AW4G.1.pdf
ACP/IPOC 2013 © OSA 2013
AW4G.1.pdf
ACP/IPOC 2013 © OSA 2013
High Performance Optical Data Links using Hybrid CAP/QAM Transmitter/Receiver Scheme 1
J. L. Wei1, J. D. Ingham1, Q. Cheng1, D. G. Cunningham2, R. V. Penty1, and I. H. White1 Centre for Photonic Systems, Electrical Engineering Division, Department of Engineering, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 0FA, UK. 2 Avago Technologies, Framlingham Technology Centre, Station Road, Framlingham, Suffolk, IP13 9EZ, UK. E-mail:
[email protected]
Abstract: We experimentally demonstrate the first optical data link at 20Gb/s using hybrid CAP4/QAM-4 with transmission over 4.3km SSMF and a power penalty ~1.5dBo at BER=10-9. The hybrid CAP-4/QAM-4 link significantly outperforms a reference PAM-4 link. OCIS codes: (060.2330) Fiber optics communications; (060.4080) Modulation
1. Introduction The need for low power consumption in the information and communications technology (ICT) sector has gained importance as the throughput of the internet continues to rise exponentially. Power efficient communication systems are therefore critical for future internet networks. In response to this issue, several promising modulation techniques including pulse amplitude modulation (PAM) [1-3], carrierless amplitude/phase (CAP) modulation [2-6], and optical orthogonal frequency division multiplexing (OFDM) [2,5] have been investigated and compared to identify cost efficient and power efficient solutions for scenarios such as datacentre optical interconnects [5] and single laser 100 Gigabit Ethernet links[1-3,7]. CAP modulation exhibits important advantages including its ability to be implemented using analogue circuits such as transversal filters for waveform shaping and modulation without the need for advanced and power hungry digital signal processing (DSP) [8]. It also exhibits excellent resistance to the baseline wander that can be an issue in typical datacom transceivers [3]. Compared with other DSP-dense modulation formats, such as optical OFDM [2], analogue implementation of CAP has the potential of being lower cost with high performance and power efficiency as there is no need for power consuming digital to analogue conversion (DAC) and ADC in the transceiver [2,5]. To facilitate the implementation of CAP, theoretical analysis has shown that the use of a quadrature amplitude modulation (QAM) receiver reduces the system sensitivity to timing jitter and improves the optical link power margin compared with a standard CAP receiver [7]. Such a hybrid CAP/QAM modulation scheme not only improves performance, but also retains the excellent power efficiency of CAP. Therefore, in this paper, for the first time, we have experimentally demonstrated a proof of concept hybrid 20 Gb/s CAP-4/QAM-4 transmitter/receiver over a single-mode fibre (SMF) link. We show that the proposed hybrid CAP-4/QAM-4 system significantly outperforms a PAM-4 system operating at a similar bit rate. 2. Experimental setup CAP-4 transmitter
10Gb/s PRBS
I
XOR
SMF A
DATA
10GHz clock 90o XOR
Delay line
Q
MZM
VOA
Low-pass filter
PD Delay line
Laser LO
XOR: exclusive or MZM: Mach-Zehnder modulator PD: Photo-diode VOA: variable optical attenuator LO: local oscillator DCA: digital communication analyser
~
B DCA
QAM-4 receiver
Fig. 1 System architecture and experimental setup for 20 Gb/s hybrid CAP-4/QAM-4 transmitter/receiver system. Figure 1 illustrates the experimental setup. A 10 Gb/s non-return-to-zero (NRZ) signal with a 27-1 PRBS is generated via a pattern generator to mimic the short run length block codes used in datacoms. Then an exclusive OR (XOR) operation is performed on the NRZ signal and a 10 GHz NRZ clock signal. Similarly, the inverted NRZ signal and the NRZ clock signal with a 90 degrees phase shift also undergo an XOR operation. In the diagram, the upper XOR output and the lower XOR output are combined with a phase shift between them. The upper XOR
