Future local area networks require seamlessly converging legacy fiber and future indoor optical wireless (OW) transmission systems, enabling continuous signal propagation across the entire fiber-OW links without intermediate optical-electrical/electrical-optical conversion and/or digital signal processing. As 850 nm is the most imperative wavelength for indoor OW applications, existing ITU-T G.652.D standard single-mode fibers (SSMFs) at such a wavelength support two propagation modes (LP 01 and LP 11 ), and the resulting differential mode delay restricts the achievable 3-dB bandwidths to <250 MHz·km. To effectively address the technical challenge, under strong mode-coupling conditions emulated by multiple cascaded optical connectors, this paper experimentally demonstrates, for the first time, a fiber-OW-converged transmission system incorporating 850-nm two-mode separators, achieving continuous 850-nm signal propagation across ∼1 km SSMF and 2.5 m OW links at bitrates of 23.5 Gbit/s. The two-mode separators are constructed using commercially available 1550-nm polarization controllers and optical couplers, offering effective LP 01 mode extractions with a purity of >99%. With such mode separators, the achievable 3-dB bandwidths of the fiber-OW-converged transmission systems are increased to 7.4 GHz, resulting in 29-fold bandwidth enhancements. Moreover, 850-nm signal propagation characteristics are thoroughly characterized experimentally and numerically in ∼1 km SSMF links with multiple cascaded optical connectors. The experimental and numerical results show that even a single additional optical connector can lead to relatively strong mode coupling, which can be further intensified by increasing the number of optical connectors.

Secure data transmission is experimentally demonstrated in a 1.67 Gb/s seamlessly converged fibre-millimeter wave (mmWave) network using the authors' newly proposed chaotic digital filter (CDF)-based physical layer security (PLS) technique. The CDF-based encryption/decryption operates by introducing noise-like, key-dependent phase variations to conventional filter impulse responses. Validation is performed in a seamlessly converged network comprising a 25 km standard single-mode fibre (SSMF) link and a 5m 36GHz mm Wave wireless link, utilizing cost-effective photonic-based mmWave generation and envelope detector-based reception. Experimental results show that the demonstrated PLS technique allows the encrypted signals to continuously flow between the fibre and radio frequency (RF) domains. The PLS technique also supports simultaneous optical and radio frequency access with almost identical BER transmission performances, and power penalties of <1 dB. To gain an in-depth understanding of the measured results, the CDFs' characteristics, including their chaotic nature, sensitivity to security keys and optimum CDF design parameters, are explored both theoretically and experimentally in detail. The optimum security key properties and CDF's filter lengths are identified, which are independent of the transmission media and major characteristics of the encrypted signals. The CDF-based PLS technique offers salient advantages of 'security-by-design', 'openness-by-design', 'dynamic security at the traffic level', and 'universal network compatibility'.

Point-to-multipoint (P2MP) optical transceivers are promising for meeting the diversified requirements of emerging applications in the 6G era. However, for intensity modulation and direct detection (IMDD) passive optical networks (PONs), the previously reported P2MP optical transceivers cannot effectively accommodate a large number of diversified applications due to their limited flexibility, high digital signal processing (DSP) complexity and latency, as well as relatively high sensitivity to transmission system impairments. To cost-effectively address the challenges, this paper proposes a novel P2MP flexible optical transceiver based on a new parallel multi-channel aggregation/de-aggregation operation and an advanced extended Gaussian function (EGF)-based orthogonal digital filter bank. In comparison with the previously reported transceivers in a >50Gbit/s@25 km IMDD upstream PON with two optical network units (ONUs), our experimental measurements show that for channel counts of 8 (>10), the proposed transceivers can enhance the maximum achievable upstream transmission capacities by >52% (>290%). The new design can also reduce the overall transmitter DSP complexity (transmitter maximum DSP latency) by >70% (>24%) and decrease the receiver multi-channel de-aggregation DSP complexity by >40%. In addition, for the proposed transceivers, the network operation flexibility and scalability are also experimentally explored in a 25 km, >52Gbit/s IMDD upstream PON with five ONUs supporting 70 independent channels of different bandwidths and latencies. It is shown that the proposed transceivers fully support flexible sleep-awake ONU operation, and for a specific ONU, doubling the activated ONU count can only increase its required minimum received optical powers by ∼1.7 dB. Furthermore, the proposed transceivers can also support the ONU count growth without considerably compromising the maximum achievable upstream transmission capacities.

