IEEE C802.16m-08/991
Project
IEEE 802.16 Broadband Wireless Access Working Group <http://ieee802.org/16>
Title
Considerations for Carrier Aggregation
Date
Submitted
2008-09-05
Source(s)
Youngsoo Yuk, Inuk Jung, Ronny
Yongho Kim and Kiseon Ryu
Voice: +82-31-450-7808
E-mail: sixs@lge.com
LG Electronic Inc.
Re:
PHY: Multi-carrier Operation; in response to the TGm Call for Contributions and Comments
802.16m-08/033 for Session 57
Abstract
This contribution covers the considerations on the carrier aggregation for supporting the
multicarrier system.
Purpose
To be discussed and adopted by TGm for use in the IEEE 802.16m SDD
Notice
Release
Patent
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IEEE C802.16m-08/991
Considerations for Carrier Aggregation
Youngsoo Yuk, Inuk Jung, Ronny Yong-ho Kim and Kiseon Ryu
LG Electronics
1. Definitions for Carrier Aggregation
Before discussing about the carrier aggregation, some concepts are defined as follows to clarify the various
concepts about carrier aggregation.
-
-
Carrier Aggregation: To transmit the data on the multiple sub-bands contiguously located by using
single RF transmitter and one BB processing (single large FFT). Lager bandwidth than 20MHz can be
supported by the carrier aggregation.
Sub-band: The independent RF band for transmitting IEEE 802.16m signal which is aggregated with
other sub-bands to form a larger bandwidth. Each sub-band maintains its original structure for
supporting MS with single carrier even it is aggregated to the larger bandwidth.
Guard Resource: Additional subcarriers to be available for transmission after carrier aggregation. The
size of guard resource depends on the bandwidth of two adjacent sub-bands.
Guard Band: the guard subcarriers not to be used for transmission which is located at the edge of
aggregated bandwidth.
Center frequency of the aggregated bandwidth: The center of the total aggregated bandwidth. It can be
located either guard resource or a sub-band. In case that it is located at the center of a sub-band, it is the
same location as the center frequency of the sub-band. The center frequency should be punctured for
reducing DC offset.
Center frequency of sub-band: The center frequency of each sub-band.
Guard
Band
Sub-band 1
Sub-band 2
Center
Frequency of
Aggregated
Bandwidth
Guard
Resource
Sub-band 3
Center
Frequency of
Sub-band 4
Sub-band 4
Guard
Band
Figure 1. Basic structure of carrier aggregation
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2. Requirement of Total Bandwidth for Carrier Aggregation
In the ITU-R requirement for IMT-Advanced [1], the minimum bandwidth to be supported is determined as
40MHz, and the extension to larger bandwidth (e.g. up to 100MHz) is encouraged. The IEEE802.16m SRD[2]
also provides the requirement of the operating bandwidth as follows:
“IEEE 802.16m shall support scalable bandwidths from 5 to 40 MHz. This bandwidth may be supported by
single or multiple RF carriers. Other bandwidths shall be considered as necessary to meet operator and ITU-R
requirements.”
The current IEEE802.16m SDD defines up to 20MHz operation, and to extend this limitation the carrier
aggregation should be considered. Though the use of multiple RF carriers can be considered, we focus on the
carrier aggregation in this contribution. First, 40MHz of bandwidth should be supported as a default. Though
about the larger bandwidth than 40MHz the feasibility of RF solutions should be examined, to achieve the
requirement of ITU-R the discussion about the larger bandwidth is also an important issue at this stage.
Though we can have various considerations, we should start from the requirement. The requirements related to
the carrier aggregation are as follows.
 Peak Data Rate : 100 Mbit/s for high and 1 Gbit/s for low mobility
 Bandwidth : to be supported by single or multiple RF carriers and to support a scalable bandwidth up to
and including 40 MHz (e.g. up to 100MHz)
 Peak Spectral Efficiency : 15 bits/s/Hz (DL), 6.75 bits/s/Hz (UL)
From these requirements, we can induce the minimum requirement for the maximum aggregated bandwidth
(FABW) as: FABW ≥ 1Gbps/15bps/Hz = 66.66MHz. Thus we should consider the bandwidth larger than 67MHz
(e.g. : 70, 80, 90, 100 MHz) .