AW4G.1.pdf
ACP/IPOC 2013 © OSA 2013
AW4G.1.pdf
ACP/IPOC 2013 © OSA 2013
output (Manchester code) is the in-phase (I) channel and the lower XOR output (modified Manchester code) is the quadrature (Q) channel [2]. The phase shift ensures that the two data streams are decorrelated and that the I and Q CAP-4 waveforms are orthogonal to each other. The combined CAP-4 signal is then amplified and combined with a DC bias to drive a Mach-Zehnder modulator (MZM) with a modulation bandwidth of 10.5 GHz. The CW laser source fed into the MZM has a wavelength of 1562.6 nm. After propagating through 4.3 km of SMF, the received optical signal passes through a variable optical attenuator and is then directly detected by a photo-diode (PD) with a bandwidth of 15 GHz followed by a RF amplifier. The received CAP signal is then processed by a QAM receiver which consists of a 10 GHz local oscillator (LO), a phase shifter, a mixer supporting up to an 18 GHz RF input, and a low-pass filter (LPF) with bandwidth of 7.2 GHz. In order to highlight the advantages of the proposed CAP-4/QAM-4 scheme, a reference PAM-4 system at a similar bit rate is also measured, although it is not depicted in Fig. 1. The PAM-4 system uses the same MZM and PD as that used in the hybrid CAP-4/QAM-4 system for a fair comparison. In addition, the PAM-4 transmitter also contains a PAM-4 generator which simply splits a 10 Gb/s NRZ signal into two paths that have a relative power difference of 6 dB and a relative delay of multiple symbol periods. These two signal paths are recombined to form the PAM-4 signal. On the receiver side, the QAM receiver shown in Fig. 1 is not included in the PAM-4 case. 3. Results and discussions Figure 2 shows the eye diagrams for the hybrid CAP-4/QAM-4 system at 20 Gb/s. The combined signal eye diagram in the CAP-4 transmitter is shown in Fig. 2(a). The recovered I and/or Q channel eye diagrams are presented in Fig. 2(b) and (c), respectively. Clearly the recovered eyes indicate error free transmission is feasible. However, it has to be noted that for the received eye diagrams, the measured I and Q channel eyes exhibit some degradation. This is due to the distortions from the mixer and imperfect filtering in the experimental QAM receiver. For comparison, Fig. 3 presents the eye diagrams of PAM-4 link at 20 Gb/s and 16 Gb/s. For the 20 Gb/s PAM-4 link, the electrical eye is open although slight distortion arises from the PAM-4 generator due to reflection. However, the eye diagram for the optical back to back (B2B) case shows severe degradation caused by modulator nonlinearity and limited optical transceiver bandwidth. Thus the 20 Gb/s PAM-4 link fails to support error free transmission and the BER of the eye shown in Fig. 3(b) is about 10-3. Only by reducing the PAM-4 bit rate to 16 Gb/s, is the eye opening improved significantly and is error free transmission possible.
(a)
(b)
(c)
Fig. 2: Measured eye diagrams for CAP-4 signals in the transmitter and QAM-4 receiver output with time resolution of 20 ps/div. (a) is obtained at point A shown in Fig. 1. (b) and (c) are the QAM-4 receiver outputs (point B) for optical B2B case with (b) being for the I channel signal and (c) being for Q channel.
(a) Electrical B2B (b) Optical B2B (c) Optical B2B Fig. 3: Eye diagrams of a 20 Gb/s PAM-4 signal of (a) electrical B2B and (b) optical B2B cases; and (c) a 16 Gb/s PAM-4 optical B2B case. The time resolution: 20 ps/div in (a) & (b); 25 ps/div in (c).
