High-fidelity indoor MIMO radio access for 5G and beyond based on legacy multimode fiber and real-time analog-to-digital-compression.

Dedicated indoor radio access network (RAN, such as C-RAN with fronthaul) will be in urgent demand for 5G and beyond ((B)5G), as it becomes more difficult for outdoor base stations to serve indoor mobile/IoT terminals due to the loss issue induced by higher carrier frequency. One cost-effective and time-saving strategy for indoor (B)5G RAN is to reuse the legacy multimode fibers (MMF) deployed in buildings and premises worldwide. In this work, we introduce the concept of indoor (B)5G fronthaul over legacy MMF based on analog-to-digital-compression (ADX), termed as ADX-RoMMF. Enabled by ADX for MIMO data compression, both high radio signal fidelity and fronthaul bandwidth efficiency can be achieved, which alleviates the limitation of low MMF bandwidth-distance product and supports decent indoor coverage. Meanwhile, its digital nature is highly compatible with low-cost optical transceivers (with nonlinearity and/or imperfection) and packet-based fronthaul networking such as time-sensitive networking. Furthermore, the ultralow latency of ADX processing meets the requirement of low-delay (B)5G fronthauling. We experimentally demonstrate an ADX-RoMMF link serving 16-channel MIMO signals with NR-class bandwidth and 1024QAM, leveraging a real-time ADX prototyped on a single-chip field-programmable radio platform. Results show that this 32Gb/s CPRI-equivalent rate can be transported over MMF distance of 850m within 1024QAM EVM requirement, which is 4-fold larger than that of conventional fronthaul compression scheme. Moreover, 500ns ADX latency overhead is also verified.

Adaptive space-time compression for efficient massive MIMO fronthauling.

One of the key features of the forthcoming fifth-generation (5G) communications is the deployment of massive multiple-input-multiple-output (MIMO) antennas to support ultra-high mobile traffic density. This scenario will pose a serious challenge on the capacity of mobile fronthaul in the centralized/cloud radio access network (C-RAN) since the required fronthaul bandwidth would linearly increase with number of antennas if conventional fronthaul interfaces (e.g., CPRI) are used. In this paper, we propose an adaptive space-time compression technique to significantly improve bandwidth efficiency of fronthaul. The technique incorporates an adaptive spatial filter to track the signal subspace and reduce the number of spatial channels, followed by adaptive quantizers to compress bandwidth of each channel in time domain. Enabled by the technique, the required fronthaul bandwidth becomes only dependent on the number of users, which is no longer proportional to the number of antennas. Moreover, compared with traditional fronthaul compression schemes in only the time domain, the flexibility of the compressor increases, and joint space-time optimization becomes feasible. On the other hand, optical fronthaul bandwidth is usually limited by cost-effective optical and electronic components. Moreover, increased reach would limit the bandwidth of IM-DD-based fronthaul (due to chromatic dispersion) as well as the received optical power. We experimentally investigate the combined optimization of a proposed space-time compressor with an optical fronthaul link. Experimental results of uplink 256-antenna fronthaul (259.5-Gb/s CPRI-equivalent rate) show that 32 users with 20MHz (30.72MSa/s) OFDM signal with lower-than-1% EVM are supported by 10GBd PAM4 optical interface.

Wavelength-interleaved MDM-WDM transmission over weakly-coupled FMF.

