Digital Filter Multiple Access Research Articles

DFT-Spread Spectrally Overlapped Hybrid OFDM-Digital Filter Multiple Access IMDD PONs.

A novel transmission technique—namely, a DFT-spread spectrally overlapped hybrid OFDM–digital filter multiple access (DFMA) PON based on intensity modulation and direct detection (IMDD)—is here proposed by employing the discrete Fourier transform (DFT)-spread technique in each optical network unit (ONU) and the optical line terminal (OLT). Detailed numerical simulations are carried out to identify optimal ONU transceiver parameters and explore their maximum achievable upstream transmission performances on the IMDD PON systems. The results show that the DFT-spread technique in the proposed PON is effective in enhancing the upstream transmission performance to its maximum potential, whilst still maintaining all of the salient features associated with previously reported PONs. Compared with previously reported PONs excluding DFT-spread, a significant peak-to-average power ratio (PAPR) reduction of over 2 dB is achieved, leading to a 1 dB reduction in the optimal signal clipping ratio (CR). As a direct consequence of the PAPR reduction, the proposed PON has excellent tolerance to reduced digital-to-analogue converter/analogue-to-digital converter (DAC/ADC) bit resolution, and can therefore ensure the utilization of a minimum DAC/ADC resolution of only 6 bits at the forward error correction (FEC) limit (1 × 10−3). In addition, the proposed PON can improve the upstream power budget by >1.4 dB and increase the aggregate upstream signal transmission rate by up to 10% without degrading nonlinearity tolerances.

Concurrent Inter-ONU Communications for Next Generation Mobile Fronthauls based on IMDD Hybrid SSB OFDM-DFMA PONs

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 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.

Experimental Demonstration of a Real-Time Digital Filter Multiple Access PON With Low Complexity DSP-Based Interference Cancellation

A four-channel, upstream, digital filter multiple access (DFMA) passive optical network (PON) is experimentally demonstrated for the first time in a 26 km single mode fiber (SMF) intensity modulation and direct detection system employing real-time optical network units (ONUs) and optical line terminal (OLT), which are dynamically reconfigurable in terms of channel spectral allocation and signal modulation parameters. To combat physical channel frequency response-induced cross channel interference (CCI) between spectrally overlaid orthogonal channels, a real-time, reconfigurable CCI cancellation (CCIC) function implemented in the OLT receiver digital signal processing (DSP), is demonstrated to significantly improve the DFMA PON performance and robustness in terms of significant reductions in total channel bit error rates (BERs) and individual subcarrier BERs, substantial increases in channel capacities, considerably reduced sensitivity to ONU synchronization, and excellent transparency to the interfering channel's signal modulation format. The tradeoff between the complexity of the CCIC function's filter, in terms of tap count (multiplier elements) and the DFMA PONs performance is optimized to determine the optimum tradeoff. It is shown that the CCIC filter only requires as few as 42 taps to achieve excellent CCIC performance. We also fully analyze the DSP complexity of the transmit and receive DFMA filters and the CCIC filters for varying DFMA channel counts and varying ratios of AD/DA converter sample rate to digital logic clock rate (parallelization factor). It is shown that DFMA filter complexity is independent of channel count and furthermore that for channel counts ≥ 4 the CCIC filter only needs to process ~10 interference-inducing samples per wanted signal sample, resulting in only ~10 multipliers per parallel filter in the CCIC filter. We have therefore successfully demonstrated an ultra-low complexity, real-time CCIC function, which considerably enhances the performance and robustness of DFMA PONs.

Hybrid DFT-Spread OFDM-Digital Filter Multiple Access PONs for Converged 5G Networks

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.

