A Joint User Selection and Resources Optimization Scheduling Algorithm for CoMP

doi:10.4236/cn.2013.53B2026

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Communications and Network, 2013, 5, 136-139

http://dx.doi.org/10.4236/cn.2013.53B2026 Published Online September 2013 (http://www.scirp.org/journal/cn)

A Joint User Selection and Resources Optimization

Scheduling Algorithm for CoMP

Xincheng Zhong, Weihai Li, Yuqin Lv

School of Electronic Engineer, Beijing University of Posts and Telecommunications, Beijing, China

Email: [email protected]

Received April, 2013

ABSTRACT

Coordinated Multi-point (CoMP) transmission technology is one of the key techniques in LTE-Advanced, Which can

share the channel and data information in multiple cells, and optimize the whole system performance. In order to opti-

mize the average sector throughput and improve the fairness of resource scheduling, a scheduling algorithm based on

the resource is mainly investigated. In this algorith m, users in the netwo rk are classified fi rstly and then we combine the

fixed resources division and flexible scheduling. System level simulation platform is set up to validate the algorithm

and the results turn out that the average throughput is better compared with the traditional scheme.

Keywords: CoMP; User Selection; Resource Scheduling; System Level Simulation

1. Introduction

As one of the key techniques in LTE-Advanced, CoMP

is discussed in detail in the 3GPP. With its effect in re-

ducing interference between neighboring cells, optimi-

zating the whole system performance of multiple cells

and expanding the coverage of high data rate, people pay

more and more attention to it[1]. Its basic principle is

through the coordination of different base transceiver

stations (BTS), the interference can be converted to use-

ful signal. According to the information can be shared

among multiple transport nodes or not, CoMP technol-

ogy can be divided into the joint processing (JP) and

beam forming[2].

Resource scheduling algorithm for JP is a research

hotspot in CoMP to further improve the system per-

formance[3,4], as the CoMP system requires the alloca-

tion of resources in multi-point coordinated cells, and

single-cell resource scheduling can’t meet the require-

ment of the system scheduling, we need to design sched-

uling algorithm which can consider interference between

adjacent BTSs. The author in [5] has investigated a us-

er-centric scheduling scheme in downlink channel to

provide coordination for majority of users, so as to im-

prove the average throughput, but the increasing of the

system complexity was not consid ered. In [6], centralized

control in CoMP system was researched and the author

proposed to use the MAC layer to transport the informa-

tion of channel status, so as to reduce the system com-

plexity. In order to avoid the waste of resources and re-

duce the scheduling complexity, collision avoidance re-

sources scheduling scheme for CoMP was proposed in

[7].

In this paper, the scheduling scheme based on re-

sources for single-user multi-input multi-output (CoMP-

SU-MIMO) is researched, which improves the traditional

scheme of dividing the users into cell-edge users and

cell-center users by the SINR values[8], and 2 or 3 sec-

tors coordination is added to reduce the waste of re-

sources, at the same time, on the basis of the scheme to

divide the frequency into fixed two parts, resources op-

timization is considered in one part to ensure profit both

edge and center users. We also set up a system level si-

mulation platform and the simulation results also verify

the effectiveness of the proposed algorithm.

The rest of paper is organized as follows: In section II,

the system model of JP in CoMP is described. Section III

introduces the specific scheduling algorithm and the si-

mulation results are showed in section IV. Then conclu-

sions are drawn in section V.

2. CoMP JP System Model

The downlink of a cellular network with M hexagonal

cells is considered and each cell is partitioned into 3 sec-

tors with K active users served within the coverage of

each sector. The total number of BTSs in the system is M.

Each BTS which corresponds to one sector is equipped

with Nt transmit antennas, while each user has Nr receive

antennas. We assume each three adjacent sectors as a

cluster[9], show as in Figure 1.

