Research on uplink coordinated transmission schemes in LTE-advanced systems

The Journal of China Universities of Posts and Telecommunications April 2011, 18(2): 72–77 www.sciencedirect.com/science/journal/10058885

http://www.jcupt.com

Research on uplink coordinated transmission schemes in LTE-advanced systems WANG Ya-feng1 ( ), WEI Guo-xing1, YANG Da-cheng1, WEI Xiang2 1. School of Information and Communication Engineering, Beijing University of Posts and Telecommunications, Beijing 100876, China 2. Faculty of Engineering and Surveying, University of Southern Queensland, Toowoomba, QLD 4350, Australia

Abstract

The investigation of inter-cell interference mitigation techniques is a key area in wireless communications. Coordinated multiple points (CoMP) transmission/reception is a candidate technique for interference cancellation in 3GPP LTE-Advanced system. However, the coordination scheme in CoMP remains a key research problem to be solved, which will have a strong influence on the performance of CoMP. In this paper, a novel coordinated transmission scheme is proposed for the uplink LTE-Advanced system. In our scheme, several base transceiver stations (BTS) and users are selected as coordination partners which form a CoMP cluster. Joint processing is used at the receiver to mitigate interference. From the perspective of coordinated partner selection, our scheme can be divided into static and dynamic coordination which are both considered to fully exploit the throughput gain of CoMP. The proposed schemes are evaluated by system level simulation and compared with the conventional LTE system based upon single cell processing. Our simulation results demonstrate that the proposed schemes attain superior performance as opposed to the conventional system in terms of cell average and cell edge throughput. Keywords CoMP, joint processing, inter-cell interference, static coordination, dynamic coordination

1

Introduction 

3GPP has started future advancement of long term evolution (LTE-Advanced) since March, 2008. The targets for this evolution are concern with significantly increased instantaneous peak data rates, decreased latency, higher average spectrum efficiency, and cell edge user throughput efficiency [1]. LTE-Advanced is based on orthogonal frequency division multiplexing (OFDM) which transmits data on orthogonal sub-carriers. So the system can eliminate intra-cell interference but still suffers from inter-cell interference (ICI). However, cell-edge users are known to experience large ICI with a frequency reuse factor equal to 1. Therefore, ICI will cause severe degradation to the system performance. In order to overcome this new challenge, the multi-cell multiple-input multiple-output (MIMO) strategy termed

Received date: 18-06-2010 Corresponding author: WANG Ya-feng, E-mail: [email protected] DOI: 10.1016/S1005-8885(10)60047-7

CoMP transmission/reception was proposed and included into the technical report of 36 series in August, 2008 [2]. CoMP is one of the candidate techniques for LTE-Advanced system to increase the cell average and cell edge throughput [3–4]. However, multi-cell coordination remains an open research problem that will strongly influence the performance of CoMP [5]. In this paper, we propose a novel coordinated transmission scheme that can mitigate ICI effectively for the uplink of the LTE-Advanced system. As shown in Fig. 1, a CoMP cluster is composed of a coordinated BTS set and a coordinated user equipment (UE) set. It is assumed in the proposed scheme that BTSs in a CoMP cluster are connected to a high layer core network and managed by a super evolved Node B (eNodeB). Super eNodeB can manage the BTSs and UEs in the CoMP cluster. BTSs in the same CoMP cluster are called coordinated BTSs which can transmit/receive signals jointly. We further assume that the super eNodeB has full channel state information for the coordinated UEs in the CoMP cluster. The coordination partners within a CoMP cluster can be selected

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from all the BTSs and UEs in the system.

Fig. 1

CoMP cluster model for LTE-advanced uplink

From the perspective of coordinated partner selection, we divide our scheme into static CoMP and dynamic CoMP. In the static CoMP scheme, the coordinated BTS set is invariant and the coordinated UE set is determined by the coordinated BTSs. While in dynamic CoMP, the coordinated BTS and UE sets change as the channel condition varies. The static and dynamic CoMP is simple and feasible to be employed in the LTE-Advanced system. Moreover, we will analyze the all-sector CoMP scheme, which is the case where all BTSs and UEs in the system coordinate together. Although it is impossible in practice, it can provide a theoretical gain upper bound that the CoMP technique can obtain. In this paper, spatial channel model extended (SCME) will be adopted in our simulation to evaluate the proposed schemes. The system level simulation results show that the proposed coordinated transmission schemes can obtain better cell average as well as cell edge throughput gain compared to the conventional LTE system. The rest of the paper is organized as follows. In Sect. 2, the proposed uplink multi-cell OFDM-MIMO CoMP system model is introduced. The static CoMP and dynamic CoMP schemes are described in detail in Sect. 3. The performance of the proposed schemes is evaluated in Sect. 4 through system level simulation. Finally, concluding remarks are drawn in Sect. 5.

