A Multi-User MIMO Resource Scheduling Scheme

8/2/2019 A Multi-User MIMO Resource Scheduling Scheme

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A Multi-user MIMO Resource Scheduling Scheme

for Carrier Aggregation Scenario

Na Lei, Caili Guo, Chunyan Feng, Yu ChenSchool of Information and Communication Engineering

Beijing University of Posts and Telecommunications

Beijing , China

E-mail: [email protected], [email protected], [email protected], [email protected]

AbstractThis paper focuses on the multi-user MIMO re-source scheduling of LTE-Advanced system with carrier aggre-gation. With the special function of a UEs primary componentcarrier (PCC), the original proportional fair (PF) schedulingscheme can result in invalid user groups for multi-user MIMOtransmission, making UEs of the user group can not be trans-mitted. A multi-user MIMO resource scheduling scheme forcarrier aggregation scenario (RSM-CA) is proposed. Comparedto PF, RSM-CA guarantees the resource on the PCC of a UE

is primarily allocated to the UE, making sure all user groupsare valid. Furthermore, frequency selective diversity, which isspecific for carrier aggregation scenario, is exploit in RSM-CAto maximize the system throughput. According to the systemlevel simulations of downlink LTE-A, RSM-CA can achieve bettersystem throughput than the original PF scheduling scheme.

Keywords-LTE-Advanced, carrier aggregation, multi-userMIMO, proportional fair

I. INTRODUCTION

In order to support wider transmission bandwidths e.g.

up to 100MHz, the LTE-Advanced system introduces the

carrier aggregation technology, where two or more component

carriers belonging to a single frequency band or different

frequency bands can be aggregated[1]. With the carrier ag-

gregation technology, it will be possible to schedule a user

(UE) on multiple component carriers simultaneously, but a UE

can not use all the component carriers in its belonging cell, a

certain UE uses only a certain set of the component carriers

in one cell according to their own aggregation abilities[2].

Therefore, different UEs have different sets of aggregated

carriers, and the control signaling is transmitted on one of

the aggregated carriers, this special carrier is called Primary

Component Carrier (PCC), which can not be deactivated, that

is to say, if a UE doesnt get a Resource Block (RB) from its

PCC during the resource scheduling procedure, it can not betransmitted. Thus, some new problems should be considered

in resource scheduling procedure.

The resource scheduling problem in carrier aggregation sce-

nario is especially severe for multi-user MIMO transmission,

which can allow more than one UE to use a RB. While using

the classic scheduling scheme, such as proportional fair (PF)

This work is supported by Chinese National Nature Science Foundation(60902047) and the Fundamental Research Funds for the Central Universi-ties (2011RC0113).

algorithm[35] , without taking care of the PCC, one or more

UEs scheduled on a RB may not be transmitted, resulting

an invalid multi-user MIMO user group. Moreover, in non

adjacent inter band aggregation scenario, where the aggregated

carriers belong to different frequency bands, the fading char-

acteristics are different between carriers, such as the path loss

and Doppler shift[6]. This can result in spectrum heterogeneity

that can be used as frequency selective diversity, which theresource scheduling scheme in one component carrier system

can not exploit to optimize the system performance.

In this paper, a multi-user MIMO Resource Scheduling

Scheme for Carrier Aggregation scenario (RSM-CA) is pro-

posed. Compared to PF, RSM-CA guarantees the resource on

the PCC of a UE is primarily allocated to the UE, making sure

all user groups are valid. In order to use frequency selective

diversity, different users have different grouping users on

different carriers aiming to maximize the system throughput.

The rest of this paper is organized as follows. Section

2 gives the system model and background of the proposed

scheduling scheme. Section 3 elaborates the multi-user MIMO

resource scheduling scheme for carrier aggregation. Section 4presents the performance of the proposed scheme, including

system level simulation assumptions and results. Section 5

concludes this paper with a summary of results.

II. SYSTEM MODEL AND BACKGROUND

A typical cell structure in LTE-A system with an eNB

(evolved Node B) and several UEs is shown in Fig.1. For

carrier aggregation scenario, the aggregated carriers are di-

vided into several categories, these categories includes eNB

CC, which is a cells total number of component carriers that

can be allocated to its belonging UEs; UE CC, which is a

subset of the eNB CC that UE selects according to its own

needs and aggregation ability[2]; Active CC, which is the UECC UE actually used in one scheduling process; Deactive CC,

corresponding to active CC, the UE CC that is not used by UE

in one scheduling process is called deactive CC; PCC, which

is the only one carrier of UE CC that is used to transmit the

control signaling, and its predefined by the system, so during

one scheduling process this carrier can not be deactived; SCC,

which is the rest of the UE CC except the PCC. In the case of

Fig.1, there are five eNB CC: C1,C2,C3,C4,C5; The UE CC

of UE1 is: C1,C2,C3, the UE CC of UE2 is: C1,C3,C4,C5;

978-1-4577-1010-0/11/$26.00 2011 IEEE


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User Group

CASE I

InvalidValid

CASE II CASE III

Fig. 3. User Marking Resource Allocation (UMRA) Procedure

component carrier scenario, all the UEs priority are calculated

and compared, but for carrier aggregation scenario, different

UEs have different UE CC, so not all the UEs priorities can

be calculated for a RB, and considering the PCCs function we

should grantee UE is primarily scheduled on its PCCs RB.

