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Bidirectional Incoherent 16QAM Transmission over Hybrid WDM/TDM Passive Optical Network Nikolaos Sotiropoulos, Student Member, IEEE, Ton Koonen, Fellow, IEEE Huug de Waardt, Member, IEEE COBRA Research Institute, Eindhoven University of Technology Den Dolech 2, P.O. Box 513,5600 MB Eindhoven, The Netherlands Tel: (+31) 402472091, Fax: (+31) 40 2455197, e-mail: [email protected] ABSTRACT There is growing demand for higher bit rates in the access domain, due to the emergence of bandwidth-hungry applications, such as Video-on-Demand and file sharing. Incoherent QAM modulation used in a hybrid WDM/TDM passive optical network (PON) is proposed as a way to achieve the necessary high bit rates. The use of multilevel modulation allows for the use of low-speed electronic and optical components and is scalable to very high bit rates (e.g. 100 Gb/s) for future applications. Along with the resource sharing and the scalability of the network architecture, this leads to a cost-efficient and future-proof solution for delivering broadband applications to end users. In particular, bidirectional transmission of a 10 Gb/s incoherent 16QAM signal has been modelled in VPI. The performance of the system in terms of BER as a function of the received power has been evaluated. Also, the impact of the input power of the upstream signal is examined. The results indicate that incoherent 16QAM is an attractive option for future optical access networks. Keywords: optical multilevel modulation, optical QAM, passive optical networks, WDM PON, TDM PON, broadband access. 1. INTRODUCTION The explosive growth of Internet has led to a demand for high bit rates for home and office users and the increasing popularity of video-related services means that even higher bit rates will be required in the near future [1]. Time Domain Multiplexed (TDM) PONs are a cost-efficient solution for providing high bandwidth to users. Combined with Wavelength Division Multiplexing (WDM), very scalable and flexible network architectures are possible [2]. The main disadvantage of TDM PONs is that, since users share the available bandwidth, the total system bit rate is the sum of the bit rate guaranteed to each user. The increasing demand for higher user bit rates means that the system bit rate will need to be very high, requiring expensive electronics and optoelectronics at the Central office (CO) and the Optical Network Unit (ONU). An interesting solution to this problem is the use of advanced multilevel modulation techniques. In contrast to the conventional OOK signal, in multilevel signals each transmitted symbol has M possible values, carrying log2M bits of information. This eases the speed requirements on electronic and optical components by the same factor (log2M). As the next generation access networks will move to bit rates higher than 10 Gb/s, this reduction can prove crucial for the development of cost-efficient, very-high-speed PONs. In order for the switch to multilevel modulation to be meaningful, the symbol rate should be at least four times lower than the bit rate. The two most obvious ways to achieve 4 bits/symbol are Differential Quadrature Phase Shift Keying with polarization multiplexing (DQPSK-POLMUX) and 16 Quadrature Amplitude Modulation (QAM). In DQPSK-POLMUX, two bits are encoded into one of the four possible phase differences between two consecutive symbols and two DQPSK signal are multiplexed in the polarization domain to achieve 4 bits/symbol transmission. DQPSK is known to be a very robust modulation format [3], but the implementation of polarization multiplexing in the access domain can be challenging. For that reason, we have focused in 16QAM modulation and in particular incoherent QAM, which does not require a local oscillator at the receiver. Incoherent 16QAM is an extension of DQPSK, encoding four bits into the phase difference of two consecutive symbols and in the amplitude of each symbol. The organization of the remainder of the paper is as follows: in Section 2, we describe in detail incoherent 16QAM and suitable transmitter and receiver structures. In Section 3, the network architecture used in the simulations is described and the results obtained are presented. Finally, in Section 4 a discussion of the results is made and some indications of our future work on the topic are given. 2. INCOHERENT QAM QAM is a modulation format widely used in electrical (wired and wireless) communications. It is a form of combined amplitude/phase modulation, in which the symbols are located at the edges of squares, as it can be seen in Fig. 1. This arrangement of symbols yields almost-optimal Euclidean distance compared to other amplitude/phase modulation formats, which means that there is lower probability of error for the same received ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ This work has been funded by the Dutch national project MEMPHIS and the NRC Photonics grant.

978-1-4244-779-/10/$26.00 ©2010 IEEE

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power and noise levels. Therefore, QAM is the most robust format for increasing the spectral efficiency of optical communication systems.

