Rectangular orthogonal digital filter banks (ODFBs) based on square-root-raised-cosine (SRRC) functions are widely utilised to realise flexible and elastic multi-channel aggregations for fixed and mobile networks. However, long digital filter lengths are required to minimize digital filtering-associated signal distortions. In this paper, based on the extended Gaussian function (EGF), a novel rectangular ODFB with excellent robustness against the short digital filter length-induced truncation effect is proposed. Optimum digital filter parameters of the EGF-based ODFBs are identified and verified in multi-channel hybrid OFDM-digital filter multiple access (DFMA) PONs based on intensity modulation and direct detection (IMDD). By making use of the identified optimum digital filter parameters, extensive comparisons of digital filter characteristics and corresponding multi-channel upstream PON performances are made between the EGF-based ODFBs and the SRRC-based ODFBs. It is shown that to achieve a similar aggregated upstream signal transmission capacity, the EGF-based ODFB reduces the digital filter DSP complexity by a factor of 4. For a digital filter length as short as 8, in comparison with the SRRC-based ODFB, the EGF-based ODFB introduces >1.5 dB (>0.8 dB) improvements in upstream receiver sensitivity for 5-bits (8-bits) DACs/ADCs, increases the aggregated upstream signal transmission capacity by >5.5%, enlarges the ONU launch power dynamic range by >2.5 dB and improves the frequency offset tolerance by a factor of >1.5. In addition, the EGF-based ODFB also enhances upstream performance robustness against ONU symbol timing offsets.

Concurrent adaptive inter-ONU and upstream communications are experimentally demonstrated in 25km@101.6Gbit/s IMDD hybrid SSB OFDM-DFMA PONs, offering a promising candidate to meet 5G and beyond networks’ requirements in terms of latency, bitrate and connection density.

Matching filter (MF)-free digital filter multiplexed (DFM) single sideband (SSB) OFDM intensity modulation and direct detection (IMDD) dual-channel transmissions of 51.25Gbit/s over 25km SSMFs are experimentally demonstrated. It is shown that both transmission system impairments and digital filter characteristic variations can only lead to <1dB transmission performance degradations. Compared with the MF-free DFM-based double sideband (DSB) OFDM technique, the SSB technique has a similar receiver DSP complexity and provides almost twice the maximum signal transmission capacity. When compared with a conventional DFM technique incorporating a dedicated shaping and matching filter pair for each channel, the present SSB technique achieves a 10-fold reduction of receiver DSP complexity and 7.82Gbit/s of additional signal transmission throughput. We demonstrate these throughput and complexity advantages by modelling and measurement.

DSP-enabled multi-channel aggregation techniques are promising for cost-effectively improving the flexibility, adaptability and elasticity of fronthaul transport networks. By utilizing orthogonal digital filtering in multi-channel aggregation in IMDD transmission systems, two DSP-enabled matching filter (MF)-free multi-channel aggregation techniques respectively based on SSB OFDM and orthogonal DSB OFDM have been reported; however, the SSB (DSB) technique has a drawback of relatively high digital filter DSP complexity (reduced adaptability to physical layer system characteristics). To effectively overcome these drawbacks associated with these two techniques, in this paper, a DSP-enabled MF-free adaptively variable SSB/DSB OFDM multi-channel aggregation technique is proposed and experimentally demonstrated, in which >72Gb/s@25 km IMDD transmissions have been achieved. This work also evaluates, for the first time, the flexibility, adaptability, and elasticity of the orthogonal digital filtering-enabled multi-channel aggregation techniques. The results show that the proposed technique not only maintains the SSB technique’s excellent adaptability but also possesses the DSB technique’s low digital filter DSP complexity features.

In PON-based mobile fronthauls, direct inter-ONU communications without passing end-user traffic to the OLT offer a promising solution for fulfilling the stringent latency and bandwidth requirements of 5G and beyond networks. In this paper, with slight modifications to the PON remote node, a concurrent inter-ONU and upstream communication technique is proposed and experimentally demonstrated in 101.6Gbit/s multipoint-to-point hybrid SSB OFDM digital filter multiple access (DFMA) IMDD PONs over 25km SSMFs. Multiple gapless inter-ONU and upstream SSB signals are aggregated by digital orthogonal filtering in each ONU transmitter. A single FFT operation is applied for demultiplex and demodulation in the OLT/ONU receivers. The results show that for both the inter-ONU and upstream transmissions, the optimum length of digital filters is 32, based on which the power penalties due to the fiber transmission and ONU channel interference are <1dB and <2dB, respectively. For the inter-ONU communications, adaptive RF spectral assignments can effectively mitigate the Rayleigh and Brillouin backscattering effects and the upstream channel fading effect, thus giving rise to >30% improvements in aggregated signal transmission capacity. In addition, detailed experimental investigations are also undertaken of the trade-off between differential ONU optical launch power dynamic range and aggregated signal transmission capacity. An approximately 1dB increase in ONU launch power dynamic range is achievable by reducing the aggregated signal transmission capacity by 5Gbit/s.

