The Research On Some Key Issues In Optical Access Network Based On Optical OFDM Technology

Access network is a bridge which connects the subscriber station network and metropolitan area/core network. With the speedup of social infomationization and the development of Internet technologies, the data business has increased rapidly and the access services became more and more diverse. Nowadays, the increasing demands for high speed access, the growing access capacity and the interpenetration of multi-services place an unprecedented challenge on access network. The existing optical fiber access network has shown a solidifying character, which can only offer fixed link speed and is difficult to meet the requirments for the emerging network technologies and broadband multimedia services. It has become a bottleneck which constrains the network to be braodband. Hence, there needs to be a new optical access network (OAN), which gets a huge capacity while maintains small granularity for resource scheduling, to meet the network operators'demands for multi-services.Access technology based on optical orthogonal frequency divided multiplexing (OFDM) can offer a flexible resource scheduling at time and frequency domain, and it has become a very attractive way for multi-subscriber access. Optical OFDM access network gets high spectral efficiency as well as large capacity, and can enable resource scheduling with different granularities, which meets the quality of service (QoS) and bandwith requirements of different services. Optical OFDM access network can not only realize large capacity optical access, but also achieve the seamless convergence of wireless/wired access styles while being compatible with the existing OAN to realize dynamic reconfigurable netwrok. Besides, it shows the perspective in long-haul access. Therefore, optical OFDM access network has been discussed widly by FSAN organization and been thought as one of the candidates for 40Gb/s passive optical network (PON).Currently, the research about optical OFDM access network has just started across the world. The ACCORDANCE Project proposed by European Union in 2010 absolutely focuses on optical OFDM access network. In China, the "125" national high technology research and development program "The Technology and System Research on Evolution of Triple Play" has supported the optical OFDM-PON;it has also been included as a key project in the national natural science foundation in 2012. In view of these above, in order to meet the demands of next generation access network, this paper mainly focuses on the key technologies of optical OFDM access network and its scheme designments. It has proposed several innovative schemes about optical OFDM access network architecture and some key technologies to improve the performance of the network. Besides, it also proposes two new technologies about improved optical OFDM access. The research work of this paper will provide important references for the practical application of optical OFDM access network.The main innovative research efforts are summarized as follows.1. Novel optical OFDM access network architecture designments and access experiments1) WDM-OFDM-PON with multicast overlayTaking advantages of the orthogonal dimensions of optical signal, three noval WDM-OFDM-PON systems with multicast overlays have been proposed in this paper. In the architecture, the P2P service is modulated onto optical OFDM signal, and the multicast services are modulated onto DPSK, PolSK and FSK signals respectively by ultilizing the different dimensions of phase, polarization and frequency. It increases the efficiency of optical carrier without affecting the performance of optical OFDM P2P signal. The experiments have been executed to verify the feasibility of the proposed schemes, and the experimental results show a small interference between the optical OFDM P2P signal and multicast overlays.2) ECDM-OFDM-PON ECDM technology gets many advantages including high code gain, secure transmission and low signal interference. A novel ECDM-OFDM-PON architecuture based on ECDM technology has been proposed for the first time, and the resource allocation and performance of upstream/downstream optical OFDM signal are also analyzed in detail. The ECDM-OFDM-PON realizes a new strategy of resource scheduling in optical OFDM access network while keeping the original architecture of OAN. In this architecture, it only needs to improve the corresponding DSP algorithms at OLT and ONU, and can improve the capacity and performance without increasing the network costs.3) Convergence access of OFDM-PON and OFDM-ROF systemThe future OAN will be a network where fiber communication technology and multi-carrier wireless communication technology coexist. Based on the optical upconversion and integrated MZM, this paper has proposed a novel convergened access network of OFDM-PON and OFDM-ROF system to simultaneously provide wired and wireless services in the same wavelength channel. In the experimental research, an integrated MZM is used to realize the modulation of 40GHz 32QAM-OFDM-ROF and OFDM-PON signals simultaneously, which can simplify the OLT structure. In the experiment, the power penalties of wired and wireless signals are both below 0.5dB.2. Researches on performance improvements in optical OFDM access network1) OBI noise suppression technologies in colorless upstream link of optical OFDM access networkIn OAN, the colorless upstream link can not only decrease the cost of ONU but also increase the convenience of maintenance. This paper has theoretically analyzed the OBI noise mechanism in the colorless uplink for optical OFDM access network with time-slot and subcarrier scheduling. It has proposed two novel schemes to suppress the OBI noise and the simulative and experimental results are accordant with the theory analysis. The first scheme has adopted the optical carrier substitution, which is executed in the optical domain and doesn't need to operate the electrical signal. The second scheme realizes the OBI noise suppression through OFDM spectrum spreading and diversity, which doesn't need to add any optical device and reduces the system cost. These two schemes have their respective characteristics. In practical application, it could choose either scheme according to the demand and network arhictecture.2) Nonlinear effect suppression technology in optical OFDM access networkThis paper theoretically analyzes the mechanism of nonlinear effect noise caused by SPM and so on, and proposes a novel scheme to suppress the nonlinear effect in the network. This scheme introduces a time domain roll factor to increase some relevance between the optical OFDM subcarriers, which can reduce the PAPR of optical OFDM signal and increase the signal threshold of nonlinear effect. Compared with other exsisting methods, this scheme doesn't need to add any redundant information and doesn't affect the system bandwidth efficiency. The experimental results show that under same BER performance, the PAPR and nonlinear effect threshold both improves 4dB with adoption of raised consine roll factor.3) OSNR reduction technology in optical OFDM access networkIn optical OFDM access network, the lower the OSNR tolerance, the larger the split ratio, transmission length and number of subscribers. However, the high-order modulation format in OFDM modulation always needs high OSNR and there is contradiction between the two issues. A new method to reduce the OSNR tolerance by Turbo channel coding has been proposed and experimentally demonstrated in this paper. The experiment shows the change of OSNR tolenrance with Turbo coding under different high-order modulation formats.3. Improved optical OFDM access technologies1) Anti-frequency-offset technology in optical OFDM access networkBased on CPM technology, a novel MAMSK-OFDM signal has been proposed for the first time. This technology ensures the subcarreries of optical OFDM signal a fast roll-off sidelobe, which can reduce the effects caused by inter-carrier interference and frequency offset. The experimental results show that the MAMSK-OFDM signal can improve the performance of optical OFDM access network to a certain extent and simplify the DSP processing at the signal receiver.2) All-optical OFDM access technologyThis paper has proposed a novel all-optical OFDM access network architecture and scheme. As a long-dated optical access technology, this scheme can improve the access capacity by two orders of magnitude while keeping the existing ODN architecture. Besides, the coherent detection at the ONU can further improve the sensitivity of the received signals and increase the number of subscribers a lot. This paper makes a deep research on the theory of all-optical OFDM access based on recirculating frequency shifter. In the experiment, an 820Gb/s all-optical MIMO OFDM access network is demonstrated, which can offer each ONU 40Gb/s access rate.In conclusion, this paper systematically investigates the optical OFDM access network architecture, analyzes the network performance, proposes several optimization solutions according to the network characteristics, and studies the new improved optical OFDM access technologies. The investigation work is elaborated in the four aspects:theoretical analysis, numerical simulations, experimental demonstration and results discussion. The research work of this paper will provide references for the practical application of optical OFDM access network.