November 1, 2013 / Vol. 38, No. 21 / OPTICS LETTERS
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Simple signal-to-signal beat interference cancellation receiver based on balanced detection for a singlesideband optical OFDM signal with a reduced guard band Jianxin Ma State Key Laboratory of Information Photonics and Optical Communications, School of Electric Engineering, Beijing University of Posts and Telecommunications, Beijing 100876, China
[email protected] Received July 29, 2013; revised August 20, 2013; accepted September 12, 2013; posted September 17, 2013 (Doc. ID 194838); published October 21, 2013 A simple signal-to-signal beat interference cancellation receiver based on balanced detection (ICRBD) with an interleaver, a 2 × 2 three-decibel optical coupler, and a balanced photodiode pair is proposed for a single-sideband optical orthogonal frequency division multiplexing (SSB-OOFDM) signal with a reduced guard band (GB). Simulation demonstration of the ICRBD for a 40 Gbit∕s 16-QAM SSB-OOFDM signal with a reduced GB was achieved successfully. © 2013 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (040.1880) Detection; (060.2630) Frequency modulation. http://dx.doi.org/10.1364/OL.38.004335
Optical orthogonal frequency division multiplexing (OOFDM) has received considerable attention due to its enhanced spectral efficiency (SE) and high dispersion tolerance since the narrowband data-bearing subcarriers are overlapped orthogonally [1]. Coherent optical (COOFDM) with higher-level quadrature amplitude modulation (QAM) has achieved an SE of 14 bit∕s∕Hz [2] and is becoming a potential candidate for future long-haul optical transmission systems [3–6]. However, lasers with very narrow linewidth are needed at both transmitter and receiver fronts and the receiver for CO-OFDM is complicated by solving the frequency offset and the phase noise. Direct detection OOFDM (DD-OOFDM) systems require a simple receiver structure because the transmitted OFDM signal is recovered by detecting the carrier and signal beat products in a square-law photodiode (PD) [7,8]. There is no need to estimate the frequency and the phase offset because they are cancelled out as the OOFDM signal and the optical carrier are beating with each other in the PD since both come from the same laser with the synchronous frequency and the phase offset. In DD-OOFDM, optical single-sideband OFDM (SSBOOFDM) is promising since it can not only overcome the inherent chromatic dispersion-induced fading effect associated with double-sideband OOFDM, but also improves the SE. However, the desired OFDM signal generated by beating the OOFDM sideband with the optical carrier is affected by the signal-to-signal beat interference (SSBI) as the guard band (GB) between the optical carrier and the OOFDM signal is small. Several methods have been proposed to minimize the penalty due to SSBI. In [8], a sufficient GB is used to avoid the spectrum overlap of the SSBI and the desired radio frequency OFDM (RF-OFDM) signal. Since the minimum GB should be equal to the bandwidth of the OOFDM, the SE is half in comparison with the CO-OFDM system. It is worth mentioning that reducing the GB of the DDOOFDM results in not only improving the SE, but also relaxation of the bandwidth requirements of the electronic components at both transmitter and receiver fronts, including the PD bandwidth, analog-to-digital 0146-9592/13/214335-04$15.00/0
conversion (ADC), and digital-to-analog conversion (DAC) sampling rates. Although the SE can be improved further by directly reducing the GB in the SSB-OOFDM, the system performance of the DD is degraded due to SSBI. Some research works have been done to address this issue [9–16]. In [11], Cao et al. have proposed the use of turbo coding to compensate for SSBI. In [12], an iterative detection is also proposed to reduce SSBI. Recently, Li et al. proposed block-wise signal-phase switching to cancel the SSBI of the specially designed OOFDM signal with two iteration schemes [13]. Moreover, in [14], the optical carrier is separated from the OOFDM signal and amplified by an erbium-doped fiber amplifier (EDFA) to suppress the SSBI; the EDFA as well as the optical hybrid used to couple the two optical tones make the scheme more complex. In [15] and [16], a beat interference cancellation receiver (BICR) has been proposed and is investigated in detail to mitigate SSBI in reduced GB SSB-OOFDM systems via one optical filter and one balanced receiver with improved tolerance to both phase noise and polarization mode dispersion (PMD). However, half of the optical power that is used to generate the SSBI only has no contribution in the received OFDM signals, so the receiver sensitivity is low. Moreover, the receiver configuration is not truly symmetric, although a balanced photodiode pair (BPD) is used. In this Letter, a simple SSBI cancellation receiver is proposed, consisting of an optical interleaver (IL), a 2 × 2 