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Article

Concurrent Direct Inter-ONU and Upstream Communications in IMDD PONs Incorporating P2MP Flexible Optical Transceivers and Advanced Passive Remote Nodes

by
Wei Jin
1,
Lin Chen
2,*,
Jiaxiang He
1,
Roger Philip Giddings
1,
Yi Huang
3,
Ming Hao
4,
Md. Saifuddin Faruk
1,
Xingwen Yi
1,
Tingyun Wang
3 and
Jianming Tang
1
1
Digital Signal Processing Centre of Excellence, School of Computer Science and Engineering, Bangor University, Bangor LL57 1UT, UK
2
College of Electronics and Information Engineering, Shanghai University of Electric Power, Shanghai 200090, China
3
Key Laboratory of Specialty Fiber Optics and Optical Access Networks, Shanghai University, Shanghai 200444, China
4
School of Automation and Information Engineering, Sichuan University of Science and Engineering, Yibin 644000, China
*
Author to whom correspondence should be addressed.
Photonics 2024, 11(11), 1021; https://doi.org/10.3390/photonics11111021
Submission received: 17 September 2024 / Revised: 20 October 2024 / Accepted: 28 October 2024 / Published: 30 October 2024
(This article belongs to the Section Optical Communication and Network)
Browse Figures
Versions Notes

Abstract

:
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.

1. Introduction

In the 5G and beyond era, ultra-reliable low-latency communication (URLLC) is increasingly critical for supporting use cases associated with factory automation, transportation, and electrical power distribution [1,2]. The International Telecommunication Union (ITU-T) has specified that the maximum end-to-end user-plane latency for 5G URLLC services is 1 ms [3]. For delivering future 6G services, even more challenging latency targets of <1 ms are envisaged, which has already been accepted by both academia and industry [4]. As a direct result, exploring cost-effective solutions capable of satisfying such an extremely low latency requirement imposes unprecedented pressure on the telecommunications R&D community worldwide.
On the other hand, from the practical network implementation point of view, intensity modulation and direct detection (IMDD) passive optical networks (PONs) offer a cost-effective solution to construct the next generation of x-haul mobile access networks [5], where standard single-mode fibers (SSMFs) are utilized to connect an optical line terminal (OLT) (interconnected to the central unit or distributed unit) and optical network units (ONUs) (interconnected to the distributed unit or remote unit). For a typical 20 km PON, a one-way fiber propagation delay is ~0.1 ms (~5 μs/km [6]), which is nonnegligible in comparison with other latency-generating factors such as forward error correction (FEC) time delay (<0.5 μs [7]).
In addition, numerous URLLC applications including autonomous vehicles, industrial automation, and telemedicine not only have stringent low latency requirements, but also impose a strong need for dynamically establishing communication connections between different users within a single mobile access network [7,8]. However, the communications between different end-users in a single PON-based mobile access network require passing the source ONU data to the OLT and then to the destination ONUs, which leads to a round-trip fiber propagation delay of ~0.2 ms (for 20 km PONs). Therefore, to effectively meet the ultra-low latency requirements of these applications, direct inter-ONU communications (also termed ONU-to-ONU communications in this paper), without passing data to the OLT and compromising upstream transmission performances, are highly desirable [9,10,11,12,13,14].
For practically achieving such direct inter-ONU communications in a PON, it is necessary to equip the remote node with the capability of redirecting and broadcasting each ONU signal to all other ONUs. For achieving this functionality, several techniques have been reported, where corresponding PON remote nodes are modified using cyclic arrayed waveguide gratings [10,11], fiber Bragg gratings [12], optical switches [13,14], passive optical couplers [9,15,16], cascaded optical interleavers [17], and their combinations [18]. The techniques based on dynamic optical switches [13,14,19] cause their remote nodes to be active, thus leading to relatively high power consumption. A ring network topology would also deliver the desirable direct inter-ONU communications [20,21], which, however, may require modifying the existing network architecture. In comparison, the optical coupler-based remote nodes are simple, cost-effective, and transparent to wavelengths [9,15,16,22]. However, for PONs with a large number of ONUs, the optical coupler-based remote nodes introduce extremely high power losses for inter-ONU communications. For example, for a PON using a 64-by-3 optical coupler for supporting the inter-ONU communications between 64 ONUs, the inter-ONU signal power losses can be as large as ~43 dB. Existing optical coupler-based techniques use extra optical amplifiers to compensate for such power loss [23], which, however, result in active remote nodes, and are thus not desirable for PON application scenarios. Considering the fact that future PONs are expected to support more ONUs, it is thus vital to explore advanced passive remote nodes with low inter-ONU signal power losses.
Apart from the aforementioned requirements to remote node modifications, IMDD point-to-multipoint (P2MP) flexible optical transceivers [15,16,24], which allow multiple low-speed transmitters to concurrently communicate with multiple high-speed receivers, are also envisaged to be used for delivering the desirable concurrent direct inter-ONU and upstream communications by dynamically and flexibly aggregating/de-aggregating multiple inter-ONU and upstream channels. The utilization of such transceivers is beneficial for reducing the overall network transceiver count [16] and further improving network scalability and upgradability [24]. For previously reported IMDD P2MP optical transceivers, multiple channels are aggregated/de-aggregated in digital signal processing (DSP) by using digital orthogonal filtering (DOF) [15,16], cascaded inverse fast Fourier transform (IFFT)/fast Fourier transform (FFT) operations [24], and a single-IFFT/FFT operation (i.e., different channels use different subcarrier groups) [25]. In comparison, DOF-based channel aggregation/de-aggregation possesses a relatively large transceiver DSP complexity for aggregating/de-aggregating a large number of channels. On the other hand, a single-IFFT/FFT operation-based alternative requires extra DSP processes or frequency guard bands (i.e., null subcarriers) for reducing interferences between signals from different ONUs. The previously reported cascaded IFFT/FFT operations for channel aggregation/de-aggregation are preferable, where DOF is also employed for reducing interferences, as the operation possesses several advantages in terms of low transceiver DSP complexity, high spectral efficiency, and additional physical layer network security (because multiple channels are aggregated in two dimensions, i.e., the time domain and the frequency domain) [24].
To effectively deliver the desirable inter-ONU communications in future PON-based mobile access networks with a large number of ONUs, in this paper, a novel concurrent direct inter-ONU and upstream communication network is proposed and experimentally demonstrated. An advanced passive remote node architecture is proposed, which, depending on the employed optical components, can be implemented using two approaches. In comparison with an X-by-3 optical coupler-based conventional remote node [15,16], these two remote node implementation approaches can reduce the inter-ONU signal power losses by >12.2 dB and >16.6 dB, respectively. In addition, the cascaded IFFT/FFT-based IMDD P2MP flexible transceivers are also used in all of the ONUs and the OLT.
The proposed network is experimentally evaluated in a 27 km, >62.47 Gbit/s upstream IMDD PON. The experimental results show that the network has three salient features, including (1) concurrently providing ultra-dense, fast, and dynamic ONU-to-ONU and upstream (ONU-to-OLT) connections each adaptively tailored according to specific requirements of an inter-ONU/upstream communications; (2) excellent tolerance to various transmission system impairments including chromatic dispersion, the Rayleigh and Brillouin backscattering effects, and the channel interference effects; and (3) supporting dynamic channel allocations without considerably compromising the channel performances.