To cost-effectively meet 6G latency requirements, concurrent upstream and direct inter-optical network unit (ONU) communication passive optical networks (PONs) based on flexible point-to-multipoint (P2MP) optical transceivers and intensity modulation and direct detection (IMDD) have been reported to enable direct communications among different ONUs within the same PON without passing data to the optical line terminal (OLT). However, the previously reported P2MP transceivers suffer from high DSP complexity for establishing ultra-dense connections. For such application scenarios, the PON’s remote nodes also have high inter-ONU signal power losses. To effectively solve these technical challenges, this paper experimentally showcases (a) new P2MP transceivers by utilizing parallel multi-channel aggregation/de-aggregation and advanced extended Gaussian function (EGF)-based orthogonal digital filter banks, along with (b) low inter-ONU signal power loss-remote nodes. By introducing these two techniques into a 27 km, >54.31 Gbit/s concurrent upstream and direct inter-ONU communication IMDD PON, comprehensive experimental explorations of the PON’s performances were undertaken for the first time. The remote node is capable of supporting 128 ONUs. The results show that the new P2MP transceivers lead to >75% (>40%) reductions in overall transmitter (receiver multi-channel de-aggregation) DSP complexity, and they can also equip the PONs with an enhanced capability of providing ultra-dense connections. The experimental results also show that the PON allows each ONU to flexibly change its upstream and inter-ONU communication channel count without considerably compromising its performance. Therefore, the PON outperforms those of previously reported works in terms of ensuring low DSP complexity, highly robust transmission performance, and enhanced capabilities of flexibly accommodating numerous applications with diverse requirements regarding traffic characteristics, thus making it suitable for ultra-dense connection application scenarios.

Employing free-running laser/envelope detection-based millimeter wave (mmWave) signal generation/detection at remote radio heads (RRHs)/user equipment (UE) offers a cost-effective solution for seamlessly integrating existing intensity modulation-direct detection (IM-DD)-dominated optical access networks and wireless networks. Such fiber-wireless convergence enables a continuous flow of signals with varying characteristics between the baseband unit (BBU) and UE across fiber and wireless network segments without the need for optical-electrical-optical (O-E-O) conversions and digital signal processing (DSP) at intermediate nodes. In this paper, we extensively investigate the performance of such a fiber-wireless converged access network employing free-running laser/envelope detection-based mmWave generation/detection in an IM-DD-based 1.67 Gbit/s transmission system with 25 km standard single-mode fiber (SSMF) and 5 m @38 GHz mmWave wireless links. Experimental results demonstrate that both mmWave frequency tunability and adaptive mmWave network coverage are achievable by just dynamically and adaptively configuring the output wavelength and power of the RRH-embedded free-running laser. Additionally, envelope detection allows RRHs to use low-cost MHz-linewidth-level free-running lasers while maintaining excellent performance stability.

For implementing next-generation radio access networks (NG-RANs) supporting services/applications in the beyond-5G (B5G) era, seamless fiber-wireless network convergence is vital for enabling heterogeneous signals of various characteristics to continuously flow between the optical and electrical domains, i.e., the baseband unit (BBU) and user equipment (UE), without optical-electrical-optical (O-E-O) conversions or digital signal processing (DSP) at any intermediate nodes. To address such challenges, this paper proposes and experimentally demonstrates, for the first time, a cost-effective fiber-wireless converged flexible and dynamic access network based on intensity modulation and direct detection (IM-DD). The demonstrated network utilizes O-E-O conversion-free Soft-reconfigurable optical add/drop multiplexers (Soft-ROADMs) at remote nodes to dynamically establish connections between the BBU and the remote radio heads (RRHs) at the sub-wavelength level. In addition, free-running laser-enabled photonic millimeter-wave (mmWave) signal generation and passive electrical envelope detector-enabled mmWave down-conversion are also used, respectively, at the RRHs and UEs to achieve mmWave frequency tunability and adaptive wireless network coverage. The network performance and optimum network configuration are experimentally explored extensively in a fiber-wireless converged access network with 3 x 1.333 Gbps dynamic BBU-UE connections over a 10 km IM-DD fiber link and a 5 m, 38 GHz mmWave wireless link. The results show that wide mmWave frequency tuning ranges and adaptive mmWave coverages are achievable by just adjusting the RRH-laser frequency and output powers.