three-decibel optical coupler (OC), and a BPD. Since the SSBI is eliminated completely, the GB of the SSBOOFDM signal can be reduced greatly and the SE is improved. The simulation results of the 40 Gbit∕s 16-QAM SSB-OOFDM signal with the GB smaller than the bandwidth of the OFDM signal show that our proposed interference cancellation receiver based on balanced detection (ICRBD) can cancel the SSBI as well as some copolar noise, and performance degradation is slim compared with the full-GB case. As the GB reduces, the error vector magnitude (EVM) of the OFDM signal detected by the ICRBD increases much slowly in comparison with the DD scheme, and the EVM remains below the forward error correct (FEC) limit even with the GB of 2 GHz. © 2013 Optical Society of America
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OPTICS LETTERS / Vol. 38, No. 21 / November 1, 2013 WG
WS
WS
EC(t)
ES(t) WG
DC
fRF
E1(t)
EC(t)
fo
I 2 t
ICRBD
WS
WS
PD1
fo+fRF
−
E2(t)
ES(t) OC
IL
WG WS
PD2
I 1 t
fRF
WS fRF
μ
jE 1 tj2
jE 2 tj2 1 0 1 − cjE C tj2 cjE S tj2 p B jp 1 − ccE C tE S t − E C tE S t C C: μα2x B @ cjE tj2 1 − cjE tj2 A C S p −jp 1 − ccE C tES t − E C tE S t (4)
DC
Fig. 1. Schematic of the SSBI cancellation receiver with balanced detection (ICRBD). IL, interleaver; OC, optical coupler; PD, photodiode.
Figure 1 shows the schematic of our proposed SSBI cancellation receiver based on balanced detection (ICRBD). The SSB-OOFDM signal can be generated by different methods, such as optical single-sideband modulation of RF-OFDM, optical double-sideband modulation of RF-OFDM with optical SSB filtering, optical modulation of a baseband OFDM signal, and addition of an optical carrier. Generally, it can be expressed as Et E C ejωo t ES
∞ X i−∞
2 4
N∕2−1 X n−N∕2
≡ E C t E S t.
3 cni Πt − iT s ejωo 2πf RF 2πf n t 5 (1)
Here, E C and ωo 2πf o are the field amplitude and the angular frequency of the optical carrier, respectively; E S is the field amplitude of the OOFDM signal; cni is the complex signal carried by the n th subcarrier in the i th OFDM symbol with N subcarriers; Πt is the pulse shaping function, which is 1 in 1; T and 0 otherwise; T is the OFDM symbol duration; f RF ωRF ∕2π is the frequency of the RF-OFDM signal; and f n n∕T ωn ∕2π is the frequency of the n th subcarrier of the baseband OFDM signal. Since −N∕2 ≤ n < N∕2, the bandwidth of the OFDM signal is W S N∕T and the GB is W G f RF − W S ∕2. In the ICRBD, an IL is used to separate the SSBOOFDM signal as the optical carrier, E C t, and the OOFDM signal, E S t, which can be expressed in a Jones matrix as Et E C tE S tT . Then, they are input into a 2 × 2 OC with the transmission matrix p p 1p−c p jp c T αx ; (2) jp c 1−c where αx and c denote the additional loss and the coupling coefficient of the OC, respectively. p is the sign, whichpis 1 or −1. For an ideal 3 dB OC, c 1∕2 and αx 2∕2. The output mixed optical fields become ! p p ! E 1 t E C t 1 − c jp c TEt αx p p E 2 t E S t jp c 1−c ! p p 1 − cE C t jp cE S t : (3) αx p p jp cE C t 1 − cE S t If the two PDs of the BPD have identical sensitivity μ, the output photocurrents converted from the optical signals become
For the two photocurrents, the first term is the DC component, which comes from the optical carrier; the third term is a linearly downconverted RF-OFDM signal, which is what we desired; and the second term is the selfbeating of the OOFDM signal, including the DC component and the beat interference between the subcarriers, viz., the SSBI, whose spectrum is arranged from zero to N − 1∕T ≈ W S , with a bandwidth of W S and reducing power spectrum density. For the SSB-OOFDM signal with a small GB, the SSBI is overlapped with the desired OFDM signal in the DD scheme. In the BPD, the two photocurrents, I 1 t and I 2 t, are subtracted and the SSBI term as well as the DC components are added destructively, whereas the desired OFDM signal is added constructively. So, the output photocurrent of the BPD becomes It I 1 t − I 2 t μα2x f1 − 2cjE C tj2 − 1 − 2cjE S tj2 p 2jp 1 − ccE C tE S t − E C tE S tg:
(5)
In the balanced detection scheme with the 3 dB OC, viz., c 1∕2, the first two terms are cancelled out completely, and so we obtain It 2μα2x RefjpEC tE S z; tg ( " N∕2−1 ∞ X X 2 2pμαx Re E C E S e cni Πt − iT s i−∞ n−N∕2
#) × ejf RF 2πf n tπ∕2 2pμα2x E C E S
∞ X
"
N∕2−1 X
i−∞ n−N∕2
Refcni ejf RF 2πf n tπ∕2 g
#
× Πt − iT s :
(6)
It can be seen that the DC components as well as the SSBI are zero, and only the OFDM signal is output with doubled amplitude. Since the SSBI is eliminated, the GB between the optical carrier and the OOFDM sideband is unnecessary theoretically. But in the practical case the IL is, usually, not the ideal brick-wall edge and the laser frequency drifts randomly around the central frequency. So some frequency gap is required to avoid corruption on the two tones as they are separated. Of course, a sharper edge of the IL and a more stable laser will mean a smaller GB.
