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Palm-shaped spectrum generation for dual-band millimeter wave and baseband signals over fiber
Optics Communications 367 (2016) 137–143
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Invited Paper
Palm-shaped spectrum generation for dual-band millimeter wave and baseband signals over fiber R. Lin a,d, Z. Feng a, M. Tang a, R. Wang a, S. Fu a, P. Shum b, D. Liu a, J. Chen c,d,n a Next Generation Internet Access National Engineering Lab (NGIA), School of Opto-Electonic Science & Technology, Huazhong University of Sci&Tech (HUST), 1037 Luoyu Road, Wuhan, 430074 China b School of EEE, Nanyang Technological University, 637553, Singapore c ZJU-SCNU Joint Research Center of Photonics, South China Normal University, Guangzhou, 510631, China d School of ICT, KTH Royal Institute of Technology, Electrum 229, Kista, 164 40, Sweden
art ic l e i nf o
a b s t r a c t
Article history: Received 18 November 2015 Received in revised form 6 January 2016 Accepted 15 January 2016 Available online 4 February 2016
In order to offer abundant available bandwidth for radio access networks satisfying future 5G requirements on capacity, this paper proposes a simple and cost-effective palm-shaped spectrum generation scheme that can be used for high capacity radio over fiber (RoF) system. The proposed scheme can simultaneously generate an optical carrier used for upstream and two bands of millimeter wave (MMW) that are capable of carrying different downstream data. The experiment results show that the proposed palm-shaped spectrum generation scheme outperforms optical frequency comb (OFC) based multi-band MMW generation in terms of upstream transmission performance. Furthermore, simulation is carried out with different dual-band MMW configurations to verify the feasibility of using the proposed spectrum generation scheme in the RoF system. & 2016 Elsevier B.V. All rights reserved.
Keywords: Millimeter wave (MMW) communication Radio over fiber (RoF) Palm-shaped spectrum generation
1. Introduction To satisfy the growing demand for high-speed wireless network [1], millimeter wave (MMW) technology becomes attractive for providing high bandwidth [2]. Single-band MMW with 10 Gb/s and beyond [3–4] has been demonstrated. Multi-band MMW technology capable of carrying data in several radio frequencies (RFs) further improves capacity. Meanwhile, radio over fiber (RoF) is a promising technique for transmitting MMW signals between the central office (CO) and the base station (BS) as it exhibits ultralow transmission loss. In the RoF system studied in [5], multi-band signals are firstly merged in electrical domain and then up-converted to optical domain by an electro-absorption modulator (EAM). It is a straightforward way to generate multi-band signals for the RoF system. However, narrow operational bandwidth of high-frequency electric components inhibits their application in the high capacity wireless transmission. In [6], the authors employ a single Mach–Zehnder modulator (MZM) in the CO and n Corresponding author at: ZJU-SCNU Joint Research Center of Photonics, South China Normal Univeristy, Guangzhou, 510631, China E-mail addresses: [email protected] (R. Lin), [email protected] (Z. Feng), [email protected] (M. Tang), [email protected] (R. Wang), [email protected] (S. Fu), [email protected] (P. Shum), [email protected] (D. Liu), [email protected] (J. Chen).
http://dx.doi.org/10.1016/j.optcom.2016.01.044 0030-4018/& 2016 Elsevier B.V. All rights reserved.
heterodyne mixing technique to realize multi-band MMW signal generation by using an optical local oscillator (LO) at the BS, resulting in a complex system configuration. Paper [7] proposes multi-band signal generation with dual-drive modulator and two separate clock signals, which requires high frequency synthesizer and synchronization. Optical frequency comb (OFC) with flat spectrum and equal frequency spacing is an alternative approach to generate multi-band MMW [8]. In this scheme only one comb line is allocated to carry downstream data and the others are used for beating to generate multiple bands of MMWs. It should be noted that in most of the existing schemes, e.g., [6–9], different MMW bands are modulated with the same data, resulting in inefficient spectrum utilization. In [10], the authors have demonstrated two-band MMW modulated with different data. This scheme is based on a dual parallel Mach–Zehnder modulator (DPMZM) followed by an MZM, leading to high insertion loss and configuration complexity in the CO. In this paper, we introduce a new concept, i.e., palm-shaped spectrum generation, where the central optical carrier looks like the middle finger in a palm with obviously higher power than the sidebands. The proposed scheme can be used in the RoF system to transmit dual-band MMW signals for downstream and baseband for upstream. In contrast to the flat OFC technique, palm-shaped spectrum generation is relatively simple and cost-effective in the CO, where only one DPMZM is employed to generate the dual-
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band MMWs and modulate different data on two bands. In this way, the efficient bandwidth increases to twice as much as that in the methods proposed in [6–9]. The central optical carrier is sent to the BS, and then reused to carry the upstream signals. Since no additional light source and wavelength management are required, the complexity and cost of the BS can be reduced compared with the RoF system using heterodyne mixing technique, such as [6]. Meanwhile, similarly as in [6–10] microwave photonics technique is implemented in our scheme for signal up-conversion, as it enables the centralized architecture, moving the signal up-conversion from the BSs to the CO. Multiple mixers, which are required in the BSs when employing electrical up-conversion, are therefore not needed anymore. Thus, the system configuration at the side of the BSs can be simplified. Moreover, multi-band MMW application requires very high radio frequency generation, e.g., beyond 60 GHz. The electrical up-conversion to such a high frequency is difficult to realize. We experimentally verify the proposed palmshaped spectrum generation and demonstrate that it obviously outperforms the OFC technique in terms of upstream transmission performance. Furthermore, simulation is carried out with different dual-band configurations, such as 40 GHz and 80 GHz, 60 GHz and 120 GHz, to validate the feasibility of using the proposed spectrum generation in the RoF system.