Therefore, we modify the scheduling policy as follows:

i = arg maxi

wiRi(t, k)

Ti(t)(2)

where wi is a selection factor, wi [0, 1], it indicateswhether to calculate UE s priority, in order to get its value,

three situations should be considered:

1) If UE hasnt have RB on its PCC, and the scheduling

RB in the current slot belongs to the UEs PCC, wi = 1;2) If UE hasnt have RB on its PCC, and the scheduling RB

in the current slot doesnt belong to the UEs PCC,wi = 0;3) If UE has have RB on its PCC, and the scheduling RB

in the current slot belongs to the UEs UE CC, wi = 1 .

Using the selection factor, UEs priority is first calculatedon its PCC to grantee its transmission.

C. Getting GU

While finding the group user (GU) of the SU in the third

step, we use a user marking resource allocation (UMRA)

procedure to guarantee the GU can be transmitted and the

user group is valid. In order to make the user group valid, we

should make sure the users in one group can be transmitted,

that is, they have all have RB on their PCC. With the second

step, we grantee the SU can be transmitted, so whether the

user group is valid depends on the GU.

As shown in Fig.3, UMRA includes three cases:

CASE I: The scheduling RB in the current slot belongs tothe GUs PCC or the GU has already have RB on its PCC.

CASE II: GU hasnt have RB on its PCC, and there is RB

left on its PCC which can be allocated to the GU, the GU is

marked in order to give it its PCCs RB in the next scheduling

slot.

CASE III: GU hasnt have RB on its PCC, and there isnt

RB left on its PCC.

The marked GU grantees it can be first scheduled in the

next scheduling slot to make its current user group valid.

Getting SU

Get a GU

Find or not?

yes

noSU use the RB

exclusively

Deciding the

transmission mode

Update priority

The SU and GU give the

largest packet size on

the RB

Does SU need

grouping?

When scheduling new

eNB CC or SU hasn t

been grouped

yes

Use the previous

groupno

The user group

valid or not?

yes

Is there marked

user?

noAllocate a RB to the

marked user on its

PCC

yes

Scheduling RB

Delect the GU

from all the

UEs

UMRA

Getting GU

no

Fig. 4. The Overall MU-MIMO-RSM-CA Process

D. Deciding transmission mode

The transmission mode is decided in the forth step, if the

packet size of the SU-MIMO mode is larger than the MU-MIMO mode, the SU-MIMO mode is selected, otherwise, the

MU-MIMO mode is selected.

E. Updating priority

In the last step, after RB k is allocated, the average data

rate Ti(t) is updated as following:

Ti(t + 1) = (1 1

Tc)Ti(t) +

1

TcRi(t, k)b(i) (3)

b(i) ={ 1 if i = i,

0 if i = i. (4)

where Tc is the observation window length of the average

transmission rate in terms of TTI (Transmission Time Inter-

val).

The overall RSM-CA process is described as Fig.4.

In Fig. 4, the ellipse area corresponding to the third step-

getting GU-in Fig. 3, and in order to use frequency selective

diversity, SU finds different GU on different carriers aiming

to maximize the system throughput.


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IV. SIMULATION RESULTS AND ANALYSIS

This section evaluates the performance of the RSM-CA.

The simulation is for downlink of LTE-Advanced system. The

bandwidth of each CC is 10MHz, and each CC has 50 RBs to

allocate to UEs. Table I summarize the simulation parameters.

TABLE ISYSTEM SIMULATION PARAMETERS[7]

Simulation Parameters Settings

Site layout 7 cells wrap-around

Inter-site distance 500 m

Minimum distance between UE and ce ll 35 m

User location Uniformly dropped in all ce lls

UE speeds of interest 3 km/h

Channel Model Spatial Channel Model

Thermal Noise Spectral Density -174 dbm/Hz

Penetration Loss 20 db

Total TX power 40 dBm (40W)

Antenna pattern 1*1

Traffic model Full buffer

Fig. 5 shows the number of invalid user groups with the

increase of user total number in five different scenarios: 1.

