Polarization-division-multiplexed QPSK versus polarization-switched QPSK

  • Description

    Using the four dimensions of the optical field (phase and quadrature components of the orthogonal polarization states) to encode and detect a signal enables to design modulation formats that present a better sensitivity than modulation formats using classical two-dimensional constellations (QAM) combined with polarization division multiplexing (PDM) [1].
    Such 4D constellations can be generated using classical polarization- multiplexed IQ modulators combined to high-speed digital-to-analog converters (DACs) that are required to generate the multilevel electrical driving signals. However, for simple constellations the use of DACs can be omitted. For instance, this is the case of polarization-switched QPSK (PS QPSK) and its polarization-division-multiplexed counterpart PDM QPSK, which makes these formats attractive for high-speed low-cost solutions. The purpose of this example is to compare the noise performance of polarization-switched QPSK and polarization-division-multiplexed QPSK.

  • Typical Results

    The performance of polarization-switched QPSK and polarization-division-multiplexed QPSK is compared for a back-to-back configuration, where noise loading is performed in front of the receiver (Figure 1). The constellation diagrams of 24-Gbaud polarization-switched QPSK and polarization-division-multiplexed QPSK are displayed after phase and frequency recovery in Figure 2 and Figure 3 for a 20-dB OSNR.
    At equal baud rate (24 Gbaud), PS QPSK presents a better performance than PDM QPSK (see Figure 4), but enables the coding of 3 bits per symbol only compared to 4 bits per symbol for PDM QPSK. For a fair comparison, performance at equal bit rates should be considered. For this, two approaches are possible: either increasing the baud rate of the PS-QPSK signal to 40 Gbaud or keeping the same baud rate for both formats, but using the additional bit available on the PDM-QPSK format to perform forward error correction.
    At an equal raw bit rate, polarization-switched QPSK presents a slightly better performance than PDM QPSK (see Figure 4). However, when using a simple Hamming (15-11) code with 27% overhead, the performance of both formats becomes similar. When considering a more efficient code, such as a LDPC (4161-437) code with 17% overhead combined with the MINSUM detection, the performance of coded PDM QPSK outperforms the one of uncoded PS QPSK [2] (see Figure 5).

  • Further Information

    Keywords: Polarization Division Multiplexing (PDM), Quadrature Phase-Shift Keying (QPSK), Polarization-switched (PS), PDM QPSK, DP QPSK, PS QPSK, coded modulation, 4D modulation, FEC, BER

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

    [1] M. Karlsson, E. Agrell, Power -Efficient Modulation Schemes, Chap. 5 in Impact of Nonlinearities on Fiber Optic Communications, Editors: Shiva Kumar, Springer, 2011
    [2] B. Krongold et al. 'Comparison Between PS-QPSK and PDM-QPSK With Equal Rate and Bandwidth', IEEE Photonics Technology Letters, Vol. 24, No. 3, February 1, 2012, pp. 203-205

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