2. P2MP Flexible Optical Transceiver and Advanced Remote Node-Enabled Concurrent Direct Inter-ONU and Upstream Communications in IMDD PONs

The schematic diagram of the proposed concurrent direct inter-ONU and upstream communication IMDD PONs is illustrated in Figure 1. The involved transceiver multi-channel aggregation/de-aggregation processes and advanced remote node architectures are also presented in Figure 2 and Figure 3, respectively. In discussing the proposed network, special attention is given to upstream communications and direct inter-ONU communications only.

2.1. P2MP Flexible Optical Transceivers

In the OLT and each ONU, a single pair of the cascaded IFFT/FFT-based P2MP flexible optical transceivers are deployed to simultaneously establish fast and dynamic multiple ONU-to-OLT and ONU-to-ONU (without involving the OLT) connections. For each ONU’s transmitter DSP, the adopted cascaded IFFT-based multi-channel aggregation operations [24] are illustrated in Figure 2a. The operations are capable of dynamically aggregating an arbitrary number of independent channels of adaptively varying bitrates, each of which can be used for either upstream communications or direct inter-ONU communications.
To briefly outline the channel aggregation operation, here by taking the i-th ONU (ONU-i) as an example, as seen in Figure 2a, to aggregate R independent channels, the total required IFFT operation count is (R-1). Assuming the first IFFT operation size is 2N, to aggregate the r-th channel (r = [2, …, R]), the required IFFT size for the (r-1)-th IFFT operation is given by 2r−1N. The (r-1)-th IFFT operation input is a function of the r-th channel data ( S r = [ s r 1 , s r 2 , … , s r 2 r − 2 N ] ) and the output data of the (r-2)-th IFFT operation ( O r − 2 = [ o r − 2 1 , o r − 2 2 , … , o r − 2 2 r − 2 N ] ). Detailed descriptions of this multi-channel aggregation technique can be found in [24].
After cyclic prefix (CP) insertion and parallel-to-serial (P/S) conversion, a complex baseband signal is produced, which consists of multiple independent upstream and direct inter-ONU communication channels. Subsequently, after a digital domain up-sampling operation (M↑) and an orthogonal digital filtering operation [24], the complex baseband signal is converted to a real-valued radio frequency signal located at a desirable radio frequency spectral region. Following a digital-to-analogue conversion (DAC) process, intensity modulation is performed to produce an optical signal containing the above-described aggregated multiple upstream and direct inter-ONU communication channels produced by ONU-i.
In the receivers, the signal detection and demodulation procedures for both the upstream and direct inter-ONU communications are similar, which include direct detection, analogue-to-digital conversion (ADC), serial-to-parallel conversion (S/P), CP removal, FFT operations, and subcarrier identification, as well as ONU signal demodulation [24]. In the receiver DSPs, after CP removal, the FFT operations are performed to separate the signals from different ONUs. For a specific ONU, assuming that the size of the last IFFT operation of the multi-channel aggregation is P and that the digital up-sampling factor is M, the required FFT operation size can be calculated by LFFT = M × P [24]. When different ONUs adopt the same FFT sizes, a single FFT operation is sufficient to separate the signals from these ONUs [24]. If different ONUs require different FFT operation sizes, multiple FFT operations, each corresponding to a specific FFT operation size, can be performed [24]. It is worth highlighting that for direct inter-ONU communications, each ONU receiver only performs the FFT operations just for separating the targeted ONUs’ signals, which considerably reduces the receiver DSP complexity. After that, the subcarriers of the targeted ONUs are identified and then delivered to the corresponding ONU signal demodulation modules for channel de-aggregations and signal demodulation. In the ONU signal demodulation module, a conventional single-tap equalizer is first used to realize subcarrier equalization. The multi-channel de-aggregation operations illustrated in Figure 2b are performed to separate the upstream and direct inter-ONU communication channels for a specific ONU. Detailed descriptions of this multi-channel de-aggregation technique can be found in [24]. Finally, the transmitted upstream or direct inter-ONU communication data can be decoded.
The proposed concurrent direct inter-ONU and upstream communications offer the following three salient features: (1) for each ONU, its upstream channel count and/or direct inter-ONU communication channel count can be changed independently and adaptively according to end-users’ requirements with reduced channel interferences [24]; (2) for each ONU, by dynamically reconfiguring the transceiver DSPs only, dynamic channel allocations are achievable without considerably compromising the channel performance, as verified in Section 3.4; and (3) according to upstream and direct inter-ONU communication traffic loads, dynamic radio frequency allocations/variations can also be made for each ONU by adaptively adjusting the adopted digital up-sampling factors and orthogonal digital filter parameters [24].
