Experimental demonstration of 24-Gb/s CAP-64QAM radio-over-fiber system over 40-GHz mm-wave fiber-wireless transmission Junwen Zhang,1,2,3 Jianjun Yu,1 Nan Chi,1,* Fan Li,2,3 and Xinying Li1 1
Department of Communication Science and Engineering, and Key Laboratory for Information Science of Electromagnetic Waves (MoE), Fudan University, 220 Handan Road, Shanghai 200433, China 2 ZTE (TX) Inc., Morristown, NJ 07960, USA 3 The School of ECE, Georgia Institute of Technology, Atlanta, GA30332, USA *[email protected]
Abstract: We propose and demonstrate a novel CAP-ROF system based on multi-level carrier-less amplitude and phase modulation (CAP) 64QAM with high spectrum efficiency for mm-wave fiber-wireless transmission. The performance of novel CAP modulation with high order QAM, for the first time, is investigated in the mm-wave fiber-wireless transmission system. One I/Q modulator is used for mm-wave generation and base-band signal modulation based on optical carrier suppression (OCS) and intensity modulation. Finally, we demonstrated a 24-Gb/s CAP-64QAM radio-over-fiber (ROF) system over 40-km stand single-mode-fiber (SMMF) and 1.5-m 38-GHz wireless transmission. The system operation factors are also experimentally investigated. ©2013 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (060.4080) Modulation.
References and links 1.
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#196650 - $15.00 USD Received 29 Aug 2013; revised 16 Oct 2013; accepted 19 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026888 | OPTICS EXPRESS 26888
13. J. L. Wei, D. G. Cunningham, R. V. Penty, and I. H. White, “Study of 100 Gigabit Ethernet using carrierless amplitude/phase modulation and optical OFDM,” J. Lightwave Technol. 31(9), 1367–1373 (2013). 14. 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(27), 26551–26556 (2011). 15. M. B. Othman, X. Zhang, L. Deng, M. Wieckowski, J. Jensen, and I. T. Monroy, “Experimental investigations of 3D/4D-CAP modulation with DM-VCSELs,” IEEE Photon. Technol. Lett. 24(22), 2009–2012 (2012). 16. G. Stepniak and J. Siuzdak, “Transmission beyond 2 Gbit/s in a 100 m SI POF with multilevel CAP modulation and digital equalization,” in Proc. of OFC 2013, paper NTu3J.5. 17. L. Tao, Y. Wang, Y. Gao, A. P. Lau, N. Chi, and C. Lu, “Experimental demonstration of 10 Gb/s multi-level carrier-less amplitude and phase modulation for short range optical communication systems,” Opt. Express 21(5), 6459–6465 (2013). 18. M. Iglesias Olmedo, Z. Tianjian, J. Bevensee Jensen, Z. Qiwen, X. Xu, and I. T. Monroy, “Towards 400GBASE 4-lane solution using direct detection of MultiCAP dignal in 14 GHz bandwidth per Lane,” in Proc. OFC 2013, paper PDP5C.10. 19. J. L. Wei, L. Geng, D. G. Cunningham, R. V. Penty, and I. H. White, “Gigabit NRZ, CAP and optical OFDM systems over POF links using LEDs,” Opt. Express 20(20), 22284–22289 (2012). 20. J. D. Ingham, R. V. Penty, and I. H. White, “40 Gb/s carrierless amplitude and phase modulation for low-cost optical data communication links,” in Proc. OFC 2011, OThZ3. 21. J. Zhang, J. Yu, F. Li, N. Chi, Z. Dong, and X. Li, “11 × 5 × 9.3Gb/s WDM-CAP-PON based on optical single-side band multi-level multi-band carrier-less amplitude and phase modulation with direct detection,” Opt. Express 21(16), 18842–18848 (2013).
1. Introduction Radio-over-fiber (ROF) technology, which can effectively integrate optical and wireless systems, is a promising solution to provide the large capacity and high flexibility. The wide variety of multimedia data and services drive the demand of wireless access rate from today’s end users [1–12]. To meet this capacity and spectrum efficiency needs, ROF fiber-wireless system are moving from the On-off keying (OOK) [1–3] to more advanced modulation formats, such as the quadrature amplitude modulation (QAM) based the sub-carrier modulation (SCM) [4–7], the orthogonal frequency division multiplexing (OFDM) [8–10] and optical complex signal transmission with in-phase/quadrature (I/Q) modulation and heterodyne up-conversion [11,12]. One candidate technique that may provide good system performance and low complexity is the carrier-less amplitude and phase (CAP) modulation based on intensity modulation and direct detection, which has been widely investigated in short-range optical access networks recently [13–17]. It allows relatively high data rate to be achieved using electrical and optical components of limited bandwidth and less complexity in both transmitter and receiver [13–17]. Since the CAP signal is generated by the orthogonal digital filters and approaching Nyquist spectrum shaping is naturally performed [17], higher spectrum efficiency can be obtained for CAP compared with ordinary QAM-SCM in previous works [4–7]. Comparing with alternative schemes such as QAM and OFDM, no electrical or optical complex-to-real-value conversion is necessary which involves a complex mixer and radio frequency (RF) source for base-band signal modulation. Neither does it require the discrete Fourier transform (DFT) that is utilized in OFDM signal generation and demodulation [16,17]. The CAP signal can be generated by using a digital filter with several taps and a higher order modulation can be realized, thus reducing the complexity of computation and system structure considerably. In [13], they have proved CAP has great potential of being with high power efficiency and with low cost compared with OFDM for short range optical transmission. In [14,15], they also have demonstrated the feasibility of CAP-based access network. In [16], systems based on CAP-64QAM are proposed with high spectrum efficiency but the bit rate is only 2.1-Gb/s. Most recently in [18], multi-level multi-band CAP (MM-CAP) has been proposed to extend the bandwidth of each channel for high capacity short range data transmission. Without using a large size of FFT/IFFT, simulation studies [19] have shown that CAP can outperform OFDM in SI POF, remaining more cost-effective and less energy consuming. On the other hand, since only orthogonal filters are used for signal generation, as proposed in [20], they have experimentally demonstrated a proof-of-concept CAP system by
#196650 - $15.00 USD Received 29 Aug 2013; revised 16 Oct 2013; accepted 19 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026888 | OPTICS EXPRESS 26889
using analog transversal filters in the transmitter and receiver. Therefore, CAP schemes have great potential for cost-effectiveness and power efficiency. Therefore, considering the large capacity and high spectrum efficiency with all these technical advantages, it is worth investigating the CAP in ROF system with fiber-wireless transmission. In this paper, we propose and demonstrate a novel CAP-ROF system based on multi-level CAP-64QAM with high spectrum efficiency for mm-wave fiber-wireless transmission. The performance of novel CAP modulation with high order QAM is investigated in the mm-wave fiber-wireless transmission system. One I/Q modulator is used for mm-wave generation and base-band signal modulation based on optical carrier suppression (OCS) and intensity modulation. Finally, we demonstrated a 24-Gb/s CAP-64QAM ROF system over 40-km stand single-mode-fiber (SMMF) and 1.5-m 38-GHz wireless transmission. The system operation factors are also investigated. To the best of our knowledge, this is the first demonstration for CAP-64QAM used for ROF system with such a high speed. 2. The CAP-ROF system architecture and the principle of CAP signal processing
Fig. 1. The mm-wave generation and signal modulation based on I/Q modulator.