Known hybrid orthogonal frequency division multiplexing-digital filter multiple access (OFDM-DFMA) PONs show promise of seamless and cost-effective convergence of optical and mobile networks for 5G and beyond. This paper reports, for the first time, a new hybrid OFDM-DFMA PON based on intensity modulation and direct detection (IMDD), obtained by modifying digital signal processing (DSP) algorithms embedded in both the OLT and ONUs. The proposed PON allows two spectrally overlapped sub-bands to occupy each individual sub-wavelength spectral region to independently transmit upstream ONU information. A model of the proposed PON is developed and its upstream transmission performances are numerically explored for different application scenarios. Compared with the previously published PON, the proposed PON doubles the number of supported ONUs and provides >1.7-fold aggregate upstream signal transmission capacity increases with <1.5 dB upstream power budget degradations. Alternately, for the same ONU count, >2.2-fold aggregate upstream signal transmission capacity increases and >0.7 dB upstream power budget improvements are achievable. The performance improvements vary by <18% for a transmission distance range as large as 50 km. In addition, the proposed PON is tolerant to finite digital filter tap length-induced channel interferences.

Aggregated 16.6Gbit/s@26km upstream hybrid OFDM-DFMA IMDD PONs utilizing low-cost optical/electrical devices are demonstrated, for the first time, which significantly improve the performance robustness and differential ONU optical launch power dynamic range, compared to published work. © 2020 The Author(s) OCIS code: (060.0060) Fiber optics and optical communications; (060.4250) Networks; 1. Introduction To realize the seamless convergence of the existing optical metro/access networks and mobile front-haul/mid- haul/back-haul networks for 5G and beyond networks, recently, a hybrid orthogonal frequency division multiplexing-digital filter multiple access (OFDM-DFMA) PON has been proposed and theoretically investigated [1,2], where for upstream signal transmission, various digital in-phase (I) shaping filters (SFs) are employed in the ONUs to dynamically locate their OFDM signals at the required sub-wavelengths. Whilst in the OLT, the OFDM signals from various ONUs are de-multiplexed and demodulated simultaneously by a single FFT operation without utilizing digital matching filters. Compared with the previously reported DFMA PON [3-5], its unique advantages are summarized as follows: 1) >100-fold reduction in the overall OLT DSP complexity when accommodating 36 ONUs [6], 2) considerable relaxation of ONU-embedded digital SF DSP complexity requirements for achieving specific transmission performances, 3) significant improvements in upstream system power budget and upstream performance robustness against both component/system impairments and channel interferences, 4) inherent transparency to existing OFDM-based 4G networks, and 5) excellent transparency to ONU design parameters. In this paper, experimental explorations of the more challenging upstream signal transmission performance of the hybrid OFDM-DFMA PONs are reported, for the first time, utilizing off-the-shelf low-cost optical/electrical devices. The experimentally measured results confirm our numerical predictions, and more importantly, reveal that the proposed PON not only has improved upstream performance robustness against transmission system impairments and channel interferences, but also possesses a large differential ONU optical launch power dynamic range (Diff- ONU OLPDR), compared to the previously published work [4]. 2. Experimental setup and measured upstream transmission performances Based on the hybrid OFDM-DFMA PON operating principle explicitly described in [1], the experimental setup of a representative two-channel upstream hybrid OFDM-DFMA PON is illustrated in Fig. 1. In the transmitter, two independent digitally-filtered analogue OFDM signals are produced by a dual-channel AWG (Keysight M8195A) operating at 30GS/s@8-bit. In the AWG-embedded Matlab program, two real OFDM signals are first generated and Fig. 1 Experimental upstream setup of the IMDD hybrid OFDM-DFMA PON without utilizing digital matching filters. Fig. 2 (a) Back-to-back and 26km SSMF transmission performance, (b) impact of channel interference on transmission performance and (c) differential ONU optical launch power dynamic range. then digitally up-sampled by a factor of M=4 (4 1), and finally digitally filtered by two in-phase SFs. The adopted SFs are constructed by using a Hilbert-pair approach [1]. The key parameters of the OFDM signals and the digital filters are listed in Table I. Prior to digital-to-analogue conversion