Inside Track : Carrier Aggregation in LTE

Carrier aggregation is one of the most distinct features of 4G systems including LTE-Advanced, which is being standardized in 3GPP as part of LTE Release 10. This feature allows scalable expansion of effective bandwidth delivered to a user terminal through concurrent utilization of radio resources across multiple carriers. These carriers may be of different bandwidths, and may be in the same or different bands to provide maximum flexibility in utilizing the scarce radio spectrum available to operators.

The key to achieving higher data rates with LTE is to enable network operators to use the technology in bandwidths wider than 20MHz. Some network operators may be lucky enough to have contiguous spectrum allocations of more than 20MHz. However, the nature of spectrum allocation over the years is such that most operators have a mix and match of spectrum within and between frequency bands. Following the redistribution of analogue TV spectrum and the provision of higher frequency spectrum, an operator might have LTE spectrum at one or more of 700MHz, 800MHz, 900MHz, 1800MHz, 1900MHz, 2100MHz, 2500MHz and 2600MHz.

Recently Telekom Austria, conducted a successful demonstration of LTE-Advanced carrier aggregation .The live demo, which included handling large file transfers with a simultaneous video stream, showcased download speeds of 580 megabits per second (Mbps), far more than twice the current 4G LTE peak rates. In combination with carrier aggregation, Telekom Austria plan to turn LTE-Advanced into a Gigabit technology that will allow a large number of users to simultaneously access high data rates within one mobile radio cell.

The U.S. market is one of the main drivers for deployment of Carrier Aggregation since the frequency spectrum is a scarce resource and very fragmented with few operators having contiguous 20 MHz spectrum generally available. The LTE Carrier Aggregation roll out will start during 2013 and we expect significant growth 2014. U.S., Korean and Japanese operators already have firm deployment plans for Carrier Aggregation. In the first phase, up to 20MHz of spectrum will be aggregated enabling subscribers to enjoy up to 150Mbps downlink data throughput, or even higher in the future.

CA (Carrier Aggregation) may be used in three different spectrum scenarios :

Intraband Contiguous CA — This is where a contiguous bandwidth wider than 20 MHz is used for CA. Although this may be a less likely scenario given frequency allocations today, it can be common when new spectrum bands like 3.5 GHz are allocated in the future in various parts of the world. The spacing between center frequencies of contiguously aggregated CCs (Component Carriers) is a multiple of 300 kHz to be compatible with the 100 kHz frequency raster of Release 8/9 and preserving orthogonality of the subcarriers with 15 kHz spacing.

Intraband Non-Contiguous CA — This is where multiple CCs belonging to the same band are used in a non-contiguous manner. This scenario can be expected in countries where spectrum allocation is non-contiguous within a single band, when the middle carriers are loaded with other users, or when network sharing is considered.

Interband Non-Contiguous CA — This is where multiple CCs belonging to different bands (e.g., 2 GHz and 800 MHz are aggregated). With this type of aggregation, mobility robustness can potentially be improved by exploiting different radio propagation characteristics of different bands. This form of CA may also require additional complexity in the radio frequency (RF) front-end of UE. In LTE Release 10, for the UL the focus is on intraband CA, due to difficulties in defining RF requirements for simultaneous transmission on multiple CCs with large frequency separation, considering realistic device linearity. For the DL, however, both intra and interband cases are considered in Release 10, while specific RF requirements are being developed.

Whereas GSM made use of only four frequency bands globally and WCDMA/HSPA requires five bands to get global coverage, LTE is deployed on more than ten bands already as of December 2012, and the number of bands will still increase. The frequency band fragmentation is a result of regional decisions on how the spectrum shall be utilized. Of course, there are initiatives to define some bands as global roaming bands. Another big challenge for Carrier Aggregation is to develop a RF front-end flexible enough to support a majority of domestic, and international, bands together with possible future Carrier Aggregation combinations

LTE Carrier Aggregation will provide mobile network operators with even greater scope to support services that hitherto would have been restricted to fixed networks, and may open up the possibility of providing a viable alternative to fixed network broadband services, particularly in rural locations where fixed broadband provision may be poor. Which is why CA is extremely important for Africa.

At the current rate of LTE Spectrum allocations it is most unlikely that the long suffering African broadband consumer will ever experience the high speed LT. By the time Regulators release the Digital Dividend spectrum in Africa ( 2015 – 2018 ), LTE Advanced will be a globally standardised and mature technology. This is the silver lining, believe it or not , since African Telcos will have the opportunity to launch LTE Advanced with all its speed benefits and refinements like Carrier Aggregation by the time they get some Spectrum.

Sadiq Malik ( Telco Strategist )


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