We present a demo for a multiple uplink access system with real-time services. Several terminals transmit and are detected simultaneously through FDMA. The system can allow latency-sensitive and best-effort applications to share the network

Bluemm, Christian

Calabrò, Stefano

Cano, Ivan N.

Caruso, Giuseppe

Huang, Rongfang

Kirchbauer, Heinrich von

Leyva, Pablo

Talli, Giuseppe

Wei, Jinlong

Wuensche, Ullrich

English

PORTO@iris (Publications Open Repository TOrino - Politecnico di Torino)

29 March 2023POLITECNICO DI TORINORepository ISTITUZIONALEFDMA Point-to-Multi-Point Fibre Access System for Latency Sensitive Applications / Bluemm, Christian; Kirchbauer,Heinrich von; Caruso, Giuseppe; Leyva, Pablo; Wuensche, Ullrich; Huang, Rongfang; Wei, Jinlong; Cano, Ivan N.;Calabrò, Stefano; Talli, Giuseppe. - ELETTRONICO. - (2022), pp. 1-4. ((Intervento presentato al convegno 2022European Conference on Optical Communication (ECOC) tenutosi a Basel, Switzerland nel 18-22 September 2022.OriginalFDMA Point-to-Multi-Point Fibre Access System for Latency Sensitive ApplicationsIEEE postprint/Author's Accepted ManuscriptPublisher:PublishedDOI:Terms of use:openAccessPublisher copyright©2022 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in anycurrent or future media, including reprinting/republishing this material for advertising or promotional purposes, creatingnew collecting works, for resale or lists, or reuse of any copyrighted component of this work in other works.(Article begins on next page)This article is made available under terms and conditions as specified in the  corresponding bibliographic description inthe repositoryAvailability:This version is available at: 11583/2974215 since: 2022-12-29T16:56:33ZIEEEFDMA Point-to-Multi-Point Fibre Access System for Latency Sensitive Applications   Christian Bluemm(1), Heinrich von Kirchbauer(1), Giuseppe Caruso (1,2), Pablo Leyva (3), Ullrich Wuensche(1), Rongfang Huang(1), Jinlong Wei(1), Ivan N. Cano(1), Stefano Calabrò(1), Giuseppe Talli(1)  (1) Huawei Technologies Duesseldorf GmbH, Munich, Germany, christian.bluemm@huawei.com  (2) Politecnico di Torino, Torino, Italy  (3) Citrobits GmbH, Munich, Germany  Abstract We present a demo for a multiple uplink access system with real-time services. Several ter-minals transmit and are detected simultaneously through FDMA. The system can allow latency-sensitive and best-effort applications to share the network. ©2022 The Author(s) Introduction Fibre based point-to-multi-point (P2MP) networks have proven to be a reliable and low cost solution for residential optical access in the form of pas-sive optical networks (PONs). The success of PONs relies significantly on the simplicity of the optical distribution network (ODN) deployed in the field, which is essentially based on power splitters creating the P2MP distribution fibre links. Besides the residential broadband application, other use cases have been proposed for P2MP networks in recent years. P2MP for machine-to-machine (M2M) networks in the framework of In-dustry 4.0 is a very demanding use case, as it re-quires data links with extremely low latency down to 1ms or less and jitter as low as 1µs [1-3]. While most M2M links will require low data rate, ser-vices like machine vision applications could po-tentially require Gb/s capacity to transmit low la-tency high definition video streams to the compu-ting units for real-time decisions [4, 5].    Fibre based P2MP networks have several ad-vantages compared to state of the art solutions in industrial networks (industrial Ethernet protocols  and several other proprietary communications buses), which are almost entirely based on point-to-point electrical interconnects, namely: intrinsic immunity to electromagnetic interferences, sim-pler management of the head end since the traffic is aggregated in a single transceiver, thinner, lighter and generally less bulky cables. All these advantages can lead to a lower overall cost. While standardised residential PON systems have been proposed for this application, their time-domain sharing of the network capacity be-tween multiple users inherently introduces high latency and jitter, which are not compatible with the M2M use cases. PON systems with a modi-fied and optimized medium access control (MAC) have been recently demonstrated where dynamic bandwidth allocation is replaced by fixed time slot allocation to each user [6]. However, some jitter remains and buffers need to be added which en-force a trade-off between latency and jitter sup-pression. Alternative solutions propose to share the P2MP network capacity via frequency division multiple access (FDMA) [6, 7]. FDMA sub-chan-nels effectively create continuous links between the optical terminal equipment (OTE) and the op-tical aggregation point (OAP), allowing for ultra-low latency and jitter.  