A dual-arm soft-ROADM drop operation with drop RF-signal phase-offset insensitive performance is demonstrated, which eliminates the need to dynamically control the drop RF-signal phase-offset. Thus, making soft-ROADMs highly attractive for future optical-wireless converged access networks.

Resource-constrained devices in security-sensitive applications require security techniques to ensure cyber-resilience, data privacy and integrity, and trust in open networks. To effectively address the above requirement, a physical layer security technique based on chaotic digital filters (CDFs), which have chaotic amplitude and phase frequency responses, is proposed and experimentally demonstrated, for the first time, in an intensity modulation and direct detection intensity (IMDD) 12Gb/s@25km standard single-mode-fiber (SSMF) optical link. The CDF-based technique provides signal protection by directly distorting signals and inducing interferences between data signals. The low-complexity and cost-efficient CDF-based security technique offers salient features of ‘security-by-design’, ‘openness-by-design’ and ․dynamic security at the traffic level’, which are of great importance for applications in heterogeneous access networks.

In order to meet the diverse needs of different services and applications in the B5G era, seamless integration of fiber and wireless network segments of next-generation radio access networks (NG-RANs), with flexible/fine channel switching granularity and ultra-low latency, is vital for enabling dynamic and continuous flow of heterogenous signals of various characteristics across different network segments without optical-electrical-optical (O-E-O) conversions or digital signal processing (DSP) at intermediate nodes. To address these requirements cost-effectively, we experimentally demonstrate 3xl.333 Gbps flexible BBU-UE connections over 10 km standard single-mode fiber (SSMF) and adaptable millimeter-wave (mmWave) wireless reach up to 50 m, using free-running laser/envelope detection-based tunable mm Wave generation/detection, and soft-ROADM-enabled dynamic sub-wavelength-level channel switching. The proposed technique requires no O-E-O conversions or DSP at intermediate nodes and offers dynamic RRH-controllable mmWave transmission with large frequency tunability and adaptive wireless network coverage.

Driven by a large number of emerging diversified services, in the 5G and beyond era, concurrent direct inter-ONU and upstream communications inside a PON-based mobile access network are highly desirable to provide dynamic, ultra-dense, and fast ONU-to-ONU (without involving an OLT) and ONU-to-OLT connections. To cost-effectively deliver highly dynamic and low latency direct inter-ONU communications, this paper proposes and experimentally demonstrates novel concurrent direct inter-ONU and upstream communications in an upstream 27 km, >62.47 Gbit/s IMDD PON. For supporting inter-ONU communications between a large number of ONUs, an advanced passive remote node is also proposed. Based on different passive optical components, this remote node can be implemented using two approaches, which can, respectively, reduce the inter-ONU signal power losses by >12.2 dB and >16.6 dB (for 128 ONUs) in comparison with existing inter-ONU communication techniques’ remote nodes. In each ONU and OLT, a single pair of cascaded IFFT/FFT-based point-to-multipoint (P2MP) flexible optical transceivers are employed to simultaneously and dynamically establish multiple ONU-to-ONU and ONU-to-OLT communications according to actual users’ requirements. Experimental results show that the proposed network has excellent robustness against various transmission system impairments, including chromatic dispersion, the Rayleigh and Brillouin backscattering effects, and the channel interference effects. For each ONU, dynamic channel allocation can be made without compromising its overall performance.

