Performance Evaluation Of Dwdm For Radio

Published Date: 02 Nov 2017

Abstract

The radio-over-fiber (RoF) system is one of the potential schemes for the future broadband wireless communication systems such as mobile communications, hotspots and suburban areas. The use of Dense Wavelength Division Multiplexing WDM system is responding to the demands for high data rate applications and reasonable mobility for broadband communication.

The work aims to investigate the performance of DWDM system utilizing Eribium Dopped Fiber amplifiers EDFAs and Dispersion Compensating Fiber DCF for different length of optical fiber and bit rates. The most effective factors causes performance degradations are the attenuation and dispersion. EDFA was introduced in the proposed system model as a solution to encounter the effects of attenuation and scattering losses, while the DCF utilized to mitigate the effect of dispersion.

The simulation was implemented using optiwsystem 7 and matlab R2011a. The results show that the use of EDFA and DCF make significantly boosts the performance of DWDM RoF system and increases the length of the fiber at which a Bit Error Rate BER of 10-9 can be obtained.

Keywords: OTDM, DWDM, EDFA, DCF, BER, Q-factor

تقييم اداء نظام DWDM ل RoF مع تعويض التعريض وEDFA

الخلاصة

الراديوية عبر الألياف (ROF) نظام هو واحد من مخططات محتملة للمستقبل نظم الاتصالات اللاسلكية ذات النطاق العريض مثل الاتصالات المتنقلة، والنقاط الساخنة ومناطق الضواحي. استخدام نظام تمازج الطول الموجي الكثيف WDM يستجيب لمطالب لارتفاع معدل البيانات التطبيقات والتنقل المعقولة للاتصال واسع النطاق.

العمل يهدف إلى مراقبة تحقيق أداء نظام DWDM باستخدام مكبرات ألياف المشوبة بالايريبيوم Eribium Dopped Fiber amplifiers DFA و فايبر تعويض التعريض DCFعبر اطوال مختلفة من الألياف البصرية ومعدلات ارسال مختلفة. العوامل الأكثر فعالية التي تسبب انهيار الأداء هي التوهين والتعريض. وقدم ال EDFA في نموذج النظام المقترح كحل لمواجهة آثار خسائر التوهين و الاستطارة، في حين أن استخدام DCF للتخفيف من تأثير التعريض.

المحاكاة تمت باستخدام برنامجي optisystem 7 and matlab R2011a. النتائج تبين أن استخدام EDFA وDCF يعزز إلى حد كبير أداء النظام DWDM RoF ويزيد طول اللبف البصري التي يمكن الحصول عندها على معدل خطأ بت BER .10-9

الكلمات الرئيسية: OTDM, DWDM, EDFA, DCF, BER, Q factor

Introduction:

Future multi-terabit/s optical core networks require optical technologies capable of managing ultra-high bit rate OTDM/DWDM (optical time division multiplexing/dense wavelength division multiplexing) channels at 160 Gbit/s or higher bit rates. OTDM (Optical Time-Division Multiplexing) is a very powerful optical multiplexing technique that deliveries very high capacity of data over optical fiber. The basic principle of this technology is to multiplex a number of low bit rate optical channels in time domain. The key functionalities in ultra-high speed network nodes are all-optical wavelength conversion, 3R-regeneration and demultiplexing of OTDM signals. Advanced optical networking techniques (optical add-drop multiplexing and optical routing) are studied in simulations and their performance evaluated considering 160 Gbit/s OTDM/DWDM channels.

Optical time-division-multiplexing (OTDM) is an important technique to overcome the electronic bottleneck and achieve single channel high bit-rate system. The commercially available electronic components are limited to around 10Gb/s data rate. Since the first 100-Gb/s OTDM transmission experiment over a 36-km fiber link was already reported in 1993 [1], OTDM was first demonstrated as early as 1968 [63], primarily as means to increase the capacity of an optical link. Since the first 100-Gb/s OTDM transmission experiment over a 36-km fiber link was already reported in 1993 [1], OTDM technologies have made a lot of progress toward much higher bit rates and much longer transmission distance [2–4]. For example, 160-Gb/s transmission over a record length of 4320 km [5] and on 2.56-Tb/s transmission over 160-km have been reported [Tao-rong 6].

