A Study of Access Methods Effect on the Performance of Two-Tier LTE Femtocell

8/6/2019 A Study of Access Methods Effect on the Performance of Two-Tier LTE Femtocell

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28thNAT NAL RAD C ENCE C NFEREN(NR C 2 )

pril 26-28, 2011, ational elecommunicat on nst tute

d f h d ff h fw k

Mo eh EI- Gen , sam SourourFaculty of E gi eeri g, Alexa dria U iversity, Alexa dria, Egypt, Mo eh_Elge [email protected]

Faculty of E gi eeri g, Alexa dria U iversity, Alexa dria, Egypt, [email protected]

BSTRACTFemtocells a e a promisi g solutio for the provisio of high i door coverage a d capacity. Fur hermore,

OFDMA-based femtocells have prove to be highly versatile whe deali g with cross-layer co-cha eli terfere ce; however, co ce s still exist related to the impact of the differe t access methods to femtocells i aoverlaid etwork. .Femtocells based o a Closed Subscribers group, where o ly device ow ers are allowed toco ect, i troduce severe i terfere ce to microcells' users. O the other ha d, Ope Access femtocells where a yuser ca co ect, does ot bri g m y adva tages to the femtocell ow er [1]. this paper, a study of the two types of access methods (Closed&Ope Access) is proposed a d a discussio of how to overcome their drawbacks by usi g the Hybrid access method is prese ted, which allows all users to co ect but u der restrictio s. The performa ce of the model is evaluated throughout system level simulatio s a d it is show thatHybrid access co tributes to seriously improve the performa ce of the whole etwork.

K LTE, Access methods, Femtocell, Dow li k. NTRODUCTION

Lo g Term Evolutio (LTE) is co sidered as o e of the most suitable tech ologies for the ure deployme tof cellula etworks, tha ks to its capability to suppor high QoS (Quality of Service) a d data rates. O the other ha d, femtocells e low-cost a d low-power cellula home base statio s that co ect mobile users to a etworkoperator by mea s of a broad ba d backhaul[2].Femtocells a e a good solutio for ma agi g the expo e tial

growth of the tra c withi the macro-cells a d for improvi g i door coverage, as show i Fig.I.

80c 60

20

2008 2009 2010 2011 2012Y aFig. 1 : Fo ecast: numbe ofunits (data ext acted om AIResea ch [3])

Si ce femtocells are paid by the e d users, they will also reduce the overall cost of the etwork deployme t for the operators. LTE femtocells have bee poi ted as a good approach for the ture deployme t of wireless broad ba d i door solutio s. However, several aspects of this ew tech ology such as the access method, mobilityma ageme t a d resource allocatio ( eque cy a d power) still eed r her i vestigatio before femtocells become widely deployed. Whe deployi g a femtocell etwork, the access method, which refers to the rights of the users whe maki g use of the femtocells, eeds to be de ed.

The mai strategies for femtocell access have bee proposed:Ope Access where all the users ca access all the femtocells of a give operator.


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performa ce of subscribers a d the level of access that is gr ted to o subscribers. Therefore, the umber of sha ed resources must be care lly tu ed to avoid a large impact i the quality of service of the Femtocellcustomers otherwise, subscribers might feel that they a e payi g for a service that is to be exploited by others.

The impact to subscribers must thus be mi imized i terms of performa ce or via eco omic adva tages.

PRO AGATION MODEL:The radio li ks betwee the Base statio s a d the users are divided i to outdoor, i door, a d outdoor to

i door li ks accordi g to the radio e viro me t. The propagatio loss L caused by slow fadi g at the dow li k ismodeled based o a T path loss model[6]:

door li k (Subscribers FAP)

= 3 d37 OSI0 Outdoor li k ( o subscribers Macrocell BS)

=149( )4/ 1SI0 1Outdoor to i door li k ( o subscribers FAP, Subscribers Macrocell BS)

L = 14 .9( )4 / 31OsI0 1OP LI 1

(1)

(2)

(3)

Where PL, d, a d f are the buildi g all pe etratio loss (dB), the amou t of tra smitter receiver sepa atio(m), a d the eque cy (MHz), respectively. S represe ts log ormal shadowi g with a sta dard deviatio of 8 dB.