November 1, 2013 / Vol. 38, No. 21 / OPTICS LETTERS
For the SSB-OOFDM signal generated in our scheme, since the optical carrier and the OOFDM sideband come from the same laser source, the frequency and the phase offset of the two tones are synchronous and can be cancelled as beating with each other. As the SSB-OOFDM signal is transmitted along the fiber, the polarization direction may rotate randomly, but the two tones can remain copolarized. In the ICRBD, both the IL and the OC are polarization-insensitive, so the two tones in E 1 t and E 2 t can be maintained copolarized easily and their beating efficiency is immune to the random polarization of the two tones. To verify the feasibility of our proposed ICRBD for the SSB-OOFDM with a reduced GB, a concept-proof simulation SSB-OOFDM link with a 40 Gbit∕s 16-QAM signal is built, as shown in Fig. 2. For comparison, a DD is also conducted. In the transmitter, the baseband OFDM signal is generated by an inverse fast Fourier transform (IFFT) module. The pseudo-random binary sequence with a word length of 215 –1 is mapped to 16-QAM data with I- and Q-branches, and is then input into the IFFT module for serial–parallel conversion and IFFT and parallel– serial conversion. The IFFT size is 256. Among the 256 subcarriers, 128 subcarriers are allocated for bearing the signals, whereas the others are zero-padded at the edges for oversampling. No cyclic prefix and pilot subcarrier are added because no fiber transmission is conducted here. After DAC, the baseband OFDM signal is upconverted by the RF, with frequency varying from 6 to 20 GHz for characterizing the influence of the GB on the signal performance. The 10-GHz-bandwidth RFOFDM signal with the 10 GHz RF is shown by the spectrum in Fig. 2(a). Then, the RF-OFDM signal is modulated on the light wave from a CW laser diode with a central frequency of 193.1 THz and a linewidth of 100 MHz, as shown in Fig. 2(b). The RF-OFDM signal drives an optical Mach–Zehnder (MZM) modulator in a pull–push pattern with a peak-to-peak voltage swing of 0.5 V π to reduce the nonlinear distortion. The carrier-to-signal power ratio (CSPR) can be adjusted flexibly via the DC bias voltage of MZM. The generated double-sideband OOFDM signal is filtered by a tunable optical filter for suppressing the lower sideband to produce the SSB-OOFDM. By Tx
LO
(a)
OFDM signal
ICRBD
Rx PD1 −
(d) MZM
IL
OC
PD2
TOF EDFA
CW LD
(b)
(c)
DD
Rx (e) PD
Fig. 2. Simulation link of the SSB-OOFDM signal with the reduced GB, detected by the ICRBD and DD. The insets are (a) the spectra of the transmitted RF-OFDM signal and the RF-OFDM signal received by (d) the ICRBD and (e) DD, (b) the optical spectrum of the CW laser diode, and (c) the transmitted SSB-OOFDM signal. CW LD, continuous-wave laser diode; LO, local oscillator; MZM, Mach–Zehnder modulator; TOF, tunable optical filter; EDFA, erbium-doped fiber amplifier; IL, interleaver; OC, optical coupler; PD, photodiode.
