Adaptation of AMO-FBMC-OQAM in optical access network for accommodating asynchronous multiple access in OFDM based uplink transmission

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1 Adaptation of AMO-FBMC-OQAM in optical access network for accommodating asynchronous multiple access in OFDM based uplink transmission Sun-Young Jung, Sang-Min Jung, Sang-Kook Han Dept. of Electrical and Electronic Engineering, Yonsei Univ. 50 Yonsei-ro, Seodaemum-gu, Seoul , Republic of Korea ABSTRACT Exponentially expanding various applications in company with proliferation of mobile devices make mobile traffic exploded annually. For future access network, bandwidth efficient and asynchronous signals converged transmission technique is required in optical network to meet a huge bandwidth demand, while integrating various services and satisfying multiple access in perceived network resource. Orthogonal frequency division multiplexing (OFDM) is highly bandwidth efficient parallel transmission technique based on orthogonal subcarriers. OFDM has been widely studied in wired-/wireless communication and became a Long term evolution (LTE) standard. Consequently, OFDM also has been actively researched in optical network. However, OFDM is vulnerable frequency and phase offset essentially because of its sinc-shaped side lobes, therefore tight synchronism is necessary to maintain orthogonality. Moreover, redundant cyclic prefix (CP) is required in dispersive channel. Additionally, side lobes act as interference among users in multiple access. Thus, it practically hinders from supporting integration of various services and multiple access based on OFDM optical transmission In this paper, adaptively modulated optical filter bank multicarrier system with offset QAM (AMO-FBMC-OQAM) is introduced and experimentally investigated in uplink optical transmission to relax multiple access interference (MAI), while improving bandwidth efficiency. Side lobes are effectively suppressed by using FBMC, therefore the system becomes robust to path difference and imbalance among optical network units (ONUs), which increase bandwidth efficiency by reducing redundancy. In comparison with OFDM, a signal performance and an efficiency of frequency utilization are improved in the same experimental condition. It enables optical network to effectively support heterogeneous services and multiple access. Keywords: OFDM, FBMC, OQAM, adaptive modulation, optical uplink transmission, sidelobes suppression, optical multiple access, asynchronous waveform. 1. INTRODUCTION Exponentially expanding various applications in company with proliferation of mobile devices make mobile traffic exploded annually. Recently, technical requirements for fifth-generation (5G) wireless communications have been discussed to support a future wireless access network, after long-term-evolution-advanced (LTE-A). For examples, Internet of things (IoT), gigabit wireless connectivity and cooperative multipoint (CoMP) were stated as key requirements for 5G [1], [2]. In order to accommodate these applications, the future access network should have high bandwidth efficiency, as well as it should support seamless integration of asynchronous multiple users. For accommodating 5G wireless communications, optical access network should also have high efficiency in spectrum utilization and effectively support the asynchronous multiple access to meet a huge bandwidth demand, while integrating various services and satisfying multiple access in perceived network resource. In the LTE-A wireless communications, orthogonal frequency division multiplexing (OFDM) has been used as a standard [3]. OFDM is a highly spectrum efficient transmission technique based on overlapped multiple subcarriers. Moreover, it could maximize total capacity within certain bit-error-rate (BER) by employing adaptive modulation [3], [4]. Furthermore, OFDM is robust against dispersion by using cyclic prefix. For all those reasons, OFDM based transmission systems have been actively researched not only wireless communications but also optical communications Next-Generation Optical Communication: Components, Sub-Systems, and Systems IV, edited by Guifang Li, Xiang Zhou, Proc. of SPIE Vol. 9389, 93890Q 2015 SPIE CCC code: X/15/$18 doi: / Proc. of SPIE Vol Q-1