Figure 1 shows the ROF signal modulation and mm-wave generation architecture in our system. Here, we use the OCS method to generate the mm-wave. OCS technique with intensity modulation has been demonstrated to show the simplicity in system configuration and good performance in long distance transmission [1,2]. Different from previous work in [1,2] based on two cascaded Mach-Zehnder modulators (MZMs), we use only one I/Q modulator for both signal modulation and mm-wave generation similar as in [3,4]. The I/Q modulator consists of two MZMs (MZM1 and MZM2) placed in parallel with two arms, and a phase shifter (PS) in one arm. The continuous-wavelength (CW) light used as optical source Ec, is split into the two arms in the I/Q modulator, while one is for the base-band CAP signal modulation by MZM1 and the other is for mm-wave tones generation by MZM2 based on OCS as shown in Fig. 1. For the upper arm with base-band signal modulation, the MZM1 is driven by the electrical CAP-mQAM signal which is generated by the digital-to-analog-convertor (DAC). Assuming the modulation is in the linearity area with a small-signal driving condition, the output of the upper arm can be expressed as E1 (t ) =
1 π 1 π Ec × cos[ (α1S BB (t ) + β1 )] ≈ Ec × [ (α1 S BB (t ) + Δβ1 )] 2 2 2 2
(1)
Here, α1 and β1 are the modulation index and DC bias of the upper MZM1, which represent the ratio of signal and DC bias voltage to the half-wave voltage, respectively. Δβ1 = β1-1, shows the bias offset. SBB(t) is the normalized base-band signal. Therefore, the power of optical carrier after base-band signal modulation is determined by the DC part Δβ1. Here, we assume small signal modulation is just for mathematic expression. The driver/modulator working condition for base-band CAP modulation is the same with other modulation formats, such as OFDM or QAM, which should be modulated in the liner region. #196650 - $15.00 USD Received 29 Aug 2013; revised 16 Oct 2013; accepted 19 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026888 | OPTICS EXPRESS 26890
For the lower arm used for mm-wave tones generation, the MZM2 is driven by the radio frequency (RF) clock signal at fs for OCS modulation. One general method is to set the MZM2 biased at the null-point at the half-wave voltage. After I/Q modulator, the base-band signal and the first-order tone are filtered out by an optical band-pass filter (OBPF) with bandwidth less than 2fs. The other undesired tones are also cut off. In this way, the final output after the OBPF can be expressed as Eout (t ) =
1 π π π Ec × [ (α1S BB (t ) + Δβ1 )] + sin( β 2 ) exp[ j (2π f s t + + ϕ ps )] (2) 2 2 2 2
Here, φps is the phase shift caused by the phase shifter in the I/Q modulator. In this way, only based-band signal and the first-order tone are kept with frequency spacing of fs. After optical beating at the photo detector (PD), the mm-wave is generated at the frequency of fs, which can be used for wireless transmission. Since the outputs of the two arms of the I/Q modulator are from the same light source and only intensity modulation is used in each arm, the laser linewidth has no obvious impact on the results. At the mm-wave receiver, the mm-wave signal is first down-converted to the base-band and then recovered by the CAP m-QAM digital signal processing (DSP) blocks. From the above analysis, we can see that the DC bias of the two MZMs is very important for signal modulation and mm-wave tones generation. Especially for the fiber-wireless with both optical and electrical devices, the linear or nonlinear impairments should also be considered with these factors. Data Stream
M-QAM Mapping
mfI mfQ
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Fig. 2. Schematic diagrams of transmitter and receiver based on CAP m-QAM for CAP signal generation and processing.
