Carrier Aggregation

Illustrating carrier aggregation in LTE. Source: Qualcomm 2016, slide 10.
Illustrating carrier aggregation in LTE. Source: Qualcomm 2016, slide 10.

Carrier Aggregation (CA) is a feature that allows a mobile device to send and receive packets from multiple carriers (aka frequencies) at the same time. Carrier aggregation as a feature is available in Wi-Fi, 3G HSPA+, LTE-Advanced and 5G networks. The many carriers are controlled by a single access point (in Wi-Fi) or base station (in cellular systems).

The figure shows an example of CA in LTE. LTE has a maximum carrier bandwidth of 20 MHz. Via CA, five carriers are combined to achieve an aggregated bandwidth of 100 MHz. CA brings higher data rate, lower latency, better user experience, and better use of network resources.

CA is possible in downlink and uplink, symmetric or asymmetric. Carriers can be aggregated across FDD and TDD bands, licensed or unlicensed bands.

Discussion

  • Could you explain intra-band versus inter-band CA?
    Intra-band versus inter-band CA. Source: Wikipedia 2022.
    Intra-band versus inter-band CA. Source: Wikipedia 2022.

    There are broadly two types of CA:

    • Intra-Band CA: The carriers belong to the same band. Carriers can be contiguous (next to each other) or non-contiguous. CA with contiguous carriers is easier to implement. Operators most likely don't have large contiguous spectrum. Hence, non-contiguous CA is a useful feature.
    • Inter-Band CA: The carriers belong to different bands. This is more complex to implement than the intra-band contiguous type. Because cellular operators have licenses to different bands, this has led to the standardization of many inter-band CA combinations to cater to industry needs.
  • What technologies use carrier aggregation?

    In HSPA+, 8 DL and 2 UL carriers can be aggregated. HSDPA Multiflow enables aggregation across cells controlled from the same RNC.

    In LTE-Advanced, up to five carriers can be aggregated for a maximum bandwidth of 100 MHz. Each carrier can use a different channel bandwidth. In LTE-Advanced Pro (Rel-13), up to 32 carriers can be aggregated for a maximum bandwidth of 640 MHz. Carriers from unlicensed bands could be aggregated.

    In 5G, intra-band and inter-band CA is possible within FR1, within FR2, and across FR1 and FR2. Up to 16 carriers can be aggregated. Maximum aggregated bandwidth is limited to 1 GHz. Each aggregated carrier can be of a different numerology.

    In both LTE and 5G, Supplemental Downlink (SDL) carriers provide additional throughput. This is nothing but CA. On the contrary, Supplemental Uplink (SUL) is not CA. In general, a UE can be assigned more downlink than uplink carriers.

    Wi-Fi's channel bonding is intra-band and contiguous. OFDM symbols from adjacent channels are combined to yield a single channel. Hence, this isn't CA as used in HSPA+/LTE/5G. However, Wi-Fi 7 introduced multi-link operation in which a device can aggregate channels from 2.4/5/6 GHz bands.

  • What are the possible deployment scenarios of CA?
    Carrier aggregation deployment scenarios. Source: Robalo and Velez 2020, fig. 2.
    Carrier aggregation deployment scenarios. Source: Robalo and Velez 2020, fig. 2.

    The figure show five possible deployment scenarios with CA using carriers F1 and F2 (assume F2 > F1):

    • Scenario 1: F1 and F2 are in the same band and have similar coverage with co-located cells. This scenario provides higher capacity and more bandwidth to the UE.
    • Scenario 2: F2 is in a higher band and hence has smaller coverage. F1 provides coverage while F2 improves the throughput.
    • Scenario 3: Cells are co-located but F2 is directed to cover the holes in F1. F1 provides coverage and mobility while F2 improves the throughput.
    • Scenario 4: F1 provides macro cell coverage while F2 Remote Radio Heads (RRHs) improve throughput at hot spots. F1 and F2 can be non-contiguous carriers in the same band or inter-band carriers.
    • Scenario 5: Similar to scenario 2, but frequency-selective repeaters are used to improve F2 coverage.

    The above scenarios suggest that the benefits of CA are many: higher throughput, better cell coverage, consistent user experience, higher energy efficiency, and higher ROI for operators.