2 [5]. However, due to rectangular window of IFFT/FFT, OFDM essentially possess sinc-shaped sidelobes, which causes multiple access interference (MAI) in uplink transmission; moreover, MAI increased as increasing users [6]. To avoid the MAI among ONUs, it is required that a tight synchronization or a large frequency guard interval between ONUs in OFDM based multiple access (OFDMA). The tight synchronization of optical uplink signals is obviously inefficient in conventional passive optical network (PON) and the large guard band waste frequency resource and reduce spectral efficiency (SE). Thus, OFDMA could not effectively support the asynchronous multiple signals in optical uplink transmission, although OFDM has a high SE. Filter bank based multicarrier (FBMC) system is realized by prototype filter which responds to subcarrier [7], which has been spotlighted in wireless communications recently, but it is still new in optical communications. By using well designed prototype filter, FBMC could suppress sidelobes and relax the tight synchronization requirement between asynchronously combined signals. The amount of sidelobes suppression and spectrum confinement are decided by forming the shape of prototype filter. Mirabbasi-Martin equation based prototype filter is estimated as the best among the known filters in terms of sidelobes suppression [8]. In optical communications, a raised cosine filter or a root raised cosine filter were applied to shape the spectrum of optical signal [9], [10], but Mirabbasi-Martin filter was firstly employed in our group. Previously, we demonstrated sidelobes suppression and interference reduction when wired- /wireless signals were converged in optical downlink transmission [11], but optical uplink transmission was not verified. In this paper, to solve the interference problem of OFDMA, FBMC based multiple access (FBMC-MA) realized by Mirabbasi-Martin prototype filter was employed in optical multiple access of asynchronous optical uplink signals. We experimentally demonstrate performance improvement of FBMC-MA by comparative analysis with OFDMA. 2. SCHEMATICS Figure 1 presents optical uplink transmission based on OFDMA and FBMC-MA in PON system. Each ONUs is independently modulated as an optical signal with their best effort having unequal electro-optic (E/O) conversion efficiency, and then passed through different optical path. At optical combining region, the optical signals are passively and asynchronously combined and transmitted through single mode fiber (SMF). In this scenario, as illustrated in Figure 1, the ONUs interfere each other due to their sidelobes in case of OFDMA and these sidelobes could be effectively suppressed by employing FBMC in optical multiple access. In order to suppress sidelobes, FBMC was realized based on digital filter at overlapping factor K=4. The frequency ONU 1 ONU 2 ONU 1 (E/O) Independent, Unequal ONU 2 (E/O) Optical path difference ONU! ONU 2 Passive optical combine (Asynchronous) COD OLT (0/E, DSP) FBMC-MA Side lobes suppression t ONU I ONU 2 Figure 1. Optical multiple access based on OFDM and FBMC in passive optical network uplink transmission. OFDM FBMC PPN based filtering (K=4) With OQAM+ overlap & sum A Channel estimation ltap equalizing F F T 4-4- OFDM Remove CP FBMC PPN based filtarina (K=4) & sum Down sample & time synchron ization 4- D o Figure 2. Block diagram of OFDM and FBMC modulation/demodulation process. Proc. of SPIE Vol Q-2

3 130 Frequency do n rl F r (a) OFDM (b) FBMC (c) OFDM Time domain With 128 subcarriers and 10 symbol frame sequence Figure 3. Modulated signals in case of OFDM and FBMC. Offset (JAM real Q imaginary iramada; atepw timzeor eaw. SI,SCR Sear Half symbol duration offset Time shift & overlap/ sine & cosine plane r0/2 co 3r /2 2r0 Figure 4. Offset QAM and offset of a half-symbol-duration. coefficients of the filter for K=4 (H 0, H 1, H 2, H 3 ) are 1, , 2/2, , respectively [7]. In order to perform FBMC based filtering, frequency domain oversampling is required for an extended FFT filtering; it increases FFT size by K times. The FFT size limited practical real time process, and it occupies most of computational complexity in OFDM process. Thus, polyphase network (PPN) structure [7] was used to maintain FFT size by using repetitive time domain characteristic. After filtering process in Figure 2, FBMC could suppress not only side lobes in spectrum but also edge power of time domain signal as illustrated in Figure 3; therefore, FBMC is robust against dispersion without cyclic prefix (CP), which could improve SE. Even though sidelobes are suppressed, orghogonality between the directly adjacent subcarriers is broken in FBMC (Figure 3(b)). In order to satisfy orthogonality among subcarriers, offset QAM (OQAM) was introduced as illustrated in Figure 4. The real part and imaginary part of FBMC was alternatively assigned at an even/odd subcarrier for OQAM process, which reduces total capacity by half. To maintain total capacity of FBMC as same as that of OFDM, an offset of half-symbol-duration was also employed with OQAM; it utilizes a repetitive property of output waveform of FBMC-OQAM and an orthogonality of sine/cosine plane. Thus, by using PPN with OQAM, total capacity and computational complexity of FBMC could be comparable with OFDM. 3. EXPERIMENT Figure 5 presents experimental setup and system parameters. Multicarrier based optical multiple access was experimentally demonstrated in case of two different ONUs in IM/DD system. Individual ONU was independently modulated by Mach-Zehnder modulator (MZM). Bias current, input RF signal power and input optical polarization were optimized according to individual MZM condition; thus, each ONU had different performance due to unequal device response. The light source wavelength of ONU1 and ONU2 were nm and nm, respectively, in order to exclude an effect from optical beating interference (OBI) [12]. The modulated optical signals from each ONU passed through different optical path, which causes multipath delay spread. At optical distribution network (ODN), these signals were combined passively and asynchronously, which means power imbalance and MAI between the multiplexed signals were included. After optical combining, the uplink signal transmitted through SMF. For performance comparison, both experiments for OFDMA and FBMC-MA were conducted under same condition. As shown in Figure 2, OFDM and FBMC signals were realized by using offline process with same manners, and as shown in Figure 5, the used parameters were equal excepting CP length and filter shape. Redundant CP (1/16 of symbol duration) was inserted in OFDM only, which decreases SE of OFDM due to increased transmitting symbol duration. Instead of CP insertion, Mirabbasi-Martin equation based filtering was induced in FBMC to suppress sidelobes and confine spectrum. In FBMC generation, PPN Proc. of SPIE Vol Q-3

