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DAPSK-OFDM-based coherent PON and IFoF heterogeneous access network with PDM
Optics Communications 456 (2020) 124678
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
DAPSK-OFDM-based coherent PON and IFoF heterogeneous access network with PDM✩ Kyoung-Hak Mun, Soo-Min Kang, Sang-Kook Han ∗,1 Department of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, South Korea
ARTICLE
INFO
Keywords: Coherent communication Intermediate frequency over fiber Differential amplitude and phase shift keying Orthogonal frequency division multiplexing Polarization division multiplexing
ABSTRACT Heterogeneous access networks with wired/wireless convergence comprise an important trend in the development of access networks because they reduce network investment and operating costs. In this paper, we propose a heterogeneous access network with a coherent passive optical network (PON) and intermediate frequency over fiber (IFoF) within a single wavelength. The coherent PON and IFoF are both DAPSK-OFDM-based and do not involve noise-compensation digital signal processing (DSP), which lowers bandwidth efficiency. DAPSKOFDM is a differential modulation scheme that eliminates channel and phase noise during the demodulation process. The coherent PON and IFoF with different transmission schemes are combined using polarization division multiplexing (PDM) and a complete heterogeneous network without crosstalk through polarization optimization and maintenance at a transmission distance less than 20 km. The proposed heterogeneous network is experimentally tested, which shows that its performance is similar to that yielded when operating each system independently.
1. Introduction Optical access network capacity demand is rapidly increasing. Continuous passive optical networks (PONs) evolution based on conventional intensity modulation and direct detection (IM/DD), and on–off keying transmission is difficult because of bandwidth and optical power limitations. Therefore, Coherent PONs have attracted attention as the next-generation optical access network technology owing to their high sensitivity and bandwidth efficiency. Orthogonal frequency division multiplexing (OFDM) can be applied to coherent PONs, which enables the efficient use of time and frequency resources and heterogeneous services within a single wavelength [1,2]. However, heterogeneous signals allocated to the OFDM subcarriers must be re-modulated to their original form after the optical network unit (ONU) receives them. Because of this inconvenience, many heterogeneous network studies are based on wavelength division multiplexing (WDM) [3]. WDM can allocate heterogeneous services to each wavelength and independently operate through a single-mode fiber. Meanwhile, owing to the excess capacity requirements of common public radio interface, radio over fiber (RoF) is emerging as the nextgeneration mobile fronthaul technology [4]. RoF carries the mobile analog signal as it is on the optical carrier and transmits it from the digital unit (DU) to the radio unit (RU). Thus, the problem of
increasing optical transmission capacity through analog–digital conversion and digital–analog conversion can be solved. For this reason, many heterogeneous network studies consider RoF with PON [5]. One of the important problems occurring in RoF is the nonlinear phenomenon, which results from analog optical transmission. A nonlinear phenomenon is a signal distortion phenomenon that generates frequency components other than the input signal frequency. The analog transmission method is more vulnerable to the nonlinear response of the system than the digital transmission method. Therefore, it is necessary to strictly set the linear characteristics of the system devices. Also, when extending the RoF system through WDM, four-wave mixing (FWM), a kind of Kerr-nonlinearities, can occur, which degrade system performance [6]. Therefore, in WDM-based RoF, the number of wavelengths used, and the optical power are limited. In this paper, we propose a heterogeneous access network of coherent PONs and intermediate frequency over fiber (IFoF) with polarization division multiplexing (PDM). IFoF is an efficient fronthaul system that uses several intermediate frequencies to transmit multiple RoF signals over a single wavelength [7]. The proposed system combines multiplexed PON and IFoF signals; in addition, it transmits them on a single wavelength. Fig. 1 shows the proposed PDM-based heterogeneous access network. The OFDM-PON signal based on the
✩ This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP; Ministry of Science, ICT & Future Planning) (No. 2019R1A2C3007934). ∗ Corresponding author. E-mail address: [email protected] (S.-K. Han). 1 Senior Member of IEEE.
https://doi.org/10.1016/j.optcom.2019.124678 Received 25 June 2019; Received in revised form 3 September 2019; Accepted 30 September 2019 Available online 3 October 2019 0030-4018/© 2019 Elsevier B.V. All rights reserved.
K.-H. Mun, S.-M. Kang and S.-K. Han
Optics Communications 456 (2020) 124678
3. DAPSK-OFDM for the coherent PON and IFoF Differential modulation schemes, such as DAPSK, map data onto the differences between adjacent symbols. DAPSK can be applied to OFDM, which is multicarrier-based modulation. When the symbol difference of the OFDM frame is used in the time domain, it is termed 𝑡 (time)DAPSK-OFDM; when the symbol difference of the adjacent subcarrier is used [10], it is termed 𝑓 (frequency)-DAPSK-OFDM. The symbols of t-DAPSK-OFDM and f-DAPSK-OFDM are as follows:
Fig. 1. The proposed heterogeneous access network.
