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Hybrid Modulation Formats for Advanced Datacommunications J. D. Ingham, J. L. Wei, R. V. Penty, and I. H. White University of Cambridge, Department of Engineering, Electrical Engineering Division 9 J. J. Thomson Avenue, Cambridge, CB3 0FA, United Kingdom Tel: +44 1223 748354, Fax: +44 1223 748342, e-mail:
[email protected] ABSTRACT Optical datacommunication links are under active consideration at rates beyond 40 Gb/s, especially at 100 Gb/s. The development of solutions that are compatible with constraints such as low-bandwidth optical sources requires advanced modulation formats. This talk will concentrate on hybrid schemes, for example combining conventional PAM modulation with an orthogonal channel taken from a CAP system. Recent real-time experimental results at 100 Gb/s will be described. Keywords: fibre optic communication systems; modulation formats. 1. INTRODUCTION Local-area network and storage-area network traffic continues to increase and this motivates the development of new datacommunication standards. In Ethernet, the IEEE P802.3bm Task Force is currently finalising the development of a second generation of 40 Gigabit Ethernet (GbE) and 100 GbE optical links. Furthermore, the IEEE P802.3bs Task Force has begun investigation of solutions for 400 GbE transmission. Advanced modulation formats offer the potential to achieve high-speed transmission despite bandwidth limitations imposed by components such as directly-modulated lasers, electro-optic modulators and photodiodes. Hybrid approaches that use a modulation format such as pulse-amplitude modulation (PAM), in conjunction with an orthogonal channel, for example from a carrierless amplitude and phase (CAP) system, are of considerable interest. Pulse shaping can be achieved using components such as transversal filters or exclusive OR (XOR) gates. 2. EXAMPLE HYBRID APPROACH FOR 100 Gb/s TRANSMISSION As an example of a hybrid approach, we will describe a real-time demonstration at 100 Gb/s using a 66.6 Gb/s PAM channel with four levels (PAM4) combined with a 33.3 Gb/s biphase channel with two levels, designated as CAP2 [1]. Figure 1 shows representative simulated eye diagrams, at the output of the transmitter, assuming a Mach-Zehnder modulator with a -3 dB bandwidth of 50 GHz.
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Figure 1: (a) PAM4 channel at 66 Gb/s; (b) CAP2 channel at 33 Gb/s. Figure 2 shows the experimental set-up for the real-time demonstration.
Figure 2. Experimental set-up for 100 Gb/s transmission. 978-1-4799-5601-2/14/$31.00 ©2014 IEEE
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ICTON 2014
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In the experiment, a 33.3 Gb/s pseudo-random binary sequence is generated by a pulse-pattern generator (PPG). The 27 – 1 PRBS is chosen to model the short-run-length line codes used in datacommunication links. One of the PPG outputs provides the data input to a high-speed XOR gate. The clock input to this XOR gate is a 33.3 GHz sinusoid, synchronized to the pattern generator clock. The CAP2 channel at 33.3 Gb/s is formed from the output of the XOR gate. The remaining pattern generator output forms the PAM4 channel at 66.6 Gb/s, by means of RF splitters, an RF phase shifter and a 6 dB attenuator. Figures 3(a) and 3(b) show the encoded PAM4 and CAP2 eye diagrams, respectively. Figure 3(c) shows the combined signal. This combined signal forms the electrical drive signal to a MZM with a -3 dBo bandwidth of approximately 35 GHz. The MZM modulates the output of a CW laser, at a wavelength of 1548 nm. After transmission over 500 m of standard single-mode fibre, a high-speed photodiode detects the signal. An erbiumdoped fibre amplifier (EDFA) is used as a preamplifier. DECODING
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Figure 3. Eye diagrams for 100 Gb/s hybrid PAM4-CAP2 transmission (approximately 6 ps/div). As examples of the decoding process, Fig. 3(d) shows the result of decoding the CAP2 signal and Fig. 3(e) shows the result of decoding the PAM4 signal. The CAP2 signal is decoded using a 2-tap transversal filter, assembled from discrete RF amplifiers, phase shifters and combiners, which is configured as a matched filter. The components of this filter have bandwidths of approximately 40 GHz. The tap coefficients of the filter are approximately 1 and -1, with a tap spacing of 15 ps. The resulting eye diagram is clearly open. The PAM4 signal is decoded using a low-pass filter with a bandwidth of 25 GHz. The resulting eye diagram would be expected to meet the BER requirements of forward-error correction [2]. 3. CONCLUSION Hybrid modulation formats are important candidates for next-generation optical datacommunication links. Realtime experimental results indicate the potential for a 100 Gb/s link, using a combination of PAM and CAP modulation. ACKNOWLEDGEMENTS This work was supported by the UK EPSRC project “INTERNET” and Avago Technologies. REFERENCES [1] J.D. Ingham, R.V. Penty, I.H. White, and D.G. Cunningham, “100 Gb/s PAM4-CAP2 real-time modulation of a single optical source for next-generation datacommunication links,” in Proc. Optical Fiber Communication Conference 2014, San Francisco, CA, USA, paper W1F.1, Mar, 2014. [2] ITU-T Recommendation G.975.1, Apx I.9, 2004.
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