A Novel Cross-Carrier Scheduling Method in
Carrier Aggregation
Liang Zeng, Qimei Cui
Key Laboratory of Universal Wireless Communication, Ministry of Education
Beijing University of Posts and Telecommunications
Beijing, China, 100876, E-mail: buptzengliang@gmail.com
Abstract—Long Term Evolution (LTE) of Universal Mobile
Telecommunications System (UMTS) Terrestrial Radio
Access and Radio Access Network is a Fourth Generation
(4G) wireless broadband technology which is capable of
delivering high data transmission rates and low latency with
reduced costs, among other promises. Carrier Aggregation
(CA) is employed to improve the capacity of the cell.
Regarding scheduling there are two main alternatives for
CA, either resources are scheduled on the same carrier as
the grant is received, or cross-carrier scheduling may be
used. A key research topic is to design efficient and reliable
Physical Downlink Control Channels (PDCCHs) that carry
the downlink scheduling assignments and uplink scheduling
grants. In Release11, the New Carrier Type characterized by
no PDCCH was put forward which has to rely on the crosscarrier scheduling. However with the increase in carriers,
CA-based HetNet with cross-carrier scheduling, in which
case the PDCCH capacity might become a limiting factor. At
the same time, the UE complexity for blind decoding is much
higher than before. To solve the problems a novel crosscarrier scheduling method is proposed in this paper to
reduce the PDCCH capacity blocking probability and UE
complexity.
Keywords—LTE-A; Carrier Aggregation (CA); PDCCH;
Carrier Indicator Field (CIF); New Carrier Type (NCT)
I. INTRODUCTION
Long Term Evolution (LTE) of UMTS Terrestrial
Radio Access and Radio Access Network is a Fourth
Generation (4G) wireless broadband technology.
In order to achieve up to 1 Gb/s peak data rate in future
IMT-Advanced mobile systems, Carrier Aggregation(CA)
technology is introduced by the 3GPP to support veryhigh-data-rate transmissions over wide frequency
bandwidths (e.g., up to 100 MHz) in its new LTEAdvanced standards.[1]. In the CA, multiple basic
frequency
blocks
called
component
carriers
(CCs).Regarding scheduling there are two main
alternatives for CA, either resources are scheduled on the
same carrier as the grant is received, or so called crosscarrier scheduling may be used. See Fig.1. [2]
Fig. 1 Carrier Aggregation and cross-carrier scheduling
The Physical Downlink Control Channel (PDCCH) is
used for downlink control information, mainly scheduling
decisions, required for reception of PDSCH (The Physical
ÄÄÄĈ+'''
Downlink Shared Channel), and for scheduling grants
enabling transmission on the PUSCH (The Physical
Uplink Shared Channel). The enhanced physical
downlink control channel (EPDCCH) is introduced in
3GPP LTE Release11.In this paper, the problem and the
solution to the problem is suitable for both PDCCH and
the EPDCCH. So the analysis is mainly based on the
PDCCH.
When the cross-carrier scheduling is been used, the
component carriers are divided into two parts, primary
component carrier and secondary component carriers. The
primary component carries the PDCCHs both for primary
component carrier and secondary component carriers
while the secondary carries carry no PDCCH. In the
presence of cross-carrier scheduling, where downlink
PDSCH or uplink PUSCH is transmitted on an
(associated) component carrier other than the PDCCH,
the carrier indicator in the PDCCH provides information
about the component carrier used for the PDSCH or
PUSCH. [3]
The PDCCH is used to carry downlink control
information (DCI) such as scheduling decisions and
power-control commands. The different types of control
information correspond to different DCI message sizes.
Whether cross-carrier scheduling is used or not is,
configured using higher-layer signalling; if cross-carrier
scheduling is not configured then no carrier indication
field is included in the DCI. Thus, most of the DCI
formats come in two “flavors”, with and without the
carrier indication field, and which “flavor” the terminal is
supposed to monitor is determined by enabling/disabling
support for cross-carrier scheduling. To signal which
component carrier a grant relates to, the component
carriers are numbered. The primary component carrier is
always given the number zero, while the different
secondary component carriers are assigned a unique
number each through UE-specific RRC signalling.
