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Proposals for Cost-Effectively Upgrading Passive Optical Networks to a 25G Line Rate Doutje T. van Veen and Vincent E. Houtsma Abstract—Bandwidth demands keep increasing, inspiring researchers to continue to find cost-effective ways to increase the line rate in PON systems. In this paper, we present and discuss proposals to upgrade 10G PONs with a 25 Gbps line rate at low cost and with low power consumption. Our proposals are based on binary NRZ and Duobinary based transmission which gained a lot of attention as solutions for upgrading PON to a 25 Gbps line rate in recent research papers as well as the standardization body IEEE. IEEE started standardizing 100G-EPON beginning of 2016. We report the latest results of our study of duobinary detection showing that it is a cost-effective technical feasible solution for 25G PON. Also, topics like forward error correction and power consumption are investigated in the context of 25G PON. Index Terms— Three-level duobinary modulation, Nonreturn-to-zero modulation, time division multiplexed passive optical networks, 25 Gbps line rate, forward error correction.
I. INTRODUCTION
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ECAUSE bandwidth demands are ever increasing, optical access researchers have always investigated ways to increase the line rate in optical access networks in costeffective ways. At the same time operators require compatibility of an upgraded passive optical network (PON) with the existing outside fiber plants for all the PON generations. So, the challenge to increase the bit-rate is to also support the same optical power budget as for the lower rate in a cost-effective way. Increasing the bit-rate means a lower dispersion tolerance, a lower signal-to-noise at the receivers and the need for more expensive premium optical parts. To overcome this challenge different solutions have been incorporated in PON, like using single mode distributed feedback laser (DFB) instead of multimode Fabry–Pérot (FP) lasers when rates became higher than 622 Mbps, introducing avalanche photodiodes (APD) instead of P-type/Intrinsic/N-type (PIN) based receivers for line-rates higher than 1 Gbps. And also, the introduction of Forward Error Correction (FEC) and electro-absorption modulated lasers (EMLs) when 10G-PON was standardized [1]. In 2011 The Full-Service Access Network (FSAN) Group concluded that increasing the line-rate beyond 10 Gbps was not cost-
D.T. van Veen is with the Fixed Networks Research Group, Nokia-BellLabs, Nokia, NJ 07974 USA (email:
[email protected]) V.E. Houtsma is with the Fixed Networks Research Group, Nokia-BellLabs, Nokia, NJ 07974 USA (email:
[email protected])
effective yet for a target deployment date of 2015 [2], and proposed for the first time to use the wavelength dimension in combination with the time dimension to increase the aggregate bitrate of the a PON, which was called Time and Wavelength Multiplexed-PON (TWDM-PON). In this PON 4 or 8 wavelengths, each carrying 10 Gbps, on a dense wavelength grid of 50-200 GHz are used to upgrade a 10 Gbps PON to 4080 Gbps. Such a system has the promise to be cost-effective because 10G technology can be reused. FSAN also decided to introduce tunability of the ONU transmitters to make them colorless, which potentially simplifies operation and maintenance of the PON [3]. Since then we learned that the requirements for the optical network units (ONUs) in the TWDM-PON are very stringent, for example tunablity over 4-8 times the grid-spacing, very high side-mode suppression to avoid crosstalk into neighboring channels, high output power to overcome insertion loss of the needed wavelength filter components, and the wavelength stability of bursted tunable dense-wavelengthdivision-multiplexed (DWDM) transmitters. These stringent requirements result in a relative high cost at short to medium term for the optical transmitter at the ONU even though it also partly uses mature 10 Gbps technology. In the meantime, researchers continued to investigate alternative schemes to achieve a higher aggregate rate PON. The main directions have been using coherent detection receivers [4,5] and high rate Time Division Multiplexed (TDM) PON using direct-detection receivers [6-12]. One type of coherent PON is an ultra-dense wavelength division multiplexing (UDWDM) PON [4], another coherent PON scheme still uses TDM [5]. The research on these high capacity PON schemes has targeted low-cost implementations. In [4] for example the authors propose a homodyne receiver which is significantly simplified since it does not make use of any DSP and uses thermally tuned DFB-lasers as the local oscillator. They achieved a 128-split PON with peak rate of 1.25 Gbps per user. One disadvantage of UDWDM-PON is that for each ONU a coherent transceiver is needed at the optical line termination (OLT) which increases the cost per user significantly. This disadvantage is addressed in the coherent TDM-PON proposals by using the time-domain. The technical challenge in this case is to implement a burst mode coherent receiver at the OLT, see for example [5]. Finally, the advantage of a TDM-based PON (coherent and directdetection) is that they support flexible bandwidth allocation and high user peak-rate. High peak rates are a requirement for