2 Proposed uplink CoMP system model

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antennas and N T omni-directional antennas, respectively. The minimum frequency allocation unit is resource block (RB) that is comprised of 12 sub-carriers. After scheduling the frequency resource, the UEs in different sectors which are allocated the same RB would cause inter-cell interference among each other [6]. The essential idea behind our scheme is to make a UE coordinate with other UEs that cause severe mutual interference. By this way, the interference caused by the UEs in the CoMP cluster can be mitigated by the multi-user MIMO detection algorithm [7]. Assume that there are U UEs and V BTSs in a CoMP cluster. The BTSs in the CoMP cluster jointly detect the signal from UEs in the same CoMP cluster using the minimum mean-square error (MMSE) algorithm. The useful signal can be detected while the ICI caused by the UEs in the CoMP cluster is mitigated. The structural diagram is shown in Fig. 2. We employ the MMSE detection algorithm at the V BTSs to process the signal from U coordinated UEs jointly. A virtual MIMO array which is composed of VN R receive antennas and UN T transmit antennas is formed.

Fig. 2 Transmission model of uplink CoMP

The signal on sub-carrier n received by CoMP cluster m can be given as

Rn

H n( m ) S ( m ) 

I

¦

i 1, i z m

H n( i ) S (i )  n

(1)

where Rn is a VN R u 1 received vector for sub-carrier n.

S (m)

(m) T [ s1( m ) , s2( m ) ,..., sUN ] is a UN T u 1 vector, representing T

the data streams sent by U UEs in the CoMP cluster m. H n( m ) ª¬ ( F1 H n( m,1 ) )VN R * NT ,...,( Fu H n( m,u) )VN R * NT ,...,( FU H n( m,U) )VN R * NT º¼

is a VN R u UN T

complex matrix, where

H n( m,u)

is a

VN R u N T complex matrix representing the channel gain A multi-cell and multi-user uplink CoMP system is briefly described in this section. Every cell is divided into three sectors. All sectors have the same antenna configuration. That is, BTSs and UEs are equipped with N R directional

between UE u and coordinated BTSs in CoMP cluster m on sub-carrier n. Fu is a diagonal matrix whose diagonal elements

denote

transmit

power

and

path

loss:

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The Journal of China Universities of Posts and Telecommunications

­ ½ ° ° diag ® PTxu PPl1 ... PTxu PPl1 ,..., PTxu PPlv ... PTxu PPlv ,..., PTxu PPlV ... PTxu PPlV ¾.  

 

  

°¯ °¿ NR NR NR

PTxu and PPlv are transmit power and path loss, respectively.

H n( i ) is a VN R u UN T complex matrix representing channel gain from interfering CoMP cluster i on sub-carrier n. S (i ) is a UN T u 1 vector represents data streams sent by interfering UEs in CoMP cluster i. n is a VN R u 1 complex Gaussian random variable with zero mean and covariance matrix V 2 ǿ , denoting the white Gaussian noise on each receive antenna. The MMSE detection is adopted and the equalization matrix can be denoted as [8] (2) Wn ( H n( m )H H n( m )  V 2 IUNT *UNT ) 1 H n( m )H

2011

3.1 Static CoMP scheme As shown in Fig. 3, sectorized antennas are employed in the LTE-Advanced system. The strongest interferences most probably come from adjacent sectors. Take sector 0 for example, the interference suffered by sector 0 would be largely reduced if we mitigate the two strongest interferences from sector 1 and sector 2 in adjacent cells. Therefore, we propose that the coordinated partners of sector 0 are sector 1 and sector 2 in the adjacent cells. In this way, the coordinated BTS set is constructed according to its geographical position and is invariant.

After MMSE detection, the signal-to-interference plus noise ratio (SINR) of the UE j in CoMP cluster m can be given by SINR n , j (Wn H n( m ) ) j , j UN T

¦

k 1, k z j

(Wn H

(m) n

UN T

2

) j , k  ¦¦ (Wn H n ) j , k  V (WnWn ) j , j 2

i

(i )

2

2

(3)

H

k 1

After signal detection, the block error ratio (BLER) can be mapped from the SINR to BLER curves provided by the link level simulation. If a packet is transmitted successfully, the data transmitted in this packet would be accounted into the UE’s throughput. Otherwise, this packet will be retransmitted until it is either dropped or received successfully. Larger SINR not only reduce the system BLER but lead to a higher modulation and coding scheme (MCS) level, which will result in a larger system throughput. The Interference over thermal noise (IoT) is employed as the interference scale and the CoMP system can also reduce it by mitigating the interference. The IoT of UE i at time t is defined as: Ii IoT(t ) (4) Ni

where I i and N i denote the total interference power and thermal noise power, respectively. The system design aims to reduce the IoT and guarantee system stability.

