Novel OBI noise reduction technique by using similar-obi estimation in optical multiple access uplink

Vol. 25, No. 17 21 Aug 2017 OPTICS EXPRESS 20860 Novel OBI noise reduction technique by using similar-obi estimation in optical multiple access uplink HYOUNG JOON PARK, SUN-YOUNG JUNG, AND SANG-KOOK HAN Department of Electrical and Electronic Engineering, Yonsei University, 134 Shinchon-dong, Seodaemun-gu, Seoul 120-749, South Korea *skhan@yonsei.ac.kr Abstract: A novel optical beat interference (OBI) noise reduction technique for optical multiple access uplink using the same wavelength is presented. The OBI noise is estimated by similar-obi and mitigated by subtracting estimated OBI noise using the digital signal processing technique. To derive similar-obi, an RF clipping tone is transmitted out of signal band. We numerically and experimentally verified the validity of proposed OBI noise reduction method in orthogonal frequency division multiplexing access (OFDMA) passive optical network (PON) uplink where the OBI noise has been reduced and signal to noise ratio (SNR) has been improved from 4.4 db to 10.6 db. 2017 Optical Society of America OCIS codes: (060.4510) Optical communications; (060.5625) Radio frequency photonics. References and links 1. G. Wunder, P. Jung, M. Kasparick, T. Wild, F. Schaich, Y. Chen, and L. Mendes, 5GNOW: non-orthogonal, asynchronous waveforms for future mobile applications, IEEE Commun. Mag. 52(2), 97 105 (2014). 2. E. Wong, Next-generation broadband access networks and technologies, J. Lightwave Technol. 30(4), 597 608 (2012). 3. N. Cvijetic, OFDM for next-generation optical access networks, J. Lightwave Technol. 30(4), 384 398 (2012). 4. N. Cvijetic, D. Qian, J. Hu, and T. Wang, Orthogonal frequency division multiple access PON (OFDMA-PON) for colorless upstream transmission beyond 10 Gb/s, IEEE J. Sel. Areas Comm. 28(6), 781 790 (2010). 5. S. M. Kang, K. H. Mun, S. M. Jung, and S. K. Han, Mitigation of Optical Interference Noise by Optical Carrier Cancelation in Self-Coherent Reflective PON Uplink Transmission, IEEE Photonics J. 9(3), 7903509 (2017). 6. S. M. Jung, K. H. Mun, S. Y. Jung, and S. K. Han, Optical-Beat-Induced Multi-User-Interference Reduction in Single Wavelength OFDMA PON Upstream Multiple Access Systems with Self-Homodyne Coherent Detection, J. Lightwave Technol. 34(11), 2804 2811 (2016). 7 S. Y. Jung, C. H. Kim, S. M. Jung, and S. K. Han, Optical Beating Interference Reduction by Using Optical Pulse Division Multiplexing in IM/DD based OFDMA-PON Uplink, ECOC 2016, Paper Th2.P2.SC7 (2016) 8. S. Y. Jung, C. H. Kim, S. M. Jung, and S. K. Han, Optical pulse division multiplexing-based OBI reduction for single wavelength uplink multiple access in IM/DD OFDMA-PON, Opt. Express 24(25), 29198 29208 (2016). 9. A. Zadok, H. Shalom, M. Tur, W. D. Cornwell, and I. Andonovic, Spectral shift and broadening of DFB lasers under direct modulation, IEEE Photonics Technol. Lett. 10(12), 1709 1711 (1998). 10. S. M. Jung, S. M. Yang, K. H. Mun, and S. K. Han, Optical beat interference noise reduction by using out-ofband RF clipping tone signal in remotely fed OFDMA-PON link, Opt. Express 22(15), 18246 18253 (2014). 11. Y. Y. Won, S. M. Jung, and S. K. Han, Suppression of optical beat interference-noise in orthogonal frequency division multiple access-passive optical network link using self-homodyne balanced detection, Opt. Fiber Technol. 20(4), 309 313 (2014). 1. Introduction In recent years, passive optical network(pon) is developing due to evolution in network services. To support upcoming network services like internet of things(iot), 5G, and datacenter network, the traffic of optical access network is becoming more complex and diverse [1,2]. Therefore, flexible and easily manageable network is required to satisfy the complex and diverse optical access network. Also, high data requirements could be embraced in optical access network. To satisfy all these needs, orthogonal frequency division multiplexing is on the rise because of its dispersion tolerance, flexible multiple access with dynamic bandwidth allocation(dba), and high spectral efficiency [3]. #293225 Journal 2017 https://doi.org/10.1364/oe.25.020860 Received 20 Apr 2017; revised 20 Jul 2017; accepted 12 Aug 2017; published 18 Aug 2017