Because the wavelengths for the upstream/inter-ONU communications are redirected and broadcasted back to all ONUs, the downstream communications can use different wavelengths for reducing interferences and adopt various transmission techniques (conventional orthogonal frequency-division multiplexing/pulse-amplitude modulation techniques and/or P2MP flexible transceiver techniques used in this paper).

2.2. Advanced Passive Remote Nodes

For PONs accommodating a large number of ONUs, the advanced passive remote node architecture without using optical filters, illustrated in Figure 3a, can be used to considerably reduce the inter-ONU signal power losses. As seen in the figure, assuming the ONU count is X, this advanced remote node just consists of a 1-by-X optical coupler (OC-1), an X-by-X optical coupler (OC-2), and X sub-structures. For each sub-architecture, its Port-1 (P1) is connected to an ONU, its P2 is connected to a port of OC-1, and its P3 and P4 are connected to OC-2. Using different optical components, as presented, respectively, in Figure 3b,c, the sub-structures can be realized using two diffident approaches, all of which are capable of delivering the same functionalities.
As seen in Figure 3a, each ONU’s inter-ONU and upstream signal is first transmitted to its corresponding sub-structure in the remote node and then split into multiple copies (the number of copies depends on optical couplers used in this sub-structure). One copy is delivered to the OLT through the sub-structure’s P2 and OC-1. One copy is transmitted to OC-2 through P3, which is then split and broadcasted to all ONUs by OC-2 through all sub-structures’ P4 ports. The optical isolators and circulators used in the advanced remote nodes’ sub-structure-1 and sub-structure-2, as shown in Figure 3b,c, allow only one direction transmission of optical signals from each ONU to be redirected to all other ONUs, thus reducing unwanted optical interferences between different signals produced by the OCs. For downstream signal transmissions, each downstream signal is broadcasted to all ONUs through OC-1 and sub-structures-embedded optical couplers.
Upstream/downstream signal power losses: In comparison with standard PONs employing a 1-by-X optical coupler at the remote nodes, the PONs equipped with the sub-structure-1 (sub-structure-2)-based advanced remote nodes lead to an extra ~6.2 dB (~4 dB) power loss for both the upstream signals and the downstream signals due to the use of 1-by-3 (1-by-2) optical couplers in the sub-structures. Such estimations are made based on the maximum insertion losses of commercial 1-by-3/1-by-2 optical couplers of ~6.2 dB/4 dB. For example, for 128 ONUs, a 128-port optical coupler in the standard PON remote node can result in a power loss of ~24.65 dB [26], and the upstream/downstream signal power losses caused by the sub-strucutre-1/sub-structure-2-based advanced remote nodes are thus ~30.85 dB (6.2 dB + 24.65 dB)/~28.65 dB (4 dB + 24.65 dB). Such power losses would be acceptable to PONs with E2-class power budgets of 35 dB [26].
Inter-ONU signal power losses: For the proposed techniques with two advanced remote node sub-structures being implemented, the resulting inter-ONU signal power losses are estimated and presented in Table 1. For comparisons, the inter-ONU signal power losses of the conventional concurrent direct inter-ONU and upstream communication techniques reported in [15,16] are also calculated and shown in Table 1. The conventional techniques use X-by-3 optical couplers at the remote nodes, as presented in Figure 3d, and the inter-ONU signals need to pass the coupler twice. It can be found that in comparison with the conventional techniques, the proposed techniques can considerably reduce the inter-ONU signal power losses when the ONU count is ≥16. For 128-ONUs, >12.2 dB (>16.6) reductions in the inter-ONU signal power losses can be obtained if the remote nodes use sub-structure-1 (sub-structure-2). In addition, for 128-ONUs, the sub-structure-1/sub-structure-2-based advanced remote nodes can result in ~37.05 dB/32.65 dB inter-ONU signal power losses, respectively. These power losses would be acceptable to PONs with E2-class power budgets [26] because inter-ONU communications have significantly shorter transmission distances in comparison with upstream communications.
It is worth mentioning that (1) for the proposed advanced remote node, a large ONU count leads to a relatively large power loss, as illustrated in Table 1. As such, in practically implementing the proposed networks, depending on the actual network deployment/designs including ONU count, fiber length, etc., the minimum network power budget requirements can be estimated. When a relatively large power budget is achievable, the proposed network can support a relatively large number of ONUs. Additionally, (2) for the proposed networks, the adopted flexible optical transceivers [24] allow each ONU to dynamically and adaptively change its inter-ONU and upstream transmission signal bit rate according to its transmission link quality (power losses, signal distortion, etc.). This allows the PONs to accommodate ONUs with largely differentiated link lengths/losses. In addition, the ONU power equalization techniques [27] would also be employed to reduce the ONU link power loss differences.