Recently mode-division-multiplexing (MDM) has been widely investigated to enhance fiber optics capacity, in which modes or mode groups in few-mode fiber (FMF) or multi-mode fiber (MMF) are exploited as different spatial channels for data transmission. For short-reach applications, significantly reducing inter-spatial-channel crosstalk to avoid coherent detection and multiple-input-multiple-output (MIMO) equalization is preferred. Currently most studies focus on the design of weakly-coupled FMFs and mode (de)multiplexers. Alternatively, in this work, a wavelength-interleaved (WI) scheme is proposed to mitigate inter-spatial-channel crosstalk by optimizing the design of direct detection (DD) MDM and wavelength-division-multiplexing (WDM) system. In weakly-coupled MDM systems, crosstalk mainly comes from the adjacent spatial channels, and the signal-to-crosstalk beat interference (SCBI) constitutes main crosstalk impairment after square-law detection. The WI scheme interleaves the WDM grids in adjacent spatial channels by half WDM channel spacing and uses an electrical low-pass filtering (ELPF) to remove out-of-band SCBI. The effectiveness of SCBI suppression is theoretically analyzed. The feasibility of WI scheme is experimentally verified by 3-mode 3-wavelength MDM-WDM transmission over 500-m OM3 MMF. Enabled by WI scheme, record 120-km 10G-per-channel MDM-WDM transmission over 2-mode FMF without MIMO equalization is successfully demonstrated. The WI scheme is promising to enhance the performance of short reach or even metro MDM optics.

Tunable optical multicasting of PDM-OFDM signals by novel polarization-interleaved multi-pump FWM scheme.

Optical multicasting that supports point-to-multipoint traffic replication can be one of the necessary techniques in next-generation all-optical elastic networks. In this paper, we propose an optical multicasting approach for polarization-division-multiplexing (PDM) orthogonal frequency division multiplexing (OFDM) signals based on a novel polarization-interleaved multi-pump (PIMP) four-wave mixing (FWM) scheme in highly nonlinear fiber (HNLF). Besides format transparency and the support of PDM signals, the scheme further enables wide spectral tunability of generated replicas. The pump frequency arrangement for the scheme is presented, which successfully prevents the replicas from being superimposed by unwanted FWM components during tuning. We experimentally demonstrate multicasting operation of a 3-band 100-Gb/s PDM-OFDM signal. With different input positions, 1.4 and 1.6 Terahertz tuning ranges of four replicas are achieved with Q-factor performance better than the forward error correction threshold. Tunable replica spacing from 100-GHz to 250-GHz are also verified. In addition, the scalability of the scheme is demonstrated via 5-pump multicasting, successfully generating a total of 14 replicas.

Quantification of MDL-induced signal degradation in MIMO-OFDM mode-division multiplexing systems.

Mode-division multiplexing (MDM) transmission over few-mode optical fiber has emerged as a promising technology to enhance transmission capacity, in which multiple-input-multiple-output (MIMO) digital signal processing (DSP) after coherent detection is used to demultiplex the signals. Compared with conventional single-mode systems, MIMO-MDM systems suffer non-recoverable signal degradation induced by mode-dependent loss (MDL). In this paper, the MDL-induced signal degradation in orthogonal-frequency-division-multiplexing (OFDM) MDM systems is theoretically quantified in terms of mode-average error vector magnitude (EVM) through frequency domain norm analysis. A novel scalar MDL metric is proposed considering the probability distribution of the practical MDM input signals, and a closed-form expression for EVM measured after zero-force (ZF) MIMO equalization is derived. Simulation results show that the EVM estimations utilizing the novel MDL metric remain unbiased for unrepeated links. For a 6 × 100 km 20-mode MDM transmission system, the estimation accuracy is improved by more than 90% compared with that utilizing traditional condition number (CN) based MDL metric. The proposed MDL metric can be used to predict the MDL-induced SNR penalty in a theoretical manner, which will be beneficial for the design of practical MIMO-MDM systems.

All-optical OXC transition strategy from WDM optical network to elastic optical network.

Elastic optical network (EON) has been proposed recently as a spectrum-efficient optical layer to adapt to rapidly-increasing traffic demands instead of current deployed wavelength-division-multiplexing (WDM) optical network. In contrast with conventional WDM optical cross-connect (OXCs) based on wavelength selective switches (WSSs), the EON OXCs are based on spectrum selective switches (SSSs) which are much more expensive than WSSs, especially for large-scale switching architectures. So the transition cost from WDM OXCs to EON OXCs is a major obstacle to realizing EON. In this paper, we propose and experimentally demonstrate a transition OXC (TOXC) structure based on 2-stage cascading switching architectures, which make full use of available WSSs in current deployed WDM OXCs to reduce number and port count of required SSSs. Moreover, we propose a contention-aware spectrum allocation (CASA) scheme for EON built with the proposed TOXCs. We show by simulation that the TOXCs reduce the network capital expenditure transiting from WDM optical network to EON about 50%, with a minor traffic blocking performance degradation and about 10% accommodated traffic number detriment compared with all-SSS EON OXC architectures.