In the process of standard effort, one purpose of carrier
aggregation is studying additional carrier types including
non-backwards compatible elements for Carrier
Aggregation [4] [5].
In Release11 the New Carrier Type(NCT)is proposed
in order to minimizing legacy control signalling and
common reference signals reduces the interference and
overhead level at low-to-medium loads, allowing for
higher end-user throughput and improving system
spectral efficiency.[6]
The New Carrier Type is characterized by no
PDCCH/PHICH/PCFICH, no PSS/SSS, and no CRS. A
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+%%22TQEGGFKPIU
contradiction for the New Carrier Type comes obviously,
which the absence of the PDCCH and the NCT’s
requirement for PDCCH. So the Cross-Carrier Scheduling
based on Carrier Aggregation is the inevitable choice.
And the legacy carrier and the NCT will be aggregated
together. The primary carrier is the legacy carrier and the
secondary is the New Carrier Type. The primary one
carries the PDCCH not only for itself but also for the New
Carrier Type.
Then a problem appears in the primary carrier. It will
carry much more PDCCHs than before. The PDCCH
capacity blocking is becoming more and more seriously.
At the same time, for a UE, the complexity of decoding is
increasing, especially the searching times in search space
is multiplied.
In this paper, a novel cross-carrier scheduling method
in carrier aggregation is proposed to reduce the PDCCH
capacity blocking probability and decrease the search
times in search space especially when the New Carrier
Type is been utilized. The remaining part is outlined as
follow. In Section II, the cause of the problem is analysed
in-depth. The solution of the problem and an example is
involved in Section III. In Section IV, the solution is
evaluated from several different aspects. We conclude the
paper in Section V.
II. PROBLEM ANALYSIS
The problem of the PDCCH/EPDCCH capacity and the
multiplied searching times is caused by several reasons.
The internal reason is that the categories of the PDCCH
format and DCI format are increasing as the continual
emergence of new demands. The external reason is that
the primary carrier multiple PDCCHs inevitably in CA
based seniors utilizing the NCT.
The PDCCH has 4 formats, see Table 1.And the
EPDCCH has 5 format (Case A and Case B), see Table 2.
[7]
TABLE 1
Number of CCEs
0
1
2
3
1
2
4
8
Number of ECCEs for one EPDCCH
EPDCCH
format
Case B
Localized
transmission
Distributed
transmission
0
1
1
1
2
2
2
4
4
3
8
8
4
-
16
The DCI is categorized into different DCI formats,
where a format corresponds to a certain message size and
usage. There are 13 formats in all in Release 11 [8].
The number of format both for PDCCH and the DCI is
increasing as the function is more than before. Each
PDCCH supports multiple formats and the format used is
a priori unknown to the terminal. Therefore, the terminal
needs to blindly detect the format of the PDCCHs. To
limit the maximum number of blind decoding attempts in
the terminal, LTE defines so-called search spaces. In
release 8/9, the number of blind decoding attempts is 44
per subframe, while for release 10 with uplink spatial
multiplexing the number is 60 assuming a single
component carrier. In CA, the number will be multiplied.
While the CA is deployed and the cross-carrier
scheduling is enabled, the requirement of the resource is
multiplied and the searching times of the search space in
the primary component carrier is multiplied too. As the
Fig.2 shows, assuming that 3 component carrier
aggregated, the need for resource available is three times
than before, 144bits at least and 1252 bits at most. At the
same time, the searching times of search space will be
120 times more than before.
Number of PDCCH
bits
72
144
288
576
FORMAT OF EPDCCH IN CASE A
Number of ECCEs for one EPDCCH
EPDCCH
format
FORMAT OF EPDCCH IN CASE B
FORMAT OF PDCCH
PDCCH format
TABLE 2A
TABLE 2B
Case A
Localized
transmission
Distributed transmission
0
2
2
1
4
4
2
8
8
3
16
16
4
-
32
Fig.2 Search space with multiple carriers
It is obvious that the problem of PDCCH capacity
blocking and the complexity of the searching times are
serious, as the more carriers are aggregated. This problem
cannot be evaded especially the NCT is introduced which
charactered with no PDCCH itself.