3. Experimental Demonstrations of Concurrent Direct Inter-ONU and Upstream Communications in IMDD PONs

In this section, a representative 27 km, 62.47 Gbit/s upstream IMDD PON capable of supporting the direct inter-ONU communications is constructed using the cascaded IFFT/FFT-based P2MP flexible transceivers [24] and off-the-shelf optical components. Spectral attention is given to the explorations of the upstream and direct inter-ONU transmission performances, the channel interference effects between different ONUs, and dynamical channel allocations.

3.1. Experimental Setups and Parameters

The experimental setups of the 62.47 Gbit/s@27 km IMDD PON offering simultaneous upstream and direct inter-ONU communications are illustrated in Figure 4, where two ONUs are considered. A dual-channel arbitrary waveform generator (AWG, Keysight M8195A) operating at 32GSample/s is used to represent these two ONUs. Each ONU produces an optical signal conveying 4 independent channels, of which, 2 high (low) bitrate channels are used for the upstream (inter-ONU) connections.
In each ONU’s transmitter DSP, the cascaded IFFT-based multi-channel aggregation process presented in Figure 2a is implemented to aggregate 4 independent channels. The IFFT sizes of the required three IFFT operations are 16/32/64. As such, the numbers of data-bearing subcarriers of the aggregated 4 channels are 8/8/16/32. Adaptive bit loading is implemented to maximize the spectral efficiency by adaptively varying each subcarrier modulation format from DBPSK to 64-QAM [24]. Following the cascaded IFFT operations, a CP containing 4 samples is inserted at the beginning of each symbol. After S/P conversion, a 4× digital up-sampling operation and a digital orthogonal filtering operation are performed successively [24]. A Hilbert-pair approach [28] is utilized to produce the required two orthogonal digital filter pairs, and the baseband prototype filter has a square-root-raised cosine profile. To effectively reduce imperfect digital filter-induced signal distortions, a digital filter length of 32 and an excess bandwidth factor of 0, which are optimized in [16], are employed in this paper. Subsequently, a signal clipping operation with a clipping ratio of 14 dB and a digital domain time delay operation are implemented successively. The digital domain time delay operation is to adjust the ONU timing for synchronizing the involved two ONUs [24]. To enable the ONUs to achieve the best transmission performances, the AWG- and oscilloscope-embedded pre-compensation functions are used to flatten the PON transmission system frequency responses [24].
The AWG outputs two analogue signals, each having a spectral bandwidth of 8 GHz and an amplitude of 900 mV. Their spectra can be found in Figure 4a,b. Each analogue signal contains 4 independent channels, and their bitrates are given in Table 2. The total bitrates of ONU1 and ONU2 are about 31.29 Gbit/s and 31.17 Gbit/s, respectively, thus resulting in an aggregated bitrate of 62.47 Gbit/s. Following the AWG, two optical intensity modulators (Thorlabs MX35D) are used to perform intensity modulation. Each optical modulator has a 15 kHz linewidth laser source and a 35 GHz Mach–Zehnder modulator (MZM). The wavelengths of the two ONUs are 1565.495 nm and 1564.678 nm, respectively, which give rise to a wavelength spacing of ~0.8 nm for effectively mitigating the optical beating interference effect [16]. Such a wavelength spacing is large enough to allow the involved two ONUs to use free-running lasers. For ONU2 occupying the high-frequency spectral region, a tunable optical filter (TOF, Yenista XTM-50 filters) is used to produce an optical single sideband signal (OSSB) for mitigating the channel fading effect [29]. The optical filter bandwidth and its passband central frequency are adjusted to ensure that the channel interference effects between different ONUs are negligible. Two Erbium-doped fiber amplifiers (EDFAs) each followed by a 0.8 nm bandwidth TOF fix the ONU output optical powers at ~6 dBm.
At the remote node, for the proof-of-concept demonstrations, a 50:50 passive optical coupler and an optical circulator are used for realizing the simultaneous upstream and direct inter-ONU communications. The optical power launched into the 25 km feeder fiber is ~6 dBm. For the upstream communications, in the OLT, the received optical signals’ spectra are plotted in Figure 4e. After direct detection, the corresponding electrical signal spectra containing eight channels are presented in Figure 4d. In addition, for the direct inter-ONU communications, the ONU-received optical and electrical signals’ spectra are also illustrated in Figure 4f and Figure 4c, respectively.
