5G Carrier Aggregation (CA) Demystified
Contents
What is Carrier Aggregation and Carrier Component
Carrier Aggregation (CA) is a technique that combines multiple radio carriers (frequency channels) into a single logical channel to boost data rates and capacity. In simple terms, it “glues together” separate chunks of spectrum so a device can use them simultaneously, effectively widening its bandwidth.
5G NR (New Radio) supports CA extensively, allowing up to 16 component carriers (CCs) in both uplink and downlink streams. Each CC is a contiguous frequency block of a certain bandwidth (the width of the channel, e.g. 20 MHz or 100 MHz). The radio spectrum is divided into frequency bands. For example, 5G uses Frequency Range 1 (FR1): from 410 MHz to 7125 MHz (sub-7 GHz), and Frequency Range 2 (FR2): from 24.25–71 GHz (also known as millimeter wave).
CA lets a 5G cell (UE) transmit on several carriers across possibly different bands, summing their bandwidths into one big virtual pipe.
- Carrier Aggregation(CA): Combining multiple component carriers into one larger channel to increase data throughput.
- Component Carrier (CC): An individual frequency carrier (e.g. a 20 MHz or 100 MHz block) that can be aggregated. One CC becomes the primary (PCell) and others are secondary (SCells).
Why CA is fundamental for 5G
I- Increased Throughput:
- 5G is all about very high speeds (multi-Gbps), huge capacity (supporting many devices), and broad coverage. CA is a foundational tool to achieve these goals. By adding together multiple carriers, a 5G cell effectively creates a much wider channel.
- For example, aggregating two 100 MHz carriers gives an effective 200 MHz channel. This directly increases peak throughput. More total spectrum means more bits per second for all users. Aggregating different bands lets operators leverage fragmented and flexible allocations.
Examples from the real world scenarios:
- Verizon combined six sub-6 GHz carriers (total ~350 MHz of spectrum) to hit 5.5 Gbps and Ericsson reports mmWave CA (8×100 MHz) reaching ~4.2 Gbps.
II- Improved Coverage
- CA can improve coverage and experience at the cell edge. For example, combining a high-frequency mm Wave carrier component (good for speed but short-range) with a low-frequency carrier (good for range), meaning UE can send uplink on the far-reaching low band while still getting high downlink speed.
- Also, CA lowers latency and congestion by giving devices more bandwidth resources (so data transfers complete faster), and it can add robustness if one band CC fades.
Types of Carrier Aggregation
CA can take several forms depending on how the carriers are arranged in frequency:
- Intra-band contiguous CA: The aggregated CCs are in the same frequency band and are adjacent (back-to-back) with no gap. For example, two adjacent 50 MHz blocks in the 3.5 GHz band. This is the simplest case, often with minimal guard-band overhead.
- Intra-band non-contiguous CA: The CCs are in the same band but separated by one or more frequency gaps. For instance, a 50 MHz block and a 100 MHz block in band n78 with some unused space between. This may require extra guard bands, but still stays within one band.
- Inter-band CA: The CCs lie in different bands. For example, one carrier in the 3.5 GHz mid-band plus another in the 700 MHz low-band. This lets a network combine very different spectrum segments.
Carrier Aggregation and FDD/TDD
FDD Carrier Aggregation
In FDD CA, each Component Carrier (CC) brings its own UL/DL pair. CA can be intra-band (same FDD band) or inter-band (different FDD bands).
- Example1: Inter-band FDD CA: Aggregating 20 MHz in Band 3 (UL 1710–1730 MHz / DL 1805–1825 MHz) with 20 MHz in Band 7 (UL 2500–2520 MHz / DL 2620–2640 MHz) yields a total of 40 MHz UL + 40 MHz DL. Devices send and receive simultaneously on both bands, doubling peak rates over a single band.
- Example2: In FDD Carrier Aggregation you simply add up the paired uplink and downlink bandwidth of each component carrier. So with your two CCs:
- CC1: 20 MHz total → 10 MHz UL + 10 MHz DL
- CC2: 30 MHz total → 10 MHz UL + 20 MHz DL
When you aggregate them:
| Direction | CC1 | CC2 | Total |
|---|---|---|---|
| UL | 10 MHz | 10 MHz | 20 MHz |
| DL | 10 MHz | 20 MHz | 30 MHz |
Uplink and downlink always use their respective paired frequencies, and scheduling aligns across all aggregated CCs.
