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OPTICS LETTERS / Vol. 39, No. 6 / March 15, 2014
Experimental demonstration of optical data links using a hybrid CAP/QAM modulation scheme J. L. Wei,1,* J. D. Ingham,1 Q. Cheng,1 D. G. Cunningham,2 R. V. Penty,1 and I. H. White1 1
2
Centre for Photonic Systems, Electrical Engineering Division, Engineering Department, University of Cambridge, 9 J J Thomson Avenue, Cambridge CB3 0FA, UK
Avago Technologies, Framlingham Technology Centre, Station Road, Framlingham, Suffolk IP13 9EZ, UK *Corresponding author: [email protected] Received November 26, 2013; revised January 16, 2014; accepted January 27, 2014; posted January 29, 2014 (Doc. ID 201976); published March 5, 2014
The first known experimental demonstrations of a 10 Gb∕s hybrid CAP-2/QAM-2 and a 20 Gb∕s hybrid CAP-4/ QAM-4 transmitter/receiver-based optical data link are performed. Successful transmission over 4.3 km of standard single-mode fiber (SMF) is achieved, with a link power penalty ∼0.4 dBo for CAP-2/QAM-2 and ∼1.5 dBo for CAP-4/ QAM-4 at BER 10−9 . © 2014 Optical Society of America OCIS codes: (060.0060) Fiber optics and optical communications; (060.4080) Modulation. http://dx.doi.org/10.1364/OL.39.001402
The need for low power consumption in the information and communications technology (ICT) sector has gained importance as the performance 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 data center optical interconnects [5] and single-laser 100 gigabit ethernet links [1–3,7]. As an alternative to quadrature amplitude modulation (QAM), CAP was originally proposed for copper wire communications as early as 1975 [8]. Instead of modulating the amplitude of two carrier waves, CAP generates a QAM-like signal by combining two multilevel signals using two filters whose impulse responses form an orthogonal Hilbert pair. The two filter outputs are the inphase (I) channel and quadrature (Q) channel, respectively. Thus, CAP can be implemented without explicit modulation and demodulation blocks or mixers, as are required in a QAM transmitter and receiver. Research on CAP grew strongly in the 1990s, due to the development of technologies required for high-speed local area networks using unshielded twisted pair wiring [9–13]. CAP was adopted in a number of asymmetric digital subscriber line applications [10]. Recently, the optical data communications community has investigated CAP [2–6,14–16]. The implementation of CAP for optical data communication links falls into two categories: digital implementation [4,14,15], and analog implementation [2,6]. As in other DSP-dense modulation formats, such as optical OFDM [2,5], the digital implementation of CAP requires DSP blocks, including analog-to-digital convertors (ADCs) and digital-to-analog convertors (DACs) in the transceivers. ADCs and DACs are often the most power consuming constituents in the transceiver of a high-speed optical data link [2,5]. Thus, digital implementation is challenging. Alternatively, analog implementation of CAP has the potential of being 0146-9592/14/061402-04$15.00/0
lower cost, with high performance and power efficiency, as it features important advantages, including its simple implementation using transversal filters [6] for waveform shaping and modulation without the need for advanced and power-hungry DSP and ADCs/DACs. The main limitation for CAP, however, is its high sensitivity to timing jitter in the receiver, because the recovered CAP signal has significant interference between the I and Q channels involved. To tackle this obstacle, a modified QAM receiver has been proposed [10–13]. Theoretical analysis has shown that the use of the CAP/QAM modulation scheme in optical data links not only significantly reduces the system sensitivity to timing jitter, but also improves the system power margin [7]. Such a hybrid CAP/QAM modulation scheme also retains the excellent power efficiency of analog CAP. Therefore, in this Letter, we analyze the hybrid CAP/ QAM system and experimentally demonstrate a proofof-concept hybrid 10 Gb∕s CAP-2/QAM-2 and 20 Gb∕s CAP-4/QAM-4 transmitter/receiver over a single-mode fiber (SMF) link. We believe that this is the first experimental demonstration of the CAP/QAM modulation scheme in optical data links implemented using analog components. Simulations based on the hybrid CAP/QAM experimental setup are shown to be in excellent agreement with the experimental results. The hybrid CAP/QAM transmitter/receiver is illustrated in Fig. 1, where a conventional CAP receiver consisting of matched filters is also drawn for comparison. In the CAP transmitter, the two bit streams are mapped into PAM symbols, denoted as Ak and Bk . yA t and yB t are the output signals of demodulators in the modified QAM receiver, which comprises mixers, a local oscillator (LO), low pass matched filters, and a symbol rotator. The output of each matched filter consists of components from both the I channel and Q channel. The I channel matched filter output contains transmitted signals from the I channel yI t and Q channel yIQ t, with the latter being the cross-channel interference (CCI): yA t yI t yIQ t; where © 2014 Optical Society of America
