Why can't NRZ be used for high-speed DWDM systems?

 

In WDM systems, as the signal rate per wavelength increases from 10 Gb/s to 40 Gb/s, and then to 100 Gb/s and higher, traditional NRZ coding is no longer applicable, and new modulation methods are needed to improve spectral efficiency.

Why can't NRZ be used for high-speed DWDM?

The Fourier transform period multiplied by the angular frequency equals 2π. As the signal rate increases, the pulse period shortens, which means the spectral width increases. The spectral width of the signal increases with the signal rate. However, DWDM has limited spectral resources.

To put it simply, DWDM stands for Dense Wavelength Division Multiplexing. Each wavelength channel is like a signal path, and these paths can be wide, ranging from 200 GHz to 50 GHz.

 

 

Each person is represented by a symbol bit, carrying information in bits. NRZ means that each person carries only one bit at a time, and this bit can be either 0 or 1.

Low data rates, such as 10 Gb/s per wavelength, are like sending a message at a leisurely pace.

If a wavelength were to reach a rate of 100 Gb/s, a tenfold increase in signal speed, that would be like delivering a message by express courier on a 600-mile journey. In movies, the rider in an express courier's 600-mile mission would often say, "Make way, make way, the imperial edict has arrived..."

The faster the speed, the wider the path it needs to remain unaffected; the spectral width of the signal increases with the signal rate.

DWDM (Driving-Wide DM) is very particular about road width; the narrower each lane, the more people can be accommodated.

This creates a contradiction:

 

Elevate Optical Transmission Capacity

↓

Increase Single-Wave Rate or Densely Pack More Waves

↓

Traditional NRZ

↙ ↘

Increase single-wave rate, Densely pack more waves,
each wave carries more  each wave carries fewer
frequencies needed  frequencies acceptable

↓ ↓

Generate Spectrum Letters
(Produce frequency combs)

↓

Spectrum Slicing

↙ ↘

Reduce Guard Band** **Increase Baud Rate

↓

Reduce Channel Spacing

↓

Total Transmission Capacity Increases

 

For example, in four-phase modulation, there's an I-pole modulation on the left and a Q-pole modulation on the right. The I-pole is either 1 or 0, and the Q-pole is also either 1 or 0. This is a sign bit, carrying two bits, each bit having two states: 1 or 0. So the information carried by this person always has 4 states (4 phases). In this way, it still spreads out, without needing a wider spectral width, but the information capacity is increased.

At a higher level, the shoulder is a polarization state, the arm is a polarization state, and each shoulder carries one bit, with either a 1 or 0 state; each arm carries one bit, also with either a 1 or 0 state. This still results in a stochastic scattering, without increasing the spectral width, but it increases the total information capacity.