellation and Forward Error Correction (FEC) scheme, but also on the amount of reference and pilot symbols spent to guarantee reliable synchronization and channel estimation[23]
illustration not visible in this excerpt
Figure 2. Schematic shows the setup of WiMAX downlink integrated in the RoF system which consist of SMF, DCF and CFBG for the increased fibre length of 792km.
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- Quote paper
- Mazin Al Noor (Author), Jonathan Loo (Author), Richard Comley (Author), 2011, Extended Mobile WiMAX Signal Transmission over RoF viaTriple Symmetrical Dispersion System SMF, DCF and CFBG, Munich, GRIN Verlag, https://www.grin.com/document/195645
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