BW 7.3 upgrade from 7.0

Hi Team,

Our client is planning to upgrade BW 7.3 from 7.0.

We have source systems ECC 6.0, SRM 701 and HCM 6.0 connected to the BW.

Do we have to upgrade source systems to some level to support BW 7.3?

Can you please provide me link or details or note that gives minimum release level for the source systems to connect with BW 7.3.

I tried search on varies documents but no luck. Really appreciate your help.

Thanks and Regards

Karth