Demonstration of all-optical MDM/WDM switching for short-reach networks.

Mode division multiplexing (MDM) has been widely investigated in optical transmission systems and networks to improve network capacity. However, the MDM receiver is always expensive and complex because coherent detection and multiplex-input-and-multiplex-output (MIMO) digital signal processing (DSP) are required to demultiplex each spatial mode. In this paper, we investigate the application of MDM in short-reach scenarios such as datacenter networking. Two-dimensional MDM and wavelength division multiplexing node structure based on low modal-crosstalk few-mode fiber (FMF) and components is proposed, in which signal in each mode or wavelength can be independently switched. We experimentally demonstrate independent adding, dropping and switching functionalities with two linearly polarized modes and four wavelength channels over a total 11.8-km 2-mode low modal-crosstalk FMFs. The structure is simple without coherent detection or MIMO DSP. Only slight penalties of receiver sensitivity are observed for all switching operations. The influence of modal-crosstalk accumulation for cascaded switching nodes is also investigated.

Cost effective wavelength reused MDM system for bidirectional mobile fronthaul.

In this paper, we propose a cost-effective wavelength-reused mode-division-multiplexing (MDM) system for high speed symmetrical bidirectional mobile fronthaul application. At the base band unit (BBU) pool, one of the spatial modes is used to transmit signal carrier while the others are used for downstream (DS) signal channels. At the remote radio unit (RRU) side, the signal carrier is split and reused as modulation carrier for all the upstream (US) signal channels after mode demultiplexing. Thanks to the low mode crosstalk characteristic of the mode multiplexer/demultiplexer (MUX/DEMUX) and few-mode fiber (FMF), the signal carrier and each signal channel can be effectively separated. The spectral efficiency (SE) is significantly enhanced when multiple spatial channels are used. Compared with other wavelength reused scheme in which the downstream and upstream be modulated in orthogonal dimension, the modulation format of both directions are independent in the proposed wavelength reused MDM system. Therefore, it can easily achieve symmetrical bidirectional transmission without residual re-modulation crosstalk. The proposed scheme is scalable to multi-wavelength application when wavelength MUX/DEMUX is utilized. With the proposed scheme, we demonstrate a proof of concept intensity modulated 4 × 25-Gb/s 16-QAM orthogonal frequency division multiplexing (OFDM) transmission over 10-km FMF using low modal-crosstalk two-mode FMF and MUX/DEMUX with error free operation. The downstream receiver sensitivity is -21 dBm while the upstream receiver sensitivity is -18 dBm for bidirectional transmission. Due to the Rayleigh backscattering and other spurious reflections, the upstream suffers 2 dB power penalty compared with unidirectional transmission without downstream. To mitigate bidirectional transmission impairments, we propose a simple and effective method to suppress Rayleigh backscattering by shifting the downstream subcarrier frequency. A receiver sensitivity improvement of up to 2.5 dB is achieved for upstream with different downstream power.

Upstream WDM-PON transmission scheme based on PDM-OOK modulation and digital coherent detection with dual-modulus algorithm.

Wavelength-division-multiplexing passive optical network (WDM-PON) is a promising architecture for next-generation access networks because of its large bandwidth, protocol transparency and scalability. In this paper, we propose a cost-effective, high-speed upstream WDM-PON scheme adopting polarization division multiplexed (PDM) on-off keying (OOK) modulation at the optical network unit (ONU) and digital coherent/self-coherent detection with a novel blind dual-modulus equalization algorithm at the optical line terminal (OLT). As such, the upstream capacity can be directly enhanced at low ONU expenditure, and receiver sensitivity as well as power budget can be also improved. Enabled by the scheme, 40-Gb/s upstream transmission in 80-km WDM-PON is experimentally demonstrated.

Experimental demonstration of EON node supporting reconfigurable optical superchannel multicasting.