The gNB treats each CC’s UL/DL pair independently but schedules them in parallel. UEs monitor multiple UL/DL carriers concurrently.
TDD Carrier Aggregation
TDD CA aggregates multiple unpaired carriers, each with its own UL/DL time-slot pattern. Carriers may be in the same band (contiguous/non-contiguous) or different TDD bands.
Example1:
- Aggregating two 100 MHz CCs in Band n41 (2.5 GHz TDD) each configured for a 70% DL / 30% UL split. Combined, the network offers 200 MHz of shared DL/UL spectrum, effectively increasing capacity proportionally.
Example2:
Let’s say, we have these 2 CCs:
- CC1: 100 MHz total, mid‑band, numerology µ = 1 (30 kHz SCS ⇒ 0.5 ms slots)
- CC2: 200 MHz total, higher band, numerology µ = 2 (60 kHz SCS ⇒ 0.25 ms slots)
Together, the UE gets a 300 MHz‑wide data highway, with DL and UL divided in time across both carriers.
Example of TDD pattern (per 10 ms frame), potentially chosen based on service/app:
Subframes 0–2: DL
Subframe 3 : UL
Subframe 4 : Guard
Subframes 5–7: DL
Subframe 8 : UL
Subframe 9 : Guard 70 % DL, 20 % UL, 10 % Guard
All CCs follow this same subframe‑level DL/UL schedule.
NoteBecause both carriers share the same subframe DL/UL pattern, you simply paint each carrier’s slots inside those subframes:
- In Subframe 0 (0–1 ms), both CC1’s two 0.5 ms slots and CC2’s four 0.25 ms slots transmit DL.
- In Subframe 3 (3–4 ms), CC1’s two slots and CC2’s four slots all switch to UL.
| Subframe # | 0 (DL) | 1 (DL) | 2 (DL) | 3 (UL) |
|---|---|---|---|---|
| CC1 | Slot 0 (DL) | Slot 2 (DL) | Slot 4 (DL) | Slot 6 (UL) |
| Slot 1 (DL) | Slot 3 (DL) | Slot 5 (DL) | Slot 7 (UL) | |
| CC2 | Slots 0,1,2,3 DL | Slots 4–7 DL | Slots 8–11 DL | Slots 12–15 UL |
In other words, using this TDD DL/UL pattern, the following occurs:
- Subframes 0, 1, 2 (0–3 ms): all slots on both CC1 (100 MHz) and CC2 (200 MHz) are in Downlink. You get the full 300 MHz of aggregate spectrum carrying DL traffic for those 3 ms.
- Subframe 3 (3–4 ms): every slot on both carriers switches to Uplink, so the full 300 MHz is available for UE‑to‑gNB transmissions for that 1 ms.
So over each 4 ms cycle you have 3 ms of DL at 300 MHz and 1 ms of UL at 300 MHz. Continuous operation just repeats that pattern.
FDD–TDD Carrier Aggregation (Cross-Duplex CA)
Mixing FDD and TDD CCs lets operators combine the coverage of FDD with the capacity of TDD mid-/high-band. This is common in Non-Standalone (NSA) and Standalone (SA) deployments.
Live Network Example (stc):
- CA Setup: 20 MHz NR FDD @700 MHz (using Ericsson Spectrum Sharing with LTE) + 100 MHz NR TDD @3.6 GHz.
- Outcome: Combined coverage and capacity; the low-band FDD extends uplink reach (critical for cell-edge), while the mid-band TDD supplies high DL throughput.
Summary
CA Type | Duplex Mode | Carriers | Total BW | Notes |
Intra-band contiguous | FDD | e.g. 2 20 MHz in Band 3 | 40 MHz UL + DL | Simple guard-band; symmetric UL/DL |
Intra-band contiguous | TDD | e.g. 2 100 MHz in n41 (70% DL /
30% UL) | 200 MHz shared | Requires slot alignment across
numerologies |
Inter-band | FDD | 20 MHz @Band 3 + 20 MHz @Band 7 | 40 MHz UL + DL | Both UL/DL paired; straight
scheduling |
Inter-band | TDD | 100 MHz @n41 + 60 MHz @n41 | 160 MHz shared | Both CCs share time division; can
differ in SCS |
Inter-band | FDD + TDD | 20 MHz @700 MHz FDD + 100 MHz @3.6
GHz TDD | 20 MHz UL + DL + | FDD PCell for control; TDD SCell for DL capacity |
100 MHz DL/UL slots |
References:
Wikipedia
3GPP
Nokia