III. SOLUTION
To solve the problem, we propose a method which to
merge the DCI so that decreasing the capacity blocking
and the searching times.
As the Fig.3 and Fig.4 show, it is an example for one
just one UE. The UE should receive the PDCCH carried
by 3 carriers respectively. In the old way, no matter
whether the #1, #2 or #3 is same or not, the DCI is
transmitted 3 times. Now we proposed that the 3 DCIs
should be merged to 1 DCI indicating 3 carriers at the
same time.
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information of component carrier via RRC signalling, and
then UE can decode the ECIF easily.
IV. FEASIBILITY ANALYSIS
In this part, we make a feasible analysis to the proposal.
Firstˈthis proposal is compatible well with the old
way. Another important consideration is the feasibility
analysis about merging DCI.
ECIF can be seen as an expanding of the CIF. An
example is shown below. See Table 4.If there are 3
carriers, and the CIF and the ECIF is both 3bits. The
ECIF expands the meaning of the CIF using the muted
code bits.
Fig. 3 DCIS for different carrier
Fig. 4 Merged DCI for different carriers
TABLE 4 EXAMPLE OF COMPATIBILITY
The CIF cannot satisfy the new demand as the CIF just
can indicate one carrier once. In order to reach the goals
of merging the DCI, Enhanced Carrier Indicator Field
(ECIF) is proposed. The ECIF can indicate several
carriers at the same time.The ECIF have two
characteristics different from CIF. See Table 3.
TABLE 3.COMPARIATION OF CIF AND ECIF
Number of bits
Function
CIF
3(fixed)
Only indicates on which
carrier the scheduled resource
is located.
ECIF
Changing dynamically
Indicates on which carrier the
scheduled resource is located
and the multiplexing
information
For example, if one UE’s CA ability is 5, the DCI in
CC1and CC2 is same; the DCI in CC4and CC5 is same.
The ECIF is 5 bits. A possible configuration for ECIF is
shown in Fig.5 and Fig.6.
Fig. 5 Five component carriers
Fig. 6 Difference between CIF and ECIF
The bits number of ECIF is changing dynamically. We
proposed that the bits number of ECIF is equal to the
number carrier. It can be proven that the bits is been
utilization in optimum.
The number of component carrier is n, and the different
combination may be exit.
Cn 1+ Cn2+ Cn3 ĂĂ + Cn n-1+ Cn n=2n-1
To satisfy the situation of 2n-1, the least number of
Binary numbers is n.
The eNodeB configure the ECIF according to the
number of component carrier, and the UE get the
CIF
ECIF
Description
CC1
001
001
Retention
CC2
010
010
Retention
CC3
011
100
Retention
CC1ǃ
ǃCC2
None
011
added
CC1ǃCC3
None
101
added
CC2ǃCC3
None
110
added
CC1ǃCC2ǃCC3
None
111
added
The consideration of merging DCI mainly depends on
two respects: carrier bandwidth, and the channel quality.
The length of DCI is influenced by the system bandwidth,
and the content of DCI is influenced by channel quality.
So the ECIf can be applied as the carrier has the similar
channel quality and the same system bandwidth.
One of the applied scenery typically is small cell
especially combining the New Carrier Type as the
component carrier. The small cell have characteristic as
below:
Big bandwidth: Using more bandwidth of 200MHz;
Low mobility: the smaller mobility of indoor scenes;
Small coverage: Indoor scenes and the using of high
frequency band lead to the smaller coverage.
The small coverage and the big bandwidth are very
suitable for the Carrier Aggregation. And the low
mobility and indoor scenes always mean that the channel
quality is steady and it is high probably the component
carriers have the similar channel quality.