AW4G.1.pdf
ACP/IPOC 2013 © OSA 2013
Fig. 4: BER versus received optical power for 20 Gb/s hybrid CAP-4/QAM-4. For comparison, the BERs of a 16 Gb/s PAM-4 signal are also presented in the optical back to back (B2B) case. The time resolution of the inset eye diagrams is 20 ps/div for hybrid CAP-4/QAM-4 and 25 ps/div for PAM-4. The BER versus received optical power curves for the various links are plotted in Fig. 4. This includes results for the I and Q channels of the 20 Gb/s hybrid CAP-4/QAM-4 link and the three eyes of the 16 Gb/s PAM-4 signal. For the hybrid CAP-4/QAM-4 scheme, two transmission cases are shown: optical B2B and, transmission over 4.3 km of SMF. For both I and Q channels, the transmission over 4.3 km of SMF case shows roughly 1.5 dBo penalty compared with the optical back to back case at a BER of 10-9. Moreover, for each transmission case, the I channel has a lower BER compared with the Q channel for a fixed received power and the Q channel shows about 0.5 dBo penalty compared with the I channel. This is mainly because the Q channel (modified Manchester code) has a spectral peak further away from the 10GHz LO carrier than I channel does. The insets in Fig. 4 show representative eye diagrams for the recovered I and Q signals obtained at the received optical power required for BERs at the level of 10-9. For comparison, the BERs of the reference 16 Gb/s PAM-4 system for the optical B2B case are also shown in Fig. 4. As reflected in the representative eye diagrams shown in Fig. 4, the middle eye has the best BER performance due to its excellent eye opening in both horizontal and vertical directions. It is important to note that the hybrid CAP-4/QAM-4 scheme has approximately 4 dBo better optical sensitivity at a BER of 10 -9 compared with that of the PAM-4 middle eye, which is attributed to the multilevel penalty of PAM-4. This indicates the superiority of the proposed hybrid CAP-4/QAM-4 modulation scheme in terms of achievable optical power margin. 4. Conclusions We have experimentally demonstrated a hybrid CAP-4/QAM-4 optical data link at 20 Gb/s. The hybrid CAP4/QAM-4 system successfully supports error free transmission over 4.3 km of standard SMF at a wavelength of 1562.6 nm. The measured optical power penalty for the 4.3 km link was ~1.5 dBo at a BER of 10 -9. In addition, the experiment also shows that the hybrid CAP-4/QAM-4 outperforms significantly a PAM-4 system at similar bit rate. Acknowledgements This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) via the INTERNET project. References [1] A. Ghiasi, Z. Wang, V. Telang, and B. Welch, “Investigation of PAM-4/6/8 Signaling and FEC for 100 Gb/s Serial Transmission,” IEEE P802.3bm 40 Gb/s and 100 Gb/s Fiber Optic Task Force, (Nov., 2012). [2] J. L. Wei, J. D. Ingham, R. V. Penty, and I. H. White, “Performance Studies of 100 Gigabit Ethernet Enabled by Advanced Modulation Formats,” IEEE Next Gen 100G Optical Ethernet Study Group, (May, 2012). [3] J. L. Wei, J. D. Ingham, R. V. Penty, I. H. White and D. G. Cunningham, “Update on Performance Studies of 100 Gigabit Ethernet Enabled by Advanced Modulation Formats,” IEEE P802.3bm 40 Gb/s and 100 Gb/s Fiber Optic Task Force. (Sep., 2012). [4] R. Rodes, M. Wieckowski, T. T. Pham, J. B. Jensen, J. Turkiewicz,, J. Siuzdak, and I. T. Monroy, “Carrierless amplitude phase modulation of VCSEL with 4 bit/s/Hz spectral efficiency for use in WDM-PON,” Opt. Express, 19, 26551- 26556 (2011). [5] J. L. Wei, J. D. Ingham, D. G. Cunningham, R. V. Penty, and I. H. White, “Performance and power dissipation comparisons between 28 Gb/s NRZ, PAM, CAP and optical OFDM systems for datacommunication applications,” J. Lightwave Technol., 30, 3273-3280 (2012). [6] J. D. Ingham, R. V. Penty, I. H. White and D. G. Cunningham, “40 Gb/s carrierless amplitude and phase modulation for low-cost optical datacommunication links,” in OFC/NFOEC11, Paper OThZ3 (2011). [7] J. L. Wei, L. Geng, R. V. Penty, I. H. White, and D. G. Cunningham, “100 Gigabit Ethernet Transmission Enabled by Carrierless Amplitude and Phase Modulation Using QAM Receivers,” in OFC/NFOEC13, Paper OW4A.5 (2013). [8] J. J. Werner, Tutorial on Carrierless AM/PM, (ANSI X3T9.5 TP/PMD Working Group, 1992&1993).