Vol. 25, No. 17 21 Aug 2017 OPTICS EXPRESS 20861 To achieve these advantages in a practical system, multiple access transmission on a single wavelength using orthogonal frequency division multiplexing access(ofdma) PON, a critical issue known as optical beat interference(obi) noise needs to be overcome. The OBI noise is generated by the product of multiple optical sources in photo-detection process. In multiple access on a single wavelength, there are a number of optical carriers from several ONUs, and the average frequencies of each optical carriers are single wavelength. However, every moment, the frequencies of optical sources are fluctuating, so there are frequency differences between several optical carriers. Therefore, optical beating interferences between each optical carriers generate unwanted broadband beat components, which are called OBI noise. Since the frequency differences between optical carriers are not quite large, the unwanted OBI noise is centered in the baseband. Thus, the OBI noise is critical obstacle for optical multiple access using the same wavelength. To mitigate the OBI noise, several techniques are proposed and demonstrated. Some techniques were proposed and demonstrated to avoid OBI noise employing coherent receiver [4 6]. However, coherent transmission is inappropriate for optical access network owing to its complexity and costineffectiveness, which is due to additional external modulator and coherent receiver. Another technique has been proposed to suppress OBI noise using optical pulse division multiplexing [7,8]. The OBI noise was mitigated by the optical pulse division multiplexing in two ONU system. However the proposed technique could be difficult to implement in more than two ONU system, since the technique is based on time division multiplexing. Also accurate timing offset between two ONU is needed. Another technique has been proposed to mitigate OBI noise using spectrum broadening effect [9 11]. To employ spectrum broadening effect, RF clipping tone was modulated inside or outside of signal bandwidth. By this technique, the OBI noise was easily broadened and suppressed, however the OBI noise was still existed. In this paper, we propose a novel technique to mitigate OBI noise in optical multiple access uplink using similar-obi estimation with out-of-band RF clipping tone. We demonstrated the proposed technique both by numerical analysis and experimental analysis. By using the proposed technique, we were able to reduce the OBI noise, which is critical issue in optical multiple access using the same wavelength. 2. Proposed OBI noise reduction technique in OFDMA PON uplink Fig. 1. Illustration of OBI noise in optical multiple access using the same wavelength. In this part, the proposed OBI noise reduction technique will be numerically demonstrated. OBI noise is the optical beating interference between two or more optical carriers from different optical network unit(onu). Therefore, with RF clipping tone, optical beating interference between an optical carrier from an ONU and RF clipping tone from another ONU would be generated and located nearby RF clipping tone as represented in Fig. 1. Since, the frequency of the RF clipping tone fluctuates with its optical carrier, the shape of OBI between each optical carriers is very similar with the shape of OBI between optical carriers and RF clipping tones. Thus, we denominated the OBI between an optical carrier and a RF clipping tone as similar-obi.

