1. Introduction

Various applications, such as the Internet of Things (IoT), require an increased number of optical network units (ONUs) as well as an increased data rate per ONU in optical access networks. Thus, the capacity and split ratio in the passive optical network (PON) standard have been increased steadily. In a conventional PON, uplink multiple access (MA) is realized by time-division multiplexing (TDM) as shown in Fig. 1(a). Each ONU that wants to transmit uplink signal is assigned a time slot. To avoid collision and congestion, time scheduling is required. Even though TDMA is a mature technique and it can support a large number of ONUs, as the split ratio increased, time scheduling becomes more complex and the idle time of ONU is increased. To increase the split ratio without an additional scheduling and ONU idle time, simultaneous MA within the same TDM slot can be employed by using multicarrier transmission, such as orthogonal frequency-division multiplexing (OFDM), as shown in Fig. 1(b). In particular, OFDM is a standard in wireless access and OFDM-PON has been actively researched in optical access due to its advantages such as dispersion robustness [1, 2].

 

Fig. 1 Single wavelength multiple access: (a) TDMA, (b) TDM-OFDMA, (c) TDM-OPDMA.

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In simultaneous MA, down link transmission from optical line terminal (OLT) to ONU is relatively stable like a point-to-point link, but uplink MA has the critical problem of optical beat interference (OBI). The center frequency of OBI is determined by the wavelength difference among uplink optical carriers, and the OBI shape is determined by the convolution among power spectral densities of the optical fields [3]. Thus, in the LD-based simultaneous MA, the OBI is generated at baseband by forming a Lorentzian shape, which interrupts proper signal detection by causing a very large intensity fluctuation. In order to avoid the OBI, some techniques have been proposed [4–9]. Carrier suppression with coherent detection [4] can avoid OBI by eliminating the beating source, which is novel but is not an intensity modulation/direct detection (IM/DD) system. Wavelength separation [5] can up-convert the center frequency of OBI from baseband to an out-of-signal band. It is effective but requires every ONU to have a different wavelength-optical source. Dual polarization-based MA [6] can avoid OBI because beating does not occur between orthogonal polarizations, but it can support only two ONUs and is limited in its ability to extend to a polarization-division multiplexing (PDM) system. Spectrum broadening [7, 8] can flatten OBI by spreading the convolution of OBI over a broad spectral range. It is simple, but the wavelength-division multiplexing (WDM) channel width and transmission distance would be limited. Recently, we proposed optical pulse-division multiplexing (OPDM)-based OBI reduction technique [9] by considering both the spectrum broadening effect and time regularity, which could effectively suppress OBI by a relatively simple method. However, none of these techniques, including OPDM [4–9], could simultaneously receive uplink MA signals with a single photo diode (PD) unless bandwidth allocation is employed.

Commonly, the performance of an optical modulator for uplink is inferior to that of downlink, i.e., the bandwidth of the uplink device is more limited. The bandwidth allocation divides up the available frequency resources of the uplink device among ONUs. That is, the allocated bandwidth of each ONU is decreases when the number of simultaneous MAs is increased. Thus, by allocating the available bandwidth of the optical device for several ONUs, a simultaneous MA with a single PD can be supported, but it degrades the utilization efficiency of optical devices and limits capacity per wavelength channel.

In this paper, we propose OPDM-based full-band simultaneous MA within same time slot in a single wavelength channel (Fig. 1(c)) to effectively reduce OBI and fully utilize the bandwidth of the optical modulator. Both in multicarrier and single-carrier signal modulations, the performance of the proposed technique was experimentally verified in an IM/DD-based PON uplink MA. After a 20-km single-mode fiber (SMF) transmission, full-band uplink signals from multiple ONUs could be simultaneously received in a single PD without bandwidth allocation, and OBI was effectively reduced.

2. Operational principle

Figure 2 shows a process schematic of the proposed full-band TDM-OPDMA. In this work, a source seeding-based colorless PON is employed. A single optical source from the remote node is fed to several ONUs and independently modulated at every ONU by an optical modulator, such as a reflective semiconductor optical amplifier (RSOA). The modulated uplink signals from different ONUs are combined at the ODN and transmitted through a single wavelength channel. Multiple signals in the same TDM slot are simultaneously detected at the OLT by a single PD. Unless an OBI reduction technique is employed, OBI is generated as described in Fig. 1(b) because the simultaneously detected optical carriers (reflected seeding sources) acted as an individually generated optical sources because of the time delay caused by the optical path differences among ONUs. To simply reduce the OBI, OPDM [9] is employed in this system on the basis of a directly modulated radio frequency (RF) clipping tone (CT). In the frequency domain, a directly modulated RF CT with a large modulation depth can broaden the optical spectrum of the seeding source by increasing the amplitude of the harmonics and generating nonlinear components. Thus, the baseband OBI can be up-converted to every harmonics and nonlinear components, thereby spreading the OBI over a broad spectral range by flattening the noise spectrum. Therefore, OBI can be considered as an additive white noise.