The optical detection and signal demodulation processes of the upstream signals and the direct inter-ONU communication signals are similar. A 40 GHz bandwidth PIN photodetector is used to convert the two optical signals to an electrical signal consisting of eight independent channels. Following the PIN, a 40 GHz@64GSample/s digital sampling oscilloscope (Keysight UXR0402A) digitizes the received electrical signal. The signal demodulation processes are conducted offline, which include S/P conversion, CP removal, 256-point FFT operation, conventional single-tap subcarrier equalization, multi-channel de-aggregation, and BPSK/QAM decoding. For the multi-channel de-aggregation process, two cascaded FFT operations with the sizes of 32 and 16 are, respectively, implemented for each ONU.
For the considered PON architectures, both the direct inter-ONU and upstream communications suffer from the Rayleigh and Brillouin backscattering effects. Because the fibers between each ONU and the optical coupler are only ~2.2 km as seen in Figure 4, the upstream communication-experienced backscattering effects arising from the broadcasted direct inter-ONU signals are negligible in comparison with the backscattering effects suffered by the direct inter-ONU communications [16]. This is further verified by Figure 4g,i, where the Stokes and anti-Stokes components of the upstream communication-suffered Brillouin backscattering effects are not obvious in comparison with the backscattered components suffered by the direct inter-ONU communications. For the inter-ONU communications, the Brillouin backscattering effects lead to a power peak at a radio frequency of ~10.8 GHz in the directly detected direct inter-ONU communication signals, as seen in Figure 4h. The results presented in Section 3.4 show that for the case considered here, both the Rayleigh and Brillouin backscattering effects can only cause negligible channel capacity reductions when upstream channels are reallocated to inter-ONU communications.

3.2. Upstream and Direct Inter-ONU Communication Performances

The measured bit error rate (BER) performances of the considered eight channels are presented in Figure 5. To explore the fiber transmission-induced performance degradation at a 20% overhead soft-decision FEC threshold at a BER of 2 × 10−2 [16], the corresponding back-to-back (B2B) transmission performances of the considered eight channels are also measured and presented in Figure 5. It can be found from the figure that: (1) for all the channels, the receiver sensitivities are ~−5 dBm for achieving their BERs at the considered FEC limit, and (2) for the upstream (direct inter-ONU) communications, the fiber transmission-induced power penalties are <1.5 dB (<0.7 dB) for all the involved channels.
Due to ONU2′s (ONU1′s) utilization of OSSB modulation (low-frequency spectral locations), ONU2 (ONU1) suffers the negligible channel fading effects, as verified by Figure 4d, where no obvious power fading dips can be found for both ONUs. The fiber nonlinearity effects are also negligible for both the upstream and direct inter-ONU communications due to the adopted low optical launch powers and relatively short transmission distances. Therefore, for the considered PONs, the major factors causing the transmission system impairments include chromatic dispersion, the Rayleigh and Brillouin backscattering effects, the channel interference effect between different ONUs, practical hardware impairments, and the interplay between the abovementioned elements. The channel interference effects between different ONUs mainly arise from the signal-to-signal beating interference (SSBI) noise by directly detecting ONU2′s OSSB signals and imperfect digital filter-induced power leakages, which are explicitly explored and discussed in Section 3.3. As discussed in Section 3.1, the upstream (direct inter-ONU) communications suffer from the negligible (relatively strong) backscattering effects. As such, for the direct inter-ONU communications, the power penalties of <0.7 dB in Figure 5a are mainly caused by the Rayleigh and Brillouin backscattering effects and chromatic dispersion. For the upstream communications, the observed <1.5 dB power penalties in Figure 5b are mainly due to chromatic dispersion. Such small power penalties imply that the proposed technique has excellent tolerance to chromatic dispersion and the Rayleigh and Brillouin backscattering effects.

