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978-4799-8422-0/15/$31.002015 IEEE

Energy Efficient Carrier Aggregation for LTE-Advanced

Basel Barakat and Kamran ArshadDepartment of Electrical, Electronic and Computer Engineering

Faculty of Engineering and Science, University of GreenwichUnited KingdomEmail: {bb141, ak52}@greenwich.ac.uk

Abstract Traditional wireless and mobile network focuses

on higher throughout, coverage and larger capacity. In future,

energy efficiency is of vital importance for wireless networks due

to a large number of connected and battery constrained mobile

devices. In Long Term Evolution Advanced (LTE-Advanced),

Carrier Aggregation (CA) is proposed to increase the

transmission bandwidth and hence data rate. This paper studies

the effect of CA on the total power transmitted by the LTE-

Advanced eNodeB based on the Orthogonal Frequency Division

Multiple Access (OFDMA) downlink while taking the users

Quality of Service (QoS) constraints into consideration. Thenumerical analysis and results show that by using CA a reduction

in total power consumption can be achieved.

Keywords Energy-efficient mobile network; LTE-A; OFDMA

downlink; Carrier Aggregation ; QoS.

I. INTRODUCTION

Today there are approximately five billion mobile phoneusers and it is estimated that the number will increase to seven

billion wireless devices in the next three years [1]. This hugegrowth in the number of wireless users is causing higher powerconsumption in the base stations (BS) or eNodeB. It is

projected that communication industrys contribution to the

ecological carbon footprint would be more than double in thenext ten years [2]. Currently there are more than four millionBS worldwide; each BS on average consumes 25MWh per year[3]. This enormous number of BS causes more than 55% of thewhole power consumption in the network [2]. Reducing the

power consumption in the base station is critical for thenetwork operator because of its economic and environmentaleffects.

After the successful deployment of LTE, communicationindustry is now focusing on the LTE-Advanced (or beyond 4G)technology. LTE-Advanced introduces additionalfunctionalities that may influence overall power consumptionof the network such as carrier aggregation, extended MIMO,

heterogeneous networks etc. Carrier aggregation allowseNodeB to transmit multiple LTE carriers each with abandwidth up to 20MHz that increases transmission bandwidthresulting higher bit rates.

Energy efficiency in LTE-Advanced is getting the centre ofattention for wireless communication researchers in the lastcouple of years. Several techniques have been proposed toreduce the power consumption, such as cell zooming, sleepmode, cognitive radio etc. According to [3] changing the cellsize (i.e. cell zooming) to adapt to the users traffic conditioncan solve the traffic imbalance problem and reduce the energy

consumption. Additionally, the following four techniques weresuggested to implement the cell zooming in the mobilenetworks: Physical adjustment of BS, BS cooperation,Relaying and BS Sleep mode. Cognitive radio was proposed toreduce the energy consumption and it was demonstrated in [4]that changing the transmission parameters according to thechannel conditions result energy conservation in the network.In [5], the authors developed a power scaling law for optimalrange adaptation. Similarly, several research studies evaluatethe performance of CA in terms of achievable data rate [6]. To

date, the majority of the studies either focuses on the bit rateperformance of CA or energy efficiency in mobile networkwithout considering CA.

In this paper, we investigate the effect of CA on the totalpower transmitted by eNodeB in single and multi-cellscenarios. We consider LTE-Advanced network havingOFDMA downlink air-interface with users in each cell havingstrict QoS requirements. We derive a general expression thatdescribes the relationship between the total power transmitted

by the eNodeB and the users assigned bandwidth. The usersdistribution in each cell is considered as a HomogeneousPoisson Point Process (HPPP) distribution.

The paper is organised as follows: system model ispresented in section II and derives the eNodeB transmit powerfor the system. We defined two different scenarios for the

performance evaluation and are presented in section III. Thenumerical results of the study and related discussion are

provided in section IV. Finally, section V concludes this paper.

II. SYSTEM MODEL

A. System Overview

We consider a downlink OFDMA based LTE-Advancednetwork with mobile users uniformly distributed in the cellwith a density of . For the sake of simplicity, we assumed thatall users are active and have same QoS requirements, so the

LTE channel bandwidth (as per standard), , can be equallydivided among users. We assumed channel noise as AdditiveWhite Gaussian Noise (AWGN) for all sub-carriers with asnoise power spectral density. According to 3GPP release 10and onwards, it is possible for the eNodeB to aggregate morethan one carrier to achieve high data rates. In our system, bothcontiguous and non-contiguous CA is supported. In thissection, we derived a general expression for the total powertransmitted by the eNodeB in a single cell scenario.

