LTE FEMTOCELLS INTERFERENCE SCENARIO

AND COEXISTENCE WITH THE BRAZILIAN DIGITAL BROADCAST SYSTEM

Jussif J. Abularach Arnez1, Luiz da Silva Mello

1, Carlos Rodriguez R

1 and Pedro Gonzalez Castellanos

2.

1Pontifical Catholic University of Rio de Janeiro, PUC-Rio

1Center for Telecommunication Studies

2Inmetro - National Institute of Metrology, Standardization and Industrial Quality

Rio de Janeiro, Brazil

jussif@cetuc.puc-rio.br, smello@cetuc.puc-rio.br, crodriguez@cetuc.puc-rio.br, pvcastellanos@inmetro.gov.br2

Abstract —The purpose of this work is to evaluate the

interference produced by LTE Femtocells operating at 700 MHz

on Digital Television (SBTD) that, in Brazil, employs the

Integrated System for Digital Broadcast, Brazilian Version

(ISDB-Tb) standard. The study considered indoor interference

from a 700 MHz Femtocell on the digital television channel

located at 695 MHz. Experimental results were compared with

simulation results obtained with the SEAMCAT program.

Keywords — ISDB-Tb; LTE femtocell; Adjacent-Channel

Interference; Monte Carlo Simulation, USRP.

I.INTRODUCTION

In the last years, the mobile telecommunications had a fast evolution, are now in its fourth generation, the well-known Long Term Evolution (LTE). In Brazil, the first LTE systems are currently being implemented at 2.5 GHz, but the available bandwidth is insufficient to answer the demand. The best to increase the spectrum available for LTE is to use the 700 MHz band, as frequencies will be released after the completion of the digital Brazilian television transition that is expected by the end of 2016. However, some coexistence issues between the LTE mobile system and the Brazilian System for Digital Television that is located at adjacent frequency bands must be addressed. Even though the systems will operate in different frequency bands, unwanted interference due to the adjacent channel leakage and the receiver filter imperfection will exist, causing performance degradation in both systems. In the case of the SBTD system, the interference from the LTE system leads to the deterioration of the digital television receiving sensitivity, causing digital television outages and eventually reducing the coverage of the digital television system. Therefore, it is important to define the requirements to protect each other from mutual interference.

Previous research evaluating the interference between a femtocell network and a digital television system to evaluate the interference and guarantee coexistence can be found in [1-6]. The main proposed of this article is to assess the adjacent channel interference in the SBTD produced by an LTE system located at 700 MHz, considering a controlled indoor environment, through computer simulations as presented in [7] and experimental measurements to evaluate the coexistence scenario.

II.THE SEAMCAT SIMULATOR

The SEAMCAT (Spectrum Engineering Advanced Monte Carlo Analysis Tool) [8] is a statistical simulation tool that uses Monte Carlo analysis to assess the interference between radio

communication systems. The methodology considers: (a) unwanted emissions, consisting of the spurious emissions and out-of band emissions of the interfering transmitter represented by the Adjacent Channel Leakage Ratio (ACLR) [9] falling within the victim’s receiver bandwidth; (b) the receiver blocking power, a combination between the Adjacent Channel Selectivity (ACS), that refers to the filter receiver capacity to avoid unwanted emissions [9], and the blocking mode, defined in this case by the Ratio Protection Mode [8].

A basic scenario is illustrated in Fig. 1. The Seamcat tool models a Victim Receiver ( ) connected to a Wanted Transmitter ( ) operating among Interferer Transmitter ( ). This type of interference may belong to the same system as the victim, a different system or a mixture of both. Interference occurs when the Carrier to Interference Ratio (i.e. ) less than the minimum allowable value. In order to calculate the victim’s , it is necessary to establish the Desired Received Signal Strength (dRSS), corresponding to “C”, and the interfering signal strength (iRSS) corresponding to the “I” [8].The desired Received Signal Strength (dRSS) is the strength of the signal received at the Victim Receiver ( ) from the Wanted Transmitter ( ), and all interfering Signal Strengths (iRSS), which is the strength of a signal from the Interfering Transmitter (it) received at the [8], this process is repeated N times.

The desired Received Signal Strength (dRSS) is given by:

(1)

where, the variables described before are [8]: ,

power supplied to the transmitter antenna; , the antenna

gain towards receiver ; , the path loss between

the transmitter and the receiver; , the operating frequency of

the ; , the antenna gain towards the transmitter .

