Trends of Optical Access Network Technologies
Optical access network technologies have experienced rapid advance in recent years amid a sustained rise in user demand for bandwidth. As EPON and GPON increasingly fall short of operational needs, operators have begun mass deployment of 10G PON. The IEEE is formulating the NG-EPON standard, while the ITU-T has started the standardization work of 50G PON.
When it comes to the method for improving system bandwidth beyond 10G PON, the industry has not yet reached a consensus on whether to increase single wavelength rates or to multiplex multiple wavelengths. Discussions are being focused on three solutions: single-wavelength 25G PON, single-wavelength 50G PON and multi-wavelength 100G PON. Single-wavelength 25G PON is easy to implement but produces only modest rate improvement over 10G PON. Single-wavelength 50G PON yields significant rate improvement but presents power budget challenges for the PON system. These challenges may be overcome by using optical amplification technology. 100G PON provides huge rate improvement but imposes high requirements for photoelectric components. Normally 100G transmission is achieved by multiplexing multiple wavelengths (four 25G wavelengths or two 50G wavelengths).
Single-Wavelength 50G PON Implementation
Single-wavelength 50G can be implemented through non-return-to-zero (NRZ) or higher-order modulation. NRZ modulation requires 50G optical components, but their development is still at the sample stage. Higher-order modulation comes in various schemes such as 4-level pulse-amplitude modulation (PAM-4), duobinary, and discrete multitone (DMT). PAM-4 and duobinary need 25G optical components. DMT is an orthogonal frequency division multiplexing (OFDM) technology that requires fast Fourier transformation (FFT). The component bandwidth of DMT is related to the modulation order of each subcarrier. The higher the modulation order, the smaller the component bandwidth and the worse the sensitivity.The modulation schemes ranked in descending order by sensitivity are DMT, PAM-4 and NRZ.
For single-wavelength 50G transmission in the optical access network, PAM-4 and duobinary are the main modulation schemes because they can reuse the already mature 25G components used in the data center industry chain.
Single-Wavelength 50G PON Based on PAM-4 Modulation
While a symbol of an NRZ signal only has two levels carrying 1-bit data, a symbol of a PAM-4 signal has four levels that carry 2-bit data. The component bandwidth of PAM-4 is only half of that of NRZ, which allows PAM-4 to use 25G components for 50G signal transmission. Although PAM-4 can reduce component bandwidth requirements, its signal reception sensitivity is 5 to 6 dB worse than NRZ.
PAM-4 signal transmission and reception usually involves digital signal processor (DSP). The data signals to be transmitted first enter the DSP for PAM-4 mapping and pre-equalization, then are converted into analog signals by a digital-to-analog converter (DAC), and finally is sent to an optical link via the optical transmitter. At the receiver side, the signals undergo a photoelectric conversion by an optical receiver, then are convert into digital signals by an analog-to-digital converter (ADC), and are finally sent to the DSP. At the DSP, digital signals goes through clock recovery and equalization. The original data signals are restored after symbol decision and demapping. To reduce physical layer requirements and improve transmission performance, forward error correction (FEC) is generally performed at the transmitting and receiving ends.
Fig. 1 shows the BER curve of 50G PAM-4 signals. The performance of the transmitter (25G EML) and receivers (APD and SOA+PIN) is tested. When the RS (255, 239) FEC algorithm is used, the receiving sensitivities are –20 dBm and –22 dBm respectively. For a 29 dB power budget to be achieved for the PON system after considering the optical link transmission penalty, the transmitter’s optical power needs to be at least 10 dBm and 8 dBm, and the transmitter needs to integrate an SOA. If the low-density parity-check (LDPC) error correcting code is used to relax the BER of the receiver to 10E-2, the receiving sensitivities are –22.5 dBm and –26 dBm respectively, and the transmitter’s optical power needs to be greater than 6.5 dBm and 3 dBm. The transmitter needs to integrate an SOA to reach 6.5 dBm, but 3 dBm can be achieved by an ordinary laser.
Single-Wavelength 50G PON Based on Duobinary Modulation
Although the duobinary code has the same symbol rate as that of NRZ code, its spectrum bandwidth is only half of the NRZ code. With duobinary modulation, data can be transmitted and received in several ways. Transmission schemes include delay addition and low-pass filtering, while reception schemes include three-level decision as well as DSP equalization and recovery. The delay addition scheme is implemented by a finite impact response (FIR) filter. After the data is delayed by one bit and the next bit is added, the output is a duobinary code. The low-pass filter features delay addition, so an NRZ signal becomes a duobinary signal after going through a low-pass filter with an appropriate bandwidth. At the receiving end, the original signal can be recovered in two ways. One is to first employ three-level decision to recover the transmitted duobinary code sequence. After the code sequence undergoes duobinary decoding, the original data is restored. The other is to treat the received duobinary data as bandwidth-restricted NRZ data and use DSP equalization to eliminate inter-symbol crosstalk to recover the original data.
Fig. 2 shows the BER curve of 50G duobinary signals. The data is recovered using DSP equalization, and the bit error performances with or without maximum likelihood sequence detection (MLSD) decision are compared. The performance of the transmitter (25G EML) and receivers (APD and SOA+PIN) is tested. When the RS (255, 239) FEC algorithm and MLSD decision are used, the receiving sensitivities are –23 dBm and –25 dBm respectively, that is, if an APD receiver is used, the transmitter must reach at least 6 dBm to support the 29 dB power budget. In this case, the transmitter needs to integrate an SOA. If a SOA+PIN receiver is used, an ordinary transmitter will meet the power budget requirement. If the LDPC FEC algorithm is used, the receiving sensitivities are increased to -24.5 dBm and -28 dBm respectively. In this case, the transmitter does not need to integrate an SOA.
Direction for Single-Wavelength 50G PON Development
The lasers, receivers, modem chips, serializer/deserializer (SerDes) devices needed for 50G PON transmission can be fully or partially reused from the data center industry chain. This is beneficial to promoting the maturity of 50G PON. However, the issues unique to the PON system, including upstream burst transmission, large dynamic range and high power budget, have yet to be resolved.
Upstream burst transmission is an issue to be addressed whether 50G PON employs PAM-4 or duobinary modulation scheme. The burst laser driver for the ONU upstream transmitter and the 25G burst linear transimpedance amplifier (TIA) for the OLT upstream receiver are still technical issues that need key breakthroughs in the industry chain. The existing 25G lasers and receivers can hardly support the PON system with a power budget of 29 dB, so it is necessary to develop higher-power lasers and lower-sensitivity receivers. When the rate rises to 50 Gbps, the receiver’s sensitivity gets worse. As its overload power remains unchanged, the receiver’s dynamic range decreases. The small dynamic range is also a problem that must be solved before 50G PON is commercialized.
Compared with the data center, the optical access network provides a longer transmission distance and requires a higher power budget. Compared with the optical transport network, the optical access network has a shorter transmission distance but needs a higher power budget because the optical distribution network (ODN) uses optical splitters. The access network is cost-sensitive because it has an enormous number of end users. To promote the development of the 50G PON industry, a balance between optical and electronic components must be achieved to produce a cost-effective solution. The data center uses PAM-4 higher-order modulation to implement high-speed signal transmission. There are numerous technologies involving designing and testing of PAM-4 modem chips and optical transceivers. The PAM-4 industry chain has already matured. Therefore, 50G PON is more likely to adopt the PAM-4 modulation scheme. The solution to burst transmission, a large dynamic range and a high power budget will be the key direction for 50G PON research in the future.
Single-wavelength 50G PON, NG-EPON, 25G PON, 50G PON, PAM-4 modulation, duobinary modulation