𝑡𝑆𝑖,𝑘 = 𝑡𝐵𝑖,𝑘 × 𝑡𝑆𝑖−1,𝑘 ,
(1)
𝑓 𝑆𝑖,𝑘 = 𝑓 𝐵𝑖,𝑘 × 𝑓 𝑆𝑖,𝑘−1
(2)
where 𝑡𝑆𝑖,𝑘 and 𝑓 𝑆𝑖,𝑘 represent a t-DAPSK and an f-DAPSK symbol of the ith frame and kth subcarrier, respectively. 𝑡𝐵𝑖,𝑘 and 𝑓 𝐵𝑖,𝑘 are differential values of adjacent symbols onto which data are mapped. 𝑡𝑆𝑖,𝑘 is obtained by multiplying the symbol before the frame by 𝑡𝐵𝑖,𝑘 , and 𝑓 𝑆𝑖,𝑘 is obtained by multiplying the symbol before a subcarrier by 𝑓 𝐵𝑖,𝑘 . The symbol 𝑅𝑖,𝑘 obtained after OFDM modulation and coherent transmission is as follows: 𝑅𝑖,𝑘 = 𝑆𝑖,𝑘 × 𝐻𝑖,𝑘 × 𝑒
𝑗𝛥𝜙𝑃 𝑁
𝑖,𝑘
(3)
+ 𝑛𝑖,𝑘
𝑗𝛥𝜙𝑃 𝑁
𝑖,𝑘 and 𝑛 where 𝐻𝑖,𝑘 , 𝑒 𝑖,𝑘 are the channel response, phase noise, and additive white Gaussian noise (AWGN), respectively. If the AWGN is negligible, the demodulation process is as follows:
Fig. 2. The proposed ONU and RU structure for polarization extraction.
𝑡𝐷𝑖,𝑘 =
coherent transmission is modulated through X-polarization, and the subcarriers are allocated to the ONUs separately. IFoF is modulated through Y-polarization and combined with the OFDM-PON signal in the polarization beam combiner (PBC). The heterogeneous signal is distributed through the optical distribution network (ODN) and input to the ONUs and the RUs; only the allocated polarization is extracted and received. Note that a differential modulation scheme (DAPSKOFDM) is used to easily resolve phase noise and channel noise in the system. In QAM-OFDM, phase and channel noise are estimated and compensated through the pilot-tone and preamble [8]. However, DAPSK-OFDM makes it possible for data to be restored without any additional compensation by mapping the data onto the phase and amplitude difference between adjacent symbols [9].
𝑡𝑅𝑖,𝑘 𝑡𝑅𝑖,𝑘−1
= 𝑡𝐵𝑖,𝑘 ×
𝑓 𝑅𝑖,𝑘
𝐻𝑖,𝑘 𝐻𝑖,𝑘−1
𝑒
×
𝐻𝑖,𝑘
𝑗𝛥𝜙𝑃 𝑁
𝑖,𝑘
𝑗𝛥𝜙𝑃 𝑁
𝑒
𝑒
,
(4)
𝑖,𝑘−1
𝑗𝛥𝜙𝑃 𝑁
𝑖,𝑘
. (5) 𝑒 The demodulation process is started by dividing adjacent symbols. In the case of t-DAPSK-OFDM, the symbols of the adjacent frames are divided. In the case of f-DAPSK-OFDM, the symbols of the adjacent subcarriers are divided. If the quasi-stationary condition is satisfied in time and frequency domains, the noise multiplied by adjacent symbols is canceled. Then, the symbol differential values 𝑡𝐷𝑖,𝑘 and 𝑓 𝐷𝑖,𝑘 can be obtained. The performance of differential modulation depends on the quasistationary condition. Therefore, the domain of differential modulation must be determined according to the degree of change of time and frequency channels. In the fixed physical channel state, the shorter the frame length of the OFDM signal and narrower the subcarrier interval, the better the quasi-stationary condition. In a coherent PON, a wide signal band of several GHz or more is used typically, and the frame length is several tens of ns. Therefore, it is appropriate to use t-DAPSKOFDM. In differential modulation, the first frame is redundant, but if the OFDM frame is transmitted continuously, this loss is insignificant. IFoF is based on the long-term evolution (LTE) standard. The OFDM subcarrier spacing is as narrow as 15 kHz, and the frame length is 64 ms, which is relatively long without considering a cyclic prefix and a header. Therefore, f-DAPSK-OFDM is appropriate. There is a loss in the first subcarrier for every frame; however, because LTE uses bandwidth of up to 20 MHz and 1200 subcarriers, the reduction in bandwidth efficiency due to subcarrier loss is minimal. The proposed heterogeneous access network combines a coherent PON based on t-DAPSK-OFDM and IFoF based on f-DAPSK-OFDM by using PDM. 𝑓 𝐷𝑖,𝑘 =
The proposed system performance was compared with the performance of an independent coherent PON and IFoF in a 20 km transmission experiment. There was almost no change in the optimized polarization at the transmission distance of the access network within 20 km, and there was no significant difference in performance between the proposed and separate access systems.
