54th International Symposium ELMAR-2012, 12-14 September 2012, Zadar, Croatia

UMTS LTE Downlink Cell Size Calculation

Winton Afri1, Sonja Zentner Pilinsky2 1University of Split, University department of vocational study, Livanjska 5, 21000 Split Croatia.

2Polytechnic of Zagreb, Vrbik 8, 10000 Zagreb Croatia sonja.zentner@tvz.hr

Abstract This paper deals with cell size calculations for UMTS LTE downlink. As LTE uses adaptable modulation technique, the possibility to use various modulation techniques in cells with different size was analyzed. At the basis of Okomura-Hata propagation model, the equation for maximal cell radius was calculated taking into account the height and gain of both base station and mobile station antenna, transmitter power and losses and receiver sensitivity and losses. Various simulations were performed to give insight into signal propagation in European urban and suburban area. At the basis of simulations it is shown for these areas that UMTS cell structure and size can be used also for LTE cells which will be covered with 64QAM modulation.

Keywords UMTS LTE, Cell Size Calculation

I. INTRODUCTION UMTS UTRAN Long Term Evolution (LTE) was defined

by the 3rd Generation Partnership Project 3GPP to the end of 2009. The first release of LTE provides peak rate of 300 Mbps and a radio delay of less than 5ms. LTE supports both frequency division duplex (FDD) and time division duplex (TDD). LTE has a significant increase in spectrum efficiency (by a factor of 2 to 3 for the uplink (UL) and 3 to 4 for the downlink) compared to the previous cellular system. In order to achieve this goal new functionalities are introduced and access schemes are selected. Orthogonal Frequency division Multiplexing (OFDM) is regarded as a key technology given its high immunity to multipath, spectral efficiency and bandwidth scalability. OFDMA based multiple access scheme has been selected for downlink (DL).

Bandwidth efficiency depends of the current modulation technique. LTE uses adaptable modulation scheme (4QAM, 16QAM, or 64 QAM) with the goal to cover large area of the cell with the most complex modulation type (64 QAM). Applied modulation scheme in each area depends of the stability of radio link (signal to noise ratio SNR).

In urban area, where we must satisfy large number of mobile users, we have small cell structure. In urban area average cell size for UMTS users is about one kilometer. This size of cell is expected also for LTE usage in urban area. For such cell size and cowering ranges, antenna systems with both mechanical and electrical downtilt are used [4].

In the paper we analyze which type of modulation can be dominantly used for covering one call in system with small cell structure, short covering distances and adaptable modulation. The emphasis of this paper is on the simulation method for calculating maximal possible cell size for each type of modulation.

Second section of the paper presents Propagation model for UMTS LTE on 2GHz. Third section of the paper considers cell size calculation by using CCIR (now ITU-R) propagation model for mobile propagation. Fourth section of the paper shows results of the mathematical simulations. The emphasis of this paper is on the simulation method of calculating cell size for UMTS LTE different modes of operations.

II. PROPAGATION MODEL FOR UMTS LTE ON 2GHZ Okomura-Hata propagation model for cellular systems in

urban and sub-urban environment was developed at the basis of Okomura s measurements and empirical data and Hata s calculations. This model was used by CCIR (Comite Consultatif International des RadioCommunication, now ITU-R) to publish a detailed model (so called CCIR model) which is an empirical formula of combined effects of free space path loss and terrain-induced path loss [2].

In our work we started our analysis from model with following parameters given in [1]:

Site to site distance: 500 m; Carrier frequency: 2.0 GHz; Carrier bandwidth: 10 MHz; Distance depended path loss:

1037.6 logL I R P1037.6 log R P10I 37.6 log

(1)

where are: R [km] Distance in km, I [dB] 128.12 dB for 2 GHz P [dB] 20 dB penetration loss

Lognormal shadowing with following parameters: 8 dB std dev, 50 m correlation distance, 0.5 correlations between sites. Channel model: 3GPP SCM, extended to 10 MHz

Equation (1) is one very simplified derivation of CCIR model which gives quick and ad-hoc view in propagation losses. For our calculations we adopted more detailed CCIR model [2] as we wanted to analyze the influence of base station and mobile station antenna heights as well as various path loss exponents on cell size. The equation we used for calculating propagation loss is

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54th International Symposium ELMAR-2012, 12-14 September 2012, Zadar, Croatia

10 10 1 2

10 1

69.55 26.16 log 13.82 log44.9 6.55 log10

T T T R R R MHzP L G G L S f h a h Bhd

L G G L ST T R R RT T R R f h a h B69 26 16 l 13 82 l10 10 1 210 10 1 210 f h a h B10 1 21 269 55 26 16 log 13 82 log1069.55 26.16 log1010 16.55 log h1110