⎡π ⎤ E out1 (t )= E in cos ( ω0 t )⋅cos ⎢ +m1 sin (ωt ) ⎥ ⎣2 ⎦ ∞
= E in ⋅cos ( ω0 t )⋅2
∑ J2n − 1 ( m1) [sin (2n−1) ωt ]. n=1
Modulation index m ¼ VRF / Vπ is introduced to simplify the expression. m1 and m2 represent modulation index of upper arm and lower arm, respectively. Jn ( . ) is Bessel function of the first kind of nth order. ω0 ¼f0/2π, where f0 represents the optical carrier frequency, and ω ¼fc/2π, where fc is the sinusoidal driving frequency. High order terms in the Jacobs Anger expansion (i.e., J2n 1, with n4 1) can be ignored due to their negligible impact on the output. Then Eq. (2) can be simplified as:
E out1 (t ) = E in ⋅J1 (m1)[sin ( ω0 − ω) t + sin ( ω0 + ω) t ].
The system schematic diagram of the proposed palm-shaped generation for RoF system is depicted in Fig.1. At the CO, DPMZM consists of a pair of child LiNbO3 MZMs and a phase modulator (PM) embedded in the two arms of a parent MZM structure. A bias voltage of DPMZM should be firstly configured in order to generate palm-shaped spectrum. The modulation of the optical carrier can be expressed by:
⎡ π ⎤ (VDC +VRF ) ⎥, E out (t )=E in cos ⎢ ⎣ 2Vπ ⎦
(1)
where Ein (t) and Eout (t) depict input and output optical field, respectively. Vπ indicates the half-wave voltage of MZM, VDC and VRF denote the bias voltage and modulation voltage, respectively. The DC bias voltage of the upper MZM should be set to minimum point of its transmission curve so that it works in the carrier suppressed mode. Jacobs Anger expansion of the output optical field of the upper arm can be depicted as:
(3)
With optical carrier suppression, the first order sidebands with band spacing of 2fc are produced from the upper-arm of the DPMZM. By adjusting the bias voltage applied on the lower arm MZM near to the maximum point of its transmission curve, the output of the lower arm can be written as:
E out2 (t )= E in ⋅{J0 ( m2 )⋅cos ( ω0 t )+J2 ( m2 )⋅[cos ( ω0 +2ω) t +cos ( ω0 −2ω) t ]} .
2. Principle
(2)
(4)
The first order sidebands are suppressed. Two second order sidebands with 4fc spacing are induced as well as the central optical carrier. As the result of combination of output from both arms, a palm-shaped spectrum with flat sidebands and higher power central carrier (as shown in Fig. 1) is obtained if the modulation indexes satisfy the following conditions: J2 (m2 )=J1 (m1) and J0 (m2 )=J2 (m2 ). As shown in Fig. 1, a continuous-wave (CW) light is divided into two beams and launched into the configured DPMZM. Data1 and Data2 representing different traffic flows are mixed with the RF source to drive the upper- and lower-arm of the DPMZM, respectively. In this way, MMW carriers with modulated signals for downstream are generated and then transmitted. At the BS, two FBGs are used to separate the corresponding sidebands and the central carrier. Detected by high speed photodetector (PD), MMW signals are generated by beating symmetrical sidebands and then broadcast by the antenna. The central carrier can be used to perform upstream transmission. There are three reasons why we choose palm-shaped spectrum instead of OFC with flat comb lines: (1) the comb spectrum
Fig. 1. Palm-shaped spectrum generation scheme for RoF system. FBG: fiber Bragg grating, PM, phase modulator, EDFA: Erbium-doped fiber amplifier, PC: polarization controller.
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generated by DPMZM requires larger driving voltage to make sure the optical power of all the frequencies are as high as that of the central wavelength [11] so that it is less energy efficient than the proposed palm-shaped spectrum generation scheme; (2) to generate the flat comb, the relationship between modulation indexes of both DPMZM arms is more stringent (i.e., J0 (m2 )=J2 (m2 )=J1 (m1), resulting in complex operation and configuration in practice; and (3) in contrast to flat comb, palm-shaped spectrum offers a higher optical sideband suppression ratio (OSSR) and hence leads to a better signal quality when reusing optical carrier for upstream transmission, while it does not downgrade MMW signals quality received at the BS.
3. Performance evaluation In this section, we firstly implement experiments to verify the proposed palm-shaped spectrum generation scheme and then compare it with OFC technique in terms of upstream transmission performance. Besides, simulation is carried out for validation of the overall RoF system using the proposed palm-shaped spectrum generation scheme.
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3.1. Experimental validation of the palm-shaped spectrum generation The experiment is implemented to validate the palm-shaped spectrum generation. The experiment setup is shown as Fig. 2(a). CW light is generated by a tunable external cavity laser (ECL) at 1550 nm and modulated by a DPMZM. The linewidth of the laser is 100 KHz. To prove the concept, a 20 GHz RF source (i.e., fc ¼20 GHz) is divided by a 50:50 power splitter and applied on both arms of the DPMZM to produce dual-band MMWs of 40 GHz and 80 GHz. At the receiver end, two first-order sidebands are selected by the programmable optical filter (POF). The POC has two passbands centering at 1549.84 nm and 1550.16 nm, both of which have 0.1 nm 3 dB bandwidth, with Gaussian shape. A 40GHz PD is employed to conduct optical-to-electrical conversion and photomixing. The photomixed output is monitored with a 43 GHz electrical spectrum analyzer (ESA). By adjusting the bias voltage of DPMZM according to analysis in Section II, a central wavelength is generated together with the sidebands, which have much lower power than the central optical carrier (see Fig. 2(b)). Two first order sidebands are extracted by the POF with OSSR larger than 35 dB, as shown in Fig. 2(c). The photomixed output with the
Fig. 2. (a) Experiment setup for spectrum generation. (b) optical spectrum of the generated palm, (c) the filtered 40 GHz sidebands at the receiver side and (d) electric spectrum of the 40 GHz MMW, (e) experiment setup for upstream transmission, (f) the filtered central carrier for upstream in the case of palm-shaped spectrum generation, (g) optical spectrum of the generated comb, and (h) the filtered central carrier for upstream in the case of flat optical frequency comb generation. POF: programmable optical filter; ESA: electrical spectrum analyzer; EDFA: Erbium doped fiber amplifier; TOF: tunable optical filter; DSO: digital sampling oscilloscope.