2. Structure of proposed ONU and RU Generally, a polarization beam splitter is used in PDM; however, the structure of the ODN is then changed, which is inefficient. To use the existing passive splitter-based ODN, polarization must be extracted from the ONU and RU. Additionally, this simplifies the polarization monitoring and optimization performed remotely in ODN. Fig. 2 shows the proposed ONU and RU structures for allocated polarization extraction. Heterogeneous signals distributed through the passive power splitter are input to the ONU and RU. Only allocated polarization can be extracted using the polarization controller (PC) and polarizer in each unit. In the optical access network with a transmission distance up to 20 km, the speed of polarization change is not high; therefore, polarization optimization and maintenance can be performed relatively easily. Provided that polarization orthogonality is maintained, the ONU performs coherent detection, and the RU performs direct detection without polarization crosstalk.
𝑓 𝑅𝑖,𝑘−1
= 𝑓 𝐵𝑖,𝑘 ×
𝐻𝑖,𝑘−1
×
𝑗𝛥𝜙𝑃 𝑁
𝑖,𝑘−1
4. Experiments Fig. 3 shows the experimental setup for the proposed system. The optical line terminator used an external cavity laser (ECL) with a center wavelength of 1549 nm and linewidth of less than 100 kHz as an optical source. The ECL was used as a local oscillator in the ONU. For the proof of concept of the proposed system, we used a self-coherent detection technique that excludes the carrier frequency offset. The LO light source was transmitted from the ECL of the OLT to the ONU 2
K.-H. Mun, S.-M. Kang and S.-K. Han
Optics Communications 456 (2020) 124678
Fig. 3. Experimental setup. Table 1 Signal specifications of the proposed system.
Subcarrier spacing First subcarrier frequency Sub-band spacing Signal bandwidth # of subcarriers in a band # of sub-bands DAPSK-order OFDM frame length
IFoF
PON
15 kHz 30 MHz 60 MHz 20 MHz (per band) 1200 12 64 64 ms
46.87 MHz – – 3 GHz 64 – Adaptive modulation 22.3 ns
through a separated optical fiber. The proposed DAPSK-OFDM focused on phase noise and channel response. t-DAPSK-OFDM was optically modulated using Mach–Zehnder modulator and set to X-polarization by the PC and input to the PBC. In DU, the IFoF signal based on f-DAPSK-OFDM was optically modulated using a direct modulation laser diode with the same wavelength with the ECL and set to Ypolarization. The IFoF signal was combined with the coherent PON signal via the PBC. The heterogeneous signal was distributed to ONU and RU through passive splitter after 20 km transmission. The received optical power at the coherent receiver in ONU was −25 dBm, and 5 dBm was received at PD in RU. ODN loss for 64 ONUs was emulated by using the variable optical attenuator before coherent receiver. The units received only the polarization assigned to each unit using the PC and the polarizer. After a coherent detection of the ONU and direct detection of the RU, the DAPSK-OFDM was demodulated. All the DSP processes, including DAPSK-OFDM modulation and demodulation, were performed by offline processing. Table 1 shows the specifications of signals used in the experiment. One band of IFoF was based on the LTE standard, and the modulation order was 64 [11]. The band spacing and the number of bands were determined by considering the cloud radio access network (C-RAN) structure and non-linearity, with reference to [7]. The coherent PON signal had a bandwidth of 3 GHz and 64 subcarriers. Adaptive modulation was applied, and 8 or 16 DAPSKs were allocated. Note that the frame length of the coherent PON signal was 22.3 ns and IFoF was 64 ms, which was much longer than that of the PON. Thus, t-DAPSK-OFDM was used for the coherent PON and f-DAPSK-OFDM was used for IFoF.
Fig. 4. Spectrums of (a) the coherent-PON channel and (b) the optimized PON signal in the proposed system.
5. Results and discussion
optimizing polarization, the crosstalk could be minimized by suppressing noise, as shown in Fig. 4(b), and it could be maintained at a transmission distance of 20 km. The SNR of the modulated DAPSKOFDM signal on the optimized channel was 30 dB maximum. In the case of IFoF, there was no noise even when the coherent PON signal was input by polarization crosstalk. This was because the received signal of the coherent PON was so weak (less than −25 dBm) that it could
Fig. 4 shows the spectrums of the coherent PON in the proposed heterogeneous system. In the coherent PON part of the PDM-based heterogeneous network which received a signal through optical mixing, polarization crosstalk generated beating noise. Thus, baseband noise was thus shown in the unpolarized channel case of Fig. 4(a). By 3
K.-H. Mun, S.-M. Kang and S.-K. Han
Optics Communications 456 (2020) 124678
Fig. 8. EVM performance of IFoF according to the sub-band index.