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March 15, 2014 / Vol. 39, No. 6 / OPTICS LETTERS
yI t
X
Ak gt−kTcosωc t−kT ·cosωc tθ⊗g−t
y0A t ≅
X Ak cosωc t − kTht − kT k
k
X
X Bk sinωc t − kTht − kT;
−
Ak cosωc kT θht−kT
X 0.5 Ak gt−kTcosωc 2t−kT θ ⊗g−t;
y0B t ≅
k
(2) in which
X
Bk cosωc t − kTht − kT
k
−
X
Ak sinωc t − kTht − kT:
(10)
k
(3)
where ht is a Nyquist pulse and gt is a square-rootraised cosine (SRRC)-shaping filter. Here, ωc 2πf c , where f c 1 αF s ∕2 is the central frequency of the shaping filters in the transmitter, α (0 < α ≤ 1) is the roll-off coefficient [9] of the shaping filter, and F s 1∕T is the symbol rate, respectively. θ is the phase offset introduced to the LO to compensate for the delay throughout the transceiver and channel. T is the symbol period for the PAM symbols. Similarly, X yIQ t Bk sinωc kT θht − kT k
X Bk gt − kT sinωc 2t − kT θ ⊗ g−t: − 0.5 k
(4) Since 2f c is larger than the bandwidth of the low pass matched filter g−t, the second terms in Eqs. (2) and (4) are negligible. Then we have X yA t ≅ Ak cosωc kT θ Bk sinωc kT θht − kT: k
(5) In a similar way, X Bk cosωc kT θ − Ak sinωc kT θht − kT: yB t ≅ k
(6) The second term in Eqs. (5) and (6) represents CCI, and is zero only if the excess bandwidth α 1 with θ 0. For α < 1, the CCI term exists in each matched filter’s output. The symbol can be recovered only if a proper delay (θ) is chosen and a symbol rotator is used, as shown in Fig. 1. The symbol rotator outputs are given by A0k t RefyA t j · yB t · ejωc kT g X Ak ht − kT; ≅
(9)
k
k
ht 0.5gt ⊗ g−t;
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Obviously, the CCI terms in Eqs. (9) and (10) can only be removed at optimum sampling points (t kT). Otherwise, the eye exhibits horizontal and vertical closure, due to the CCI [12]. For a CAP-4 system, the simulated recovered I channel eye diagrams are shown in Fig. 1 for QAM receiver (upper) and CAP receiver (lower), respectively. Figure 2(a) illustrates the hybrid CAP/QAM 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 degree phase shift also undergo an XOR operation. The Q channel XOR input NRZ has a relative time delay of a sufficiently large integer number of symbol periods to the I channel bit source. The XORs are used to mimic pulse-shaping filters. This is an efficient approach, since the outputs of XORs have passband spectra that are similar to those generated by classical SRRC pulse-shaping filters [2]. As indicated in Fig. 2(b), although the I and Q channel XOR outputs exhibit different spectra, their combination is comparable with the spectrum of a SRRC shaping filter [9]. There are two configurations in the setup: for the 10 Gb∕s CAP-2 case, only the upper XOR output is electrically amplified, and then combined with a DC bias to drive a Mach–Zehnder modulator (MZM). For 20 Gb∕s CAP-4, the upper XOR output and the lower XOR output are combined with a phase shift between them. The upper XOR output (Manchester code) is the I channel and the lower XOR output (modified Manchester code) is the Q channel [2]. The phase shift ensures that the two data streams are de-correlated 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 MZM. The MZM has a modulation
(7)
k
B0k t ImfyA t j · yB t · ejωc kT g X Bk ht − kT: ≅
(8)
k
For comparison, we also give the equations of the output of a standard CAP receiver, based on [13]:
Fig. 1. Hybrid CAP/QAM transmitter/receiver scheme. For comparison, a standard CAP receiver is also shown. The signals from a QAM4 and a CAP4 I channel after the corresponding receiver are plotted.
OPTICS LETTERS / Vol. 39, No. 6 / March 15, 2014
Normalized Intensity (A. U.)
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1 0.8
I, experiment Q, experiment I+Q, experiment SRRC spectrum
0.6 0.4
Fig. 3. Hybrid CAP-2/QAM-2 eye diagrams. (a)–(c) are simulation results, and (d)–(f) are experimental measurements with time resolution of 20 ps∕div. (a) and (d) are obtained at point A, as shown in Fig. 2, (b) and (e) are at point D, obtained for optical back-to-back (B2B) case, and (c) and (f) are at point D, by replacing the QAM-2 receiver with a matched filter for the optical B2B case and (f) is from [16].
