Demonstration of all-digital burst clock and data recovery for symmetrical 50 Gb/s/$\lambda$ PON based on low-bandwidth optics

Driven by the rapid development of cloud servicesbusiness servicesand 5G wireless backhaulthe ever increasing capacity demands of access network grows pretty fast. Passive optical networks (PONs) are deployed worldwide as a cost-effective technology for access networks [1]. The IEEE 802.3ca Task Force has released 25 Gb/s and 50 Gb/s Ethernet PON (EPON) standards by bonding two wavelength channels to reach its 50 Gb/s target [2]and the ITU-T single wavelength 50 Gb/s PON in project G. hsp took a big step towards becoming a reality as consent during the ITU-T SG15 plenary meeting on April 232021 [3]. Four-level amplitude modulation (PAM-4) is the most potential modulation format for high-capacity PON systems due to its simpler digital signal processing (DSP) architecture and lower energy consumption [4]. There have been several works on demonstration of 50G PON by using the PAM-4 as the modulation formats for the next generation PON [5][6][7][8]. In [6][8]the downlink and uplink for a symmetrical PON are separately studied with different fiber links. The transmission between the two links is independent. Besidesin these worksthe uplink transmission is studied in continuous-modewithout considering data burst-mode. The transceivers in optical links require a high speed clock and data recovery algorithm to extract a synchronous clock and recover the received data. While for the upstream in PONs with the symbol rate of 25 GBaudthe fast synchronization remains a significant technical challenge due to the burst-mode nature of the traffic. A sufficiently short settling time is required to ensure the data is correctly recovered. Burst clock and data recovery (BCDR) has not yet been reported on symmetrical single-wavelength 50 Gb/s PAM-4 PON over the same fiber link based on bandwidth limited optics.

Various BCDR techniques have been proposed for Non-Return-to-Zero (NRZ)signalssuch as phase locked loops (PLL)gated-voltage controlled oscillators (G-VCO) and over-sampling [1][9]. Several research works have been reported for BCDR in PON upstream link. In [10]10.3 Gb/s burst-mode 3R receiver incorporating a full automatic gain control optical receiver and 82.5-GS/s over-sampling clock and data recovery (CDR) for 10G-EPON was developedand the receiver sensitivity of −30.1 dBm at the BER of 10⁻³ and the upstream power budget of 37.6 dB are successfully achieved. In [11]a hybrid burst-mode CDR employing high-speed samplers and a G-VCO was presented for 10Gbps PON and a large tolerance to the pulse distortion over 0.64 unit interval (UI) was successfully shown with the G-VCO supported half-rate sampling technique. Howeverthe G-VCO CDR has the disadvantages of no jitter rejection and reduced pulse-width distortion (PWD) tolerance [12]. The over-sampling CDR suffers from high power consumption and large integrated circuit (IC) chip area [13][14]. The PLL implementations rely on analog building blocks and require too many transitions to lock [15][16]. For PAM-4 signalstraditional methods are more challengingsince there are more transition types for PAM-4 signal than for NRZ signal. Insteadan all-digital phase detection and interpolation-based CDR can avoid such a complicated setupallowing a free-running analog-to-digital converter (ADC) and ensuring a fast CDR for base-band signals [17][18]. Among many all-digital re-timing algorithmsthe squaring timing recovery as a feed-forwardnon-data-aided methodcan be implemented efficiently for clock phase estimation due to the high loop bandwidth [17][19]. Input signal is sampled at a fixed rate by a free running oscillator and absolute timing offset is determined from the given samples of the signal by BCDR for symbol detection to recover the burst clock and data [17].

In this paperall-digital BCDR for symmetric single-wavelength 50Gb/s PAM-4 PON uplink burst transmission based on bandwidth limited optics has been experimentally demonstrated. Instead of traditional continuous-mode uplinkburst-mode upstream link is utilized in PON transmission for more accurate evaluation. This is the first BCDR demonstration for symmetric 50 Gb/s/$\lambda$ PAM-4 PON downstream and upstream over the same fiber link at O-band. Firstwe elaborate on the principle of BCDR based on squaring timing recovery algorithm. Subsequentlythe setup of a symmetric 50G PON uplink burst transmission system based on bandwidth limited devices is presented. Thenthe uplink burst frame is designed based on the system. Finallythe performance of downlink continuous-mode and uplink burst-mode transmission are discussedrespectively. The results show that the evaluation of symmetrical 50 Gb/s/$\lambda$ PON performance by uplink burst-mode has 1 dB power budget loss less than uplink continuous-mode.