Fig. 5. Spectrums of direct detected IFoF signal according to polarization states.
polarization crosstalk despite polarization optimization. Fig. 7 represents a bits-loading profile where 8-DPSK and 16-DAPSK were allocated according to EVM results. Bits-loading was performed based on the FEC limit (2 × 10−3 ). In the baseband three subcarriers, 4 bits were allocated for the DAPSK-OFDM-PON only, whereas 3 bits were allocated for the DAPSK-OFDM-PON with IFoF. Also, the performance of the DAPSKOFDM-PON only case was better in some high-bandwidth subcarriers. Bandwidth efficiency was 3.634 bits/s/Hz for DAPSK-OFDM only, and 3.556 bits/s/Hz for DAPSK-OFDM with IFoF. The total throughput was 10.9 Gbps and 10.6 Gbps, respectively. There was a performance difference due to polarization crosstalk, but the difference was rather small — 0.0078 bits/s/Hz and 0.3 Gbps. According to 3GPP specification, the EVM requirement for 64 QAMOFDM in the RoF system was 8% [12]. Considering the differential modulation penalty, the EVM requirement for 64 DAPSK-OFDM was 6.3%. Fig. 8 shows the EVM performance of IFoF according to the subband index. For IFoF with DAPSK-OFDM-PON, the EVM was up to 2% higher than IFoF only; however, the EVM requirement of 6.3% was satisfied for all the bands. Despite some polarization crosstalk penalty, there was no significant difference in performance, and the required EVM could be satisfied. The capacity of each IFoF sub-band was 112.5 Mbps and the total throughput was 1.35 Gbps.
Fig. 6. EVM profiles of the DAPSK-OFDM-PON in the proposed system.
6. Conclusion We have proposed a heterogeneous access network based on DAPSKOFDM and PDM. t-DAPSK-OFDM was used for a coherent PON, and bandwidth efficiency was optimized by adaptive modulation. f-DAPSKOFDM was used in the case of IFoF according to the LTE standard; it was based on the narrow frequency spacing characteristic because it could not satisfy the quasi-stationary condition in the time domain. There was no significant difference in the performance of the proposed heterogeneous network when polarization optimization was used in contrast to independent network operation. The proposed scheme effectively combines a coherent access network and the IM/DD based IFoF and is therefore useful for convergence of future wired/wireless access networks.
Fig. 7. Bits-loading profiles according to EVM results.
not be detected directly. However, there was SNR reduction due to the polarization crosstalk of the IFoF signal, which could be minimized by the polarization optimization as shown in Fig. 5. The SNR of up to 30 dB was obtained. The state of polarization could be maintained through performance monitoring, including the maximum SNR. Coherent PON and IFoF transmission experiments and performance measurements were performed through optimized heterogeneous PDM channels. For a coherent PON, adaptive modulation was used to maximize bandwidth efficiency. An error vector magnitude (EVM) profile was measured by transmitting 4-DPSK-OFDM for bits-loading, which is shown in Fig. 6. In the case of DAPSK-OFDM-PON with IFoF, the EVM was up to 3% higher in the baseband than when only the DAPSKOFDM-PON signal was transmitted. This was due to some remaining
References [1] W. Shieh, I. Djordjevic, Orthogonal Frequency Division Multiplexing for Optical Communications, Academic Press, Burlington, MA, 2009. [2] N. Cvijetic, OFDM for next-generation optical access networks, J. Lightwave Technol. 30 (4) (2012) 384–398. [3] S. Xu, S. Xu, Y. Tanaka, Dynamic resource reallocation for 5G with OFDMA in multiple user MIMO RoF-WDM-PON, in: Proceedings Asia-Pacific Conference on Communications, APCC, Kyoto, 2015, pp. 480–484. [4] Y. Tian, K.L. Lee, C. Lim, A. Nirmalathas, 60 GHz analog radio-over-fiber fronthaul investigations, J. Lightwave Technol. 35 (19) (2017) 4304–4310. 4
K.-H. Mun, S.-M. Kang and S.-K. Han
Optics Communications 456 (2020) 124678 [9] K.H. Mun, S.M. Jung, S.M. Kang, S.K. Han, Channel equalization and phase noise compensation free DAPSK-OFDM transmission for coherent PON system, IEEE Photonics J. 9 (5) (2017) 1–9. [10] H. Wang, D. Kong, Y. Li, J. Wu, J. Lin, Performance evaluation of (D)APSK modulated coherent optical OFDM system, Opt. Fiber Technol. 19 (2013) 242–249. [11] M. Gong, Y. Ji, H. Han, X. Lin, Two-dimensional differential demodulation for 64–DAPSK modulated OFDM signals, in: Proc. IEEE CCNC, Las Vegas, NV, USA, Jan. 2010, pp. 1–5. [12] Base Station (BS) radio transmission and reception, Tech. Spec. Group Radio Access Netw., 3GPP TS 36.104 v. 12.5.0, Rel. 12, Oct. 2014.
