Aalborg Universitet
Carrier Aggregation for LTE-Advanced
Functionality and Performance Aspects
Pedersen, Klaus Ingemann; Frederiksen, Frank; Rosa, Claudio; Nguyen, Hung Tuan; Garcia,
Luis Guilherme Uzeda; Wang, Yuanye
Published in:
I E E E Communications Magazine
DOI (link to publication from Publisher):
10.1109/MCOM.2011.5783991
Publication date:
2011
Document Version
Early version, also known as pre-print
Link to publication from Aalborg University
Citation for published version (APA):
Pedersen, K. I., Frederiksen, F., Rosa, C., Nguyen, H. T., Garcia, L. G. U., & Wang, Y. (2011). Carrier
Aggregation for LTE-Advanced: Functionality and Performance Aspects. I E E E Communications Magazine,
49(6), 89-05. https://doi.org/10.1109/MCOM.2011.5783991
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Carrier Aggregation for LTE-Advanced: Functionality
and Performance Aspects
Klaus Ingemann Pedersen, Luis Guilherme Uzeda Garcia, Hung Nguyen,
Yuanye Wang, Frank Frederiksen, Claudio Rosa
Abstract
– Carrier aggregation (CA) is one of the key features for LTE-Advanced. By means of
CA, users gain access to a total bandwidth of up to 100 MHz in order to meet the IMT-Advanced
requirements. The system bandwidth may be contiguous, or composed of several non-contiguous
bandwidth chunks, which are aggregated. This paper presents a summary of the supported CA
scenarios as well as an overview of the CA functionality for LTE-Advanced with special
emphasis on the basic concept, control mechanisms, and performance aspects. The discussion
includes definitions of the new terms primary cell (PCell) and secondary cell (SCell),
mechanisms for activation and deactivation of CCs, and the new cross-CC scheduling
functionality for improved control channel optimizations. We also demonstrate how CA can be
used as an enabler for simple yet effective frequency domain interference management schemes.
In particular, interference management is anticipated to provide significant gains in
heterogeneous networks, envisioning intrinsically uncoordinated deployments of home base
stations.
I. Introduction
The first version of long term evolution (LTE) was completed in March 2009 as part for
3GPP Release-8 (Rel-8) [1]. LTE is based on flat radio access network architecture without a
centralized network component, offering flexible bandwidth options ranging from 1.4 MHz to 20
MHz using orthogonal frequency division multiple access (OFDMA) in the downlink and single-
carrier frequency division multiple access (SC-FDMA) in the uplink [1]. Multiple-input-multiple-
output (MIMO) up to order 4x4 are supported for the downlink, while only single layer
transmission is supported in the uplink. In March 2008, 3GPP started a new study item in order to
further develop LTE towards LTE-Advanced targeting the IMT-Advanced requirements as
defined by the International Telecommunications Union (ITU) [2]-[5]. The LTE-Advanced study
item was closed March-2010. The outcome was a set of new radio features, which are currently
being standardized to become part of LTE-Advanced in 3GPP Rel-10.
Carrier aggregation (CA) is one of the main features for LTE-Advanced in Rel-10 for
meeting the peak data rate requirements of IMT-Advanced, namely 1 Gbps and 500 Mbps for the
downlink and uplink, respectively [6]. This paper provides a thorough overview of CA for LTE-
Advanced, while elucidating its impact on the overall system design and performance. Although
we primarily focus on CA for the downlink of frequency division duplex systems, CA is
supported in the uplink as well as in time division duplex systems [7].
CA is designed to be backward compatible, meaning that legacy Rel-8 and Rel-9 users
should still be able to co-exist with LTE-Advanced on at least part of the total bandwidth. Thus,
each individual spectrum chunk, denoted component carrier (CC), inherits the core physical layer
design and numerology from LTE Rel-8. Nevertheless, the introduction of CA for LTE-Advanced
does include new functionalities and modifications to the link layer and radio resource
management (RRM) framework. In our description of such modifications for LTE-Advanced, we
assume that the corresponding LTE Rel-8 design is known by readers, who may otherwise refer
to [1], [8], [9] for additional information.