An OTDM system consists of the following key sub-systems: ultra-short pulse generator, OTDM multiplexer, tributary clock extraction and high-speed optical switch to perform demultiplexing. There are several optical-signal-processing technologies available for each part, and EAM is one of the optimum terminal equipments as optical gate and a clock recovery device in the receiver [Tao-rong, 4].

Overall, successful demonstration of OTDM up to 400Gb/s has brightened the future of commercial OTDM. This system has the advantage of operating only on a single wavelength. It is possible of running OTDM on a number of existing WDM channels, which improves the overall data capacity. It is purely digital and compliant with the concepts of all-digital network. With rapid advancement in semiconductor technology and integration techniques, it will eventually make possible to manufacture compact, stable and higher performance components for commercial OTDM system [Jeffrey Huang EE558 Spring 99]. Hybrid WDM/OTDM networks have been proposed to move data between WDM and OTDM networks, and various subsystems have been demonstrated at 40 Gbit/s including WDM-to-OTDM and OTDM to-WDM translators, OTDM transmitters, and OTDM add–drop multiplexers [Lavanya]. OTDM and WDM are considered as the bases of second generation optical networks. Since OTDM and WDM can be used within the same network, they are complementary technologies in that a single fiber strand can be transmitting a several WDM signals, and each single WDM wavelength can contain OTDM multiplexed data. Using WDM and OTDM together is called an All Optical Network (AON). AON increase the efficiency and throughput while decreasing delay and errors. DWDM takes WDM, one of the second-generation optical network technologies, and takes it further.

This research work carried and examined the feasibility of using efficient photo receivers in the Central Office, in a WDM-PON network during the downstream direction. This paper is structured as follows. A brief outline of coding techniques in section II. Section III describes about the recent challenges and current PON technologies existing in the TDM and WDM PON. The simulation techniques and the values of the parameters are outlined under the heading performance evaluation in section IV. This is followed by Section V which describes

the results and discussions and finally section VI conclusions.

Optical Time-Division Multiplexing OTDM System

In time-division multiplexing and demultiplexing, each of the baseband data streams is given a series of time slots on the multiplexed channel. A multiplexer (MUX) is responsibe of assembling the higher bit-rate bit stream from several baseband streams while a demultiplexer (DEMUX) does the opposite job to reconstruct replica of the bit streams at the original lower bit rate by separating bits in the multiplexed stream. This technique is applied to optical system to multiplex and demultiplex optical signals as done in electrical systems.

In Figure (1), a schematic diagram of an N channel OTDM transmission system is shown. An optical pulse train from a laser diode is splitted into N paths. In each path, the pulse train is individually modulated by an electrical data signal forming in N optical RZ format data channels. Each of branches is delayed by a fraction of the clock period and synchronized to allow passive multiplexing to sum up an individual data stream. Here, the multiplexer is most simply implemented using passive fiber couplers with appropriate optical delays between the channels. To avoid crosstalk between these interleaved channels, the laser source must be able to generate optical pulses of duration < 1/N of the clock period.

To multiplex an optical signal with period T ps to channel N, the required time delay ∆τ for each path is:

Where ∆τi is the time delay for ith path. For example, for multiplexing the optical pulse train of 10 Gbit/s to 40 Gbit/s, the period T is 25 ps, so the time delay is 3.125 ps, and the difference of the fiber length is 0.2 mm.

Goal of OTDM is to increase of the aggregate rate BOTDM = NBch into the Tb/s-range (T~1ps). Where, N is the number of time-channels and Bch is the channel bit rate.

