Identification of Interferers in Het-Net in LTE-A systems Based on FeICIC with Cell

Range ExpansionMir Yasir Umair

School of Electronics and Information Engineering Beihang University

Beijing, China miryaserumair@gmail.com

Dengkun Xiao Beijing Institute

Huawei Technologies Co., Ltd Beijing, China

xiaodengkun@huawei.com

Yang Dongkai School of Electronics and Information Engineering

Beihang University Beijing, China

edkyang@buaa.edu.cn

Fauzi Fanny School of Electronics and Information Engineering

Beihang University Beijing, China

fauzi.fanny@yahoo.com

Abstract— In LTE-Advanced (LTE-A), heterogeneous networks (HetNet) comprising of pico and femto cells are laid onto the traditional well planned macro cells, are extensively investigated for improving the overall system throughput. Cell range expansions (CRE) are effectively used in HetNets to enhance the system coverage as well as the cell-edge throughput. In a HetNet there is a severe interference specifically to the cell edge users of pico cell from the macro cell which makes the cell identification and cell selection, basic procedures in LTE-A, very complex. To mitigate this degradation, suitable Inter-Cell Interference Coordination (ICIC) techniques are required to be deployed. Recently, for LTE and LTE-A eICIC techniques have been further upgraded to Further enhanced ICIC (FeICIC). This paper investigates one of the side conditions of Radio Resource Management (RRM) design, that is, to find the 1st and 2nd interferers with CRE 9 db using FeICIC. The validated results of this simulation are approved in the #RAN4 64 meetings and will be pushed for the 3GPP specifications. Keywords: cell identification; Further enhanced ICIC; Heterogeneous networks; LTE-Advanced; RRM

I. Introduction: Heterogeneous network is a hot topic in the field of cellular communication in which Lower Power Nodes (LPN) are introduced in the LTE and LTE-A systems termed as pico cells and femto cells . These relay nodes aim to extend the coverage, improve the capacity and the system throughput in the next-generations wireless systems. At the platform of 3rd Generation Partnership Project (3GPP) HetNets are extensively investigated due to the ever increasing

data traffic demand. The traditional modern systems are no longer able to cater a consistent throughput with in a cell. In a homogenous environment the User Equipments (UEs) closer to the macro cells enhanced nodeB (eNB) were able to achieve better throughputs and the UEs on the edges of the cell had poor quality of link with the eNB. This inconsistent access of resources to the UEs laid the deployment of Hetnets. The requirements for LTE-A were agreed upon in [1] and the work item (WI) specifications on LTE-Advanced started from Release 10.

Fig. 1. A macro and pico heterogeneous network with range

extension

HetNet is a favorable solution to the aforementioned problem in the homogenous wireless environment. HetNet is a network containing macros and low power nodes such as pico network nodes, Home evolved Node Bases (HeNBs) / Closed Subscriber Group (CSG) cells, and femto and relay nodes, which can

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share the same frequency band [2]. In order to take the full benefit from HetNet deployments, some challenges are needed to be addressed which include interference management, mobility and backhauling. The interference can cause serious problems to some of the most basic procedures, for accessing resources from the LTE Cell, like cell search and cell identification.

In this paper we focus on the interference issues which can lead to the degradation of overall system throughput. During the 3GPP meetings before RAN1 59bis it was discussed in detail in some of the contributions [4][5][6]that traditional Rel 8 cell selection methods may remain uncomfortable for the HetNets. The UE selection methods are significantly dependant on the UE measurements of Reference Signal Received Power (RSRP) so it is expected that very few UEs will choose the LPNs as their serving nodes. This can result in an unbalanced cell load for HetNets. A new scheme RE was proposed which can help UEs to select LPNs as the serving nodes to increase the cell splitting gains where a positive offset can be applied to the RSRP measured from pico-eNBs. This is mathematically expressed as,

