FUTURE DIRECTIONS
7.2 Novel Hybrid 10G/1G Coexisted TDM-PON
7.2 Novel Hybrid 10G/1G Coexisted TDM-PON
In the near future, TDM-PON is good solution for increasing data requirements.
Recently, both IEEE 802.3 and FSAN view 10G PON as a next generation access option.
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In 10G PON, technical challenges are locating in high speed burst mode receiver of OLT.
Currently the commercial maximum burst receiving speed is 1.25 Gbps. There are two ways to deal with this challenge: one way is to achieve technical breakthrough in 10G burst mode clock and data recovery (CDR); another way is to trunk several lower speed channels as upstream channel by imitating 10G Ethernet configuration. Unlike other access networks, high speed TDM-PONs are point-to-multipoint networks capable of delivering services over 20 to 40 kilometers of single-mode fiber and have been intensively discussed to meet the upcoming enormous bandwidth requirement. A simple and cost-effective evolution path from current TDM-PONs to next-generation high speed TDM PONs is highly required without changing the legacies of the current PON infrastructure.
In this section, a novel and efficient 10G bidirectional TDM-PON structure using reflective colorless ONUs to increase bandwidth while reducing system cost, all-optical signal preprocessors for mitigating system impairments and providing intelligent control for data transmitting and receiving management. The carrier-suppressed centralized lightwaves to provide a global clock information and enable instantaneous clock recovery and enforced synchronization for the upstream bursty packets without using traditionally required clock and data recovery (CDR) circuits. The reflective colorless ONU is cost-effective since it contains both amplification and modulation capabilities, and optical carrier is simply provided by the centralized light source at CO, which makes network management and maintenance easier. Such reflective ONU can contain four piped sub-units, and each of them carries 2.5 Gb/s data to realize 10 Gb/s upstream. On the other hand, the burst-mode receiver at CO can be technically challenging since they should
have a wide dynamic range for the different incoming bursty signals from different ONUs, and should also retrieve precise and sufficient clock information to synchronize these upstream packets with extremely short guard times. Such requirements will be even more severe when 10G TDM-PON is considered. Therefore, all-optical signal preprocessors and carrier-suppressed centralized lightwaves are proposed to achieve data rate transparent power equalization in front of the electrical receiving stage, and provide simple detection of continuous and low-jitter clock information, respectively.
7.2.1 Network Architecture and Wavelength Plan
Figure 7.2 shows the proposed hybrid 10G/1G coexisted TDM-PON architecture. In the CO, centralized lightwaves are employed to provide optical carriers for both downstream and upstream simultaneously, which are combined by a WDM coupler and sent to RN with a feeder fiber span. In the RN, a 1 × N optical splitter is used for distributing the whole traffic to each ONU. The 10G/1G transmitter (TX) module contains both 10G and 1G downstream signal at different wavelength bands. In the meantime, 10G upstream traffic is realized by delivering four continuous wave channels at wavelengths λ1~λ4 from CO, and modulating them using reflective ONUs. Each 10G ONU contains four reflective transmitters connected with a 1 × 4 add/drop filter, and can be viewed as a pipe-line upstream module. Thus, 10G upstream can be retrieved by directly modulating the optical carriers of four piped reflective transmitters at λ1~λ4 with electrically demultiplexed 2.5 Gb/s patterns. Moreover, using a reflective semiconductor optical amplifier (RSOA) is one of cost-effective solutions to achieve both amplification and modulation functionalities in the ONU.
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In addition, traditional 1G ONU modules at wavelength λx can be seamlessly plugged in this system. As for upstream receiver at CO, four 2.5G (λ1~λ4) and one 1G (λx) upstream channels are demultiplexed and individually fed into burst-mode receivers. Since the optical carrier for 10G upstream traffic is solely provided by the centralized lightwaves, their wavelength and power level can be easily supervised and adjusted at the CO.
Furthermore, a media access controller (MAC) with dynamic bandwidth allocation (DBA) can be equipped at the CO to allow efficient bandwidth sharing of the single-feeder fiber.
A forward error correction (FEC) coding can be applied for upstream data modulation to obtain more coding gain and mitigated power budget. Figure 7.3 displays the wavelength plans for (a) the traditional EPON/GPON and (b) the proposed 10G/1G coexisted TDM-PON. In the proposed TDM-PON, only four wavelengths ranging from 1305 to 1315 nm is required for 10G upstream (US), which are entirely and precisely governed by the CO, while a portion of the enhancement band in EPON/GPON from 1540 to 1545 nm is utilized for 10 G downstream (DS). Note that the remained upstream and downstream
Figure 7.2: Proposed hybrid 10G/1G coexisted TDM-PON architecture using reflective ONUs.
wavelength ranges (dash line) are available for existing 1G ONUs in EPON/GPON, which makes coexistence of 10G/1G TDM-PON systems a nature combination.
7.2.2 Experimental Setup and Results
Figure 7.4 illustrates the configuration of the 10G TDM-PON access system considered in our experiment. For simplicity, C-band components had been used such as RSOA for feasibility demonstration, and employed a single RSOA with four input signals to simulate four piped upstream. In the CO, a single DFB laser (LDd) at 1557 nm served as the transmitter for downstream, and four laser sources (LDu1~u4) were used to realize centralized lightwaves for colorless ONU to reduce system cost. These four CW lasers were set to be upstream wavelengths from 1550.00 nm to 1552.4 nm with 100 GHz spacing. The LDd was fed to a Lithium Niobate intensity modulator to obtain a 10 Gb/s OOK signal for downstream traffic with PRBS of 231-1 word length. The combined downstream data and four CW sources using a coupler were fed into 15-km SSMF and used a 1 × 4 splitter to share the traffic for more than four ONUs. Another 5-km SSMF was set between RN and ONU to simulate the distribution fiber spans in real market
Figure 7.3: Wavelength plans for (a) traditional EPON/GPON and (b) proposed hybrid 10G/1G coexisted TDM-PON reflective ONUs.