1) Considering the serving BTS m, and user k in clus-

ter c severed in the CoMP mode, the received signal is

given as:

X. C. ZHONG ET AL. 137

Figure 1. The system model of CoMP JP.

()() ()() ()

()()

kmk kknk kk

jkj j

ytHtwstH twst

HtwstN















 (1)

where the first item in the right-hand-sid e(RHS) of equa-

tion(1) presents the transmission signal of BTS m, Hm,k(t)

denotes the NrNt channel matrix from BTS m to user k,

sk(t) denotes the signal from BTS m to user k, wk pre-

sents the precoding matrix of sk(t). The second item pre-

sents the coordinated signal from the BTSs of cluster c,

Hn,k(t) denotes the channel matrix form other BTSs in

cluster c to user k. The third item presents the interfer-

ence signal from the BTSs out of the cluster c to user k.



Assuming that each BTS has perfect channel state in-

formation(CSI) of all the users in the sector coverage,

and BTS within the same cluster can fully share CSI and

user data, the SINR of user k is given as:

()

kmkmn nkn

nCnm

jjk j

pHwpH w

SINR NpHtw















(2)

where pk presents the transmit power from BTS m to user

k, pn is the transmit power from other BTSs in cluster c to

user k, and pj presents the interference power from BTSs

out of cluster c to user k.

2) Considering user k in cluster c severed in the No-

CoMP mod e, and th e ser vin g BTS is m, then the received

signal is given as:

()() ()() ()

()()

kkknk

jkj j

ytHtwstH twst

HtwstN















(3)

where the second item is not the coordinated signal but

the interference signal from BTSs of the cluster c to user

k, then the SINR of user k is given as:

0, ,

()

kmkm

jjk jnnkn

jnCnm

pH w

SINR

NpHtw pHw











(4)

where presents the transmit power from BTS m to user k,

is the interference power from other BTSs in cluster c to

user k, and i presents the interference power from

BTSs out of cluster c to user k.

3. The Proposed Scheduling Algorithm

3.1. User Selection

Proposed in [8], the network is divided into a number of

disjoint clusters, where each cluster contains 3 adjacent

sectors, and all of the users are divided into cell-center

users and cell-edge users according to the SINR, which

work in the No-CoMP and CoMP mode respectively.

When in CoMP mode, all the three sectors in the cluster

send signals to one user at the same time. But fixed three-

sectors coordination can’t guarantee the best scheduling

of resources, as shown in Figure 2, when the channel

situation is not good enough between the user and the

BTS, the sector of the BTS is not suitable to coordinate

with the adjacent sectors. Considering this situation, two-

sectors CoMP mode is added.

1) three-sectors CoMP user: According to the equa-

tions (2) and (4), we can describe the SINR of user k

working in No-CoMP and CoMP mode as SINRno-CoMP

and SINRthree-CoMP, then user k works in three-sectors

CoMP mode if

three CoMPNo CoMP

SINR SINR



 (5)

2) two-sectors CoMP user: According to the equation

(2), considering user k works in CoMP mode with two

sectors which contain better channel gain in the cluster,

we can describe the SINR of user k as SINRtwo-CoMP, then

user k works in two-sectors CoMP if

no CoMPtwo CoMPno CoMP

SINR SINRSINR

 .





 (6)

3) cell-center user: Other users are cell-center users

which wor k in No -C oMP mode.

3.2. Resources Optimization Scheduling

In traditional scheduling scheme, the frequency band is

divided into two parts, one for CoMP mode and the other

for No-CoMP mode. PF scheduling algorithm is imple-

mented separately in each frequency zone for different

Figure 2. An example of clusterd network.