3 Proposed coordinated transmission schemes From the perspective of coordinated partner selection, the CoMP system model proposed in Sect. 2 can be classified into static CoMP and dynamic CoMP.

Fig. 3 Cell structure of static CoMP

The static CoMP is a network-centric scheme and the coordinated UE set is determined by the coordinated BTSs jointly. We choose UEs occupying the same RB in the three adjacent sectors to be the coordinated UEs. The coordinated UEs in a CoMP cluster transmit the signal independently, which will be processed jointly at the BTS side. Channel state information of coordinated UEs is perfectly known at the super eNodeB and the MMSE detection algorithm is used to mitigate the interference. The SINR of every UE in the CoMP cluster can be obtained by Eq. (3). In spite of its simplicity, the static CoMP scheme can mitigate the strongest interferences and significantly improve the system performance. 3.2 Dynamic CoMP scheme We assume there are M coordinated UEs and N coordinated BTSs in a CoMP cluster. Evidently, a NN R u MN T virtual MIMO array is formed. M and N are arbitrary variables that satisfy the condition NN R ! MN T .We will introduce the steps of dynamic CoMP in detail in this section. Dynamic CoMP is a user-centric scheme in which every UE can determine the partners with which to form a CoMP cluster according to the channel condition. Without loss of generality, we take the process of UE i for

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WANG Ya-feng, et al. / Research on uplink coordinated transmission schemes in LTE-advanced systems

example. The coordinated BTSs selection procedure is described as follows: 1) UE i detects the pilot signal of one candidate BTS. 2) If all the candidate BTSs have been detected, go to Step 3), else go back to Step 1). 3) Selecting N candidate BTSs which have the strongest detected signal as the coordinated BTSs for UE i. After the selection of coordinated BTSs, UE i should select M  1 coordinated UEs. The coordinated UEs selection procedure is detailed as follows: 1) The coordinated BTS set detects the pilot signal of one candidate UE jointly using the maximal ratio combing (MRC) algorithm; 2) If all the candidate UEs have been detected, go to Step 3), else go back to Step 1); 3) Selecting M  1 candidate UEs which have the strongest detected signal to coordinate with UE i. When the selection of coordinated UEs is completed, M UEs transmit signal on the allocated RB independently. As shown in Fig. 4, the BTSs in the CoMP cluster detect the signal of UE i jointly and employ the MMSE algorithm to mitigate the interference caused by the coordinated UEs. The SINR of every UE in the system can be obtained by Eq. (3). The performance of dynamic CoMP improves as M or N increases. This is because that more interfering power would be mitigated if more UEs join in the CoMP cluster, and more diversity gain would be achieved if more BTSs join in the cluster.

Fig. 4 Transmission model of uplink dynamic CoMP

3.3

All-sector CoMP scheme

As illustrated in Fig. 5, there are 21 sectors in this scenario and the UEs can only receive signals from these sectors. Signals from other sectors can be negligible because the distance is too far. If all the 21 sectors join in a CoMP cluster, then all the ICI would be mitigated completely. Therefore, obviously this scheme would achieve the greatest system

75

performance.

Fig. 5 Cell structure of all-sector CoMP

However, the scheduling result of each sector should be reported to the super eNodeB and the information exchange will largely increase the burden of the backbone. Another restriction is that the BTS can not accomplish the channel estimation accurately because the cooperative area is too large. As a result, this scheme can not be used in practical applications, rather than serving as a theoretical gain limit for the CoMP system.