3.3. Channel Interference Effect

For the proposed technique, due to the digital filtering operation in the transmitters, spectral guard bands are not used between adjacent ONUs to improve the spectral utilization efficiency, as seen in Figure 4d. In addition, in the considered PONs, to effectively mitigate the channel fading effects, low-frequency ONU1 and high-frequency ONU2 employ optical double sideband (ODSB) and OSSB signals, respectively. As such, it is easy to understand that channel interferences between different ONUs occur mainly due to SSBI arising from directly detecting ONU2′s OSSB signals and the imperfect digital filter-induced power leakages.
To investigate the channel interference effects between different ONUs and further evaluate the resultant transmission performance degradations, based on the experimental parameters used in obtaining Figure 5, the power penalties caused by the channel interference effects between the considered two ONUs are measured and illustrated in Figure 6, which presents the receiver sensitivity variations of each channel of an ONU with the other ONU’s 4 channels being switched on and off [16]. For ONU2, the channel interference effects can only lead to negligible power penalties of <0.6 dB for both the upstream and direct inter-ONU communications. This indicates that for the considered digital filter length of 32, the imperfect digital filter-induced channel interference effects are negligible, which agrees with the results obtained in [24]. However, in comparison with ONU2, ONU1 suffers extra power penalties of <1.8 dB (2.35 dB–0.57 dB = 1.8 dB). This is mainly caused by the ONU2-produced SSBI effects, which locate at the baseband spectral regions after direct detection, and are spectrally overlapped with ONU1′ signals [29].
For practical implementation, to further mitigate the SSBI-induced channel interference effects, conventional SSBI estimation and cancellation techniques [30,31] may be employed in the receiver DSPs to jointly cancel the SSBI effect. On the other hand, other channel fading mitigation techniques/solutions may also be applied to replace the adopted optical filter-based OSSB signal generation techniques, including IMDD system linearization techniques [32], modified OSSB techniques with reduced SSBI [33], dispersion compensating fibers/devices [34], chirp-managed lasers [35], and O-band optical transmissions [36].
In addition, as seen in Figure 6, it can also be found that for each ONU, CH3 suffers a relatively large power penalty in comparison with all other channels of the same ONU. Such a relatively high power penalty suffered by CH3 is a result of the interplay between the channel interference effects and the transceiver-embedded cascaded IFFT/FFT operations [24].

3.4. Dynamic Channel Allocation

For the proposed current upstream and direct inter-ONU communications, for each ONU, dynamic channel allocation can be made adaptively according to actual user communication requirements. To comprehensively evaluate the dynamic channel allocation-induced impacts on the channel performance, three cases are considered: for Case I (Case III), for each ONU, all the involved four channels are allocated for the upstream (direct inter-ONU) communications. For Case II, for each ONU, CH1/CH2 (CH3/CH4), is used for the direct inter-ONU (upstream) communications.
For each considered case, the maximum achievable aggregated overall signal transmission capacity is measured and presented in Figure 7a. Adaptive bit-loading is used for each channel to achieve the maximum aggregated bitrates [24]. The received optical power is fixed at −2 dBm for all the considered cases. It can be found that for the considered three cases, their aggregated overall signal transmission capacity differences are <2.2 Gbit/s, thus indicating that dynamic channel allocation has negligible impacts on aggregated overall signal transmission capacity.
It is also interesting to find from the figure that the highest aggregated overall signal transmission capacity is achievable when all the channels are allocated for upstream communications. When more channels are allocated for direct inter-ONU communications, the aggregated overall signal transmission capacity is slightly reduced. To understand the physical mechanisms behind the observed overall signal transmission capacity variations, for each case, the corresponding channel capacities of each ONU are presented in Figure 7b,c. It can be found that for each ONU, in comparison with Case I’s CH3/CH4 (Case III’s CH1/CH2), Case II’s CH3/CH4 (CH1/CH2) can deliver similar transmission capacities. This implies that dynamic channel allocations can only slightly influence the performances of the channels that the channel reallocation operations are activated, and have negligible negative impacts on all other channels. As discussed in Section 3.1, in comparison with the upstream communications, the direct inter-ONU communications suffer from relatively lower chromatic dispersion, while the Rayleigh and Brillouin backscattering effects are, however, more pronounced. Therefore, it is easy to understand that when an upstream channel is reallocated to direct inter-ONU communications, its channel transmission capacity is slightly reduced mainly due to the backscattering effects.
The advantages associated with the proposed PONs in terms of overall network throughput, latency, flexibility, scalability, adaptability, and network resource utilization efficiency are more pronounced in comparison with existing PONs with long transmission distances and/or high optical powers. Technically speaking, our previous research [16] indicates that (1) increasing the fiber length (either the distribution fiber length or the feeder fiber length) can enhance the chromatic dispersion effect and the Rayleigh and Brillouin backscattering effects, thus resulting in relatively large signal distortions and lower signal-to-noise ratios (SNRs). Additionally, (2) when the fiber lengths are >25 km and the ONU optical launch powers are <6 dBm, an ONU launch power increase can just lead to a slight increase in the Rayleigh and Brillouin backscattering effects. When the ONU optical launch powers are >6 dBm, a further ONU launch power increase can result in a nonnegligible power increase in the backscattered Brillouin stoke components. In practically implementing the proposed network, the optimum ONU launch powers and the maximum achievable transmission performance/capacity should be adaptive to the actual network status.
For the proposed network, the power budget can be considerably enhanced by: (1) all the techniques capable of significantly improving IMDD PON transmission performances, as mentioned in Section 3.3; (2) high-sensitivity avalanche photodiodes (APDs) adopted in the receiver [37]; (3) relatively high ONU launch powers [37]; and (4) optical preamplifiers prior to optical–electrical conversion in the receiver [37].