)1(}

,{maxarg

##1#

#1

offsetpicomacrojmacro

macrojjselected

RERSRP

RSRPj

+

=

+≤≤+

≤≤

Where, #macro and #pico denotes the set of measured macro and pico cells, respectively. This approach is referred to as the pico-cell Range Extension (CRE) [3]. By introducing this bias in the cell selection, more UEs are pushed to the pico-layer as shown in Fig. 1. Before the #RAN4 63 AH for FeICIC, the side conditions of RRM design have been widely discussed based on 9db CRE. These conditions include Signal to Interference Noise Ratio (SINR), number of strongest interferers and their strengths. But so far these three metrics have not been agreed in the 3GPP meetings. Remainder of the paper is organized as follows. Section 2 includes the problem analysis followed by the system level assumptions in the section 3. Simulation results are discussed in section 4. Finally the conclusion is presented in section 5 .

II. Problem Analysis In this section we investigate the configurations which are the cause of disagreement between various

companies, on the metrics mentioned in Section I. Later in the paper we consider the suitable assumptions so that the results can be validated and can be agreed by all the 3GPP delegate companies. They are as follows. A. Scenario configuration In 36.814[9], #4b(4) and #1(4) are suggested for HetNets deployments. But #4b(4) is suitable for the increasing the Hotspots where the UEs are distributed uniformly and are placed much more closer to the pico eNB. On the other hand, in the case of #1(4) UEs are distributed randomly and by simulation results in [10] it is found that the SINR is lower in #1(4). So in this paper we assume #1(4) scenario is more suitable and can be regarded as a benchmark.

B. Pico Configuration In the system level simulation few cases of pico base stations are considered with different transmit powers and Inter-Site Distances (ISD). Theoretically, SINR is lower in the case of 24dbm pico transmit power with 500 m of ISD than 30dbm pico transmit power with 1732 m of ISD. This has also been proved in [10]. However in this paper, for the sake of comprehensive results, our simulations encompass cases for both pico transmit power and ISD.

C. UE selection The choice of the type of UE is another significant issue which needs to be addressed for suitable findings. From the view point of system throughput enhancement, there is a need for selection of either CRE UE or all Pico UEs so that there is a reference for all the cases to be simulated. Moreover, often 5%-tile or 25%-ile are evaluated to investigate the side conditions. In this paper, as a reference for the side conditions evaluations, we assume to take 5%-ile of all pico UEs.

III. System level assumptions For this paper we intend to align with the baseline system level assumptions for FeICIC. The result of these alignments can also be used as an input to the link level simulations later which is our future work. Initially the interference conditions for FeICIC were discussed in the RAN4 #62bis. In [7], the system level assumptions have been discussed and agreed upon to investigate the interference levels. So in this paper we evaluated the baseline cases only. The parameters involved in simulations are listed in Table 1

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TABLE 1. SIMULATIONS ASSUMPTIONS [8]

Parameter Setting

Deployment scenario

Reuse Rel-10 deployment scenarios: #4b(4) – configuration #4b with 4 pico nodes per macro area, #1(4) – configuration #1 with 4 pico nodes per macro area

PCI assignment Macro cells: lanned PCIs with 3-reuse per macro site (baseline) Pico cells: Random PCIs for pico cells (baseline)

ISD 500 m and 1732 m Cell selection offset

9 dB

Maximum eNodeB transmit power

Macro: 46 dBm Pico: 24 dBm and 30 dBm

Subframe alignment

SFN-aligned

Frequency / bandwidth

2GHz, 10 MHz

Antenna gains & configuration

Macro: three-cell, 14 dBi incl. connector loss, 3D pattern Pico: omni, 5 dBi incl. connector loss UE: omni, 0 dBi

Es/Iot calculation

per RE, before interference mitigation

Traffic model Full buffer, full load Load In non-ABS: full load

In ABS: signal/channel-dependent and RE-dependent

ABS configuration

Almost Blank Subframe (ABS) pattern is the same in all cells using ABS.

Path loss Baseline: Macro to UE: L= 128.1+37.6log10(R), Pico to UE:

RL 10log7.367.140 += , R in km Here, it should be noted that many contributions from different companies using the 3GPP meetings platform have been investigated to find the most suitable baseline cases.