Vol. 25, No. 17 21 Aug 2017 OPTICS EXPRESS 20862 Fig. 2. The proposed OBI noise reduction technique scheme. Fig. 3. Modulated signal and RF tone in (a) ONU1 and (b) ONU2. In proposed technique, similar-obi is extracted using high pass filter and down converted into baseband as shown in Fig. 2. Then, the power of similar-obi is matched with the power of OBI noise. Lastly, similar-obi is subtracted from received signal in OLT. We used digital signal processing(dsp) to implement this technique, however the technique can be implemented using analog circuit. In DSP, digital high pass filter was used, and multiplying cosine signal with the same frequency of RF tone was employed instead of local oscillator. Power matching was implemented by digitally multiplying similar-obi with the power ratio between similar-obi and the OBI noise. Also subtracting was also digitally progressed. By this process, we could mitigate OBI noise. Since there are more than two ONUs in an optical access network, the OBI noise will be the sum of beating interference between any two ONUs. In our proposed technique, for the OBI noise generated by any two ONUs, the pair similar-obi always exists. Therefore, multiple ONU case could be proved by demonstrating two ONU case. Thus, we numerically demonstrate for just two ONUs; ONU1 and ONU2. To analyze numerically, signal of ONU1 and ONU2 is abbreviated as S1 and S2, respectively, as shown in Fig. 3. Also, the DC bias of ONU1 and ONU2 is abbreviated as B. RF clipping tone of ONU1 and ONU2 is also abbreviated as T. We can represent the optical signals O and ONU 1 O from ONU1 and ONU2 as, ONU 2 () exp( ω ) O t = B+ S + T j t (1) ONU 1 1 O1 () exp( ω ) O t = B+ S + T j t (2) ONU 2 2 O 2 where ω and ω are the optical carrier frequencies from ONU1 and ONU2, respectively. O1 O 2 In photo detector(), optical power is measured by square-law detection. Therefore, measured signal O is represented as, ( exp ( ω ) exp( ω )) 2 1 1 2 2 O = B+ S + T j t + B+ S + T j t (3) O O Since exp ( j( ω ω ) t ) term could not be detected by, O can be represented as, Using Maclaurin series, ( 2B 2 ) exp ( ( ω ω ) ) O = + S + S + T + B+ S + T B+ S + T j t (4) 1 2 1 2 O can be approximated as,

Vol. 25, No. 17 21 Aug 2017 OPTICS EXPRESS 20863 1 ( S + T) 1 ( S + T) O = B+ S + S + T + B + 2 B 8 B 1 ( S + T) 1 ( S + T) + exp j ω ω t 2 B 8 B 1 1 2 (2 2 ) (1 ( ) ( ) ) 1 2 2 2 2 (1 ( ) ( ) ) ( ( ) ) Because the value of 2nd term in the Maclaurin series is low, all the products of 2nd term except the product of 0th term and 2nd term are omitted. Then, O can be represented as, O = (2B+ S + S + 2 T) 1 2 2 2 S + S 2S S S S 1 2 1 2 1 2 + ( B+ + T + ) exp( j( ω ω ) t) 2 8B There are many terms in Eq. (6), however every terms could be separated into two groups; baseband group and RF band group. As shown in Fig. 2, the terms of baseband group are located in the baseband and the terms of RF band group are located in RF band, which is around the RF clipping tone. The term B exp( j( ω ω ) t) is the dominant OBI noise term. Therefore, we can estimate the OBI noise with similar-obi term T exp( j( ω ω ) t), which has the same frequency fluctuation ( ω ω ) with OBI noise. Since the power and center frequency of OBI noise and similar-obi are different, power and frequency tuning is needed. Therefore, we numerically demonstrated the mitigation of OBI noise by the proposed technique using similar-obi. 3. Experimental setup (5) (6) Fig. 4. Experimental setup. Numerical analysis showed how the OBI noise could be mitigated by the proposed OBI noise reduction technique. To experimentally demonstrate the proposed technique, experiment was implemented as a proof of concept employing two ONUs. The experimental scheme is shown in Fig. 4. Continuous wave optical source was generated by distributed feedback laser(dfb- LD), of which center frequency is 1550 nm. Bias current of laser was 52.2 ma. The Optical source was split into two and each source became the optical source of ONU1 and ONU2. Since each optical sources for ONU1 and ONU2, has to be incoherent to be practical, we used 20 km fiber. To match the power of optical sources, variable optical attenuator was used. Mach Zehnder modulator(mzm) is polarization dependent device, so polarization controller is used before MZM. Polarization is controlled to get the highest output power, which was 12.7 dbm in the experiment. The S21 bandwidth of MZM was 15 GHz. In arbitrary waveform generator, signal with RF clipping tone for ONU1 and ONU2 is modulated separately and transmitted to each MZM. The subcarrier spacing in OFDM was 150 khz and RF clipping tone was located at out of signal band, 2 GHz. The power of RF clipping tone was same as the power of OFDM signal. MZM converted electric signal to optical signal. The optical signal from ONU1 and ONU2 is combined by 3 db coupler and then detected in.

