Carrier Aggregation

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4G Carrier Aggregation (CA) is a technique used in LTE-Advanced (LTE-A) to increase the peak data rate (i.e., maximum available speed) of a 4G LTE network. It is one of the key techniques used to enable the very high data rates of 4G to be achieved. By combining more than one carrier together, either in the same or different bands it is possible to increase the bandwidth available and in this way increase the capacity of the link.

With radio spectrum a rare commodity, Carrier aggregation has also helped to utilize smaller spectrum allotments which only support smaller channel widths and where traditional LTE would lead to very low peak data rates. Carrier aggregation is supported by both formats of LTE, namely the FDD and TDD variants. This ensures that both FDD LTE and TDD LTE are able to meet the high data throughput requirements placed upon them.

By aggregating multiple channels together a mobile network operator can increase the total available bandwidth of a single transmission, and thereby increase the bit rate and capacity of the network. Carrier aggregation requires many different features to be implemented to enable it to operate effectively, as the two channels are likely to have very different characteristics, especially if they are on different bands.

Before LTE-A, a mobile network operator could only use additional LTE bands to increase capacity (i.e., total number of users), by distributing traffic across multiple bands. While this is an effective way to provide more airtime to any one particular user and thereby improving available data rates during busy periods, it does not increase the peak data rate. 

The target figures for data throughput in the downlink is 1 Gbps for 4G LTE Advanced. Even with the improvements in spectral efficiency it is not possible to provide the required headline data throughput rates within the maximum 20 MHz channel. The only way to achieve the higher data rates is to increase the overall bandwidth used. IMT Advanced sets the upper limit at 100 MHz, but with an expectation of 40 MHz being used for minimum performance. For the future it is possible the top limit of 100 MHz could be extended. 

It is well understood that spectrum is a valuable commodity, and it takes time to re-assign it from one use to another in view - the cost of forcing users to move is huge as new equipment needs to be bought. Accordingly as sections of the spectrum fall out of use, they can be re-assigned. This leads to significant levels of fragmentation.

Wireless network providers all over the world use cellular frequency bands to deliver 4G LTE voice and data services to their customers. Because your mobile device is compatible with your carrier’s frequency bands, this technology enables you to make phone calls, send and receive text messages, and access the internet. 

The bands used by mobile carriers are numbered sequentially and represent “blocks” of a frequency range. LTE supports different channel bandwidths and as a result a different number of resource blocks can be supported. A band can be shared by several carriers, or it may be assigned exclusively to one cellular carrier. In addition, a frequency range may be comprised of multiple bands; for instance, 700 MHz includes LTE bands 12, 13, 17 (and more) used in the U.S. and Canada. 

To adequately meet customers’ different needs, cell phone carriers have several bands from high to low frequencies. For example, AT&T offers a range of frequencies from 2300 MHz, which is designed for densely populated urban areas; to 700 MHz, which performs better over long distances in rural areas. A variety of frequencies and bands are needed so each carrier can provide the best cellular coverage and capacity to all of their mobile subscribers.