PAM is not very good for long-distance transmission. Why is it so?

[McMurry]

Homework Statement

Just came across the statement that PAM is not very good for long-distance transmission.
Why is it so?
Thanks.

Homework Equations

The Attempt at a Solution

Is it because the output is of infinite length?

 

[xcvxcvvc]

what the heck is pam and what class is this for? (is it for applied electromagnetics with an intro to transmission line theory?)

 

[McMurry]

what the heck is pam and what class is this for? (is it for applied electromagnetics with an intro to transmission line theory?)

Ops.

PAM = pulse amplitude modulation
for digital communication.