To satisfy the continuously increasing bandwidth requirements from individual and business end-users, Optical Orthogonal Frequency Division Multiplexing (OOFDM) has been widely considered as one of the strongest contenders for high-speed Local Area Networks (LANs) and Next Generation (NG)-Passive Optical Networks (PONs). The aim of the PhD dissertation research is to extensively explore the feasibility of implementing the OOFDM technique in the aforementioned application scenarios.Special attention is given to investigating the transmission performance of Adaptively Modulated Optical OFDM (AMOOFDM) in cost-sensitive Directly-Modulated DFB Lasers (DML)-based, Intensity-Modulation and Direct-Detection (IM/DD) systems utilising Multi-Mode Fibres (MMFs) or Single-Mode Fibres (SMFs). The impacts of a wide range of different parameters of various components involved in the AMOOFDM modems on the transmission performance of the un-amplified IM/DD AMOOFDM signals are thoroughly explored, in single-channel MMF/SMF systems, and Dense Wavelength Division Multiplexing (DWDM) PON systems. The validity of numerical simulations presented in the thesis is also rigorously verified by comparing with real-time experimental measurements conducted in Bangor University.
Following an extensive review of OOFDM operating principles as well as challenges and opportunities of existing transmission techniques associated with LANs and NGPONs, detailed investigations are first undertaken of the impact of Adaptive Cyclic Prefix (ACP) on the transmission performance of AMOOFDM over DML-based, IM/DD systems using both MMFs and SMFs. Three ACP mechanisms are identified, each of which can, depending upon the system properties, affect significantly the AMOOFDM transmission performance. In comparison with AMOOFDM having a fixed cyclic prefix duration of 25%, AMOOFDM with ACP can not only improve the transmission capacity by a factor of >2 (> l.3) for >l000m MMFs (<80km SMFs) with I dB link loss margin enhancement, but also relax considerably the requirement on the DFB modulation bandwidth.
Fu1ther investigations of the AMOOFDM technique in multi-channel NG-PON
systems are also undertaken. It is shown that AMOOFDM not only significantly
reduces the nonlinear WDM impairments induced by the effects of cross-phase
modulation and four-wave mixing but also effectively compensates for the DML-induced frequency chirp effect. In comparison with conventional OOFDM using an identical signal modulation format across all the subcarriers, AMOOFDM improves the maximum achievable signal transmission capacity of a central WDM channel by a factor of 1.3 and 3.6 for 40- and 80-km standard SMFs, respectively, with the corresponding dynamic input optical power ranges being extended by approximately 5dB. In addition, AMOOFDM also causes the occurrence of cross-channel complementary modulation format mapping among various WDM channels, leading to considerably improved transmission capacities for all individual WDM channels.
To further maximize the OOFDM transmission performance and simultaneously
improve the system flexibility and performance robustness, the widely adopted
adaptive loading algorithms including Bit Loading (BL), Power Loading (PL), and
Bit-and-Power Loading (BPL) are compared in terms of transmission performance for different transceiver parameters in both SMF and MMF systems. As different MMF links reveal large variations in both the 3dB bandwidths and the system frequency responses, any explorations of the topic of interest over a specific MMF Iink are not adequate. Hence, statistical investigations of the performance of these three algorithms are undertaken over 1000 statistically constructed worst-case MMF links. The thesis tenders for the first time complete evaluations of adaptive loading algorithms over MMF and SMF-based systems. It is shown that the BPL (PL) algorithm always offers the best (worst) transmission performance. The absolute transmission capacity differences between these algorithms are independent of signal bit rate and increase with both transmission distance and digital-to-analog converter/analog-to-digital converter (DAC/ADC) sampling rate. More importantly, numerical results also indicate that, for PON systems and worst-case MMF links of less than 300 m, in comparison with the most sophisticated BPL algorithm, the simplest PL algorithm is sufficiently effective in escalating the OOFDM system performance to its maximum potential. The effectiveness of the PL algorithm can be further improved when a large number of subcarriers are utilized. On the other hand, for relatively transmission systems with their 3-dB bandwidths being much less than the transmitted OOFDM signal spectrum, the BPL algorithm has to be adopted. The aforementioned results have great potential for practical cost-effective OOFDM transceiver architecture design.
Having explored OOFDM in cost-sensitive LANs and PONs, further investigations of the OOFDM technique are conducted in transparent optical Metro/Regional
Reconfigurable Optical Add/Drop Multiplexer (ROADM)-based networks without
employing optoelectronic regeneration. In Metro/Regional networks using ROADMs, one significant source of penalty is signal degradation due to transmission through cascaded ROADMs, referred to as the filter concatenation effect. The use of AMOOFDM to improve the network tolerance to the filter concatenation effect is theoretically investigated. It is shown that, compared to the conventional identical modulation OOFDM, adaptive modulation can not only improve the maximum achievable transmission capacity by up to 60%, but also enhances the network tolerance to the filter impairments. More importantly, it is shown that the utilization of adaptive modulation is essential due to its vital tolerance to the filter misalignment penalty.