The 5G networks targeting to provide high connection density at ultra-high speed with very low latency is envisioned to meet the growing traffic demand associated with ever-increasing bandwidth-intensive applications and services emerging in the near future. To meet the stringent 5G requirements, passive optical networks (PONs) are considered as one of the most competitive candidates to seamlessly integrate independently developed traditional optical access/metro networks and mobile fronthaul (MFH)/backhaul (MBH) networks into a converged fixed-mobile cloud access network (CAN). To enable the PONs to support such convergence, it is of great advantage if the PONs are equipped with not only flexible, elastic, and dynamically reconfigurable networking features but also inherent transparency to major network design characteristics such as signal modulation formats, different signal detection schemes, flexible wavelength division multiplexing (WDM)-grids, diversified network topologies and various multiple access techniques. Moreover, to enable high spectral efficient MFHs/MBHs capable of supporting the dynamic traffic with variable signal bandwidths, it is crucial if the PONs can also provide elastic bandwidth allocations enabling a flexible and simple redistribution of the available overall transmission bandwidth among different users with improved quality of service (QoS). Furthermore, since the access networks are cost-sensitive, the use of low-cost and low-complexity transmission systems such as intensity-modulation and direct-detection (IMDD) and transceiver components such as 10G-class narrow bandwidth optical devices are preferred in these PONs to improve the access network cost-effectiveness and reduce the overall transceiver complexity.
The study presented in this PhD dissertation research covers a diversified range of physical-layer fundamental technologies required for achieving flexible, adaptable, dynamically reconfigurable and converged fixed-mobile CANs. Extensive explorations are undertaken of the technical feasibility and performance of advanced optical transceiver-embedded digital signal processing (DSP)-enabled digital filtering techniques, which are to be implemented in the future-proof and cost-effective PON-based converged fixed-mobile CANs. These techniques include: i) hybrid orthogonal frequency division multiplexing-digital filter multiple access (OFDM-DFMA) IMDD PONs utilizing spectrally overlapped digital orthogonal filtering, ii) discrete Fourier transform (DFT)-spread spectrally overlapped hybrid OFDM-DFMA IMDD PONs, iii) hybrid single sideband (SSB) OFDM-DFMA PONs, and iv) experimental demonstrations of 30Gb/s/λ digital orthogonal filtering-multiplexed multiple channel transmissions over IMDD PON systems utilizing 10G-class optical devices. For each individual technique, a technical summary is given below:

The previously reported hybrid OFDM-DFMA PONs offer a promising solution for seamlessly converging optical and mobile networks for 5G. However, these PONs based on IMDD halves the aggregate upstream signal transmission capacity and overall spectral efficiency since each sub-wavelength just conveys a single upstream sub-band signal produced by an optical network unit (ONU). To offer a spectrally efficient PON-based converged CANs, a new hybrid OFDM-DFMA PON utilizing spectrally overlapped digital orthogonal filters (DOF) is proposed and extensively investigated, where the DSP algorithms embedded in both the ONUs and optical line terminal (OLT) are modified to allow two spectrally overlapped sub-bands to occupy the same sub-wavelength. This is achieved by multiplexing multiple independent OFDM channels using both in-phase (I-phase) and Q-phase (Q-phase) ONU-embedded dynamically reconfigurable and adaptive DOFs. In the OLT, similar to the hybrid OFDM-DFMA PONs, the single fast Fourier transform (FFT) operation is applied. For channel demultiplexing/demodulating, independent equalization is performed on the lower sideband (LSB) and upper sideband (USB) subcarriers of a single sub-wavelength, and the two received spectrally overlapped OFDM sub-bands are demultiplexed by summing and subtracting the LSB and USB subcarriers of the same sub-wavelength. As the proposed PONs signal recovery process in the OLT is still free from digital matching filters (MF), thus the new PONs maintain all the unique features associated with the earlier reported hybrid OFDM-DFMA PONs including i) simplified network as its reduce the OLT-DSP hardware/software complexity, thus leading to cost reductions ii) provide flexible, elastic, and dynamically reconfigurable DSP-enabled network, and iii) backward compatibility with existing OFDM-based 4G networks as they are inherently transparent to underlying signal modulation formats and signal bit rates. More importantly, in comparison with the conventional hybrid OFDM-DFMA PONs, the proposed PON doubles the number of supported ONUs and provides >1.7-fold increases in aggregate upstream signal transmission capacity with <1.5dB upstream power budget degradations. On the other hand, for supporting the same ONU count, the aggregate upstream signal transmission capacity increases by >2.2-fold and achieves >0.7dB upstream power budget improvements. The proposed PON’s performance improvements vary by <18% for transmission distances of up to 50km, and tolerant to finite digital filter tap length-induced channel interferences.

