Design of Directly Modulated Long-Reach PONs ... - OSA Publishing

848 J. OPT. COMMUN. NETW./VOL. 5, NO. 8/AUGUST 2013

Alves et al.

Design of Directly Modulated Long-Reach PONs Reaching 125 km for Provisioning of Hybrid Wired–Wireless Quintuple-Play Service Tiago M. F. Alves, Maria Morant, Adolfo V. T. Cartaxo, and Roberto Llorente

Abstract—Design tools for the quintuple-play service provisioning in a long-reach passive optical network (LR-PON) using off-the-shelf directly modulated lasers (DMLs) are experimentally demonstrated. The quintupleplay service including broadband wired Internet, phone/ voice data, wireless high-definition TV, wireless data, and home security/control is provided by a radio-over-fiber bundle of multi-format orthogonal frequency division multiplexing (OFDM) signals. This bundle occupies a multioctave band and consists of a full-standard worldwideinteroperability-for-microwave access signal, a long-term evolution signal, two ultra-wideband channels, and an ad hoc OFDM signal providing gigabit Ethernet connectivity. Dynamic centralized impairment compensation, fixed optical dispersion compensation, proper selection of the multiplexed signal level applied to the DML, and unbalanced power sharing between the different OFDM-based signal formats are shown as effective design tools for the delivery of the quintuple-play service to end-users. The experimental validation confirms that the DMLs are an effective solution to deliver the quintuple-play service to end-users 125 km away from the central office. Additionally, it is shown that the directly modulated LR-PON employing negative residual dispersion is able to provide the quintuple-play service to end-users located between 75 and 125 km away from the central office with an error vector magnitude (EVM) margin, relative to the EVM stated in each signal standard, by more than 1 dB. Therefore, the proposed directly modulated LR-PON is a cost-effective alternative to networks using external modulation for the provision of wireless and wired quintuple-play service to end-users using a single hybrid network. Index Terms—Directly modulated lasers; Fiber optic systems; Long-reach passive optical networks; Orthogonal frequency-division multiplexing; Wired–wireless convergence.

Manuscript received March 13, 2013; revised June 7, 2013; accepted June 7, 2013; published July 15, 2013 (Doc. ID 186959). Tiago M. F. Alves (e-mail: [email protected]) and Adolfo V. T. Cartaxo are with the Group of Research on Optical Fibre Telecommunication Systems (GROFTS) of Instituto de Telecomunicações, Department of Electrical and Computer Engineering, Instituto Superior Técnico, Technical University of Lisbon, Lisbon 1049-001, Portugal. Maria Morant and Roberto Llorente are with the Nanophotonics Technology Centre, Universidad Politécnica de Valencia, Camino de Vera s/n. Valencia 46022, Spain (e-mail: [email protected]). http://dx.doi.org/10.1364/JOCN.5.000848

1943-0620/13/080848-10$15.00/0

I. INTRODUCTION

N

owadays, high-data-rate wireless services such as ultra-wideband (UWB) for high-definition (HD) audio and video content distribution, long-term evolution (LTE) for phone/voice provisioning, or wireless interoperability for microwave access (WiMAX) for wireless data transmission and home security are receiving particular attention from the optical fiber communication systems community due to the high bandwidth demand required by these services [1,2]. In order to provide enough bandwidth to the end-users, the full integration of wireless and wired services in a single hybrid optical access network is required [2–4]. Radio-over-fiber networks, where both wireless and wired services are delivered to customers’ premises, have been appointed as a powerful solution to meet the requirements envisioned by the optical-wireless convergence [2,5]. In radio-over-fiber networks, the end-users benefit from the huge bandwidth provided by the optical fiber and from the mobility enabled by the wireless networks. Also, the operators may benefit from the centralized management techniques recently proposed for these networks where the fully standard signals supporting the different services are generated and managed only at the central office [6]. Recently, the coexistence of orthogonal frequencydivision multiplexing (OFDM)-based LTE, WiMAX, and UWB signals and a custom OFDM signal providing capacity compatible with standard gigabit Ethernet (GbE) service was proposed and demonstrated in a novel integrated wavelength-division multiplexing (WDM) longreach (LR) passive optical network (PON) for the provision of the quintuple-play service to the end-users [6]. The proposed architecture complies with the goals defined by the Full Service Access Network organization in the Next Generation PON Task Group (NG-PON) specification, such as the provision of carrier-class solutions with the capability of smooth migration between system generations and the reuse of the legacy fiber plant [7]. In addition, the proposed LR-PON is indicated for the NG-PON architectures due to its large coverage area, high flexibility, cost efficiency, ability for future integration of new standards, and centralized management capabilities. © 2013 Optical Society of America

Alves et al.

