S pecial Topic Field Transmission of 100G and Beyond: Multiple Baud Rates and Mixed Line Rates Using Nyquist-WDM Technology Zhensheng Jia, Jianjun Yu, Hung-Chang Chien, Ze Dong, and Di Huo
Field Transmission of 100G and Beyond: Multiple Baud Rates and Mixed Line Rates Using Nyquist-WDM Technology Zhensheng Jia, Jianjun Yu, Hung-Chang Chien, Ze Dong, and Di Huo (ZTE USA, Morristown, NJ 07960, USA)
Abstract In this paper, we describe successful joint experiments with Deutsche Telecom on long-haul transmission of 100G and beyond over standard single mode fiber (SSMF) and with in-line EDFA-only amplification. The transmission link consists of 8 nodes and 950 km installed SSMF in DT’s optical infrastructure. Laboratory SSMF was added for extended optical reach. The first field experiment involved transmission of 8 × 216.8 Gbit/s Nyquist-WDM signals over 1750 km with 21.6 dB average loss per span. Each channel, modulated by a 54.2 Gbaud PDM-CSRZ-QPSK signal, is on a 50 GHz grid, which produces a net spectral efficiency (SE) of 4 bit/s/Hz. We also describe mixed-data-rate transmission coexisting with 1T, 400G, and 100G channels. The 400G channel uses four independent subcarriers modulated by 28 Gbaud PDM-QPSK signals. This yields a net SE of 4 bit/s/Hz, and 13 optically generated subcarriers from a single optical source are used in the 1T channel with 25 Gbaud PDM-QPSK modulation. The 100G signal uses real-time coherent PDM-QPSK transponder with 15% overhead of soft-decision forward-error correction (SD-FEC). The digital post filter and 1-bit maximum-likelihood sequence estimation (MLSE) are introduced at the receiver DSP to suppress noise, linear crosstalk, and filtering effects. Our results show that future 400G and 1T channels that use Nyquist WDM can transmit over long-haul distances with higher SE and using the same QPSK format. Keywords coherent detection; field trial; coherent optical OFDM; Nyquist WDM; MLSE
1 Introduction
T
he demand for bandwidth is being driven by more and more video streaming and proliferation of cloud computing, social media, and mobile data delivery [1]-[4]. Along with the need for reduced cost per bit per hertz, this trend has led to high-speed underlying optical transmission interfaces. At present, 100 Gbit/s long-haul systems, whether being developed or deployed, are all based on a single-carrier polarization-division multiplexed quadrature phase-shift keying (PDM-QPSK) modulation format that is associated with coherent detection and digital signal processing (DSP) [5]-[6]. Spectral efficiency (SE) over a conventional 50 GHz optical grid is 2 bit/s/Hz; thus, the system capacity is around 10 Tbit/s in a fiber C-band transmission window. Corresponding standardization for client-side 100GE, transport layer optical transport unit 4 (OTU4), and key electro-mechanical aspects has been completed to make end-to-end system connection and interoperability
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ZTE COMMUNICATIONS September 2012 Vol.10 No.3
possible [7]. Optical transmission at 100G+ per channel is the logical next step to sustaining future traffic growth. Much research has been done on trading off capacity, data rate, and optical reach (Fig. 1) [8]-[9]. Optical time-division multiplexing (OTDM) is one of the classic approaches to increasing the channel data rate. A recent experiment with OTDM resulted in a symbol rate of 1.28 Tbaud and a single-channel bit rate of 10.2 Tbit/s with return-to-zero (RZ) modulation [10]. Another recent experiment with OTDM resulted in 640 Gbaud and a line rate of 1.28 Tbit/s with non-return-to-zero (NRZ) modulation [11]. However, the commercial reality is that OTDM provides limited system stability and compactness and is generally considered a useful interim technique to explore the limits of high-bit-rate transmission. It was replaced by electrical time-division multiplexing (ETDM) when the bandwidth of optoelectronic components allowed the desired bit-rate. Spatial-division multiplexing (SDM) that uses multicore fiber (MCF) or few-mode fiber (FMF) coupled with multiple-input multiple-output (MIMO) signal processing is