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A novel technique for mitigating the optical beat interference (OBI) noise in an optical orthogonal frequency division multiple access passive optical network (OFDMA-PON) uplink transmission is presented. By using an out of signal band RF clipping tone to the optical seed carrier, the OBI noise has been reduced and the resulting throughput and spectral efficiency has been improved. As an experimental verification, we demonstrate that the spectral efficiency of 23 km and 50 km have been doubled in the OFDMA-PON uplink transmission.
© 2014 Optical Society of America
1. Introduction
In recent years, the network resource management of optical networks has become important with the advent of next-generation network services such as software-defined networking (SDN) and the Internet of Things (IoT). To support these advanced network services effectively, the advancement of next-generation access networks will need the flexible network resource management to deal with complex network functionality [1]. Not only improve the data capacity, but it will should provide resource management functions such as flexible data rate and efficient multiple access. The orthogonal frequency division multiple access passive optical network (OFDMA-PON) has been spotlighted as the next-generation optical access networks with various merits of orthogonal frequency-division multiplexing (OFDM) such as high spectral efficiency, the transparency of subcarriers and adaptive modulation for each subcarrier, it can also provide dynamic bandwidth allocation for efficient multiple access and strong scalability with wavelength-division multiplexing (WDM) [2].
To achieve these advantages in a practical system, a serious issue known as optical beat interference (OBI) noise needs to be overcome, especially in the uplink transmission. The OBI noise is the interference noise generated from the optical beating among the multiple adjacent lightwaves in the photo-detection process. By the square law detection of the photo detector (PD), the multiple adjacent light waves generate unwanted broadband beat components which are centered on the wavelength differences. This was also the main obstacle for realizing the subcarrier multiplexing PON (SCM-PON) [3].
In the OFDMA-PON system, a multiple access operation can be performed by allocating subcarrier sub-channels of an OFDM frame to each user. These sub-channels are allocated over the same nominal wavelength in the single wavelength OFDMA uplink transmission. Consequently, it generates intense OBI noise, and this broadly generated interference noise overwhelms the signal bandwidth. A technique has been proposed to avoid OBI noise by using optical carrier suppression and coherent detection [4]. However, wavelength stability is required in the optical network unit (ONU) due to the optical carrier suppression and it uses an additional external modulator such as a Mach-Zehnder modulator and an optical device such as an optical interleaver; this leads to a cost-ineffective ONU. Another technique for using multiple optical sources with different wavelengths at each ONU has been proposed [5, 6]. Unfortunately, it would create a heavy burden for the network provider due to increasing network repair and maintenance costs which come from a complex inventory configuration consisting of different optical and electrical devices. Additionally, it cannot provide multiple access operation on the same nominal wavelength light waves, leading to a scalability limitation with WDM. Another technique has been proposed which uses a spectral broadening effect of the optical carrier to avoid OBI noise [7–9]. A simple and effective spectral broadening technique which modulates the RF clipping tone to the laser has been proposed for SCM-PONs [7, 9]. It easily reduces the OBI noise, but it generates a lot of unwanted nonlinear components on the signal bandwidth of OFDM frame.
In this paper, a novel scheme which applies an out-of-band RF clipping tone to the optical seed carrier in the optical line terminal (OLT) is proposed in order to reduce OBI noise in the remotely fed optical uplink transmission of an OFDMA-PON system. The proposed transmission system is based on a 1-GHz reflective semiconductor optical amplifier (RSOA) used as colorless ONUs; discrete multitone (DMT) modulation is utilized to generate real-valued OFDM signals. The spectral efficiency for uplink transmission has improved more than two times by using the proposed scheme.
2. Schematics
Figure 1 illustrates the concept of the proposed OBI mitigation technique in OFDMA-PON uplink transmission based on a remotely-fed RSOA. As shown in Fig. 1, when the RF clipping tone modulates the optical seed source directly, there is a periodic variation in the carrier density in the laser diode (LD). Then, the refractive index of the LD’s active layer is changed dynamically with varying carrier density. Because the resonant condition of the LD depends on the refractive index, the generated optical wave has dynamically varying optical center wavelength. With the large signal modulation (modulation index > 1), it also generates nonlinear harmonics components. As a result, the launched optical carrier, which is fed to the RSOA at the ONU, was spectrally broadened. And this spectral broadening effect is a result of these two mechanisms and can be varied for different RF clipping frequency and power.
