Thursday, 20 October 2016

Carrier Aggregation (CA) and Dual Carrier (DC) enhancements in Release-13

Recently I posted a summary whitepaper of 3GPP Release-13 by 5G Americas. This article from NTT Docomo technical journal complements that nicely and provides in depth analysis of selected features.

The article (embedded below) focuses on Carrier Aggregation (CA),Dual Carrier (DC) enhancements, LAA and LWA. In this post, I am going to restrict the discussion to CA and DC.

The following is from the magazine article:

Carrier Aggregation (CA):

Up to Release 12 CA, a maximum of 5 LTE carriers called “Component Carriers” (CCs) could be configured for a User Equipment (UE). This enables a maximum 100 MHz bandwidth for data communications, which achieves a theoretical peak data rates of approximately 4 Gbps, assuming eight Multiple Input Multiple Output (MIMO) layers and 256 Quadrature Amplitude Modulation (QAM) for downlink, and 1.5 Gbps assuming four MIMO layers and 64QAM for uplink.

In Release 13, the maximum number number of CCs that can be configured for a UE simultaneously was increased to 32 to archive higher data transmission rates with wider bandwidths. This enables a maximum 640-MHz bandwidth for data transmission, achieving peak data rates of approximately 25 Gbps for downlink with 8 MIMO layers and 256QAM, and 9.6 Gbps for uplink with 4 MIMO layers and 64QAM.
Release 13 introduced the new function to enable PUCCH configuration for a Secondary Cell (SCell) in addition to the PCell in uplink CA. When CA is performed with this function, CCs are grouped together either with the PCell or SCell with PUCCH (PUCCH-SCell). UE sends UCI for CCs within each group by using the PCell or PUCCHSCell. With this new function, uplink radio resource shortages can be resolved by offloading UCI from macro cell to the small cells while keeping the macro cell as the PCell.

Dual Carrier (DC):

Release 12 designed DC to achieve user throughput comparable with that of CA by aggregating multiple CCs across two eNBs. In release 13, DC was further enhanced with higher uplink throughput and more flexible deployment.

In DC, separate eNBs allocate uplink resources independently for a UE. Hence, Release 13 addresses how to allocate adequate uplink resources on multiple CCs for UE. Typically, eNB calculates the required uplink resources based on the uplink buffer amount reported from UE. In DC, since both eNBs calculate the amount of uplink resources based on the report and allocate them to the UE independently, excess uplink resource allocation over actual amount of remaining data will occur. In particular, with small data packets, if resources are allocated by both eNBs, the UE may send all data to only one of them, and send padding (meaningless bit strings) to the other eNB, which wastes radio resources.

To prevent the excess uplink resource allocation for the small data packets described above, new uplink transmission control methods were introduced. In Release 13 DC, UE buffer status reporting and uplink data transmission are controlled based on the amount of uplink data buffered in the UE.

If the amount of the buffered data is smaller than the threshold configured by the eNB, the UE performs buffer status reporting and uplink data transmission only to one of the eNBs, just like DC in Release 12. In contrast, if the amount of the buffered data is larger than the threshold, the UE transmits to both eNBs. This buffer size-based mechanism solves the uplink resource over-allocation problem since only one eNB is aware of the buffered data and allocates resources when the amount of the buffered data is small.

The paper is embedded as follows:

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