This is because of the reduction of the minimal distance between two points of the constellation, which reduces the resilience to channel impairments. For instance, going from a PDM-QPSK up to a PDM-16QAM transmission doubles the data rate at the cost of an optical reach divided by a factor of 5.
Modulation is the process of encoding information onto a carrier signal, which is then transmitted over a communication channel. The choice of modulation scheme can have a significant impact on the reach of a system, which refers to the maximum distance over which a signal can be transmitted and still be reliably received.
There are several ways in which changing modulation can improve the reach of a system:
Increased spectral efficiency:
Modulation schemes that allow for higher data rates within the same bandwidth can improve the reach of a system by enabling more data to be transmitted over the same distance.
This is achieved by using more complex modulation schemes that allow for more bits to be transmitted per symbol.
Improved resistance to noise:
Some modulation schemes, such as frequency-shift keying (FSK) and phase-shift keying (PSK), are more robust to noise and interference than others.
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