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Universal (RM551E & RM520N)

Carrier Aggregation (CA) & Secondary Carriers

Primary cell and secondary component carrier (SCC1–SCC3) monitoring with bandwidth metrics.

4 min read · Updated

Carrier Aggregation (CA) & Secondary Carriers

Carrier Aggregation combines multiple frequency bands simultaneously (e.g. B2 + B66 + n41) to multiply overall data transmission bandwidth.

QManager RM551E Cellular information screen
RM551E: Every active carrier with its band, channel, PCI and live signal quality.

Understanding Carrier Aggregation (CA)#

Quectel RM520N and RM551E modems support multi-carrier aggregation:

  • Up to 4x Downlink CA in LTE.
  • Up to 3x Downlink CA in 5G NR (NR-CA), including Sub-6 FDD+TDD aggregation.

PCC vs SCC1-SCC3 Breakdown#

In QManager's Carrier Aggregation widget:

  • PCC (Primary Component Carrier): Handles control signaling, uplink transmission, and downlink.
  • SCC1, SCC2, SCC3 (Secondary Component Carriers): Provide supplemental downlink bandwidth.

The dashboard displays:

  • Band & Channel: e.g., PCC: B66 (20 MHz) + SCC1: B2 (15 MHz) + SCC2: n41 (100 MHz).
  • Total Aggregated Bandwidth: e.g., 135 MHz Total Channel Width.
  • Modulation Scheme: e.g., 256-QAM or 64-QAM.

Why Does CA Disappear When Idle?#

T-Mobile SA 4xCA & 5xCA EFS Patches#

In modern 5G Standalone (5G SA) networks—especially T-Mobile USA (PLMN 310260 / 310410)—carriers deploy multi-band NR aggregation combining multiple mid-band and low-band channels simultaneously (such as n41 + n41 + n71 + n25 or n41 + n41 + n77 + n71).

The Missing RRC Capability Problem#

Stock factory firmware on many Quectel modules (including early RM520N and RM551E builds) contains conservative default Radio Resource Control (RRC) capability tables that advertise support for only 2xCA or 3xCA in Standalone mode, preventing the network from assigning 4-carrier or 5-carrier aggregation.

Upstream EFS Patch Injection Workflow#

As reverse engineered and documented by Cameron Thompson (iamromulan) in the Cellular Modem Wiki:

  1. Purging Stale Carrier Combos (AT+QNVFD): Stale or conflicting carrier aggregation definitions must first be removed from the Qualcomm EFS partition:
    AT
    AT+QNVFD="/nv/item_files/modem/lte/rrc/cap/combos"
  2. Injecting Custom CA Combo Tables (AT+QNVFW): The custom T-Mobile SA 4xCA/5xCA binary table is written directly into non-volatile EFS using Quectel's NV write command:
    AT
    AT+QNVFW="/nv/item_files/modem/nr5g/rrc/cap/ca_combos",<binary_payload>
  3. Overriding RRC Capability Limits: Re-enables wide-bandwidth channel aggregation across non-contiguous n41 carriers and n25/n71 carrier combos.
  4. Rebooting Baseband:
    AT
    AT+CFUN=1,1

Screens in QManager#

QManager RM520N Cellular and radio information screen
RM520N: Cellular Information lists every active carrier with its band, channel, and signal.