In this paper the on-the-field performance of a WDM-PON optical access providing quintuple-play services using orthogonal frequency division multiplexing (OFDM) modulation is evaluated in a real fiber-to-the-home (FTTH) network deployed by Towercom operator in Bratislava (Slovakia). A bundle of quintuple-play services comprising full-standard OFDM-based signals (LTE, WiMAX, UWB and DVB-T) and an ad-hoc OFDM-GbE signal is transmitted in coexistence per single user. Both downstream and upstream transmission performances are evaluated in different on-the-field long-reach optical link distance configurations. Four wavelength multi-user transmission of quintuple-play OFDM services is demonstrated exceeding 60.8 km reach in standard single mode fiber.
© 2014 Optical Society of America
Next-generation optical access networks need to deal with high capacity services providing high bitrate per-user to a continuously increasing number of users. The use of orthogonal frequency division multiplexing (OFDM) modulated transmission has been proposed as a good solution to achieve long-reach transmission altogether with high capacity in fiber-to-the-home (FTTH) access networks . The radio-over-fiber (RoF) transmission of fully standard OFDM-based radio signals in coexistence with wired OFDM data signal enables the integration of wireless in the optical access, mutualizing network equipment at the central office. This simplifies network operation and increases the added-value for telecommunications operators . In this paper, an optical architecture based on converged optical access and in-building distribution forming a deep FTTH access network as depicted in Fig. 1 is proposed. The integration of the in-building optical distribution with the FTTH network enables an extended capacity of the access network . Also, from the operator’s point-of-view, deep FTTH decreases the OpEx costs by reducing the active elements in actual cooper-based access networks . In this work, several OFDM-based radio signals are transmitted simultaneously including full-standard 3GPP long term evolution (LTE), IEEE 802.16 worldwide interoperability for microwave access (WiMAX), ECMA-368 ultra-wide band (UWB) and ETSI terrestrial digital video broadcasting (DVB-T). An ad-hoc I/Q OFDM signal is transmitted in coexistence with these fully standard radio signals forming a OFDM bundle to provide quintuple-play services comprising broadband wired Internet, voice, high-definition TV, wireless data and home security services. The proposed access architecture follows next-generation long-reach PON (NG-PON2) definition proposed by FSAN . Using this approach, the optical network termination (ONT) is simplified as no re-modulation or frequency upconversion is required. At the ONT, the wireless signals are radiated following the relevant standard in the corresponding regulated frequencies and received by fully standard devices. In this paper, the on-the-field operation of a multi-user wavelength division multiplexing passive optical network (WDM-PON) with integrated in-building optical and electrical distribution is demonstrated in a real FTTH network.
2. On-the-field demonstration
The FTTH network employed for the on-the-field demonstration is deployed in Bratislava by Towercom operator and is currently providing service to approximately 14.000 households. Figure 2 shows the map of the on-the-field tests where the OLT is located at Towercom tower premises –Fig. 2(a)– and it is connected to the on-the-field fiber distribution –Fig. 2(b)– using B-Lite MB SP1358 microcable containing 12 micro-bundles of 12 fibers each –Fig. 2(c)–. At the distribution point –Fig. 2(d)–, the signal is routed to a test flat in Bratislava central district where an open-house demonstrator –Fig. 2(e)– shows the quintuple-play functionalities as represented in Fig. 2(f).
Figure 3 shows the experimental setup installed in the operator premises for evaluating the performance of the RoF provision of quintuple-play services using the integrated FTTH and in-building network. The quintuple-play bundle comprises the following OFDM signals:
- • Two neighboring DVB-T channels generated with Ikusi MAC-401 generators at central frequencies 762 MHz and 770 MHz (UHF ch57 and ch58) configured according to ETSI standard with 8 MHz bandwidth, 8k 64QAM carriers, 1/32 guard interval and 7/8 code rate . Each DVB-T channel provides four TV programs.
- • An OFDM-GbE signal generated with an I/Q OFDM modem developed by Fibernova Systems S.L. to provide Gbit Ethernet (GbE) capabilities. The OFDM-GbE signal at 1.5 GHz occupies 1 GHz bandwidth . This signal provides bi-directional GbE connectivity and also includes the upstream management data.