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Fig. 3. Simulated optical spectrum diagrams that are measured at the different points marked in Fig. 1 (frequency range: 193.04 THz to 193.16 THz, X-axis scale: 20 GHz/div, Y-axis scale: 10-dB/div): (a) output spectrums of upper-arm of DPMZM; (b) output spectrum of lower-arm of DPMZM; (c) received palm-spectrum at the BS after modulation and transmission over 25 km fiber, (d) reflected central carrier from FBG1, (e) spectrum passing through FBG2 (4fc), (f) reflected spectrum of FBG2 (2fc).
spectral range of 43 GHz is displayed in Fig. 2(d). A clear and stable 40 GHz carrier frequency can be observed with more than 40 dB RF spurious suppression ratio (RFSSR). Experiment setup shown in Fig. 2(e) is for evaluation of
upstream transmission performance using the central optical carrier of the palm-shaped spectrum. On-off-keying (OOK) modulation is considered here. A tunable optical filter (TOF) is used to select the central optical carrier. The 3 dB bandwidth of the TOF is
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0.2 nm. We have observed that the impact of the length of pseudo random bit sequence (PRBS) on the transmission performance is negligible when PRBS 215–1 or the higher order is used. To keep the experimental and simulation time in a reasonable scale, PRBS 215–1 is used. Fig. 2(b) shows the palm-shaped spectrum. After 25 km single mode fiber (SMF) transmission, a PD and digital sampling oscilloscope (DSO) are employed to obtain the eye-diagram. The spectrum with the filtered central carrier and detected eye-diagram are shown in Fig. 2(f). The eye is widely open thanks to the large OSSR of 41.2 dB. For comparison purpose, flat OFC spectrum generation is also tested. We maintain the output power of the laser constant and then adjust the bias and driving voltages of the DPMZM in order to generate a flat comb spectrum and repeat the transmission experiments as in the case of palm-shaped spectrum generation. The entire comb spectrum is shown in Fig. 2 (g). In both palm and OFC spectrum generation, the optical power of sidebands used for the downstream is quite close. Moreover, the similar SSR of the sidebands (i.e., 17.1 dB in the palm-shaped spectrum and 16.8 dB in the OFC) can be observed. At the receiver side, as shown in Fig. 2(c) the crosstalk between the central carrier and the sidebands can be eliminated by using a proper filter. That means the beating results between corresponding sidebands will not be affected by the central carrier. Thus, the similar downstream performance in palm and OFC can be obtained. The filtered central carrier from the comb as well as the eye-diagram of the upstream signals is shown in Fig. 2(h). Degraded extinction ratio of the eye-diagram is observed because of the worse OSSR in OFC. Besides, it is worth noting that with the same input optical power, the optical power of central carrier in the palm-shaped spectrum is larger than 0 dBm and the sidebands are close to –20 dBm, while the optical power of each carrier in comb is obviously lower (less than –23 dBm). 3.2. Simulation of the overall RoF system VPItransmissionMaker™ Optical Systems (v.9.2) is adopted to validate the feasibility of the overall RoF system using the proposed palm-shaped spectrum generation scheme. Vπ of both MZMs in the DPMZM is set to be 3.0 V. The bias voltages of upperand lower- arm are 3.0 V and 0 V, respectively. Driving voltages applied to the upper- and lower-arm are 0.26 V and 1.42 V. It should be noted that both driving voltages could be much less than Vπ. In contrast to the single-MZM scheme presented in [7] which requires approximately 1.5 times of Vπ, our scheme consumes much less driving power. Simulation setup is the same as the diagram shown in Fig. 1. The spectrums measured at the different points marked in Fig. 1 are plotted in Fig. 3. Two different sources with PRBS 215-1, i.e., Data1 and Data2, are modulated on the upper- and lower-arm, respectively. OOK is implemented for all sidebands with a bit rate of 2.5 Gb/s. The output spectrums of upper- and lower-arm of DPMZM after data modulation are shown in Fig. 3(a) and (b), respectively. The received spectrum at the BS is shown in Fig. 3(c). At the BS, two FBGs are used to separate the optical carrier from dual-band MMW. FBG1 has 80% reflection ratio and 3 dB bandwidth of 0.2 nm. The signal reflected by FBG1, i.e. the middle finger of the palm, is indicated in Fig. 3(d). The signal passing through FBG1 is then injected to FBG2. The reflection ratio and 3 dB bandwidth of FBG2 are 80% and 0.64 nm, respectively. 80 GHz MMW is obtained after passing through FBG2 (see Fig. 3(e)), and 40 GHz MMW reflected by FBG2 is shown in Fig. 3(f). The middle finger of the palm (i.e., central optical carrier) is then adopted for upstream transmission, modulated by PRBS 215 –1 with data rate of 2.5 Gb/s. After 25 km transmission on SMF, upstream signal is detected by a PD at the CO. For the experiments in lab, the polarization controller (PC) is often employed to maintain good transmission performance when using external
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MZM modulators. In the real system, the use of PC can be avoided by adopting polarization-insensitive modulators (such as commercially available electro-absorption modulator [12]). In the proposed system, for simplicity no dispersion management or compensation is implemented. The transmission distance is limited by the bit walk-off, Δτ , which represents the time shift of the code edge carried by the sidebands [13]. For the considered non-return zero (NRZ) signals, Δτ can be calculated as
Δτ=λ2DΔfL,
(5)
where λ , D, Δf, and L represent the wavelength of the optical carrier, fiber dispersion, MMW frequency, and the fiber transmission distance. With a given data rate B, the code width can be obtained:
τ=1/B.