Wavelength-Division Multiplexing (WDM)

Wavelength-division multiplexing (WDM) gives better utilization of the large bandwidth of optical fiber and can increase the capacity of the cable network. Through WDM, signals from two or more line systems are transmitted over the same fiber [Hani]. The signal from different sources which combined by a multiplexer and fed into an optical fiber, channels combined are separated in the receiver unit by a demultiplexer and detected by photodetector [Hani]. The WDM devices at the transmitting unit is essentially a power combining referred to as a multiplexer. The device at the receiver unit is called a demultiplexer and should ideally separate out various channels with negligible loss and signal distortion. a large number of channels can be combined and separated with angularly dispersive multiplexing elements. At the output of the multiplexer, these light rays become co-linear and can be easily launched simultaneously into an optical fiber. At the receiver, a WDM works in exactly in the reverse fashion, directing light beams of various wavelengths from a fiber into their respective channels [Hani].

The main goal in any communication system is to increase the transmission distance. Loss and dispersion are the main factor that cause signal degradations and affect fiber-optical communication being the high-capacity develops. It is easy to see that the dispersion become the major factor that restricts long distance fiber-optical transfers as the bit rate increases [Zou, Bo-ning].

several dispersion compensation technologies were proposed[Bo-ning, Djafar]. Amongst the various techniques proposed in the literature, the ones that appear to hold immediate promise for dispersion compensation and management could be broadly classified as: dispersion compensating fibers (DCF), chirped fiber Bragg gratings (FBG), and high-order mode (HOM) fiber [Jianjun].

Optical amplifiers are used to maintain the correct signal power to keep sufficient signal to noise ratio for acceptable bit-error rate. In such systems, fiber dispersion can be managed in many ways. One method is to balance the average group velocity dispersion in the whole system such that it is zero referred to dispersion management and another is to use soliton transmission techniques. Demultiplexing and clock recovery allows the input optical signal to be split into the discrete channels.

The third-order dispersion will influence the transmission system seriously as the bit rate of a single exceeds 40 Gbit/s and degrades the system performance. To deal with this problem, one can use DCF to compensate both the second-order dispersion and third-order dispersion of the SMF to extend the transmission distance in the system. The condition for a fiber link containing two kinds of fibers of length L1 and L2 to form the dispersion management is [94] :

Where β2 j and β3 j are second and third-order dispersion parameters for the fiber of length indicated.

Passive Optical Networks PON

During the year 1980’s only Passive optical networks (PONs) were developed. Since it is cheap way to implement, PON has received great interest. It is used as a cost effective method for sharing fiber infrastructure to business premises, curb, and home etc. The PON architectures use the passive components, which potentially reduces the cost and maintenance since it is point to multi point transport network.

PONs uses optoelectronics so they characterized to have low power consumption, except laser amplifiers and photo receivers. Gigabit PON GPON has found to improve bandwidth factor by four through maintenance and security issues [Rajalakshmi, Baca]. There are several architectures of PON using different modulation schemes like TDM, WDM and hybrid using both TDM/WDM.

The TDM PON is a point to multipoint architecture shown in Figure (2). The packets were broadcasted by the Optical Line Terminal OLT in the downstream direction. It is passed through a 1: N optical splitter and it is extracted by the designated Optical Network Unit ONU. The data is sent in the form of packets and each user transmits after a definite time delay. The same time delay is utilized at the destination ONU to distinguish the packets meant for it.

WDM PON

WDM PONs as shown in Figure (3), has been widely researched as a potential technology. This PON uses multiple wavelengths in a single fiber to multiply the capacity without increasing the data rate. But a single wavelength is assigned for TDM PON. A TDM PON provides moderate bandwidth but more channels [Rajalakshmi].

The PON architecture consist of a single mode fiber which connects a Central Office CO to the network distribution unit which consist of passive optical splitters or/and Multiplexers and Demultiplexer. Figure (3) represents WDM PON architecture. The Optical Line Terminal OLT housed in the CO contains a set of tunable laser sources or fixed wavelength laser sources used to transmit the downstream traffic to Optical Network Unit ONU. Each user has been assigned a fixed frequency at which the laser operates. The frequency allotment can be permanent or it can be based on the requirement of bandwidth demanded. The data is then given to a multiplexer which combines all the data together and sends it through the optical fiber of lengths varying from 20km to 100km.

The Optical fiber that comes from the central office is connected to a passive WDM Demultiplexer. The function of it is to split the light depending on the wavelength and to transmits the same to the corresponding ONU. The ONU us again an optoelectronic component and converts the light signal to electrical signal and the data is retrieved [Rajalakshmi].

