Single login for multiple domain

I have multiple website, some are host in sub-domain and some are host in other(cross-domain) domain . now the my requirement is if user is registered in abc.com(main-site) ,he can login in my all websites without going to another website login page so i want only 1 login page for every websites , my all websites build in wordpress and i used user synchronization plugin for user data sync , with the help of this plugin data of wp_users,and wp_usersmeta data is same in all the database ,like username ,password and email etc.. is same .