Lightwave Logic Inc (LWLG)

[Photonics_Guy]

Difference between NRZ and PAM4 Modulation Schemes

I'm sure many of your eyeballs roll back when terms like NRZ and PAM4 modulation are mentioned. Here is my attempt to take this out of the tinfoil hat arena to a layman's perspective.

What is modulation and why do we need it?

Modulation: Modulation is a process through which audio, video, image or text information is added to an electrical or optical carrier signal to be transmitted over a telecommunication or electronic medium. Modulation enables the transfer of information on an electrical signal to a receiving device that demodulates the signal to extract the blended information.

What does NRZ modulation mean?

NRZ Modulation: "Non-Return-To-Zero" modulation has two logic levels. A time domain representation of this signal format is shown along with its associated eye diagram. NRZ (aka PAM2) modulation encodes one digital bit per clock cycle. The signal levels are either fully on or fully off. Hence, it has superior noise immunity as compared to other modulation schemes.



What does PAM4 modulation mean?

PAM4 Modulation: "4-Level Pulse-Amplitude" modulation has four logic levels. A time domain representation of this signal format is shown along with its associated eye diagram. PAM4 modulation encodes two digital bits per clock cycle. As it has four distinct amplitude levels, it is more sensitive to amplitude noise that may confuse the detector and give erroneous bits (increased bit error rate or BER) as compared to NRZ.




What are the basic components of a Photonics Transmission Link?

A basic photonics transmission link used to encode a modulation protocol, send the modulated signal over a transmission line, and receive/detect/transform the signal to a demodulated signal as shown below:



What does a digitally modulated signal look like on a photonics transmission link?

A photonics transmission line used to transmit digital information encodes a digital waveform on a optical laser signal (continuous wave) by amplitude modulating (AM modulation) the laser signal, which is then sent over a fiber optic cable to a remote detector. The detector then recovers the 'envelope' of the modulated waveform and transforms it into a demodulated RF signal as shown below.