  • What's the basic terminology used in LTE and 5G NR carrier aggregation?
    Examples of component carriers in downlink and uplink. Source: ShareTechNote 2022.
    Examples of component carriers in downlink and uplink. Source: ShareTechNote 2022.

    Each aggregated carrier is called a Component Carrier (CC). One of them is called Primary Component Carrier (PCC) and the rest are called Secondary Component Carriers (SCCs). The respective cells are called PCell and SCells. Both PCell and SCell are considered as serving cells. There's one HARQ entity per serving cell.

    PCC is essential since cell search/selection, system information acquisition and initial random access happen on it. PCC is UE-specific; that is, different UEs can have different carriers as their PCC. In the uplink, only the PCC carries PUCCH (unless two PUCCH groups are configured). Uplink SCCs provide only PUSCH. In the downlink, PCC carries both PDSCH and PDCCH. Downlink SCCs carry PDSCH and optionally PDCCH.

    CA can be used together with Dual Connectivity (DC). A UE may be connected to a Master Cell Group (MCG) and a Secondary Cell Group (SCG). CA can happen in each cell group.

  • How are CA band combinations named?
    Naming of CA band combinations. Source: Adapted from Roessler et al. 2012
    Naming of CA band combinations. Source: Adapted from Roessler et al. 2012 and ETSI 2022f.

    Figure shows examples of LTE CA. CA_1C operates in band 1 with 101-200 number of aggregated resource blocks for CA class C. The maximum aggregated channel bandwidth is 40 MHz from two CCs. Moreover, CA_1C has two Bandwidth Combination Sets (BCSs). Each BCS allows for a certain combination of channel bandwidth in MHz per CC: (15 + 15, 20 + 20) for BCS0 and (5/10/15 + 20, 20 + 5/10/15/20) for BCS1.

    CA_25A-25A indicates non-contiguous intra-band CA, hence "25A" is repeated. CA_1A-5A indicates inter-band CA involving bands 1 and 5, both applicable to CA class A.

    In 5G NR, CA bandwidth classes are defined separately for FR1 and FR2. We note a few CA NR band combinations:

    • Intra-band Contiguous in FR1: CA_n78B (20 + 50)
    • Intra-band Non-Contiguous in FR1: CA_n77(3A) (3 CCs, 2 BCSs, 1 combination per BCS)
    • Inter-band in FR1: CA_n1A-n77A, CA_n1A-n77(3A) (2 non-contiguous CCs in n77)
    • Inter-band in FR1 and FR2: CA_n66-n77-n260 (3 bands), CA_n1-n3-n8-n77-n257 (5 bands)

    Band combinations can be browsed online: LTE CA, NR FR1, NR FR2 and NR FR1-FR2

  • Which LTE or 5G NR protocol layers are affected by CA?
    DL L2 structure with CA configured. Source: ETSI 2022d, fig. 6.7-1.
    DL L2 structure with CA configured. Source: ETSI 2022d, fig. 6.7-1.

    The main impact of CA is on MAC and PHY. RLC needs to have larger buffers to handle higher data rates but otherwise RLC is unaffected by CA. RRC signalling is modified. For example, once the UE is connected on the PCell, SCells are set up via RRC signalling.

    MAC distributes the RLC PDUs among the many CCs. Each CC handles one transport block per TTI. Each CC is associated with it's own HARQ entity, though all CCs are handled by a single MAC entity.

  • What are some additional features of LTE and 5G NR CA?
    Scheduling and control signalling in CA. Source: Adapted from Dahlman et al. 2018, fig. 7.10-7.11.
    Scheduling and control signalling in CA. Source: Adapted from Dahlman et al. 2018, fig. 7.10-7.11.

    With cross-carrier scheduling, the UE monitors a single PDCCH (for example, on the PCC). This will contain the scheduling information for all the CCs.

    Feedback signalling such as HARQ acknowledgements of all CCs are sent on the uplink of the PCell. This suits the asymmetric design of CA in downlink and uplink. However, this may overload the single uplink carrier carrying the signalling. It's therefore possible to configure two PUCCH groups. The cell carrying the signalling in the second group may be called Primary Second Cell (PSCell).

