Carrier Aggregation in LTE Advanced

What is carrier aggregation and how this will help in improving bandwidth and throughput?

ITU set specific requirements for IMT Advanced compliant technology should fulfill. Some of these requirements include at least 40 MHz bandwidth, peak spectral efficiency of 15 bit/s/Hz in downlink and 6.75 bit/s/Hz in uplink and control plane and user plane latency of less than 100 and 10 ms respectively. Carrier aggregation is supported for both FDD and TDD.

Carrier aggregation is a LTE Advanced feature to increase aggregate bandwidth in order to improve data throughput in 4G mobile communication systems. Carrier aggregation in LTE was first introduced in 3GPP release 10 and further improvements are ongoing in Release 11 and onward. To make carrier aggregation backward compatible carriers used in this new feature are R8/R9 carriers.

  • A Rel-10 UE with reception and/or transmission capabilities for CA can simultaneously receive and/or transmit on multiple Component Carriers (CCs) corresponding to multiple serving cells.
  • A Rel-8/9 UE can receive on a single Component Carrier and transmit on a single Component Carrier corresponding to one serving cell only.

LTE Carrier Aggregation

Carrier Aggregation is supported for both contiguous and non-contiguous Component Carriers with each Component Carrier limited to a maximum of 110 Resource Blocks in the frequency domain using the Rel-8/9 numerology.

  • The number of DL Component Carriers that can be configured depends on the DL aggregation capability of the UE.
  • The number of UL Component Carriers that can be configured depends on the UL aggregation capability of the UE.
  • It is not possible to configure a UE with more UL Component Carriers than DL Component Carriers.
  • In typical TDD deployments, the number of Component Carriers and the bandwidth of each Component Carrier in UL and DL is the same.

The spacing between centre frequencies of contiguously aggregated CCs shall be a multiple of 300 kHz. This is in order to be compatible with the 100 kHz frequency raster of Rel-8/9 and at the same time preserve orthogonality of the subcarriers with 15 kHz spacing.

Carrier Aggregation in Layer 2

In case of Carrier Aggregation the multi-carrier nature of the physical layer is only exposed to the MAC layer for which one HARQ entity is required per serving cell.

In both uplink and downlink, there is one independent hybrid-ARQ entity per serving cell and one transport block is generated per TTI per serving cell in the absence of spatial multiplexing. Each transport block and its potential HARQ retransmissions are mapped to a single serving cell.

The reception timing difference at the physical layer of DL assignments and UL grants for the same TTI but from different serving cells (e.g. depending on number of control symbols, propagation and deployment scenario) does not affect MAC operation. A UE should cope with a relative propagation delay difference up to 30 microseconds among the component carriers to be aggregated in inter-band non-contiguous CA. This implies that a UE should cope with a delay spread of up to 31.3 micro seconds among the component carriers monitored at the receiver, since the BS time alignment is specified to be up to 1.3 microseconds.

Layer 2 Structure for downlink with CA configured

Layer 2 structure for downlink with carrier aggregation

Layer 2 Structure for uplink with CA configured

layer 2 structure for uplink with carrier aggregation

RRC Carrier Aggregation

When CA is configured, the UE only has one RRC connection with the network. At RRC connection establishment/reestablishment/handover, one serving cell provides the NAS mobility information (e.g. TAI), and at RRC connection reestablishment/handover, one serving cell provides the security input. This cell is referred to as the Primary Cell (PCell).

In the downlink, the carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC) while in the uplink it is the Uplink Primary Component Carrier (UL PCC).

Depending on UE capabilities, Secondary Cells (SCells) can be configured to form together with the PCell a set of serving cells. In the downlink, the carrier corresponding to an SCell is a Downlink Secondary Component Carrier (DL SCC) while in the uplink it is an Uplink Secondary Component Carrier (UL SCC).