Figure 2 shows the schematic diagrams of transmitter and receiver based on CAP m-QAM for fiber-wireless transmission. CAP is a multilevel and multi-dimensional modulation format proposed by Bell Labs for short range communication [13–17], which is similar to QAM signal, but does not require a RF carrier source. Different from QAM or OFDM intensity modulation, CAP does not use a sinusoidal carrier to generate two orthogonal components I and Q. The two dimensional CAP can be generated by using two orthogonal filters as fI and fQ shown in Fig. 2 as the filter pair. The original bit sequence is first mapped into complex symbols of m-QAM (m is the order of QAM), and then the mapped symbols are up-sampled to match the sample rate of shaping filters. The sample rate of shaping filters is determined by the data baud rate and DAC sample rate. For CAP generation, the I and Q components of the up-sampled sequence are separated and sent into the digital shaping filters respectively. The outputs of the filters are subtracted to be combined together as S(t) after DAC to drive the upper MZM1. At the receiver side, the received signal after down-conversion is fed into two matched filters to separate the I and Q components. The orthogonal and matched filter pairs fI(t), fQ(t), mfI(t) and mfQ(t) are the corresponding shaping filters and form a so-called Hilbert pair in transmitter and receiver. The two orthogonal filters are constructed by multiplying a square root raised cosine pulse with a sine and cosine function respectively, as described in [21]. Assuming sI(t) and sQ(t) are the I and Q data after #196650 - $15.00 USD Received 29 Aug 2013; revised 16 Oct 2013; accepted 19 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026888 | OPTICS EXPRESS 26891
QAM mapping and up-sampling, then the combined output signal S(t) of CAP signal can be expressed as
S (t ) = [ sI (t ) ⊗ f I (t ) − sQ (t ) ⊗ f Q (t )]
(4) n
n
At the receiver, generally, we have the matched filters with relations as mfI (t) = fI (-t), and mfQn(t) = fQn(-t). In this way, for the CAP receiver, the I and Q data after matched filter pair can be expressed as rI (t ) = R (t ) ⊗ mf I (t ),
rQ (t ) = R(t ) ⊗ mf Q (t )
(5)
where R(t) is the CAP signal after down-conversion at the receiver and rI (t)and rQ(t) are the output after matched filter pair. After down-sampling, a linear equalizer is employed for the complex signal and a decoder is utilized to obtain the original bit sequence. 3. Experimental setup and results
Fig. 3. Experimental setup. (AWG: arbitrary waveform generator; ECL: external cavity laser; EDFA: Erbium-doped fiber amplifier; TOF: tunable optical filter; TA: tunable attenuator; EA: electrical amplifier; ADC: analog-to-digital-convertor)
As a proof of concept, the experimental setup of the 24-Gb/s CAP-64QAM ROF system over 40-km SMMF and 1.5-m 38-GHz wireless transmission is shown in Fig. 3. One external cavity laser (ECL) at 1550.10-nm is used as CW light source. The I/Q modulator is commercially available with two MZMs placed in parallel with two arms. The upper arm MZM is driven by the 4-Gbaud CAP-64QAM signal, which is generated by the 12GSa/s commercial arbitrary waveform generator (AWG). The data sequence is first mapped to the 8 level 64-QAM I and Q signals with 16x211 symbols. Then, the 16 sets of 8-level I and Q sequences are up-sampled to 3 Sa/symbol and filtered by the orthogonal Hilbert filters pair. The filters are finite impulse response (FIR) filters with length of 10 symbols each. The roll-off coefficient is 0.2 and the excess bandwidth is set to 15%. In this way, the 4-Gbaud base-band CAP-64QAM signal is generated by the AWG. The lower arm which is used for mm-wave tones generation is operated at OCS condition. The lower arm MZM2 is biased at null-point, and driven by the 38-GHz RF signal. One OBPF with 0.5-nm 3-dB bandwidth is used for the base-band signal and first-order tone filtering. The optical spectrum of the output of lower arm MZM2 with mm-wave tones and the output after I/Q modulator with both base-band signal and mm-wave tones are shown in Fig. 4 as (a) and (b). From Fig. 4(a), we can see an obvious carrier suppression result with more than 50-dB first-tone to carrier power ratio. Figure 4(c) show the result after OBPF, where only base-band
#196650 - $15.00 USD Received 29 Aug 2013; revised 16 Oct 2013; accepted 19 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026888 | OPTICS EXPRESS 26892
signal and the left side-band first order mm-tone are kept with carrier spacing of 38-GHz. The power ratio of the carrier ((left side-band first order mm-tone) to base-band (CSR) is about 18-dB. After that, the generated optical mm-wave signal is then launched into the 40-km SSMF, which has about 10-dB loss and 17-ps/km/nm chromatic dispersion (CD) at 1550nm without optical dispersion compensation. One Erbium-doped fiber amplifier (EDFA) is used before the fiber to compensate the fiber loss. mm-wave tones
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Fig. 4. The optical spectrum of (a) the output of lower arm MZM2 with mm-wave tones, (b) the output after I/Q modulator with both base-band signal and mm-wave tones, and (c) the output after OBPF with only base-band signal and the left side-band first order mm-tone.
For wireless transmission, the optical base-band signal is up-converted by the optical beating at the 50-GHz PD. The generated 38-GHz mm-wave is first amplified by an electrical amplifier (EA) with bandwidth of about 10-GHz and then broadcasted through a horn antenna with a gain of 25-dBi. At the wireless receiver side, the broadcasted wireless signal is received by another 40 GHz horn antenna with 25-dBi gain. The received signal is then directly sampled by a 120-GSa/s high-speed oscilloscope with 45-GHz bandwidth for analog-to-digital conversion (ADC). Figure 5(a) shows the FFT spectrum of 38-GHz mm-wave signal after ADC. We can see that the mm-wave signal carrying 24-Gb/s CAP 64-QAM signal centered at 38-GHz occupies less than 9-GHz bandwidth. 0
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Fig. 5. The FFT spectrum of (a) the received 24-Gb/s CAP-64QAM mm-wave signal at 38-GHz carrier after ADC and (b) the down-converted base-band 24-Gb/s CAP-64QAM signal.
After ADC, the off-line DSP is employed. First, the signal is down converted to base-band signal by multiplying a synchronous cosine or sine function. Then, the down-converted base-band signal is resampled to 16-GSa/s. Figure 5(b) shows the FFT spectrum of the down-converted base-band CAP 64-QAM signal with 16-GSa/s. It is worth noting that here we use digital down-conversion after the high speed ADC to demonstrate the ROF system. For practical use, an analog down-conversion can be employed using RF local oscillator (LO) to relax the high speed ADC requirement. After that, the re-sampled 16-GSa/s signal passes through the orthogonal matched filters pair to obtain the I and Q data. Noting that, the matched FIR filters are now at 4Sa/symbol and with 10 symbols each. Then, the I and Q signals are down-sampled to 2Sa/symbol before the linear equalization. Here, four real-valued, 31-tap, T/2-spaced butterfly configured adaptive digital FIR filters, based on decision-directed least mean squares (DD-LMS) algorithm, are used for signal equalization and recovery.