 

Fig. 2 Schematic of the proposed full-band TDM-OPDMA in a source seeding system.

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Meanwhile, in the time domain, the RF CT causes periodic intensity variation in the optical seeding source as shown in Fig. 2(1). We call this source the optical pseudo pulse train (OPT), whose periodicity is determined by the frequency of the RF CT. The OPT is seeded from remote node to ONUs, and then the envelope of the OPT is modulated by the uplink signal at each ONU (Figs. 2(2)–2(3)). The uplink signals can be either multicarrier (analog) signals, such as OFDM, or single-carrier (digital) signals, and all ONUs use full-band without bandwidth allocation. Under near-orthogonal conditions, multiple uplink signals (modulated OPT) from different ONUs are overlapped with a staggered arrangement in the time domain as shown in Fig. 2(4), which could minimize the level of the flattened OBI.

 

Fig. 3 Experimental setup to demonstrate full-band TDM-OPDMA and detected OPTs without uplink signal modulation: (a) Waveforms of OPTs before combining. (b) Waveforms of combined OPTs at some position (P1–P4) by controlling the optical delay line.

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Fig. 4 RF spectra of the received signals: (a) OBI and reduced OBI at some positions. Received signal of (a) ONU1 and (b) ONU2 at P1. (MC: multicarrier, SC: single-carrier)

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Whereas the ONU signal bands were separated by using bandwidth allocation in the previous OPDM technique, in this work, the signal bands of ONUs are completely overlapped, which causes serious interference among ONUs. Thus, an optical pulse interference cancellation process is required after photo detection to distinguish the simultaneously detected OPTs transmitted from individual ONUs in a single PD. As shown in Fig. 2(5), the received signals are down-sampled, which nullifies samples except certain periodic samples. For considering the phase of OPT, some groups are composed by cyclically shifting the initial phase of down-sampling. The periodicity and the number of groups can be determined at OLT by considering down-sampling rate and pattern of OPT. The maximum correlation group for an individual ONU is selected by comparing the down-sampled signal of each group with preambles commonly used for channel estimation in OFDM. After searching the OPT correlation peaks of each ONU, the selected samples are processed by conventional demodulation process, such as a channel estimation and an equalization in the case of OFDM.

 

Fig. 5 Error vector magnitude of the simultaneous MA signals according to subcarrier index by varying optical delay in multicarrier-based full-band TDM-OPDMA: (a) ONU1, (b) ONU2.

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3. Experiments

The experimental setup for the source seeding-based PON uplink MA is presented in Fig. 3. A decision feedback laser diode (DFB-LD) was used as a seeding source, which was directly modulated by the RF CT generated by a vector signal generator (VSG). Two RSOAs acted as ONUs, and the envelope of the seeding source was modulated by uplink signals generated by an arbitrary waveform generator (AWG). Two polarization controllers (PCs) were inserted after the RSOAs to exclude the polarization effect on OBI reduction, which could potentially extend to the PDM system. To cause path-length differences among the ONUs, an additional 2.5-km fiber was inserted. An optical delay line was inserted at the ONU1 side to vary the delay offset of OPT between ONUs. A 3-dB coupler acted as the ODN. For WDM extendibility, a 50-GHz OBPF was used to emulate a wavelength demultiplexer, and a 20-km SMF was inserted to emulate the access network. A single PD was used as an OLT receiver, which verifies simultaneous MA within the same TDM slot. The signal bandwidth was 1 GHz with RSOAs whose 3-dB bandwidth was less than 500 MHz.

In this work, both analog and digital signal modulations were investigated. For analog signal, OFDM was used. The number of subcarriers was 128, and each subcarrier was equally modulated by 4-QAM. Hermitian symmetry was employed in OFDM generation to obtain a real-value signal for the intensity modulation. To easily observe the signal and noise level, OFDM sidelobes were suppressed by employing filter bank-based multicarrier (FBMC) [10]. A preamble whose subcarriers were equally modulated by 4-QAM was inserted to estimate channel state, which was used not only in single-tap equalization but also in the optical pulse interference cancellation. For digital signal, OOK was used. Without single-tap equalization, a training sequence was inserted to synchronize and find a maximum correlation group.

4. Results and discussion

Figures 3(a) and 3(b) show waveforms of the received OPTs when the uplink signals were not modulated. The reflected OPTs of each ONU can be detected at the OLT as shown in Fig. 3(a). Although a seeded OPT was generated at the same single source, the reflected OPTs from each ONU have power variation due to different paths and device response. The combined OPTs can be observed as shown in Fig. 3(b) according to optical delay, which was controlled by the optical delay line at the ONU1 side. With increasing optical delay, combined OPTs could be shifted from P1 to P4, which is repeated cyclically according to optical delay because of the periodic property of OPT.