(6)
Assuming the maximum tolerable walk-off amount equals to 70% of the code width [14], and D is 17 ps/nm km, the maximum transmission distance can be obtained as:
L_ max=(70%τ)/(Δf ⋅D).
(7)
When fc ¼20 GHz, Δf is determined by the second sidebands frequency difference, i.e., 80 GHz, the maximum transmission distance of the system is around 25 km with the data rate of 2.5 Gb/s. Similarly, we can obtain that when fc ¼ 30 GHz (i.e., 120 GHz MMW) the maximum transmission distance for the system is about 19 km. In the simulation, we test the cases with the maximum transmission distance. The bit error rate (BER) curve and eye diagram of the downconverted electrical MMW signals are depicted in Fig.4 with fc ¼20 GHz and fc ¼ 30 GHz, respectively. In the case of fc ¼ 20 GHz, 25 km SMF is implemented. The power penalty between back-toback (b2b) and 25 km SMF transmission to reach a BER of 10 5 are 0.6 dB and 1.8 dB for 40 GHz (Data2) and 80 GHz (Data1) MMW, respectively. When fc ¼30 GHz, the power penalty between b2b and 19 km SMF transmission at BER ¼10 5 is 1.5 dB and 2.7 dB for 60 GHz (Data2) and 120 GHz (Data1) MMW. It can be seen that the higher frequency MMW is, the larger power penalty is obtained. It is mainly due to the dispersion induced distortion. To further increase the system capacity, advanced modulation format, such as quadrature phase shift keying (QPSK), discrete multi-tone (DMT), etc., can be considered. Using these modulation formats, with the same data ratea longer symbol period can be obtained compared to OOK. To some extent, it helps to increase the maximum transmission distance of the MMW signals that is constrained by the fiber dispersion. On the other hand, transmission over the fiber has negligible impact on upstream BER performance (see Fig. 4(e) and (f)). Although the central carrier is also modulated with downstream Data1, it does not significantly affect upstream transmission since BER r 10–5 can still be achieved after the fiber transmission when the received optical power at the CO is low ( –23 dBm).
4. Conclusion We propose a palm-shaped spectrum generation scheme for RoF system offering dual-band MMW signals for downstream and reusing the optical carrier for upstream. Only a single DPMZM is used to generate and modulate two bands of MMW, which is easy to implement and cost effective. Dual-band MMWs with different data for downstream are generated by direct beating between symmetric sidebands of the palm while the optical carrier (i.e., the middle finger of palm) can be used for upstream transmission at the BS. The proof-of-concept experiment validates the proposed
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Fig. 4. BER performance and eye diagrams at the corresponding measuring points: (a) downstream signal of 40 GHz MMW (Data2), (b) 80 GHz MMW (Data1) and (c) upstream in case of fc ¼ 20 GHz, and (d) downstream signal of 60 GHz MMW (Data2), (e) 120 GHz MMW (Data1) and (f) upstream in case of fc ¼ 30 GHz.
spectrum generation scheme can provide obviously higher OSSR and hence better transmission performance in upstream in comparison with the optical frequency comb, palm-shaped spectrum. Meanwhile, the feasibility of using the proposed palm-shaped spectrum generation in the RoF system is verified by carrying out simulation with different configurations of MMW bands.
Acknowledgments This work was supported by the National High-tech R&D Program of China (863 Program) under Grant no. 2013AA013402, the National Natural Science Foundation of China (NSFC) under Grants no. 61331010, the Program for New Century Excellent Talents in
R. Lin et al. / Optics Communications 367 (2016) 137–143
University (NCET-13-0235), and Göran Gustafsson Foundation.
References [1] Cisco visual networking index: forecast and methodology, 2013–2018, Cisco, San Jose, CA, USA, 2014. [2] C. Lim, et al., Fiber-wireless networks and subsystem technologies, J. Light. Technol. vol. 28 (4) (2010) 390–405. [3] X. Pang, et al., 25 Gbit/s QPSK hybrid fiber-wireless transmission in the W-band (75–110 GHz) with remote antenna unit for in-building wireless networks, IEEE Photonics J. 4 (3) (2012) 691–698. [4] A. Hirata, et al., 120-GHz-band millimeter-wave photonic wireless link for 10 Gb/s data transmission, IEEE Trans. Microw. Theory Tech. 54 (5) (2006) 1937–1944. [5] K. Ikeda, et al., Simultaneous three-band modulation and fiber-optic transmission of 2.5G b/s baseband, microwave-, and 60GHz-band signals on a single wavelength, J. Light Technol. 21 (12) (2003) 3194–3202. [6] L. Zhang, et al., Generation of multiband signals in a bidirectional wireless over fiber system with high scalability using heterodyne mixing technique, IEEE Photonics Technol. Lett. 24 (18) (2012) 1621–1624.