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The total power transmitted from the eNodeB for a number of users according to the Poisson distribution, to fulfilthe QoS have a random value, so expectation of can beused. In this case, the expectation of over random userslocations given is defined by law of iterated expectation,then can be expressed as

| (13)

, (14)where . denote the expectation operator.From the HPPP distribution properties the inner expectation in(14) can be expressed as

, (15)For the sake of simplicity, assume circular shape of the cellwith radius R, so the users distribution inside the cell have a

probability density function of 2/. Then the expectation of (the inner expectation)assuming both and are knowncan be obtained by

, (16)To calculate the expectation of knowing both rand weused the value in (12), then the expectation can be expressed as

, (17)Then the total power transmitted,, can be expressed as

2 2 (18)Finally by substituting the value of in (18), the generalexpression of can be written as

(19)From (19) it is obvious that the channel bandwidth has a

significant effect on the total power transmitted by theeNodeB under given QoS constraints. Moreover, the cell

radius R also has impact on the as evident from (19).The number of users is proportional to the cell area under the

assumption that the users are uniformly distributed throughoutthe cell.

III. SYSTEM TESTING SCENARIOS

The performance of the overall system in terms of theeffect of CA on the total power transmitted by the eNodeB istested under two scenarios: single-cell and multi-cell scenario.

A.

Single Cell ScenarioThe first scenario is an extreme case of a single cell

network. The users are uniformly distributed in the cell and thecell QoS requirements are assumed to be equal for all the users.The users are also assumed to be active and receiving datafrom the eNodeB at all time. The power transmitted by theeNodeB varies with respect to the cell radius. For this scenario,we study the effect of aggregating two component carriers onthe power transmitted by the eNodeB.

B.Multi-cell Scenario

In the second scenario we assumed a multi-cell networkand cells covers a pre-defined area. It is assumed in thisscenario that all cells are identical i.e. each cell has the same

properties (e.g. users distribution, noise power etc.) as with thecell in the single cell scenario. The total power transmitted byall the cells that covers the area is calculated by the summationof the power transmitted by each cell. The total powertransmitted by all eNodeB is studied according to the cellsradius with all the sub-carrier combinations of aggregating twosubcarriers.

Fig. 1 shows the relation between the number of cell tocover an area and the cell radius. Its obvious from Fig. 1 thatthe number of cells increases when the cell radius is smaller.In multi-cell scenario the number of cells is related to the areathat they cover as shown in Fig. 1. To efficiently cover the pre-defined area with a circular shape cells, 80 cells of 200m radius

are needed, on the other hand, nine cells of 600m radius canefficiently cover it or four cells of 1000m radius.

IV.NUMERICAL RESULTS AND DISCUSSION

In this section, the overall performance is obtainednumerically using the following parameters. We assumedchannel path loss exponent as 3 and the minimum datarate per user (i.e. QoS) is C= 150 Kbit/sec. The total number ofactive users,, are determined by the area covered and theusers density. It was also assumed that the primary sub-carrier

bandwidth is 5MHz, and all the sub-carriers that had beendefined by 3GPP for LTE-A is tested to be secondary sub-carrier. Without loss of generality, coding and modulation

coefficient is assumed to be 1.The noise power is a function of the channel bandwidth

and the noise temperature,, where is equal to with asBoltzmann constant (= 1.38 10-23m2kg s-2K-1). The pathloss at the reference point is equal to -55dB, the distance

between the reference point and the eNodeB is equal to100m. The total area that the cells cover in the second scenariois equal to 10000 km. The outage probability is assumed to be10and the numbers of resource block assigned to each userare assumed to be the division of the total number of resource

blocks by the number of users.

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The CA combination that has been used isub-carriers aggregation (due to thelimitations). The primary component carrierthe secondary sub-carrier is one of the carrdefined in the LTE-A standard. The primsub-carriers combinations shown in Table 1scenarios.