#196650 - $15.00 USD Received 29 Aug 2013; revised 16 Oct 2013; accepted 19 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026888 | OPTICS EXPRESS 26893
Figure 6 shows the bit-error-ratio (BER) results of the 24-Gb/s CAP 64-QAM signal versus the received optical power in back-to-back (BTB) case without fiber or wireless transmission, after 40-km SMMF transmission and after 40-km SMMF and 1.5-m wireless transmission. The optical power is measured after a tunable attenuator (TA). A 0.9nm TOF is used before PD front-end to remove excess amplified spontaneous emission (ASE) noise after EDFA. We can see that the required received optical power at BER of 3.8x10−3 for the 7% hard-decision FEC limit in BTB case is about −25dBm. Less than 1-dB power penalty after 40-km fiber transmission is observed at the BER of 3.8x10−3 compared to BTB case. However, about 2-dB power penalty is introduced by the 1.5-m wireless transmission due to the wireless transmission power loss and impairments. The constellations of the processed CAP 64-QAM signal at −22dBm received power after 40-km fiber transmission without and with 1.5-m wireless transmission are shown in the insets (i) and (ii) of Fig. 6, respectively. Therefore, these results clearly demonstrate the feasibility of the proposed high spectrum efficiency CAP 64-QAM ROF fiber-wireless transmission system. It is worth noting that, as a proof of concept, we only measured the results with 1.5-m wireless link. We believe longer distance of wireless transmission can be realized by getting larger received optical power or by using electrical amplifiers in the wireless transmitter and receiver sides. BTB 40Km Fiber only 40Km Fiber and 1.5 m Wireless
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To further study the ROF system performance, we also test and measured the impact of some system factors on the BER results as mentioned in section 2. First, we study the impact of DC bias 1 of the upper arm MZM1 for base-band signal modulation. As analyzed above, the offset Δβ1 = β1-1, shows the bias offset and determines the power of carrier or the power of DC component of the modulated signal. 0.01
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Fig. 7. (a) The optical spectrum of base-band signal and mm-wave tone under different DC bias 1 offset, 0.8V and 0V; (b) The BER versus the offset of DC bias 1 after 40-km SSMF and 1.5-m wireless transmission.
#196650 - $15.00 USD Received 29 Aug 2013; revised 16 Oct 2013; accepted 19 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026888 | OPTICS EXPRESS 26894
Figure 7(a) shows the optical spectrum of base-band signal and mm-wave tone under different DC bias 1 offset, 0.8V and 0V. We can see that, the power of optical carrier of the base-band signal gets larger when the DC bias 1 offset is not zero. About 7.5-dB power difference exists between zero and 0.8 V bias offset. Figure 7(b) shows the BER results versus the offset voltage of DC bias 1. We can see that the BER get worse for larger offset and the optimal offset should be zero, which means the bias of MZM1 should be at null point. We believe there are two main reasons. First, the modulation is at the optimal linear area when the MZM is biased at null-point. Second, the large power optical carrier is also up-converted to mm-wave, which reduces the signal-to-noise-ratio (SNR) after passing the gain-limited optical and electrical amplifiers. The impact of the offset of DC bias 2 on BER performance is shown in Fig. 8(a). The results are similar to that in Fig. 7(b). As analyzed in section 2, the DC bias 2 should be set at null-point for carrier suppression. The offset of DC bias 2 can worsen the carrier suppression output, which introduces the signal crosstalk and in-band carrier. Again, the large power mm-wave signal carrier reduces the SNR after passing the gain-limited electrical amplifiers. Figure 8(b) shows the impact of CSR (the left first order side-band tone to the base-band signal as shown in Fig. 4(c)) on the BER performance after fiber and wireless transmission. Here, we adjust the CSR of the output optical signal after I/Q modulator before the EDFA and fiber by using the OBPF to adjusting the power of carrier and base-band signal. We can see that, the CSR should be larger than 10-dB. The optimal CSR is about 18-dB and slight BER deterioration is observed for CSR larger than 20-dB due to the gain competition when passing the EDFA.
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Fig. 8. (a) The BER versus the offset of DC bias 2 after 40-km SSMF and 1.5-m wireless transmission; (b) The BER versus the CSR after 40-km SSMF and 1.5-m wireless transmission.
4. Conclusion
We propose and demonstrate a novel CAP-ROF system based on multi-level CAP 64-QAM with high spectrum efficiency for mm-wave fiber-wireless transmission. The performance of novel CAP modulation with high order QAM is investigated in the mm-wave fiber-wireless transmission system. One I/Q modulator is used for mm-wave generation and base-band signal up-conversion based on carrier-suppression modulation. Finally, we demonstrated a 24-Gb/s CAP-64QAM ROF system over 40-km SMMF and 1.5-m 38-GHz wireless transmission. To the best of our knowledge, this is the first demonstration for CAP-64QAM used for ROF system with such a high speed. Acknowledgments
This work was partially supported by the NHTRDP (973 Program) of China (Grant No. 2010CB328300), and NNSF of China (No. 61250018, No. 61177071), NHTRDP (863 Program) of China (2011AA010302, 2012AA011302), The National Key Technology R&D Program (2012BAH18B00), Key Program of Shanghai Science and Technology Association (12dz1143000)), and the China Scholarship Council (201206100076).
