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vide mobile broadband at much higher speeds than are available through the 3G service. LTE is relevant to PMR applications too, because there are proposals for ‘TETRA over LTE’, where LTE could provide an overlay network for high-speed data in some areas. While LTE with its OFDM

Carrier aggregation L

TE (Long Term Evolution) is fast becoming accepted as the route forwards to pro-

tion (CA) is being developed for LTE. It will operate on both the FDD and TDD formats of LTE Advanced. CA offers a way of providing

high bandwidth by concurrent use of multiple carriers. Although it adds complexity, especially to the handset, additional levels of inte- gration may be able to implement it without undue increase in costs. Using CA, several channels are

technology provides improve- ments in spectral efficiency, to achieve the LTE Advanced head- line rate of 1Gbit/s in the down- link and 500Mbit/s in the uplink, 100MHz of spectrum will be required. For the farther future, even higher bandwidths are being talked about. Te problem with these higher

bandwidths is that spectrum has become the major bottleneck for cellular and wireless systems. Some bands are only 10MHz wide and cannot accommodate the maximum LTE Advanced channel bandwidth of 20MHz. Tis has resulted from the difficul- ties in reassigning spectrum where users are already established. Even with the digital dividend being reaped in many countries, large expanses of contiguous spectrum will not become available. To overcome this difficulty, a scheme known as carrier aggrega-


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combined, and the data is sent across the wider bandwidth pro- vided. Te data must be sched- uled across the different channels, but this can be achieved by using processing at either end, with rel- evant signalling over the radio in- terface. Not only does CA provide an in-

creased data throughput, but it can also improve improved overall per- formance by using techniques such as load-balancing and interference co-ordination to improve the over- all efficiency of the network.

Aggregation formats Carriers can be aggregated in vari- ous ways, depending upon how the carriers are located and sepa- rated within the overall frequency spectrum. Te different types of carrier aggregation are important because they require different techniques to enable them to op- erate correctly (diagram, above).


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Intra-band carrier aggregation: contiguous component carriers

Band A

Intra-band carrier aggregation: non-contiguous component carriers

Band A Band B

Inter-band carrier aggregation

Band A Band B

Bundled bands: aggregating channels will enable tomorrow’s technologies to deliver large bandwidths without having to wait for huge blocks of spectrum to be cleared of all their occupants

Intra-band carrier aggrega-

tion takes place within a single band. Te carriers may be spaced in two ways, and this has an im- pact on the complexity of the handset and processing required. • Contiguous: carriers are located next to one another. From the RF viewpoint, this form of chan- nel aggregation may be seen as a single enlarged channel. Ac- cordingly only one transceiver is required within the handset to support it, although the process- ing complexity is increased over a single standard channel. From the base-station viewpoint, multi-carrier operation, even if non-aggregated, is already a re- quirement in many instances, requiring little or no change to the RF elements of the design. Software upgrades would natu- rally be required.


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Bands identified for LTE services, for FDD (left) and TDD (above)

• Non-contiguous: no longer can the multi-carrier signal be treat- ed as a single one. Two transceiv- ers are required, but this adds complexity, particularly to the handset where space, power and cost are prime considerations.

• Inter-band non-contiguous: this uses separate bands, and is likely to be needed because of band fragmentation. It demands the use of multiple transceivers within the handset, with the usual impact on cost, perform- ance and power. Additional complexities follow from the re- quirements to reduce intermod- ulation and cross-modulation from the two transceivers. Current standards allow up to

five 20MHz carriers to be aggre- gated, although in practice two or three are likely to be the limit. When carriers are aggregated,

each one is referred to as a com- ponent carrier. It may be either a primary component which car- ries the main signalling, or the secondary component carrier of which there may be one or more. With Sprint in the US plan-

ning the first LTE Advanced de- ployments for 2013, and others soon to follow, carrier aggregation will soon be in widespread use to enable the headline data through- put rates to be achieved.

LAND mobile November 2011

Ian Poole introduces an up-and-coming technique which will help LTE deliver the enormous data capacity promised by next-generation wireless data services

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