0.2 0 0
5
10 15 Frequency (GHz)
20
Fig. 2. (a) Experimental setup for hybrid CAP/QAM transmitter/receiver. The dashed block is only for CAP-4/ QAM-4 case. (b) Simulated spectra of the signals at points A and B in (a). For comparison, the simulated spectra of the typical SRRC pulses with roll-off coefficient of 1 are also plotted with dashed lines.
bandwidth of 10.5 GHz while 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 then directly detected by a photodiode (PD) with a bandwidth of 15 GHz followed by a RF amplifier. The CAP signal is then processed by a QAM receiver consisting of a 10 GHz LO, a phase shifter, a mixer supporting an 18 GHz RF input, and a low-pass filter (LPF) with bandwidth of 7.2 GHz. To gain an insight into the experimental results, numerical simulations of the 10 Gb∕s and 20 Gb∕s hybrid CAP/ QAM experimental setup depicted in Fig. 2 are performed. The simulation model contains all the components shown in Fig. 2. The model includes an XOR gate with a Gaussian frequency response with a bandwidth of about 10 GHz and a MZM modeled with a Gaussian response and with a 20%–80% rise time of 20 ps. The LPF is modeled as a super Gaussian filter of order 6 with bandwidth equal to 0.72 times the baud rate, to simulate the steep filtering profile of the LPF used in the experiment. For comparison, a CAP-2 receiver that uses matched filters is also considered. The matched filter is simply modeled as a bi-phase square impulse response. Figure 3 shows the eye diagrams for hybrid CAP-2/ QAM-2 obtained at several points in the signal path, as shown in Fig. 2. Both simulation results and experimental measurements are presented for comparison. As shown in Fig. 3, the experimental measurements agree very well with the simulation results. Note that the measured eye of the CAP-2 receiver signal is from [16], where the residual interference from a NRZ signal leads to eye distortion compared with the simulated results, although the distortion is not significant. It is very interesting to compare the eye diagram of the CAP-2 receiver signal with that of the QAM-2 receiver signal. The horizontal eye opening of the measured QAM-2 receiver signal is about
0.9 unit intervals (UI) and, thus, shows significant improvement compared with the measured CAP receiver signal, which has a horizontal eye opening of about 0.25 UI, indicating at least a 3-fold increase in tolerance to timing jitter. Figure 4 presents the measured bit error ratio (BER) versus received optical power performance for the hybrid CAP-2/QAM-2 system for an optical back-to-back (B2B) case and transmission over 4.3 km of SMF. Error-free transmission can be achieved for both cases, and a power penalty of ∼0.4 dBo is obtained at BER of 10−9 for 4.3 km SMF transmission. For comparison, simulation results for the optical B2B system, based on the experimental setup with a 10GBASE-LRM receiver [5], are also plotted as a reference. This agrees well with the experiment. Figure 5 shows the simulated and measured eye diagrams for the hybrid CAP-4/QAM-4 system at 20 Gb∕s. The simulated and experimental eye diagrams are in good agreement. However, the measured I and Q channel eyes in QAM receivers exhibit some additional degradation compared with the simulations. This is due to the distortion from the mixer and imperfect filtering in the experimental QAM-4 receiver. Also, comparing Figs. 5(c) and 5(d), the Q channel horizontal eye opening is worse compared with the I channel, indicating that the I channel might exhibit better performance. The BER versus received optical power is plotted in Fig. 6 for the I and Q channels of the 20 Gb∕s hybrid CAP-4/QAM-4 link. For the hybrid CAP-4/QAM-4 scheme,
Fig. 4. BER versus average received optical power for hybrid CAP-2/QAM-2. Both optical B2B and transmission through 4.3 km SMF are considered. The inset eye diagrams are obtained at point D shown in Fig. 2 with a time resolution of 20 ps∕div. For comparison, simulation results for optical B2B are presented.
March 15, 2014 / Vol. 39, No. 6 / OPTICS LETTERS
Fig. 5. Eye diagrams for hybrid CAP-4/QAM-4. (a)–(d) are simulation results, and (e)–(h) are experimental measurements with time resolution of 20 ps∕div. (a) and (e) are obtained at point B, (b) and (f) are obtained at point C. (c), (d), (g), and (h) are the QAM-4 receiver outputs for optical B2B case with (c) and (g) being for the I channel signal, and (d) and (h) being for the Q channel.
