Back in July 2014 I wrote a post with a link over to Anandtech with a great introduction to the components and chips that sit in front of the baseband modem chip of a modern smartphone. Now the team over at Anandtech has greatly expanded on this and have written another article on the topic.
If you are interested but new to the topic it's worth to read the older post first as it includes a great diagram that shows how the components are connected together and to then continue with their latest post.
And once you've done that it's worth having a look at the tear-down of the iPhone 6 over at iFixit where they show how most of the components discussed in the posts above are used in this device. By my standards, the iPhone 6 has the most advanced modem front end to date as it supports 20 LTE bands, more than any other device currently on the market I'm aware of.