ZTE Communications

More documents

Recommendations

Info

High Spectral Efficiency 400G Transmission Xiang Xiang Zhou Zhou (AT&T Labs-Research, Middletown, NJ 07748, USA) Abstract This paper gives an overview of the generation and transmission of 450 Gb/s wavelength-division multiplexed (WDM) channels over the standard 50 GHz ITU-T grid at a net spectral efficiency (SE) of 8.4 b/s/Hz. The use of nearly ideal Nyquist pulse shaping, spectrally-efficient high-order modulation format, distributed Raman amplification, distributed compensation of ROADM filtering effects, coherent equalization, and high-coding gain forward error correction (FEC) code may enable future 400G systems to operate over the standard 50 GHz grid optical network. Keywords spectral efficiency; optical; modulation format; coherent 1 Introduction O ptical transport costs have traditionally been lowered by increasing per-channel data rates and spectral efficiency (SE). With 100 Gb/s wavelength-division multiplexed (WDM) technology being made commercially available in 2010, research is now being conducted on per-channel bit rates beyond 100 Gb/s that have spectral efficiencies greater than 2 b/s/Hz. It is likely that 400 Gb/s will be the next-generation transport standard [1]. From an historical point of view, increasing the transport interface rate in proportion to SE has minimized the cost per bit transmitted. In line with this trend, SE of 8 b/s/Hz might be needed for future 400 Gb/s systems (Fig. 1). Such SE enables future 400 Gb/s systems to operate over existing optical networks with 50 GHz WDM channel spacing; therefore, it is very attractive from a carrier’s perspective. Because of several limitations, transmitting 400 Gb/s per-channel signals on the 50 GHz WDM grid is very challenging. First, according to Shannon theory, 11.76 dB signal-to-noise ratio (SNR) is required for an 8 b/s/Hz 400 Gb/s system, and this is higher than in current 2 b/s/Hz 100 Gb/s systems (without taking fiber nonlinearity into consideration). Second, fiber nonlinearity limits the allowable launch power and, consequently, the achievable signal SNR. Furthermore, a higher-SE modulation format is less tolerant of fiber nonlinearity because of the reduced Euclidean distance. Third, non-ideal passband shapes from optical network components, such as the widely used reconfigurable S pecial Topic High Spectral Efficiency 400G Transmission Xiang Zhou add/drop multiplexer (ROADM), cause significant channel narrowing. Finally, a high-SE modulation format is less tolerant of laser phase noise, which may introduce extra penalty. Spectrally efficient, high-order coherent modulation formats, coherent detection, transmitter- and receiver-side digital signal processing (DSP), distributed Raman amplification, high-coding-gain forward error correction (FEC) code, and new low-loss, low-nonlinearity fibers have all been considered as enabling technologies for next generation high-speed transport systems. These technologies are currently being explored by the research community. Single-channel bit rates beyond 100 Gb/s have been demonstrated using single-carrier high-order coherent modulation formats (up to 448 Gb/s) [2] and multicarrier-based high-order coherent modulation formats (up to 448 Gb/s [3],[4] and 1.2 Tb/s [5]). For WDM transmission, the following have been demonstrated using polarization-division-multiplexed (PDM)-16 QAM, digital coherent detection, distributed Raman amplification, and new ultralarge-area fiber (ULAF): 50 GHz-spaced, 10 × 224 Gb/s over 12 × 100 km with SE of 4 b/s/Hz [6]; 70 GHz-spaced, 10 × 456 Gb/s over 8 × 100 km with SE of 6.1 b/s/Hz [7]; and 100 GHz-spaced, 3 × 485 Gb/s over 48 × 100 km with SE of 4 b/s/Hz [8]. Recently, 400G transmission over the standard 50 GHz WDM grid has been demonstrated using PDM-32 QAM [9]-[11]. In [10], 8 × 450 Gb/s WDM signals transmitted over 400 km of ULAF fiber and passing through one 50 GHz grid wavelength-selective switch (WSS)-based ROADM was March 2012 Vol.10 No.1 <strong>ZTE</strong> COMMUNICATIONS 03
S pecial Topic High Spectral Efficiency 400G Transmission Xiang Zhou SE (b/s/Hz) 8 7 6 5 4 3 2 1 0 0 Deployed Projected In 2011-2012 100 200 300 Transport Interface Rate (Gb/s) SE: spectral efficiency demonstrated with net SE of 8 b/s/Hz. This was the first demonstration of a 400G WDM system over the standard 50 GHz grid optical network. In [11], the transmission reach was extended to 800 km by introducing a broadband optical spectral-shaping technique to compensate for ROADM filtering effects. This is the longest transmission distance beyond 4 b/s/Hz that has been demonstrated for WDM SE. Key enabling technologies and experimental results are reviewed in the following sections. In section 2, the 450 Gb/s PDM-Nyquist-32 QAM transmitter is described. In section 3, the coherent receiver and DSP algorithms are presented. In section 4, two WDM transmission experiments and back-to-back are presented. In section 5, a summary is given. 