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adjusting the bias voltage of the MZM, the bandwidth of the optical filter, and the pump optical power properly, transmitted SSB-OOFDM signals with a CSPR of 1.2 dB and an optical power of 5.4 dBm are generated with different GBs. The CSPR is close to the optimum value [17]. Figure 2(c) is the optical spectrum of an SSB-OOFDM signal with a GB of 5 GHz. It can be seen that the GB between the optical carrier and the OOFDM signal is only half of the bandwidth of the OOFDM signal. The generated SSB-OOFDM signal is then injected into the proposed ICRBD. In the ICRBD, the IL with a sharp edge at 193.102 THz separates the optical carrier at 193.1 THz and the OOFDM signal at 193.110 THz. A 2 × 2 three-decibel OC is used to couple with the optical carrier and the OOFDM signal with equal power but with relative phase shifts of 90 deg and −90 deg for the two outputs; the two combined signals from the OC are fed to the BPD. After opto-electrical conversion and subtraction with sensitivity of 1 mA∕mW, photocurrent with only the RF-OFDM signal is generated, as shown by the spectrum in Fig. 2(d). It can be seen the SSBI component as well as the DC component are eliminated by the ICRBD. The RF spectrum of the 10-GHz-bandwidth SSB-OOFDM signal with the 5 GHz GB detected by DD is also given in Fig. 2(e) for comparison. It can be seen that the SSBI generated by DD overlapped with the desired OFDM signal in the frequency domain. Comparing the spectra in Figs. 2(d) and 2(e), we can also observe that the white noise at a higher frequency range is also cancelled out, which will be investigated in the future. The RF-OFDM signal photocurrent is coherently demodulated to the baseband OFDM signal with I- and Q-branches. Then, following the reverse routines as in the OFDM signal generation, including ADC, serial– parallel conversion, and FFT and parallel–serial conversion, the 16-QAM signal is obtained. The EVM versus optical power curves for the 40 GHz SSB-OOFDM signal with the 5 GHz GB detected by the ICRBD and DD are demonstrated in Fig. 3 along with constellation diagrams. From the EVM curves, we can see that there are error floors for the ICRBD at 10% with an optical power of −10 dBm and for DD at 30% with a power of −15 dBm, and the ICRBD reduces the EVM at least 20% when the optical power of the SSB-OOFDM signal is larger than −15 dBm. The error floor of the OFDM signal by DD appears before that of the signal by the ICRBD because of the SSBI or the crosstalk from the baseband by DD. While the latter error floor is for both cases mainly due to the MZM’s nonlinearity. The constellations of DD are blurred greatly by the SSBI, as shown in Figs. 3(a) and 3(b), whereas the constellations of the ICRBD in Figs. 3(c) and 3(d) show clear points with good convergence. This is attributed to the fact that the SSBI overlapped with the OFDM signal in the DD scheme cannot be filtered out in the coherent demodulation. In the ICRBD, the SSBI can be cancelled out completely even if it is overlapped with the OFDM signal. Compared with the curve of Q-factor versus launch power of the BICR in Fig. 4 of [16], it can be seen that both cases are limited by the ASE noise of the EDFA and the laser source at a lower optical power. In the BICR case in [16], the Q-factor reduces due to fiber nonlinearity as the launch power increases, when the launch optical
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Fig. 3. EVM versus optical power and constellation diagrams of the OFDM signal received with the ICRBD and DD.
power is higher than −1 dBm. Since the SSB-OOFDM signal is transmitted over the fiber, the degradation caused by the fiber nonlinearity reduces the Q-factor as the launch power increases. In this Letter, since the SSBOOFDM signal is not transmitted over the fiber, there is no transmission penalty due to fiber nonlinearity as the optical power increases. Of course, if the SSBOOFDM signal is transmitted over the optical fiber, the EVM will also increase at higher launch optical powers due to fiber nonlinearity. To check the capability of the ICRBD to reduce the GB of the SSB-OOFDM, the 40 Gbit∕s 16-QAM SSB-OOFDM signal with the optical power of 5.4 dBm is detected by the ICRBD and DD for comparison, with the GB varying from 1 to 15 GHz and, correspondingly, RF varying from 6 to 20 GHz. The EVMs calculated from the simulated constellation diagrams are shown in Fig. 4. It can be seen that the EVMs of the ICRBD and DD are almost same as the GB is equal to or larger than the bandwidth of the OFDM signal, which can also be observed from the constellation diagrams in Figs. 4(c) and 4(d), since the SSBI is located outside of the bandwidth of the OFDM signal and its impact can be eliminated by filtering out in the coherent demodulation in DD. As the GB is reduced to a value smaller than the bandwidth of the OOFDM signal, the EVM of DD increases rapidly and is bigger than 20% as GB 7 GHz, as shown in the constellation diagram in Fig. 4(b), whereas the EVM of the ICRBD increases much slowly until the GB is reduced to 3 GHz, which is similar
Fig. 4. EVM versus GB for the ICRBD and DD, with constellations.
