R&D and marketing are usually a couple of steps ahead of what's really deployed in live LTE networks so I thought I'd write a post today and the state of the art I can currently see in live networks and relate that to a number of recent press releases about new features and what I think we will realistically see next in networks.
From what I can see, the state of the art in live LTE networks is a maximum theoretical downlink speed of 300 Mbit/s. In some countries like Germany, this is achieved by bundling two 20 MHz downlink carriers for a combined 40 MHz channel. According to this post over at Gigaohm a network operator in Korea achieves similar speeds by combining 3 carriers, one with 20 MHz and two with 10 MHz. So while that requires more sophisticated devices the end result is the same. Network operators in the US probably do carrier aggregation as well these days but since they mostly use 10 MHz channels, with some exceptions, they are not quite reaching those theoretical speeds yet.
So, what are the next steps? Like in Korea, network operators in other parts of the world also have spectrum in more than two frequency bands so it is likely that 3 carrier aggregation (3CA) will be used in other parts of the world to go beyond the theoretical 300 MBit/s, soon. In Europe for example, some network operators could bundle resources in the 800, 900, 1800, 2100 and 2600 MHz band as recent spectrum auctions have left them with ample resources for now to expand their services in the frequency domain. Bundling 3 carriers with 10, 20 and 20 MHz would result in a theoretical top speed of 375 Mbit/s and should somebody be lucky enough to have three 20 MHz carriers, it would result in 450 Mbit/s.
On the mobile device side, most devices sold today are not carrier aggregation capable. This is still the domain of the high end devices such as the Galaxy S6. This is likely to change, however, and I expect that in 18 months from now, 2CA will be the status quo.
In the meantime, high end devices will have gone to 3CA. Qualcomm has recently announced an LTE Category 12 downlink modem that can bundle 3 carriers in downlink which also supports 256QAM modulation for an additional speed boost to the 64QAM currently used close to the base station. Instead of 450 Mbit/s, a theoretical top speed of 600 Mbit/s is thus possible when combining three 20 MHz carriers. Few network operators, however, are likely to have three carriers with such a bandwidth available. Also, 256QAM is only reachable very very close to the base station so I wouldn't count on it making a lot of a difference in real live. For me the most interesting part of the announcement is that the chipset will also support LTE Carrier Aggregation in the Uplink direction. So far, Uplink speeds even in networks using 20 MHz carriers are limited to 16QAM and a single carrier which results in a theoretical peak of 50 Mbit/s. The new Qualcomm modem will support 64QAM in uplink direction and bundling two 20 MHz carriers for a theoretical uplink speed of 150 Mbit/s, which makes it a LTE category 13 uplink device.
While the numbers are certainly impressive, I don't think I need 600 Mbit/s in the downlink on a mobile device anytime soon. As usage of networks increases, however, the main benefit I see of the various advances in LTE carrier aggregation is that more advanced devices will be able to send and receive data over a broader channel, which is important as networks are filling up. It would of course also be possible to make non-carrier aggregation devices hand-over to other frequency bands if a carrier is already highly loaded. However, measuring the availability of other carriers is a non trivial task as well so going the full mile isn't much of an additional trouble.