two transmission cases are shown: optical B2B and transmission over 4.3 km of SMF. For both the I and Q channels, transmission over 4.3 km of SMF shows roughly 1.5 dBo penalty compared with the optical B2B 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 agrees with the theoretical prediction using the eye diagram comparison shown in Fig. 5, and is mainly attributed to the I and Q spectral difference shown in Fig. 2(b). The insets in Fig. 6 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 . In Fig. 7, for comparison with the QAM receiver case, eye diagrams due to a normal CAP-4 receiver consisting of two matched filters are plotted. In this simulation, the in-phase matched filter is modeled as an ideal biphase impulse response, the quadrature matched filter impulse response has a T/4 relative time delay to the in-phase counterpart. In the experiment, transversal filters with 5 taps and normalized tap values of [-1 -1 0.78 0.78 0] and [-0.75 0.81 1 -0.95 -0.11] with tap spacing of 25 ps are used for I and Q, respectively. The recovered eye diagrams are shown in Fig. 7. Obviously, both simulated and measured eye diagrams show worse eye opening compared with their QAM receiver counterparts,
Fig. 6. BER versus average received optical power for 20 Gb∕s hybrid CAP-4/QAM-4. Both optical B2B and transmission over 4.3 km SMF are considered. The inset eye diagrams are obtained at point D shown in Fig. 2 with a time resolution of 20 ps∕div.
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Fig. 7. Eye diagrams for CAP-4 using CAP receiver. (a) and (b) are simulation results (the eye is marked in red), and (c) and (d) are experimental measurements with time resolution of 20 ps∕div. For the optical B2B case, (a) and (c) are for the I channel matched filter outputs, and (b) and (d) are for Q channel.
indicating the advantage of the QAM receiver over the CAP receiver. We have analyzed a hybrid CAP/QAM system and experimentally demonstrated a 10 Gb∕s hybrid CAP-2/ QAM-2 and a hybrid 20 Gb∕s CAP-4/QAM-4 optical data link. Both hybrid CAP/QAM systems successfully support 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 is ∼0.4 dBo for CAP-2/ QAM-2 and ∼1.5 dBo for CAP-4/QAM at a BER of 10−9 . This Letter is based on research supported by the UK EPSRC via the INTERNET project. References 1. A. Ghiasi, IEEE 802.3 Next Generation 40 Gb/s and 100 Gb/s Optical Ethernet Study Group Interim Meeting (IEEE, 2012). 2. J. L. Wei, J. D. Ingham, R. V. Penty, and I. H. White, IEEE 802.3 Next Generation 40 Gb/s and 100 Gb/s Optical Ethernet Study Group Interim Meeting (IEEE, 2012). 3. J. L. Wei, J. D. Ingham, R. V. Penty, I. H. White, and D. G. Cunningham, IEEE P802.3bm 40 Gb/s and 100 Gb/s Fiber Optic Task Force Interim Meeting (IEEE, 2012). 4. R. Rodes, M. Wieckowski, T. T. Pham, J. B. Jensen, J. T. Turkiewicz, J. Siuzdak, and I. T. Monroy, Opt. Express 19, 26551 (2011). 5. J. L. Wei, J. D. Ingham, D. G. Cunningham, R. V. Penty, and I. H. White, J. Lightwave Technol. 30, 3273 (2012). 6. J. D. Ingham, R. V. Penty, I. H. White, and D. G. Cunningham, in Optical Fiber Communication Conference and Exposition/National Fiber Optic Engineers Conference (2011), paper OThZ3. 7. J. L. Wei, L. Geng, D. G. Cunningham, R. V. Penty, and I. H. White, in Optical Fiber Communication Conference and Exposition/National Fiber Optic Engineers Conference (2013), paper OW4A.5. 8. D. D. Falconer, Bell Laboratories, Tech. Memo (1975). 9. J. J. Werner, Tutorial on carrierless AM/PM, ANSI X3T9.5 TP/PMD Working Group (1992 and 1993). 10. W. Y. Chen, G.-H. Im, and J. J. Werner, AT&T and Bellcore contribution to ANSI T1E1.4/92-149 (1992). 11. L. M. Garth, J. Yang, and J. J. Werner, in International Conference on Communications (1999), Vol. 3, pp. 1531–1536. 12. I. L. Thng, A. Cantoni, and Y. H. Leung, IEEE Trans. Commun. 48, 396 (2000). 13. A. H. Abdolhamid and D. A. Johns, International Symposium on Circuits and Systems (1998), Vol. 4, 316. 14. M. I. Olmedo, T. Zuo, J. B. Jensen, Q. Zhong, X. Xu, and I. T. Monroy, Optical Fiber Communication Conference (Optical Society of America, 2013), paper PDP5C.10. 15. L. Tao, Y. Wang, Y. Gao, A. P. T. Lau, N. Chi, and C. Lu, Opt. Express 21, 6459 (2013). 16. S. H. Lee, L. Geng, J. D. Ingham, R. V. Penty, I. H. White, and D. G. Cunningham, Conference on Lasers and ElectroOptics (2011), paper CThO3.
