Study On Key Techniques In The Wavelength Stacked Next Generation Passive Optical Network System

With the popularity of the emerging broadband services of terminal users, such as Internet Protocol Television(IPTV), High-definition Television(HDTV), large file sharing, cloud storage/computing, social network and wireless backhaul, etc, the demands of bandwidth in access networks are increasing rapidly, which drives the development of next generation access networks. To address this issue, research institutions at home and abroad have expended great efforts on studying the cost-effective, high-speed, flexible bandwidth allocation and “future-proof” next generation access network, and the low-cost passive optical network(PON) is considered as one of the most promising candidates. However, among numerous technique solutions for next generation passive optical network(NG-PON), wavelength stacked PON systems have attracted a great deal of research and development interest, due to their capable of providing the advantages as following: 1) compatibility with legacy PON systems by reusing the existing optical distribution network(ODN); 2) make full use of the existing techniques of PON system, such as taking the advantages of Wavelength Division Multiplexing(WDM), Orthogonal Frequency Division Multiplexing(OFDM) and time division multiplexing(TDM) techniques for cost-saving; 3) easy network capacity upgrading through adding new wavelength channels; 4) providing the multiple granularity access ability for satisfying the full service access requirements of NG-PON system. Our thesis focuses on the researches of the key techniques of wavelength stacked PON systems, including(Stacked WDM-based OFDM-PON, SWDM-OFDM-PON) and(wavelength stacked TDM-PON or Time- and Wavelength- Division Multiplexing PON, TWDM-PON) systems, and the details are discussed as follows:1. Key technique of system architecture in Wavelength-stacked TDM-PON(TWDM-PON) systemAs for basic technology solution for passive optical network stage2(NG-PON2), TWDM-PON is widely studied. Nevertheless, to achieve the cost-effective investment costs whilst maintaining good system performance, TWDM-PON deployments are recommended to process the following technique challenges:1)high-speed and low-cost colorless transmitters; 2) the dispersion distortion induced by longer transmission distance and high transmission speed; 3) high system power budget with long-reach transmission system. In this thesis, we focus on these three points and demonstrate two system schemes to realize high performance TWDM-PON system:1) Low-cost, high-speed colorless upstream transmitters based transmission system: We propose to use low-cost, thermally tuned, directly modulated distributed feed-back laser(DFB) laser as colorless upstream transmitter. As for the distortion caused by the chirp of laser and fiber dispersion, we use the delay interferometer(DI) for spectral shaping, and experimentally demonstrate a symmetric 40-Gb/s TWDM-PON system with the capability of simultaneous downstream differential phase shift keying signal demodulation and upstream signal chirp management based on DI. Furthermore, this spectral shaping scheme is also verified in the test-bed of ZTE Corporation.2) High power budget and long reach TWDM-PON system: In this thesis, we proposeand experimentally demonstrate a symmetric 40-Gb/s DFB-based TWDM-PONsystem with maximum 100 km transmission leach by using a single DI to mitigatethe distortion caused by chirp and fiber dispersion. We also study the influences ofdifferent free spectral range of DI and the wavelength shift of laser on system performance.2. Key algorithms of wavelength stacked OFDM-PON(SWDM-OFDM-PON)The signal synchronization and channel estimation algorithms are necessary in the SWDM-OFDM-PON system. However, these algorithms are limited by the system noise, which result in the inaccurate signal synchronization and system channel characteristics. On the other hand, the resource allocation algorithms in media access control(MAC) is also a key research point of PON system. To deal with these issues, we propose several novel algorithms in this thesis, which are described as below:1) The symbol timing synchronization algorithm based on correlation theory: we use the specifically designed training sequence with the strong sequence correlation character after IFFT to achieve peak value for obtaining the initial position of OFDM symbol. This scheme is very simple and easy to implement. Furthermore, it can overcome the disadvantage of the fuzzy localization in common OFDM-based system.2) The symbol timing synchronization algorithm based on the M-sequence: We propose and experimentally demonstrate a novel scheme to achieve the accurate synchronization by using the characters of strong autocorrelation and Gaussian robust of M-sequence. Compared to the existing symbol timing synchronization algorithms in optical OFDM system, this method can achieve high timing accuracy and high noise tolerance, so it is very suitable for the OFDM-based PON system.3) The channel estimation algorithm with the capability of resistance to Gaussian noise: We propose and experimentally demonstrate a novel channel estimation algorithm by using the average operation based on the traditional least square method. The performance of the Gaussian-noise tolerance is also studied by the experiment and simulation. In comparison with the existing channel estimation algorithms in OFDM-PON system, the proposed scheme can achieve a significant performance improvement. Besides, it is very simple and easy to implement.4) The resource allocation algorithms in the OFDM-PON system: To the best of our knowledge, it is the first time to establish the resource allocation scheduling model in the OFDM-PON system. Based on this model, we propose two algorithms for jointing sub-carrier and time slot allocating, and investigate their performances by simulation. The results verified that our proposed algorithms have a good performance and low computational complexity.3. Key technique of system architecture in SWDM-OFDM-PONAs with other PON techniques, for the SWDM-OFDM-PON system, achieving the high system performance with cost-effective configurations is the final aim. To achieve this goal, there are some challenges in the PON system, such as high power budget, long reach, multi-service access and so on. In this thesis, we present three novel schemes to solve the above-mentioned issues, which are discussed as following:1) Power budget improved symmetric 40-Gb/s long reach SWDM-OFDM-PON: We propose and experimentally demonstrate a SWDM-OFDM-PON system which has the capability of transmitting 10-Gb/s per wavelength downstream OFDM signals and directly modulated upstream on-off keying(OOK) signals over 100 km single mode fiber without any repeater. This system has the virtues of simplicity, scalability, and compatibility with the existing ODN. On the other hand, using OFDM technique relaxes the bandwidth requirement of optoelectronic devices, so that the system cost can be reduced.2) Adaptive Rate, multi-rate coexistent downstream transmission system of SWDM-OFDM-PON: Based on the flexible tunable bandwidth character of OFDM signals, we experimentally demonstrate a novel adaptive rate and 1.25-Gb/s, 2.5-Gb/s, 5-Gb/s, 10-Gb/s data rates coexistent transmission in SWDM- OFDM-PON system.3) Offset-QAM based OFDM(OQAM-OFDM) and its application in SWDM-OFDM-PON: For the first time, we use the OQAM-OFDM modulation format in this PON system for improving the system performance. We demonstrate the performance of this scheme by simulation and experiment. Compared to the system based on traditional OFDM and discrete fourier transform spread orthogonal frequency division multiplexing(DFT-SOFDM), our scheme achieves higher receive sensitivity without any guard interval, therefore improving the transmission efficiency and system feasibility.