The conceptual simplified scheme of the proposed WDM LR-PON architecture, connecting the optical line termination (OLT), installed at the central office, to the optical network unit (ONU), and installed at the users’ premises, is depicted in Fig. 1. A WDM LR-PON structure is used to serve different ONUs with different wavelengths to ensure that different contents can be delivered to different users without significant bandwidth requirements at the ONU side [8]. At the OLT, the wireless (LTE, WiMAX, and UWB) and the wired OFDM-GbE signals are frequency-division multiplexed in the electrical domain to generate the bundle of OFDM signals. The spectrum of the bundle of OFDM signals occupies a frequency range between 1 and 4.8 GHz (multi-octave range), as shown in the inset of Fig. 1. The signals are transmitted along the LR-PON preserving their native frequency and modulation features. With this, there is no need of upconversion or transmodulation of the wireless signals at each ONU, leading to a fully transparent access network. The feeder fiber connects the central office of the network operator to a distribution point at the remote node (RN). In order to deal with future reach requirements in the optical access network, solutions exploiting mid-span reach extenders, deployed at the RN to achieve longer reaches (>60 km), were proposed in the past [9,10]. The different customers are connected to the RN via distribution fibers in a fiber-to-the-home approach. At the ONU, a single photodetector is used for optoelectrical conversion of both wireless and OFDM-GbE signals with cost savings for the network operator [11]. At the user’s premises, the OFDM-GbE signal provides wired connectivity with standard Ethernet cabling, and the wireless signals (LTE, WiMAX, and UWB) are radiated directly to the terminal devices. The multi-octave bundle of OFDM signals (OFDM-GbE, LTE, WiMAX, and UWB) was successfully transmitted along the proposed LR-PON employing a novel centralized impairment compensation approach and reaching an OLT–ONU distance of 100 km [12]. Experimental demonstration of a WDM LR-PON transmission employing three

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wavelengths to provide quintuple-play services to three users located at different distances from the central office was reported in [13]. In both works, the electro-optic conversion realized at the OLT was accomplished using a conventional Mach–Zehnder modulator (MZM). However, the MZM is not the most attractive solution to be employed in access networks as it remarkably increases the network cost. Directly modulated lasers (DMLs) have been proposed as an alternative and cost-effective solution to external modulation [14]. In this paper, we propose different techniques to deal with the challenge of DML-based networks associated with the degradation induced by the fiber dispersion and laser chirp on the double sideband optical signals [15,16]. Some PON architectures employing DMLs have been proposed in the recent past [17,18]. These works demonstrated successfully the transmission of high-data-rate traffic (≥10 Gbit∕s) in PONs reaching 20 km. In [19], the transmission of a 12 Gbit∕s adaptively modulated optical OFDM signal, occupying the band between 3 and 9 GHz, along a 100 km long PON was reached by using the favorable interplay conditions between the laser chirp and the fiber dispersion. Direct modulation has also been proposed for provisioning of wired–wireless services in access networks [16,20–22]. In [20], the transmission of a triple-play service [global system for mobile communications (GSM), WiMAX, and GbE] in a directly modulated PON reaching 26 km was experimentally demonstrated. In [21], a system comprising a 15 Gbit∕s baseband OFDM signal and three OFDM bands (maximum traffic rate of 11.2 Gbit∕s) for wired and wireless connectivity, respectively, was proposed and demonstrated in a 20 km long PON using one wavelength for the wired and another wavelength for the wireless signal transmission. This system requires two transmitters and, as a consequence, significantly increases the overall cost of the network, leading to some constraints for a deep market penetration. Contrarily, a low-cost PON system using direct modulation of off-the-shelf verticalcavity surface-emitting lasers (VCSELs) was proposed in

Fig. 1. Simplified scheme of a hybrid WDM LR-PON for provision of wireless and wired signals to users’ premises.


