
precoding can be omitted in laboratory experiments using pseudo-random bit
sequences (PRBS), since differential precoding does not change a PRBS [215].
The main advantage from using DPSK instead of OOK comes from a 3-dB
receiver sensitivity improvement [9, 216], which can be intuitively understood
from Figure 2.17, showing that the symbol spacing for DPSK is increased by
ffiffiffi
2
p
compared to OO K for fixed average optical power [214]. This increased
symbol distance makes DPSK accept a
ffiffiffi
2
p
larger standard deviation of the optical
field noise than OOK for equal BER, which translates into a 3-dB reduction in the
required OSNR. Dependin g on modulation waveforms, ...