Nokia has recently sold 450 patents to Sisvel, an italian Intellectual property manager. Of those, 350 patents spanning across 33 families encapsulate key wireless technologies like GSM, UMTS/WCDMA, and LTE. Although Nokia nor Sisvel are going into details of the deal, it's expected that Nokia will retain the rights to use these patents, and likely collect on some of the ongoing royalties, but Sisvel will likely get a cut of royalties in exchange for actively protecting the patents in the courtroom.
It might seem natural that Nokia hold onto these patents with an iron fist and squeeze every penny-worth of royalties out of them, but legal costs can add up; by incentivizing an outside source to do the trolling while possibly still getting some of the royalties themselves, Nokia will be able to focus their resources (both legal and monetary) on Windows Phone. It will be interesting to see how aggressive Sisvel is in protecting the patents, and if they're able to lock down any big licensing deals that Nokia wasn't able to. The last big case Nokia won was against Apple, so I have a hard time imagining that Nokia has left any stone unturned.