Figure 1. 16QAM constellation diagram. In electrical communications, QAM is demodulated coherently, i.e. a local oscillator at the receiver is mixed with the incoming signal and the absolute phase of the received signal is available. In an optical access network, however, it is highly desirable that the ONU is laser-free, so the demodulation of the QAM signal must be performed incoherently. In that case, only the phase difference between two consecutive symbols is known. This means that the information must be suitably encoded before transmission, similarly to DQPSK. This encoding is called phase pre-integration, first introduced in [4] and transported in the optical domain in [5]. For the kth QAM symbol, the pre-integration operation is:

ak e jφk → ak e j(φk +φk −1) the amplitude remains the same, but the phase of the previous symbol is added to that of the current one. The main functionalities of an incoherent QAM transmitter are shown in Fig. 2a. After a QAM encoder creates QAM symbols from incoming bits, phase pre-integration is performed. The signal is then split into its real and imaginary part (In-phase and Quadrature components). Both I and Q components are predistorted, to account for the nonlinear transfer characteristic of the modulators. The modulators used are two single-drive Mach-Zehnder modulators. After a pi/2 phase shift in the Q path, the two signals are combined to create the transmitted incoherent QAM signal. RZ carving can then be performed by an extra MZ modulator, if necessary. The incoherent QAM receiver is shown in Fig. 2b. Incoherent modulation formats use interferometric-based demodulation. An interferometric receiver consists of two asymmetric Mach-Zehnder interferometers, followed by two balanced detectors. Each MZ interferometer has a time delay equal to one symbol period and a phase shift of 0 and pi/2 respectively. A separate photodiode is also needed, to produce an estimate of the amplitude of the received signal. The output currents of the balanced detectors are proportional to cos ∆φ ,sin ∆φ , where ∆φ is the phase difference between the current and the previously received symbol. Using these two metrics, the inverse tangent is computed and an estimation of the phase difference is produced, which is an estimation of the phase of the original QAM symbol, before the pre-integration operation. Since amplitude estimation is also available, from the single photodiode, the transmitted symbol can be reconstructed at the receiver. A QAM decoder then maps the complex symbol into the four corresponding bits.

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Figure 2a. Incoherent QAM transmitter.

Figure 2b. Incoherent QAM receiver.

3. SIMULATION SET-UP AND RESULTS 3.1 Simulation Set-up VPI was used to model bidirectional transmission of a 10 Gb/s incoherent 16QAM signal over a WDM/TDM PON with 20 km of fiber and splitting ratio of 32. The network configuration can be seen in Fig. 3. There is central provision of unmodulated carriers from the CO which are modulated by the upstream data, allowing for a source-less, cost-efficient ONU. The downstream signal and the carrier are multiplexed and transmitted over 20 km of feeder fiber. After the fiber, the signals pass through a WDM demultiplexer, which forwards them to the TDM PON they serve. In the TDM PON, the signals pass through a passive splitter and reach the ONU. There, both signals are amplified by a SOA (chosen because of its suitability for integration) and a filter separates the

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downstream signal, which is led to the receiver, and the carrier, which is led to the transmitter. The carrier is modulated by the upstream data and the upstream signal then passes through the same path and reaches the CO, where it is detected and demodulated.

Figure 3. WDM/TDM PON schematic.

Figure 4. Received constellation diagram.

3.2 Results The first aspect of the system design that was investigated was the optimal power of the unmodulated carrier. As the upstream signal is more sensitive than the downstream, the carrier power was optimised for the upstream transmission. With a constant downstream input power of -8.6 dBm at the fiber, resulting to a BER of around 10 −11 , the upstream BER as a function of the carrier input power is shown in Fig. 5. The increase of the carrier power leads to improved BER for the upstream signal, until the nonlinear effects start to degrade the signal at around 1.5 dBm input power. The lowest BER occurs for 0 dBm and is 5×10-7, without FEC. Using the above specified values, the BER as a function of the received power was evaluated, for laser linewidth values of 200 kHz. Since the transmitter and the receiver were custom-made (not part of the VPI library), it was not possible to use the BER estimation tools of VPI and error counting leads to prohibitive simulation times. The approach we followed was to use the EVM of the received signal as an estimation of its SNR, and use the SNR to obtain a BER value assuming a Gaussian distribution of the noise. In reality, the noise is not Gaussian, as it can be observed in the received constellation diagrams (Fig. 4), but it affects more intensely the phase of the received signal. This leads to an underestimation of the error probability, especially for low received power. However, the results are comparable with experimental results (e.g. 40 Gb/s 16QAM transmission over 200 km [6]), which leads us to consider the Gaussian approximation as a good first indication of the system performance for medium and high receiver power, until experimental results become available. The BER curves for back-to-back and 20 km of fiber are shown in Fig. 6. The power penalty for the downstream signal is negligible. For the upstream signal, however, there is larger power penalty. It is mainly caused by the nonlinear interactions in the fiber, with SPM and XPM being the most important, as the signal is phase-modulated. Further simulations, with the nonlinear effects turned off, showed that the power penalty becomes negligible in that case. Besides the penalty because of the fiber transmission, a significant degradation of the performance for the upstream signal is observed, compared to the downstream. The reason for this degradation is the second amplifier that lies in the upstream path, which adds noise to the signal. When the noise from the preamplifier at the CO is turned off, the BER of the upstream signal is comparable to that of the downstream, with a slight degradation caused by the double transmission distance. This is expected, since the tight packing of the QAM symbols in the signal space, compared to OOK and QPSK, means that QAM signal is more sensitive to noise. However, even with this sensitivity degradation, error-free operation for the upstream signal can be achieved, as the BER can be well below the FEC limit, which is around 10−3 . -2