In this real-time demonstration, we present a solution for an FDMA P2MP system (Fig. 1), us-ing an integrated coherent receiver (ICR) to de-tect FDMA sub-channels, generated by the trans-mitters (Txs) in the OTE. FPGA-based prototyp-ing platforms with compatible analog to digital converters (ADC) and digital to analog converters (DAC) implement low-latency real-time pro-cessing [8]. The demonstration focuses on the uplink from the OTE to the OAP, which has the additional challenge of having to maintain an ac-curate control of the OTE wavelengths to avoid interference between the sub-channels. The demonstration implements a closed loop wave-length control and registration protocol for the OTEs, and is capable of carrying Ethernet (ETH) traffic with low and deterministic latency between the OTE and OAP.  FDMA for Time-Sensitive P2MP Access Net-works As an alternative to conventional time-domain multiple access networks, we propose an FDM  Fig. 1: P2MP access network system utilizing low latency FDMA sub-channels in the upstream. TxICRLOUpstream DataDownstream DataOAPTxRxDataOTE 1OTE 2OTE NODN1x N approach in which the terminals can have slightly different wavelengths and transmit simultane-ously without needing to compete for the network. This could be of particular interest in applications that require a deterministic latency or whose transmission time overlaps within certain periods. In order to maintain the terminal equipment in a simple manner, we employ a directly modulated laser (DML) with a TEC control to stabilise the wavelength. Several OTEs are collocated as close as possible in the wavelength domain. A coherent Rx at the OAP detects simultaneously all the uplink signals and also extracts the spec-tral distance of the terminals to the local oscillator (LO) frequency. With this information from the OAP, we can fine-tune the OTE lasers. To avoid interference among the terminals, we propose the following registration procedure: Initially, the terminal heats to the maximum in order to move its emission frequency up to the registration spec-tral band (Fig. 2). With a low-frequency tone, the OTE communicates its serial number to the OAP, which measures the OTE wavelength and sends it back through the broadcast downlink channel. The OAP also informs the terminal to which spec-tral channel it should tune. Once the terminal knows its wavelength and the target channel, it cools down following the empirical rule of 0.1nm/°C.   Demo Description and user interfaces For the demonstration setup (Fig. 3), we focus on the uplink. Firstly, we test the system with pseudo random binary sequences (PRBS) of several lengths. We generate the sequences directly in the FPGA and encode them with a consistent overhead byte stuffing (COBS) [9] for efficient, re-liable and unambiguous packet framing which allows the receiver (Rx) OAP to recover from malformed packets. Subsequently, we apply a G.709 forward error correction (FEC) encoder [10]. Next, the samples are differentially encoded and filtered by means of a squared-root Nyquist filter. A digital differen-tiator is implemented, but can be bypassed de-pending on the modulation format. Finally, an up-sampling stage is added to match a 72 GSa/s DAC with 6-bit physical resolution, which con-verts to analog waveforms that either directly modulate a DFB laser or an optical modulator. The inverted output from the DAC modulates an-other Tx which acts as a second terminal.  Thanks to the differentiator, we can modulate the DFB in phase and get a binary phase modu-lated signal [11] which is sent through 20km of standard single-mode fibre. With an optical vari-able attenuator (VOA), we control the received optical power after the fibre. At the central aggre-gation point, a 20-GHz ICR detects both OTE sig-nals. For reduced complexity and number of com-ponents in the Rx, we use the ICR in heterodyne mode by displacing the LO wavelength from the Tx one, and detect only the in-phase components of the two polarizations through a pair of 68 GSa/s ADC with 6-bit resolution. The Rx can easily be extended to intradyne operation, detect-ing also the quadrature components and increas-ing the available optical bandwidth.  Fig. 3 illustrates how samples from the ADCs are collected by two frontend FPGAs and pro-cessed by a backend FPGA. Before the Rx DSP, a downconversion stage translates each terminal signal to baseband and allows to separate them in frequency. Next, we apply downsampling and then decode the signals with an interferometer. Afterwards, the samples pass through a single-output 7-tap equalizer whose coefficients are adapted according to the constant modulus algo-rithm (CMA). A slicer then decides on the binary information and the bits are FEC decoded. When sending ETH packets, the frames are also COBS decoded. Finally, the bit error ratio (BER) is com-puted by direct counting of errors. The wavelength of the DFBs is set and con-trolled in temperature by means of a TEC and a control circuit. The latter can be implemented ei-ther with analogue components or digitally. For  Fig. 2: Registration procedure for OTE controlled from OAP  Fig. 3: End-to-end real time demo setup with FPGA-based DSP implementations  the communication with the DFB and control ele-ments, we employ a Raspberry Pi processor.  