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[7] Y. Hsueh, et al., A novel bidirectional 60-GHz radio-over-fiber scheme with multiband signal generation using a single intensity modulator, IEEE Photonics Technol. Lett. 21 (18) (2009) 1338–1340. [8] C. Zhang, et al., A full-duplex WDM-ROF system based on tunable optical frequency comb generator, Opt. Commun. 344 (2014) 65–70. [9] R. Zhang, et al., A multi-band access radio-over-fiber link with SSB optical millimeter-wave signals based on optical carrier suppression modulation Part. 3, Opt. Switch. Netw. 18 (2015) 235–241. [10] Q. Chang, et al., Simultaneous generation and transmission of downstream multiband signals and upstream data in a bidirectional radio-over-fiber system, IEEE Photonics Technol. Lett. 20 (3) (2008) 2007–2009. [11] Q. Wang, et al., Ultra-flat optical frequency comb generator using a singledriven dual-parallel Mach–Zehnder modulator, Opt. Lett. 39 (10) (2014) 3050–3053. [12] Coretk, “10G LR-EAM-1550 datasheet”, available online: 〈http://www.coretk. com/CataLog/cata_img/FILE/934350250/CIP/25/25_164_1139885243.pdf〉. [13] J. Ma, et al., Fiber dispersion influence on transmission of the optical millimeter-waves generated using LN-MZM intensity modulation, J. Light Technol. 25 (11) (2007) 3244–3256. [14] L. Christina, et al., Impact of chromatic dispersion on 60 GHz radio-over-fiber transmission LEOS2008, 21st Annual Meeting IEEE Lasers Electron – Optical Society (2008) 89–90.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Invited Paper
Palm-shaped spectrum generation for dual-band millimeter wave and baseband signals over fiber R. Lin a,d, Z. Feng a, M. Tang a, R. Wang a, S. Fu a, P. Shum b, D. Liu a, J. Chen c,d,n a Next Generation Internet Access National Engineering Lab (NGIA), School of Opto-Electonic Science & Technology, Huazhong University of Sci&Tech (HUST), 1037 Luoyu Road, Wuhan, 430074 China b School of EEE, Nanyang Technological University, 637553, Singapore c ZJU-SCNU Joint Research Center of Photonics, South China Normal University, Guangzhou, 510631, China d School of ICT, KTH Royal Institute of Technology, Electrum 229, Kista, 164 40, Sweden
art ic l e i nf o
a b s t r a c t
Article history: Received 18 November 2015 Received in revised form 6 January 2016 Accepted 15 January 2016 Available online 4 February 2016
In order to offer abundant available bandwidth for radio access networks satisfying future 5G requirements on capacity, this paper proposes a simple and cost-effective palm-shaped spectrum generation scheme that can be used for high capacity radio over fiber (RoF) system. The proposed scheme can simultaneously generate an optical carrier used for upstream and two bands of millimeter wave (MMW) that are capable of carrying different downstream data. The experiment results show that the proposed palm-shaped spectrum generation scheme outperforms optical frequency comb (OFC) based multi-band MMW generation in terms of upstream transmission performance. Furthermore, simulation is carried out with different dual-band MMW configurations to verify the feasibility of using the proposed spectrum generation scheme in the RoF system. & 2016 Elsevier B.V. All rights reserved.
Keywords: Millimeter wave (MMW) communication Radio over fiber (RoF) Palm-shaped spectrum generation
1. Introduction To satisfy the growing demand for high-speed wireless network [1], millimeter wave (MMW) technology becomes attractive for providing high bandwidth [2]. Single-band MMW with 10 Gb/s and beyond [3–4] has been demonstrated. Multi-band MMW technology capable of carrying data in several radio frequencies (RFs) further improves capacity. Meanwhile, radio over fiber (RoF) is a promising technique for transmitting MMW signals between the central office (CO) and the base station (BS) as it exhibits ultralow transmission loss. In the RoF system studied in [5], multi-band signals are firstly merged in electrical domain and then up-converted to optical domain by an electro-absorption modulator (EAM). It is a straightforward way to generate multi-band signals for the RoF system. However, narrow operational bandwidth of high-frequency electric components inhibits their application in the high capacity wireless transmission. In [6], the authors employ a single Mach–Zehnder modulator (MZM) in the CO and n Corresponding author at: ZJU-SCNU Joint Research Center of Photonics, South China Normal Univeristy, Guangzhou, 510631, China E-mail addresses: [email protected] (R. Lin), [email protected] (Z. Feng), [email protected] (M. Tang), [email protected] (R. Wang), [email protected] (S. Fu), [email protected] (P. Shum), [email protected] (D. Liu), [email protected] (J. Chen).
http://dx.doi.org/10.1016/j.optcom.2016.01.044 0030-4018/& 2016 Elsevier B.V. All rights reserved.
