Comparison Between PS-QPSK and PDM-QPSK With Equal Rate and Bandwidth

Polarization-switched quadrature phase-shift keying (PS-QPSK) is a recently introduced 4-D signal constellation that has a power efficiency (and sensitivity) advantage over polarization-division multiplexed QPSK (PDM-QPSK). Using a polarization-rotated form of PS-QPSK, we show that it is equivalent to a short block code, and assert that basic information theory explains its advantage over PDM-QPSK. We consider the practicality of PS-QPSK by considering its performance after a forward error correction (FEC) is applied, as this is the true measure of a modulation format's power efficiency. When fairly compared with the same data rate, bandwidth, and transmit power, simulations show that PS-QPSK is actually not as efficient as PDM-QPSK when a sufficient FEC is applied to achieve the very low BERs required in practice.