[5] C.H. Yeh, C.W. Chow, Heterogeneous radio-over-fiber passive access network architecture to mitigate Rayleigh backscattering interferometric beat noise, Opt. Express 19 (7) (2011) 5735–5740. [6] A. Panda, D.P. Mishar, Nonlinear effect of four-wave mixing for WDM in radio-over-fiber systems, J. Electron. Commun. Eng. Res. 2 (4) (2014) 1–6. [7] C. Han, M. Sung, S.H. Cho, H.S. Chung, S.M. Kim, J.H. Lee, Performance improvement of multi-IFoF-based fronthaul using dispersion-induced distortion mitigation with IF optimization, J. Lightwave Technol. 34 (20) (2016) 4772–4778. [8] X. Yi, W. Shieh, Y. Tang, Phase estimation for coherent optical OFDM, IEEE Photonics Technol. Lett. 19 (12) (2007) 919–921.
5
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
DAPSK-OFDM-based coherent PON and IFoF heterogeneous access network with PDM✩ Kyoung-Hak Mun, Soo-Min Kang, Sang-Kook Han ∗,1 Department of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, South Korea
ARTICLE
INFO
Keywords: Coherent communication Intermediate frequency over fiber Differential amplitude and phase shift keying Orthogonal frequency division multiplexing Polarization division multiplexing
ABSTRACT Heterogeneous access networks with wired/wireless convergence comprise an important trend in the development of access networks because they reduce network investment and operating costs. In this paper, we propose a heterogeneous access network with a coherent passive optical network (PON) and intermediate frequency over fiber (IFoF) within a single wavelength. The coherent PON and IFoF are both DAPSK-OFDM-based and do not involve noise-compensation digital signal processing (DSP), which lowers bandwidth efficiency. DAPSKOFDM is a differential modulation scheme that eliminates channel and phase noise during the demodulation process. The coherent PON and IFoF with different transmission schemes are combined using polarization division multiplexing (PDM) and a complete heterogeneous network without crosstalk through polarization optimization and maintenance at a transmission distance less than 20 km. The proposed heterogeneous network is experimentally tested, which shows that its performance is similar to that yielded when operating each system independently.
1. Introduction Optical access network capacity demand is rapidly increasing. Continuous passive optical networks (PONs) evolution based on conventional intensity modulation and direct detection (IM/DD), and on–off keying transmission is difficult because of bandwidth and optical power limitations. Therefore, Coherent PONs have attracted attention as the next-generation optical access network technology owing to their high sensitivity and bandwidth efficiency. Orthogonal frequency division multiplexing (OFDM) can be applied to coherent PONs, which enables the efficient use of time and frequency resources and heterogeneous services within a single wavelength [1,2]. However, heterogeneous signals allocated to the OFDM subcarriers must be re-modulated to their original form after the optical network unit (ONU) receives them. Because of this inconvenience, many heterogeneous network studies are based on wavelength division multiplexing (WDM) [3]. WDM can allocate heterogeneous services to each wavelength and independently operate through a single-mode fiber. Meanwhile, owing to the excess capacity requirements of common public radio interface, radio over fiber (RoF) is emerging as the nextgeneration mobile fronthaul technology [4]. RoF carries the mobile analog signal as it is on the optical carrier and transmits it from the digital unit (DU) to the radio unit (RU). Thus, the problem of
increasing optical transmission capacity through analog–digital conversion and digital–analog conversion can be solved. For this reason, many heterogeneous network studies consider RoF with PON [5]. One of the important problems occurring in RoF is the nonlinear phenomenon, which results from analog optical transmission. A nonlinear phenomenon is a signal distortion phenomenon that generates frequency components other than the input signal frequency. The analog transmission method is more vulnerable to the nonlinear response of the system than the digital transmission method. Therefore, it is necessary to strictly set the linear characteristics of the system devices. Also, when extending the RoF system through WDM, four-wave mixing (FWM), a kind of Kerr-nonlinearities, can occur, which degrade system performance [6]. Therefore, in WDM-based RoF, the number of wavelengths used, and the optical power are limited. In this paper, we propose a heterogeneous access network of coherent PONs and intermediate frequency over fiber (IFoF) with polarization division multiplexing (PDM). IFoF is an efficient fronthaul system that uses several intermediate frequencies to transmit multiple RoF signals over a single wavelength [7]. The proposed system combines multiplexed PON and IFoF signals; in addition, it transmits them on a single wavelength. Fig. 1 shows the proposed PDM-based heterogeneous access network. The OFDM-PON signal based on the
✩ This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP; Ministry of Science, ICT & Future Planning) (No. 2019R1A2C3007934). ∗ Corresponding author. E-mail address: [email protected] (S.-K. Han). 1 Senior Member of IEEE.
https://doi.org/10.1016/j.optcom.2019.124678 Received 25 June 2019; Received in revised form 3 September 2019; Accepted 30 September 2019 Available online 3 October 2019 0030-4018/© 2019 Elsevier B.V. All rights reserved.