#196650 - $15.00 USD Received 29 Aug 2013; revised 16 Oct 2013; accepted 19 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026888 | OPTICS EXPRESS 26895
Department of Communication Science and Engineering, and Key Laboratory for Information Science of Electromagnetic Waves (MoE), Fudan University, 220 Handan Road, Shanghai 200433, China 2 ZTE (TX) Inc., Morristown, NJ 07960, USA 3 The School of ECE, Georgia Institute of Technology, Atlanta, GA30332, USA *[email protected]
Abstract: We propose and demonstrate a novel CAP-ROF system based on multi-level carrier-less amplitude and phase modulation (CAP) 64QAM with high spectrum efficiency for mm-wave fiber-wireless transmission. The performance of novel CAP modulation with high order QAM, for the first time, is investigated in the mm-wave fiber-wireless transmission system. One I/Q modulator is used for mm-wave generation and base-band signal modulation based on optical carrier suppression (OCS) and intensity modulation. Finally, we demonstrated a 24-Gb/s CAP-64QAM radio-over-fiber (ROF) system over 40-km stand single-mode-fiber (SMMF) and 1.5-m 38-GHz wireless transmission. The system operation factors are also experimentally investigated. ©2013 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (060.4080) Modulation.
References and links 1.
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#196650 - $15.00 USD Received 29 Aug 2013; revised 16 Oct 2013; accepted 19 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026888 | OPTICS EXPRESS 26888
13. J. L. Wei, D. G. Cunningham, R. V. Penty, and I. H. White, “Study of 100 Gigabit Ethernet using carrierless amplitude/phase modulation and optical OFDM,” J. Lightwave Technol. 31(9), 1367–1373 (2013). 14. 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(27), 26551–26556 (2011). 15. M. B. Othman, X. Zhang, L. Deng, M. Wieckowski, J. Jensen, and I. T. Monroy, “Experimental investigations of 3D/4D-CAP modulation with DM-VCSELs,” IEEE Photon. Technol. Lett. 24(22), 2009–2012 (2012). 16. G. Stepniak and J. Siuzdak, “Transmission beyond 2 Gbit/s in a 100 m SI POF with multilevel CAP modulation and digital equalization,” in Proc. of OFC 2013, paper NTu3J.5. 17. L. Tao, Y. Wang, Y. Gao, A. P. Lau, N. Chi, and C. Lu, “Experimental demonstration of 10 Gb/s multi-level carrier-less amplitude and phase modulation for short range optical communication systems,” Opt. Express 21(5), 6459–6465 (2013). 18. M. Iglesias Olmedo, Z. Tianjian, J. Bevensee Jensen, Z. Qiwen, X. Xu, and I. T. Monroy, “Towards 400GBASE 4-lane solution using direct detection of MultiCAP dignal in 14 GHz bandwidth per Lane,” in Proc. OFC 2013, paper PDP5C.10. 19. J. L. Wei, L. Geng, D. G. Cunningham, R. V. Penty, and I. H. White, “Gigabit NRZ, CAP and optical OFDM systems over POF links using LEDs,” Opt. Express 20(20), 22284–22289 (2012). 20. J. D. Ingham, R. V. Penty, and I. H. White, “40 Gb/s carrierless amplitude and phase modulation for low-cost optical data communication links,” in Proc. OFC 2011, OThZ3. 21. J. Zhang, J. Yu, F. Li, N. Chi, Z. Dong, and X. Li, “11 × 5 × 9.3Gb/s WDM-CAP-PON based on optical single-side band multi-level multi-band carrier-less amplitude and phase modulation with direct detection,” Opt. Express 21(16), 18842–18848 (2013).
1. Introduction Radio-over-fiber (ROF) technology, which can effectively integrate optical and wireless systems, is a promising solution to provide the large capacity and high flexibility. The wide variety of multimedia data and services drive the demand of wireless access rate from today’s end users [1–12]. To meet this capacity and spectrum efficiency needs, ROF fiber-wireless system are moving from the On-off keying (OOK) [1–3] to more advanced modulation formats, such as the quadrature amplitude modulation (QAM) based the sub-carrier modulation (SCM) [4–7], the orthogonal frequency division multiplexing (OFDM) [8–10] and optical complex signal transmission with in-phase/quadrature (I/Q) modulation and heterodyne up-conversion [11,12]. One candidate technique that may provide good system performance and low complexity is the carrier-less amplitude and phase (CAP) modulation based on intensity modulation and direct detection, which has been widely investigated in short-range optical access networks recently [13–17]. It allows relatively high data rate to be achieved using electrical and optical components of limited bandwidth and less complexity in both transmitter and receiver [13–17]. Since the CAP signal is generated by the orthogonal digital filters and approaching Nyquist spectrum shaping is naturally performed [17], higher spectrum efficiency can be obtained for CAP compared with ordinary QAM-SCM in previous works [4–7]. Comparing with alternative schemes such as QAM and OFDM, no electrical or optical complex-to-real-value conversion is necessary which involves a complex mixer and radio frequency (RF) source for base-band signal modulation. Neither does it require the discrete Fourier transform (DFT) that is utilized in OFDM signal generation and demodulation [16,17]. The CAP signal can be generated by using a digital filter with several taps and a higher order modulation can be realized, thus reducing the complexity of computation and system structure considerably. In [13], they have proved CAP has great potential of being with high power efficiency and with low cost compared with OFDM for short range optical transmission. In [14,15], they also have demonstrated the feasibility of CAP-based access network. In [16], systems based on CAP-64QAM are proposed with high spectrum efficiency but the bit rate is only 2.1-Gb/s. Most recently in [18], multi-level multi-band CAP (MM-CAP) has been proposed to extend the bandwidth of each channel for high capacity short range data transmission. Without using a large size of FFT/IFFT, simulation studies [19] have shown that CAP can outperform OFDM in SI POF, remaining more cost-effective and less energy consuming. On the other hand, since only orthogonal filters are used for signal generation, as proposed in [20], they have experimentally demonstrated a proof-of-concept CAP system by
#196650 - $15.00 USD Received 29 Aug 2013; revised 16 Oct 2013; accepted 19 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026888 | OPTICS EXPRESS 26889
using analog transversal filters in the transmitter and receiver. Therefore, CAP schemes have great potential for cost-effectiveness and power efficiency. Therefore, considering the large capacity and high spectrum efficiency with all these technical advantages, it is worth investigating the CAP in ROF system with fiber-wireless transmission. In this paper, we propose and demonstrate a novel CAP-ROF system based on multi-level CAP-64QAM with high spectrum efficiency for mm-wave fiber-wireless transmission. The performance of novel CAP modulation with high order QAM is investigated in the mm-wave fiber-wireless transmission system. One I/Q modulator is used for mm-wave generation and base-band signal modulation based on optical carrier suppression (OCS) and intensity modulation. Finally, we demonstrated a 24-Gb/s CAP-64QAM ROF system over 40-km stand single-mode-fiber (SMMF) and 1.5-m 38-GHz wireless transmission. The system operation factors are also investigated. To the best of our knowledge, this is the first demonstration for CAP-64QAM used for ROF system with such a high speed. 2. The CAP-ROF system architecture and the principle of CAP signal processing
Fig. 1. The mm-wave generation and signal modulation based on I/Q modulator.
