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Cost effective architecture for symmetric full-duplex colorless WDM-PONs using DPSK downstream and OOK Re-modulated upstream Data. signals.


In recent years, traffic trends in access networks have shifted from simple web based services to video based interactive and multimedia services. The traditional network cannot cope with this continuous growing demand for high data rate and bandwidth. Therefore a newly designed access network is required to meet the user expectations. The wavelength division multiplexed passive optical network (WDM-PON) is considered to provide broadband access for next generation networks. Reducing the transmitter cost is the most crucial issue in the deployment of WDM-PONs [1-2]. In such a system, the architecture design and technique of millimeter wave generation for upstream and downstream signals play an important role in efficient real network deployment [3-5]. The network architecture with centralized light source at the central office (CO) and with data re-modulation using downstream signal received at the base station (BS) is an attractive solution for low cost implementation of the upstream transmitter. It requires no wavelength management and needs no expensive wavelength specific light source at the base station. On the other hand differential phase shift keying (DPSK) is a non-coherent kind of phase shift keying technique. The receiver doesn't require any local oscillator for data recovery from carrier phase, which enable them to be cost-effective and simple in configuration. The DPSK modulation format could keep high Extinction Ratio(ER) and provide higher tolerance to fiber non linear impairment and chromatic dispersion, thus resulting in improved system power budget [6]. DPSK as compared to ON-OFF keying (OOK) has the benefit of 3 dB lower OSNR to reach a given BER [7], which can helps in extending the transmission distance, reduce power requirements and simplified circuitry configuration that results in a cost effective network. Hence for a colorless WDM-PON network, DPSK modulation format is used for the downstream transmission and OOK can be used for the upstream transmission.

Recently, several schemes have been proposed to achieve full duplex transmission over a single fiber in WDM-PONs. Injection locked Fabry-Parot laser diode (FP-LDs) is used to achieve a low cost WDM-PON [8]. However it has some limitation due to high injection power requirement. The generation of upstream signal based on semiconductor optical amplifier (SOA) is reported [9]. However it has Extinction Ratio limitation and degrades downstream data. A WDM-PON scheme based on Reflective semiconductor optical amplifier (RSOA) is discussed in [10] which inhabit data rate limitation due to limitation of RSOA. Double Sides band (DSB) and single side band (SSB) modulation techniques for generation of optical millimeter and centralized wave is presented in [11]. However they have limitation due to high frequency bandwidth and low receiver sensitivity respectively. A directly modulated distributed feedback (DFB) laser is used for cost effective efficient DPSK transmission in a PON with 10-Gbps downstream and 2.5-Gbps upstream [12]. A full-duplex 10-Gbps WDM-PON scheme using NRZ-DPSK signal for downstream and NRZ-OOK signal for upstream is reported in [13]. However the DPSK signal in this scheme is generated by conventional pulse carving approach. Pulse carver needs an additional Mach-Zehnder modulator which increases the cost of the transmitter design and will also reduce the power of optical signal, thus affecting the overall power budget of the optical system.

In this paper we propose and demonstrate the generation of downstream signal using a single Mach-Zehnder modulator without using the pulse carver. The data is combined with electrical clock by combiner before getting modulated by light source using a single Mach-Zehnder modulator. The downstream signal can be subsequently re-modulated at base station for OOK upstream signal at symmetric rate of 10-Gbps. The scheme does not need the pulse carver at DPSK transmitter and can reduce deployment cost significantly. The rest of the paper structure is as given. Section 2 describes the working principles and network architecture. Section 3 presents the simulation setup and operation. Section 4 discusses the transmission performance and analysis and section 5 concludes the paper.


The proposed WDM-PON architecture is shown as in Figure-1. A CO consists of a distributed feedback laser diode (DFB-LD) arrays which offers the wavelength from 11 to X4 for downstream data. By choosing an appropriate biased voltage for Mach-Zehnder modulator (LiNbO3-MZM) and applying a super imposed electrical signal of clock and data, the LiNbO3-MZM allows to generate an optical signal which is RZ data modulated using a single modulator. The generated downstream RZ-DPSK signal is multiplexed with other downstream 10-Gbps channels. The multiplexed signal is transmitted over 25 Km standard single mode fiber (SSMF). On the other end the de-multiplexer is used to demultiplex the downstream signals and send them to respective BS. At the BS, a portion of the downstream received optical power is tapped off by a half power splitter. The constant intensity downstream DPSK signal is demodulated by a 1-bit delayed interferometer (DI) and balanced photo diodes. The rest of the downstream optical signal is re-modulated by an intensity modulation technique of 10-Gbps RZ-OOK. The generated upstream signal is transmitted back to the CO using SSMF through a complete path.