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Figure 6. BER vs. received power.

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4. CONCLUSIONS In this paper, the performance of incoherent 16QAM in the context of a WDM/TDM passive optical network was investigated through extensive simulations in VPI. The results indicate that error-free operation can be achieved for both downstream and upstream signals. The optimal input power for the unmodulated carrier was found and the reasons for the power penalties observed in the upstream signal were investigated and identified. The results obtained indicate that incoherent QAM is an attractive option for future high-speed optical access networks. The reduction in bandwidth requirements in components could lead to more cost-efficient implementations of 10 Gb/s PONs. Moreover, new applications (such as 3D TV) are foreseen to require a move to 40 and eventually 100 Gb/s access networks. Incoherent QAM can be easily scaled to such bit rates, whereas OOK systems are impractical at such high bit rates. The proposed modulation format has two main drawbacks. The first one is that it requires more complex transceivers, compared to OOK. However, the increasing popularity of multilevel modulation and the continuous improvement of optical integration technology can lead to mass-produced, compact and low-cost QAM transceivers. The second limitation is the requirement of low-linewidth lasers. This limitation is overcome by the laser-free design of the ONU and the resource sharing enabled by TDM. In the network architecture proposed, only two lasers are needed to serve the 32 users of each TDM PON. The next step of our work is to experimentally validate the results obtained through simulations for 10 Gb/s transmission. Future work will include investigation of the potential for 40 and 100 Gb/s transmission in an access network. Furthermore, ways to improve the performance of the system to enable a higher splitting ratio and higher bit rates will be explored. In that direction, the implementation of the Maximum Likelihood Differential Sequence Detection algorithm looks particularly promising. REFERENCES [1] Heron, R. W.; Pfeiffer, T.; van Veen, D. T.; Smith, J.; Patel, S. S.; , “Technology innovations and architecture solutions for the next-generation optical access network,” Bell Syst. Tech. J., vol. 13, pp .163-181, 2008. [2] Grobe, K.; Elbers, J.-P.; , “PON in adolescence: from TDMA to WDM-PON,” Communications Magazine, IEEE , vol. 46, no. 1, pp. 26-34, Jan. 2008. [3] Wree, C.; Leibrich, J.; Eick, J.; Rosenkranz, W.; Mohr, D.; , “Experimental investigation of receiver sensitivity of RZ-DQPSK modulation using balanced detection,” Optical Fiber Communications Conference, 2003. OFC 2003 , pp. 456- 457, vol.2, 23-28 March 2003 [4] Simon, M.; Huth, G.; Polydoros, A.; , “Differentially Coherent Detection of QASK for Frequency-Hopping Systems--Part I: Performance in the Presence of a Gaussian Noise Environment,” Communications, IEEE Transactions on , vol. 30, no. 1, pp. 158-164, Jan. 1982. [5] Kikuchi, N.; Sasaki, S.; , “Incoherent 40-Gbit/s 16QAM and 30-Gbit/s staggered 8APSK (amplitude- and phase-shift keying) signaling with digital phase pre-integration technique,” Digest of the IEE/LEOS Summer Topicals 2008, pp. 251-252. [6] Mori, Y.; Chao Zhang; Igarashi, K.; Katoh, K.; Kikuchi, K.; , “Unrepeated 200-km transmission of 40Gbit/s 16-QAM signals using digital coherent optical receiver,” Opto-Electronics and Communications Conference, 2008 and the 2008 Australian Conference on Optical Fibre Technology, OECC/ACOFT 2008, pp. 1-2, 7-10, July 2008.

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