When the uplink signals are detected at the receiver, the OBI noise is dispersed from the DC to a higher frequency band because the main beating contributors have a broader optical spectrum. This dispersion of the OBI noise enhances the signal-to-noise ratio (SNR) of the uplink transmission signals. Therefore, the effect of the OBI noise on the signal bandwidth is drastically reduced. However, when the RF clipping tone is used, the RF clipping tone and its harmonics generate intermodulation components with uplink signals, as shown in Figs. 2(a) and 2(b).
Fig. 2 Intermodulation effect with (a) low frequency RF clipping tone and (b) out-of-band RF clipping tone.
As already mentioned before, there were previous works to reduce OBI noise which use a low frequency (within 3-dB cut-off frequency of LD) RF clipping tone to create a spectral broadening effect in SCM system. In SCM system, it uses multiple intermediate frequencies for each multiple access ONUs, these intermediate frequencies upconvert each access signals to different intermediate frequencies. As a result, these up-conversions make guard bands between the uplink signals and the low frequency RF clipping tone and its harmonics components. In the OFDMA uplink transmission (shown in Figs. 2(a) and 2(b)), an RF clipping tone and its harmonics are also used to generate optical spectral broadening effect, and as a results, it also generated unwanted intermodulation components. On contrary, the OFDMA system has a dense and broad-baseband RF spectrum for its orthogonal subcarriers. Therefore, when a low frequency RF clipping tone is used to create a spectral broadening effect like in SCM system, the unwanted intermodulation components are generated in signal bandwidth of OFDMA system, as shown in Fig. 2(a). This interference greatly degrades the signal performance. To avoid interference with the unwanted intermodulation components, the used frequency of RF clipping tone need to be far away from the signal bandwidth (the frequency of RF clipping tone has been set higher than two times the signal bandwidth), as shown in Fig. 2(b).
In addition to the intermodulation components interference with the baseband signal, there is another technical issue needs to be considered. These unwanted intermodulation components can reduce the modulation efficiency of baseband uplink signals because of the dither effect by re-modulation process with ROSA. Therefore, the frequency and power for the out-of-band RF clipping tone should be optimized to create enough spectral broadening effect and avoid interference with the intermodulation components. Because the LD has a limited frequency response, if the clipping tone was located within a 3-dB range of the LD’s frequency response, the optical spectrum becomes effectively broadened. However, when the tone is located out of the 3-dB range, the optical spectrum broadening effect is saturated and less effective. Out of the 3-dB bandwidth the RF clipping tone which used in our proposed scheme, RF tone itself is also modulated less effectively, even though the power of the clipping tone is relatively high. These less effective RF tone modulation work as advantage in proposed system, because this low effective RF tone modulation reduces the dither effect which reduce the modulation efficiency of uplink signal at the ONU. To maximize the modulation efficiency of the uplink signal at the ONU, the intermodulation effect should be minimized; in other words, the RF clipping tone should be less effectively modulated
3. Experiments and results
Figure 3 shows the experimental setup for the proposed scheme. A distributed feedback laser (DFB) with a center wavelength of 1554.348 nm was used as a CW light source for the OLT.
The DFB-LD was directly modulated by a 6.7 GHz RF clipping tone with 18.5 dBm (modulation depth was 120%). The 3-dB cut-off frequency of DFB LD was 3.5GHz. So, even we drove the 6.7GHz RF tone with modulation depth of 120%, the modulated RF tone and its nonlinear harmonics were relatively small. Nonlinear harmonics generated higher than 3rd were generated lower than system noise floor. This RF frequency was found to optimize the spectral broadening effect of the optical CW source and reduce the dithering effect of the baseband signal to RF clipping tone. Figures 4(a) and 4(b) show the measured spectral broadening effect which is created by applying an RF clipping tone. We captured the spectra after the directly modulated DFB-LD (point ‘A’ in Fig. 3). The linewidth of the optical source broadened to almost 0.3 nm with the optimized RF clipping tone power. Figure 5 shows the RF spectrums of the received signal without modulation at the RSOAs (DC bias only). We received the signal at point ‘B’ in Fig. 3 and checked the RF spectra by using an RF spectrum analyzer. A black triangle represents the noise floor without OBI noise. A red square represents the beating interference noise caused by two different uplink optical fields. A blue circle represents the RF spectrum when the clipping tone was used. As shown in Fig. 5, by using an RF clipping tone, the unwanted OBI noise is broadly dispersed; an increase of only 5 dB in the noise floor has been observed.
