The present document relates to optical transmission
systems. In particular, the present document relates
to a transmitter and/or a receiver for optical polarization
switches (PS) QPSK (quadrature phase shift keying)
signals. An apparatus (500) for an optical transmitter
of an optical polarization switched M-PSK signal comprising
a first and a second polarization, with M = 2m and
m≥ 1 is described. The apparatus (500) comprises mapping
means (200) configured to map, at a time instant
k=1,...,K, m information bits to an M-PSK constellation
point, thereby generating a first symbol at time instant k
for the first polarization; assign, at the time instant k, one
further information bit by generating a second symbol at
time instant k for the second polarization, wherein the
second symbol at time instant k corresponds to the first
symbol at time instant k, or corresponds to the constellation
point opposite the constellation point of the first
symbol at time instant k; and repeat the mapping and
assigning steps for all k=1,...,K to generate a sequence
of first and second symbols for the first and second polarization,
respectively. The apparatus (500) further comprises
decorrelation means (501, 502) configured to
decorrelate the first symbol at time instant k and the second
symbol at time instant k, for all k=1,...,K