X. C. ZHONG ET AL.

138

work mode. However, this scheme can hardly guarantee

the fairness within the system while improving the sys-

tem throughput at the same time. In addition, the tradi-

tional schemes arrange the CoMP frequency zone at the

beginning of the whole frequency band, which may not

be the best RBs to cell-edge UEs for CoMP-SU-MIMO

transmission. A kind of resources optimization schedul-

ing scheme is proposed in this paper, The basic idea is

the combining of the fixed partition and flexible sched-

uling of resources. One part of the resources are used to

ensure the demand of the cell-center users and in the

other part, starting from each resource block(RB), the

most appropriate way of scheduling is selected. In this

way, cell-center users which are the larger part can get

sufficient resources and at the same time, the cell-edge

users can also get the most appropriate scheduling re-

sources. System performance and fairness is balanced

and the main steps are as follows:

1) For all the RBs in the network, we take a specific

proportion to be used for the cell-center users, as shown

in equation (7),



is the ratio of factors which can set

different values according to the state of channel, so that

the fixed RBs used for the cell-center users in different

clusters can be adjusted according to the different load.

No CoMP

NoCoMP threeCoMP twoCoMP

Num

Num NumNum











 (7)

2) Assume the set of all the cell-edge users in the co-

ordination cluster is c, the set of cell-center users in

sector 1 is 1, the set of cell-center users in sector 2 is

, and the set of cell-center users in sector 2 is



2 3



3) The set of fixed RBs which are allocated to the

cell-center users is 1



, and the other flexible RBs are in

the set of 2



4) RBs in 1



are selected for cell-center users, ac-

cording to the PF scheduling algorithm, for the RBs in



, calculate the highest priorities c、1、2

p、3 of

the users in c、1

、2、

p pp

3



respectively, and the

selected users are k, j1, j2, j3.

5) We compare them as follows

ppp p







 (8)

If the left-hand-side of (8) is no less than the

right-hand-side, the system will perform the CoMP mode

in the RB and user k is scheduled for transmission

through BTSs coordination with two or three sectors;

otherwise, the three BTSs of cluster c will operate single

sector No-CoMP transmissions for users j1, j2, j3 inde-

pendently.

6) The schedule of RBs is completed, and if all the

RBs are allocated, the schedule is over, otherwise, a=a+1,

the RB continues to be scheduled. The flow chart of the

proposed scheme is as Figure 3.

4. Simulation Results

In this section, the performance of the proposed algo-

rithm is evaluated through our syste m level si mulation. A

cellular system with 19 hexagonal cells is assumed, and

the distance between base stations is 500 m. The channel

bandwidth is 10 Mhz with 50 RBs and each one is 180

khz with 12 subcarriers. We choose the number of an-

tennas to be Nt=Nr=2. Each BTS has perfect CSI of all

the users in the sector, and BTSs within the same cluster

an fully share CSI and user data.

Figure 4 compares the normalized user throughput

CDF of Round Robin No-CoMP and r esources optimiza-

tion CoMP, and th e result illustrates the use of CoMP can

greatly improve user throughput in hexagonal cells with

the coordination of the different base stations.

Figure 5 compares the normalized user throughput

CDF of three differ ent CoMP scenarios which are Round

Robin CoMP, the traditional fixed frequency CoMP and

the resources optimization CoMP. The result illustrates,

due to the resources optimization algorithm, the majority

cell-center users can get fixed resources and at the same

time, each other RB is able to choose the optimal sched-

uling according to channel state, the throughput of the

users is effectively improved and resource utilization is

optimized.

Figure 3. The flow chart of the proposed scheme.

X. C. ZHONG ET AL.

139

6. Acknowledgements

This paper is supported by Beijing Municipal Science &

Technology Commissio n (Grant No . Z11110006651 1007)

and Fundamental Research Fund for the Central Univer-

sities(2013RC0203).

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A joint user selection and resources optimization sched-

uling algorithm for CoMP is proposed in this paper, the

algorithm adds the flexible two or three sectors coordina-

tion in user selection at the same time, the scheme of

resources allocation is improved. By the system level

simulation, the algorithm is compared with the No-

CoMP scenario and other two CoMP scenarios, and the

results verify the effectiveness of the proposed algorithm.

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