Additionally, we discuss the potential of CA as an enabler for new frequency domain
interference management schemes, providing attractive gains for heterogeneous environments
with dense deployment of small base station nodes (e.g. pico or home base stations). For
example, a fully distributed interference management concept with a CC resolution, called
autonomous component carrier selection (ACCS) has been proposed in [10].
A set of system level performance results are presented in order to demonstrate the
benefits of CA. In particular, we focus on comparing the performance of N separate LTE Rel-8
carriers versus using CA of N carriers. The performance comparison is presented for a dynamic
birth-death traffic model to illustrate how the performance varies with the offered traffic per cell.
Performance results for heterogeneous networks with dense deployment of small base station
nodes are also presented in order to illustrate the potential of the developed ACCS concept.
The rest of the paper is organized as follows; Section II outlines the scenarios and basic
assumptions for CA configurations. The CA functionality and impact on radio resource
management (RRM) algorithms is described in Section III. Section IV addresses interference
management on a carrier resolution, followed by presentation of performance results in Section
V. Finally, Section VI recapitulates the main findings and points out to future work.
II. CA scenarios and CC types
The maximum supported bandwidth for LTE-Advanced of 100 MHz can be achieved via
CA of 5 CCs of 20 MHz as illustrated in Fig. 1a. Thus, an LTE-Advanced user supporting such
high bandwidths can be served simultaneously on all 5 CCs. The bandwidth of each CC follows
the LTE Rel-8 supported bandwidth configurations, meaning 1.4, 3, 5, 10, 15, and 20 MHz. The
aggregated CCs may be contiguous as illustrated in Fig. 1a, or non-contiguous as depicted in Fig.
1b. Notice also from the example in Fig.1b that the aggregated CCs can in principle also have
different bandwidths. The support for both contiguous and non-contiguous CA of CCs with
different bandwidths offers significant flexibility for efficient spectrum utilization, and gradual
re-farming of frequencies previously being used by other systems such as e.g. Global System for
Mobile Communications (GSM) or Code Division Multiple Access (CDMA). From an
implementation and physical layer perspective, contiguous CA is easier, in the sense that it can be
realized with a single Fast Fourier Transform (FFT) and a single Radio Frequency (RF) unit,
while non-contiguous CA in most cases requires multiple RF chains and FFTs. The non-
contiguous CA cases have additional implications; the radio network planning phase and the
design of the RRM algorithms need to take into account that different CCs will exhibit different
path loss and Doppler shifts. For example, Doppler shift influences on the ability to gain from
frequency domain packet scheduling within a CC [8].
Carrier #1 Carrier #2 Carrier #3 Carrier #4 Carrier #5
LTE-Advanced bandwidth
Carrier #1 Carrier #3Carrier #2
1.8 GHz 2.1 GHz 2.6 GHz
20 MHz10 MHz10 MHz
(A)
(B)
20 MHz 20 MHz 20 MHz 20 MHz 20 MHz
Fig. 1: Example of carrier aggregation scenarios: Contiguous aggregation of 5 component carriers with
equal bandwidth (A) and non-contiguous aggregation of component carriers with different bandwidths.
Notice that for LTE Rel-8 with frequency division duplex (FDD), uplink and downlink
carriers are always paired with options for defining the frequency duplex distance and bandwidth
through system information signaling. With CA it is also possible to have asymmetric
configurations, so there for example is multiple downlink CCs configured for a UE and only one
uplink CC. The linking between uplink and downlink configured CCs is signaled to the UE with
higher layer signaling. For each LTE-Advanced user, a CC is defined as its Primary cell (PCell)
[7]. Different users may not necessarily use the same CC as their PCell. The PCell can be
regarded as the anchor carrier for the terminal and is thus used for basic functionalities such as
radio link failure monitoring. If more than one CC is configured for user, the additional CCs are
denoted as Secondary Cells (SCells) for the user.
III Functionality and terminology
III-A Protocol stack
Figure 2 shows an overview of the downlink user plane protocol stack at the base station,
as well as the corresponding mapping of the most essential RRM functionalities for CA. Each
user has at least one radio bearer, denoted the default radio bearer. The exact mapping of data to
the default bearer is up to the operator policy as configured via the Traffic Flow Template (TFT).