[22] for provisioning of the quintuple-play service to users’ premises. A maximum reach of 20 km was achieved in the proposed PON. In [16], the transmission of an OFDM-GbE signal for Internet connectivity and three UWB bands for wireless HDTV provisioning in LR-PONs reaching 100 km was demonstrated. Common to all these works on directly modulated PONs is that the performance degradation due to the fiber dispersion and the DML’s chirp is the main constraint. In this work, the transmission of a multi-octave bundle of wired and wireless OFDM-based signals providing the quintuple-play service, coexisting in a single wavelength, along a directly modulated WDM LR-PON is proposed, analyzed, and demonstrated experimentally. Fixed optical dispersion compensation, which is shared by all ONUs, is proposed to mitigate the degradation induced by the fiber dispersion and the DML’s positive chirp on the performance of the multi-octave bundle of OFDM signals. Compared with the work reported in [16], the fixed optical dispersion compensation, the suitable design of the electrical power distribution (EPD) between the coexisting multi-format OFDM signals applied to the DML, and the use of a dynamic centralized impairment compensation technique enabled by inserting a set of radio-frequency (RF) pilots close to each OFDM signal band for channel sounding are the main design tools used to achieve the successful provisioning of quintuple-play service to end-users located between 75 and 125 km away from the central office.

Alves et al.

II. EXPERIMENTAL SETUP Figure 2(a) shows the experimental setup used for the laboratory demonstration of the proposed WDM LR-PON architecture employing DMLs. As this work is a first proof of concept, the setup depicted in Fig. 2(a) emulates the WDM LR-PON of Fig. 1 in a single-end-user operation. As the proposed system is WDM-based, each user is served by a different wavelength. Therefore, multiple ONUs are easily addressed in the proposed WDM LR-PON architecture by using a set of optical sources with different wavelengths and an arrayed waveguide grating at the RN in order to separate the wavelengths delivered to the different ONUs. In addition, attention is focused only on the transmission impairments suffered by the downstream signals. Although only the downstream path is addressed, the analysis holds when the bidirectional operation is considered because the proposed LR-PON does not use a reflective solution at the ONU. Therefore, the impact of the reflective effects, such as the Rayleigh backscattering effect, on the quintuple-play service is negligible [23]. The bundle of OFDM signals offers the quintuple-play service to the end-users by employing two standard UWB channels, a LTE signal, a WiMAX signal, and a custom OFDM signal providing GbE connectivity. All the signals use quadrature phase-shift keying (QPSK) mapping, and the multi-format OFDM signals are transmitted in

Fig. 2. (a) Experimental setup emulating a WDM LR-PON employing DML for transmission of the bundle of OFDM signals. (b) Schematic diagram of the OFDM transmitter used to generate the different OFDM-based signals.

Alves et al.