V. BENEFITS AND INFLUENCE
The benefits are obvious, which are reducing the
PDCCH capacity blocking and decreasing the searching
times in searching space. It is analysed specifically that
how much of the benefits are in this part.
The benefits of this method varies from the number of
the subframe, the size of the PDCCH formats, the number
of the component carriers and the number of the merged
carriers and so on. The first, we consider the gain of the
block capacity vs. size of the PDCCH format. Then we
consider the gain varies from the number of merged
carriers.
As mentioned above, we group the component carrier
according to the channel quality. The Channel-Quality
Indicator (CQI) indicates the channel quality. And the
value of the CQI is from integer 0 to 15, 16 levels in all.
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The component carriers are grouped according to the CQI.
The value of the CQI from 1 to 6 is grouped and the value
of CQI from 7 to 9 is grouped. The value of the CQI from
10 to 15 is grouped.
The single-downlink simulation only covers the link
between one base-station and one user equipment.
Assume that the number of the component carrier and the
number of merged component carriers is fixed. There are
3 component carrier and 2 of them are be merged. The
number of the PDCCH is only 1. The number of the
subframe is 100.The result of bits number vs. PDCCH
format is below. See Fig.7.
From the Fig.7, we can see that the saving bits are
more as the size of format is bigger. The saving bits are
115k just in 100subframes just for 2 merged component
carriers for format 3.
Then the gain varies from the number of the merged
component carriers is analysed. Assume that the PDCCH
format is fixed and just one UE and just one PDCCH for
each component carrier. The PDCCH format is format 3.
The number of the PDCCH format is 1. The number of
the subframe is 100. The result of the search times vs.
number of carriers is below. See Fig.8. The result of the
bits number vs. number of carriers is below. See Fig.9.
From the Fig.8 we can conclude that the searching
times is decreasing using ECIF and the searching times
decreasing more as the number of merged component
carriers increased. As the number of emerged component
carriers is 5, the searching times is decreasing 12000
times.
From the Fig.9 we can conclude that the bits number is
decreasing using ECIF and the bits number decreasing
more as the number of merged component carriers
increased. As the number of emerged component carriers
is 5, the bits number is decreasing 115200 bits in all.
As the Fig.8 and the Fig.9 only consider 1 UE and 1
format, the gain will be much more while the more UEs
and PDCCHs are more.
Fig.8 Search times vs. Number of carriers
Fig.9 Bits number vs. Number of carriers
VI. CONCLUSIONS
In this paper we present a novel cross-carrier
scheduling method in carrier aggregation.it decreases the
average number of blind decoding and reduces the
PDCCH capacity blocking probability effectively.
Besides, the proposed ECIF is compatible with the CIF
well. In the future 3GPP standard effort, with the increase
of the aggregated CCs, and the extensive utilization of the
NCT, the performance gain in the proposed method will
be more significant.
REFERENCES
[1] Guangxiang Yuan, "Carrier aggregation for LTE-advanced
mobile Communication systems," IEEE Comm. Magazine,
February, 2010, pp. 88-93.
[2] http://www.3gpp.org/Carrier-Aggregation-explained
[3] Erik Dahlman, Stefan Parkvall, Johan Skold, "4G LTE/LTEAdvanced for Mobile Broadband," Elsevier, 2012.
[4] RP-110451, "LTE Carrier Aggregation Enhancements WID",
Nokia Corporation, Nokia Siemens Networks.
[5] RP-110732,"Update to LTE Carrier Aggregation Enhancements
WID", Nokia Corporation, Nokia Siemens Networks.
[6] 3GPP TS 36.213 Physical layer procedures
[7] 3GPP TS 36.211 Physical Channels and Modulation
[8] RP-122028, " Updated WI proposal: New Carrier Type for LTE ",
Ericsson.
[9] R1-113131, “Summary of email discussion on downlink control
signaling enhancements for CA ", Nokia Corporation.
[10] 3GPP TS 36.213 Multiplexing and channel coding.
Fig.7 Bits number vs. PDCCH format
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