heterodyne mixing technique to realize multi-band MMW signal generation by using an optical local oscillator (LO) at the BS, resulting in a complex system configuration. Paper [7] proposes multi-band signal generation with dual-drive modulator and two separate clock signals, which requires high frequency synthesizer and synchronization. Optical frequency comb (OFC) with flat spectrum and equal frequency spacing is an alternative approach to generate multi-band MMW [8]. In this scheme only one comb line is allocated to carry downstream data and the others are used for beating to generate multiple bands of MMWs. It should be noted that in most of the existing schemes, e.g., [6–9], different MMW bands are modulated with the same data, resulting in inefficient spectrum utilization. In [10], the authors have demonstrated two-band MMW modulated with different data. This scheme is based on a dual parallel Mach–Zehnder modulator (DPMZM) followed by an MZM, leading to high insertion loss and configuration complexity in the CO. In this paper, we introduce a new concept, i.e., palm-shaped spectrum generation, where the central optical carrier looks like the middle finger in a palm with obviously higher power than the sidebands. The proposed scheme can be used in the RoF system to transmit dual-band MMW signals for downstream and baseband for upstream. In contrast to the flat OFC technique, palm-shaped spectrum generation is relatively simple and cost-effective in the CO, where only one DPMZM is employed to generate the dual-
138
R. Lin et al. / Optics Communications 367 (2016) 137–143
band MMWs and modulate different data on two bands. In this way, the efficient bandwidth increases to twice as much as that in the methods proposed in [6–9]. The central optical carrier is sent to the BS, and then reused to carry the upstream signals. Since no additional light source and wavelength management are required, the complexity and cost of the BS can be reduced compared with the RoF system using heterodyne mixing technique, such as [6]. Meanwhile, similarly as in [6–10] microwave photonics technique is implemented in our scheme for signal up-conversion, as it enables the centralized architecture, moving the signal up-conversion from the BSs to the CO. Multiple mixers, which are required in the BSs when employing electrical up-conversion, are therefore not needed anymore. Thus, the system configuration at the side of the BSs can be simplified. Moreover, multi-band MMW application requires very high radio frequency generation, e.g., beyond 60 GHz. The electrical up-conversion to such a high frequency is difficult to realize. We experimentally verify the proposed palmshaped spectrum generation and demonstrate that it obviously outperforms the OFC technique in terms of upstream transmission performance. Furthermore, simulation is carried out with different dual-band configurations, such as 40 GHz and 80 GHz, 60 GHz and 120 GHz, to validate the feasibility of using the proposed spectrum generation in the RoF system.
⎡π ⎤ E out1 (t )= E in cos ( ω0 t )⋅cos ⎢ +m1 sin (ωt ) ⎥ ⎣2 ⎦ ∞
= E in ⋅cos ( ω0 t )⋅2
∑ J2n − 1 ( m1) [sin (2n−1) ωt ]. n=1
Modulation index m ¼ VRF / Vπ is introduced to simplify the expression. m1 and m2 represent modulation index of upper arm and lower arm, respectively. Jn ( . ) is Bessel function of the first kind of nth order. ω0 ¼f0/2π, where f0 represents the optical carrier frequency, and ω ¼fc/2π, where fc is the sinusoidal driving frequency. High order terms in the Jacobs Anger expansion (i.e., J2n 1, with n4 1) can be ignored due to their negligible impact on the output. Then Eq. (2) can be simplified as:
E out1 (t ) = E in ⋅J1 (m1)[sin ( ω0 − ω) t + sin ( ω0 + ω) t ].
The system schematic diagram of the proposed palm-shaped generation for RoF system is depicted in Fig.1. At the CO, DPMZM consists of a pair of child LiNbO3 MZMs and a phase modulator (PM) embedded in the two arms of a parent MZM structure. A bias voltage of DPMZM should be firstly configured in order to generate palm-shaped spectrum. The modulation of the optical carrier can be expressed by:
⎡ π ⎤ (VDC +VRF ) ⎥, E out (t )=E in cos ⎢ ⎣ 2Vπ ⎦
(1)
where Ein (t) and Eout (t) depict input and output optical field, respectively. Vπ indicates the half-wave voltage of MZM, VDC and VRF denote the bias voltage and modulation voltage, respectively. The DC bias voltage of the upper MZM should be set to minimum point of its transmission curve so that it works in the carrier suppressed mode. Jacobs Anger expansion of the output optical field of the upper arm can be depicted as:
(3)
With optical carrier suppression, the first order sidebands with band spacing of 2fc are produced from the upper-arm of the DPMZM. By adjusting the bias voltage applied on the lower arm MZM near to the maximum point of its transmission curve, the output of the lower arm can be written as:
E out2 (t )= E in ⋅{J0 ( m2 )⋅cos ( ω0 t )+J2 ( m2 )⋅[cos ( ω0 +2ω) t +cos ( ω0 −2ω) t ]} .
2. Principle
(2)
(4)
The first order sidebands are suppressed. Two second order sidebands with 4fc spacing are induced as well as the central optical carrier. As the result of combination of output from both arms, a palm-shaped spectrum with flat sidebands and higher power central carrier (as shown in Fig. 1) is obtained if the modulation indexes satisfy the following conditions: J2 (m2 )=J1 (m1) and J0 (m2 )=J2 (m2 ). As shown in Fig. 1, a continuous-wave (CW) light is divided into two beams and launched into the configured DPMZM. Data1 and Data2 representing different traffic flows are mixed with the RF source to drive the upper- and lower-arm of the DPMZM, respectively. In this way, MMW carriers with modulated signals for downstream are generated and then transmitted. At the BS, two FBGs are used to separate the corresponding sidebands and the central carrier. Detected by high speed photodetector (PD), MMW signals are generated by beating symmetrical sidebands and then broadcast by the antenna. The central carrier can be used to perform upstream transmission. There are three reasons why we choose palm-shaped spectrum instead of OFC with flat comb lines: (1) the comb spectrum
Fig. 1. Palm-shaped spectrum generation scheme for RoF system. FBG: fiber Bragg grating, PM, phase modulator, EDFA: Erbium-doped fiber amplifier, PC: polarization controller.
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generated by DPMZM requires larger driving voltage to make sure the optical power of all the frequencies are as high as that of the central wavelength [11] so that it is less energy efficient than the proposed palm-shaped spectrum generation scheme; (2) to generate the flat comb, the relationship between modulation indexes of both DPMZM arms is more stringent (i.e., J0 (m2 )=J2 (m2 )=J1 (m1), resulting in complex operation and configuration in practice; and (3) in contrast to flat comb, palm-shaped spectrum offers a higher optical sideband suppression ratio (OSSR) and hence leads to a better signal quality when reusing optical carrier for upstream transmission, while it does not downgrade MMW signals quality received at the BS.