to the BICR with a higher order filter in [16]. But the EVM still remains below the FEC limit even after the GB is reduced to 2 GHz. Under the FEC limit of EVM 16.3% for 16-QAM, the ICRBD can reduce the GB of the SSBOOFDM from 7.6 to 2 GHz and so the spectrum efficiency is improved greatly. The GB limit of 2 GHz is because the edge of the IL filter damages the OOFDM signal, as can be seen from the constellation in Fig. 4(a). A sharper edge filter can reduce the GB further. In summary, we have proposed and demonstrated a novel detection scheme for an SSB-OOFDM signal with an SSBI cancellation receiver based on balanced coherent detection. Using this method, the DC and the SSBI can be cancelled out, and the GB of the SSB-OOFDM signal can be reduced greatly. Compared with DD, the SE of the SSB-OOFDM signal is improved, although the complexity of the receiver is increased to some degree. The ICRBD can be used not only in OFDM-based optical access networks, but also in single-polarization long-haul optical transmission systems. This work is supported in part by the Program for New Century Excellent Talents in University through grant no. NECT-11-0595, the Specialized Research Fund for the Doctoral Program of Higher Education (grant no. 20100005120014), and the Fundamental Research Funds for the Central Universities of China (grant no. 2013RC0209). References 1. X. Liu, S. Chandrasekhar, T. Lotz, P. Winzer, H. Haunstein, S. Randel, S. Corteselli, B. Zhu, and D. W. Peckham, in Optical Fiber Communication Conference and Exposition (Optical Society of America, 2012), paper PDP5B.3. 2. T. Omiya, M. Yoshida, and M. Nakazawa, Opt. Express 21, 2632 (2013). 3. Q. Yang, Z. He, Z. Yang, S. Yu, X. Yi, and W. Shieh, Opt. Express 20, 2379 (2012). 4. X. Yi, N. Fontaine, R. Scott, and S. Yoo, J. Lightwave Technol. 28, 2054 (2010). 5. Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, J. Lightwave Technol. 28, 308 (2010). 6. J. Yu, Z. Dong, and N. Chi, IEEE Photon. Technol. Lett. 23, 1061 (2011). 7. J. Armstrong, IEEE J. Lightwave Technol. 27, 189 (2009). 8. A. J. Lowery and J. Armstrong, Opt. Express 14, 2079 (2006). 9. I. V. Djordjevic and B. Vasic, Opt. Express 14, 3767 (2006). 10. W. R. Peng, X. Wu, V. R. Arbab, K. Feng, B. Shamee, L. C. Christen, J. Y. Yang, A. E. Willner, and S. Chi, J. Lightwave Technol. 27, 1332 (2009). 11. Z. Cao, J. Yu, W. Wang, L. Chen, and Z. Dong, IEEE Photon. Technol. Lett. 22, 736 (2010). 12. W.-R. Peng, B. Zhang, K.-M. Feng, X. Wu, A. E. Willner, and S. Chi, J. Lightwave Technol. 27, 5723 (2009). 13. A. Li, D. Che, X. Chen, Q. Hu, Y. Wang, and W. Shieh, Opt. Lett. 38, 2614 (2013). 14. B. J. C. Schmidt, Z. Zan, L. B. Du, and A. J. Lowery, in Optical Fiber Communication Conference (Optical Society of America, 2009), paper PDPC3. 15. W. Peng, I. Morita, and H. Tanaka, in European Conference and Exhibition on Optical Communication (ECOC, 2010), paper Tu.4.A.2. 16. S. A. Nezamalhosseini, L. R. Chen, Q. Zhuge, M. Malekiha, F. Marvasti, and D. V. Plant, Opt. Express 21,15237 (2013). 17. C. Lim, M. Attygalle, A. Nirmalathas, D. Novak, and R. Waterhouse, IEEE Trans. Microwave Theory Tech. 54, 2181 (2006).