Table 1 LTE-ADVANCED CARRIERS CO

5 MHzSecondary Sub-car

1.4 3 5 1

CA combination

(MHz)6.4 8 10 1

Fig. 1. Relation between number of cells and cell r

Fig. 2 shows the total power transmittedfulfil the QoS constrains for different cell rad

scenario. It is noticeable from Fig. 2 and theof eNTP, the effect of CA on the power trans

According to Fig. 2 the channel bandproportional to the total transmit power undeconstraints. The maximum bandwidth that cuser according to the 3GPP LTE-Advanced5MHz primary carrier is the 25MHz combinthe power transmitted to fulfil the QoS cons

by using a 5MHz sub-carrier is equal to 7radius of 600m. On the country the power traggregating two carriers to achieve a 25MHto 28.27 dBW to fulfil the same QoS constrasame cell radius.

In the single cell scenario aggregating twthe power transmitted by the eNodeB trequirements at a cell radius of 600m baggregating the 5 MHz and the 20 MHz cneeded to meet the QoS requirements is equa36% of the power needed by using a 5MHz s

of the 8th IEEE GCC Conference and Exhibition, Mu

n this paper is twophysical deviceis the 5 MHz andiers that had beenry and secondarywere used in both

BINATION

ries (MHz)

0 15 20

5 20 25

adius.

by the eNodeB toiuses in single cell

umerical analysisission.

idth is inverselygiven users QoS

n be assigned to astandard with thetion. For instance,trains without CA7.74 dBW at cellnsmitted by usingper user is equalns and also on the

o carriers, reducedo fulfil the QoS

49.47dBW. Byarriers, the powerl to approximatelyingle carrier.

Fig. 2. Total power transmitted by the

Although the larger the ceneeded to cover the area. The

the predefined area in the multwhen the cells have a larger ra

Aggregating two carriersreduced the power needed toinstance the total power transthe QoS in the multi-cell scena nine 600m cell radius is apOn the country, by aggregatin

power needed to fulfil the sa125 dBW. In the multi cell scethe QoS requirements is reduccarriers.

Fig. 3. Total power transmitted by the

scat, Oman, 1-4 February, 2015

eNodeB in a singel cell network

ll radius is, the more power isotal power transmitted to cover

i-cell scenario is still minimisedius as shown in Fig. 3.

in the multi-cell scenario alsoeet the QoS requirements, foritted by the eNodeB to fulfil

rio by using a 5 MHz carrier inroximately equal to 344 dBW.the 5MHz and the 20 MHz the

me QoS requirements is equalnario the power needed to meeted by 65% by aggregating two

eNodeB in a multi-cell network

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V. CONCLUSION

In this paper, we investigated the effect of CA on anOFDMA downlink of LTE-Advanced. A general expressionfor the total power transmitted by the eNodeB with strict QoSconstraints was derived. Afterwards, two different scenarioswere studied and in each scenario two component carriers wereaggregated and compared with the single sub-carrier powertransmission. The general expression showed the effect of the

cell radius and the carrier bandwidth on the energy efficiencyof the overall system. Numerical results show that the CA canreduce the transmission power to fulfil the QoS constraints in

both scenarios by approximately 65% and hence improving theoverall energy efficiency of the system.

Acknowledgment

The research leading to these results was derived from theUniversity of Greenwich Research & Enterprise InvestmentProgramme grant under agreement number RAE-ES-01/14.

REFERENCES

[1] G. Auer, V. Giannini, C. Desset, I. Godor, P. Skillermark, M. Olsson, et

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[2] F. Daquan, J. Chenzi, L. Gubong, L. J. Cimini, Jr., F. Gang, and G. Y.Li, "A survey of energy-efficient wireless communications,"Communications Surveys & Tutorials, IEEE, vol. 15, pp. 167-178, 2013.

[3] N. Zhisheng, W. Yiqun, G. Jie, and Y. Zexi, "Cell zooming for cost-efficient green cellular networks," Communications Magazine, IEEE,vol. 48, pp. 74-79, 2010.

[4] D. Grace, C. Jingxin, J. Tao, and P. D. Mitchell, "Using cognitive radioto deliver 'Green' communications," in Cognitive Radio OrientedWireless Networks and Communications, 2009. CROWNCOM '09. 4thInternational Conference on, 2009, pp. 1-6.

[5] L. Shixin, Z. Rui, and L. Teng Joon, "Optimal Power and Range

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[6] SOHEIL ROSTAMI and KAMRAN ARSHAD. A Novel Spectrum-Aggregating Technique in Cognitive Radio Networks, Internationalconference on Advance Computing and Communication Systems,Decemmber 2013, Dehli, India.