10

1 2

10 1

69.55 26.16log

13.82 log

44.9 6.55 log log10

MHZ

km

L dB f

h a h

h d B

10 MHZf1069.55 26.16log M

1 213.82 log h a h1 21 2

10 16.55 log log1010 1 kmh d Blog1011 km (2)

where h1 and h2 are base station and mobile station antenna heights in meters, respectively, dKm is the link distance in kilometers, fMHz is the central frequency in megahertz, and parameters a and B equal respectively

2 10 2 101.1 log 0.7 1.56 log 0.8MHz MHza h f h f101.1 log 0.7 1.56 log 0.8MHz2 10101 1 log 0 7 1 56 log0 7 1 56 log10 2 1010 22 102 (3)

1030 25 log % of area covereds by buildingsB 1030 25 log % 10 (4)

Equation (2) is recognizable as the Hata model for medium-small city propagation conditions, supplemented with a correction factor B which is shown in Fig.1.

5 10 15 20 25-5

0

5

10

15

20

25

30

B

% of area covered of building

Low

cove

ring

Hig

hco

verin

g

Real interval of building density

Figure 1. Correction factor B as function

of the % area covered of building.

A. Path loss exponent calculation In offered model for calculating path loss in (1), path loss exponent is 3.76. But in real situation path loss exponent depends of the base station antenna height. For that reason and for better understanding of the influence of different conditions to the path loss calculation we will use CCIR formula and path loss exponent calculation given in equation (2). By using equation (2) we can express path loss exponent as; (5)

0 10 20 30 40 50 60 70 803.2

3.4

3.6

3.8

4

4.2

4.4

4.6

4.8

hBTS(m)

Figure 2. Path loss exponent as function of the BS antenna heights

III. CELL SIZE CALCULATION For calculations of cell size we have taken as a first

condition the theoretical limit, which says that received power PR has to be higher or equal to receiver sensitivity SR

R RP SRSR (6)

In case when the receiving power is on the threshold (receiver sensitivity SR is always defined for some BER e.g.-BER=10-6 and modulation type e.g.- BPSK1/2) maximal link distance which means maximal cell radius is achieved.

More strict and prompt calculations have to take into account also the feeding, feeder losses, antennas gain etc. By using CCIR model (2) and typical link values such as transmit power, transmit feeder loss, antenna gain etc., we have derived maximal cell distance or cell radius as

(7)

where are:

PT[dBm] Transmit power, LT[dB] Transmit feeder loss, GT [dBi] Transmit antenna Gain, GR [dBi] Receiver antenna Gain, LR [dB] Receiver feeder loss, SR [dBm] Sensitivity of receiver which equals received

power PR in our case fMHz [MHz] Carrier frequency h1 [m] Base station antenna height h2 [m] Mobile station antenna height B [dB] Building density correction factor from

eq. (4) a(h2) [dB] Correction factor from eq.(3)

10 144.9 6.55 log /10h10 16.55 log / 10 1 11 44.9

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54th International Symposium ELMAR-2012, 12-14 September 2012, Zadar, Croatia

For our simulations we have used some parameters given in model used in [1]:

Base station power 46 dBm (40W); Terminal power 23 dBm (0.2W).

The UMTS-LTE transmitter [3] is based on conventional Orthogonal Frequency Division Multiplexing (OFDM) system structure. The blocks of digital data (traffic) are paralleled and mapped in the complex data blocks using different modulation techniques 4QAM, 16QAM, and 64 QAM respectively. Each complex data block, also referred to as symbol, is attached to an individual sub-carrier. These modulation schemas use also UMTS-LTE Pilot Structure (the proposed LTE pilot insertion), zero padding, IFFT, and cyclic prefix.

Different types of modulation and different redundancy coding cause different threshold. In our simulations we use the following receiver threshold values (BER10-6) calculated from empirical data we had [5]:

SR(BPSK1/2) = -99 [dBm] SR(QPSK1/2) = -96[dBm] SR(QPSK3/4) = -94 [dBm] SR(16QAM1/2) = -90 [dBm] SR(16QAM3/4) = -87 [dBm] SR(64QAM2/3) = -82 [dBm] SR(64QAM3/4) = -80 [dBm]

IV. SIMULATIONS RESULTS In all our simulations we adjusted all parameters as close

as possible to proposed UMTS LTE standards and terrain conditions for European urban area. Some results of our simulations are given in this chapter. These results quite good describe the situation in LTE systems and give good insight into conditions where we can expect to use 64QAM and where we have to use other types of modulation.

In the Fig. 3. one can see how maximal distance changes with different percent of area covering by building with type of modulation as parameter. In the Fig. 4. and 5. it is shown how maximal covering distance is influenced by base station antenna heights with different types of modulation as parameter.

When we analyze the obtained results, from Fig.3 we can see that for small cells used in urban areas (typically below 1 km radius) 64QAM can be used. When we analyzed the height of base station antennas we obtained the following results:

In suburban or some urban areas where the percentage of area covered by building is below or at 10% the antenna height should be above 20 m to cover the cell with radius of 2 km. Very good results were also obtained for areas with very high percentage of covering by buildings (i.e. 25% of cove-rage) where with base stations antennas placed above 20 m, cells with radius of 1 km can be covered with 64QAM.