OPTICS LETTERS / Vol. 39, No. 6 / March 15, 2014
Experimental demonstration of optical data links using a hybrid CAP/QAM modulation scheme J. L. Wei,1,* J. D. Ingham,1 Q. Cheng,1 D. G. Cunningham,2 R. V. Penty,1 and I. H. White1 1
2
Centre for Photonic Systems, Electrical Engineering Division, Engineering Department, University of Cambridge, 9 J J Thomson Avenue, Cambridge CB3 0FA, UK
Avago Technologies, Framlingham Technology Centre, Station Road, Framlingham, Suffolk IP13 9EZ, UK *Corresponding author: [email protected] Received November 26, 2013; revised January 16, 2014; accepted January 27, 2014; posted January 29, 2014 (Doc. ID 201976); published March 5, 2014
The first known experimental demonstrations of a 10 Gb∕s hybrid CAP-2/QAM-2 and a 20 Gb∕s hybrid CAP-4/ QAM-4 transmitter/receiver-based optical data link are performed. Successful transmission over 4.3 km of standard single-mode fiber (SMF) is achieved, with a link power penalty ∼0.4 dBo for CAP-2/QAM-2 and ∼1.5 dBo for CAP-4/ QAM-4 at BER 10−9 . © 2014 Optical Society of America OCIS codes: (060.0060) Fiber optics and optical communications; (060.4080) Modulation. http://dx.doi.org/10.1364/OL.39.001402
The need for low power consumption in the information and communications technology (ICT) sector has gained importance as the performance 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 data center optical interconnects [5] and single-laser 100 gigabit ethernet links [1–3,7]. As an alternative to quadrature amplitude modulation (QAM), CAP was originally proposed for copper wire communications as early as 1975 [8]. Instead of modulating the amplitude of two carrier waves, CAP generates a QAM-like signal by combining two multilevel signals using two filters whose impulse responses form an orthogonal Hilbert pair. The two filter outputs are the inphase (I) channel and quadrature (Q) channel, respectively. Thus, CAP can be implemented without explicit modulation and demodulation blocks or mixers, as are required in a QAM transmitter and receiver. Research on CAP grew strongly in the 1990s, due to the development of technologies required for high-speed local area networks using unshielded twisted pair wiring [9–13]. CAP was adopted in a number of asymmetric digital subscriber line applications [10]. Recently, the optical data communications community has investigated CAP [2–6,14–16]. The implementation of CAP for optical data communication links falls into two categories: digital implementation [4,14,15], and analog implementation [2,6]. As in other DSP-dense modulation formats, such as optical OFDM [2,5], the digital implementation of CAP requires DSP blocks, including analog-to-digital convertors (ADCs) and digital-to-analog convertors (DACs) in the transceivers. ADCs and DACs are often the most power consuming constituents in the transceiver of a high-speed optical data link [2,5]. Thus, digital implementation is challenging. Alternatively, analog implementation of CAP has the potential of being 0146-9592/14/061402-04$15.00/0
lower cost, with high performance and power efficiency, as it features important advantages, including its simple implementation using transversal filters [6] for waveform shaping and modulation without the need for advanced and power-hungry DSP and ADCs/DACs. The main limitation for CAP, however, is its high sensitivity to timing jitter in the receiver, because the recovered CAP signal has significant interference between the I and Q channels involved. To tackle this obstacle, a modified QAM receiver has been proposed [10–13]. Theoretical analysis has shown that the use of the CAP/QAM modulation scheme in optical data links not only significantly reduces the system sensitivity to timing jitter, but also improves the system power margin [7]. Such a hybrid CAP/QAM modulation scheme also retains the excellent power efficiency of analog CAP. Therefore, in this Letter, we analyze the hybrid CAP/ QAM system and experimentally demonstrate a proofof-concept hybrid 10 Gb∕s CAP-2/QAM-2 and 20 Gb∕s CAP-4/QAM-4 transmitter/receiver over a single-mode fiber (SMF) link. We believe that this is the first experimental demonstration of the CAP/QAM modulation scheme in optical data links implemented using analog components. Simulations based on the hybrid CAP/QAM experimental setup are shown to be in excellent agreement with the experimental results. The hybrid CAP/QAM transmitter/receiver is illustrated in Fig. 1, where a conventional CAP receiver consisting of matched filters is also drawn for comparison. In the CAP transmitter, the two bit streams are mapped into PAM symbols, denoted as Ak and Bk . yA t and yB t are the output signals of demodulators in the modified QAM receiver, which comprises mixers, a local oscillator (LO), low pass matched filters, and a symbol rotator. The output of each matched filter consists of components from both the I channel and Q channel. The I channel matched filter output contains transmitted signals from the I channel yI t and Q channel yIQ t, with the latter being the cross-channel interference (CCI): yA t yI t yIQ t; where © 2014 Optical Society of America
