LTE-Advanced carrier aggregation

Modes of carrier aggregation

3GPP, short for 3rd Generation Partnership Project and the standardization body behind LTE, is addressing IMT-Advanced requirements by standardizing LTE-Advanced as part of its Release 10.

LTE-Advanced comprises a set of features, while one of the most demanded feature is carrier aggregation (CA). LTE-Advanced allows the aggregation of at a maximum five component carriers up to 20 MHz of bandwidth to attain a total transmission bandwidth of up to 100 MHz. As none of the service providers owns continuous spectrum of 100 MHz three different modes of carrier aggregation exist: intra-band contiguous and non-contiguous as well as inter-band carrier aggregation. 3GPP’s RAN Working Group 4 (RAN4), responsible for setting performance requirements, initially limits the aggregation to two component carriers only.

The signaling towards an end user device for carrier aggregation affects only certain layers, not the entire protocol stack. For instance, the device is permanently connected via its PCC to the serving Primary Cell (PCell). Non-Access Stratum (NAS) functionality such as security key exchange and mobility information are provided by the PCell. All secondary component carriers are understood as additional transmission resources. For the Packet Data Convergence Protocol (PDCP) and Radio Link Control (RLC) layer carrier aggregation signaling is transparent. A terminal is configured on the Radio Resource Control (RRC) layer, to handle secondary component carriers. Moreover, on RRC the parameters of the SCell(s) are configured. The Medium Access Control (MAC) layer acts as multiplexing entity for the aggregated component carriers as they are activated or deactivated by MAC control elements. In case of activation in subframe n, than 8 subframes (8 ms) later the resources are available to the device and it can check for scheduling assignments. While the MAC acts as multiplexer, note that each component carrier has its own Physical Layer (PHY) entity, providing channel coding, HARQ, data modulation and resource mapping.

Carrier aggregation is one key enabler of LTE-Advanced to meet the IMT-Advanced requirements in terms of peak data rates. It is a highly demanded feature from a network operator perspective, since it enables also the aggregation of different spectrum fragments. However, the major design challenge is on terminal side. Support of higher bandwidths and aggregation of carriers in different frequency bands increase complexity of transceiver circuits, including component design like wideband power amplifiers, highly efficient switches and tunable antenna elements. Further the additional functionality provided to PHY/MAC layer and the adaptations to the RRC layer need to be thoroughly tested. Rohde & Schwarz offers a multitude of solutions for testing LTE-Advanced carrier aggregation implementations. The broad offering includes signal generators and analyzers for performing physical layer tests on base stations, mobile devices or components as well as base station emulators for physical layer and protocol tests for all kind of wireless devices and chipsets.

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