- • A 3GPP LTE signal, working in frequency division duplex (FDD) , is generated in the 2.6 GHz band with an Agilent E4438C electrical signal generator (ESG).
- • An IEEE 802.16 WiMAX signal  at 3.5 GHz, initially generated with a E4438C and then with a Ruggedcom RuggedMAX WiN7200 base station for wireless data and security provision via a wireless security camera installed at customer premises.
- • An ECMA-368 UWB channel at 3.96 GHz with 528 MHz bandwidth  generated with a Wisair transmitter. UWB provides HD audio and video broadcasting.
With this configuration, the aggregated downstream (DS) bitrate per user is 1.45 Gbit/s. Figure 4(a) shows the generated electrical spectrum for DS –measured at point (1) of Fig. 3– that includes extra RF-pilots in the free-spectrum for channel sounding monitoring .
The different OFDM services are combined considering the specific power levels which minimizes mutual interference. The bundle is pre-compensated to minimize the crosstalk and improve the performance after long-reach optical transmission using the information extracted from the RF-pilots and included in the upstream (US) OFDM-GbE, as proposed in . Figure 4(b) shows the US spectrum from the bi-directional OFDM-GbE, LTE and WiMAX signals, measured at point (2) of Fig. 3. According to the regulated wireless bands, the LTE US signal is located at 2.57 GHz and the WiMAX US at 3.47 GHz.
A multi-user demonstration is performed using 4 Mach-Zehnder modulators operating at quadrature bias point fed by continuous wave lasers in the 100 GHz ITU grid at optical channels ch33 (1550.92 nm), ch34 (1550.12 nm), ch35 (1549.32 nm) and ch36 (1548.51 nm). These four wavelengths are combined by a thin film dense wavelength division multiplexer with 100 GHz (0.8 nm) channel spacing. For the upstream direction, a directly modulated laser (DML) working at the optical channel ch43 (1542.94 nm) is used. The DML has 4 GHz bandwidth and threshold Ith = 8.1 mA and bias Ibias = 30 mA currents (Fitel FRL15DDAA).
At the remote node (RN) optical amplification using Erbium doped fiber amplifier (EDFA) is included following FSAN long-reach extension definition. Optical amplification can be avoided in short reach fiber deployments. Optical circulators are used to address the downstream and upstream paths. At the RN, each user wavelength is demultiplexed to a distribution fiber till customer premises.
As it can be observed in Fig. 1, an adapter is located at customer premises to connect the access network to the in-building optical and electrical distribution networks. As depicted in detail in Fig. 3, the adapter comprises a 3 dB optical power splitter to connect both the optical and the electrical in-building networks. From one side, for the optical in-building network, the adapter is directly connected to the in-home optical distribution comprising 100 m of Corning ClearCurve® bend-insensitive single mode fiber (BI-SMF). The optical in-building distribution arrives to the ONT where the OFDM-GbE is received by the I/Q OFDM modem including filtering (VLF-400+ and VHF-880+) and the LTE, WiMAX and UWB signals are filtered (VHF-1500+) and radiated together to the final user (ensuring that the UWB spectral mask is met at the antenna). In addition, the signal quality is measured at the ONT with a real-time oscilloscope Agilent DSO91304A to ensure that the EVM at the transmitter antenna meets the wireless standard requirements. From the other side, for the electrical in-building distribution, the DVB-T signal is photodetected, filtered, amplified and directly transmitted along the legacy coaxial network available in the building. In the on-the-field experiments, 20 m of 75 Ω coaxial network is used. This approach enables the simultaneous low-power distribution over coaxial to supply power to USB devices . The signal is received with a DVB-T distribution and low-power micro-module (DDPM) and measured using R&S ETL TV analyzer. Low-power voltage (5.5 V) is distributed in the coaxial cabling supplying power to a DVB-T receiver that can be connected to a legacy analog TV set.