Hybrid OFDM-Digital Filter Multiple Access PONs

Hybrid orthogonal frequency division multiplexing (OFDM) digital filter multiple access (DFMA) passive optical networks (PONs) are, for the first time, proposed and extensively investigated, where digital signal processing (DSP) based software-reconfigurable digital filtering is employed for each individual OFDM signal from various optical network units (ONUs), and a single fast Fourier transform operation and its following data recovery processes are implemented in a pipelined approach in the optical line terminal. As a direct result of combining both OFDMA and DFMA PON characteristics, the proposed hybrid OFDM-DFMA PON still maintains highly desirable features associated with DFMA PONs, more importantly, it also offers a number of unique advantages including great relaxation of digital filter DSP complexity and considerable reduction in transceiver cost, significantly enhanced upstream PON performance and its robustness and flexibility, as well as excellent backward compatibility with existing OFDM-based 4G networks. In this paper, an analytical hybrid OFDM-DFMA PON model is developed, and numerical simulations are performed to verify the validity of the proposed technique and also to examine the upstream hybrid OFDM-DFMA PON performance characteristics over representative 25-km standard single mode fibre IMDD PON systems. It is shown that in comparison with the DFMA PONs, the hybrid OFDM-DFMA PON can improve the upstream ONU EVM performance by >10 dB for relatively large received optical powers, and also increases the differential ONU launch power dynamic range by as large as 16 dB.

All-optical VPN utilizing DSP-based digital orthogonal filters access for PONs

Utilizing digital filtering-enabled signal multiplexing and de-multiplexing, a cost-effective all-optical virtual private network (VPN) system is proposed, for the first time to our best knowledge, in digital filter multiple access passive optical networks (DFMA-PONs). Based on the DFMA technology, the proposed system can be easily designed to meet the requirements of next generation network’s flexibility, elasticity, adaptability and compatibility. Through dynamic digital filter allocation and recycling, the proposed all-optical VPN system can provide dynamic establishments and cancellations of multiple VPN communications with arbitrary traffic volumes. More importantly, due to the employment of DFMA technology, the system is not limited to a fixed signal format and different signal formats such as pulse amplitude modulation (PAM), quadrature amplitude modulation (QAM) and orthogonal frequency division multiplexing (OFDM) can be used. Moreover, one transceiver is sufficient to simultaneously transmit upstream (US)/VPN data to optical line terminal (OLT) or other VPN optical network units (ONUs), thus leading to great reduction in network constructions and operation expenditures. The proposed all-optical VPN system is demonstrated with the transceiver incorporating the formats of QAM and OFDM, which can be made transparent to downstream (DS), US and VPN communications. The bit error rates (BERs) of DS, US and VPN for OFDM signals are below the forward-error-correction (FEC) limit of 3.8×10−3 when the received optical powers are about −16.8 dBm, −14.5 dBm and −15.7 dBm, respectively.

Performance Tolerance of IMDD DFMA PONs to Channel Frequency Response Roll-Off

The impacts of the channel frequency response roll-off effect on upstream optical OFDM transmission performance in digital filter multiple access (DFMA) passive optical networks (PONs) based on intensity modulation and direct detection (IMDD) are investigated, in terms of BER versus received optical power performance, optical network unit (ONU) transmission capacity, ONU launch power variation (LPV) range, and inter-ONU sample timing offset. The effect of signal bandwidth is also explored by comparing tolerance with the channel roll-off effect for two different signal bandwidths. Over all of the aforementioned aspects, good agreements are obtained between numerical simulations and experimental measurements. The results provide important insights into channel roll-off-induced performance dependencies to facilitate cost-effective designs of both DFMA transceivers and IMDD DFMA PONs.

Energy-Efficient Scheme Based on Sub-Band Grouping and Allocating for Digital Filter Multiple Access Adopted PON

An energy-efficient optical access network scheme based on sub-band grouping and allocating is proposed and studied for the digital filter multiple access adopted passive optical network (DFMA-PON). In this scheme, sub-bands generated in digital sdomain are grouped as band groups and properly allocated for optical network unit (ONU) transmissions to reduce the transmission time. Based on this, longer sleeping time can be provided to each ONU, and some more energy can be saved. Numerical evaluations indicate that comparing with basic DFMA-PON, significant energy is saved by the proposed scheme, and a promoted energy saving performance can be achieved by properly arranging the number of sub-bands assigned in a band group. Furthermore, the technology improvement and its impact on the performance of the scheme are also investigated. It is demonstrated that comparing with the conventional network, the proposed scheme possesses prominent energy saving potential. It can be considered as a potential solution for future energy-efficient optical access network.