Fig. 1. Typical Victim Link and Interfering Link Scenario [8].

978-1-4799-0543-0/13/$31.00 ©2013 IEEE

The unwanted interference is a function of transmission

power, antenna gains, the power control gain and the

propagation loss. The blocking probability is given by the sum

of all interference signals considering the blocking attenuation

of the victim receiver. The equations for the calculations of

the unwanted interference and the blocking probability are

detailed in [8].

III.SIMULATION SCENARIOS

The simulations scenarios were performed by generation

of 80,000 events that ensure the stability of the results

obtained [8]. The parameters corresponding of the Brazilian

System for Digital Television were obtained from [10], [11],

[12] and are summarized in the Table I. The HeNB femtocell

parameters were obtained from [7], [9], [13-18] and are

summarized in the Table II. It is important to consider the

spectrum emission mask correspond to the HeNB, and the

characteristics of the digital terrestrial television receiver

blocking mask equal to -29 dB [11]. Fig. 2 shows the HeNB

Femtocell emission mask obtained from [9].

The propagation models employed in the simulations

scenarios correspond to the ITU-R Rec. P.1546-4 model for

propagation over land, considering the Broadcasting Digital

System mode [19] and the Hata-Short Range Devices Model

described in [8], [20].

TABLE I. DIGITAL TELEVISION BROADCASTING PARAMETERS

SBTD Parameters

Frequency Band [MHz] 695 (CH 50)

Power [dBm] 69

Bandwidth [MHz] 5.7

Modulation Schemes QPSK , 16 QAM, 64 QAM

Digital Television Receiver Sensitivity [dBm] -89.22 ; -82.42 ; -77.42

Coverage Radius [km] 42

Height Digital TV Broadcast Tower [m] 600

TABLE II. FEMTOCELL PARAMETERS

HeNB Femtocell

Frequency Band [MHz] 700 (FDD)

Power [dBm] 20; 15; 10; 5

Bandwidth [MHz] 10

Power Control Step Size [dB] 0.5

Minimum Threshold [dBm] -10

Dynamic Range [dB] 20

Height (m) 3

Fig. 2.- Home eNodeB Spectrum Emission Mask 20 dBm [9].

Macrocell

Network

Femtocell

Network

HeNB

SEPARATION

DISTANCES

Digital TV Broadcast Tower

DTT

RECEIVER

INDOOR

ENVIRONMENT

Fig. 3. Digital TV Broadcast Tower and Femtocell Scenario

Fig. 3 illustrates the simulation scenario, which consists of

two distinct systems. The first one, the “Victim System Link”

corresponds to a fixed digital broadcasting antenna located at

695 MHz with a 5.7 MHz bandwidth system and a coverage

radius approximately of 42 km [12], in which the digital

terrestrial television receiver is randomly located inside the

digital TV coverage area, with different average separation

distances between 2 to 25 meters from the LTE femtocell

location. The “Interfering System”, corresponds to an indoor

HeNB femtocell operating at the 700 MHz frequency band

and an LTE indoor User Equipment placed randomly into the

femtocell coverage area, which was set to 100 meters.

IV. EXPERIMENTAL SCENARIO

The experimental interference scenario considers two

systems situated in a controlled indoor environment in a

typical digital television broadcaster mode as described in

[10], [21]. The first system, the “Victim System” consists of a

SBTD transmitter and receiver pair [10-12]. On the

transmission side, the digital television signal generator is

tuned to 695 MHz, with a bandwidth equal to 5.7 MHz, as

defined by ABNT [10] and a transmission power level equal

to -5 dBm considering different modulation (64 QAM, 16

QAM, QPSK). On the reception side, consists of a Set Top

Box (STB) and a television set in order to allow the subjective

(qualitative) performance evaluation. In addition, a spectrum

analyzer was used with the Victim System to allow

quantitative performance evaluation of the received signal

(e.g., in terms of BER, MER and received power levels).