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system. A thin-film filter has been used to divide downstream signal and four CW sources. These four CW sources were simultaneously injected into an RSOA and then modulated at 2.5 Gb/s with a PRBS length of 231-1 as four upstream channels with an aggregate capacity of 10 Gb/s.
Figure 7.5(a) illustrates the optical spectrum of the combined downstream signal and four CW sources after optical coupler as inset (i) in Figure 7.4. Figure 7.5(b) presents the received optical spectrum for upstream over 20 km transmission as inset (ii) in Figure 7.4.
Both of spectra were obtained by an optical spectral analyzer (OSA) with a RB = 0.01. It can be seen that good optical signal to noise ratio more than 35 dB for upstream over 20 km transmission has been attained.
Figure 7.5: Received optical spectra: (a) combined downstream signal and four CW sources as inset (i) in Figure 7.4; (b) upstream signal over 20-km SSMF as inset (ii) in Figure 7.4
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Figure 7.4: Experimental setup for TDM-PON.
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The BER curves and the corresponding eye diagrams for downstream and upstream with back-to-back and over 20-km SSMF are shown in Figure 7.6(a) and (b), respectively. The power penalty caused by downstream and upstream transmission was less than 0.2 dB.
The large OSNR, clear eye and negligible receiving power penalty demonstrated the feasibility for this proposed novel TDM-PON architecture. In the future, wavelengths will be switched to S-band and O-band for downstream and upstream channels in adherence to the EPON/GPON standards.
7.2.3 10G TDM-PON using Optical Carrier Suppression and Separation Scheme
Based on previous results, a novel and cost-efficient 10G TDM-PON employing clock embedded centralized lightwaves has been proposed to realize instantaneous 10G burst-mode clock recovery while reducing the system cost and enabling easy network management, as shown in Figure 7.7. Downstream traffic is easily achieved by externally modulating the laser diode (LDd) with 10 Gb/s data sequence, while the upstream traffic is realized by delivering clock embedded wave using OCSS technique. In the CO, a single DFB laser (LDd) is used as the designated transmitter for downstream service and
-25 -24.5 -24 -23.5 -23 -22.5 -22 -21.5 US 20 km (1550nm) US 20 km (1550.8nm ) US 20 km (1550nm) US 20 km (1550.8nm )
Figure 7.6: BER curves and corresponding eye diagrams: (a) upstream and (b) (b) downstream with B-T-B and over 20 km transmission
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four DFB lasers (LDu1~u4) are fed into a DAM to realized OCS and source-free ONUs, the optical spectrum is displayed in Figure 7.8(a). Meanwhile, each wavelength (LDu1~u4) with internal clock will deliver to ONU and send back to CO as upstream traffic.
Therefore, the expensive bust-mode receivers will no longer be required in CO. Figure 7.8(b) presents the received optical spectrum for combined downstream and four CW sources after OCS over 20 km transmission with a RB = 0.01 on an optical spectral analyzer as set inset (ii) in Figure 7.7. The received corresponding eye diagrams for back to back and over 20 km transmission of downstream and upstream are shown from Figure 7.8(c) to 7.8(f), respectively. It can be seen that eye diagrams are clear and good OSNR more than 40 dB for downstream and upstream over 20 km SSMF are obtained. By using centralized clock source originated from the OLT or the CO in the proposed architecture, the complex and expensive CDR circuits are not required by conventional burst-mode receivers. Therefore, the simplicity in transmission design has been achieved while reduce the system cost in the TDM-PON. These preliminary test results showed the feasibility for this proposed 10G TDM-PON configuration.
Figure 7.7: Experimental setup for TDM-PON using OCSS scheme.
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7.2.4 Summary
This investigation presents and experimentally demonstrates a novel TDM-PON using central office controlled reflective ONUs for reducing system cost, increasing the bandwidth, and making coexistence of 10G/1G PON feasible. 10 Gb/s upstream is achieved by 4 piped 2.5 Gb/s sub-units with negligible power penalty less than 0.2 dB.
We believe that this proposed hybrid 10G/1G coexisted scheme is a promising low-cost Figure 7.8: Received optical spectra: (a) one of four CW sources after OCS, as inset (i) in Figure 7.7. (b) Combined downstream signal and four CW sources over 20 km SSMF, the inset (ii) in Figure 7.7. Downstream: Back to back
50 ps/div 50 ps/div Downstream: over 20km SSMF
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200 ps/div 200 ps/div Upstream: Back to back
200 ps/div 200 ps/div Upstream: over 20km SSMF
(f) (e)
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solution for the future upgrade of current TDM-PON system. In the future, OCSS technique will be used within the TDM-PON; therefore, no complex and expensive burst-mode receiver would be required. Furthermore, one wavelength with 10 Gb/s upstream would be replaced four wavelengths with 2.5 Gb/s per channel to realize cost-effective 10G TDM-PON system.