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Dual connectivity DC and carrier aggregation CA

3GPP Rel. 15 & 16 Enhancements in 5G NSA Dual Connectivity (DC) and Carrier Aggregation (CA)

10/21/2022 | Icon feather-clock 8 minutes

Dual connectivity DC and carrier aggregation CA can be simultaneously combined, configured in LTE and 5G to allow a UE to utilize multiple sectors.
However the connectivity delay is always an issue in cases of mMTC massive IoT or eMBB services. 3GPP in rel 15 and 16 allows for some enhancements to speed up the CA and DC connectivity with early idle mode measurement configuration to UE.
Following document explains the procedure.

Carrier Aggregation (CA) and Dual Connectivity (DC) overview

Dual connectivity DC and carrier aggregation CA can be simultaneously combined, configured in LTE and 5G to allow a UE to utilize multiple sectors.
However the connectivity delay is always an issue in cases of mMTC massive IoT or eMBB services. 3GPP in rel 15 and 16 allows for some enhancements to speed up the CA and DC connectivity with early idle mode measurement configuration to UE.
Following document explains the procedure:

Carrier aggregation (CA) is a well-known technology, specially designed to improve the expected user throughput by simultaneously configuring UE to be connected with multiple carriers. Honestly speaking CA is perhaps among the most worldwide appreciated and used functionalities in LTE and 5G with multiple carriers to be aggregated at up to 140 MHz maximum bandwidth in LTE and up to 800 MHz in 5G mmW.

Dual Connectivity (DC) on the other hand, is another well-known technology globally adapted to allow UE to be simultaneously connected to two sectors from two separate base stations, one anchor known as the master node (MN) and one secondary node (SN). DC allows also a UE to simultaneously transmit and receive data over MN/SN as well as a number of multiple component carriers (CCs) from two cell groups, i.e. MCG and SCG. According to 3GPP there are several different DC variants including DC between two serving nodes operating in the same technology (LTE-LTE or NR-NR ), or DC in different radio access technologies (LTE-NR known also as ENDC).

In a typical network service delivery UE is in idle mode and then UE switched to connected mode by proper signaling interaction (RACH, AS/NAS signaling and session/DRB establishment) to a single serving sector. UE is then configured by connected sector to start layer 1 and layer 2 measurements on neighbor sectors and carriers. If proper measurement reports are reported then CA carriers and SN addition for ENDC is about to be configured and start using them.

Apart from mobile broadband (MBB) traffic there are also other types of services, like broadband IoT services, where devices need to connect and shortly send a broadband measurement report frequently enough to cause signaling overload. An important factor then to facilitate those idle to connected mode interactions is to minimize the transition delay to setup CA and DC. Generally speaking, by combining DC and CA, the target is to allow UE to drastically increase the maximum bandwidth and eventually the expected throughput with the minimum transition delays.

3GPP Rel. 15 & Rel. 16 enhancements in DC/CA transitional delays

According to 3GPP LTE Rel-15, based on the documentation RP-182006 entitled as “Signalling for euCA (Enhancing LTE CA Utilization)”, 3GPP proposed an enhancement to the initial configuration of CA, known as enhanced utilization of carrier aggregation (euCA), according to which UE can be early configured to perform early measurements while in idle/inactive state on other potential component carriers. According to that proposal eNB may assign UE to do measurements during IDLE mode, that the network can use for when the UE enters CONNECTED mode. Rephrasing the proposed functionality, UE is able to report the measurement results when establishing (or resuming the stand Alone (SA) connection) to the primary cell (PCell) when entering connected state. eNB will make use of these  measurements to fast configure the UE with secondary cells (SCells) for CA.

The procedure includes the RRC Connection Release message including an extra information element “early measurement configuration” and the T331 timer “Upon receiving RRC Release message with measIdle Duration”, and according to RP-182006 follows as below:

While T331 is running, the UE shall:

1>  perform the measurements in accordance with the following:

2>  for each entry in measIdleCarrierListEUTRA within VarMeasIdleConfig:

3>  if UE supports carrier aggregation between serving carrier and the carrier frequency and bandwidth indicated by carrierFreq and allowedMeasBandwidth within the corresponding entry;

4>  perform measurements in the carrier frequency and bandwidth indicated by carrierFreq and allowedMeasBandwidth within the corresponding entry;

4>  if the measCellList is included:

5>  consider PCell and cells identified by each entry within the measCellList to be applicable for idle mode measurement reporting;

4>  else:

5>  consider PCell and up to maxCellMeasIdle strongest identified cells whose RSRP/RSRQ measurement results are above the value(s) provided in qualityThreshold (if any) to be applicable for idle mode measurement reporting;

4>  store measurement results for cells applicable for idle mode measurement reporting within the VarMeasIdleReport;

3>  else:

4>  do not consider the carrier frequency to be applicable for idle mode measurement reporting;

1>  if validityArea is configured in VarMeasIdleConfig and UE reselects to a serving cell whose physical cell identity does not match any entry in validityArea for the corresponding carrier frequency:

2>            stop T331;

The aforementioned discussed procedure is illustrated in the following signaling flow diagram, according to 3GPP TSG-RAN WG2 Meeting #105bis , (LTE_NR_DC_CA_enh-Core) Early measurements Signaling in two solution proposals:

Solution1: 

UE is configured with a measurement configuration upon going to IDLE and the UE performs the measurement in IDLE mode using this configuration.

When the connection is established again, the UE will check if the target supports early measurement reporting by reading the SIB and if so, it will indicate that it has early measurement reporting in msg5 (i.e. RRC Setup Complete).

Once the security is activated, the network may request the UE via UE Information Request to send the early measurements and the UE does so using UE Information Response message.

3GPP Rel. 15 & 16 Enhancements in 5G NSA Dual Connectivity (DC) and Carrier Aggregation (CA)

Solution2: 

In both NR and in LTE, measurement configurations in RRC_CONNECTED may be provided before security is activated, while measurements shall not be reported until security is activated. Hence, during an IDLE to CONNECTED transition, the UE first needs to complete the initial security activation to be able to transmit a measurement report.

However, for early measurements, it has been argued that this requirement could be revisited (e.g. R2-1902014, Fast setup MR-DC and NR CA with early measurement reporting, LG Electronics Inc., RAN2 #105] and Figure 2 shows such a solution where the UE reports the idle measurements even before security activation.

The initial steps are like solution 1 but the UE will indicate that it has early measurement reporting in msg3 (i.e. RRC Setup Request) instead of msg5.

The network requests the UE, via an indication in the msg4 (i.e. RRC Setup), to send the measurement report and the UE does so by including it in msg5 or multiplexing it with msg5.

One possible advantage (claimed in [12]) of sending the measurements before security activation is that if the network receives the measurements before sending RRC Reconfiguration, it can use that information to configure CA/DC in the first reconfiguration message, thereby reducing the latency required to setup CA/DC (while in solution 1, two extra RTTs are required before CA/DC can be configured via the early measurements).

3GPP Rel. 15 & 16 Enhancements in 5G NSA Dual Connectivity (DC) and Carrier Aggregation (CA)

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5G SA, Dual connectivity, 5G DC, ENDC, MRDC, NRDC, Carrier Aggregation CA, #3gpp, 3GPP TSG RAN, 5G NSA, 5GC, LTE, 5GS, 4G, 3GPP Release16,

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