of each OFDM signal, a digital-domain time delay operation is applied to adjust the ONU signal timing for synchronization and an extra 1.5* oversampling operation is also performed for each digitally-filtered OFDM signal. As such, for each analogue signal produced, its signal bandwidth/bitrate is 5GHz/8.3Gbit/s, which gives rise to an overall upstream signal transmission bandwidth of 10GHz and an aggregated upstream signal bitrate of 16.6Gbit/s. To perform optical intensity modulation, a 10GHz EML and a 20GHz MZM are used for the low frequency channel (CH-1) and high frequency channel (CH-2) respectively. To alleviate the optical beating interference effect in the OLT, a minimum wavelength space of 0.28nm between these two ONUs is employed. After a 50:50 passive OC, the fiber launch power is 4.7dBm. After 26km SSMF transmission, a 25GHz linear PIN and an electrical amplifier, a digital sampling oscilloscope (DSO) captures and digitalizes the received electrical signal at a sampling speed of 25GS/s, and finally a signal demodulation process is performed off-line, which includes signal resampling [5], signal synchronization, serial-to- parallel conversion (S/P), CP removal, a single 128 (32*4) point FFT operation, signal sideband identification, sideband processing and data recovery. In the signal sideband identification process after the FFT, the 64 subcarriers in the positive frequency bin are classified into two groups each containing 32 subcarriers (corresponding to one OFDM signal). In each group, its 16 low/high frequency subcarriers occupy the lower sideband (LSB)/the upper sideband (USB) of the signal. As the LSB and USB of a specific OFDM signal convey identical data, for each identified OFDM signal, the sideband processing is then applied to improve signal transmission performance, where a conjugate operation is first performed for the USB subcarriers and subsequently a phase compensation operation is then implemented for both the LSB and USB subcarriers, and finally the subcarriers in the LSB and USB of the same signal are directly summed. The upstream transmission performances and the received signal spectra of the considered hybrid OFDM-DFMA PONs before and after upstream transmitting the aggregated 16.6Gbit/s over 26km SSMF are plotted in Fig. 2(a). The results show that the fiber transmission-induced power penalties at BERs of 1*10-3 are < 0.5dB for both channels, indicating that fiber transmission nonlinearities have negligible impacts on the BER performances of different channels. This agrees very well with our theoretical predictions [1]. The channel interference-induced performance degradations are explored in Fig. 2(b), where the BER performances of each channel with the other channel present/omitted in the optical domain are illustrated. As expected from our numerical results [1], the channel interference-induced power penalties are <1dB. Furthermore, by utilizing the system parameter setting similar to that adopted in Fig. 2(a), the Diff-ONU OLPDR [1] over 26km SSMF transmission is examined in Fig. 2(c), where the received optical power is fixed at -3.5dBm. Fig. 2(c) shows that the Diff-ONU OLPDR as large as 6.7dB is obtainable. In addition, it is also worth highlighting the following two aspects: 1) in Fig. 2, the similarities of the observed ONU OLPDRs and the BER performances between these two considered ONUs with different optical intensity modulators suggest that the hybrid OFDM-DFMA PON has excellent robustness against variations in ONU transceiver designs, and 2) all the subcarriers in each channel have very similar EVM performances, as shown in Fig. 2(b), this implies the effectiveness of the sideband processing adopted in the OLT.

In this paper, we propose hybrid discrete Fourier transform (DFT)-spread orthogonal frequency division multiplexing (OFDM)-digital filter multiple access (DFMA) passive optical networks (PONs) for the first time to the best of our knowledge. In this case, digital filtering based on digital signal processing is applied to individual DFT-spread OFDM signals from various optical network units, and a single fast Fourier transform operation and its following data recovery processes are implemented in a pipelined approach in the optical line terminal. The proposed networks maintain all salient features associated with previously reported hybrid OFDM–DFMA PONs. More importantly, they additionally reduce the upstream signal peak-to-average power ratios by less than 2 dB. As a direct result, in comparison with the hybrid OFDM–DFMA PONs, the proposed PONs increase the upstream system power budget by more than 3 dB and reduce the minimum required digital-to-analog converter/analog-to-digital converter quantization bits by at least 1 bit.