their native format along the network without any type of transmodulation or up or downconversion in order to keep the network transparency and enable the integration of new standards in the free band in the near future [24]. The parameters of each OFDM format are shown in Table I. The spectrum of the OFDM-signal bundle occupies the frequency range between 1 and 4.8 GHz. The error vector magnitude (EVM) requirements at the transmitter antenna stated in the current LTE, WiMAX, and UWB standards are also shown in Table I as a reference. These EVM limits are defined in the standards as the maximum acceptable EVM at the wireless transmitter output and are used in this work as maximum acceptable EVM levels at the ONU output (before being radiated in the wireless network). The EVM limits are different for the LTE, WiMAX, and UWB wireless signals as follows: i) the different signals are used to support services requiring different quality; ii) the maximum wireless reach and, consequently, the robustness to the transmission impairments of each signal are quite different; and iii) the codes employed in the different wireless signals present different error correction capabilities. In the case of OFDM-GbE signal, the EVM limit corresponds to a bit error ratio (BER) of 10−4 (within the forward error correction threshold) in noiseimpaired systems. The wireless transmission of LTE, WiMAX, and UWB signals at the users’ premises is not considered in the experiments as the goal of this work is to verify if the quality of the OFDM-signal bundle after optical transmission along the directly modulated LR-PON is still acceptable. Meeting the EVM requirements at the transmitter antenna (ONU output) means that a conventional wireless link can be established with a full-standard receiver device. The different OFDM-based signals are generated and combined offline using Matlab. Figure 2(b) shows the schematic diagram of the OFDM transmitter used to generate each OFDM signal. After generation of the OFDM bundle, nine RF pilots are inserted close to the spectrum of each OFDM signal for dynamic channel sounding [12,13]. The schematic spectrum of the bundle of OFDM signals and of the RF pilots is depicted in Fig. 3. The information provided by the channel sounding is used to predistort the multioctave signal bundle. The electrical signal corresponding to the OFDM bundle is generated by an arbitrary waveform generator operating at 20 Gsamples∕s. The signal is converted to the optical domain by using a low-cost multi-quantum-well distributed-feedback laser characterized

Fig. 3. OFDM-based signal bundle and RF pilots inserted at the OLT for broadband channel sounding.


by a threshold current of Ith 8.1 mA, a bias current of Ib 30 mA, a chirp parameter of 2.6, frequency modulation efficiency of 20.2 MHz∕mA, and an intensity modulation (IM) response bandwidth of about 4 GHz [16]. An RF amplifier and a variable electrical attenuator are used to set the modulation index of the signal applied to the DML. The modulation index is defined as m I RMS ∕Ib − Ith , where IRMS is the root mean square (RMS) current of the signal applied to the DML. The double sideband multi-format OFDM signal is launched into the feeder standard single-mode fiber (SSMF), with a dispersion parameter of 17 ps∕nm∕km and an attenuation coefficient of 0.19 dB∕km. An average optical power of −1 dBm is imposed at the feeder fiber input in order to reduce the nonlinear transmission effects in the SSMF. A feeder fiber length of 75 km is considered between the OLT and the RN. At the RN, an optical amplifier compensates for the fiber loss and a dispersion compensating module (DCM), based on etalon technology, is used to adjust the residual dispersion of the link. The DCM used in the experiments is an off-the-shelf Civcom 10G multi-channel M-DCM device with full C-band coverage. In the case of a practical WDM LR-PON, if a channel spacing of 50 GHz is used and considering that a bandwidth of 32 nm is available in the C-band, then this DCM device is able to compensate simultaneously for the wavelengths used to serve 80 ONUs. A noise loader is used to set the optical signal-tonoise ratio, defined in a 0.1 nm reference bandwidth, to 30 dB. The wavelength router (which, in a WDM LR-PON, is used to separate the wavelengths delivered to the different ONUs) is implemented through an arrayed waveguide grating with −3 dB bandwidth of 0.6 nm. The average optical power at the input of the distribution fiber is −1 dBm. The distribution fiber connects the RN to the ONU, and different distribution fiber lengths, ranging between 0 and 50 km, are assessed. Thus, the total LR-PON reach under analysis ranges between 75 and 125 km. At the ONU side, a variable optical attenuator at the positive–intrinsic– negative (PIN) input is used to set the average optical power to −14 dBm regardless of the distribution fiber length employed. With this, similar balance between the amplified spontaneous emission noise power and the noise power introduced by the electrical part of the receiver is ensured in all the measurements. The signal is photodetected by a 10 GHz PIN with an integrated transimpedance amplifier. The photodetected signal is filtered (−3 dB bandwidth of 4.85 GHz), sampled by a real-time oscilloscope, and processed for EVM calculation. The amplitude of the RF pilots is also extracted at the ONU and sent back to the OLT to estimate the characteristic of the predistorter [12,13]. In a practical bidirectional network, the transmission of the information of the RF pilots from the ONU to the OLT can be performed using the bidirectional connectivity provided by the wired OFDM-GbE signal [13]. Among the different signals of the OFDM bundle, the UWB signals are expected to be the most impaired by the LR-PON limitations [12]. As UWB signals are used for audio/video broadcasting only, our attention is focused on the downstream path of the WDM LR-PON.