Figure 1 shows the ROF signal modulation and mm-wave generation architecture in our system. Here, we use the OCS method to generate the mm-wave. OCS technique with intensity modulation has been demonstrated to show the simplicity in system configuration and good performance in long distance transmission [1,2]. Different from previous work in [1,2] based on two cascaded Mach-Zehnder modulators (MZMs), we use only one I/Q modulator for both signal modulation and mm-wave generation similar as in [3,4]. The I/Q modulator consists of two MZMs (MZM1 and MZM2) placed in parallel with two arms, and a phase shifter (PS) in one arm. The continuous-wavelength (CW) light used as optical source Ec, is split into the two arms in the I/Q modulator, while one is for the base-band CAP signal modulation by MZM1 and the other is for mm-wave tones generation by MZM2 based on OCS as shown in Fig. 1. For the upper arm with base-band signal modulation, the MZM1 is driven by the electrical CAP-mQAM signal which is generated by the digital-to-analog-convertor (DAC). Assuming the modulation is in the linearity area with a small-signal driving condition, the output of the upper arm can be expressed as E1 (t ) =
1 π 1 π Ec × cos[ (α1S BB (t ) + β1 )] ≈ Ec × [ (α1 S BB (t ) + Δβ1 )] 2 2 2 2
(1)
Here, α1 and β1 are the modulation index and DC bias of the upper MZM1, which represent the ratio of signal and DC bias voltage to the half-wave voltage, respectively. Δβ1 = β1-1, shows the bias offset. SBB(t) is the normalized base-band signal. Therefore, the power of optical carrier after base-band signal modulation is determined by the DC part Δβ1. Here, we assume small signal modulation is just for mathematic expression. The driver/modulator working condition for base-band CAP modulation is the same with other modulation formats, such as OFDM or QAM, which should be modulated in the liner region. #196650 - $15.00 USD Received 29 Aug 2013; revised 16 Oct 2013; accepted 19 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026888 | OPTICS EXPRESS 26890
For the lower arm used for mm-wave tones generation, the MZM2 is driven by the radio frequency (RF) clock signal at fs for OCS modulation. One general method is to set the MZM2 biased at the null-point at the half-wave voltage. After I/Q modulator, the base-band signal and the first-order tone are filtered out by an optical band-pass filter (OBPF) with bandwidth less than 2fs. The other undesired tones are also cut off. In this way, the final output after the OBPF can be expressed as Eout (t ) =
1 π π π Ec × [ (α1S BB (t ) + Δβ1 )] + sin( β 2 ) exp[ j (2π f s t + + ϕ ps )] (2) 2 2 2 2
Here, φps is the phase shift caused by the phase shifter in the I/Q modulator. In this way, only based-band signal and the first-order tone are kept with frequency spacing of fs. After optical beating at the photo detector (PD), the mm-wave is generated at the frequency of fs, which can be used for wireless transmission. Since the outputs of the two arms of the I/Q modulator are from the same light source and only intensity modulation is used in each arm, the laser linewidth has no obvious impact on the results. At the mm-wave receiver, the mm-wave signal is first down-converted to the base-band and then recovered by the CAP m-QAM digital signal processing (DSP) blocks. From the above analysis, we can see that the DC bias of the two MZMs is very important for signal modulation and mm-wave tones generation. Especially for the fiber-wireless with both optical and electrical devices, the linear or nonlinear impairments should also be considered with these factors. Data Stream
M-QAM Mapping
mfI mfQ
Up sampling
Down sampling
fI fQ
Equalizations
+ -
Add
M-QAM De-Mapping
DAC
Data Stream
Fig. 2. Schematic diagrams of transmitter and receiver based on CAP m-QAM for CAP signal generation and processing.
