Very high sensitivities can be reached employing 2-PSK based binary sequences for the coherent modulation of an optical carrier. Homodyne detection can be employed, attaining an improvement of 3 dB with respect to usual ultimate sensitivity limits calculated for M-FSK transmission with heterodyne detection, when M tends to infinity. Considering a real receiver, a transmission efficiency as high as 0.4 bit/photon can be reached, at the expense of a bandwidth expansion factor smaller than 6.
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