I've already stumbled a couple of times over the term "HetNet", a 3GPP work item in LTE-Advanced and an abbreviation for "Heterogeneous Networks". However, it has so far always remained somewhat of a mystery work item to me and I wasn't really sure what people where after here. But after watching this 4 minute HetNet Intro video from Ericsson it's much clearer now.
In essence, one of the aims of HetNets as explained in the video is to make the best use of the spectrum in areas where macro and pico base stations will be deployed in parallel. When a mobile is close to a pico base station it should use it, obviously, and fall back to the macro network once it leaves the coverage area or once the macro network can serve it better in terms of downlink throughput. However, there is a grey zone in which the downlink is better served from the macro network while the uplink would be better served by the pico base station. This imbalance is caused by the higher transmit power of the base station compared to the mobile and the pico cell. In the uplink, on the other hand, distance matters more due to the limited output power of the mobile device.
So one of the things required for efficient HetNets is a way to handle the uplink and downlink data transmissions of a mobile device to/from/with different base stations. Quite a tricky thing as close co-operation between the pico and macro layer is required.
And for more information on the topic, I can highly recommend this recent whitepaper from Ericsson on Heterogeneous Networks. Bring some time though to read it, this is not a 5 minute pitch.