3. Performance evaluation In this section, we firstly implement experiments to verify the proposed palm-shaped spectrum generation scheme and then compare it with OFC technique in terms of upstream transmission performance. Besides, simulation is carried out for validation of the overall RoF system using the proposed palm-shaped spectrum generation scheme.
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3.1. Experimental validation of the palm-shaped spectrum generation The experiment is implemented to validate the palm-shaped spectrum generation. The experiment setup is shown as Fig. 2(a). CW light is generated by a tunable external cavity laser (ECL) at 1550 nm and modulated by a DPMZM. The linewidth of the laser is 100 KHz. To prove the concept, a 20 GHz RF source (i.e., fc ¼20 GHz) is divided by a 50:50 power splitter and applied on both arms of the DPMZM to produce dual-band MMWs of 40 GHz and 80 GHz. At the receiver end, two first-order sidebands are selected by the programmable optical filter (POF). The POC has two passbands centering at 1549.84 nm and 1550.16 nm, both of which have 0.1 nm 3 dB bandwidth, with Gaussian shape. A 40GHz PD is employed to conduct optical-to-electrical conversion and photomixing. The photomixed output is monitored with a 43 GHz electrical spectrum analyzer (ESA). By adjusting the bias voltage of DPMZM according to analysis in Section II, a central wavelength is generated together with the sidebands, which have much lower power than the central optical carrier (see Fig. 2(b)). Two first order sidebands are extracted by the POF with OSSR larger than 35 dB, as shown in Fig. 2(c). The photomixed output with the
Fig. 2. (a) Experiment setup for spectrum generation. (b) optical spectrum of the generated palm, (c) the filtered 40 GHz sidebands at the receiver side and (d) electric spectrum of the 40 GHz MMW, (e) experiment setup for upstream transmission, (f) the filtered central carrier for upstream in the case of palm-shaped spectrum generation, (g) optical spectrum of the generated comb, and (h) the filtered central carrier for upstream in the case of flat optical frequency comb generation. POF: programmable optical filter; ESA: electrical spectrum analyzer; EDFA: Erbium doped fiber amplifier; TOF: tunable optical filter; DSO: digital sampling oscilloscope.
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Fig. 3. Simulated optical spectrum diagrams that are measured at the different points marked in Fig. 1 (frequency range: 193.04 THz to 193.16 THz, X-axis scale: 20 GHz/div, Y-axis scale: 10-dB/div): (a) output spectrums of upper-arm of DPMZM; (b) output spectrum of lower-arm of DPMZM; (c) received palm-spectrum at the BS after modulation and transmission over 25 km fiber, (d) reflected central carrier from FBG1, (e) spectrum passing through FBG2 (4fc), (f) reflected spectrum of FBG2 (2fc).
spectral range of 43 GHz is displayed in Fig. 2(d). A clear and stable 40 GHz carrier frequency can be observed with more than 40 dB RF spurious suppression ratio (RFSSR). Experiment setup shown in Fig. 2(e) is for evaluation of
upstream transmission performance using the central optical carrier of the palm-shaped spectrum. On-off-keying (OOK) modulation is considered here. A tunable optical filter (TOF) is used to select the central optical carrier. The 3 dB bandwidth of the TOF is
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0.2 nm. We have observed that the impact of the length of pseudo random bit sequence (PRBS) on the transmission performance is negligible when PRBS 215–1 or the higher order is used. To keep the experimental and simulation time in a reasonable scale, PRBS 215–1 is used. Fig. 2(b) shows the palm-shaped spectrum. After 25 km single mode fiber (SMF) transmission, a PD and digital sampling oscilloscope (DSO) are employed to obtain the eye-diagram. The spectrum with the filtered central carrier and detected eye-diagram are shown in Fig. 2(f). The eye is widely open thanks to the large OSSR of 41.2 dB. For comparison purpose, flat OFC spectrum generation is also tested. We maintain the output power of the laser constant and then adjust the bias and driving voltages of the DPMZM in order to generate a flat comb spectrum and repeat the transmission experiments as in the case of palm-shaped spectrum generation. The entire comb spectrum is shown in Fig. 2 (g). In both palm and OFC spectrum generation, the optical power of sidebands used for the downstream is quite close. Moreover, the similar SSR of the sidebands (i.e., 17.1 dB in the palm-shaped spectrum and 16.8 dB in the OFC) can be observed. At the receiver side, as shown in Fig. 2(c) the crosstalk between the central carrier and the sidebands can be eliminated by using a proper filter. That means the beating results between corresponding sidebands will not be affected by the central carrier. Thus, the similar downstream performance in palm and OFC can be obtained. The filtered central carrier from the comb as well as the eye-diagram of the upstream signals is shown in Fig. 2(h). Degraded extinction ratio of the eye-diagram is observed because of the worse OSSR in OFC. Besides, it is worth noting that with the same input optical power, the optical power of central carrier in the palm-shaped spectrum is larger than 0 dBm and the sidebands are close to –20 dBm, while the optical power of each carrier in comb is obviously lower (less than –23 dBm). 