Figure 2 shows the schematic diagrams of transmitter and receiver based on CAP m-QAM for fiber-wireless transmission. CAP is a multilevel and multi-dimensional modulation format proposed by Bell Labs for short range communication [13–17], which is similar to QAM signal, but does not require a RF carrier source. Different from QAM or OFDM intensity modulation, CAP does not use a sinusoidal carrier to generate two orthogonal components I and Q. The two dimensional CAP can be generated by using two orthogonal filters as fI and fQ shown in Fig. 2 as the filter pair. The original bit sequence is first mapped into complex symbols of m-QAM (m is the order of QAM), and then the mapped symbols are up-sampled to match the sample rate of shaping filters. The sample rate of shaping filters is determined by the data baud rate and DAC sample rate. For CAP generation, the I and Q components of the up-sampled sequence are separated and sent into the digital shaping filters respectively. The outputs of the filters are subtracted to be combined together as S(t) after DAC to drive the upper MZM1. At the receiver side, the received signal after down-conversion is fed into two matched filters to separate the I and Q components. The orthogonal and matched filter pairs fI(t), fQ(t), mfI(t) and mfQ(t) are the corresponding shaping filters and form a so-called Hilbert pair in transmitter and receiver. The two orthogonal filters are constructed by multiplying a square root raised cosine pulse with a sine and cosine function respectively, as described in [21]. Assuming sI(t) and sQ(t) are the I and Q data after #196650 - $15.00 USD Received 29 Aug 2013; revised 16 Oct 2013; accepted 19 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026888 | OPTICS EXPRESS 26891
QAM mapping and up-sampling, then the combined output signal S(t) of CAP signal can be expressed as
S (t ) = [ sI (t ) ⊗ f I (t ) − sQ (t ) ⊗ f Q (t )]
(4) n
n
At the receiver, generally, we have the matched filters with relations as mfI (t) = fI (-t), and mfQn(t) = fQn(-t). In this way, for the CAP receiver, the I and Q data after matched filter pair can be expressed as rI (t ) = R (t ) ⊗ mf I (t ),
rQ (t ) = R(t ) ⊗ mf Q (t )
(5)
where R(t) is the CAP signal after down-conversion at the receiver and rI (t)and rQ(t) are the output after matched filter pair. After down-sampling, a linear equalizer is employed for the complex signal and a decoder is utilized to obtain the original bit sequence. 3. Experimental setup and results
Fig. 3. Experimental setup. (AWG: arbitrary waveform generator; ECL: external cavity laser; EDFA: Erbium-doped fiber amplifier; TOF: tunable optical filter; TA: tunable attenuator; EA: electrical amplifier; ADC: analog-to-digital-convertor)
As a proof of concept, the experimental setup of the 24-Gb/s CAP-64QAM ROF system over 40-km SMMF and 1.5-m 38-GHz wireless transmission is shown in Fig. 3. One external cavity laser (ECL) at 1550.10-nm is used as CW light source. The I/Q modulator is commercially available with two MZMs placed in parallel with two arms. The upper arm MZM is driven by the 4-Gbaud CAP-64QAM signal, which is generated by the 12GSa/s commercial arbitrary waveform generator (AWG). The data sequence is first mapped to the 8 level 64-QAM I and Q signals with 16x211 symbols. Then, the 16 sets of 8-level I and Q sequences are up-sampled to 3 Sa/symbol and filtered by the orthogonal Hilbert filters pair. The filters are finite impulse response (FIR) filters with length of 10 symbols each. The roll-off coefficient is 0.2 and the excess bandwidth is set to 15%. In this way, the 4-Gbaud base-band CAP-64QAM signal is generated by the AWG. The lower arm which is used for mm-wave tones generation is operated at OCS condition. The lower arm MZM2 is biased at null-point, and driven by the 38-GHz RF signal. One OBPF with 0.5-nm 3-dB bandwidth is used for the base-band signal and first-order tone filtering. The optical spectrum of the output of lower arm MZM2 with mm-wave tones and the output after I/Q modulator with both base-band signal and mm-wave tones are shown in Fig. 4 as (a) and (b). From Fig. 4(a), we can see an obvious carrier suppression result with more than 50-dB first-tone to carrier power ratio. Figure 4(c) show the result after OBPF, where only base-band
#196650 - $15.00 USD Received 29 Aug 2013; revised 16 Oct 2013; accepted 19 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026888 | OPTICS EXPRESS 26892
signal and the left side-band first order mm-tone are kept with carrier spacing of 38-GHz. The power ratio of the carrier ((left side-band first order mm-tone) to base-band (CSR) is about 18-dB. After that, the generated optical mm-wave signal is then launched into the 40-km SSMF, which has about 10-dB loss and 17-ps/km/nm chromatic dispersion (CD) at 1550nm without optical dispersion compensation. One Erbium-doped fiber amplifier (EDFA) is used before the fiber to compensate the fiber loss. mm-wave tones
0.02 nm
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1551
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Fig. 4. The optical spectrum of (a) the output of lower arm MZM2 with mm-wave tones, (b) the output after I/Q modulator with both base-band signal and mm-wave tones, and (c) the output after OBPF with only base-band signal and the left side-band first order mm-tone.
For wireless transmission, the optical base-band signal is up-converted by the optical beating at the 50-GHz PD. The generated 38-GHz mm-wave is first amplified by an electrical amplifier (EA) with bandwidth of about 10-GHz and then broadcasted through a horn antenna with a gain of 25-dBi. At the wireless receiver side, the broadcasted wireless signal is received by another 40 GHz horn antenna with 25-dBi gain. The received signal is then directly sampled by a 120-GSa/s high-speed oscilloscope with 45-GHz bandwidth for analog-to-digital conversion (ADC). Figure 5(a) shows the FFT spectrum of 38-GHz mm-wave signal after ADC. We can see that the mm-wave signal carrying 24-Gb/s CAP 64-QAM signal centered at 38-GHz occupies less than 9-GHz bandwidth. 0
0
Power(dBm) (dB) Power
Power (dBm) Power (dB)
20
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20 30 40 Frequency (GHz)
50
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8
Fig. 5. The FFT spectrum of (a) the received 24-Gb/s CAP-64QAM mm-wave signal at 38-GHz carrier after ADC and (b) the down-converted base-band 24-Gb/s CAP-64QAM signal.
After ADC, the off-line DSP is employed. First, the signal is down converted to base-band signal by multiplying a synchronous cosine or sine function. Then, the down-converted base-band signal is resampled to 16-GSa/s. Figure 5(b) shows the FFT spectrum of the down-converted base-band CAP 64-QAM signal with 16-GSa/s. It is worth noting that here we use digital down-conversion after the high speed ADC to demonstrate the ROF system. For practical use, an analog down-conversion can be employed using RF local oscillator (LO) to relax the high speed ADC requirement. After that, the re-sampled 16-GSa/s signal passes through the orthogonal matched filters pair to obtain the I and Q data. Noting that, the matched FIR filters are now at 4Sa/symbol and with 10 symbols each. Then, the I and Q signals are down-sampled to 2Sa/symbol before the linear equalization. Here, four real-valued, 31-tap, T/2-spaced butterfly configured adaptive digital FIR filters, based on decision-directed least mean squares (DD-LMS) algorithm, are used for signal equalization and recovery.