Incorporating O-E-O conversion-free Soft-ROADMs and free-running laser-based tunable mmWave generations with RF envelope detection, a cost-effective optical-wireless-converged RAN supporting flexible ultra-dense BBU-UE connectivity without O-E-O conversion and DSPs at any intermediate nodes is proposed and experimentally demonstrated, achieving 2x2.4-Gbit/s dynamic connections over 10-km fiber and 2-m @38GHz mmWave transmissions.

A novel MIMO-based I/Q Crosstalk Mitigation technique, is demonstrated to effectively mitigate soft-ROADM drop RF signal phase-offset and fiber-induced chromatic-dispersion effects. Thus, simplifying the practical implementation of soft-ROADM-based Point-to-Multipoint 5G fronthaul.

A dual-arm IQ soft-ROADM drop element is demonstrated to dynamically and adaptively drop any IQ channel pair, from a single optical wavelength containing multiple sub-wavelengths, whilst maintaining an acceptable BER independent of drop RF signal phase offset. Such soft-ROADMs simplify the practical implementation of Point-to-Multipoint 5G fronthauls.

This letter presents the first real-time experimental demonstration of concurrent upstream and inter-ONU communications in a hybrid OFDM DFMA PON, enabled by a simple low-cost alteration to the remote node. Real-time FPGA-based DSP, incorporating a 128-pt FFT and a joint sideband processing technique, is used to demultiplex different sub-wavelength channels at the ONU and OLT receivers. The simple modification in the remote node removes the need for direct user-to-user data to pass via the OLT and core network thus providing ultra-low latency inter-ONU connectivity to support a variety of newly emerging latency sensitive 5G services. The presented PON is validated with two subwavelength bands, each capable of carrying one of two possible orthogonal channels ( $I$ or $Q$ ). The dynamic channel and subcarrier allocation allows flexible allocation of PON capacity between upstream and inter-ONU links for dynamic on-demand capacity allocation and also performance optimisation according to the different length dependent link characteristics. Moreover, the backscattering effect associated with upstream signals is shown to have negligible effect on the BER performance of the inter-ONU communications.

In this work, a prior-training-free and low-complexity modulation format identification (MFI) scheme, based on amplitude histogram distributions, was proposed and demonstrated, both numerically and experimentally, for autonomous digital coherent receivers. In the proposed scheme, after having performed power normalization, incoming polarization division multiplexed (PDM) signals were classified into QPSK, 8QAM, 16QAM, 32QAM and 64QAM signals, according to their ratios. Ratios were defined according to specific features of their amplitude histograms. The proposed MFI scheme used only amplitude information. As such, it was insensitive to carrier phase noise. Furthermore, the proposed scheme did not require any prior information, such as optical signal-to-noise ratio (OSNR). The performance of the proposed MFI scheme was numerically verified using 28GBaud PDM-QPSK/-8QAM/-16QAM/-32QAM/-64QAM signals. The numerical simulation results showed that the proposed scheme was able achieve a 100% correct identification rate for all five modulation formats when their OSNR values were higher than the thresholds corresponding to the 20% FEC correcting bit error rate (BER) of 2.4 × 10−2. To further explore the effectiveness of the proposed MFI scheme, proof-of-concept experiments in 28GBaud PDM-QPSK/-8QAM/-16QAM, and 21.5GBaud PDM-32QAM transmission systems were also undertaken, which showed that the proposed scheme as robust against fiber nonlinearities. To explore the scheme’s feasibility for use in practical transmission systems, the computational complexity analysis of the proposed scheme was conducted. It showed that, compared with relevant MFI schemes, the proposed MFI scheme was able to significantly reduce the computational complexity.

Point-to-multipoint flexible optical transceiver-enabled concurrent direct inter-ONU and upstream communications are experimentally demonstrated in >62.47Gbit/s@27km IMDD PONs. The PONs flexibly and adaptively establish simultaneous ONU-to-ONU and ONU-to-OL T communication connections according to end-user's dynamic requirements.