Vol. 25, No. 17 21 Aug 2017 OPTICS EXPRESS 20864 The transitionn time of was 115 ps, 3-dB bandwidth was 3.5 GHz. and conversion gain was 1100 V/W at 1550 nm. The detected signal was transmitted to digital phosphor oscilloscope(dpo), of which sample rate is 50 GS/s, then the proposed technique is implemented by using DSP. 4. Results and discussions In first, we experimentally demonstrated the proposed technique without data signal in ONU1 and ONU2. Only RF clipping tone was modulated inn each MZMs. The RF spectrum of received signal is shown in Fig. 5(a). By DSP, similar-obi components are extracted and frequency shifted to baseband. The waveform of processed similar-obi components is represented in Fig. 5(c) for 10000 samples. The waveform of extracted OBI noise using low pass filter is represented in Fig. 5(b) for 10000 samples. The x-axis and y-axis of the graph are samples and amplitude. It can be confirmed that the overall shape of OBI noise and similar-obi is almost the same. Therefore, it could bee verified thatt the OBI noise can be mitigated using similar-obi. Fig. 5. RF spectrum of received signal and waveforms of (a) OBI noise andd (b) similar-obi Fig. 6. RF spectra of OBI noise (a) without using proposed technique and (b) using proposed technique. As shown in Fig. 6(a), the OBI noise is represented in frequency domain. It can be verified that OBI noise is serious in low frequency area and it iss distributed over wide frequency area, which is wider than 350 MHz. By employing the proposed OBI noise reduction technique, the OBI noise is effectively mitigated more than 10 db, as shown in Fig. 6(b). Since OBI noise was mitigated, frequency spectrum is flattened. There is some component remaining at the low frequency band in Fig. 5(b). Thee reason for remaining component is that power matching of the OBI noise andd similar-obi is not perfectly accurate due to AWGN and mechanical issues such as resolution in DPO. However, suppression of

Vol. 25, No. 17 21 Aug 2017 OPTICS EXPRESS 20865 OBI noise more than 20 db in low frequency band means the accuracy of power matching is more than 99%. The noise level around 550 MHz was worsened employing the proposed technique, because the high frequency components of similar-obi could not be observed, which is covered with AWGN as shown in Fig. 5(a). Therefore, the performance of the proposed technique could be improved in low noise situation. Fig. 7. RF spectra and constellation of signal (a) without using proposed technique and (b) using proposed technique. To confirm the performance of the proposed OBI noise reduction technique when data signal is transmitted, OFDM signal is modulated in each MZM. For ONU1, frequency band from 0.11 GHz to 0.14 GHz is allocated and for ONU2, frequency band from 0.14 GHz to 0.18 GHz is allocated. As shown in Fig. 7(a), signal to noise ratio(snr) is 4.4 db at the start of ONU1 frequency band due to high OBI noise. The error vector magnitude(evm) of the signal was 70%. However, by employing the proposed OBI noise reduction technique, SNR is 10.5 db at the start of ONU1 frequency band as shown in Fig. 7(b). The SNR enhancement was more than 6 db. The EVM of transmitted signal was 33%, so 4QAM signal could be transmitted satisfying 10 3 of BER performance. The signal performance enhancement is also shown in constellation. Although, SNR improvement by proposed technique is experimentally verified, residual OBI noise is located at low frequency band. Residual OBI noise could be more mitigated by improving accuracy and using devices, which have better performance. 5. Conclusion We proposed a novel OBI noise reduction technique using similar-obi subtraction in OFDMA PON uplink. To implement the proposed method, RF clipping tone is needed, so spectrum broadening effect was also existed. However, employing novel OBI noise reduction technique, the OBI noise was suppressed more than 10 db at low frequency band. The signal performance in terms of SNR was improved 6 db at low frequency band. Therefore, data capacity can be improved employing high order modulation format. Moreover, OBI noise reduction technique could be implemented in any optical multiple access uplink using single wavelength. We believe that the proposed technique would be useful for future optical access network where optical multiple access uplink is arranged by using the same wavelength. Funding ICT R&D program of MSIP/IITP, Republic of Korea [2017-0-01676].