PARAMETERS

OF THE

TABLE I OFDM-GBE, LTE, WIMAX,

Parameters

III. OPTIMIZATION OF THE RESIDUAL DISPERSION MODULATION INDEX

WiMAX

UWB 2

UWB 3

1.5 1000 132 4 1.2 × 103 128 81 8 39 QPSK −11

2.6 31 71.4 × 103 4.7 × 103 33.3 2048 1190 12 846 QPSK −15.1

3.5 23 12.6 × 103 1.4 × 103 30.4 256 192 8 56 QPSK −20

3.96 528 312.5 70.1 640 128 100 12 16 QPSK −14.5

4.49 528 312.5 70.1 640 128 100 12 16 QPSK −14.5

AND

In order to keep the system cost as low as possible, the experimental setup of Fig. 2 was first implemented without using optical dispersion compensation (the DCM of the RN was removed). However, the experimental results obtained in that situation revealed that it is not possible to achieve Normalized intensity response [dB]

UWB SIGNALS

LTE

In this section, the design of the OFDM-signal bundle level applied to the DML is performed experimentally to identify the best operation point of the system in terms of balance between signal-to-noise ratio and distortion introduced by the DML nonlinearity. Our target is to ensure that EVM-compliant levels (the EVM limit of each standard is presented in Table I) are achieved in all the OFDM-based signals at the output of an ONU of a LRPON reaching, at least, 100 km of optical fiber. Additionally, as the positive chirp introduced by the DML affects the performance of the multi-format signal bundle when combined with fiber dispersion [16], rather than performing the optimization in back-to-back operation, the OFDMbundle level is optimized considering a distribution fiber of 25 km (the OLT–ONU distance is 100 km).

EVM-compliant levels in all the signals of the OFDM bundle (UWB bands were the most impaired signals). This is attributed mainly to the combined effect of the fiber dispersion and chirp introduced by the DML, and also due to the reduced IM response bandwidth of the DML. Figure 4 depicts the normalized intensity response of the directly modulated LR-PON with reach of 100 km and considering different residual dispersion levels. The residual dispersion is defined as the result of adding the accumulated (positive) dispersion of the transmission fiber and the (negative) dispersion introduced by the DCM. The intensity response was obtained by applying a sinusoidal signal to the 100 km long directly modulated LR-PON and by changing the dispersion-compensated level of the DCM. In addition, the frequency response of the link was obtained ensuring that the level of the input signal is low enough to keep all the generated distortion components below the noise level at the receiver side. Figure 4 shows that the intensity response of the link is remarkably affected by the residual dispersion. For instance, if the residual dispersion of the link is changed from 1700 ps∕nm (uncompensated case) to −500 ps∕nm, an amplitude gain close to 7 dB may be achieved at the frequency of 3.5 GHz (central frequency of WiMAX signal). Also, the amplitude gain exceeds 7 dB for signals located between 3 and 5 GHz if the residual dispersion of the link is changed from −300 to −500 ps∕nm. In a second step, and from the conclusions of Fig. 4, the performance of the bundle of OFDM signals was assessed considering the optical dispersion compensator included at the RN. Though the insertion of a dispersion compensator in the network increases the overall network cost, the additional cost of the compensator, installed at the RN, can be shared by all the users served by the WDM LR-PON. Hence, this is still a cost-effective solution.

~7 dB

3

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OFDM-GbE

Central frequency [GHz] Nominal bandwidth [MHz] Symbol duration [ns] Guard time [ns] Maximum bit rate [Mb/s] Number of subcarriers (fast Fourier transform size) Number of data subcarriers Number of pilot subcarriers Number of guard subcarriers Symbol mapping EVM standard limit [dB]

6 −600 ps/nm 4 2 0 −2 −4 −6 −500 ps/nm −8 1700 ps/nm −10 −12 −300 ps/nm −14 0 1 2

Alves et al.