MU-MIMO without CA, i.e. there is only one CC in the

system; 2. CA-MU-MIMO with 2 CCs, i.e. each UE can

aggregate 2 CCs, using original PF scheduling scheme to do

resource schedule; 3. CA-MU-MIMO with 3 CCs, i.e. each UE

can aggregate 3 CCs, using original PF scheduling scheme

to do resource schedule; 4. RSM-CA with 2 CCs, i.e. each

UE can aggregate 2 CCs, using the proposed RSM-CA to do

resource schedule; 5. RSM-CA with 3 CCs, i.e. each UE can

aggregate 3 CCs, using the proposed RSM-CA to do resource

schedule. We can see that, the number of invalid user groupdecreases with the increment of the number of aggregated CC,

because there are more resource can be allocated to UEs. So

for scenario 1, when user total number is 160, there are about

30 user groups are invalid, and the number linearly increases

with the increment of user total number, when the system

can aggregate one more CC, there are 50 more user groups

(100 more UEs) can be transmitted. On the other side, while

using the original PF scheduling scheme, when user total

number exceeds RB number, some UEs can not get RB on

their PCC, resulting in invalid user group. As shown in Fig.

5, when user total number exceeds 200 with 2 CCs or user total

number exceeds 300 with 3 CCs, some user groups can not

be transmitted, and the number of invalid user groups linearlyincreases with the increment of user total number. Compared

to scenario 2 and 3, for scenario 4 and 5, the proposed RSM-

CA makes sure there is no invalid user group and all the

scheduled UEs can be transmitted.

For the last four scenarios of Fig. 5, Fig. 6 shows the total

throughput of the system. The system throughput increases

when there are more CC aggregated, because more UEs can

get resource to be transmitted. When the number of UEs

exceeds the number of RBs (i.e. 200 for 2 CC and 300 for 3

CC), some UEs can not get RBs to transmit, the system can

only satisfy a certain number of UEs, which is not larger than

the total resource number, so the throughput stops to increase.

Compared to PF, RSM-CA can have more total throughput

(about 20 Mbps) because there are more valid groups can

be transmitted and frequency selective diversity is exploited

to maximize the system throughput. Some fluctuations can

be seen from Fig.6, that is because each time the user

total number changes, we re-drop the UEs randomly and

with their positions become different their channel state are

different, which leads to different receiving SINRs, when most

UEsSINR are very low, the throughput of more UEs may be

smaller than less UEs .

150 200 250 300 350 4000

50

100

150

User total number

Invalidusergroupnumber

MUMIMO without CA

CAMUMIMO with 2 CCs

CAMUMIMO with 3 CCs

RSMCA with 2 CCs

RSMCA with 3 CCs

Fig. 5. The Number of Invalid User Group with The Increase of User TotalNumber in Five Different Scenarios

100 150 200 250 300 350 40040

60

80

100

120

140

160

User total number

Totalthroughput/Mbps

CAMUMIMO with 2 CCs

CAMUMIMO with 3 CCs

RSMCA with 2 CCs

RSMCA with 3 CCs

Fig. 6. The Total Throughput of The System for Four Different Scenarios

V. CONCLUSION

A multi-user MIMO resource scheduling scheme is pro-

posed in this pager for LTE-Advanced system with carrier


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aggregation technology. Simulation results demonstrate that

the RSM-CA can achieve better system throughput than the

original PF scheduling scheme, especially when the number

of UEs exceeds the number of RBs. The proposed scheme

grantees all scheduled UEs can be transmitted according to

3GPP standard and makes use of frequency selective diversity

aiming to maximize the system throughput.

REFERENCES

[1] 3GPP, 3rd Generation Partnership Project; Technical SpecificationGroup Radio Access Network; Further Advancements for E-UTRA;Physical Layer Aspects(Release 9),,in TR 36.814 V0.4.1, ed, 2009.

[2] 3GPP, 3rd Generation Partnership Project;Technical SpecificationGroup Radio Access Network;Evolved Universal Terrestrial Radio Ac-cess (E-UTRA);Carrier Aggregation;Base Station (BS) radio transmis-sion and reception(Release 10), , in TR 36.808, ed, 2010.

[3] W. C. Chung, et al., A low-complexity beamforming-based schedulingto downlink OFDMA/SDMA systems with multimedia traffic, Wireless

Networks, vol. 17, pp. 611-620, 2011.[4] S. Jagabathula and D. Shah, Fair Scheduling in Networks Through

Packet Election Information Theory, IEEE Transactions on, vol. 57,pp. 1368-1381, 2011.

[5] J. W. Jung, et al., Group Based Proportional Fairness Schedulingwith Imperfect Channel Quality Indicator in OFDMA Systems, IEICE

Transactions on Communications, vol. 94, pp. 599-602, 2011.[6] 3GPP, Doppler Impact of Higher Carrier Frequencies on LTE-A Up-link , R1-090283 2009.

[7] 3GPP, 3rd Generation Partnership Project;Technical SpecificationGroup Radio Access Network;Physical layer aspect for evolved Uni-versal Terrestrial Radio Access(UTRA)(Release 7), ,in TR 25.814, ed,2006.

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A Multi-User MIMO Resource Scheduling Scheme