Fig. 4 Optical spectra of optical seed carrier (a) without clipping tone and (b) with clipping tone.
In the experiments, we used DMT as a modulation format to transmit a real-valued baseband version of an OFDM signal under the IM/DD-based transmission system. The calculated DMT signal was loaded into a 6.4 Gs/s arbitrary waveform generator (AWG: Tektronix 7122B). The number of FFT size was 512 with Hermitian symmetry. So, the number of effective subcarriers was 256 ranging from DC to 3.2 GHz. And the cyclic prefix was 8 samples per each DMT frame. The signal bandwidth was also optimized to maximize the data throughput with adaptive modulation based on a water-filling algorithm.
In OFDMA uplink transmission, the time synchronization is also very critical issue which solved in the other work [2]. To avoid timing mismatch problem and focus to the effect of OBI noise, we modulated only one of the RSOAs and drove DC current for the other RSOA. Used RSOAs have similar frequency response characteristics (f3db ~1GHz) but different optical gain characteristics. To create identical optical gain and modulation responses for each ONU, we operated these RSOAs in different operation conditions (RSOA1: 20 °C, 60 mA; RSOA2: 25 °C, 80 mA). And these operation conditions are optimized for each RSOA by checking the transmission performance separately. RSOA 1 was driven by DMT signals with a modulation depth of 100% and RSOA 2 was driven by only DC current. And polarization controllers (PC) were used to control input polarization to optimize RSOA performance. The magnitude of the DMT signal from the AWG was optimized by using a variable electrical attenuator (VEA) and an RF amplifier for a full modulation. In this experiment, we used theunidirectional standard single mode fiber (SSMF) link to eliminate all of the possible Rayleigh backscattering effects. The dispersion parameter of SSMF was 18ps/nm∙km at 1550nm. The transmission length difference between each ONU and OLT was 5 km to break the coherence between the uplink signals. The input optical power in the receiver was maintained at −4.5 dBm for experimental consistency. The received DMT signal was captured by a digital phosphor oscilloscope (DPO: Tektronix 72004C) sampling at 25 Gsample/s and evaluated by offline processing.
Figure 6 shows the RF spectra of the received signal after the transmission. To analyze the OBI noise in the frequency region, we compared the RF spectrum for each case. Figures 6(a) and 6(b) show only the baseband spectrum without uplink signals in cases without and with clipping tones, respectively. Even though there were no uplink signals, it shows severe unwanted beating components from the DC to a higher frequency region. These components completely interfere with the signal bandwidth which leads to performance degradation. However, with the out-of-band RF clipping tone, the beating components are dispersed broadly and there is only a slight noise floor increase. Figures 6(c) and 6(d) show the spectra of the signal bandwidth with uplink signals in cases without and with clipping tones, respectively.
Fig. 6 RF spectra of received signals. In case of without uplink signals (a) without RF clipping tone and (b) with RF clipping tone. In case of with uplink signals (c) without RF clipping tone and (d) with RF clipping tone. Spectra of RF clipping tone region (e) without uplink signals and (f) with uplink signals.
Because the DMT signal is the sum of the multiple orthogonal subcarriers which has very narrow frequency spacing, the nonlinear components and its distortion cannot be distinguished in the spectrum. Figures 6(e) and 6(f) show the spectrum around the RF clipping tone frequency in cases with and without uplink signals, respectively. From Figs. 6(e) and 6(f), we can observe that the baseband DMT signals, which have a 3.2 GHz signal bandwidth, also dithered to the RF clipping tone frequency. This degrades the modulation efficiency in the baseband.