IV. SIMULATION RESULTS For all the configurations in the previous section, system level simulations were performed, the values of the 1st interferer SNR are shown in Table 2.

TABLE 2. FIRST INTERFERER VALUES

From the Table 2 it can be deduced that the SNR value of the 1st interferer for all the cases approaches to 4db approximately and it is the most suitable assumption for the side conditions.

Table 3 summaries the key values for the difference of the first two strongest interferers simulated for all the cases. These values are taken from the 20%-ile of their CDF. In all the cases the difference is approximately 2db. This value is significant for finding the value for 2nd interferer. All the simulation results are depicted in Fig. 2 to Fig. 5. These results are all obtained by the system level simulations. Our platform simulator logged the three strongest interferers that a UE received. It later calculated the difference between the 1st and 2nd interferer and the difference of 1st and 3rd interferer. Our primary objective for using these figures is to calculate the difference between the 1st and 2nd interferers. Hence, we can ignore the second curve in the figures, which depicts the difference between the 1st and 3rd interferers.

TABLE 3. DIFFERENCE OF FIRSRT-SECOND INTREFERER

Cases ISD Pico Tx Power

Difference of 1st-2nd interferer at 20%tile

Case 1 500m 24dbm 1.99 Case 2 1732m 1.72 Case 3 500m 30dbm 2.07 Case 4 1732m 2.02

Fig. 2. Difference of Interferers-Case1

Cases ISD Pico Tx Power

1st interferer SNR

Case 1 500m 24dbm 4.706 Case 2 1732m 4.312 Case 3 500m 30dbm 4.403 Case 4 1732m 4.153

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Fig. 3. Difference of Interferers-Case2

Fig. 4. Difference of Interferers-Case3

Fig. 5. Difference of Interferers-Case4

From the Table 3 the value of 2nd interferer can easily be deduced as we have found the value for the 1st interferer earlier. It is obvious that for all cases most suitable value for the 2nd interferer is 2db

V. CONCLUSION In this paper we have investigated the side conditions for the RRM design test cases using FeICIC in a HetNet with CRE 9db using system level simulations with baseline cases. In a HetNet for LTE-A systems where pico cells are laid onto macro cells, accessing the resources becomes very complicated due to the inter cell interference. For interference management it is very significant to know the strongest interference levels from the neighboring cells. In this paper first we evaluated the 1st and 2nd interferer values using FeICIC. On the basis of our simulations, we recommend to define the 1st and 2nd interferers as 4 db and 2 db respectively. ACKNOWLEDGMENT This work is derived from cooperation project of Huawei Technologies Co., Ltd. and Beihang University

REFERENCES [1] 3GPP, TR36.913 (V9.0.0), “Requirements for further advancements for E-UTRA (LTE-Advanced),” Dec. 2009. [2] Rose Qingyang Hu, Yi Qian, Sastri Kota, Giovanni Giambene, “HetNets––A New Paradigm for Increasing Cellular Capacity and Coverage,” IEEE Wireless Communication, Jun. 2011. [3] Yuanye Wang and Klaus I. Pedersen “Performance Analysis of Enhanced Inter-cell Interference Coordination in LTE-Advanced Heterogeneous Network,” Vehicular Technology Conference (VTC Spring), 2012 IEEE 75th [4] [RI-083813, "Range expansion for efficient support of heterogeneous networks," Qualcomm Europe; [5] RI-094225, "DL Performance with Hotzone Cells," Qualcomm Europe; [6] RI-094882, "Importance of Serving Cell Selection in HetNets," Qualcomm Europe [7] R4-122229, “System simulation assumptions for intra-frequency FeICIC studies”, Ericsson, ST-Ericsson Huawei, HiSilicon, TSG-RAN4 #62bis, March 2012 [8] R4-122449, ” Further system level simulations for FeICIC with 9 dB cell range expansion”, Qualcomm Incorporated, TSG-RAN WG4 #63, May 2012 [9] Further considerations on interference conditions for feICIC, Renesas Mobile Europe Ltd [10] Additional considerations on interference levels for FeICIC core requirements 36.101 v. Qualcomm Incorporated

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