Elastic optical networks (EON) based on optical superchannel enables higher spectral flexibility, in which the network nodes should provide multiple all-optical functionalities to manipulate bandwidth-variable data traffic. In this paper, we propose and demonstrate an EON node structure supporting reconfigurable optical superchannel multicasting. The node structure incorporates a shared multicasting module, which performs reconfigurable selection of target incoming/outgoing superchannels/replicas and leverages a group of nonlinear devices to satisfy multiple multicast requests. Moreover, an optical comb is utilized to efficiently provide and manage all pump resources for multicasting with potential cost reduction and phase noise inhibition. Based on the node structure, we experimentally demonstrate polarization division multiplexing (PDM) superchannel multicasting scenarios with different replica amount, input/output locations, and modulation formats. Less than 0.7 dB optical signal-to-noise ratio (OSNR) penalties are demonstrated in multiple multicasting scenarios.

Novel MDM-PON scheme utilizing self-homodyne detection for high-speed/capacity access networks.

In this paper, we propose a cost-effective, energy-saving mode-division-multiplexing passive optical network (MDM-PON) scheme utilizing self-homodyne detection for high-speed/capacity access network based on low modal-crosstalk few-mode fiber (FMF) and all-fiber mode multiplexer/demultiplexer (MUX/DEMUX). In the proposed scheme, one of the spatial modes is used to transmit a portion of signal carrier (namely pilot-tone) as the local oscillator (LO), while the others are used for signal-bearing channels. At the receiver, the pilot-tone and the signal can be separated without strong crosstalk and sent to the receiver for coherent detection. The spectral efficiency (SE) is significantly enhanced when multiple spatial channels are used. Meanwhile, the self-homodyne detection scheme can effectively suppress laser phase noise, which relaxes the requirement for the lasers line-width at the optical line terminal or optical network units (OLT/ONUs). The digital signal processing (DSP) at the receiver is also simplified since it removes the need for frequency offset compensation and complex phase correction, which reduces the computational complexity and energy consumption. Polarization division multiplexing (PDM) that offers doubled SE is also supported by the scheme. The proposed scheme is scalable to multi-wavelength application when wavelength MUX/DEMUX is utilized. Utilizing the proposed scheme, we demonstrate a proof of concept 4 × 40-Gb/s orthogonal frequency division multiplexing (OFDM) transmission over 55-km FMF using low modal-crosstalk two-mode FMF and MUX/DEMUX with error free operation. Compared with back to back case, less than 1-dB Q-factor penalty is observed after 55-km FMF of the four channels. Signal power and pilot-tone power are also optimized to achieve the optimal transmission performance.

Multicasting based optical inverse multiplexing in elastic optical network.

Optical multicasting based inverse multiplexing (IM) is introduced in spectrum allocation of elastic optical network to resolve the spectrum fragmentation problem, where superchannels could be split and fit into several discrete spectrum blocks in the intermediate node. We experimentally demonstrate it with a 1-to-7 optical superchannel multicasting module and selecting/coupling components. Also, simulation results show that, comparing with several emerging spectrum defragmentation solutions (e.g., spectrum conversion, split spectrum), IM could reduce blocking performance significantly but without adding too much system complexity as split spectrum. On the other hand, service fairness for traffic with different granularity of these schemes is investigated for the first time and it shows that IM performs better than spectrum conversion and almost as well as split spectrum, especially for smaller size traffic under light traffic intensity.

Experimental demonstration of 400 Gb/s optical PDM-OFDM superchannel multicasting by multiple-pump FWM in HNLF.

OFDM superchannel that consists of multiple low speed individually-modulated subbands has been proposed for high speed optical transmission and flexible optical networks with multiple data rate accommodation. In this work, we investigate the feasibility of superchannel multicasting and verify it utilizing multiple-pump FWM in highly nonlinear fiber. 400 Gb/s PDM-OFDM superchannel that consists of ten subbands is successfully delivered from one superchannel to up to seven different superchannels with error free operation. Pump power and signal power are also optimized to achieve the optimal multicasting performance.