4. Conclusions

This paper proposed a cost-effective concurrent direct inter-ONU and upstream communication IMDD PON with an advanced passive remote node. In comparison with existing PONs, the advanced PON considerably improves the overall network throughput, flexibility, scalability, and adaptability, thus giving rise to a significantly enhanced network resource utilization efficiency. In addition, it also significantly reduces the latency and power loss for inter-ONU communications. Furthermore, as each ONU and OLT utilize a single pair of the cascaded IFFT/FFT-based P2MP flexible optical transceivers, multiple ONU-to-ONU and ONU-to-OLT connections can be established simultaneously and dynamically, each of which is tailored to meet the requirements of a specific direct inter-ONU or upstream communication service.
The proposed network has been experimentally explored in a 27 km IMDD upstream PON with two ONUs, where the low-frequency and high-frequency ONUs, respectively, utilize a conventional intensity modulation-produced ODSB signal and an optical filter-produced OSSB signal. The experimental results have shown that for both upstream and direct inter-ONU communications, the impacts of the transmission system impairments including chromatic dispersion, the Rayleigh and Brillouin backscattering effects, and the channel interference effects are negligible. Moreover, for each ONU, dynamic channel allocation can be made according to actual end-user communication requirements without considerably compromising its transmission capacity.

Author Contributions

Conceptualization, W.J., L.C. and Y.H.; methodology, J.T. and T.W.; software, R.P.G. and J.H.; validation, L.C. and W.J.; formal analysis, M.S.F. and X.Y.; investigation, M.H.; data curation, L.C.; writing—original draft, W.J.; writing—review and editing, L.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work was partially funded by the Sichuan Science and Technology Program (SSTP) (2023YFH0067) and China Scholarship Council (CSC) (202008310010), partially funded by the European Regional Development Fund through Welsh Government, partially funded by the North Wales Growth Deal through Ambition North Wales, Welsh Government and UK Government, partially funded by the Science and Technology Commission of Shanghai Municipality Project Grant (SKLSFO2021-02, SKLSFO2022-04), partially funded by the National Natural Science Foundation of China (NSFC) (62475144), and partially funded by the Natural Science Foundation of Shanghai (22ZR1423000).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Photonics 11 01021 g001
Figure 1. Schematic diagram of the proposed concurrent direct inter-ONU and upstream communication techniques in IMDD PONs. The DSP-enabled multi-channel aggregation/de-aggregation techniques are presented in Figure 2. The advanced remote node (ARN) architectures are given in Figure 3.
Figure 1. Schematic diagram of the proposed concurrent direct inter-ONU and upstream communication techniques in IMDD PONs. The DSP-enabled multi-channel aggregation/de-aggregation techniques are presented in Figure 2. The advanced remote node (ARN) architectures are given in Figure 3.
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Figure 2. Operating principle of the cascaded IFFT/FFT-based multi-channel aggregation/de-aggregation technique embedded in the P2MP flexible optical transceivers. (a) Multi-channel aggregation, (b) multi-channel de-aggregation.
Figure 2. Operating principle of the cascaded IFFT/FFT-based multi-channel aggregation/de-aggregation technique embedded in the P2MP flexible optical transceivers. (a) Multi-channel aggregation, (b) multi-channel de-aggregation.
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Figure 3. (a) Advanced passive remote node architectures for PONs accommodating X ONUs, and (b)/(c) two practical advanced remote node sub-structure implementations (Sub-structure-1/Sub-structure-2). (d) Conventional remote nodes reported in [15,16].
Figure 3. (a) Advanced passive remote node architectures for PONs accommodating X ONUs, and (b)/(c) two practical advanced remote node sub-structure implementations (Sub-structure-1/Sub-structure-2). (d) Conventional remote nodes reported in [15,16].