V DA TI E MODULATION AND ODING:Adaptive modulatio a d codi g (AMC) has bee proposed i most of the curre t wireless sta dards toa chive higher system capacity, adaptive modulatio is used for maximizi g the data throughput of the subcarriers that a e allocated to the users. This is based o the S NR measureme ts of each sub carrier followed by the applicatio of the modulatio scheme that maximizes the spectral e cie cy while at the same timemai tai i g a acceptable BER. The E uses the CQ (Ch nel Quality formatio ) to tell the etwork of the highest data rate it ca accept. Based o this i formatio , the scheduler chooses a codi g a d modulatio formatfor the ext TT (Tra smissio Time terval) that would work better i poor co ditio s[7], The Dow li k S NR requireme ts are show i Table 1. Where impleme tatio ma gi ( M) is i cluded to accou t for the differe cei S NR requireme t betwee theory a d practicable impleme tatio[8], d the dow li k bit rates (Mbps) for LTE accordi g to the AMC used a d the cha el ba dwidth a e show i Table 2[9].

ModulationQPSK

16QAM

64QAM

AMC 1.6 MHzQPSK 1 2 0.8

16 QAM 1 2 1.516 QAM 3/4 2.3

64 QAM 3/4 3.5

a e ownTbl 1 D r k S R RCoding

1 21 23/ 4

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eqUirements

a e own .pe It atesTbl 2 D r k b ( M b ) ps3.0 MHz 5.0 MHz 10 MHz

2.2 3.7 7.44.4 7.4 14.96.6 11.1 22.3

9.9 16.6 33.5

SINR+IM (dB)4 .5

.914.321.5

15 MHz 20 MHz11.2 14.922.4 29.933.6 44.8

50.4 67.2


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v I ULATION PROCEDURES:The proposed access method is to be tested i a reside tial a ea, which is ra domly placed withi the ce tral

cell of a seve cells etwork, with 3 buildi gs, which co tai femtocells; each buildi g co tai s 20 simulta eoussubscribers with the right for ma i g use of the femtocell resources. A umber of simulta eous outdoor users edistributed ra domly across the street a d these e called o -subscribers, as show i g.2.

The etwork operates at a ce tre eque cy of 3.5 GHz, Both the Macro d the femto base statio s share thesame ba dwidth. The macro-cell dow li k tra smit power is set such that the received sig al to oise ratio at thecell edge is 20 dB, simulatio s is made for ba dwidths of 10 MHz (50 Resource Blocks) a d 15 MHz (75Resource Blocks).

x x

x x

x x

F t lS i &_ o0c. "jn o -su b scrib ers - cQ c <

"So O: cl0 ' \o

x

Fig.2. Snapshot ofthe simulation

Case (1) Closed Access: O ly subscribers e allowed to co ect to Femtocells, whereas o -subscribers a eco ected to Macrocells. The relatio s betwee the throughput a d Outage due to S NR a d lack of resources, for both the subscribers a d o -subscribers, a d the umber of o -subscribers a e discussed to show the be e tsa d drawbacks of the closed access method.

Case (2) Ope Access: Both subscribers d o -subscribers a e equally allowed to co ect to Femtocells, the rest of o -subscribers are the allowed to co ect to Macrocells. The relatio s betwee the throughput a dOutage due to S NR a d lack of resources, for both the subscribers a d o -subscribers, a d the umber of osubscribers are discussed to show the drawbacks of the ope access method.

Case (3) Hybrid Access: A part of the Resources of the Femtocells, referred to as , is reserved for the osubscribers, the rest of o -subscribers a e the allowed to co ect to Macrocells. The rest of the Resources reserved for subscribers.

Matlab system level simulatio is made. The locatio of the street is ra domly chose by the program, a d so

the locatio s of the o -subscribers i the street a d the subscribers withi the buildi gs.Usi g the propagatio loss model, losses betwee mobile users a d base statio s are calculated accordi g to the dista ce.