K.-H. Mun, S.-M. Kang and S.-K. Han
Optics Communications 456 (2020) 124678
3. DAPSK-OFDM for the coherent PON and IFoF Differential modulation schemes, such as DAPSK, map data onto the differences between adjacent symbols. DAPSK can be applied to OFDM, which is multicarrier-based modulation. When the symbol difference of the OFDM frame is used in the time domain, it is termed 𝑡 (time)DAPSK-OFDM; when the symbol difference of the adjacent subcarrier is used [10], it is termed 𝑓 (frequency)-DAPSK-OFDM. The symbols of t-DAPSK-OFDM and f-DAPSK-OFDM are as follows:
Fig. 1. The proposed heterogeneous access network.
𝑡𝑆𝑖,𝑘 = 𝑡𝐵𝑖,𝑘 × 𝑡𝑆𝑖−1,𝑘 ,
(1)
𝑓 𝑆𝑖,𝑘 = 𝑓 𝐵𝑖,𝑘 × 𝑓 𝑆𝑖,𝑘−1
(2)
where 𝑡𝑆𝑖,𝑘 and 𝑓 𝑆𝑖,𝑘 represent a t-DAPSK and an f-DAPSK symbol of the ith frame and kth subcarrier, respectively. 𝑡𝐵𝑖,𝑘 and 𝑓 𝐵𝑖,𝑘 are differential values of adjacent symbols onto which data are mapped. 𝑡𝑆𝑖,𝑘 is obtained by multiplying the symbol before the frame by 𝑡𝐵𝑖,𝑘 , and 𝑓 𝑆𝑖,𝑘 is obtained by multiplying the symbol before a subcarrier by 𝑓 𝐵𝑖,𝑘 . The symbol 𝑅𝑖,𝑘 obtained after OFDM modulation and coherent transmission is as follows: 𝑅𝑖,𝑘 = 𝑆𝑖,𝑘 × 𝐻𝑖,𝑘 × 𝑒
𝑗𝛥𝜙𝑃 𝑁
𝑖,𝑘
(3)
+ 𝑛𝑖,𝑘
𝑗𝛥𝜙𝑃 𝑁
𝑖,𝑘 and 𝑛 where 𝐻𝑖,𝑘 , 𝑒 𝑖,𝑘 are the channel response, phase noise, and additive white Gaussian noise (AWGN), respectively. If the AWGN is negligible, the demodulation process is as follows:
Fig. 2. The proposed ONU and RU structure for polarization extraction.
𝑡𝐷𝑖,𝑘 =
coherent transmission is modulated through X-polarization, and the subcarriers are allocated to the ONUs separately. IFoF is modulated through Y-polarization and combined with the OFDM-PON signal in the polarization beam combiner (PBC). The heterogeneous signal is distributed through the optical distribution network (ODN) and input to the ONUs and the RUs; only the allocated polarization is extracted and received. Note that a differential modulation scheme (DAPSKOFDM) is used to easily resolve phase noise and channel noise in the system. In QAM-OFDM, phase and channel noise are estimated and compensated through the pilot-tone and preamble [8]. However, DAPSK-OFDM makes it possible for data to be restored without any additional compensation by mapping the data onto the phase and amplitude difference between adjacent symbols [9].