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Figure 4. Experimental setup of a 27 km, 62.47 Gbit/s upstream IMDD PON with inter-ONU communications. (a–d): Electrical signal spectra. (e,f): Optical signal spectra. (g–i): The Brillouin backscattering effects with all the channels being switched off. Blue/Orange: electrical/optical signal spectra.
Figure 4. Experimental setup of a 27 km, 62.47 Gbit/s upstream IMDD PON with inter-ONU communications. (a–d): Electrical signal spectra. (e,f): Optical signal spectra. (g–i): The Brillouin backscattering effects with all the channels being switched off. Blue/Orange: electrical/optical signal spectra.
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Figure 5. Transmission performances of (a) inter-ONU communications and (b) upstream communications.
Figure 5. Transmission performances of (a) inter-ONU communications and (b) upstream communications.
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Figure 6. Impact of the channel interference effects between ONUs adopting different optical signal modulation techniques. CH1/CH2: direct inter-ONU communication. CH3/CH4: upstream communication.
Figure 6. Impact of the channel interference effects between ONUs adopting different optical signal modulation techniques. CH1/CH2: direct inter-ONU communication. CH3/CH4: upstream communication.
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Figure 7. (a) Maximum achievable aggregated network capacities for three different channel allocation cases, and (b)/(c) corresponding channel capacity for ONU1/ONU2. The received optical power: −2 dBm. US: upstream communication.
Figure 7. (a) Maximum achievable aggregated network capacities for three different channel allocation cases, and (b)/(c) corresponding channel capacity for ONU1/ONU2. The received optical power: −2 dBm. US: upstream communication.
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Table 1. Remote node-induced power losses for inter-ONU signals.
Table 1. Remote node-induced power losses for inter-ONU signals.
ONU COUNT
(X)		X = 4X = 8X = 16X = 32X = 64X = 128
Advanced Remote Nodes
(Sub-structure-1)		19.7
(=2 × 6.2 +
7.3)		23.15
(=2 × 6.2 +
10.75)		26.43
(=2 × 6.2 +
14.03)		29.73
(=2 × 6.2 +
17.33)		33.9
(=2 × 6.2 +
21.5)		37.05
(=2 × 6.2 +
24.65)
Advanced Remote Nodes
(Sub-structure-2)		15.3
(=2 × 4
+ 7.3)		18.75
(=2 × 4
+ 10.75)		22.03
(=2 × 4
+ 14.03)		25.33
(=2 × 4
+ 17.33)		29.5
(=2 × 4
+ 21.5)		32.65
(=2 × 4
+ 24.65)
Conventional Remote Nodes [Figure 3d]14.6
(=2 × 7.3)		21.5
(=2 × 10.75)		28.06
(=2 × 14.03)		34.66
(=2 × 17.33)		43
(=2 × 21.5)		49.3
(=2 × 24.65)
Note: In this table, the power losses (in dB) are calculated mainly based on optical coupler insertion losses [26]. The power losses of optical isolators and optical circulators are small (<1 dB) and thus ignored.
Table 2. Channel bitrate (GBPS).
Table 2. Channel bitrate (GBPS).
Inter-ONUUpstream
ONU1CH1CH2CH3CH4
3.763.886.8216.82
ONU2CH1CH2CH3CH4
3.883.887.4116
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Jin, W.; Chen, L.; He, J.; Giddings, R.P.; Huang, Y.; Hao, M.; Faruk, M.S.; Yi, X.; Wang, T.; Tang, J. Concurrent Direct Inter-ONU and Upstream Communications in IMDD PONs Incorporating P2MP Flexible Optical Transceivers and Advanced Passive Remote Nodes. Photonics 2024, 11, 1021. https://doi.org/10.3390/photonics11111021

AMA Style

Jin W, Chen L, He J, Giddings RP, Huang Y, Hao M, Faruk MS, Yi X, Wang T, Tang J. Concurrent Direct Inter-ONU and Upstream Communications in IMDD PONs Incorporating P2MP Flexible Optical Transceivers and Advanced Passive Remote Nodes. Photonics. 2024; 11(11):1021. https://doi.org/10.3390/photonics11111021

Chicago/Turabian Style

Jin, Wei, Lin Chen, Jiaxiang He, Roger Philip Giddings, Yi Huang, Ming Hao, Md. Saifuddin Faruk, Xingwen Yi, Tingyun Wang, and Jianming Tang. 2024. "Concurrent Direct Inter-ONU and Upstream Communications in IMDD PONs Incorporating P2MP Flexible Optical Transceivers and Advanced Passive Remote Nodes" Photonics 11, no. 11: 1021. https://doi.org/10.3390/photonics11111021

APA Style

Jin, W., Chen, L., He, J., Giddings, R. P., Huang, Y., Hao, M., Faruk, M. S., Yi, X., Wang, T., & Tang, J. (2024). Concurrent Direct Inter-ONU and Upstream Communications in IMDD PONs Incorporating P2MP Flexible Optical Transceivers and Advanced Passive Remote Nodes. Photonics, 11(11), 1021. https://doi.org/10.3390/photonics11111021

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