HeNB

(Femtocell)

USRP N210

F= 700 MHz

Spectrum Analyzer

Rohde & Schwar

MS8901A

Digital Television

Signal Generator

F= 695 MHz (CH 50)

Evaluated Values

* BER

*MER

*Received Power

NOTEBOOK

SET TOP BOX

SUBJECTIVE EVALUATION

Power

Splitter

VICTIM SYSTEM

INTERFERING

SYSTEM

TV

IN

OU

T

OU

T

OUT

Fig. 4 Digital TV Broadcast Signal Generator and Femtocell Scenario

The “Interfering System” is an LTE Femt

of an USRP N210 [22] operating at 700 MHz

WBX daughterboard [22]. The USRP trans

bandwidth OFDM signal using the BPSK

transmitting different power levels and diffe

bands. Fig. 4 illustrates the experimental setup

V. TEST PERFORMED AND REQU

Digital TV signals with different modulati

QAM, 64 QAM) were transmitted to evaluate

in presence of interference. In addition,

assessment was carried on at different Interfe

Victim System separation distances, between

The main requirement to be satisfied, accordin

BER value which must be less than

MER, in turn, must result on a minim

interference level. Also, it is important

subjective evaluation of the received signal

interpret BER and MER fluctuation asso

interference scenario envisioned.

VI. SIMULATION AND EXPERIMENT

The simulation results considered d

femtocell transmitting power levels varying fr

dBm and different modulations (64 QAM

QPSK) used in the SBTD. For each

and the values wer

experimental results considered different tr

power level from 17 dBm to -3 dBm.

For each measurement, BER and MER

assessed and subjective evaluations were also

5-8 present the simulation results for 700 MH

MHz and 705 MHz, showing that when the Q

scheme is used the probability of adjac

decreases. In contrast, when the 16 QAM

modulation is used, the probability of adjac

increases, showing the worst scenario whe

modulation scheme is used.

Figs. 9 to 11 illustrate experimental resu

protection ratio between Interfering and Victi

femtocell power control for each modulation

TV reception according to the BER and MER

each frequency location. As expected, the

independently of the modulation scheme cor

MHz. When a QPSK modulation scheme is us

the interference produced by the USRP decre

when a 16 QAM and 64 QAM modulation

interference is considerably increased sho

scenario when the 64 QAM modulation sche

interference evaluation also consider MER a

showing an interference level when BER is h

parameters (e.g. ). The results are co

subjective evaluation as we observed digit

outages in this condition.

tocell, consisting

z connected to a

smits a 10 MHz

K modulation at

erent frequencies

up.

UIREMENTS

ions (QPSK, 16

e their robustness

the interference

ering System and

n 2 to 25 meters.

ng to ABNT, is a

[9-11]. The

mum intersymbol

to considerer a

l performance to

ociated to each

TAL RESULTS

different HeNB

rom 20 dBm to 5

M, 16 QAM and

simulation the

re obtained. The

ransmitter USRP

parameters were

o considered. Fig.

Hz, 701 MHz, 703

QPSK modulation

cent interference

M or 64 QAM

cent interference

en the 64 QAM

ults considering a

m Systems and a

n to assure digital

R requirements at

e best scenario,

rresponds to 705

sed in the SBTD,

eases. In contrast,

n are chosen, the

owing the worst

eme is used. The

and BER values,

higher the require

onsistent with the

al TV reception

Fig. 5 Probability of Adjacent Channel Interf

Fig. 6 Probability of Adjacent Channel Interf

Fig. 7 Probability of Adjacent Channel Interf

Fig. 8 Probability of Adjacent Channel Interf

ference at 700 MHz.

ference at 701 MHz.

ference at 703 MHz.

ference at 705 MHz.

Fig. 9 Femtocell Power Level at different Frequency Offs

Fig. 10 Femtocell Power Level at different Frequency Off

Fig. 11 Femtocell Power Level at different Frequency Off

VII. CONCLUSIONS

Experimental and simulations resultsconsistent results with the worst scenario fometers separation distance, assuming the hpower level. As expected, QPSK modulation iadjacent interference by the LTE femtocell MHz for a 12 meters separation distance.modulation presents the results as BER valuemaximum admissible level considering a seplower than 18 meters. The 16 QAM modulatiacceptable BER value in 703 MHz at 12 mdistance. For separation distances above simulations results indicate the interferendecreases. The simulations result shown tmodulation schemes, operation of LTE femtocwith a 18 meters separation distance guaranteebetween LTE femtocells and SBTD.

set for QPSK

ffset for 16 QAM

ffset for 64 QAM

s have shown or 700 MHz at 2 higher femtocell s less sensitive to operating at 701 . The 64 QAM s were above the paration distance on scheme offers

meters separation 18 meters, the

nce considerably that, for all the cells at 705 MHz es the coexistence

ACKNOWLED

This work is supported by In(though a MSc scholarship) and b573939/2008-0 (INCT-CSF).

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