4

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frequency [GHz] Fig. 4. Normalized intensity response of the 100 km long directly modulated LR-PON for different residual dispersion levels. Results obtained without dispersion compensation (continuous line) and with a residual dispersion of −300 ps∕nm (dashed line), of −500 ps∕nm (dotted line), and of −600 ps∕nm (dashed–dotted line).

Contrary to LR-PONs employing chirpless MZMs, in which the residual dispersion of the link should be set close to zero to maximize the mitigation of the dispersioninduced power fading [25], the positive chirp introduced by the DML is an additional distortion source and, consequently, the residual dispersion of directly modulated links that leads to optimized system operation must be carefully analyzed, as evidenced in Fig. 4. In addition, the quality of

Alves et al.


the quintuple-play service provided to the end-users is also affected by the EPD between the different OFDM-based signals of the signal bundle applied to the DML. In the following, the EVM of the multi-format OFDM signals coexisting in the directly modulated LR-PON of Fig. 2 is evaluated for different residual dispersion and modulation index levels, and considering the OFDM signals, combined using two EPD sets: i) a balanced situation, where the total power of the multiplexed signal is equally shared by the UWB, LTE, WiMAX, and OFDM-GbE signals, and ii) an unbalanced situation, where the power percentage attributed to UWB signals is 70%, LTE signal is 8%, WiMAX signal is 10% and OFDM-GbE signal is 12%. This unbalanced EPD set was selected because, from a coarse testing procedure where the performance of a few EPD sets was assessed, better performance of the OFDM signals was achieved. Compared to the balanced EPD, in the unbalanced EPD, part of the power of the LTE, WiMAX, and OFDM-GbE signals is transferred to UWB signals as they are the most affected by the transmission impairments and the limited laser bandwidth (4 GHz) [16]. It should be stressed that this EPD adjustment cannot be considered as a subcarrier power-loading technique, commonly employed in other OFDM-based optical communication systems [26,27]. This is because i) the subcarrier power loading technique uses information of the signal-tonoise ratio of a given received OFDM subcarrier to optimize the power level of that subcarrier at the transmitter side, and, in the network considered in this work, the information of the different subcarriers of the wireless signals is not accessible in order to keep the ONU as cheap as possible and to ensure network transparency, and ii) the subcarrier power-loading technique is not anticipated in the wireless standards.

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Fig. 5. Measured spectra of the bundle of OFDM signals at the output of (a) and (b) the RN and (c) and (d) the receiver LPF. Results obtained for (a) and (c) balanced EPD situation, and (b) and (d) unbalanced EPD situation. The total reach of the LR-PON is 100 km, the modulation index is 10%, and the residual dispersion of the link is −500 ps∕nm.

LR-PON is 100 km; the modulation index is 10%; and the residual dispersion, Cres , of the link is −500 ps∕nm (the DCM is adjusted to compensate for 2200 ps∕nm of dispersion). Additionally, the spectra measured in the balanced and unbalanced EPD situations are presented. Figures 5(a) and 5(b) show that i) the optical spectra are composed of the double sideband version of the bundle of OFDM signals, and ii) the higher frequencies of the multiplexed signal present higher amplitude levels as a result of the digital precompensation applied at the OLT side for compensation of the power fading induced by fiber dispersion and DML chirp (which affects the signal amplitude only after the photodetection process) and the PIN and receiver LPF bandwidth limitations. The comparison between the spectra at the output of the receiver LPF presented in Figs. 5(c) and 5(d) shows the impact of the two EPD approaches. It can be seen that, compared with the balanced case, part of the power of the LTE, WiMAX, and OFDM-GbE signals is transferred to UWB signals when the unbalanced EPD is considered. Figure 6 shows contour plots of the EVM of the different OFDM signals as a function of the modulation index and residual dispersion of the directly modulated LR-PON employing balanced power sharing. The inspection of Fig. 6 shows that the performance of the OFDM signals is remarkably dependent on the modulation index. This is due to the limitations by noise for low modulation indexes and by the combined effect of the residual dispersion and DML chirp, as they induce the generation of significant distortion components for high modulation indexes. In addition, Fig. 6 shows that the performance of the OFDM signal transmission in the compensated LR-PON is weakly affected by the residual dispersion level. Indeed, a 340 ps∕nm variation of the residual dispersion around the optimum residual dispersion level that minimizes the EVM of each OFDM-based signal leads to an EVM increase, in each signal, not exceeding 1 dB. This dispersion tolerance indicates that the proposed directly modulated LR-PON may be able to support ONUs located at SSMF distances varying 20 km with EVM degradation not exceeding 1 dB. It should be noted that negative residual dispersion levels are required in order to reduce the degradation effect of the positive chirp introduced by the DML. Figure 6 shows also that the EVM of UWB band 2 is slightly worse than the corresponding limit (−14.5 dB, as presented in Table I). UWB band 2 exhibits worse performance as a result of the centralized compensation approach and of the increase of signal degradation with frequency due to distortion caused by chirp and chromatic dispersion. The EVM of the other signals is quite below the EVM limits. The UWB band 3, LTE, WiMAX, and OFDMGbE signals present EVM margins of 2.7, 11.5, 5.1, and 3.9 dB, respectively. These results suggest that EVMcompliant levels may still be achieved in all the signals of the LR-PON employing DMLs provided that a fraction of the power of LTE, WiMAX, and OFDM-GbE signals is allocated to the UWB signals.