In addition, for more efficient design some options should be considered.
A. Bandwidth of Sub-band
We can consider which bandwidths are used for aggregating among 5, 10 and 20MHz. If we consider all 3 types,
we should consider too many options of configuration for carrier aggregation. Thus it is possible to exclude
5MHz for carrier aggregation or to restrict the use of 5MHz sub-band for some limited case (e.g. 10MHz with
two 5MHz band) for more efficient control.
B. Bandwidth of Total Aggregated Bandwidth
About the granularity of the total bandwidth, 10MHz seems to be efficient for reducing the complexity.
The total bandwidth is deeply related to the FFT size. In the point of view of complexity, FFT should be applied.
Thus the bandwidth of 10, 20, 40 and 80 MHz should be considered at first. From the above calculation of
required bandwidth, 80MHz is enough to support 1Gbps of peak data rate.
For another bandwidths (30, 50, 60, 70, 90, 100 MHz), we can have multiple options for transmission.
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Assume that MS can support maximum 8192 FFT then all other band can be received by removing the
additional band after receiving the signal of 80MHz though this method has problem of the dummy operation
and the loss of SNR for large LNA bandwidth. More discussions are required to determine whether we support
the bandwidth larger than 80MHz (90, 100 MHz) as an aggregated bandwidth in the point of terminal
complexity. The 16384-FFT should be performed and the required complexity is too high. As a matter of fact, it
is a problem of implementation. However, the implementation loss also should be considered.
Another approach is to use multiple RF transceivers. (e.g. 50MHz = 40 + 10, 60MHz = 40 + 20 MHz, 100 MHz
= 80 + 20 MHz or 60 + 40 MHz …). In this case, though the guard of each aggregated band cannot be utilized,
we can minimize the loss above mentioned. Even it requires additional complexity, the combination of oversampling for FFT and rate-conversion filter also can be considered, and it is the problem of the implementation.
C. Spectral Shape considering Transmitter Spectral Mask
Last, we should consider the transmitter spectral mask for designing the bandwidth. As we already mentioned in
[2], the spectral mask of the aggregated bandwidth should be different from that of the non-aggregated
bandwidth. The Figure 2 shows the difference between two cases.
dBr
0
A
B
C
D
10MHz
4.75
5.45
9.75
14.75
20MHz
9.5
10.9
19.5
29.5
Aggregated
20MHz
9.75
10.45
14.75
19.75
-25
-35
-50
A B
C
D
MHz
20MHz
250 kHz
10MHz
10MHz
Spectral Mask for
Single-carrier System
500 kHz
Spectral Mask for Aggregated System
Figure 2. Requirement of Spectral Mask for the aggregated bandwidth
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In addition, the symmetric property may help the design of the TX and RX filters. Figure 3 shows the problem
of the asymmetric configuration. We can have two solutions for this problem.
First is about the configuration order of sub-bands. We can use the symmetric spectral mask if we use the edge
sub-bands having same size. (e.g. 60MHz with 10+20+10+10+10 MHz (Figure 3(a) not 10+20+10+20
MHz(Figure 3(b)). The second method is to use the fixed spectral mask for lowest sub-band even for every case
(figure 3(c)). In this case, we can use more guard resource if larger sub-band is applied for the edge of the
aggregated bandwidth.
Spectral Mask
Symmetric
Mask
Guard
Band
Frequency
Center
Frequency
Guard
Resource
Sub-band
(a) Symmetric Edge Sub-band
Asymmetric
Mask
(b) Asymmetric Edge Sub-band with
asymmetric spectral mask
Symmetric
Mask
(c) Asymmetric Edge Sub-band with symmetric
spectral mask (additional guard resource)
Figure 3. Considerations for Spectral Mask for the aggregated bandwidth
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IEEE C802.16m-08/991
In spite of the development of RF devices, it is possible not to support larger RF bandwidth having the same
edge shape as that of the narrow bandwidth. It is also a problem of cost of MS terminal.