    In Rel-16, unaligned frame boundary and Tx switching are allowed. This provides the UE more transmission opportunities and utilize effectively both its Tx chains. In Rel-17, the PUCCH carrier can be dynamically selected to reduce latency. Moreover, HARQ retransmissions can be on a different carrier.

    In LTE Rel-15 and 5G NR Rel-16, enhanced utilization of Carrier Aggregation (euCA) was introduced. This allows the UE to perform early measurements so that SCCs can be activated quickly when RRC connection is resumed from an inactive state.

Milestones

2009
Evolution of HSPA carrier aggregation. Source: Anritsu 2015, fig. 1.
Evolution of HSPA carrier aggregation. Source: Anritsu 2015, fig. 1.

HSPA+ Rel-8 is released. This introduces a feature called multicarrier (aka dual-carrier). Two adjacent 5 GHz carriers can be combined to offer a UE higher data rates. Rel-9 (2010) allows non-adjacent carriers to be combined this way. It also brings dual-carrier feature to uplink. This expands to four DL carriers in Rel-10 (2011) and eight DL carriers in Rel-11 (2012). With eight carriers, a UE is able to obtain about 168 Mbps on 40 MHz of aggregated bandwidth, and 672 Mbps when combined with 4x4 MIMO.

2011
Evolution of LTE carrier aggregation. Source: Brydon 2014.
Evolution of LTE carrier aggregation. Source: Brydon 2014.

Rel-10 of LTE-Advanced comes out. This is the first LTE release to introduce CA (intra-band contiguous and inter-band). This standardizes 5 x 20 MHz CA, though in practice it's limited to only two carriers. Rel-11 (2012) of LTE-Advanced introduces support for intra-band non-contiguous CA. Rel-13 (2016) of LTE-Advanced Pro allows 32 CCs that can accommodate unlicensed bands as well. Overall, CA in LTE along with other enhancements (256QAM and 4x4 MIMO) enables Gigabit LTE.

Mar
2015
LTE carrier aggregation across FDD and TDD bands. Source: Roessler et al. 2014, fig. 2-24.
LTE carrier aggregation across FDD and TDD bands. Source: Roessler et al. 2014, fig. 2-24.

Rel-12 of LTE-Advanced reaches functional freeze. This release includes the capability to aggregate carriers across FDD (paired spectrum) and TDD (unpaired spectrum) bands. This benefits operators who have licenses to both FDD and TDD bands. In fact, operators demonstrated FDD+TDD CA at the MWC 2014.

2021
5G CA across low, mid and high bands. Source: Kaur and Karlberg 2021, fig. 1.
5G CA across low, mid and high bands. Source: Kaur and Karlberg 2021, fig. 1.

3GPP starts to focus on the specifications for 5G Standalone CA in FR1 with more than two CCs. Meanwhile, Ericsson claims to support CA in both 5G SA and NSA, with DL 8CC and UL 2CC in mmWave band, 2CC in mid-band, and 3CC in low-band. Qualcomm supports CA via its Snapdragon X65 5G modem.

Mar
2022
PUCCH carrier switching. Source: Williamson et al. 2022, fig. 4.
PUCCH carrier switching. Source: Williamson et al. 2022, fig. 4.

Rel-17 reaches stage 3 functional freeze. In 5G NR, PUCCH Carrier Switching is possible whereby the PUCCH carrier can be dynamically selected to reduce latency. HARQ retransmissions can use a different carrier from the original transmission.

May
2022

Nokia and Optus achieve 3 CC CA over a 5G SA network. This uses the bands 2100 MHz (FDD), 2300 MHz (TDD) and 3500 MHz (TDD). They state that their Samsung S22 customers will soon be able to experience this in commercial service. In August, Nokia and BT Networks demonstrate 4 CC CA over a 5G SA network. Four carriers combined are in the bands 2.1 GHz, 2.6 GHz, 3.4 GHz and 3.6 GHz.

Oct
2022
Top 10 most popular CA band combinations in LTE. Source: HB Radiofrequency 2022.
Top 10 most popular CA band combinations in LTE. Source: HB Radiofrequency 2022.

