Digital Signal Processing for Dual-Carrier Transmission

  • Description

    Dual-carrier transmission consists in transmitting two channels with a reduced channel spacing (Figure 1) and in detecting them using a single digital coherent receiver. The spacing between the channels (carriers) is a trade-off between the bandwidth of the receiver's ADCs and the crosstalk between the channels, which depends on the pulse shaping and the stability of the lasers. In this example, we consider the generation and the detection of a 448-Gb/s dual-carrier DP-16QAM. Digital signal processing required to recover the dual-carrier signal after propagation over up to 1000 km of SSMF-based nondispersion-managed links is performed using the DSP library toolkit.

  • Typical Results

    The dual-carrier signal is generated using two dual polarization 16QAM transmitters (28 Gbaud). Pulse shaping is performed digitally with square-root-raised-cosine FIR filters (0.15 roll-off). The lasers are assumed to be stable so that the carrier spacing remains 32 GHz during the simulated time window. Different input powers are considered in order to optimize the system performance. Each span consists of 80 km of SSMF and of an EDFA (4 dB NF) compensating exactly for the span loss. The signal is stored after each span (see Figure 2).

    The signal is received using a polarization-diversity coherent receiver including four ADCs working at 84 Gs (50 GHz bandwidth) followed by a DSP unit (Figure 3). The signal processing includes the following steps:

    • Digital resampling (4 samples per symbol)

    • Joint frequency domain CD equalization

    • Carrier separation (performed by down- or upconverting the signal)

    • Low-pass filtering (downsampling to 2 samples per symbol)

    • Carrier frequency recovery

    • Adaptive filtering (MMA MIMO) to optimize the receiver response and reduce crosstalk between carriers

    • Carrier-phase estimation (blind phase search algorithm)

    The system performance (BER) vs. input power is reported in Figure 4 for 400 and 800 km transmission length.

  • Further Information

    Keywords: 16QAM, mQAM, superchannel, DSP, dual carrier

    Similar demonstrations are available in VPItransmissionMaker Optical Systems and on the VPIphotonics Forum.

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