#196650 - $15.00 USD Received 29 Aug 2013; revised 16 Oct 2013; accepted 19 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026888 | OPTICS EXPRESS 26893
Figure 6 shows the bit-error-ratio (BER) results of the 24-Gb/s CAP 64-QAM signal versus the received optical power in back-to-back (BTB) case without fiber or wireless transmission, after 40-km SMMF transmission and after 40-km SMMF and 1.5-m wireless transmission. The optical power is measured after a tunable attenuator (TA). A 0.9nm TOF is used before PD front-end to remove excess amplified spontaneous emission (ASE) noise after EDFA. We can see that the required received optical power at BER of 3.8x10−3 for the 7% hard-decision FEC limit in BTB case is about −25dBm. Less than 1-dB power penalty after 40-km fiber transmission is observed at the BER of 3.8x10−3 compared to BTB case. However, about 2-dB power penalty is introduced by the 1.5-m wireless transmission due to the wireless transmission power loss and impairments. The constellations of the processed CAP 64-QAM signal at −22dBm received power after 40-km fiber transmission without and with 1.5-m wireless transmission are shown in the insets (i) and (ii) of Fig. 6, respectively. Therefore, these results clearly demonstrate the feasibility of the proposed high spectrum efficiency CAP 64-QAM ROF fiber-wireless transmission system. It is worth noting that, as a proof of concept, we only measured the results with 1.5-m wireless link. We believe longer distance of wireless transmission can be realized by getting larger received optical power or by using electrical amplifiers in the wireless transmitter and receiver sides. BTB 40Km Fiber only 40Km Fiber and 1.5 m Wireless
0.01
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Received Power (dBm) Fig. 6. The BER results versus receiver optical power for BTB case, after 40-km fiber transmission only and after 40km fiber and 1.5m wireless transmission.
To further study the ROF system performance, we also test and measured the impact of some system factors on the BER results as mentioned in section 2. First, we study the impact of DC bias 1 of the upper arm MZM1 for base-band signal modulation. As analyzed above, the offset Δβ1 = β1-1, shows the bias offset and determines the power of carrier or the power of DC component of the modulated signal. 0.01
DC Bias 1 Offset = 0.8 V DC Bias 1 Offset = 0 V
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Fig. 7. (a) The optical spectrum of base-band signal and mm-wave tone under different DC bias 1 offset, 0.8V and 0V; (b) The BER versus the offset of DC bias 1 after 40-km SSMF and 1.5-m wireless transmission.
#196650 - $15.00 USD Received 29 Aug 2013; revised 16 Oct 2013; accepted 19 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026888 | OPTICS EXPRESS 26894
Figure 7(a) shows the optical spectrum of base-band signal and mm-wave tone under different DC bias 1 offset, 0.8V and 0V. We can see that, the power of optical carrier of the base-band signal gets larger when the DC bias 1 offset is not zero. About 7.5-dB power difference exists between zero and 0.8 V bias offset. Figure 7(b) shows the BER results versus the offset voltage of DC bias 1. We can see that the BER get worse for larger offset and the optimal offset should be zero, which means the bias of MZM1 should be at null point. We believe there are two main reasons. First, the modulation is at the optimal linear area when the MZM is biased at null-point. Second, the large power optical carrier is also up-converted to mm-wave, which reduces the signal-to-noise-ratio (SNR) after passing the gain-limited optical and electrical amplifiers. The impact of the offset of DC bias 2 on BER performance is shown in Fig. 8(a). The results are similar to that in Fig. 7(b). As analyzed in section 2, the DC bias 2 should be set at null-point for carrier suppression. The offset of DC bias 2 can worsen the carrier suppression output, which introduces the signal crosstalk and in-band carrier. Again, the large power mm-wave signal carrier reduces the SNR after passing the gain-limited electrical amplifiers. Figure 8(b) shows the impact of CSR (the left first order side-band tone to the base-band signal as shown in Fig. 4(c)) on the BER performance after fiber and wireless transmission. Here, we adjust the CSR of the output optical signal after I/Q modulator before the EDFA and fiber by using the OBPF to adjusting the power of carrier and base-band signal. We can see that, the CSR should be larger than 10-dB. The optimal CSR is about 18-dB and slight BER deterioration is observed for CSR larger than 20-dB due to the gain competition when passing the EDFA.
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Fig. 8. (a) The BER versus the offset of DC bias 2 after 40-km SSMF and 1.5-m wireless transmission; (b) The BER versus the CSR after 40-km SSMF and 1.5-m wireless transmission.
4. Conclusion
We propose and demonstrate a novel CAP-ROF system based on multi-level CAP 64-QAM with high spectrum efficiency for mm-wave fiber-wireless transmission. The performance of novel CAP modulation with high order QAM is investigated in the mm-wave fiber-wireless transmission system. One I/Q modulator is used for mm-wave generation and base-band signal up-conversion based on carrier-suppression modulation. Finally, we demonstrated a 24-Gb/s CAP-64QAM ROF system over 40-km SMMF and 1.5-m 38-GHz wireless transmission. To the best of our knowledge, this is the first demonstration for CAP-64QAM used for ROF system with such a high speed. Acknowledgments
This work was partially supported by the NHTRDP (973 Program) of China (Grant No. 2010CB328300), and NNSF of China (No. 61250018, No. 61177071), NHTRDP (863 Program) of China (2011AA010302, 2012AA011302), The National Key Technology R&D Program (2012BAH18B00), Key Program of Shanghai Science and Technology Association (12dz1143000)), and the China Scholarship Council (201206100076).
#196650 - $15.00 USD Received 29 Aug 2013; revised 16 Oct 2013; accepted 19 Oct 2013; published 30 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026888 | OPTICS EXPRESS 26895