To discuss the performance of the proposed WDM-PON system we establish a model for simulation using optisystem without using pulse carver [14] as shown in Figure-1. A 10-Gbps pseudorandom bit stream (PRBS) data of order [2.sup.7] -1 is superimposed by a 5GHz clock using a combiner. The superimposed signal is externally modulated by a LiNbO3-MZM to generate RZ-DPSK downstream data signal. Four continuous light waves with a launch power of 0 dBm are generated by four distributed feedback (DFB) lasers at wavelengths of 1552.52nm, 1552.12nm, 1551.72nm, and 1551.32nm for four different channels respectively. They are multiplexed and transmitted over SSMF. At the BS, a 3dB optical splitter divides downstream into two parts. A portion of the downstream data was demodulated by delay interferometer (DI) and balanced detectors. The rest of the downstream optical signal is re-modulated by an intensity modulation technique of 10-Gbps RZ-OOK. The generated upstream signal is transmitted back to the CO through SSMF where it is detected by photo detector.


The bit error rate (BER) as a function of received optical power for both the downstream and upstream transmission for channel-1 and channel-3 is shown as in Figure-2 using back to back scenario. In the downstream, the 10-Gbps RZ-DPSK data signal provides a BER of [10.sup.-9] at received power of -37 dBm, while in the upstream, the 10-Gbps RZ-OOK data signal provides a BER of [10.sup.-9] at a received power of -25 dBm. The BER as a function of received optical power for downstream and upstream data signal after traversing 25 km SSMF is shown as in Figure-3. In the downstream, the 10-Gbps RZ-DPSK data signal provides a BER of [10.sup.-9] at received power of -36 dBm, while in upstream, the 10-Gbps RZ-OOK data signal provides a BER of [10.sup.-9] at a received power of -24 dBm . A small power penalty of 1dB relative to B2B scenario has been experienced at BER of [10.sup.-9] after traversing of 25 km SSMF. Such a small power penalty could largely be attributed to chromatic dispersion, however the constant performance of the downstream signal clearly illustrate the applicability of such a cost effective DPSK transmitter for implementation in future WDM-PONs.

The average power penalty of all the four multiplexed RZ-DPSK downstream signals at a BER of [10.sup.-9] is about 0.5 dB after transmission of 25 Km in a single mode fiber without any signal amplification and dispersion compensation management. On the other hand, the average power penalty for the four multiplexed intensity modulated (OOK) upstream signal at a BER of [10.sup.-9] is less than 1dB after the corresponding upstream transmission over a complete path without any signal amplification and dispersion compensation management is shown as in Figure-04.Therefore, it is evident from the above results that an error free transmission can be achieved for both downstream and upstream direction. The insets of Figure-5 show the corresponding optical eye diagram for downlink and uplink channels. The eyes are clear and wide open.


In conclusion, we proposed and demonstrated a cost effective, data symmetric and centralized light wave WDM-PON architecture using a single Mach-Zehnder Modulator for downstream RZ-DPSK transmitter. The downstream data signal is re-modulated using RZ-OOK for upstream data signal. An error free full-duplex transmission over 25 Km SMF with lower BER and improved receiver sensitivity is achieved. The power penalties of 10-Gbps downstream and upstream data signals at BER of [10.sup.-9] are 0.5 dB and 1 dB respectively.


The financial support from National Basic Research program of China with No. 2010CB328300, National Natural Science Foundation of China with No. 61077050, 61077014, 60932004, BUPT Young Foundation with No.

2009CZ07 is gratefully acknowledged. The project is also supported by the Fundamental Research Funds for the Central Universities with No.2011RC0307, 2011RC0314 and open foundation of state key laboratory of optical communications technologies and networks (WRI) with No.2010-OCTN-02.


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Yousaf Khan (1), Muhammad Idress (1) Noaman Ahmad Khan (2), Jahanzaib Khan (2), Shahid Latif (2)

(1) Key Laboratory of Information Photonics and Optical Communications, BUPT, Beijing, China

(2) Department of Electrical Engineering, Iqra National University, Peshawar, Pakistan,, drnoaman@inu.,,
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Author:Khan, Yousaf; Idress, Muhammad; Khan, Noaman Ahmad; Khan, Jahanzaib; Latif, Shahid
Publication:International Journal of Emerging Sciences
Article Type:Report
Geographic Code:9CHIN
Date:Mar 1, 2013
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