carrier aggregation

carrier aggregation with two different cells.

In a 4G-LTE environment the available bandwidth is split into channels, or component carriers. The standard sizes of these are 1.4, 3, 5, 10, 15 or 20MHz. The operator will split their available bandwith into these component channels and a user will have that channel available - so 20MHz is the maximum available. To get more space, or indeed use the narrower channels better, some way to combine these component carriers will be needed.

To increase the data rate for the given user there are only two options namely, widen the channel or combine channels. This latter option is available in 4G+ and called carrier aggregation. This allows the various channels to be combined up to a maximum of 5 channels. So there is a theoretical maximum channel size of 100MHz. This is theoretical as the MNOs have only 35MHz blocks of bandwidth available to the 2.6GHz 4G network.

• Intra-Band Contiguous Channel - Allow the user access to the space at one side of the current allocation. So for example, if the user has a 5MHz channel, they can use one of the adjoining 5MHz channels to get a single 10MHz channel. In addition there is no requirement for space between the two channels so effectively a single 10MHz channel. This is called channel bonding.
• Intra-Band Non-Contiguous Channel - add another channel within the same bandwidth for the user's data transfer. This effectively doubles the space available to that user. But as these channels are not contiguous there is some overhead in delivering the data this way. The MNO has to manage the data between the two separate blocks. Additionally each channel will have some "dead" space to provide separation between the different channels.
• Inter-Band Channel - It is also possible to link one channel in one band with another in a second band. So the user will be using two different band to send or receive data. To achieve this there will be two (or more) serving cells for the data transmission. The data will take a different path. One of the cells is labelled the primary cell and it is used to manage the data from the other secondary cells. Whilst this may cause some overhead it does allow the MNO to share traffic between all their available bandwidth.

Note that different carrier aggregation can be implemented on the up and down links min order to make best use of the available bandwidth

With these basics quite complex carrier aggregation can be used. For the operator a large contiguous bandwidth makes channel aggregation easier and more efficient. Currently the 4G and 5G bandwidth allocated to operators is small and spread over various bands making carrier aggregation both difficult and necessary. It is generally accepted that a contiguous allocation of 100MHz of bandwith would make the transmission of data more efficient. THREE, with their purchase of UKBroadband managed to acquire a significant amount of contiguous bandwidth for 5G thus making their bandwidth management through carrier aggregation simpler. OfCom is promoting some refactoring of broadband in the UK to give more contiguous space.