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harmonics and intermodulation distortion components due to the residual dispersion and chirp of the DML varies with frequency. Second, the combined effect of fiber dispersion and chirp introduced by the DML leads to an

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Figure 6 shows that optimized residual dispersion and modulation indexes cannot be simultaneously achieved for UWB, LTE, WiMAX, and OFDM-GbE signals. Two effects contribute to this behavior. First, the power of the

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(e)

Fig. 7. Contour plot of the EVM, in decibels, of (a) UWB band 2, (b) UWB band 3, (c) LTE, (d) WiMAX, and (e) OFDM-GbE signals as a function of the modulation index and of the residual dispersion of the LR-PON. Results measured in the unbalanced EPD situation.

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IM response that is translated to signal frequencydependent amplitude gains at the ONU side after photodetection [16].

Contrary to the variation of the intensity response of the link with the residual dispersion observed in Fig. 4, Figs. 6 and 7 show that the EVM of the different OFDM signals is weakly affected by the residual dispersion variation. This low dependence is an advantage enabled by the dynamic centralized impairment compensation technique that acts to counterbalance the received RF power dependence of the OFDM signals induced by the residual dispersion. For instance, if the RF power of the signals located at higher frequencies decreases (as is the case shown in Fig. 4 when the residual dispersion changes from −500 to −600 ps∕nm), the channel sounding realized by the RF pilots inserted close to the OFDM signal band detects that power decay. In that situation, the predistorter gain applied to the multi-format OFDM signal at the OLT side attributes higher gain to the OFDM signals located at higher frequencies (UWB signals) at the expense of a gain reduction of the signals at lower frequencies (OFDM-GbE signal) to maintain the same modulation index of the signal applied to the DML. However, Fig. 4 shows that, when the residual dispersion leads to an intensity response of the link that attenuates the higher frequencies, that intensity response presents, at the lower frequencies, gain relative to the higher frequencies. This gain balances the power reduction suffered by the signals located at the lower frequencies induced by the predistorter characteristic.

IV. PERFORMANCE OF THE MULTI-FORMAT OFDM-BASED SIGNALS FOR DIFFERENT LR-PON REACHES In this section, the EVM of the OFDM-signal bundle is evaluated experimentally for different LR-PON reaches and considering the unbalanced EPD introduced in Section III. Particularly, OLT–ONU distances of 75, 85, 100, 110, and 125 km are assessed considering the modulation index set to 10% and the DCM fixed to compensate for 2200 ps∕nm regardless of the LR-PON reach (it corresponds

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EVM [dB]

Figure 7 shows contour plots similar to the ones depicted in Fig. 6 but considers the unbalanced power sharing situation. The results of Fig. 7 confirm that EVMcompliant levels are achieved in all the OFDM-based formats, as predicted before. Additionally, Fig. 7 shows that, as in the balanced case, the performance of the OFDM signals when unbalanced power sharing between the multi-format OFDM signals is employed is also weakly affected by the residual dispersion. However, Fig. 7 shows that, contrary to the balanced case, ONUs located at SSMF lengths varying 20 km (residual dispersion variation of 340 ps∕nm) lead to EVM degradation exceeding 1 dB. On the other hand, and as also observed for the balanced power sharing situation, Fig. 7 shows that the optimum operation of each signal is not achieved for the same modulation index. However, it is possible to select a modulation index that allows obtaining a good compromise between the EVM of the different signals, as is demonstrated in Section IV.