Thus we can consider two options for supporting multicarrier with dull TX/RX receiver.
First is to use multiple narrow RF. Especially about 80MHz, assuming 10% of total guard band, the guard band
size of one side should be 4MHz. However, if we want to use 10MHz sub-band at the edge, the guard band
should be reduced to 1MHz. Though it is a problem of implementation, two bands of 40MHz with two RF
carriers can be considered to support 80MHz band.
Second method is not to use the resources of edge bands which are not included in the flat-gain region shown in
figure 4. We can simply solve this problem with scheduling optimization. However, for the broadcast channels
such as SCH, PBCH/SBCH, we cannot solve the problem with scheduling. From this reason, it is considered as
a possible solution to install the partially configured carriers at the edge sub-bands.
If SCH and PBCH/SBCH do not occupy the whole bandwidth of sub-band, fully configured carriers can be
installed at the edge bands with limited burst scheduling.
In addition, due to above restriction, the legacy system (IEEE802.16e) is not recommended to be installed at the
edge sub-bands.
Symmetric
Mask
Figure 4. The example case with dull spectral mask
3. Aggregating Multiple Sub-bands
The configuration of carrier aggregation is related to various parameters of the sub-bands of the total aggregated
bandwidth. For example, according to the order of various sub-bands and the center frequencies and so on, so
many options of carrier aggregation structures can be considered with fully flexibility. Even the flexibility
provides system efficiency to the system operators, even with limited flexibilities enough efficiency can be
supported as well. On the other hand, the flexibility requires overhead to transmit the information about the
configurations.
To minimize the system information to be transmitted for supporting carrier aggregation, the transmission of the
composite index can be an efficient solution. We’ve already proposed the concept of MCI (Multicarrier
Configuration Index) at the last meeting [2]. The basic concept of MCI is based on the use of the finite number
of predetermined configuration. MCI is a simple index to indicate a configuration among these predetermined
configurations.
For the example of the important system information, the total bandwidth, the center frequency, the sub-band
configuration, guard resource type and supportable MS type etc can be considered. By applying some design
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IEEE C802.16m-08/991
rules, we can simply choose the predetermined configurations among the various configurations.
We propose the use of MCI which represents the total bandwidth and the sub-band configuration order. By
using some design rules, we can simply describe whole configuration structure only with MCI.
The examples of the aggregation rules are as follows:
 The DC sub-carrier of the total aggregated bandwidth is located on the center frequency of the total
bandwidth.
 The center frequency of total bandwidth should be the center frequency of a sub-band or be located
between two sub-bands for removing DC offset.
 The multiple sub-bands are aggregated the center outwards.
 The center-frequency of each sub-band is shifted toward the center of the total band for maintaining the
spacing between the sub-carriers in the different sub-bands.
 The shifted frequency offset should be smaller than the subcarrier spacing specified in Table 2 of
section 11.3 OFDMA parameters of SDD [3].
In addition above rules, we can consider the restriction on the total number of sub-bands within an aggregated
bandwidth, and it can be discussed at the next stage.
Figure 5 shows the examples of the configuration by adopting the above rules. By using these rules, we can
acquire the detailed configuration with such system information; The total bandwidth, lowest frequency of the
total bandwidth, the number of sub-bands and the order of sub-bands.
Among those information, the last two or three parameters can be replaced by MCI. Basically MCI indicates the
simple order of the multiple sub-bands. The total bandwidth can be either included in MCI or not. Table 1
shows the example of the MCI for some configuration (Even single carrier case, MCI can be used instead of the
bandwidth as shown in Table 1).
It is efficient to minimize the number of configurations (size of MIC) for reducing the amount of system
information and the implementation effort.