Figures 4(a)–4(c) show the RF spectra of the received MA signals. Without the proposed technique, OBI is generated at baseband by forming a Lorentzian shape as shown in Fig. 4(a). It can be effectively flattened by employing the proposed technique owing to the spectrum broadening effect of RF CT, but the noise level of the flattened OBI is determined by OPT interference. The noise level varies according to optical delay offset between ONUs, and the difference between the minimum (P1) and maximum (P3) noise level is approximately 20 dB, which is determined by electrical power of RF CT. The lowest level is obtained when the OPTs of ONUs are overlapped with a staggered arrangement (P1), which is called as the near-orthogonal condition. At the P2 position, the noise level is slightly increased approximately 3 dB compared to that of P1 as shown in Fig. 4(a). It is expected that uplink signals will be detected although some part of the OPTs are overlapped as in P2 (Fig. 3(b)). Thus, the near-orthogonality is not too strict condition to accommodate MA. Figures 4(b) and 4(c) present the spectra of the received signal of ONU1 and ONU2, respectively, when the uplink signal was modulated. Both multicarrier and single-carrier signals were demonstrated with the same signal bandwidth, and all ONUs transmitted full-band without bandwidth allocation. The baseband (DC to 1 GHz) signal is the primary signal that we detected, and the up-converted signal (over 1.5 GHz) is a replica caused by the 2.5-GHz RF CT, which is filtered out by an electrical low pass filter (LPF).

In order to observe the channel state of full-band TDM-OPDMA, error vector magnitude (EVM) according to subcarrier index was measured by varying the optical delay. Figures 5(a) and 5(b) show the EVM of the simultaneously detected full-band signals of ONU1 and ONU2, respectively. Due to 2.5-GHz RF CT, the channel EVM shows a cyclic property with 400-ps periodicity by varying the optical delay. The subcarrier index is proportional to frequency. It is clear that all subcarriers experience a similar noise level owing to the flattened OBI, although the EVM at high frequency is more deteriorated than that at low frequency because of the frequency response characteristics of the optical/electrical devices. Uplink signals of both ONUs are similarly affected by OPT interference in the entire frequency bands, because the uplink signals were modulated on an OPT envelope. Thus, except for the device response, the channel of full-band TDM-OPDMA is frequency nonselective, which provides the potential to transmit a single-carrier signal without a channel equalizer.

In order to evaluate the performance of the proposed full-band TDM-OPDMA, bit error rate (BER) was measured by varying the optical delay. Figures 6(a) and 6(b) show the BER of simultaneous MA signals in the case of multicarrier and single-carrier transmission, respectively. As mentioned for the channel state, OPT interference has a cyclic property depending on the RF CT frequency. Thus, the BER has variation with 400-ps periodicity. The near-orthogonal condition can be satisfied at approximately 300-ps and 700-ps optical delay. By assuming forward error correction (FEC), the BER target was 2 × 10⁻³ in this work. Around the near-orthogonal region, the simultaneous MA ONUs satisfy the signal quality criteria for both analog and digital transmission. Therefore, as long as near-orthogonality is satisfied, the proposed optical pulse interference cancellation can mitigate OPT interference with reduced OBI even without bandwidth allocation. In Figs. 6(a) and 6(b), the possible delay range (to satisfy signal quality) to OPT period ratio is approximately 1/4 for multicarrier and approximately 1/3 for single-carrier, respectively. It demonstrates that full-band simultaneous multiple access of two ONUs is possible and uplink capacity can be improved by at least twice. It also means that a TDM system with a split ratio of 1:64 can be scaled to at least 1:128. The ratio can be extended by employing pulse patterning in RF CT, which would further relax the condition for near-orthogonality and improve the split ratio by several times.

 

Fig. 6 Bit error rate of the simultaneous MA signals by varying optical delay in full-band TDM-OPDMA: (a) Multicarrier transmission. Subset: constellations. (upper: ONU1, lower: ONU2). (b) Single-carrier transmission. Subset: eye diagrams. (upper: ONU1, lower: ONU2).

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5. Conclusion

By employing the proposed full-band TDM-OPDMA in IM/DD-based single-wavelength MA, OBI can be effectively reduced and the bandwidth of optical modulators can be fully utilized with a single PD without dividing the frequency bandwidth. Moreover, it can be extended to WDM, PDM, and TDM systems by making a new multiplexing factor for an improved capacity. In particular, by hybridizing with TDM, it can increase split ratio markedly without additional time scheduling and ONU idle time. Furthermore, the split ratio and the possible transmission range can be further improved and extended by employing pulse patterning in OPT. Therefore, the proposed full-band TDM-OPDMA can be an effective solution for IM/DD-based single-wavelength simultaneous MA uplink transmission.