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8 Digital Optical Fiber Links
8.5 Higher-Order Signal Modulation Formats
The individual channel capacity of fiber communication links using on–off keying
(OOK) has steadily increased to 10-Gb/s rates per wavelength. Combined with dense
wavelength division multiplexing or DWDM technology (see Chap. 10), this has
resulted in transmission rates of 1 Tb/s and beyond per fiber. To increase the singlefiber capacity even more, different modulation formats with higher spectral efficiencies are being tested and deployed [40]. This section discusses the concept of spectral
efficiency and describes some multilevel modulation being used in high-speed highcapacity telecom links.
8.5.1 Concept of Spectral Efficiency
Spectral efficiency is a measure of how efficient an optical modulation scheme is at
using the available fiber frequency spectrum. This efficiency parameter is given as
the number of bits transmitted per second per hertz of optical frequency, that is, it is
measured in units of bits/s/Hz.
Transceivers operating at 10 Gb/s per channel with simple OOK modulation
occupy only a portion of the standard 50-GHz DWDM channel grid spacing. Thus,
much of the 50-GHz channel is unused, which results in a relatively low spectral
efficiency of 0.2 bits/s/Hz. For transceivers employing the 100-Gb/s modulation
techniques described in Sect. 8.5.4, ten times the capacity is transmitted in the same
50-GHz channel spacing, resulting in a spectral efficiency of 2 bits/s/Hz. Modulation schemes for 400-Gb/s transmission can achieve spectral efficiencies of up to
8 bits/s/Hz.
8.5.2 Phase Shift Keying or IQ Modulation
For high-speed data rates, information can be transmitted using a quadrature modulation technique. In this technique the information signal is separated into two parts,
one part is called the in-phase component (designated by I) and the other is an outof-phase component (designated by Q). This means that the in-phase component (I)
will be the real value of the signal and the out-of-phase component (Q) will be a
phase-shifted version of the signal. This method is referred to as phase-shift keying
(PSK) or IQ modulation. Therefore rather than turning the optical power on and off
as is done in OOK modulation, PSK techniques use changes of the phase of the
optical carrier to encode the data.
Differential phase shift keying (DPSK) is the simplest PSK format. DPSK carries
the information in the optical phase. Optical power appears in each DPSK bit slot
and can occupy the entire slot for NRZ-DPSK, or it can occupy part of the slot in
8 Digital Optical Fiber Links
8.5 Higher-Order Signal Modulation Formats
The individual channel capacity of fiber communication links using on–off keying
(OOK) has steadily increased to 10-Gb/s rates per wavelength. Combined with dense
wavelength division multiplexing or DWDM technology (see Chap. 10), this has
resulted in transmission rates of 1 Tb/s and beyond per fiber. To increase the singlefiber capacity even more, different modulation formats with higher spectral efficiencies are being tested and deployed [40]. This section discusses the concept of spectral
efficiency and describes some multilevel modulation being used in high-speed highcapacity telecom links.
8.5.1 Concept of Spectral Efficiency
Spectral efficiency is a measure of how efficient an optical modulation scheme is at
using the available fiber frequency spectrum. This efficiency parameter is given as
the number of bits transmitted per second per hertz of optical frequency, that is, it is
measured in units of bits/s/Hz.
Transceivers operating at 10 Gb/s per channel with simple OOK modulation
occupy only a portion of the standard 50-GHz DWDM channel grid spacing. Thus,
much of the 50-GHz channel is unused, which results in a relatively low spectral
efficiency of 0.2 bits/s/Hz. For transceivers employing the 100-Gb/s modulation
techniques described in Sect. 8.5.4, ten times the capacity is transmitted in the same
50-GHz channel spacing, resulting in a spectral efficiency of 2 bits/s/Hz. Modulation schemes for 400-Gb/s transmission can achieve spectral efficiencies of up to
8 bits/s/Hz.
8.5.2 Phase Shift Keying or IQ Modulation
For high-speed data rates, information can be transmitted using a quadrature modulation technique. In this technique the information signal is separated into two parts,
one part is called the in-phase component (designated by I) and the other is an outof-phase component (designated by Q). This means that the in-phase component (I)
will be the real value of the signal and the out-of-phase component (Q) will be a
phase-shifted version of the signal. This method is referred to as phase-shift keying
(PSK) or IQ modulation. Therefore rather than turning the optical power on and off
as is done in OOK modulation, PSK techniques use changes of the phase of the
optical carrier to encode the data.
Differential phase shift keying (DPSK) is the simplest PSK format. DPSK carries
the information in the optical phase. Optical power appears in each DPSK bit slot
and can occupy the entire slot for NRZ-DPSK, or it can occupy part of the slot in