Table 1. The example of MCI for carrier aggregation
0
1
2
3
4
5
6
7
…
N
Total
Bandwidth
5MHz
10MHz
10MHz
20MHz
20MHz
40MHz
40MHz
40MHz
…
80MHz
…
~100MHz (?)
MCI
Number of
Sub-bands
1
1
2
1
2
4
3
2
The order of Sub-bands (bitmap :
0:5MHz, 1: 10MHz, 2:20MHz)
0
1
00
2
00
0000
010
11
4
1111
….
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Center frequency
(Carrier frequency)
Df1+Df2
Df1
Df1-
Frequency shifting
for alignment
Frequency shifting
for alignment
Total BW for aggregation
(a) The overall center frequency is located between the sub-bands
Center frequency
(Carrier frequency)
Frequency shifting
for alignment (Df1)
Frequency shifting
for alignment
Total BW for aggregation
(b) The overall center frequency is the same frequency of the sub-band
located at the center
Figure 5 The examples of the configuration of the carrier aggregation according to the configuration rules.
References
[1] ITU-R Working Party 5D, DRAFT REPORT ON REQUIREMENTS RELATED TO TECHNICAL
SYSTEM PERFORMANCE FOR IMT-ADVANCED RADIO INTERFACE(S) [IMT.TECH],
Document 5D/TEMP/89, 2008-06-30
[2] IEEE 802.16m-08/802r3, The PRU Configuration for BW Aggregation, 2008-07-16
[3] IEEE 802.16m-08/003r4, The Draft IEEE 802.16m System Description Document, 2008-07-29
Text Proposal for the IEEE802.16m SDD
============================== Start of Proposed Text =================================
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IEEE C802.16m-08/991
19.x The Bandwidth Characteristics
19.x.1 Bandwidth Requirement for Carrier Aggregation
The bandwidths of sub-band for carrier aggregation are 10 and 20MHz. The use of 5MHz is F.F.S.
The total bandwidths of the carrier aggregation are 20MHz, 40MHz, 80MHz, and the supporting of other
bandwidths is F.F.S.
The center frequency In case of larger FFT size than the total aggregated bandwidth is applied (e.g. 50, 60 …
MHz ), the center frequency of the aggregated bandwidth is located at the center of the FFT band.
The same size of bandwidth is applied to the two edge bands.
19.y The Carrier Aggregation
Figure 19.x shows the definitions related to the carrier aggregation.
-
-
Carrier Aggregation: To transmit the data on the multiple sub-bands contiguously located by using
single RF transmitter and one BB processing. Lager bandwidth than 20MHz can be supported by the
carrier aggregation.
Aggregated Bandwidth: The total bandwidth after carrier aggregation.
Sub-band: The independent RF band for transmitting IEEE 802.16m signal which is aggregated to form
the larger bandwidth.
Guard Resource: Additional subcarriers to be available for transmission after carrier aggregation. The
size of guard resource depends on the bandwidth of two adjacent sub-bands.
Guard Band: the guard subcarriers not to be used for transmission which is located at the edge of
aggregated bandwidth.
Center frequency of the aggregated bandwidth : The center of the total aggregated bandwidth. It can be
located either guard resource or a sub-band. In case that it is located at the center of a sub-band, it is the
same location as the center frequency of the sub-band. The center frequency should be punctured for
reducing DC offset.
The center frequency of sub-band: The center frequency of each sub-band.
19.y.1 Configuration of carrier aggregation
The multiple carriers are aggregated the center outwards. The center frequency of each band is shifted with
predetermined frequency offset for alignment of subcarrier spacing.
The pre-determined configurations of the carrier aggregation are used for reducing the implementation efforts.
The pre-determined configuration is specified by the parameters such as total bandwidth, lowest frequency of
the total bandwidth, the number of sub-bands and the order of sub-bands.
To inform the configuration of the aggregated bandwidth, BS transmits these parameters to MSs in forms of
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Multicarrier Configuration Index.
============================== End of Proposed Text =================================
10