3. Experimental results
3.1 Multi-user performance evaluation
The performance of the network in multi-user operation is evaluated using four downstream wavelengths following the 100 GHz grid. Figure 5(a) shows the optical spectrum measured at the OLT –point (3) of Fig. 3–. The quality of the OFDM-based signals is measured at customer premises after 10.8 km FTTH access network. The OFDM-GbE and the full-standard wireless signals are received at the ONT after 100 m of BI-SMF. The DVB-T signals are transmitted through 20 m of coaxial cabling and connected to a television to receive the TV programs. The EVM of the signals measured in each downstream wavelength in simultaneous transmission is represented in Fig. 5(b) and we can observe only small deviations between the different wavelengths. The maximum EVM deviation was obtained for OFDM-GbE signal with a difference of 7.1% between optical channels ch35 and ch33 due to the different laser performance and transmitter optical power levels as it can be observed in Fig. 5(a) optical spectrum. This difference is smaller in WiMAX signals where the EVM difference for the same channels is only 2%.
3.2 Long-reach performance evaluation
The performance of the quintuple-play service provision when the network reach increases was also evaluated by measuring the quality of the OFDM-based signals for different optical links (L). The feeder and distribution fiber links were configured to have different FTTH network distances: L = 10.8 km comprising Lfeeder = 5.4 km + Ldistribution = 5.4 km with installed on-the-field fiber losses of 4.75 dB; L = 35.8 km with Lfeeder = 25 km + Ldistribution = 10.8 km and 12.5 dB total losses; L = 60.8 km with Lfeeder = 50 km + Ldistribution = 10.8 km having 20.79 dB losses; and L = 85.8 km formed by Lfeeder = 75 km + Ldistribution = 10.8 km giving 28.72 dB losses.
The measurements were done in multi-user configuration by transmitting simultaneously four downstream signals as shown in Fig. 5(a). For simplicity, Fig. 6 shows the measured EVM in the DS path at the center optical channel ch35 (wavelength λDS = 1549.32 nm) for the different optical links (L). The performance of the other channels follow the same behavior as depicted in Fig. 5(b). As shown in Fig. 6(a), all the wireless signals analyzed at the ONT meet the quality requirements defined at the antenna by the different wireless standards (EVMLTE<12.5% , EVMWiMAX<6% , EVMUWB<18.84% ) after 60.8 km of SSMF and integrated 100 m BI-SMF in-building distribution. In addition, the OFDM-GbE measured bit error rate (BER) was 5.5·10−3 for this distance.
At the DDPM, the DVB-T signals are also received correctly after 60.8 km FTTH network with integrated in-building electrical distribution of 20 m legacy coaxial with a EVMDVB-T<5.83% (which gives a modulation error ratio MER<21 dB at the TV plug). The measured DVB-T BER was 1.6·10−6 and the audio and video programming was received properly at the TV. Figure 6(b) shows the measured DS constellations for all the services after 60.8 km of optical transmission.
In the US, the signals are detected with a 10 Gb/s APD-TIA receiver. Figure 7 shows the US results and received constellations. Adequate performance of the upstream operation is confirmed after 60.8 km enabling bi-directional communication through the network.
This paper reports the first on-the-field WDM-PON multi-user transmission of quintuple-play OFDM-based services in a real FTTH network of Towercom operator in Bratislava (Slovakia). Radio-over-fiber transmission in deep FTTH access networks enables the provision of an ad-hoc I/Q OFDM with GbE capabilities with simultaneous full-standard OFDM-based signals (DVB-T, LTE, WiMAX and UWB) meeting the wireless spectral mask with EVM-compliant levels at the transmitter antenna.
Multi-user operation is demonstrated in a four downstream wavelength configuration operating in the 100 GHz ITU grid with excellent performance. The experimental results confirm the successful service provision at 60.8 km optical reach including integrated optical and electrical deep in-building distribution including 100 m BI-SMF and 20 m for DVB-T distribution in the coaxial cabling already deployed at costumer premises.
This work was partly funded by the European Commission FP7 ICT-4-249142 FIVER project. Support from Spain National Plan project MODAL TEC2012-38558-C02-01 and Generalitat Valenciana VALi+D postdoc program APOSTD/2013/030 is acknowledged.
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