Experimental demonstration of a DSP-based cross-channel interference cancellation technique for application in digital filter multiple access PONs.

A DSP-based cross-channel interference cancellation (CCIC) technique with initial condition-free, fast convergence and signal modulation format independence, is experimentally demonstrated in a two-channel point-to-point digital filter multiple access (DFMA) PON system based on intensity-modulation and direct-detection (IMDD). The CCIC-induced transmission performance improvements under various system conditions are fully investigated for the first time. It is shown that with one iteration only the CCIC technique can achieve a reduction in individual OFDM subcarrier BERs of more than 1000 times, an increase in transmission capacity by as much as 19 times and an increase in optical power budget by as much as 3.5dB. The CCIC technique thus has the potential to drastically improve the transmission performance of DFMA PONs.

Experimental Demonstration of Upstream Transmission in Digital Filter Multiple Access PONs With Real-Time Reconfigurable Optical Network Units

Digital filter multiple access (DFMA) passive optical networks (PONs) exploit centralized software-defined networking (SDN) controller-managed and transceiver-embedded digital orthogonal filters in individual optical network units (ONUs) to enable end-users to adaptively and dynamically access and share a common fiber transmission medium. DFMA PONs have the potential of not only equipping NG-PONs with sufficient network operation flexibility, adaptability, elasticity, and reconfigurability, but also providing highly desirable backward compatibility with current PON standards. In this paper, for the first time to our knowledge, multipoint-to-point upstream signal transmission in intensity-modulation and direct-detection (IMDD) DFMA PONs is experimentally demonstrated using two real-time, reconfigurable, optical orthogonal frequency division multiplexing-modulated ONUs and an offline optical line terminal. The experimental demonstrations show that each ONU achieves a similar upstream bit error rate performance, excellent tolerance to inter-ONU sample timing offset, and a relatively large ONU launch power variation range.

Multiple Channel Interference Cancellation of Digital Filter Multiple Access PONs

By making use of the digital signal processing (DSP) based software-reconfigurable digital orthogonal filtering, digital filter multiple access passive optical networks (DFMA PONs) have demonstrated great potential for offering excellent backward compatibility with existing PONs and supporting future cloud access networks. However, intensity modulation and direct detection (IMDD) DFMA PONs suffer from imperfect channel orthogonality-induced cross-channel interference. In this paper, a DFMA channel interference cancellation (DCIC) technique is, for the first time, proposed and extensively investigated, to significantly improve the downstream and upstream performance of the IMDD DFMA PONs. A comprehensive DCIC theoretical model is developed, and through fitting with experimental measurements, the developed theoretical model is rigorously verified and a set of accurate transceiver/system parameters is identified. It is shown that DCIC increases the aggregated upstream signal transmission capacity by a factor of > 2 and extends the differential optical network unit (ONU) launch power dynamic range by approximately 14 dB. Such significant performance improvements are achieved after just one iteration. Other salient DCIC advantages include ONU count-independent low DSP complexity, small latency and transparency to signal modulation format, signal bit rate, and initial system operation conditions.

Digital Filter Multiple Access PONs With DSP-Enabled Software Reconfigurability

Digital filter multiple access (DFMA) passive optical networks (PONs) are, for the first time to our knowledge, proposed and extensively investigated, where digital signal processing (DSP)-enabled, software-reconfigurable, digital orthogonal filtering is employed in each individual optical network unit (ONU) and the optical line terminal to enable all ONUs to dynamically share the transmission medium under the control of the centralized software-defined controller and the transceiver-embedded DSP controllers. The DFMA PONs fully support software-defined networking with the network control further extended to the physical layer. As digital filtering is the fundamental process at the heart of the proposed DFMA PONs, the filtering-induced tradeoffs among upstream transmission capacity, filter design flexibility, and filter DSP complexity are examined, based on which optimum filter design guidelines are identified for various application scenarios. Furthermore, the performance characteristics of the DFMA PONs are also numerically explored in terms of maximum achievable upstream transmission capacity, differential ONU launch power dynamic range, and ONU count-dependent minimum received optical power.