2 many operators. Our work and that of others on the topic has focused on TDM-based PON with direct-detection receivers, avoiding optical amplification at the ONU, and on using limited bandwidth optical components to enable 25G and 40G to keep the cost low, see for example [6-12]. In January 2016, the IEEE P802.3ca 100G-EPON Task Force [13] started to standardize 25/50/100 Gbps PON based on a 25 Gbps line-rate, also using direct-detection based PON. Recently in FSAN/ITU-T discussion about increasing line rates beyond 10 Gbps also started. In this paper, we will focus on using Non-Return-To-Zero (NRZ) and Electrical Duobinary (EDB) based direct-detection schemes as they have gained most traction at the current time due to their high potential to be a cost-effective and technical feasible way to implement a 25 Gbps linerate for PON. This paper is an extension of our invited conference paper [14]. We extended the conference paper by adding an overview of our latest experimental results on 25 Gbps PON based on EDB, we added new results from an investigation on high gain Forward Error Correction (FEC) to increase optical power budget, and we included a discussion on power consumption aspects in relation to the selection of a suitable 25G signaling scheme. II. AVAILABILITY OPTICAL COMPONENTS PON economics always targets maintaining or ideally reducing the cost per user when upgrading to the next generation. Therefore, the main reason why NRZ and EDB are considered for 25G line rate in PON is due to existing 4x25G Ethernet standards for data-centers. This standard has enabled a NRZ-based 25G optical and electronic component ecosystem. For EDB additionally even higher volume and mature 10G APD technology can be used to further lower cost. 10G APD technology is already used in 10G-PON ONUs. Most common 25G receiver technologies applied in datacenters are PIN-based. The 100GBASE-ER4 flavor of the standard supports 40 km and is the only version which would require APD-based or SOA/PIN-based optical receivers. However, since the 40 km version of the standard is mostly targeting enterprise core aggregation and service provider transport applications, the required volumes of 25G APD are currently very low and will remain relatively low in at least the medium term. This means that in the short and medium term the volumes of 25G APDs will likely not increase to the current 10G APD levels. Other factors that play a role are yield numbers, manufacturing and packaging cost of 25G APDs relative to 10G APDs. It is still a question if the market will drive the cost of 25G APDs down to an acceptable level for PON. Alternatively, using a Semiconductor Optical Amplifier (SOA) to optically pre-amplify a PIN-based receiver can be considered but this type of receiver has the disadvantage of a much higher power consumption due to needing a thermal electrical cooler (TEC). For lowest cost, the PIN and SOA need to be integrated, but also this technology is still very premium because of low volumes and the fact that the
technology is not very mature yet, like the 25G APD case. At short and medium term, the most cost-effective solution for 25G line rate in PON will likely be based on limited bandwidth parts, like high volume 10G APDs. III. DUOBINARY DETECTION The lowest complexity modulation scheme is NRZ, which has also been the only signaling scheme that has been standardized for PON so far. Upgrading 10G NRZ to 25G NRZ means that you need a wider bandwidth receiver and transmitter. For example, for lowest power consumption and to achieve a large enough optical power budget, you need a 25G APD. However, as mentioned in the previous section 25G APDs are still at very low volumes. The second lowest complexity modulation scheme is duobinary detection. In this scheme, you still transmit NRZ modulated light, but at the receiver the NRZ signal is detected and converted to a 3-level signal due to the low-pass filtering of the receiver. Duobinary signaling was proposed by A. Lender in 1963 [15]. We proposed to use duobinary detection in PON for the first time in 2012 [6]. It can be created by a delay-and-add filter, which we approximate by a low-pass filter with -3 dB point at ¼*bitrate in the form of the APD-based optical receiver in our duobinary detection scheme. The 3-level signal can be decoded using two threshold slicers and an exclusive OR gate (XOR) when differential encoding (precoding) is used at the transmitter [16], see also Fig. 1.
Fig. 1. Duobinary decoder based on XOR
Below we will summarize our experimental results on the technical feasibility of 25 Gbps duobinary detection without electronic equalization for PON. In all the shown experiments we used standard single mode fiber (SSMF) because this is the fiber which is required in all the currently standardized PONs. The first two experiments we show are in the C-band to investigate the chromatic dispersion tolerance of our duobinary detection scheme. In Fig. 2 we show results of using a10 Gbps EML (-3 dB at 9 GHz) and a 10 Gbps APD/TIA (-3 dB at 7.5 GHz) at 1556 nm. In the same figure, we also show the 10 Gbps NRZ performance using the exact same EML/APD combination. It can be seen that we have about 5 dB optical power penalty between 25G EDB and 10G NRZ using the same optical parts for the back-to-back case (B2B).
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Bit Error Rate [-]
EDB detection APD/TIA using a 10 Gbps EML at 1556 nm
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10
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10 -7 10 -8 10-9 10-10 10-11 10-12 10
Back-to-back After 10 km After 15km After 20 km After 30 km -34
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10 Gbps NRZ using same EML
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Received Optical Power [dBm] Fig. 2. B2B 25 Gbps EDB and 10 Gbps NRZ using 1556 nm 10G EML and 10G APD-based receiver. EDB also shown over 10, 15, 20, and 30 km SSMF.
This is the penalty that needs to be overcome to achieve the same optical power budget at 25 Gbps when using EDB. We note that the experimentally found penalty is just one dB off from the theoretical expected penalty between 10G NRZ and 25G NRZ of 4 dB. In addition, the 4.2 dB Chromatic Dispersion (CD) penalty for 20 km transmission, as shown in Fig. 2, needs to be mitigated. The 10G EML result can be improved by using a wider bandwidth transmitter to eliminate intersymbol interference (ISI) resulting from limited bandwidth (due to the 10G EML we used) or by moving the 25 Gbps transmission to the O-band for low dispersion operation with SSMF.
In Fig. 4 we show 25 Gbps EDB using 1308 nm O-band 10G EML with the same 10G APD over different lengths of fiber. Now the CD penalty becomes even insignificant as expected. In this figure we also show the BER after FEC for 40 km transmission. The B2B in the O-band has about 0.5 dB penalty compared to C-band. This is expected in the case of an APD based on InP. A longer O-band wavelength was not investigated due to unavailability of such 10G EML. But we expect
