LTE networks are up and running for five years now and we have certainly come a long way in terms of speed, stability and usable devices since 2009. The next step in the race for ever faster speeds is Carrier Aggregation (CA), i.e. the simultaneous use of several LTE carriers in different bands. While a lot has been specified to have a lot of flexibility in practice I mainly see the following CA deployments in the field today:
South Korea and the US seem to be the countries with the most pressing need for CA as for various reasons they are limited to 10 MHz carriers. Verizon for example has thus started deploying carrier aggregation of two 10 MHz carrier, one in the 700 MHz band and one in the 1700/2100 AWS for a combined bandwidth of 20 MHz.
In Europe, Germany seems to be the country most interested in Carrier Aggregation. Here, operators already have 20 MHz carriers on air in the 1800 MHz and 2600 MHz band (bands 3 and 7). In addition, three operators have a 10 MHz carrier in the 800 MHz band (band 20). In other words, they use carrier aggregation to go beyond the 20 MHz they already have. One network operator combines spectrum in their 800 MHz and 2600 MHz for a total carrier bandwidth in the downlink direction of 30 MHz. Another operator is about to aggregate resources in the 1800 MHz and 2600 MHz band for a total of 40 MHz, i.e. twice the bandwidth that is aggregated by Verizon in the US.
So far, only few devices support Carrier Aggregation but by the end of 2014 I expect that it will be quite a handful so from my point of view this is the state of the art in deployed networks at the moment. Looking a bit into the future there are a couple of further enhancements in the pipeline. On the one hand, data transmission rates could be increased by using more than the two antennas on the base station and mobile device side. 4x4 MIMO has been trialed already but the difficulty is how to get more than the 2 antennas per sector on rooftops without increasing the size and weight of the antennas unduly. On the mobile device side there's a similar dilemma, perhaps not so much in weight but in available space for even more antennas. Time will tell. And also a bit further down the road is carrier aggregation with three independent component carriers. 3GPP has just recently standardized the new device categories 9 and 10 for the purpose with a theoretical maximum downlink speed of 450 Mbit/s (20 MHz = 150 Mbit/s, 40 MHz = 300 Mbit/s, 60 MHz = 450 Mbit/s). This whitepaper by Nomor research contains some interesting details on this.