Hello Hamza, 

There was a thread sometime ago. Check the link.

https://www.researchgate.net/post/What_is_the_physical_meaning_of_negative_group_delay_in_microwave_filters

Dear Hamza,

a negative group delay means that your filter is non causal, which is not implementable in practice.

Dear Hamza,

Please follow the link:

It is really useful and answer your question both theoretically and experimentally:http://homepages.spa.umn.edu/~sourabh/ngd.pdf

Best wishes