The use of two quadrature phase shift keying (QPSK)-modulated subcarriers over a wideband direct detection optical communications system with a RF bandwidth efficiency of 1 b/s/Hz is reported. It is shown that two subcarriers each carrying 2 Gb/s QPSK, compactly spaced at just twice the data rate, can achieve a receiver sensitivity of -19.5 dBm over 48 km without crosstalk between subcarriers. This represents a receiver sensitivity improvement of 5 dB over a 4-Gb/s single QPSK subcarrier system.>
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Hill et al. (1990) studied this question.
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