System Simulation and results

The performance of OTDM system and hybrid OTDM/DWDM system will be evaluated here for different bit rates and lengths of SMF. The proposed 4 users OTDM system is shown in Figure (4). A CW laser diode with a frequency of 193.1 THz is used. The splitter divides the optical power between the four OLTs. Each subsystem included in the OLT includes a PRBS generator a specific rate and a RZ Machzehnder modulator. In each branch there is a time delay with time of [(1/bit rate)* i/N)] where i = 0, 1,…, N. So each user capable of transmitting information at a specific time slot. Then combining the output from each user by the optical power combiner and sent throught a single mode fiber SMF. At the end of the SMF of variable length, the multiplexed signal is amplified by an eribuim dopped fiber amplifier EDFA. The emerging signal is splitted and distributed among the ONUs at the receiver side and time delay unit is used at each ONU in order to synchronous with that of the OLT. Each user is allowed to access the network at certain time slot with delay time which limits the throughput of each user. Then, the signal is detected and demodulated at each ONU to extract the original information. Table lists the main parameters and settings of proposed OTDM system.

Figures (5 and 6) shows the Q factor and BER variations of four users OTDM with length of SMF without any amplification for bit rates of 1, 2.5 and 4 Gbps per user which give total bit rate of 4, 10 and 16 Gbps respectivey. Figures (7 and 8) shows the Q factor and BER variations of four users OTDM with length of SMF with EDFA amplifier of 20 dB gain and noise figure of 4 for bit rates of 1, 2.5 and 4 Gbps per user which give total bit rate of 4, 10 and 16 Gbps respectivey.

From the results it can be

Hybrid OTDM/DWDM system

Figure (9) is schematic diagram of a Hybrid OTDM/DWDM PON system which a combination of OTDM and DWDM for increasing the number of users and the rate of transmission of the system. In the figure, four OTDM systems each with four users so there are sixteen users as a total. The output of each OTDM, which is a time multiplexing of the output of its four OLTs, is at different wavelength and wavelength multiplexed by a WDM multiplexer with four input ports. The output of the WDM is at a bite rate of (4*4*bit rate). After passing through the SMF and amplified by the EDFA, the multiplexed signals are demultiplexed by a WDM demultiplexer with four output ports and each wavelength is directed into its OTDM demultiplexing unit. The power of each wavelength is distributed among its four ONU systems in which the signal are detected and demodulated. Table (2) lists the parameters of the four hybrid OTDM/ four WDM system.

Figures (10 and 11) shows The Q factor and BER variations of four users OTDM/ four WDM system with length of SMF with EDFA amplifier for bit rates of 1, 2.5 and 4 Gbps per user which give total bit rate of 16, 40 and 64 Gbps respectivey at the output of DWM multiplexer.

160Gbps hybrid OTDM/DWDM system

The SMF has a second-order dispersion β2 of -20.8 ps2/km (dispersion coefficient D = 16.75 ps / nm km) and third-order dispersion β3 of 0.1 ps3 / km. The second-order dispersion β2 of the DCF is 117.9 ps2 / km (dispersion coefficient D = -95 ps / nm km) and the length can be determined by Eq. (4-4) as 8.82 km. The third-order dispersion β3 is decided by:

and we chose β3 –0.56 ps3 / km to compensate the third-order caused by SMF.

The compensation configuration used in the work is post-compensation as shown in Figure( ). The SMF exhibits an optical attenuation of 0.2 dB/km and the attenuation of the DCF is 0.6 dB/km. The parameters of SMF and DCF for simulation are listed in Table 1. The gain of the EDFA is 15.3 dB and its’ noise figure is 2 dB. Because the core of the DCF is smaller than SMF, the effective area is small. It leads to serious nonlinear effect. When the input power is large, the self-phase modulation (SPM) will mainly limit the transmission distance. However, when the input power is small, the signal to noise ratio (SNR) of the EDFA will be degraded and limit the transmission distance.

The 160 Gbit/s multiplex the optical signal from 10 Gbit/s to 160 Gbit/s as shown in Figure ( ).