3.2. Simulation of the overall RoF system VPItransmissionMaker™ Optical Systems (v.9.2) is adopted to validate the feasibility of the overall RoF system using the proposed palm-shaped spectrum generation scheme. Vπ of both MZMs in the DPMZM is set to be 3.0 V. The bias voltages of upperand lower- arm are 3.0 V and 0 V, respectively. Driving voltages applied to the upper- and lower-arm are 0.26 V and 1.42 V. It should be noted that both driving voltages could be much less than Vπ. In contrast to the single-MZM scheme presented in [7] which requires approximately 1.5 times of Vπ, our scheme consumes much less driving power. Simulation setup is the same as the diagram shown in Fig. 1. The spectrums measured at the different points marked in Fig. 1 are plotted in Fig. 3. Two different sources with PRBS 215-1, i.e., Data1 and Data2, are modulated on the upper- and lower-arm, respectively. OOK is implemented for all sidebands with a bit rate of 2.5 Gb/s. The output spectrums of upper- and lower-arm of DPMZM after data modulation are shown in Fig. 3(a) and (b), respectively. The received spectrum at the BS is shown in Fig. 3(c). At the BS, two FBGs are used to separate the optical carrier from dual-band MMW. FBG1 has 80% reflection ratio and 3 dB bandwidth of 0.2 nm. The signal reflected by FBG1, i.e. the middle finger of the palm, is indicated in Fig. 3(d). The signal passing through FBG1 is then injected to FBG2. The reflection ratio and 3 dB bandwidth of FBG2 are 80% and 0.64 nm, respectively. 80 GHz MMW is obtained after passing through FBG2 (see Fig. 3(e)), and 40 GHz MMW reflected by FBG2 is shown in Fig. 3(f). The middle finger of the palm (i.e., central optical carrier) is then adopted for upstream transmission, modulated by PRBS 215 –1 with data rate of 2.5 Gb/s. After 25 km transmission on SMF, upstream signal is detected by a PD at the CO. For the experiments in lab, the polarization controller (PC) is often employed to maintain good transmission performance when using external
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MZM modulators. In the real system, the use of PC can be avoided by adopting polarization-insensitive modulators (such as commercially available electro-absorption modulator [12]). In the proposed system, for simplicity no dispersion management or compensation is implemented. The transmission distance is limited by the bit walk-off, Δτ , which represents the time shift of the code edge carried by the sidebands [13]. For the considered non-return zero (NRZ) signals, Δτ can be calculated as
Δτ=λ2DΔfL,
(5)
where λ , D, Δf, and L represent the wavelength of the optical carrier, fiber dispersion, MMW frequency, and the fiber transmission distance. With a given data rate B, the code width can be obtained:
τ=1/B.
(6)
Assuming the maximum tolerable walk-off amount equals to 70% of the code width [14], and D is 17 ps/nm km, the maximum transmission distance can be obtained as:
L_ max=(70%τ)/(Δf ⋅D).
(7)
When fc ¼20 GHz, Δf is determined by the second sidebands frequency difference, i.e., 80 GHz, the maximum transmission distance of the system is around 25 km with the data rate of 2.5 Gb/s. Similarly, we can obtain that when fc ¼ 30 GHz (i.e., 120 GHz MMW) the maximum transmission distance for the system is about 19 km. In the simulation, we test the cases with the maximum transmission distance. The bit error rate (BER) curve and eye diagram of the downconverted electrical MMW signals are depicted in Fig.4 with fc ¼20 GHz and fc ¼ 30 GHz, respectively. In the case of fc ¼ 20 GHz, 25 km SMF is implemented. The power penalty between back-toback (b2b) and 25 km SMF transmission to reach a BER of 10 5 are 0.6 dB and 1.8 dB for 40 GHz (Data2) and 80 GHz (Data1) MMW, respectively. When fc ¼30 GHz, the power penalty between b2b and 19 km SMF transmission at BER ¼10 5 is 1.5 dB and 2.7 dB for 60 GHz (Data2) and 120 GHz (Data1) MMW. It can be seen that the higher frequency MMW is, the larger power penalty is obtained. It is mainly due to the dispersion induced distortion. To further increase the system capacity, advanced modulation format, such as quadrature phase shift keying (QPSK), discrete multi-tone (DMT), etc., can be considered. Using these modulation formats, with the same data ratea longer symbol period can be obtained compared to OOK. To some extent, it helps to increase the maximum transmission distance of the MMW signals that is constrained by the fiber dispersion. On the other hand, transmission over the fiber has negligible impact on upstream BER performance (see Fig. 4(e) and (f)). Although the central carrier is also modulated with downstream Data1, it does not significantly affect upstream transmission since BER r 10–5 can still be achieved after the fiber transmission when the received optical power at the CO is low ( –23 dBm).
4. Conclusion We propose a palm-shaped spectrum generation scheme for RoF system offering dual-band MMW signals for downstream and reusing the optical carrier for upstream. Only a single DPMZM is used to generate and modulate two bands of MMW, which is easy to implement and cost effective. Dual-band MMWs with different data for downstream are generated by direct beating between symmetric sidebands of the palm while the optical carrier (i.e., the middle finger of palm) can be used for upstream transmission at the BS. The proof-of-concept experiment validates the proposed
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Fig. 4. BER performance and eye diagrams at the corresponding measuring points: (a) downstream signal of 40 GHz MMW (Data2), (b) 80 GHz MMW (Data1) and (c) upstream in case of fc ¼ 20 GHz, and (d) downstream signal of 60 GHz MMW (Data2), (e) 120 GHz MMW (Data1) and (f) upstream in case of fc ¼ 30 GHz.
spectrum generation scheme can provide obviously higher OSSR and hence better transmission performance in upstream in comparison with the optical frequency comb, palm-shaped spectrum. Meanwhile, the feasibility of using the proposed palm-shaped spectrum generation in the RoF system is verified by carrying out simulation with different configurations of MMW bands.
Acknowledgments This work was supported by the National High-tech R&D Program of China (863 Program) under Grant no. 2013AA013402, the National Natural Science Foundation of China (NSFC) under Grants no. 61331010, the Program for New Century Excellent Talents in
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University (NCET-13-0235), and Göran Gustafsson Foundation.
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