From the data captured by the FPGA, we com-pute the received optical spectrum as plotted in Fig. 4. This information in the Rx is useful to con-tinuously monitor the emission wavelength of each user and close the control loop. Fig. 5 shows the stability measurements of the DFBs when these are not modulated.   Fig. 6 and Fig. 7 depict the Rx graphical user interfaces (GUI) of OTE1 and OTE2, respectively.  In particular, the BER before and after FEC is shown. We limit the optical power into the Rx to be just below the FEC threshold and proof that the error correction is functioning.    In order to show that the two users can be detected simultaneously, we tune the LO wave-length close to the one of OTE1. OTE2 wave-length is 5 GHz separated as seen in Fig. 4. The FPGA detects both users simultaneously.  In the demonstration of live services, a video signal from a HD camera is encoded and sent through ETH to OTE1. Fig. 8 shows a picture of the setup. The optical signal is generated in OTE1 and then transmitted to the OAP, The decoded signal is sent to an ETH card which communi-cates with a PC to decode and display the video. We test the proper detection of the video signal in different intermediate frequencies by tuning the DFB wavelength. We count the total latency as sum of clock cycles (4 ns) that the FPGA requires for all the OTE and OAP operations and can con-firm that number by measurements. The total pro-cessing latency with our current implementation, combining Tx DSP, Rx DSP, FEC en- and de-coder is exactly 2.516 µs. For end-to-end trans-mission, there comes on top an additional ETH switching and data processing delay of up to 7.2 µs plus queuing delays and an additional fibre delay of approximately 5 µs/km [2]. In our FPGA implementation or equivalent ASIC implementa-tions, the data processing jitter is deterministic and defined only by the clock cycle period, which is 4ns in our case. This demo shows that the system keeps low and constant latencies among several users with minimal jitter.   Conclusions In this FPGA-based real time transmission demonstration, we present the operation of the uplink of a P2MP access network system utilizing low latency FDMA sub-channels. The demon-stration shows the streaming of a live HD video through Ethernet. The demonstration also imple-ments a closed loop wavelength control and reg-istration protocol for the OTEs.  Fig. 4: Detected Rx spectrum  Fig. 5: Wavelength control and stability measurements   Fig. 6: Rx GUI for OTE1  Fig. 7: Rx GUI for OTE2  Fig. 8: Experimental setup with Tx platform and camera on the right hand side and Rx platform on the left hand side  ICR   Camera   DML   OTE 1 (Tx)   OAP (Rx)  References [1] L. Dembeck, N. Benzaoui, W. Lautenschlaeger and Y. Pointurier, "Is the optical transport network of 5G ready for industry 4.0?," in Proc. ECOC, 2019 [2] T. Pfeiffer, P. Dom, S. Bidkar, F. Fredricx, K. Christodou-lopoulos and R. Bonk, "PON going beyond FTTH [In-vited Tutorial]," in Journal of Optical Communications and Networking, vol. 14, no. 1, pp. A31-A40, 2022 [3] Baumer, "White Paper: 10GigE: High-speed for your ma-chine vision task.", https://www.baumer.com/me-dias/__secure__/Baumer_10-GigE-Vi-sion_WP_EN_190312.pdf?mediaPK=8937924624414, accessed on 10 May 2022 [4] K. Christodoulopoulos, S. Bidkar, W. Lautenschlaeger, Th. Pfeiffer and R. Bonk, “Demonstration of Industrial-grade Passive Optical Network,”  in Proc. OFC 2022, Tu2G.6 [5] Bar-Niv, A. and Jonsson, R. , "Network architecture use cases for data rates beyond 10Gbs", 2020 [6] Z. Zhou, J. Wei, K. A. Clark, E. Sillekens, C. Deakin, R. Sohanpal, Y. Luo, R. Slavík, and Z. Liu, "Multipoint-to-point data aggregation using a single receiver and fre-quency-multiplexed intensity-modulated ONUs," in Proc. OFC 2022, Tu2G.4. [7] J. Tabares, S. Ghasemi, J. C. Velasquez, J. Prat, “Co-herent ultra-dense WDM-PON enabled by complexity-re-duced digital transceivers,” J. Lightwave Technol., vol. 38, pp. 1305-1313.,2019  [8] Micram Microelectronic GmbH, “USPA Brochure”, 2021 [9] S. Cheshire and M. Baker, "Consistent overhead byte stuffing," in IEEE/ACM Transactions on Networking, vol. 7, no. 2, pp. 159-172, 1999 [10] ITU-T G.975 Series G “Digital sections and digital line system - Optical fibre submarine cable systems”, 2000 [11] Keang-Po Ho, “Phase-modulated optical communication systems”, Springer Science & Business Media, 2005   

FDMA Point-to-Multi-Point Fibre Access System for Latency Sensitive Applications

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FDMA Point-to-Multi-Point Fibre Access System for Latency Sensitive Applications

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