What is DMT?

 

The optical module white paper mentions a DMT format:

 

5G mid-transmission optical module types

Line Rate	Module Type	Distance	Wavelength	Modulation Format	Optical Chip / Detector
25 Gb/s	SFP28	40 km	1310 nm	NRZ	EML + APD
	QSFP28 SFP56	10 km	1310 nm	PAM4	DFB + PIN / EML + PIN
50 Gb/s	QSFP28 BiDi	10 km	1270 / 1330 nm	PAM4	DFB + PIN / EML + PIN
	QSFP28 SFP56	40 km	1310 nm	PAM4	EML + APD
	QSFP28 BiDi	40 km	1295.56 nm / 1309.14 nm	PAM4	EML + APD
100 Gb/s	QSFP28	10 km	CWDM / LWDM	NRZ	DFB + PIN / EML + PIN
	QSFP28	40 km	LWDM	NRZ	EML + APD
	QSFP28	10 / 20 km	DWDM	PAM4, DMT	EML + PIN
100/200/400G	CFP2 – DCO	80 ~ 1200 km	DWDM	PM QPSK, 8/16QAM	IC – TROSA – ITLA
200/400G	QSFP-DD / OSFP	2 / 10 km	LWDM	PAM4	EML + PIN

 

We know about basic modulation formats like NRZ and PAM4. What is the concept of DMT? 

DMT, discrete multitone.

Tone, in essence, is frequency. When my daughter plays the guzheng, I tune it for her by checking if the frequency at which each string resonates with the cavity when plucked matches the standard frequency. A pitch meter is essentially a frequency detector.

 

 

Multi-tone refers to modulation across multiple frequency bands, meaning multiple subcarriers. Typically, it's assumed that each subcarrier uses the same modulation format.

Discrete multi-tone means that each subcarrier can be modulated independently, without strict adherence to the same format.

Why? Because attenuation is greater at higher frequencies than at lower frequencies, so choosing the appropriate modulation format makes signal transmission easier.

Our NRZ modulation is like a single string being played as fast as possible.

Multi-tone modulation is like a single instrument (or optical fiber) where each string can transmit a 1010 modulation signal. Furthermore, because of the different pitches, the receiver filters the signals through narrowband frequency filtering, ensuring that the demodulation is independent of each other.

Therefore, multi-tone modulation can achieve a higher bit transmission rate with very low bandwidth.

Discrete multi-tone modulation (DMT) can combine the characteristics of different tones and select discrete modulation formats of various types.