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March 15, 2014 / Vol. 39, No. 6 / OPTICS LETTERS
yI t
X
Ak gt−kTcosωc t−kT ·cosωc tθ⊗g−t
y0A t ≅
X Ak cosωc t − kTht − kT k
k
X
X Bk sinωc t − kTht − kT;
−
Ak cosωc kT θht−kT
X 0.5 Ak gt−kTcosωc 2t−kT θ ⊗g−t;
y0B t ≅
k
(2) in which
X
Bk cosωc t − kTht − kT
k
−
X
Ak sinωc t − kTht − kT:
(10)
k
(3)
where ht is a Nyquist pulse and gt is a square-rootraised cosine (SRRC)-shaping filter. Here, ωc 2πf c , where f c 1 αF s ∕2 is the central frequency of the shaping filters in the transmitter, α (0 < α ≤ 1) is the roll-off coefficient [9] of the shaping filter, and F s 1∕T is the symbol rate, respectively. θ is the phase offset introduced to the LO to compensate for the delay throughout the transceiver and channel. T is the symbol period for the PAM symbols. Similarly, X yIQ t Bk sinωc kT θht − kT k
X Bk gt − kT sinωc 2t − kT θ ⊗ g−t: − 0.5 k
(4) Since 2f c is larger than the bandwidth of the low pass matched filter g−t, the second terms in Eqs. (2) and (4) are negligible. Then we have X yA t ≅ Ak cosωc kT θ Bk sinωc kT θht − kT: k
(5) In a similar way, X Bk cosωc kT θ − Ak sinωc kT θht − kT: yB t ≅ k
(6) The second term in Eqs. (5) and (6) represents CCI, and is zero only if the excess bandwidth α 1 with θ 0. For α < 1, the CCI term exists in each matched filter’s output. The symbol can be recovered only if a proper delay (θ) is chosen and a symbol rotator is used, as shown in Fig. 1. The symbol rotator outputs are given by A0k t RefyA t j · yB t · ejωc kT g X Ak ht − kT; ≅
(9)
k
k
ht 0.5gt ⊗ g−t;
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Obviously, the CCI terms in Eqs. (9) and (10) can only be removed at optimum sampling points (t kT). Otherwise, the eye exhibits horizontal and vertical closure, due to the CCI [12]. For a CAP-4 system, the simulated recovered I channel eye diagrams are shown in Fig. 1 for QAM receiver (upper) and CAP receiver (lower), respectively. Figure 2(a) illustrates the hybrid CAP/QAM 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 degree phase shift also undergo an XOR operation. The Q channel XOR input NRZ has a relative time delay of a sufficiently large integer number of symbol periods to the I channel bit source. The XORs are used to mimic pulse-shaping filters. This is an efficient approach, since the outputs of XORs have passband spectra that are similar to those generated by classical SRRC pulse-shaping filters [2]. As indicated in Fig. 2(b), although the I and Q channel XOR outputs exhibit different spectra, their combination is comparable with the spectrum of a SRRC shaping filter [9]. There are two configurations in the setup: for the 10 Gb∕s CAP-2 case, only the upper XOR output is electrically amplified, and then combined with a DC bias to drive a Mach–Zehnder modulator (MZM). For 20 Gb∕s CAP-4, the upper XOR output and the lower XOR output are combined with a phase shift between them. The upper XOR output (Manchester code) is the I channel and the lower XOR output (modified Manchester code) is the Q channel [2]. The phase shift ensures that the two data streams are de-correlated 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 MZM. The MZM has a modulation
(7)
k
B0k t ImfyA t j · yB t · ejωc kT g X Bk ht − kT: ≅
(8)
k
For comparison, we also give the equations of the output of a standard CAP receiver, based on [13]:
Fig. 1. Hybrid CAP/QAM transmitter/receiver scheme. For comparison, a standard CAP receiver is also shown. The signals from a QAM4 and a CAP4 I channel after the corresponding receiver are plotted.
OPTICS LETTERS / Vol. 39, No. 6 / March 15, 2014
Normalized Intensity (A. U.)
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1 0.8
I, experiment Q, experiment I+Q, experiment SRRC spectrum
0.6 0.4
Fig. 3. Hybrid CAP-2/QAM-2 eye diagrams. (a)–(c) are simulation results, and (d)–(f) are experimental measurements with time resolution of 20 ps∕div. (a) and (d) are obtained at point A, as shown in Fig. 2, (b) and (e) are at point D, obtained for optical back-to-back (B2B) case, and (c) and (f) are at point D, by replacing the QAM-2 receiver with a matched filter for the optical B2B case and (f) is from [16].
