Hi Md. Nasir Uddin
Higher the capacity lesser is the sensitivity.
Sdh radios are more sensitive than PDH as you can check them by their SENSITIVITY.
Typically PDH has a threshold of -72dbm while those for SDH is in between -67 to -69dbm.
The reason is noise floor of the radio.Due to complex modulation techniques the noise floor is greater in higher capacity radios as a result of which their sensitivity decreases.