Fig 6. and 7. show three dimensional situations i.e. maximal distance of covering for different base station antenna heights and percentage of area covering by building.

5 10 15 20 250

2

4

6

8

10

12

14

16

18

% of area covered by building

d max

-max

imal

cell

radi

usin

km

BPSK1/2QPSK1/2

16QAM1/216QAM3/4

QPSK3/4

64QAM1/264QAM3/4

h1=40 mh2=1.8 mPT=46 dBmGT=18 dBf=2 GHz

Figure 3. Result of simulations; Change of maximal covering

distance as function of % of area covering by building with type of modulation as parameter

0 2 4 6 8 10 12 14 16 18

0

10

20

30

40

50

60

70

80

90

100

h 1B

ase

stat

ion

ante

nna

heig

hts

inm

dmax - maximal cell radius in km

BPSK

1/2

QPSK

1/2

16Q

AM1/

2

16Q

AM3/

4

QPSK

3/4

64Q

AM1/

2

64Q

AM3/

4

g=10%h2=1.8 mPT=46 dBmGT=18 dBf=2 GHz

Figure 4. Result of simulations; dependence of maximal covering distance on base station antenna heights for 10% of area covering by building and different types of modulation

0 1 2 3 4 5 6 7 8 9

0

10

20

30

40

50

60

70

80

90

100

h 1B

ase

stat

ion

ante

nna

heig

hts

inm

dmax - maximal cell radius in km

BPSK

1/2

QPSK

1/2

16Q

AM1/

2

16Q

AM3/

4

QPSK

3/4

64Q

AM1/

2

64Q

AM3/

4

g=25%h2=1.8 mPT=46 dBmGT=18 dBf=2 GHz

Figure 5. Result of simulations; dependence of maximal covering distance on base station antenna heights for 25% of area covering by building and different types of modulation

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54th International Symposium ELMAR-2012, 12-14 September 2012, Zadar, Croatia

0 5 10 15 20 25 050

1000

20

40

60

80

100

120

h1% density

dmax

Figure 6. Result of simulations; How will maximal covering distance change as function of base station antenna heights for modo0 (BPSK1/2) and % of area covering by building

0

510

1520

25

0

50

1000

5

10

15

20

25

30

% densityh1

dmax

Figure 7. Result of simulations; How will maximal covering distance change as function base station antenna heights for modo6 (64QAM3/4) and % of area covering by building

V. CONCLUSIONS In the paper we calculated maximal cell radius for

covering cells with different modulation types in LTE cellular

systems. By using CCIR path loss model for 2 GHz we have made mathematical calculation model for different conditions. Simulations have been performed by using Matlab program.

In many articles and LTE system descriptions which describe radio planning for UMTS LTE one can find the following statement: LTE deliver optimum performance in a cell size of up to 5 km. It is still capable of delivering effective performance in cell size of up to 30 km radius. Optimal Cell size depends of terrain conditions and many different factors such as frequency, bandwidth, and interference influence of the neighbouring cells. In this article we used only intra-cell values, conditions and parameters.

In model we used typical values for terrain conditions of European urban and suburban areas and UMTS radio links. Obtained results of simulations describe cell size for different conditions very realistic and show that for LTE systems same base station antenna locations as in UMTS systems can be used if we want to obtain coverage with the most complex modulation type i.e. 64QAM.

Limiting radius of covering the whole cell with 64QAM modulation was in all cases at or above 1 km, which is typical radius for UMTS cells. Thus, LTE cells with radius of approximately 1 km can be covered with 64QAM and maximal spectral efficiency can be obtained.

REFERENCES [1] D. Astely, E. Dahlaman, A. Furuskar, Y. Jading, M.Lindstrom, and S.

Parkvall, Ericsson Research; LTE: The Evolution of Mobile Broadband, IEEE Communications Magazine April 2009., p.p. 44-51

[2] J.S. Lee, L.E. Miller; CDMA System Engineering Handbook Artech House, Boston, London, 1988. pp. 165-250

[3] A. Osman and A. Mohammed; Performance Evaluation of a Low-Complexity OFDM UMTS-LTE System Department of signal Processing of Engineering, Blekinge Institute of Technology, Ronneby, Sweden, IEEE In Vehicular Technology Conference, 2008. VTC Spring 2008. IEEE (2008), pp. 2142-2146

[4] M. imko, D. Wu, Ch. Mehlfuhrer, J. Eiler and D. Liu; Implementation Aspect of Channel Estimation for 3GPP LTE Terminals Published in proc. 17th European Wireless Conference (EW 2011), April, Vienna, Austria

[5] Mehrnoush Masihpour and Johnson I Agibinya; Planning of WiMAX and LTE Networks, University of Technology, Sydney Australia, 2011.

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