4 PM - power monitor VOA -variable optical attenuator AWG - arbitrary waveform generator DPO - digital phosphor oscilloscope VSG - vector signal generator VSA - vector signal analyzer 1.5GHz BW, 20NUs,N =128 (a) AMO -OFDM (b) AMO -FBMC nm db 36S DFB -LD nm +S.SdBm PC VOA Unused in B2B case DPO 50GS/s P BdBm VOA -14dBm VOA 3 Figure 5. Experimental setup and used parameters. based digital filtering and OQAM with the offset of half-symbol-duration were used to maintain computational complexity and total capacity compared to OFDM. The number of subcarrier was 128 and IFFT/FFT size was 256 due to Hermitian symmetry for intensity modulation in both OFDM and FBMC. The used signal bandwidth was 1.5GHz and initial modulation order for preamble was 2 (4QAM) without adaptive modulation. The preamble was used to estimate channel state for adaptive modulation. According to the feedback channel state information (CSI) obtained from preamble, different bits were allocated to corresponding subcarriers based on water-filling algorithm. By employing adaptive modulation, total achievable capacity could be maximized within a certain BER; in this experiment, target BER was assuming Reed-Solomon (255, 239) forward error correction (FEC). The preamble was also used for single-tap equalizer as well as a correlation based synchronization process in receiver side (OLT). In order to evaluate the effect caused from power imbalance that could be possibly generated in PON system, ONU1 power was attenuated just before optical combiner (3dB coupler), while the PD input power was maintained for fair comparison. Moreover, to observe the influence resulted from optical path difference, a short optical fiber was inserted before optical combining. 4. RESULTS AND DISCUSSIONS Figure 6 shows a spectrum hole when ten subcarriers were nulled-out to observe sidelobes. Figures (a) and (b) present the spectrum hole shape of OFDM and FBMC, respectively. Sinc-shaped sidelobes are obviously expressed in case of OFDM. In case of FBMC, the sidelobes are effectively suppressed, as well as a lower noise floor and steeper spectrum wall are obtained. Figure 7 presents channel error vector magnitude (EVM) estimated from preamble before adaptive modulation. Figures 7 (a) and (b) express results when signals were merged with optical power balance, while Figures (c) and (d) include power imbalance between merged ONU signals at ODN. When signals power was equally multiplexed, channel EVM of ONUs was also balanced because the amount of interference is small compared to main lobe (subcarrier) power in both Figure 6. Spectrum hole of (a) OFDM and (b) FBMC [11] Proc. of SPIE Vol Q-4