2 450 Gb/s PDM-Nyquist 32-QAM Transmitter To overcome limited digital-to-analog converter (DAC) bandwidth, a frequency-locked five-subcarrier generation method is used to create the 450 Gb/s per-channel signal [10], [11]. Fig. 2 shows the demonstrated 450 Gb/s PDM-Nyquist-32 QAM transmitter. The ★ 400 output from a continuous-wave (CW) laser with line width of approximately 100 kHz is split by a 3 dB optical coupler (OC). One output is sent to a Mach-Zehnder modulator (MZM-1) driven by a 9.2 GHz clock in order to generate two 18.4 GHz-spaced subcarriers per channel (the two first-order signal components) that are offset from the original wavelengths by ± 9.2 GHz. After an erbium-doped fiber amplifier (EDFA) and a 12.5/25 GHz interleaver filter (ILF), the original wavelengths and second-order harmonics are suppressed by more than 40 dB relative to the first-order components (Fig. 2a). The signal is then equally split between two outputs of a 04 <strong>ZTE</strong> COMMUNICATIONS March 2012 Vol.10 No.1 ◀Figure 1. Projected demand for SE for the next-generation transport standard. Laser 9.2G Clock MZM1 VOA EDFA ILF Original 50 GHz Spaced Signal EDFA: erbium-doped fiber amplifier ILF: 12.5/25 GHz interleaver filter MZM: Mach-Zehnder modulator polarization beam splitter (PBS) prepared by a polarization controller (PC). The two subcarriers on one PBS output are sent to an IQ modulator (IQ MOD1), driven by a pre-equalized 9 Gbaud Nyquist 32-QAM signal with 2 15 - 1 pseudorandom pattern length. The Nyquist pulse shaping has roll-off factor of 0.01, and the digital Nyquist filter has a tap length of 64. Fig. 3 shows the Nyquist filter impulse response used in this experiment and the resulting eye diagram of the generated 32-QAM baseband signal in one quadrature. Frequency-domain based pre-equalization [12] is used to compensate for the band-limiting effects of the DACs, which have 3 dB bandwidths less than 5 GHz at 10 bit resolution and a 24 GSa/s sample rate. Fig. 4 shows the relative amplitude spectra of the generated 9 Gbaud Nyquist 32-QAM baseband electrical drive signals (after DACs) with and without pre-equalization. The filtering effects caused by the DACs are compensated for using frequency-domain-based digital pre-equalization. A second Mach-Zehnder modulator (MZM-2) driven by a 9.2 GHz clock is placed at the second PBS output to generate first-order signal components at 0 GHz and 18.4 GHz offsets from the original wavelength. After MZM-2, the signals pass through two 25/50 GHz interleavers to suppress the 0 GHz signal components and the unwanted harmonics (Fig. 2b). The second ILF re-inserts the original CW signal (from the second 3 dB OC output), resulting in three 18.4 GHz-spaced subcarriers from the original wavelength. These three subcarriers pass through an IQ modulator (IQ MOD2) that is driven by a second pre-equalized 9 Gbaud Nyquist 32-QAM signal with 2 15 -1 pseudorandom pattern length and originating from a second DAC. Then, the sets of two and three 45 Gb/s subcarriers are passively combined and polarization multiplexed with 20 ns relative delay. This results in a 450 Gb/s signal that occupies a spectral width of 45.8 GHz, sufficiently confined to be placed on the 50 GHz OC: optical coupler PBS: polarization beam splitter PC: polarization controller ▲Figure 2. 450 Gb/s PDM-Nyquist 32-QAM transmitter. Power (dBm) PC PBS 12.5/25 G PC Power (dBm) 0 -20 -40 -20 -40 -60 >40 dB -60 -50 -25 0 25 Freq. Offset (GHz) 9.2G Clock MZM2 >33 dB 25/50 G PC ILF ILF -80 -50 -25 0 25 Freq. Offset (GHz) (a) (b) 50 50 D/A Converters I Q IQ MOD 1 IQ MOD 2 I Q D/A Converters OC Pre-Equalized 9 Gbaud Digital Nyquist 32 QAM POL MUX VOA: variable attenuator EDFA Pre-Equalized 9 Gbaud Digital Nyquist 32 QAM
Page 1 and 2: www.zte.com.cn/magazine/English ISS
Page 3 and 4: 34 ..... Design of a Silicon-Based
Page 5: S pecial Topic Guest Editorial of 1
Page 9 and 10: S pecial Topic High Spectral Effici
Page 11 and 12: λ1 S pecial Topic High Spectral Ef
Page 13 and 14: S pecial Topic Greater than 200 Gb/
Page 21 and 22: S pecial Topic Spatial Mode-Divisio
Page 27 and 28: S pecial Topic Exploiting the Faste
Page 33 and 34: S pecial Topic Super-Receiver Desig
Page 35 and 36: S pecial Topic Super-Receiver Desig
Page 37 and 38: S pecial Topic Design of a Silicon-
Page 43 and 44: S pecial Topic The Key Technology i
Page 49 and 50: S pecial Topic Compensating for Non
Page 51 and 52: S pecial Topic Compensating for Non
Page 53 and 54: S pecial Topic 1 Tb/s Nyquist-WDM P
Page 55 and 56: S pecial Topic 1 Tb/s Nyquist-WDM P
Page 57 and 58:
R esearch Papers Hardware Architect
Page 59 and 60:
Page 61 and 62:
Page 63 and 64:
Page 65 and 66:
Page 67 and 68:
R esearch Papers A Histogram-Based
Page 69 and 70:
Page 71 and 72:
Page 73:
show all

ZTE Communications

Create successful ePaper yourself

Delete template?

Save as template?