0.2 0 0
5
10 15 Frequency (GHz)
20
Fig. 2. (a) Experimental setup for hybrid CAP/QAM transmitter/receiver. The dashed block is only for CAP-4/ QAM-4 case. (b) Simulated spectra of the signals at points A and B in (a). For comparison, the simulated spectra of the typical SRRC pulses with roll-off coefficient of 1 are also plotted with dashed lines.
bandwidth of 10.5 GHz while 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 then directly detected by a photodiode (PD) with a bandwidth of 15 GHz followed by a RF amplifier. The CAP signal is then processed by a QAM receiver consisting of a 10 GHz LO, a phase shifter, a mixer supporting an 18 GHz RF input, and a low-pass filter (LPF) with bandwidth of 7.2 GHz. To gain an insight into the experimental results, numerical simulations of the 10 Gb∕s and 20 Gb∕s hybrid CAP/ QAM experimental setup depicted in Fig. 2 are performed. The simulation model contains all the components shown in Fig. 2. The model includes an XOR gate with a Gaussian frequency response with a bandwidth of about 10 GHz and a MZM modeled with a Gaussian response and with a 20%–80% rise time of 20 ps. The LPF is modeled as a super Gaussian filter of order 6 with bandwidth equal to 0.72 times the baud rate, to simulate the steep filtering profile of the LPF used in the experiment. For comparison, a CAP-2 receiver that uses matched filters is also considered. The matched filter is simply modeled as a bi-phase square impulse response. Figure 3 shows the eye diagrams for hybrid CAP-2/ QAM-2 obtained at several points in the signal path, as shown in Fig. 2. Both simulation results and experimental measurements are presented for comparison. As shown in Fig. 3, the experimental measurements agree very well with the simulation results. Note that the measured eye of the CAP-2 receiver signal is from [16], where the residual interference from a NRZ signal leads to eye distortion compared with the simulated results, although the distortion is not significant. It is very interesting to compare the eye diagram of the CAP-2 receiver signal with that of the QAM-2 receiver signal. The horizontal eye opening of the measured QAM-2 receiver signal is about
0.9 unit intervals (UI) and, thus, shows significant improvement compared with the measured CAP receiver signal, which has a horizontal eye opening of about 0.25 UI, indicating at least a 3-fold increase in tolerance to timing jitter. Figure 4 presents the measured bit error ratio (BER) versus received optical power performance for the hybrid CAP-2/QAM-2 system for an optical back-to-back (B2B) case and transmission over 4.3 km of SMF. Error-free transmission can be achieved for both cases, and a power penalty of ∼0.4 dBo is obtained at BER of 10−9 for 4.3 km SMF transmission. For comparison, simulation results for the optical B2B system, based on the experimental setup with a 10GBASE-LRM receiver [5], are also plotted as a reference. This agrees well with the experiment. Figure 5 shows the simulated and measured eye diagrams for the hybrid CAP-4/QAM-4 system at 20 Gb∕s. The simulated and experimental eye diagrams are in good agreement. However, the measured I and Q channel eyes in QAM receivers exhibit some additional degradation compared with the simulations. This is due to the distortion from the mixer and imperfect filtering in the experimental QAM-4 receiver. Also, comparing Figs. 5(c) and 5(d), the Q channel horizontal eye opening is worse compared with the I channel, indicating that the I channel might exhibit better performance. The BER versus received optical power is plotted in Fig. 6 for the I and Q channels of the 20 Gb∕s hybrid CAP-4/QAM-4 link. For the hybrid CAP-4/QAM-4 scheme,
Fig. 4. BER versus average received optical power for hybrid CAP-2/QAM-2. Both optical B2B and transmission through 4.3 km SMF are considered. The inset eye diagrams are obtained at point D shown in Fig. 2 with a time resolution of 20 ps∕div. For comparison, simulation results for optical B2B are presented.
March 15, 2014 / Vol. 39, No. 6 / OPTICS LETTERS
Fig. 5. Eye diagrams for hybrid CAP-4/QAM-4. (a)–(d) are simulation results, and (e)–(h) are experimental measurements with time resolution of 20 ps∕div. (a) and (e) are obtained at point B, (b) and (f) are obtained at point C. (c), (d), (g), and (h) are the QAM-4 receiver outputs for optical B2B case with (c) and (g) being for the I channel signal, and (d) and (h) being for the Q channel.