5 I Í i I! UNUl UNUZ ONU1 UNUZ UNU1 UNUZ ONU1 UNUZ 40% 40% 40% 0% 30% OFOM (-4: -4)!Y1^ FBMC(.4 : -4) FBMC(-12: -4) 30% 30% 0% OFOM (-12: -4)?0% 20% 20% 0% 10% 10% 10% 10% 0% 0% 0 71 Al QI Al PI II, 17 Al ei Bi mi im in Figure 7. Channel EVM of OFDM and FBMC when powers from ONUs were balanced/imbalanced at ODN i M [ 1", IMO II NNNNNIII NENNNN!' I [i77'}}{.,i 1ot/111a I "[SCiI'i1r,rc.i i r1,. -o-fbmc(-4 : -4) --OFDM (-4: -4) o I I I -o-fbmc(-12 : -4) --OFDM(-12 : -4) U ii Figure 8. Bit-loading profiles for adaptive modulation when multiplexed signal power was balanced/imbalanced at ODN. ONU1 and ONU2. By comparing (a) and (b), even though EVM of OFDM and FBMC seems to be similar, it could be observed that FBMC has better EVM performance because of interference reduction due to sidelobes suppression; this EVM performance variation leads difference in total capacity. Figure 7 (c) and (d) present channel EVM when the ONU1 power was attenuated to evaluate effect of power imbalance. In both (c) and (d), signal-to-noise ratio (SNR) of ONU1 was decreased due to smaller input power, thus, channel EVM of both OFDM and FBMC was degraded. However, in case of OFDMA, interference from ONU2 was obviously expressed as shown in (c). As attenuating ONU1 power, the signal power was getting smaller but sidelobes power from ONU2 was maintained, which leads degradation of SNR as well as signal-to-interference ratio (SIR). Therefore, ONU1 was only affected by SNR degradation in FBMC- MA, although the power imbalance was generated at ODN. On the other hand, in case of OFDMA, ONU1 was additionally affected by SIR degradation if multiplexed power was imbalanced. Moreover, this SIR degradation due to sidelobes interference is getting worse as power variation is increased. In Figure 8, bit-loading profiles based on channel EVM were presented when the multiplexed ONUs power was balanced and imbalanced. The number of loaded bits was decided based on CSI shown in Figure 7. By applying waterfilling algorithm, subcarrier which has better channel EVM was allocated larger bit, while subcarrier corresponding worse channel was allocated smaller bit. In case of OFDMA with power imbalance shown in Figure 7 (c), it couldn t be loaded any bit (zero-bit loading) at the boundary region due to a very bad CSI. The zero-bit allocated boundary subcarrier means that it required frequency guard interval between ONU in order to maintain the system quality of service. In case of FBMC, because FBMC-MA had better channel EVM compared to OFDMA, loaded bit of FBMC was larger than that of OFDM especially in boundary region. This effect was becoming more critical when a large power variation between multiplexed asynchronous ONU was occurred. In addition, although converged optical power was equal, the allocated number of bit was smaller in OFDM compared to FBMC because of fractional sampling synchronization error. Even though the fractional sampling error is generated in both OFDM and FBMC, but FBMC could not only suppress sidelobes but also reduce this effect. Therefore, by employing FBMC based optical multiple access in PON system which combines signals passively and asynchronously, MAI between asynchronous ONUs could be effectively reduced and frequency utilization efficiency and total transmission capacity could be increased. Proc. of SPIE Vol Q-5

6 ii i I 2m_OFDM_ONU1 2m OFDM_ONU2...I..., I ;... ``..W ,...g Ì imam % 8% 6% 4% 2% 0% 2m_FBMC_0 NU Mq 2m _FBMC_ONU2 7CCMR... ua_au..ouua_ Figure 9. Channel EVM when a 2 m optical fiber inserted in ONU1 before optical combining. Figure 9 shows delay effect due to different optical path before ODN. In PON system, it is impossible that signals from each ONU are passed through SMF with an equal length. Moreover, a small difference in optical path length causes a large multipath delay spread because light transmit with high velocity and optical fiber communications use broader signal bandwidth compare to wireless communications. As shown in Figure 9 (a) and (b), OFDMA has a large interference in boundary region between ONU, but there is almost no interference in FBMC-MA case. Due to optical path difference, ONUs lost a synchronism with each other, and become an asynchronous relation. In OFDMA case, due to sidelobes, inter-channel-interference (ICI) between ONU is generated within whole subcarriers, especially boundary subcarrier which has larger sidelobes. On the other hand, due to sidelobes suppression, FBMC-MA is robust against multipath delay spread compared to OFDMA, although same phenomenon is occurred. In order to avoid this interference caused from optical path difference, OFDM requires a large CP according to length difference; this leads degradation of SE and total capacity because CP is redundancy. Thus, FBMC-MA improved SE compared to OFDMA due to robustness against optical path difference without CP. 5. CONCLUSION OFDM has high SE and many advantageous properties, but it essentially possesses the sinc-shaped sidelobes, which hinders OFDM based multiple access in optical uplink transmission. In order to accommodate future access network like IoT, the seamless integration of asynchronous optical signals is required. However, the sidelobes of OFDM cause MAI between ONUs and the amount of interference is proportional to the number of ONUs. By adapting AMO-FBMC- OQAM in PON based optical uplink transmission, we experimentally verified that sidelobes problems of OFDM were effectively suppressed and MAI was efficiently reduced between asynchronous ONU. Therefore, it is experimentally demonstrated that performance improvement in terms of total capacity and SE through FBMC compared to OFDM. We expect that AMO-FBMC-OQAM could be an effective candidate for future access network. ACKNOWLEDGEMENT This work was supported by the ICT R&D program of MSIP/IITP, Republic of Korea. [ ] REFERENCES [1] Wunder, G. et al., 5GNOW: Non-orthogonal, asynchronous waveforms for future mobile applications, IEEE Communications Magazine, 52(2), , (2014). [2] Wunder, G. et al., 5GNOW: Challenging the LTE design paradigms of orthogonality and synchronicity, in Proc. 77th IEEE Veh. Technol. Spring Conf., Dresden, Germany, Jun. 2013, 1 5, Proc. of SPIE Vol Q-6

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