Sig al to i terfere ce a d oise ratio (S NR) is calculated for each mobile user, a d so modulatio tech ique,codi g rate a d throughput are calculated.

the closed access method case, each of the 60 subscribers co ects to its ow F AP, while o -subscribers tries to co ect to the earest macro base statio , those o -subscribers who are 't able to co ect to the macro base statio because it is lly occupied a e co sidered outage due to lack of resources.

the Ope access method case, each of the 60 subscribers a d the o -subscribers tries to co ect to earestFAP, whe the FAP is lly occupied o -subscribers try to co ect to the ea est macro base statio , thosesubscribers d o -subscribers who a e 't able to co ect to the etwork because it is lly occupied eco sidered outage due to lack of resources.

the Hybrid access method case, a por io of the resources of each FAP(n) is reserved for o -subscribers

while the rest of the resources are reserved for the subscribers. the 60 subscribers a d the 150 o -subscribers try to co ect to earest FAP, whe the FAP is lly occupied o -subscribers try to co ect to the ea est macro


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base statio , those subscribers a d o -subscribers who a e 't able to co ect to the etwork because it is llyoccupied a e co sidered outage due to lack of resources.

For all the three me tio ed cases there is a other reaso for outage which is the S NR: whe resources have

already bee assig ed to the mobile user, but it ca ot co ect to the etwork due to its low S NR resulti g om poor sig al ( d/or) high i terfere ce.Relatio s betwee the throughput a d Outage due to S NR a d lack of resources for both the subscribers ,

o -subscribers a d the overall, a d a e discussed to show how fa the hybrid access methods a ects thedrawbacks a d the be e ts of both the two other methods.

V I ULATION SULTS:A study of the awbacks of each case d their solutio usi g Hybrid access method is proposed.

Case (1): t has bee oticed that: From g.3.1, both the throughput per o -subscriber (Summatio of the throughputs for all osubscribers divided by the umber of o -subscribers) d the overall throughput per user (Summatio of the throughputs for all users divided by the total umber users) decrease as the umber of o -subscribers i creases for both ba dwidths (10MHz, 15MHz).

From g.3.2, both the total perce tage outage (due to lack of resources a d the outage due to S NR)for o -subscribers a d for the overall users i crease, as the umber of o -subscribers i creases for both ba dwidths (10MHz, 15MHz). This is because i closed access method o ly subscribers e allowed to co ect to the femto access poi s, a d o -subscribers are o ly allowed to co ect to the macro base statio of the cell u der study , a d this causes:

1. A great outage i o -subscribers caused by lack of resources , because a limited umber of resource blocks a e available, 50R.B. for MHz d 75R.B. for 15MHz, for osubscribers.

great outage i o -subscr bers caused by terfere ce o e g bo i g fe to access poi ts, because the probability of shari g the same resource block i creases (i.e. the probability of i terfere ce).

,_ 7 0

, -- - - - - - - - - - - - - - _ .

. K. .

k ,

'Qso ' ,. " 4 0 .J: =.

=. ; : = :- .. . + - - - - - - - - - - - * . t t t

.) Non subsc ib lO Hz. v ra lO Hz

on s sc b 15 z- v ra z

10 8 ------ . . - . 7p . - t I M 6 :: ;' , 1. fIso . "C " L 0, _ _______ ___0

Numb of non subscr rs.). Non-subscribe lOMHz Non-subscribe SMHz

e MH - SMH

Fig. 3: Closed access method. (1) The relatio betwee the both the overall a d the o -subscribers throughput,d the umber of o -subscribers,(2) The relatio betwee the overall a d the o -subscribers outage a d the umber of o -subscribers.

Case (2): t has bee oticed that: From g . , both the throughput per subscriber (Summatio of the throughputs for all subscribersdivided by the umber of subscribers) a d the overall throughput per user (S atio of the throughputs for all users divided by the total umber users) decrease as the umber of osubscribers i creases for both ba dwidths ( MHz, 15MHz). From g .2, both the total perce tage outage (due to lack of resources a d the outage due to S NR)for subscribers a d for the overall users i creases, as the umber of o -subscribers i creases for both ba dwidths (10 z, 15MHz). This is because i ope access method both subscribers a d o -subscribers a e allowed to co ect to the femto access poi ts, a d this causes:

1. The subscribers cha ce to co ect to the FAP decreases a d Outage due to lack of resources i subscribers i creases.


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2. The average throughput of the subscribers decreases, a d so the overall average throughputdecreases also.