two transmission cases are shown: optical B2B and transmission over 4.3 km of SMF. For both the I and Q channels, transmission over 4.3 km of SMF shows roughly 1.5 dBo penalty compared with the optical B2B 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 agrees with the theoretical prediction using the eye diagram comparison shown in Fig. 5, and is mainly attributed to the I and Q spectral difference shown in Fig. 2(b). The insets in Fig. 6 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 . In Fig. 7, for comparison with the QAM receiver case, eye diagrams due to a normal CAP-4 receiver consisting of two matched filters are plotted. In this simulation, the in-phase matched filter is modeled as an ideal biphase impulse response, the quadrature matched filter impulse response has a T/4 relative time delay to the in-phase counterpart. In the experiment, transversal filters with 5 taps and normalized tap values of [-1 -1 0.78 0.78 0] and [-0.75 0.81 1 -0.95 -0.11] with tap spacing of 25 ps are used for I and Q, respectively. The recovered eye diagrams are shown in Fig. 7. Obviously, both simulated and measured eye diagrams show worse eye opening compared with their QAM receiver counterparts,
Fig. 6. BER versus average received optical power for 20 Gb∕s hybrid CAP-4/QAM-4. Both optical B2B and transmission over 4.3 km SMF are considered. The inset eye diagrams are obtained at point D shown in Fig. 2 with a time resolution of 20 ps∕div.
1405
Fig. 7. Eye diagrams for CAP-4 using CAP receiver. (a) and (b) are simulation results (the eye is marked in red), and (c) and (d) are experimental measurements with time resolution of 20 ps∕div. For the optical B2B case, (a) and (c) are for the I channel matched filter outputs, and (b) and (d) are for Q channel.
indicating the advantage of the QAM receiver over the CAP receiver. We have analyzed a hybrid CAP/QAM system and experimentally demonstrated a 10 Gb∕s hybrid CAP-2/ QAM-2 and a hybrid 20 Gb∕s CAP-4/QAM-4 optical data link. Both hybrid CAP/QAM systems successfully support 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 is ∼0.4 dBo for CAP-2/ QAM-2 and ∼1.5 dBo for CAP-4/QAM at a BER of 10−9 . This Letter is based on research supported by the UK EPSRC via the INTERNET project. References 1. A. Ghiasi, IEEE 802.3 Next Generation 40 Gb/s and 100 Gb/s Optical Ethernet Study Group Interim Meeting (IEEE, 2012). 2. J. L. Wei, J. D. Ingham, R. V. Penty, and I. H. White, IEEE 802.3 Next Generation 40 Gb/s and 100 Gb/s Optical Ethernet Study Group Interim Meeting (IEEE, 2012). 3. J. L. Wei, J. D. Ingham, R. V. Penty, I. H. White, and D. G. Cunningham, IEEE P802.3bm 40 Gb/s and 100 Gb/s Fiber Optic Task Force Interim Meeting (IEEE, 2012). 4. R. Rodes, M. Wieckowski, T. T. Pham, J. B. Jensen, J. T. Turkiewicz, J. Siuzdak, and I. T. Monroy, Opt. Express 19, 26551 (2011). 5. J. L. Wei, J. D. Ingham, D. G. Cunningham, R. V. Penty, and I. H. White, J. Lightwave Technol. 30, 3273 (2012). 6. J. D. Ingham, R. V. Penty, I. H. White, and D. G. Cunningham, in Optical Fiber Communication Conference and Exposition/National Fiber Optic Engineers Conference (2011), paper OThZ3. 7. J. L. Wei, L. Geng, D. G. Cunningham, R. V. Penty, and I. H. White, in Optical Fiber Communication Conference and Exposition/National Fiber Optic Engineers Conference (2013), paper OW4A.5. 8. D. D. Falconer, Bell Laboratories, Tech. Memo (1975). 9. J. J. Werner, Tutorial on carrierless AM/PM, ANSI X3T9.5 TP/PMD Working Group (1992 and 1993). 10. W. Y. Chen, G.-H. Im, and J. J. Werner, AT&T and Bellcore contribution to ANSI T1E1.4/92-149 (1992). 11. L. M. Garth, J. Yang, and J. J. Werner, in International Conference on Communications (1999), Vol. 3, pp. 1531–1536. 12. I. L. Thng, A. Cantoni, and Y. H. Leung, IEEE Trans. Commun. 48, 396 (2000). 13. A. H. Abdolhamid and D. A. Johns, International Symposium on Circuits and Systems (1998), Vol. 4, 316. 14. M. I. Olmedo, T. Zuo, J. B. Jensen, Q. Zhong, X. Xu, and I. T. Monroy, Optical Fiber Communication Conference (Optical Society of America, 2013), paper PDP5C.10. 15. L. Tao, Y. Wang, Y. Gao, A. P. T. Lau, N. Chi, and C. Lu, Opt. Express 21, 6459 (2013). 16. S. H. Lee, L. Geng, J. D. Ingham, R. V. Penty, I. H. White, and D. G. Cunningham, Conference on Lasers and ElectroOptics (2011), paper CThO3.