Point-to-multipoint (P2MP) transceivers offer a promising solution to transform present point-to-point optical access networks into scalable and flexible P2MP networks capable of dynamically meeting, in a cost-effective and high energy consumption efficiency manner, the requirements associated with 5G-Advance and beyond networks, including large signal transmission capacity, fast and dense connection, high network flexibility/adaptability and low latency. However, the previously reported P2MP optical transceivers based on either coherent XR optics or IMDD digital filter multiplexing (DFM) techniques are not suitable for implementing in low-latency and highly cost-sensitive IMDD-dominated optical access networks. This paper proposes, experimentally demonstrates and optimises a novel P2MP flexible transceiver incorporating a new cascaded IFFT/FFT-based multi-channel aggregation/de-aggregation technique and an orthogonal digital filtering technique. The performances of the proposed technique are extensively evaluated experimentally in an upstream 55.3 Gb/s @25 km IMDD PON. It is shown that in comparison with the conventional DFM transceivers, the proposed transceivers can reduce the transmitter DSP complexity by a factor that approximates to aggregated channel count, and simultaneously offer additional physical layer network security, without requiring long digital filter lengths and greatly compromising upstream transmission performances/spectral efficiencies as well as differential ONU launch power dynamic ranges.

For optical field recovery and linear dispersion compensation, we propose a performance-enhanced linearization algorithm, termed adaptive hybrid multi-constraint iteration algorithm (MCIA), which does not require any physical modifications to standard configurations of intensity-modulation and direct-detection (IM/DD) transmission systems. To improve the sensitivity to the residual inter-symbol interference (ISI) effect, we introduce, after fiber backward-propagation, a linear feed-forward equalizer (FFE) pair into the proposed algorithm. To improve the sensitivity to fiber dispersion estimation errors, we utilize a two-stage dispersion estimator coupled with the G-S iteration. After 100-Gb/s PAM-4 signal transmissions over 400-km fibers, the simulation results show that the MCIA offers a 1.5-dB optical signal-to-noise ratio (OSNR) gain and a 1-dB optical power budget improvement compared with the decision-directed data-aided iterative algorithm (DD-DIA), for highly dispersive IM/DD transmissions. By performing adaptive dispersion estimation, the MCIA has higher tolerance to estimation errors in fiber length. Moreover, for cases subject to large dispersion, the usage of the embedded FFE pair not only desensitizes the MCIA on the limited bandwidth effect, but also accelerates the convergence performance for reaching lower BERs. We experimentally demonstrate that the proposed algorithm can support 150-Gb/s PAM-4 transmissions over 25-km standard single mode fibers (SSMF), where just a 7-tap FFE-pair is required. For 150 Gb/s transmissions, the tolerance to fiber length estimation error is increased from 0.9 km to 20 km.

A structured sampling scheme for fiber optical sensing in the digital domain is proposed and demonstrated experimentally, which improves the fiber optical sensing sensitivity by 4dB without any hardware modification compared with conventional pulse-based system.

This paper presents the first real-time experimental demonstration of a hybrid OFDM DFMA PON. The presented PON is validated for two channels ( I or Q ) occupying two subwavelength bands. The hybrid OFDM DFMA PON eliminates the need for a dedicated matching filter at the receiver for each individual channel, by employing a single FFT operation to recover all channels, thereby achieving lower computational complexity. The hybrid OFDM DFMA PON used in conjunction with a joint sideband processing technique, when compared to a DFMA PON under the exact same operating conditions, is shown to offer increased performance, including a lower received optical power of up to 1.2 dB for the adopted FEC limit at a given bit rate. The experimentally demonstrated PON is also proven to be significantly more tolerant to the symbol timing offset effect at the OLT receiver. Furthermore, compared to the DFMA PON ’ s OLT receiver, the implemented FFT-based receiver is shown to operate at a drastically reduced logic clock rate and with significantly lower DSP complexity. The hybrid OFDM DFMA PON is thus shown to have numerous advantages over the DFMA PON.