4 System level simulation and results In order to simulate and evaluate the performance of the proposed coordinated transmission schemes in multi-cell multi-user MIMO-OFDM system, a system level simulation platform based on LTE-Advanced is established. We consider the scenario where each UE undergoes fast and large scale fading. The fast fading of UEs can be generated via the SCME model [9]. The propagation model can be given by PL 37.6lg D  128.1 , where D is the

distance in kilometer between the BTS and the UE. The shadowing fading is lognormal distributed with mean 0 and standard derivative 8 dB, and correlation distance is 50 m. The macro cellular hexagonal grid model composed of one-tier 7 cells is considered in our simulation and each cell is divided into 3 sectors. The wrap-around technique is employed in cellular configuration to simulate the infinite region. Our LTE-Advanced system level simulation platform is developed by C++ language. The main system simulation parameters are listed in Table 1. The scenarios are as follows: the traditional LTE system using single cell processing, the proposed static CoMP scheme, the proposed dynamic CoMP scheme (M=10, N=5; M=15, N=5, M and N is cardinality of coordinated UE set and coordinated BTS set, respectively) and finally, all-sector

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The Journal of China Universities of Posts and Telecommunications

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CoMP scheme. The traditional LTE system is an MIMO-OFDM cellular system based upon single cell processing. Table 1 Main system simulation parameters Parameters Carrier frequency System bandwidth Neighboring subcarrier spacing FFT size Number of available subcarriers UE distribution Site layout Antenna pattern Traffic type Site distance Propagation model BS total Tx power MIMO channel model Antenna configuration Penetration loss HARQ UE speed Sub-frame duration

Values 2 GHz 10 MHz 15 kHz 1 024 600 Uniformly 7 cells, 3 sectors/ cell A(T )

 min ª¬12 T T 3dB , Am º¼

T3dB

PL

70q, Am

20 dB

Full buffer 500 m 37.6 lg D  128.1 , D is in km 43 dBm SCME 2u1 20 dB Chase combining 3 km/h 1 ms

As shown in Figs. 6 and 7, static CoMP can provide significant gains in the sense of both average cell throughput and cell-edge throughput compared to the traditional LTE system. This can be explained by Fig. 8. We take the interference to sector C into account. I1 and I2 denote the interference from the adjacent sectors in adjacent cells, while I3 and I4 represent the interference from sectors in the same cell. Assuming single cell processing, I1+I2 contribute 47% of the overall interference, whereas I3+I4 contribute 23 %. And the 30 % remain interference is caused by other sectors. Obviously, static CoMP can suppress 47 % of the total interference. Therefore, the UEs is able to achieve significant SINR as well as throughput gains.

Fig. 6 Cell average throughput comparsion

Fig. 7 Cell edge throughput comparsion

Fig. 8 The distribution of interference to sector C assuming single cell processing

Compared to static CoMP, dynamic CoMP will result in larger throughput gains as shown in Figs. 6 and 7, which is due to the fact that channel condition variation is taken into consideration when constructing the CoMP cluster in the dynamic method. As M increases, the CoMP cluster can eliminate more interference and thus increase the cell average throughput and the cell-edge throughput greatly. However, these benefits come at the expense of more complexity and heavier burden to the backhaul. In all-sector CoMP, as all the sectors in the system join in a single CoMP cluster, the ICI would be eliminated completely. So the system throughput of all-sector CoMP can be considered as the theoretical limit of the CoMP system, although it is impossible to apply in practice owing to its high complexity. Fig. 9 shows the cumulative distribution function (CDF) of system IoT. It is observed that static CoMP can reduce the IoT and provide about 3 dB gain as opposed to the traditional LTE system. It is clear in Fig. 8 that 47% interference would be mitigated in static CoMP, leading to 3 dB IoT gain in this scheme. Obviously, the result of Fig. 9 is completely consistent with that of Fig. 8. In dynamic CoMP, the IoT reduces as the size of the CoMP cluster becomes larger because more interference would be mitigated as the number of coordinated UE increases. So the performance of dynamic CoMP will be closer to all-sector CoMP as more UEs join in

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WANG Ya-feng, et al. / Research on uplink coordinated transmission schemes in LTE-advanced systems

the CoMP cluster.

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Acknowledgements This

work

was

(2009ZX03003-004-01),

supported and

the

by

the

National

Key

Project

Natural

Science

Foundation of China (60811120097).

References

Fig. 9 CDF curves of IoT

5

Conclusions

A novel uplink coordinated transmission scheme is proposed in this paper. This scheme can be classified into static CoMP and dynamic CoMP in terms of coordination partner selection. The proposed CoMP schemes using the MMSE algorithm can mitigate strong ICI by sharing the channel information among coordinated BTSs. These schemes are evaluated based on the established LTE-Advanced multi-cell MIMO-OFDM system level simulation platform. Simulation results demonstrate that static CoMP provides great cell average throughput gain as well as cell-edge throughput gain compared to the conventional LTE system. Dynamic CoMP would approach the theoretical limit as more UEs join in the CoMP cluster, albeit at the expense of increased system complexity.

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(Editor: ZHANG Ying)