It's seen that most LTE networks use 2-band CA combinations. The two most popular combinations are CA_3A-7A (1800 MHz and 2600 MHz) and CA_3A-20A (1800 MHz and 800 MHz), achieving an aggregated bandwidth of 40 MHz. The most popular 3-band combination is CA_3A-7A-20A, achieving an aggregated bandwidth of 60 MHz.

References

  1. 3GPP. 2015. "Release 12." Specifications & Technologies, 3GPP. Accessed 2022-10-13.
  2. Anritsu. 2015. "Understanding Carrier Aggregation." White paper, Anritsu, March. Accessed 2022-10-14.
  3. Brydon, Alastair. 2014. "Evolution of LTE-Advanced Carrier Aggregation." Blog, Unwired Insight, March 25. Accessed 2022-10-15.
  4. Dahlman, Erik, Stefan Parkvall, and Johan Skold. 2018. "5G NR: The Next Generation Wireless Access Technology." Academic Press. Accessed 2021-02-14.
  5. Davidson, Andy. 2022. "Pushing the limits of Wi-Fi performance with Wi-Fi 7." OnQ Blog, Qualcomm, February 14. Accessed 2022-10-28.
  6. Deek, L., E. Garcia-Villegas, E. Belding, S.-J. Lee and K. Almeroth. 2014. "Intelligent Channel Bonding in 802.11n WLANs." IEEE Transactions on Mobile Computing, vol. 13, no. 6, pp. 1242-1255, June. doi: 10.1109/TMC.2013.73. Accessed 2022-10-28.
  7. Deng, C., X. Fang, X. Han, X. Wang, L. Yan, R. He, Y. Long, and Y. Guo. 2020. "IEEE 802.11be Wi-Fi 7: New Challenges and Opportunities." IEEE Communications Surveys & Tutorials, vol. 22, no. 4, pp. 2136-2166. doi: 10.1109/COMST.2020.3012715. Accessed 2022-10-28.
  8. ETSI. 2022a. "TS 138 101-1: 5G; NR; User Equipment (UE) radio transmission and reception; Part 1: Range 1 Standalone." V17.6.0, August.Accessed 2022-10-15.
  9. ETSI. 2022b. "TS 138 101-2: 5G; NR; User Equipment (UE) radio transmission and reception; Part 2: Range 2 Standalone." V17.6.0, August. Accessed 2022-10-15.
  10. ETSI. 2022c. "TS 138 101-3: 5G; NR; User Equipment (UE) radio transmission and reception; Part 3: Range 1 and Range 2 Interworking operation with other radios." V17.6.0, September. Accessed 2022-10-10.
  11. ETSI. 2022d. "TS 138 300: 5G; NR; NR and NG-RAN Overall description; Stage-2." V17.1.0, August. Accessed 2022-10-10.
  12. ETSI. 2022e. "TS 136 300: LTE; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2." V17.1.0, August. Accessed 2022-10-15.
  13. ETSI. 2022f. "TS 136 202: LTE; Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio transmission and reception." V17.6.0, August. Accessed 2022-10-15.
  14. ETSI. 2022g. "TS 137 340: Universal Mobile Telecommunications System (UMTS); LTE; 5G; NR; Multi-connectivity; Overall description; Stage-2." V17.1.0, August. Accessed 2022-10-10.
  15. Ericsson. 2020. "5G carrier aggregation for better deployment." Ericsson, April 27. Updated 2022-09-07. Accessed 2022-10-10.
  16. Ghazaleh, F. 2016. "The Introduction of Carrier aggregation." Slides, ITU Conference, Netherlands. Accessed 2022-10-28.
  17. Grilli, F. 2021. "Why carrier aggregation is needed for 5G, and the latest Qualcomm Technologies breakthroughs making it possible." OnQ Blog, Qualcomm, May 21. Accessed 2022-10-28.
  18. HB Radiofrequency. 2022. "LTE Carrier Aggregation." HB Radiofrequency. Accessed 2022-10-14.
  19. Hytonen, V., O. Puchko, T. Hohne and T. Chapman. 2012. "Introduction of Multiflow for HSDPA." 5th International Conference on New Technologies, Mobility and Security (NTMS), pp. 1-5. doi: 10.1109/NTMS.2012.6208684. Accessed 2022-10-28.