UWB #2 UWB #3 LTE WiMAX OFDM−GbE

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LR−PON reach [km] Fig. 8. EVM of the multi-format OFDM-based signals as a function of the LR-PON reach.

to a residual dispersion of −500 ps∕nm when the total LRPON length is 100 km). Figure 8 depicts the EVM of UWB, LTE, WiMAX, and OFDM-GbE signals as a function of the LR-PON reach. Figure 8 shows that the EVM of the different signals remains almost unchanged (the EVM fluctuation is lower than 0.6 dB) for LR-PON reaches between 75 and 100 km. However, when the reach of the LR-PONs exceeds 100 km, a significant degradation of the EVM of UWB, LTE, WiMAX, and OFDM-GbE signals is observed. This degradation may achieve 2 dB if an LR-PON with 125 km is considered. This degradation behavior is attributed to the combined effect of the fiber dispersion and the chirp introduced by the DML and suggests that the degradation induced by this joint effect increases when the residual dispersion approaches zero (the residual dispersion of the 125 km long LR-PON is 125 km × 17 ps∕nm∕km− 2200 ps∕nm −75 ps∕nm). Nevertheless, the comparison between the EVM results of Fig. 8 and the EVM limits of the corresponding OFDM signal presented in Table I shows that EVM-compliant levels are achieved for directly modulated LR-PONs with maximum reach ranging between 75 and 125 km. For a LR-PON with 125 km, the EVM margins for UWB band 2 and 3, LTE, WiMAX, and OFDM-GbE signals are 1.8, 3.2, 4.0, 1.1, and 1.1 dB, respectively. These EVM margins indicate that proper wireless links can be established at the customer’s premises after radiation at the ONU, enabling the quintuple-play service provisioning employing full-standard low-cost end-user devices.

V. CONCLUSIONS Design of LR-PONs using multi-format OFDM-based signals and DMLs has been experimentally demonstrated. Adequate selection of the signal level applied to the DML, usage of a fixed optical dispersion compensator that can be shared by all ONUs, and unbalanced power sharing between the different OFDM signals have been demonstrated as suitable design tools to achieve EVM-compliant provisioning of two UWB channels, a LTE, a WiMAX, and an OFDM-GbE signal coexisting in a directly modulated LR-PON with maximum reach of 125 km.


It has been shown that the bundle of OFDM signals can be delivered to the users’ premises using the directly modulated LR-PON infrastructure with an EVM margin exceeding 1 dB for all the signals and for all LR-PON reaches (between 75 and 125 km) analyzed. These results suggest that LR-PONs employing DMLs are a powerful and costeffective alternative to LR-PONs using external modulation for the provision of the quintuple-play service to the end-users.

ACKNOWLEDGMENTS This work was supported in part by the Fundação para a Ciência e a Tecnologia from Portugal under the projects PEst-OE/EEI/LA0008/2011 and TURBO-PTDC/EEATEL/ 104358/2008. Support from Spanish National Plan projects TEC2009-14250 ULTRADEF and TEC2012-38558-C02-01 MODAL is also acknowledged. This work was also supported by the European FIVER-FP7-ICT-2009-4249142 project. M. Morant’s work is supported by the Generalitat Valenciana VALi+D postdoc program. Authors would like to thank José Morgado for providing valuable contributions to this paper.

REFERENCES [1] N. Cvijetic, A. Tanaka, Y. Huang, M. Cvijetic, E. Ip, Y. Shao, and T. Wang, “4 G mobile backhaul over OFDMA/TDMAPON to 200 cell sites per fiber with 10 Gb∕s upstream burst-mode operation enabling