Figure 7 shows the variations of the achievable data rate as a function of the input optical power of the preamplifier in the OLT. In this figure, total throughput includes the redundancy of the DMT frame like cyclic prefix. Without the OBI noise reduction, the maximum achievable data rate was only 3.8 Gbits/s even in the case of the optical back-to-back transmission with adaptive modulation on DMT signals. By applying the out-of-band clipping tone signal, the achievable data rate improved from 3.8 Gb/s to 9.8 Gb/s, when the input optical power was higher than −3 dBm in the optical back-to-back transmission. In the 20 km transmission and 50 km transmission, the achievable data rates improved from 3 Gb/s to 8.1 Gb/s and 2.8 Gb/s to 6.4 Gb/s, when the input optical powers were higher than −8 dBm and −11 dBm, respectively. In 23km and 50km transmission, we didn’t use additional optical amplifier to meet same optical power with the case of optical back-to-back, because we want to avoid the additional noise figure comes from the optical amplifier. In each case, the spectral efficiency was enhanced by more than two times. Consequently, the proposed technique was able to provide the reliable 8 Gb/s and 6 Gb/s DMT transmission in 23 km and 50 km, respectively. And the total throughput could be improved with the enough optical power in each transmission.
Fig. 7 Total data throughput and spectral efficiency for input optical power of preamplifier in the case of a b2b, 23 km, 50 km transmission.
Figures 8(a) and 8(b) show the bit/power loading profile and its estimated SNR for each subcarrier. All the profiles were measured for the maximum transmission performance in each transmission length. In the loading process, a probe signal, which contained uniform bits (4 QAM) and power among the entire DMT subcarriers, was first transmitted to evaluate the channel response before the loading process, and the number of bits and power were allocated to every DMT subcarrier based on the evaluated channel response. It was verified that more bits were allocated to subcarriers which had high SNR performance in the loading process. On the other hands, relatively lower bits were loaded to subcarriers which had lower SNR performance in the loading process. The power level was allocated to optimize the signal performance in every OFDM subcarrier with a given bit number. The average BER among the entire DMT frame was less than 10−3. This means that it was able to transmit the adaptively loaded DMT signal in the proposed scheme, which satisfied the forward error correction (FEC) limit. In both with/without RF clipping tone cases, some of subcarriers at high frequency region had no bit in the transmission. This is because, as represented in the SNR evaluation, it had not enough SNR to allocate even a single bit into these subcarriers. In the view of the OBI noise effect, Fig. 8(a) shows that the maximum allocated bits for subcarrier were only 2 bits for each transmission length in the case of without RF clipping tone. This is because the OBI noise degrades the SNR severely in the signal bandwidth. However, in the case of with the RF clipping tone (Fig. 8(b)), the maximum allocated bits for subcarrier was 6 bits for each transmission length by the virtue of the OBI noise reduction effect.
Fig. 8 Bit/Power-loading Profile and SNR for each subcarrier in case of best transmission performance (a) without RF clipping tone and (b) with RF clipping tone.
In view of the optical spectral efficiency, RF clipping tone may decrease the spectral efficiency because of its spectral broadening effect. But in the access network, especially at the ONU, it is important to maximize the transmission efficiency with the cost effective device which has limited RF frequency response. Therefore, in the proposed system, the OBI noise reduction with RF clipping tone provides advantage in view of transmission throughput, even with sacrificing of optical spectral efficiency.
4. Conclusion
We have demonstrated an OBI reduction technique in an RSOA-based OFDMA uplink transmission by using an out of signal bandwidth RF clipping tone. By virtue of this technique, the OBI noise effect was significantly reduced, and as a result, the spectral efficiency increased by almost two times in the transmission. This concept would be useful for relaxing the critical OBI noise problem in the uplink transmission of the OFDM-based optical access network which can provide multiple access operations using the same nominal wavelength.
Acknowledgments
This work was supported by the Industrial Strategic Technology Development Program of KEIT (10041775) funded by the Ministry of Trade, Industry & Energy, Korea.
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