Carrier aggregation (CA) in LTE

Background 

The demand for higher peak and higher average throughput in mobile devices has always existed. Services like YouTube, video calls and live streaming require high data speeds. In order to meet the current throughput demands and increase LTE bandwidth, a very promising feature has been introduced in LTE Advanced (3GPP rel 10 onwards)  known as carrier aggregation.

Introduction

• CA is a cost effective way for operators to utilize their fragmented spectrum spreaded across different or same bands in order to improve end user experience
• The basic idea of carrier aggregation is to increase user throughput by sending data simultaneously over two carriers
• Regular cell known as primary cell (PCell) is combined/aggregated with logical cell (known as Secondary cell or SCell), serving the same cell site. Each aggregated carrier is known as component carrier, CC
• Example: Operator A has a LTE network deployed using its 5MHz spectrum in band 3. Maximum throughput available in any cell with 2x2 MIMO is 36.2 Mbps. Operator A also has 5 MHz unused spectrum in band 5. Now the throughput can be doubled by combining both 5 MHz spectrums from two different bands using carrier aggregation 

• The PCell is the main carrier with which UE will communicate i.e. RRC/NAS messages exchange, measurement, RACH etc. PCell always remains active in RRC Connected mode while SCell is activated/deactivated whenever required e.g. when high throughput is required
• PCell has PDCCH in downlink and PUCCH in uplink but SCell has only PDCCH in downlink
• 'RRC Connection Reconfiguration' procedure is used to add/remove SCell
• As per 3GPP standardization, maximum of five component carrier can be aggregated with each component carrier having bandwidth of 1.4, 3, 5, 10, 15 or 20 MHz. The maximum possible aggregated bandwidth can be 100 MHz (20 x 5)

• CA aggregation is available for both FDD and TDD and the feature is backward compatible which means that the users from rel8 and rel9 can connect to CA capable site by utilizing part of total bandwidth
• The commercial deployment of carrier aggregation has already started since 2013

Three are three types of carrier aggregation

Intra-Band Contiguous CA

When two or more component carriers belong to same frequency band and they are contiguous. There must be spacing of 300 khz x N between two contiguous component carriers (N is integer). This is the simplest form of CA aggregation from operators perspective

Intra-Band Non-Contiguous CA

When two or more component carriers belong to same frequency band but they are separated by one or more frequency gaps

Inter-Band Non-Contiguous CA

When two or more component carriers belong to different frequency bands.
This type of CA is implemented by operators who own fragmented spectrum