7I .* " '* =.-WF < : .50 ; > 1* e t

_4 - : ;..__________=. " ._ -.2 . - .f2 7 2

Numbe o o subscr be s.). u sc b lOMH ... v OMH

ubsc b 15MHz ve 15MHz

175 2

rr ----------------


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-

,90

7 0 ' . ; -I " :f ------------ -6 - '

.",-

5" ,- 60ii'" - .

.. ie. . ii

..... ., _ -

W -

_

! 40- ) 30 - .- -- -- -- -- - - , 2i e X ,

b 40 _-._ _ 0f ' - 0f

' - ubscrib r OM z

e

nubscrib r 5M z

e M

0 bs r b r OM z

v ra lO Hz

0 4 D 0 8nubs r b r l5M z

v ra 5 HzFig. 5: Hybrid access method. (3) The relatio betwee the both the overall a d the subscribers throughput, a d ,(4)

The relatio betwee the overall a d the subscribers outage , a dn.

By compa i g g . with g.5.3 a d g .2 with g.5 we ca otice how the drawbacks of ope accesswere solved by Hybrid access.

From the above results, it has bee oticed that the la ger the b dwidth is the better the performa ce is.t has also bee oticed that the value of n should be ca e lly tu ed so that it ca improve the performa ce of

the o -subscribers a d does t reduce the performa ce of the subscribers i the same time.

V ONCLUSIONS:This paper i troduced the three types of access methods, the closed, the ope , a d the hybrid access method;

both closed a d ope access methods suffer om disadva tages.Due to cross-tier i terfere ce, the closed access method reduces the performa ce of the o -subscribers, as it

decreases the throughput a d i creases the outage.Although, the ope access method improves the performa ce of the etwork, it reduces the performa ce of the

subscribers, as it shares their ow resources with others. this paper, Hybrid access method was proposed as a solutio of the above me tio ed problems, it hassolved the problem of the closed access method by improvi g the performa ce of o -subscribers, a d solved the problem of the ope access method by improvi g the perfo a ce of subscribers.

Fi ally, Hybrid access method bri gs together the best of both worlds (closed a d ope access); if it is wellemployed we ca get the be e ts a d overcome the drawbacks of both worlds.

EFERENCES[1] Alva o Valcarce, David L pez-P rez, Guillaum De La Roche, a d Jie Zha g Limited access to OFD

femtocel s, Perso al, door d Mobile Radio Commu icatio s Symposium (P MRC), Tokyo, Jap ,September2009.

[2] David L pez-P rez, Alvaro Valca ce, Guillaum De La Roche, E. Liu, a d Jie. Zha g,Access Methods toWi Femtocel s: A downlink system-level case stu i 11th EEE te atio al Co fere ce oCommu icatio Systems, Nov. 2008.

[3] S. C law, PR and he Poten ial E ect on Fem ocel MarketsFemtoCells Europe, AB resea ch, Lo do ,UK,Ju e 2008.

[4] Guillaume de la Roche, Alvaro Valca ce, David L pez-P rez, a d Jie Zha g"Access Control Mechanisms for Femtocel s" EEE Commu icatio s Magazi e Volume: 48. Ja ua y 2010, Page(s): 33 - 39.

[5}Jie Zha g a d Guillaume de la Femtocel s: Technologies and Deployment"2010 Joh Wiley &So s ,Ltd., chapter:9 , page(s): 281-282 .

[6] Ha -Shi Jo, Cheol Mu , Ju e Moo , d Jo g-Gwa Yook" nterference Mitigation Using Uplink PowerControl for Two- er Femtocel Networ ", EEE Tr sactio s o wireless commu icatio s , Vol. 8, No.10, October 2009, Page(s):4906 4910.

[7] Ajay R. Mishra " Cellular technologies for emerging markets 2G, 3G and b ond Joh Wiley &So s,Ltd, 2010, Chapter 5, page: 132.

[8] Stefa ia Sesia, ssam Tou k, Matthew Ba er "LTE The UMTS Long Term Evolution om Theo toPractice t,Joh Wiley&So s Ltd, 2009, Chapter 22, Page 521.


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[9] Ha i Holma d A tti Toskala "LTE for U S OFD and SC- Based Radio Access " ,2009 JohWiley &So s Ltd, Chapter 9, Page(s): 213 214.

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A Study of Access Methods Effect on the Performance of Two-Tier LTE Femtocell