As the hybrid OFDM-DFMA PONs utilizing spectrally overlapped DOF is OFDM-based, the clipping/quantization noise in analogue-to-digital converters (ADC) and digital-to-analogue converters (DAC) induced by the high peak-to-average power ratios (PAPR) is identified to be an important factor limiting the maximum achievable performance. To overcome this limitation, the DFT-spread spectrally overlapped hybrid OFDM-DFMA IMDD PONs are proposed by employing the DFT-spread technique in each ONU and the OLT. It is shown that the proposed PON maintains all the unique features associated with the previously reported hybrid OFDM-DFMA PONs, whilst improving the upstream transmission performance to its maximum potential. More importantly, it reduces the PAPR by ≥2 dB, thus resulting in a 1 dB reduction in the optimum signal clipping ratio. As a direct consequence, the proposed PON demonstrates excellent tolerance to low DAC/ADC bit resolution and can reduce the minimum required bit resolution by at least 1 bit. In addition, >1.4 dB improvement in upstream power budget and up to 10% increase in aggregate upstream signal transmission capacity can also be achieved without degrading nonlinearity tolerances.

For cost-sensitive application scenarios, a large number of ONUs simultaneously accommodated by the PONs inevitably increase the overall transmission bandwidth and transceiver DSP complexity and cost. To further enhance the spectral efficiency and transceiver cost-effectiveness, the use of SSB as an alternative to double sideband (DSB) in the hybrid OFDM-DFMA PONs is proposed and explored. In this technique, without the use of the Hilbert transform operation in the ONUs, multiple SSB OFDM channels produced are multiplexed using software-reconfigurable DOFs, while in the OLT, a MF-free single FFT operation similar to the PONs mentioned above is used to simultaneously demultiplex and demodulate the signal. Extensive investigations show that, compared to the hybrid DSB OFDM-DFMA PONs, the proposed SSB PONs achieves >2dB reductions in the PAPRs of digitally filtered OFDM signals, giving rise to >2dB decrease in optimum signal clipping ratio and >1 bit reduction in the minimum resolution bit required by the ADC/DAC. Moreover, for fixed spectral bandwidths, the SSB PON increases the maximum upstream transmission capacity by a factor of approximately 2 and increase the upstream system power budget by >1.2dB without degrading its differential ONU optical launch power dynamic range. On the other hand, for fixed upstream signal transmission capacities, the SSB PON halves the overall signal transmission bandwidth and improves the upstream power budget by >2.5dB compared to the hybrid DSB OFDM-DFMA PONs.

To practically implement PONs for the seamless convergence of traditional optical access/metro networks and MFH/MBH networks for cost-sensitive 5G application scenarios, commercially available off-the-shelf and low-cost narrowband 10G-class optical devices are used to experimental demonstrates 30Gb/s/λ DOF-multiplexed six independent frequency gapless channels transmission systems over >25 km standard single mode fibre (SSMF) IMDD PON link. Experimental results show the independence of the channel locations in the digital filter space, all channels have very similar transmission performances when simple adaptive channel power loading is implemented in the digital domain. In addition, under various transmission system configurations, >0.2dB negative power penalties are observed for all the channels mainly due to the interaction between the transmission system-associated negative chromatic dispersion and the intensity modulation-induced frequency chirp. Moreover, the demonstrated systems also exhibit excellent performance robustness over a wide range of transmission distances of up to 45km.