In addition to the default radio bearer, users may have additional bearers configured. There is one
Packet Data Convergence Protocol (PDCP) and Radio Link Control (RLC) per radio bearer,
including functionalities such as robust header compression (ROHC), security, segmentation,
outer automatic repeat request (ARQ), etc. Thus, the PDCP and RLC are the same as in LTE Rel-
8 [1],[8],[9]. The interface between the RLC and the Medium Access Control (MAC) is referred
to as logical channels. There is one MAC per user, which controls the multiplexing (MUX) of
data from all logical channels to the user, and how this data is transmitted on the available CCs.
As illustrated in Fig. 2, there is a separate Hybrid ARQ (HARQ) entity per CC, which essentially
means that transmitted data on CC #X shall also be retransmitted on CC #X in case prior
transmission(s) are erroneous. The interface between the MAC and physical layer (PHY) layer –
denoted transport channels – is also separate for each CC. The transport blocks sent on different
CCs can be transmitted with independent modulation and coding schemes, as well as different
MIMO coding schemes. The latter allows that data on one CC is transmitted with open loop
transmit diversity, while data on another CC is sent with dual stream closed loop pre-coding.
Thus, there is independent link adaptation per CC to benefit from optimally matching the
transmission on different CCs according to the experienced radio conditions, i.e. corresponding to
frequency domain link adaption on a CC resolution. The system also allows for using different
transmit power settings for the CCs, so that they in principle could have different levels of
coverage as also discussed in [7].
ROHC
Security
Segmentation
Outer ARQ
MUX
HARQ
PHY
CC #1
HARQ
PHY
CC #2
HARQ
PHY
CC #N
ROHC
Security
Segmentation
Outer ARQ
ROHC
Security
Segmentation
Outer ARQ
Radio Bearer
#1
Radio Bearer
#2
Radio Bearer
#K
....
....
....
....
PDCP
RLC
MAC
PHY
Admission
Control
QoS Manager
CC
Configuration
Link Adaptation
&
HARQ manager
CC #1
Dynamic Packet Scheduler &
CC activation / de-activation
Link Adaptation
&
HARQ manager
CC #2
Link Adaptation
&
HARQ manager
CC #N
....
Radio Resource Management (RRM) functions
Fig. 2: Overview of the downlink user plane architecture (left side) and the corresponding RRM
algorithms (right side).
The LTE Rel-8 control plane protocol stack also applies to LTE-Advanced with multiple
CCs, meaning that there is one Radio Resource Control (RRC) per user, independent of the
number of CCs. Similarly, idle mode mobility procedures of LTE Rel-8 also apply in a network
deploying CA. It is also possible for a network to configure only a subset of CCs for idle mode
camping.
III-B RRM considerations
The RRM framework for LTE-Advanced has many similarities with that of LTE Rel-8
[9]. Admission control is performed at the base station prior to establishment of new radio
bearers, and the corresponding quality of service (QoS) parameters are configured. The QoS
parameters are the same for LTE Rel-8 and LTE-Advanced, and are thus CC independent – see
more information in [1], [8], [9]. However, a new RRM functionality is introduced with LTE-
Advanced, which we refer to as CC configuration in the following. The latter functionality
configures a CC set for each user. The CC set is the collection of CCs where the user may
afterwards be scheduled. The CC set is configured to the users with RRC signaling. The CC
configuration functionality is an important apparatus for optimizing the system performance, as
well as limiting the power consumption for the users. The latter originates from the fact that the
power consumption per user increases with the number of CCs that a user has to receive (i.e.
increases with bandwidth it needs to process). The overall framework for the CC configuration is
illustrated in Fig. 3, where an example of input information is illustrated. For each user, QoS
parameters, radio bearer configuration, and terminal capability are useful a priori knowledge for
determining the CC set. Legacy Rel-8 users naturally only support one CC, and shall therefore
only be allocated on a single CC. For optimal system performance, it is desirable to have