𝑡𝑅𝑖,𝑘 𝑡𝑅𝑖,𝑘−1
= 𝑡𝐵𝑖,𝑘 ×
𝑓 𝑅𝑖,𝑘
𝐻𝑖,𝑘 𝐻𝑖,𝑘−1
𝑒
×
𝐻𝑖,𝑘
𝑗𝛥𝜙𝑃 𝑁
𝑖,𝑘
𝑗𝛥𝜙𝑃 𝑁
𝑒
𝑒
,
(4)
𝑖,𝑘−1
𝑗𝛥𝜙𝑃 𝑁
𝑖,𝑘
. (5) 𝑒 The demodulation process is started by dividing adjacent symbols. In the case of t-DAPSK-OFDM, the symbols of the adjacent frames are divided. In the case of f-DAPSK-OFDM, the symbols of the adjacent subcarriers are divided. If the quasi-stationary condition is satisfied in time and frequency domains, the noise multiplied by adjacent symbols is canceled. Then, the symbol differential values 𝑡𝐷𝑖,𝑘 and 𝑓 𝐷𝑖,𝑘 can be obtained. The performance of differential modulation depends on the quasistationary condition. Therefore, the domain of differential modulation must be determined according to the degree of change of time and frequency channels. In the fixed physical channel state, the shorter the frame length of the OFDM signal and narrower the subcarrier interval, the better the quasi-stationary condition. In a coherent PON, a wide signal band of several GHz or more is used typically, and the frame length is several tens of ns. Therefore, it is appropriate to use t-DAPSKOFDM. In differential modulation, the first frame is redundant, but if the OFDM frame is transmitted continuously, this loss is insignificant. IFoF is based on the long-term evolution (LTE) standard. The OFDM subcarrier spacing is as narrow as 15 kHz, and the frame length is 64 ms, which is relatively long without considering a cyclic prefix and a header. Therefore, f-DAPSK-OFDM is appropriate. There is a loss in the first subcarrier for every frame; however, because LTE uses bandwidth of up to 20 MHz and 1200 subcarriers, the reduction in bandwidth efficiency due to subcarrier loss is minimal. The proposed heterogeneous access network combines a coherent PON based on t-DAPSK-OFDM and IFoF based on f-DAPSK-OFDM by using PDM. 𝑓 𝐷𝑖,𝑘 =
The proposed system performance was compared with the performance of an independent coherent PON and IFoF in a 20 km transmission experiment. There was almost no change in the optimized polarization at the transmission distance of the access network within 20 km, and there was no significant difference in performance between the proposed and separate access systems.
2. Structure of proposed ONU and RU Generally, a polarization beam splitter is used in PDM; however, the structure of the ODN is then changed, which is inefficient. To use the existing passive splitter-based ODN, polarization must be extracted from the ONU and RU. Additionally, this simplifies the polarization monitoring and optimization performed remotely in ODN. Fig. 2 shows the proposed ONU and RU structures for allocated polarization extraction. Heterogeneous signals distributed through the passive power splitter are input to the ONU and RU. Only allocated polarization can be extracted using the polarization controller (PC) and polarizer in each unit. In the optical access network with a transmission distance up to 20 km, the speed of polarization change is not high; therefore, polarization optimization and maintenance can be performed relatively easily. Provided that polarization orthogonality is maintained, the ONU performs coherent detection, and the RU performs direct detection without polarization crosstalk.
𝑓 𝑅𝑖,𝑘−1
= 𝑓 𝐵𝑖,𝑘 ×
𝐻𝑖,𝑘−1
×
𝑗𝛥𝜙𝑃 𝑁
𝑖,𝑘−1
4. Experiments Fig. 3 shows the experimental setup for the proposed system. The optical line terminator used an external cavity laser (ECL) with a center wavelength of 1549 nm and linewidth of less than 100 kHz as an optical source. The ECL was used as a local oscillator in the ONU. For the proof of concept of the proposed system, we used a self-coherent detection technique that excludes the carrier frequency offset. The LO light source was transmitted from the ECL of the OLT to the ONU 2
K.-H. Mun, S.-M. Kang and S.-K. Han
Optics Communications 456 (2020) 124678
Fig. 3. Experimental setup. Table 1 Signal specifications of the proposed system.
Subcarrier spacing First subcarrier frequency Sub-band spacing Signal bandwidth # of subcarriers in a band # of sub-bands DAPSK-order OFDM frame length
IFoF
PON
15 kHz 30 MHz 60 MHz 20 MHz (per band) 1200 12 64 64 ms
46.87 MHz – – 3 GHz 64 – Adaptive modulation 22.3 ns
through a separated optical fiber. The proposed DAPSK-OFDM focused on phase noise and channel response. t-DAPSK-OFDM was optically modulated using Mach–Zehnder modulator and set to X-polarization by the PC and input to the PBC. In DU, the IFoF signal based on f-DAPSK-OFDM was optically modulated using a direct modulation laser diode with the same wavelength with the ECL and set to Ypolarization. The IFoF signal was combined with the coherent PON signal via the PBC. The heterogeneous signal was distributed to ONU and RU through passive splitter after 20 km transmission. The received optical power at the coherent receiver in ONU was −25 dBm, and 5 dBm was received at PD in RU. ODN loss for 64 ONUs was emulated by using the variable optical attenuator before coherent receiver. The units received only the polarization assigned to each unit using the PC and the polarizer. After a coherent detection of the ONU and direct detection of the RU, the DAPSK-OFDM was demodulated. All the DSP processes, including DAPSK-OFDM modulation and demodulation, were performed by offline processing. Table 1 shows the specifications of signals used in the experiment. One band of IFoF was based on the LTE standard, and the modulation order was 64 [11]. The band spacing and the number of bands were determined by considering the cloud radio access network (C-RAN) structure and non-linearity, with reference to [7]. The coherent PON signal had a bandwidth of 3 GHz and 64 subcarriers. Adaptive modulation was applied, and 8 or 16 DAPSKs were allocated. Note that the frame length of the coherent PON signal was 22.3 ns and IFoF was 64 ms, which was much longer than that of the PON. Thus, t-DAPSK-OFDM was used for the coherent PON and f-DAPSK-OFDM was used for IFoF.