  20. Kaur, Amareet and Björn Karlberg. 2021. "What, Why and How: the Power of 5G Carrier Aggregation." Blog, Ericsson, June 24. Accessed 2022-10-10.
  21. Malhotra, Dheeraj. 2020. "Carrier Aggregation." Blog, NR LTE related tech oriented blog, May 9. Accessed 2022-10-10.
  22. Montojo, Juan. 2022. "Just in: 3GPP completes 5G NR Release 17."OnQ Blog, Qualcomm, March 24. Accessed 2022-10-28.
  23. Nokia. 2022. "5G Carrier Aggregation explained." Nokia, October 4. Accessed 2022-10-10.
  24. Qualcomm. 2010. "HSPA+ is Here! What's Next?" White paper, Qualcomm, March. Accessed 2022-10-14.
  25. Qualcomm. 2016. "Delivering on the LTE Advanced promise." Presentation, Qualcomm, March. Accessed 2022-10-14.
  26. Robalo, D. and F. J. Velez. 2020. "Carrier Aggregation with Multi-band Scheduling in LTE-Advanced Networks." Univ. of Waterloo, September 14. Accessed 2022-10-15.
  27. Roessler, A., S. Merkel, and M. Kottkamp. 2012. "Carrier aggregation – (one) key enabler for LTE-Advanced." News, Rohde & Schwarz GmbH. Accessed 2022-10-15.
  28. Roessler, A., J. Schlienz, S. Merkel, and M. Kottkamp. 2014. "LTE- Advanced (3GPP Rel.12) Technology Introduction." White paper, Rohde & Schwarz. Accessed 2022-10-13.
  29. Rohde & Schwarz. 2015. "Evolution of Carrier Aggregation (3GPP Releases 10 to 13)." Poster, Rohde & Schwarz. Accessed 2022-10-15.
  30. Rugeland, P. and J. Bergqvist. 2020. "Key insights: Early measurements for improved carrier aggregation and dual connectivity setup." Blog, Ericsson, October 1. Accessed 2022-10-28.
  31. ShareTechNote. 2022. "4G/LTE - LTE Advanced: Carrier Aggregation." ShareTechNote. Accessed 2022-10-10.
  32. Telcoma. 2022. "Carrier Aggregation and its Challenges in 5G NR." Telcoma. Accessed 2022-10-14.
  33. Tomás, Juan Pedro. 2022. "Nokia, Optus claim world’s first 3CC Carrier Aggregation 5G SA data call." RCS Wireless News, May 4. Accessed 2022-10-14.
  34. Wieland, Ken. 2022. "BT, Nokia showcase four-channel aggregation over 5G SA." Light Reading, August 8. Accessed 2022-10-14.
  35. Wikipedia. 2022. "Carrier aggregation." Wikipedia, July 13. Accessed 2022-10-14.
  36. Williamson, R., G. D'Aria, and Ruyue YN Li. 2022. "5G TDD Uplink." White paper, V1.0, NGMN Alliance, January 17. Accessed 2022-10-28.

Further Reading

  1. Roessler, A., S. Merkel, and M. Kottkamp. 2012. "Carrier aggregation – (one) key enabler for LTE-Advanced." News, Rohde & Schwarz GmbH. Accessed 2022-10-15.
  2. Nokia. 2022. "5G Carrier Aggregation explained." Nokia, October 4. Accessed 2022-10-10.
  3. Ericsson. 2020. "5G carrier aggregation for better deployment." Ericsson, April 27. Updated 2022-09-07. Accessed 2022-10-10.
  4. Anritsu. 2015. "Understanding Carrier Aggregation." White paper, Anritsu, March. Accessed 2022-10-14.
  5. Ghazaleh, F. 2016. "The Introduction of Carrier aggregation." Slides, ITU Conference, Netherlands. Accessed 2022-10-28.

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Devopedia. 2022. "Carrier Aggregation." Version 3, October 29. Accessed 2022-10-29. https://devopedia.org/carrier-aggregation
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Last updated on
2022-10-29 05:18:14