Some people do exclusively use the term "patent troll" for non-practicing entities, and pretty much everyone would agree that they are patent trolls, but there's plenty of companies that do practice in a field and use patents to prevent competition, even for obvious implementations of old technology, and many call those patent trolls, too.

Signal extraction technology has been used in pulse oximiters since the 90's, and the patents that covered their innovation have long expired, with new patents covering descriptions of using the old technology with modern devices, which is innevitable, not innovative.