Fig. 4. Spectrums of (a) the coherent-PON channel and (b) the optimized PON signal in the proposed system.
5. Results and discussion
optimizing polarization, the crosstalk could be minimized by suppressing noise, as shown in Fig. 4(b), and it could be maintained at a transmission distance of 20 km. The SNR of the modulated DAPSKOFDM signal on the optimized channel was 30 dB maximum. In the case of IFoF, there was no noise even when the coherent PON signal was input by polarization crosstalk. This was because the received signal of the coherent PON was so weak (less than −25 dBm) that it could
Fig. 4 shows the spectrums of the coherent PON in the proposed heterogeneous system. In the coherent PON part of the PDM-based heterogeneous network which received a signal through optical mixing, polarization crosstalk generated beating noise. Thus, baseband noise was thus shown in the unpolarized channel case of Fig. 4(a). By 3
K.-H. Mun, S.-M. Kang and S.-K. Han
Optics Communications 456 (2020) 124678
Fig. 8. EVM performance of IFoF according to the sub-band index.
Fig. 5. Spectrums of direct detected IFoF signal according to polarization states.
polarization crosstalk despite polarization optimization. Fig. 7 represents a bits-loading profile where 8-DPSK and 16-DAPSK were allocated according to EVM results. Bits-loading was performed based on the FEC limit (2 × 10−3 ). In the baseband three subcarriers, 4 bits were allocated for the DAPSK-OFDM-PON only, whereas 3 bits were allocated for the DAPSK-OFDM-PON with IFoF. Also, the performance of the DAPSKOFDM-PON only case was better in some high-bandwidth subcarriers. Bandwidth efficiency was 3.634 bits/s/Hz for DAPSK-OFDM only, and 3.556 bits/s/Hz for DAPSK-OFDM with IFoF. The total throughput was 10.9 Gbps and 10.6 Gbps, respectively. There was a performance difference due to polarization crosstalk, but the difference was rather small — 0.0078 bits/s/Hz and 0.3 Gbps. According to 3GPP specification, the EVM requirement for 64 QAMOFDM in the RoF system was 8% [12]. Considering the differential modulation penalty, the EVM requirement for 64 DAPSK-OFDM was 6.3%. Fig. 8 shows the EVM performance of IFoF according to the subband index. For IFoF with DAPSK-OFDM-PON, the EVM was up to 2% higher than IFoF only; however, the EVM requirement of 6.3% was satisfied for all the bands. Despite some polarization crosstalk penalty, there was no significant difference in performance, and the required EVM could be satisfied. The capacity of each IFoF sub-band was 112.5 Mbps and the total throughput was 1.35 Gbps.
Fig. 6. EVM profiles of the DAPSK-OFDM-PON in the proposed system.
6. Conclusion We have proposed a heterogeneous access network based on DAPSKOFDM and PDM. t-DAPSK-OFDM was used for a coherent PON, and bandwidth efficiency was optimized by adaptive modulation. f-DAPSKOFDM was used in the case of IFoF according to the LTE standard; it was based on the narrow frequency spacing characteristic because it could not satisfy the quasi-stationary condition in the time domain. There was no significant difference in the performance of the proposed heterogeneous network when polarization optimization was used in contrast to independent network operation. The proposed scheme effectively combines a coherent access network and the IM/DD based IFoF and is therefore useful for convergence of future wired/wireless access networks.
Fig. 7. Bits-loading profiles according to EVM results.
not be detected directly. However, there was SNR reduction due to the polarization crosstalk of the IFoF signal, which could be minimized by the polarization optimization as shown in Fig. 5. The SNR of up to 30 dB was obtained. The state of polarization could be maintained through performance monitoring, including the maximum SNR. Coherent PON and IFoF transmission experiments and performance measurements were performed through optimized heterogeneous PDM channels. For a coherent PON, adaptive modulation was used to maximize bandwidth efficiency. An error vector magnitude (EVM) profile was measured by transmitting 4-DPSK-OFDM for bits-loading, which is shown in Fig. 6. In the case of DAPSK-OFDM-PON with IFoF, the EVM was up to 3% higher in the baseband than when only the DAPSKOFDM-PON signal was transmitted. This was due to some remaining
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